Topic: Jetting on a bored out cb550

[Ottawa_550F]

Hello.  Have been lurking around this site for awhile and really enjoy the great information.  I will apologize in advance for the hugeness of this post!  I am trying to get this bike running perfectly and have been messing with the carbs for awhile.   I have a 75 CB550F with a 592cc kit.  There is also a mild cam - I believe it was ground by Kenny Harmon and is a DS grind.  There are aftermarket valve springs and titanium retainers. 

The pistons do have a dome with cutouts for the valves and were made by ART.  61mm I believe.  The head has been mildly ported and the machinist did take about .001" off the deck of the head.  This has led to a change in compression from 1-4 which I may have to fix by having the top of the cylinder deck machined to match the head.  Not sure if these #s are way out but it tests like this:
             1     2      3     4
Dry      170   180  195  210
Wet     185   210   200  230

I am running pods and a new MAC 4-1 pipe.   I have rebuilt the carbs with keyster kits.  I just put the MAC on but before that there was a really old 4-1 pipe similar to a mac but rusted out.  The mains I drilled the mains to 118 using a pin vice.  #38 pilots.  The clip was in the 2nd from lowest position.  The bike would start but not idle too well.  There was no power through the mid range and I could not get it up past 3/4  throttle.  The plugs were sooty around the thread and dark at the center.

After installing the new pipe I purchased some new jets and drilled them to 107 - #58 bit.  The clip is now in the 2nd from top position(stock I believe) .  Now the bike does not start very well cold but does idle after a good warmup ride.  There is power through 1/4 - 3/4 throttle position but it fades still towards WOT.  The plugs are white at the center with no deposits at all and the bike does seem to run really hot on just a short spin.  The #1 and #2 plugs had a very slight tan color, I guess because of the higher compression in those cylinders??

 I remember some time ago that Rob Llloyd had said something about a similarly modified 550.  His mains were at 118 and pilots at 38.   I am thinking of dropping the clip another notch to the 3rd from top and drilling the mains again.  I am trying to decide between 109 - the #57 bit or going all the way to #56 again with the new jets that I have already drilled once (but they are new so the o-rings should be good).  Any comments would be appreciated - especially in regards to how the cam and/or piston kit would affect how much fuel the engine wants to run well.  Thanks for reading!

Adam

[Noel]

Great mods, IMO. I can't tell you how they should affect fueling, but you certainly do have the symptoms of pretty lean running. If it was my bike, I would continue drilling in one-gauge increments, and I would definitely drop your needles as well. It's a pain, but I'd recommend drilling and dropping as separate steps with plug chops in between, so that you learn the exact effects of both.

FWIW, I'm playing the same game right now. I'm actually enjoying the process. I did replace those @#$! float bowl screws with deep socket allen screws, though, which makes getting the bowls off much faster and easier.

[Noel]

But on re-reading I have a few questions: did you change the pipe and the jetting at the same time, or did you run the new pipe first to see how it affected your fueling?

And when you say the plugs were sooty with the original jetting, were those new plugs that appeared sooty after a plug chop? Or were they older plugs that had been in the bike for a while? I ask because it seems as though -- according to the way it ran -- that it was lean then, too, and I wonder if you may have been mislead by old plugs.

[Ottawa_550F]

Thanks for the reply Noel.   I did the rejetting from 118 down to 107 with the old pipe on.  But the clip was still in 2nd from bottom position with the old pipe.  The bike was lean then too.  The new pipe is likely flowing better as well I guess.  And the plugs were new last year and had a couple plug chops on them from when I had the 118's in and the clip 2nd from bottom.  I am going to put a brand new set in this weekend and drill to 109 and take it from there.  The reason I am thinking of dropping the clip is because it started better before I moved the clip up two notches when I put the new pipe on. 

Adam

[Noel]

FWIW, you can't really expect plugs to get lighter. I think it's okay to do plug chops on used plugs as long as you are trying to go richer/darker, but going from too rich/dark toward leaner/lighter don't work, IMO.

[pepsquad]

otowa were did you get your rebuild kit?

[Ottawa_550F]

I bought the kit, cam and valve springs/retainers from a local guy around 1986.  The bike was basically stripped down and put away in 1988 as I was never able to get it to run well.  It always ran hot with the kit and cam.  Back then I had no clue how to fix carbs and never even took them apart. 

[Ottawa_550F]

Ok well I used a #57 bit to drill the mains to 109 and dropped the clip to the 3rd from top position.  The bike was running better throughout but still a bit off.  Today I connected my new synch guages and found that at around 1000 rpm the vaccuum is only around 5-6  - I did manage to adjust them to all be the same.  Rode the bike and it is performing ok but not great.  I ended up having to screw the idle adjustment screw all the way in to maintain idle though.  What should the vaccuum be at?  The compression is good so I am not sure what the problem is.  Any help would be much appreciated. 

[TwoTired]

What spark plugs are you using.  And, why aren't you checking spark plug deposits for combustion conditions?

More details on your carbs?
- Float height setting?
- Carb body stamping?
- original slide needles?
- new orings for the mains?

Have you a test track for WOT throttle plug chops?

Cheers,

[Bodi]

There's lots of country roads around Ottawa for WOT chops
That's the first step: you can only get the main dialed in at wide fricking open. Find a nice country road, straight and cop-free. Go flat out for at least 30 seconds, a minute if you can. Kill switch, coast to stop and run through the gears as you slow, clutch in.
Once you get a nice tan color at WOT then work on 1/2 throttle (mark some tape on the throttle grip at 1/2 slide open). This will set the needle height roughly. Then look at 3/4; hopefully it's good, otherwise you need to change the needle taper... not hard it it's lean (just polish the clip end 1/3 of the needle with a drill and brasso a while) but tougher if it's rich.
Now 1/4 throttle and below. You may have to change the pilot jets, often they actually have to be smaller than stock with better airflow. The air screw (or fuel screw, whatever you have) affects up to about 1/2 throttle but makes more difference as you approach idle throttle.
If it is impossible to get decent plug readings throughout the throttle range, or you have consistent stumbling or surging at low throttle... you may need to change the slide cutout. This is not something I've had to so but I've heard from racers with highly modded machines that if you know what you're doing it will fix those persistent carb problems when nothing else will.
You're going to get really good at pulling and replacing those carbs!

[Ottawa_550F]

Thanks for the replies.  The bike is out in the country and I have just taken it out for a rip.  The plugs have a nice tan to them now so I think the jetting is very close.  They are 069A carbs with new kits and new, drilled jets(109) so the orings should be good.  The float height is at 22mm.  The bike is running pods and a 4-1 mac.  Is it normal to have the idle adjustment screw all the way in just to maintain a 1000 rpm idle?

[Ottawa_550F]

Oh and Too Tired, the plugs are D7EA.  Thanks!

[TwoTired]

The plugs have a nice tan to them now so I think the jetting is very close. 

Tan is good.

I don't know your level of expertise. So, if you already know this, forget I posted it.
The spark plugs show an average of the combustion conditions over the throttle positions used during the run. The carburetor meter fuel based on the throttle position, with different aspects of the carb coming into play at certain throttle positions.  main jet - 3/4 to WOT, Throttle valve 1/4 to 3/4, and slow jet- idle to 1/4 throttle positions (crude delineations).  With this arrangement, it is possible to run sooty rich at 1/3 throttle and lean at WOT, and an average run might show tan spark plug deposits. This is why plug chop on clean plug and at marked throttle setting are important to know what aspects of the carb's tune are matched to your engine requirement, and which ones are not.  The chart below shows what parts of the carb dominate mixture at various settings.

They are 069A carbs with new kits and new, drilled jets(109) so the orings should be good. 

Some on this list have reported differences in part shapes between OEM and aftermarket kit pieces, notably with the throttle valve needles.  Honda used 3 or 4 needle profiles in the same style carbs to accommodate changes in displacement and exhaust differences.  No telling what needle profile you require with your engine modifications,  However, if your kit needles have a different profile than those supplied by Honda, you have yet another variable to juggle during your tune process.

The float height is at 22mm.  The bike is running pods and a 4-1 mac.  Is it normal to have the idle adjustment screw all the way in just to maintain a 1000 rpm idle?

No.  However, if each slide adjuster is biased toward one end of it's adjustment range, it could cause your idle screw symptoms.

Cheers,

[Ottawa_550F]

Thanks Two Tired.  I have some knowledge but am basically a hack when it comes to fine tuning the carbs.  That is interesting about the needle taper.  I have some new plugs and will do the plug chops.  I do think the synch screws may be near the end of their adjustment as I was trying to turn them to give higher vacuum numbers.  Do you think 5cm-hg is way low?  I am wondering what I should do to get the idle speed up.  If I turn the synch screws back towards the middle of their settings the vacuum would probably show around 3-4 cm on the gauge.  The manual states that it should be in the 16-24 cm-hg.  The valves have been adjusted and there are new rings and good compression so I am somewhat confused by the low vacuum. 

[TwoTired]

I do think the synch screws may be near the end of their adjustment as I was trying to turn them to give higher vacuum numbers. 

All the synch adjusters do is make the mechanical linkage actuate the slides evenly so that each slide is giving an even amount of air to each cylinder.

Do you think 5cm-hg is way low?

Certainly would be on a stock bike.
Camshaft grinds that work well at high RPM, usually don't work well at low speed, and have poor vacuum charateristics at low speed.  Do you know your cam grind specifications?   How far after BDC does the intake valve close relative to the stock grind?

I am wondering what I should do to get the idle speed up.  If I turn the synch screws back towards the middle of their settings the vacuum would probably show around 3-4 cm on the gauge.  The manual states that it should be in the 16-24 cm-hg.  The valves have been adjusted and there are new rings and good compression so I am somewhat confused by the low vacuum. 

If you turn all the synch screws in the same amount, the vacuum should be same on all carbs.  Turning just one will lower the vacuum on that carb until the others are adjusted to the equivalent position.

The manual's vacuum specification is for stock engines with stock cam, pistons, air filter and exhaust.  Still think it applies to your bike? 
I imagine little of the tuneup specs from the manual apply to your bike, including:
-Tappet clearance
-ignition timing
-Ignition advance curve
-Air bleed screw setting
-slow jet size
-Throttle valve needle size, taper, and position
-main jet size
-Emulsion tube hole size and placement
-and possibly, spark plug heat range. (D8EA ?)

Further, if you have higher compression and notice poor spark plug life or effectiveness, you may also need a better ignition system, too.  Such as, gl1000 coils (3.4 ohms) , double springing your points, and increased spark plug gap.

The way you pose questions makes me wonder if you are confusing the synch adjusters with the idle air bleed screws on the sides of the carbs.  Tell me that's not so, and I am over analyzing the posts.

Cheers,

[Ottawa_550F]

Thanks TT -  The only info I know about the cam is that it has KH and DS stamped on one end and the fellow who sold it to me way back in the 80's said it was a 'mild' cam that would improve performance at higher rpm... so that may be why the vacuum reads low...I never really thought about that so thanks for the insight.  I also agree that many of the tuning specs from the manual will not apply to this bike now.   I do have a set of Dyna coils along with a dyna ignition.   The air screws are set at 1 turn out.   

 I am not confusing the air screws with the synch adjusters, although I guess what confuses me is the idle speed problem.   I did not make any large adjustments to any of the synch screws and the bike was idling at 1000 rpm with plenty of screw length left on the idle adjuster before I synched.   After setting up the synch guages I noted that each guage read around 3-6 cm-hg and I adjusted them to be at 5.  When I would adjust one, the bike idle would drop enough that I would have to screw in the idle screw to keep the bike running at 1000 rpm.  By the end of this process the idle adjustment screw was all the way in and the bike idling at 1000.    I will run the plug chops and see how close the jetting is but I am not sure what to do about the idle speed. 

[crazypj]

With a similar set up ( had a PIPER cam, JAMA exhaust) I was running 122.5 mains, needle 2nd clip, float height at max. I had about 215 psi compression an all cylinders though. Never had any problems starting (except when first built, starter would barely turn motor until it got a few thousand miles, kickstart works well though ) std pilots, air screw1 turn out (could have done with one size bigger pilot) use choke. check ignition timing.
PJ

[Ottawa_550F]

Thanks for the info Crazypj.  I am going to do some WOT plug chops this weekend to see how close I am with the 109's.  Do you remember what your vacuum readings were when you synched that bike?

[crazypj]

I have Honda gauges. I always set everything around 15~20cm/hg. If its lower it means you dont have much vacuum so the slides cannot be closed or cam timing is off. I think you should re-check valve clearances and ignition timing first. Do compression test with throttle wide open and if you can find one, do a leakdown.
All cylinders should be within 10% of each other

[Ottawa_550F]

Thanks Crazypj.  I will re-check the valves and do a compression test.   And the leakdown is a really good idea.  I will try to find a leakdown tester as well.  I am learning about carb tuning as I go here so all the help is appreciated!

[Ottawa_550F]

Thanks everyone for the advice.  Today I synched the bike and took it for a WOT plug chop with new plugs.  The new plugs were sooty from the synch process but white from the WOT plug chop.  Is it possible to lean out the mixture or make it run too rich at idle just by synching the bike?  I did a bench synch 1st and made small adjustments to get it synched.  While doing the test the bike ran fine and idled well.  I am thinking I now have the synch setup properly so the bike is running leaner.  I was getting some tan spots on the plug before I synched the bike.  I guess larger mains and perhaps a smaller pilot jet?  Thanks again all,

Adam