Topic: CB550K - Early 4-into-4 exhaust with PD Carbs - UPDATE

[Dave Voss]

In a recent thread about the power output of the late 77-78 CB550K, it was mentioned that the stock 4-into-4 exhaust for those 77-78 model years was more restrictive than the earlier model years, and that the later PD carbs were tuned specifically to work best with this system.

My 1978 CB550K has the earlier CB550K 4-into-4 exhaust system, as evidenced by the tapered tips, in lieu of the straight tips (must have been replaced at some point in time by a previous owner).  My carbs are the stock PDs that are correct for a 78 model year, they have been recently removed and cleaned and adjusted, and I have verified that they have the stock 42 slow and 90 main jets, also I'm using the stock air box with a UNI filter cartridge (similar to stock).

Now that I have put about 500 miles or so on this motorcycle since completing quite a bit of refurbishment work, I can say that it doesn't seem to have any tuning issues, and power output seems adequate.  My spark plugs appear neutral (not rich or lean) with only very light deposits as is expected, so I'm tempted to just forget about it and enjoy it the way it is, but perhaps it could/would run even better with slightly larger main jets, if in fact the early exhaust system has less back pressure and it is leaning out the mixture a bit?

Does anyone have any direct experience with this particular setup?

[uksparky]

Dave i have the same set up as you 4x4 exhausts 78 550k, same spark plug cond, exept i have the pods...idle air bleed 2.5 turns out....runs perfect idles smooth. If your bike runs ok i would ride it and forget about re jetting

[Bankerdanny]

Dave, Don't be one of those, "if it ain't broke, fix it 'till it is" guys.

There is thread after thread from owners who can't get their bikes to idle right or rev freely etc. You have a bike that has none of those issues and yet you are thinking maybe it can be 'better'.

Put down the screw driver and step away from the carb's and nobody get's hurt. Ride and enjoy.

[Dave Voss]

Dave, Don't be one of those, "if it ain't broke, fix it 'till it is" guys.

There is thread after thread from owners who can't get their bikes to idle right or rev freely etc. You have a bike that has none of those issues and yet you are thinking maybe it can be 'better'.

Put down the screw driver and step away from the carb's and nobody get's hurt. Ride and enjoy.

All good points, and overall good advice, however I've learned that sometimes the difference between 'good/adequate' and 'great/optimum' is the result of fiddling around a bit after things already seem to be right, even when it's against my better judgement to just leave it alone.  I figured there might be others who have already 'been-there-done-that' to offer up their experiences.

Years ago I owned a (stock) 1975 model year CB550K, and I recall it having just a bit more power than the 1978 model year CB550K that I have now, but it's a hard comparison to make with the time separation.  My one thought is that I *might* be running just a bit too lean at higher rpms (6k~8k) if the exhaust is a bit too free flowing, and I'm not currently riding at that sustained rpm often enough to be able to see it on the spark plugs.  I'm not sure how best to know, apart from experimenting with slightly larger main jets, and observing power output against spark plug condition and fuel economy.

[cosmicvision]

Hey Dave, I have the exact opposite setup, a '74 550k that someone put a late exhaust on at some point. All stock otherwise. My plug deposits are always a nice neutral tan if I'm crusing but around town on acceleration & such they're a little darker than is ideal. It's the difference between adequate & optimum that you mention.

May be a far-fetched notion...but if you were ever interested in reverting to the proper pipes, I would be interested in swapping, granted we both find the condition of both sets acceptable. Mine are in nice shape for an everyday rider. I've wondered before if there'd ever be someone with a late bike & early pipes looking to have the proper setup, I like the trumpet tips of the earlies. Don't know how much it would be to ship those things back & forth but I thought since we're all sitting here behind our keyboards anyway I would throw it out there!!

[Dave Voss]

My plug deposits are always a nice neutral tan if I'm crusing but around town on acceleration & such they're a little darker than is ideal.

Thanks Joshua, your opposite application (from mine) sounds like it affects your optimum state of tune by about the same amount, but in the opposite direction, possibly some confirmation that there is indeed a flow difference between the early and late stock exhaust.

I also like the look of the early tapered exhaust pipes, and mine are still in pretty nice shape (it's possible that they were purchased as NOS and installed mid-life on this motorcycle) so I'd rather tweak the carbs a bit (if it turns out to be beneficial) than to fit the 'correct' later exhaust.  FWIW, I think the shipping costs would be quite high as well.

[cosmicvision]

Right on, from what I can see in your avatar they do look nice. Good excuse to size up those jets eh?

Also quite interesting to confirm that the differences in exhaust systems will throw the ideal tune in a subtle way. Cheers!

[TwoTired]

Years ago I owned a (stock) 1975 model year CB550K, and I recall it having just a bit more power than the 1978 model year CB550K that I have now, but it's a hard comparison to make with the time separation.  My one thought is that I *might* be running just a bit too lean at higher rpms (6k~8k) if the exhaust is a bit too free flowing, and I'm not currently riding at that sustained rpm often enough to be able to see it on the spark plugs.  I'm not sure how best to know, apart from experimenting with slightly larger main jets, and observing power output against spark plug condition and fuel economy.

Um.   Were you the same weight years ago?  Are the tires the same size/weight as your recollection?

I haven't tried the configuration you have.  I do have a 78 Cb550K with stock components.  I don't thing it is as fast as my 74 K or my F models.  I do know that the fastest Cb550 I have is probably my stock 77 CB550F model.  I don't know exactly why.  And, I'm not taking it apart just to find out.

I would expect a 78 CB550K to run lean with early exhaust pipes, particularly with ethanol diluted gasoline and a Uni foam filter.

Do make certain the Uni filter is not over oiled.

To verify fuel metering correctness, you have three choices:
1. Put it on a dyno with an exhaust sniffer.  It will give you a fuel map, which you can address with the report.

2. A test track, brand new spark plugs and WOT acceleration runs to top speed, followed by hitting the kill switch, pulling in the clutch , and coming to a stop. Pull out each spark plug and "read" the deposit pattern to determine current fuel mixture suitability.  Adjust the main jet until the pattern is correct.  Then, repeat the testing with half to 2/3rd throttle runs, reading clean plugs between runs and spark plug inspections.

3. Try guessing different jetting/adjustments until you feel it is good enough, or you run out of patience. 

[Stev-o]

I had a '77 550K with stock exhaust and it didn't seem as fast as my '76K with an F header.
The bike was in very good cond and all original but I just didn't like it for some reason, I think the exhaust was a big part.  Sold it and never looked back. 

[TwoTired]

When these bikes were newish, many thought it worthwhile to make mods, in order to eek out performance and perhaps surprise other bike models/owners.
I just don't get doing that to a 30-40 year old machine.  You aren't going to make it remotely as fast as a modern bike without gobs of money.

Why not appreciate them for what they are?  I happen to like my not so fast 550s, every bit as much as the "fastest" one.  Of course, my racing days are well behind me, now.  But, they all have their thrill factor, even if not extreme.

Just a thought...

[Stev-o]

Good suggestion TT. 
I tend to like the bike that runs the best, ie: always starts right up, good acceleration, no issues or break downs, just fun to ride.  My 550 fits that bill, K4 most of the time. 

[Dave Voss]

Lots of great suggestions, comments, and sage advice, thank you.  TT & Stev-o, I'm on the same page as you about enjoying and appreciating these motorcycles for what they are, as opposed to what they could be, and I've long since lost interest in maximum power, top speed, etc. as I would much rather have a smooth running well adjusted reliable ride.  Oh, and yes I weigh about the same as I did then, give or take 5 lbs.

I started this thread because I had a suspicion that I might not have the proper carb settings, in spite of the fact that the engine seemed to be running alright so far.  Well, I decided to try a test of sorts on my way home from work this afternoon (30-mile commute, mostly highway miles), so I selected 4th gear and ran the engine at 7k with about 3/4 throttle while on the freeway, with the intention of checking the condition of the plugs for evidence of lean running once I got home.

After about 10 minutes of running like that, the engine began to loose power, and it was difficult to maintain the speed of traffic, so I had to limp along at a slower speed, thankfully I was nearly at the end of my freeway miles, and ready to take a two-lane road toward home.  The engine was still running and responsive below 1/2 throttle so I decided to keep going the remaining miles home.  While waiting at a few red lights, I could smell the heat from the engine and hear what sounded like the tell-tale 'ticking' of a compression leak, presumably a failed head gasket.

After shutting it off at home, it became obvious that the engine was running *very* hot, and it took much longer than usual to cool off enough to touch.  I removed the spark plugs, and this time they all appeared much ligher in color than before, the cores especially were very white, which seemed to confirm a lean running condition.  With the plugs out, I ran a compression check:  1: 60psi, 2: 85psi, 3: 120psi, 4: 120psi.  Several weeks ago, all four cylinders were up around 140psi.  So, it appears that the head gasket failed, probably the sound that I heard before shutting it off.

I'm thinking that the extended higher rpm run at more than half throttle in combination with a lean running condition with the earlier exhaust and UNI filter overheated the engine enough to blow the head gasket.  The stock slow jet size is 42 (should be fine) but the stock main jet size is 90, which must be close enough at lower rpm and part throttle, typical of most of the riding that I've been doing since getting this motorcycle running several weeks ago.  In conjunction with performing a head gasket replacement, I'd like to find a set of larger main jets, perhaps size 100 like the earlier carbs used as stock.

Ok, what else should I be thinking about here...?

[scottly]

Try squirting some motor oil into the #1 and 2 cylinders and rechecking compression. Also, make sure the tappets are adjusted reasonably close to spec on those two jugs.

[TwoTired]

I wouldn't expect an original head gasket to fail, even with high heat.  I, more likely, suspect valves burnt.  Do check tappet clearance before your next comp test.

A few drops of oil, just enough to wet the rings is a good idea at this point after another dry test.  Too much oil will change the volume of the compression space, and may lead you to falsely suspect rings.

However, if something were really damaged by heat, I'd expect signs of melted metal on the spark plug tips and insulator.

It could be that the heat cooked the rings enough to make them stuck in the lands.  I did that once with low oil.  And, in a couple hundred miles it straightened itself out, after refilling the case.

Don't forget to hold the throttle wide open during the Comp. test.

[scottly]

While waiting at a few red lights, I could smell the heat from the engine and hear what sounded like the tell-tale 'ticking' of a compression leak, presumably a failed head gasket.

After one too many high RPM excursions, my 750 developed a sudden loss of power, accompanied by a distinct "ticking" noise. One of the tappet adjusters snapped off with the lock-nut, allowing the remaining piece to back away from the valve stem..

[Dave Voss]

Ok, here is an update as of early this morning.  With the engine totally cold from sitting overnight, I performed another dry compression check, and this time all four cylinders were between 130~150psi.  At this point, I'm pretty sure the valve clearances tightened up while the engine was running too hot, and compression was being lost as a result, also causing the noise that I heard (most likely compression loss through the exhaust).  All of the valves had some clearance this morning, but I'll need to verify that the adjustments are still correct.

So, those who said the head gasket might not be the problem were right (thanks for the sanity check) which is great because I wasn't really looking forward to taking this engine apart at this time.  Now, I need to address the hot running condition, which I believe I can solve with a set of larger main jets, since the spark plugs have the appearance of lean running after yesterday's issue.  I know the PD carbs use different jets, and I removed one to try to match at one of a few local motorcycle shops, hopefully I'll find what I need.  If not, I may resize these 90s to 100s using a jet drill kit.

[TwoTired]

Now, I need to address the hot running condition, which I believe I can solve with a set of larger main jets, since the spark plugs have the appearance of lean running after yesterday's issue.  I know the PD carbs use different jets, and I removed one to try to match at one of a few local motorcycle shops, hopefully I'll find what I need.  If not, I may resize these 90s to 100s using a jet drill kit.

Glad it doesn't seem too serious.

Don't expect the main jet to solve all the mixture problems.  It's contribution dominates in the 3/4 to full open throttle range.  The mid-range is dominated by the slide needles.

Further, the PD carb's venturi shape is different then the early carbs.  I've long suspected this was one reason why it can use a much smaller main jet, as it probably creates more vacuum with the same air volume passing through it. 

I'm going to suggest that a #95 main jet be used and to raise your slide needles one notch.  And then, check plug for deposits.  If you drill (not really recommended, but), you can always drill them larger if needed.  Drilling them smaller is far more difficult.

Another, suggestion is to examine the float levels.  There are indications that the 78s needed a 12.5mm float height rather the 14.5mm often found.  Raising the float level would make it easier to lift fuel into the carb bore and this would enrich the entire operating range.

I'm also going to suggest you alter the carb number stamping, as that number describes the stock internal set up, which you plan to change.  Not many people do that and make liars out of the carb markings. (stealth markings?)

[Dave Voss]

Glad it doesn't seem too serious.

Don't expect the main jet to solve all the mixture problems.  It's contribution dominates in the 3/4 to full open throttle range.  The mid-range is dominated by the slide needles.

Further, the PD carb's venturi shape is different then the early carbs.  I've long suspected this was one reason why it can use a much smaller main jet, as it probably creates more vacuum with the same air volume passing through it. 

This is interesting, because the engine has been running really well at part throttle, and my early spark plug readings after several hundred miles of light cruising at lower and middle rpm ranges didn't really indicate any issues, it was only once I began to cruise at more than half throttle in the higher rpm range that the lean running condition showed up.

I'm going to suggest that a #95 main jet be used and to raise your slide needles one notch.  And then, check plug for deposits.  If you drill (not really recommended, but), you can always drill them larger if needed.  Drilling them smaller is far more difficult.

Since the engine ran lean enough to overheat as it did, do you think that #95 main jets in place of #90 main jets is enough of a change?  I would rather err on the side of being a bit too rich to avoid overheating again, but perhaps I'm just being overly cautious given yesterday's issue.  I don't like to resize jets either (primarily to avoid having to keep track of that type of change), but have on occasion when I couldn't find any.

Raising the needles will primarily affect midrange mixture, correct?  Is this the reason that you are suggesting a smaller increase in the main jet size, so that in combination with raising the slide needle, the mixture will be richer from midrange up through full throttle, as opposed to primarily being richer at 3/4-full throttle?  Midrange performance hasn't been an issue so far, maybe just change the main jets first before adjusting the slide needles?

Another, suggestion is to examine the float levels.  There are indications that the 78s needed a 12.5mm float height rather the 14.5mm often found.  Raising the float level would make it easier to lift fuel into the carb bore and this would enrich the entire operating range.

I remember reading about the discrepancies regarding the recommended float level for the 1978 model year, and so when I cleaned these carbs I set them to 13mm.  When I first opened up these PD carbs, the float level was set to 22mm (mis-applied spec from the earlier carbs), explaining why the engine would not idle at all.

I'm also going to suggest you alter the carb number stamping, as that number describes the stock internal set up, which you plan to change.  Not many people do that and make liars out of the carb markings. (stealth markings?)

You're right, not many folks think to alter something like that, although I do record what I've done so that I'll have a reference for future work, and of course for any future owners.

[TwoTired]

This is interesting, because the engine has been running really well at part throttle, and my early spark plug readings after several hundred miles of light cruising at lower and middle rpm ranges didn't really indicate any issues, it was only once I began to cruise at more than half throttle in the higher rpm range that the lean running condition showed up.

Please don't confuse engine RPM with throttle twist grip position.  These are mechanical slide carbs where the engine RPM reacts to throttle settings dependent on loading conditions (rear wheel torque both positive and negative), actual RPMs achieved are load related not specifically throttle position related. 
Mark your throttle, so you may glance at it while operating, in order to know which metering system in the carb is the dominant contributor.  You may well have been cruising at one half throttle position, which would make the jet needle dominant and the main jet, minor.
Another example is how much throttle position movement is need to achieve red line in neutral?  (Hint, it isn't WOT. )

I'm going to suggest that a #95 main jet be used and to raise your slide needles one notch.  And then, check plug for deposits.  If you drill (not really recommended, but), you can always drill them larger if needed.  Drilling them smaller is far more difficult.

Since the engine ran lean enough to overheat as it did, do you think that #95 main jets in place of #90 main jets is enough of a change?  I would rather err on the side of being a bit too rich to avoid overheating again, but perhaps I'm just being overly cautious given yesterday's issue.  I don't like to resize jets either (primarily to avoid having to keep track of that type of change), but have on occasion when I couldn't find any.

Raising the needles will primarily affect midrange mixture, correct?

Yes, for about 1/4 to 3/4 throttle position.

Is this the reason that you are suggesting a smaller increase in the main jet size, so that in combination with raising the slide needle, the mixture will be richer from midrange up through full throttle, as opposed to primarily being richer at 3/4-full throttle? 

I think you've got the idea and approach I'm recommending.

Midrange performance hasn't been an issue so far, maybe just change the main jets first before adjusting the slide needles?

That is the easiest/quickest thing to do, for sure.  Particularly, if you can source replacement jet sets.  But, the other adjustments are there for a reason.  And, I have a feeling you're focused on RPM as a range instead of throttle position as the range to be addressed.  Also note that, engines can make the same power lean of peak as well as rich off peak/ mixture-wise.

I've observed that people tend to be reactionary.  If one touches something hot, one doesn't move it just far enough away to curtail heating.  One generally moves it way way far away.  Certainly further than necessary.  Same sort of thing happens in room conditions.  If its too cold, people with set the stat way up, then latter when it is too hot they will set the stat way down.  Usually, they learn not to make such large changes at first attempt (at least, some do.)

You've had a similar scare with your engine, and the natural reaction is to overreact.

Anyway, I don't think a 100 size main jet supplies the same amount of fuel in the PD carbs as it does in the earlier style carbs.  I'm expecting the #95 main jet in the PD to supply the same as #100 in the earlier style carbs, which was the proper setting associated with earlier 4 into 4 pipes and earlier carbs. Additionally, the slide needle for the early carbs was set at 4th position from the top, right next to the most rich position.   Where are the slide needle currently placed?  I believe 3rd position was stock, if I recall.

Design changes aren't always quick and easy.

Best of luck!

[Dave Voss]

Please don't confuse engine RPM with throttle twist grip position.  These are mechanical slide carbs where the engine RPM reacts to throttle settings dependent on loading conditions (rear wheel torque both positive and negative), actual RPMs achieved are load related not specifically throttle position related.

Good point, I'm aware that throttle position and engine rpm are not synched, it just happens that more throttle and higher rpm were both in play this time.

You may well have been cruising at one half throttle position, which would make the jet needle dominant and the main jet, minor.

That makes sense, the engine may have been running a bit lean throughout, but started running much more lean once I began pushing it a bit more.

I'm going to suggest that a #95 main jet be used and to raise your slide needles one notch.  And then, check plug for deposits.

I visited four different motorcycle shops this afternoon (all the type with a bin-box of carb jets somewhere on a back shelf) and was able to find (4) size #95 and (4) size #100 main jets, I'll start with the size #95 as per your suggestion.

The slide needle for the early carbs was set at 4th position from the top, right next to the most rich position.   Where are the slide needle currently placed?  I believe 3rd position was stock, if I recall.

I just removed the slide and needle from carb #4, and the clip is in the 2nd groove from the top (the second leanest setting).  I will move the clips to the 4th groove from the top.  With this discovery, I'm feeling better about the size #95 main jets being a wiser choice.

Thanks so much for all of your tips and help, since my last CB550K had the correct stock carbs, exhaust, intake, air box, and air filter, I didn't need do anything other than make sure carbs were clean, and it always ran well.  This time around feels a bit more like traditional tuning, made easier by the knowledge here!

[scottly]

Check your actual float levels with the "clear tube" method. Very easy with the PD carbs.
Also, consider going a size up on the pilot jet. Even if the bike was completely stock, today's oxygenated fuel requires about 4.5% more fuel flow to achieve the proper air/fuel ratio.

[Dave Voss]

Check your actual float levels with the "clear tube" method. Very easy with the PD carbs.

Yes, I probably will.  While the carbs were apart today to swap jets and move needle clips, I did take the opportunity to verify the float level, at least statically.

Also, consider going a size up on the pilot jet. Even if the bike was completely stock, today's oxygenated fuel requires about 4.5% more fuel flow to achieve the proper air/fuel ratio.

Since I have a '78 model year, the slow jets are already size #42, do some people run larger sizes?

[Dave Voss]

On Friday, I sourced a set of size #95 main jets, installed them in place of the stock size #90 main jets, and moved to slide needle clips to the 4th groove, as per the advice given above.  Afterward, I verified the valve clearances, ran the engine, and re-synched the carbs.

Saturday, it was just too hot to ride (~105 F) so I waited until Sunday to test ride the new settings, and after a few short rides, there is a definite difference in the smoothness and responsiveness of the engine.  It's not like it felt 'bad' before, but it is definitely 'better' now.  After ~10 miles of riding, I made a few aggresive starts, and ran at full throttle a few times, and this is where the biggest difference is, the engine now feels like it has that little bit of power back that I suspected it should have.

When I returned home, I noticed fresh wet oil on the front, left, and right side of the cylinders, and oil splatter in several other places.  I wiped everything clean and went for another ride, and all of those areas were wet again with fresh oil.  It's definitely leaking from the head gasket, on three sides.  A month or so ago, I removed the valve cover for inspection, and I retorqued the head nuts at that time, so I know they are tight.

Unfortunately, it looks like last week's hot running did in fact cause the head gasket to fail, but in the form of oil leaks, rather than compresion leaks as I first suspected.  At the rate it is currently leaking oil, it's not fit to be riden very far.  It sounds like it's not that big a job to change the head gasket, but while the head is removed, I'll take the opportunity to check / lap the valves, and replace other convenience gaskets and seals.

More downtime, but at least once this latest list of work is complete, I believe I'll have a well-sorted machine that's ready for extended service.

[TwoTired]

There is an oring near each end of the cylinder block that can weep at the head interface, but it is not part of the head gasket.  So, it is unlikely the head gasket proper has failed.  If you insist on replacing the orings, you will need the head gasket, though, as well.  Most of my 550's weep there.  It's no big deal.  Just wipe it off once in a while.  Rather rare that they weep bad enough to trail oil.  Mostly it just looks unsightly.
But, if it is leaking oil elsewhere besides those ends, then you probably have cam cover pucks leaking.  There are 6 of them and they can let a lot of oil out the engine.   You had to move these when you re-torqued the head nuts (not sure why you did that), and if not renewed almost always leak.  They also harden with heat and become more like hard washers.  It's not very difficult to change these.  Far less effort than going down to the head gasket, can be done in an hour or two.  Just be wary of the adjuster to valve stem interference while installing the cam cover, to prevent bend valve stems.  There is a warning note in the shop manual about this.

But, I'm guessing you really really want to take this puppy apart. 

[Dave Voss]

There is an oring near each end of the cylinder block that can weep at the head interface, but it is not part of the head gasket.  So, it is unlikely the head gasket proper has failed.  If you insist on replacing the orings, you will need the head gasket, though, as well.  Most of my 550's weep there.  It's no big deal.  Just wipe it off once in a while.  Rather rare that they weep bad enough to trail oil.  Mostly it just looks unsightly.

Before the hot running last week, I didn't see any oil leaks at the head gasket, only a minor weep on the left side coming from under the chrome end cap covering the rocker shafts.  I replaced the o-rings on the cover posts, and that seemed to seal it up.

But, if it is leaking oil elsewhere besides those ends, then you probably have cam cover pucks leaking.  There are 6 of them and they can let a lot of oil out the engine.   You had to move these when you re-torqued the head nuts (not sure why you did that), and if not renewed almost always leak.  They also harden with heat and become more like hard washers.  It's not very difficult to change these.  Far less effort than going down to the head gasket, can be done in an hour or two.  Just be wary of the adjuster to valve stem interference while installing the cam cover, to prevent bend valve stems.  There is a warning note in the shop manual about this.

Well, when I say 'retorque' what I mean is that I 'checked' the torque to be sure none were lose, as many used motorcycles that I've acquired had lots of loose fasteners, including cylinder head nuts.  I did remove the pucks, and they were still pliable, so I didn't think they required renewal, but maybe I should plan on that too, although they didn't seem to be the source of any external leaks before last week's issues developed.  Yes, the valve cover can be tricky to reinstall, but I know to go slow to make sure all of the followers are above the valve stems to avoid damage.

But, I'm guessing you really really want to take this puppy apart. 

I don't mind taking it apart, but wouldn't do so uneccesarily just for kicks.  In this case, with all of the possible individual oil leak sources, I'm thinking it would be a good idea to just replace all of the gaskets and seals in the top end to stop them all at once in the same wrenching session.  The timing is just bad with the GZ Rally.

Thanks a lot for all of your help sorting through this.