Author Topic: UPDATED - 78 CB550 problems (Read 3293 times)

petersan · « **on:** April 18, 2014, 10:58:22 am »

I'm helping a friend sort through some issues on his 78 550. The bike would not consistently run without applying some choke or cranking up the idle. After some work (which I'll describe) - the bike still doesn't want to run and I'm suspecting it's a carb issue.

As long as he's had the bike, it's had 2-1 exhaust and pods and he said it's always run poorly (and had some smoke out of the exhaust). It doesn't look like any PO jetted the carbs for this set up (PD46C - still has 42 slow and 90 main jets, not sure of needle position, the floats were set to ~14mm, though I've seen in several posts here that these carbs should be 12.5mm) - though someone had been in the carbs, since the fixed top of the #2 carb had been swapped into carb #1 (I disassembled the carbs and swapped it back to it's original position in #2).

The carbs have been thoroughly cleaned - all ports passages and jets are clear - and bench synched. Compression on all 4 cyls is ~140 psi (within 5%). The cam chain tensioned and valves set. Dyna ignition is operational - get spark at all 4 cyls.

After doing this work, I still couldn't get the bike to continue to run. I ended up swapping the carbs (and intakes) off of my 76 550 and it started right up and ran ok (I didn't take the time to sync my carbs on his engine) - so I was able to verify that the dyna was appropriately timed for his bike.

I pulled my carbs and intakes off - checked all the ports on his carbs again and put them back on his bike AND the bike still won't stay running. I can get his bike started with his carbs - it just won't continue to run.

Could the jetting be that anemic for his setup (again: 2-1 exhaust and pod filters) that the bike won't run? OR could the ~2mm difference in float height be enough to keep his bike from running?

Thoughts? I'm sure someone has seen something I've missed. Thanks!

TwoTired · « **Reply #1 on:** April 18, 2014, 11:16:44 am »

I'll start by asking if the pilot jets were pulled and cleaned. And, to what position are the Idle Mixture Screws set.

This is to address getting it to idle. Tweaking the fast idle cam screw in relation to the choke position, may also be a related issue during engine warm up.

With the unspecified pods and exhaust change (no they aren't all the same), I would expect the mains need to be enlarged AND the slide needle needs to be raised in order to correct the mid-range and WOT throttle mixtures.
Without this, the engine runs way hot, and makes the valve guide seals hard and leaky. Can also contribute to valve guide rapid wear.

If it's blue-ish smoke out the exhaust, particularly at idle, it's passing oil, probably from the valve guides/seals.

petersan · « **Reply #2 on:** April 18, 2014, 11:28:50 am »

Thanks TT -

Yes - pilot jets (carefully) pulled and cleaned - Idle mixture screws were set to stock - at least what I could find here as being stock 1.5 turns out, though 2 of the carbs did not have the tiny washer that sits between the o-ring and the spring, I need to track 2 of these down, but I didn't think it would keep the bike from running.

The fast idle cam screw does engage the throttle slightly when the bike is choked, but without having the bike running/idling, I'm not sure how exactly it should be set.

Yes, I would think the mains should be larger also, though I didn't think this would keep the bike from running either - would just make for lean(er) operation. I've asked and the current owner does not have the airbox or original exhaust.

TwoTired · « **Reply #3 on:** April 18, 2014, 11:53:42 am »

With the carb throat pressure change due to pods, the mixture screw would need to be turned outward to enrich the idle mixture. The book settings only apply to the stock bike with its particular presented inlet pressures and it's high pressure exhaust system.
One caution: due to the very shallow mixture screw angle, any high force closure of the screws can damage/distort the screw tip and/or the seat in the carb body. I've seen extreme cases where it was so tight, it broke off the screw tip leaving it embedded in the metering hole, completely shutting of pilot circuit function.

The IMS change might be drastic, like 5 turns outward. Plug deposit color may be an informative tell towards what the engine needs. Particularly interesting is if all the plugs show identical coloring. If not, carb vacuum balance or induction leaks from couplers or orings on the manifolds might need to be addressed.

Are the carbs level? One function of the stock air induction is support at the rear of the carbs. With pods, the carbs can be allowed to droop.
Hardened couplers can also be a challenge to install fully at each end. Wintergreen oil and some patience can help that issue. But, is at cross purposes for carb support issues.

petersan · « **Reply #4 on:** April 18, 2014, 12:08:01 pm »

Yes, I know that the IMS's recommended 1.5 turns out is for stock induction - I just didn't think it would keep the bike from running, but I will definitely try turning them out. I'm used to my 76k 550 where I can get the bike running, then attend to the idle mix screws and balance the carbs. I was hoping to get this bike running so I could do the carb balance.

And I have noticed the fine tip/point of these screws and will be careful about only lightly seating the screw before turning them out - thanks for the warning. Do you think missing the tiny washer for two of the IMS's will be problematic?

The carbs are pretty level - and they seat completely into the rubber couplers with little difficulty - also, the clamps seem to securely fix the carbs and the intakes. I've not been able to keep the bike running long enough to test if there are airleaks through these connections - though even when the clamps are very tight, they don't seem to have bottomed out on themselves (if that makes sense). Do you think that the minor float height difference between what's set and what's stock (set at ~14mm, stock 12.5mm) would make a difference (I've seen varying opinion on this issue when reading about these specific carbs)?

Unfortunately, when we did the tune up we ditched the old plugs . . . I had hoped that getting the carbs cleaned would get the bike back to good operating condition, I'll keep old plugs around (and labeled) for the next time. I'll check the new plugs to see if there's enough color on them to read anything. Thanks again TT!

TwoTired · « **Reply #5 on:** April 18, 2014, 12:29:51 pm »

Have checked for header pipe heat? Are all cylinders firing? How much choke is needed to get it to run? Is the choke fully closing the butterflies?

Were the carb floats installed upside down?

Compression, spark, and fuel/air. What is missing? If it runs at all, the spark plug deposits can be informative.

petersan · « **Reply #6 on:** April 18, 2014, 12:43:24 pm »

All headers heat up once the bike fires - after it runs for 15-20 seconds it stops (all pipes equally hot) and then it takes me a few minutes before I can get it fired again - and again it dies shortly after, rinse, and repeat. I should mention, I was worried about fuel flow out of the tank - petcock cannot be rebuilt (riveted), but the flow is quite strong - and I do have a new inline filter. All four bowls get fuel quickly after turning petcock on.

Choke needs to be full - and yes, the butterflies fully close. I've verified this several times by pulling the filters on 1 and 4.

Floats are not upside down - I did see that this was an issue with several people - though I'd read through all those posts before cleaning out the carbs, so was careful to not repeat the error.

Compression, check. Spark, check. Fuel, check. Air, check - in fact, probably too much air!

I'll be in the garage tomorrow, might adjust floats back to 12.5mm (stock for the '78 PD46C as far as I can tell), then I'll recheck all passages/jets since the carbs will be off the bike. I'll turn the IMS out a turn at a time and see if I can't get this bike running.

I haven't checked the main jets to see if they swap with those in the 750 - but I have an assortment of 750 jets from 100 up to 130 (in increments of 2.5). If they'll swap over, maybe I should load in the 100s and see if that helps?

Can't seem to thank you enough TT! And I'll be glad to clarify any more points/questions.

harisuluv · « **Reply #7 on:** April 18, 2014, 12:59:07 pm »

Can you get it to stay idle by adjusting the idle adjust screw? I ask because I haven't seen it mentioned yet.

petersan · « **Reply #8 on:** April 18, 2014, 01:09:20 pm »

Fair question, harisuluv. When the bike is running (choke on), the idle screw acts like you would expect - but after running for a brief time, the bike just dies.

The more I talk (and think about it) the more I think this must be a fuel starvation issue (lean) - I'm going to set the floats to 12.5mm and see if the 750 jets don't fit in the emulsion tubes for this 550.

harisuluv · « **Reply #9 on:** April 18, 2014, 01:32:20 pm »

I know you're already cleaned the idle passages, but like TwoTired, I would look there first. Where you're having problems is idle jet/air screw territory. Those idle passages are incredibly small. I can clean one out and see perfectly through to the other side when you hold it up to light, and then run a small wire through again and look through and it can looked clogged.

Also, you might want to look at the castings on the carbs, the x46A has 38 slow jets (this is the one with 14.5mm float height spec)
and the PD46C comes with 42 slow jets (these have the 12.5mm float height spec)

So you might be running lean right at the slow jet, which would explain your problem. Did you have a chance to check the sizes of the slow jets?

petersan · « **Reply #10 on:** April 18, 2014, 01:38:17 pm »

Well, plan on pulling the carbs tomorrow anyway - so I'll check the passages again, though, yes, I've checked all of them several times.

These are the PD46C carbs - which I understand are correct for the 78 550. The slow jets are 42 (mains are 90) - and yes the passages are small, though the few times I've pulled them I continue to see light through them. I understand the 12.5mm float height is stock - though I have seen some conflicting posts here claiming that people have pulled factory built carbs apart and measured 14.5mm (ok, maybe I'm not remember this exactly right). I plan on resetting these floats to 12.5mm tomorrow.

Harisuluv - do you know if these mains will swap directly with those for the 750s?

Thanks

TwoTired · « **Reply #11 on:** April 18, 2014, 01:53:43 pm »

Quote from: petersan on April 18, 2014, 12:43:24 pm

Compression, check. Spark, check. Fuel, check. Air, check

Oops. Not be over argumentative, but if you have all four, then the bike MUST run!

Quote from: petersan on April 18, 2014, 12:43:24 pm

I'll be in the garage tomorrow, might adjust floats back to 12.5mm (stock for the '78 PD46C as far as I can tell), then I'll recheck all passages/jets since the carbs will be off the bike. I'll turn the IMS out a turn at a time and see if I can't get this bike running.

A higher float height will help richen the mixture over all throttle positions.
...But, it shouldn't be preventing the engine from running. My 78 Cb550K had the floats set to 14.5mm from the factory. Of course, it is all stock, FWIW.

It dies after it warms up? Did you check for internal modifications like drilled jets or other enlarged metering orifices. Such mods would show soot deposits on the plugs. Which, in turn would make spark issues on top of carburetion issues. Even new plugs can get sooty in short order with enough mods and that will foul spark reliability.

On the thought track regarding the pilot circuit. Did you "prove" liquid flow between all 4 openings in the pilot circuit? In the bare casting, there is ; the air jet, the pilot jet, the ims, and the carb throat exit port.

One way to look at it, is the pilot circuit provides the engine's minimal needs. The slide needle and main just augment the needs when the engine is asked to do more than just tick over.

BTW, the petcock CAN be overhauled if needed. I posted regarding this some time back. Drill the "rivet", drill a hole, tap, replace the internal rubber bits, and reassemble with screws. News at 11. Bwahahahah

harisuluv · « **Reply #12 on:** April 18, 2014, 01:55:46 pm »

Ok, the PD46C float height is 12.5mm.

Yes the main jets are compatible, however the emulsions tubes are different.

If you are taking them off you may want to consider shimming the needles, with pods you will probably have lackluster performance, or at least a flat spot. The PD46C should have non adjustable needles, but you can shim them. Usually it's on clip down (raises the needle).

petersan · « **Reply #13 on:** April 18, 2014, 02:01:56 pm »

TT - I agree - I appear to have all four, but something is missing, hence the post - and the benefit of the intelligence of the masses, rather than the two of us staring at the bike in my garage.

And I agree, I thought the bike would run with floats at ~14mm.

It's difficult for me to tell without having other (verified) stock 42 pilot and 90 main - that the jets would be modified - I haven't looked at the plugs since they went in, but I'll see how they read. So you suspect I could be rich, if there was some modification made?

Regarding the pilot circuit - yes, I've verified liquid flow (both carb liquid and air) in the pilot circuit - I've done it several times and will check again tomorrow (the venting is also working).

Petcock - I recall seeing something about drilling rivets in the past, but since fuel flowed freely, I didn't think the petcock was causing problems.

Shimming needles - I might look into this if I can get the bike operational - though harisuluv, I might need choke parts if I do it, since I had a hard time getting the choke assembly back together.

Thanks for all the help!

TwoTired · « **Reply #14 on:** April 18, 2014, 02:35:50 pm »

Quote from: petersan on April 18, 2014, 02:01:56 pm

It's difficult for me to tell without having other (verified) stock 42 pilot and 90 main - that the jets would be modified - I haven't looked at the plugs since they went in, but I'll see how they read. So you suspect I could be rich, if there was some modification made?

Yes, particularly with a lot of operation on full choke (or a lot of choke). Anyway, it is low effort to look at plug deposits. And, they can tell a lot about how it runs or how it died from the last run. Too rich - soot, too lean - clean as new. You are using D7EA, right? Cold op. is a #$%* with D8's in the 550.

strynboen · « **Reply #15 on:** April 18, 2014, 02:54:35 pm »

try to remove that ekstra inline filter..they can give flow problems..

petersan · « **Reply #16 on:** May 12, 2014, 10:26:28 am »

After several attempts at getting the bike to run with the original carbs, I finally found a relatively cheap set of stock replacement carbs. I cleaned them, installed on the bike and it fired right up - and idled pretty well. (I'm still unsure why the original carbs won't work on the bike - but at least it's running now).

I now have two questions.

1. Set at the stock float height, #3 leaks out of the over flow. Adjusting the float height way out of spec, I can't get it to stop (set close to 20+mm rather than 12.5mm). I took the carbs off and when I remove the float and fuel valve on that carb, I can see that the seat where the valve sits, looks like it has some damage, I'm suspecting the scuffs/scratches in the seat are letting fuel pass. Is it possible to pull these seats and replace (as with other carbs)?

2. I took the tank off to try to balance/sync the carbs. After the bike ran for a few minutes, it started to stall out - almost as if it were running out of gas. I checked the fuel delivery and it was fine, carbs had gas. But in working on the bike, I noticed the aftermarket coils for both 1-4 and 2-3 were very hot (nearly too hot to touch). I replaced with some stock coils I had laying around and the bike fired right up. I suspect the aftermarket coils are heating and then failing at temperature. But it left me wondering, how hot should a functioning set of coils get?

calj737 · « **Reply #17 on:** May 12, 2014, 11:23:13 am »

1. I'd it possible the float is installed upside down in carb #3?

2. Coils should not get hot like that. Have you checked the reg/rec to insure the proper voltage is being managed? And double-check the connections on those coils.

It is possible, but less likely, the heat was a result of the engine running while tuning but standing still. Remember these are "air-cooled" engines and do need airflow over their surfaces to cool down. You'd be surprised how hot they can get while idling away...

petersan · « **Reply #18 on:** May 12, 2014, 11:31:07 am »

Thanks calj737 -

1 - I'm sure the float is not installed upside down, before I tore into these carbs I read up on the forum and was alerted to the issue by some other threads. All the other floats are set to 12.5mm (and installed in the same orientation) and stop the fuel flow to the bowl correctly - I'm pretty sure the scratches in the fuel seat is what's causing the problem - now wondering if there's a way to swap them out.

2. I didn't think coils should be getting that hot - I can try checking the reg/rec to see that they're operating correctly. The bike does have a dyna ignition, but I only turn the key on when I'm going to run the bike, so I don't think the coils are heating due to the bike being on but not running (as I've seen in some other threads).

Also - the bike was only running a few minutes as I was trying to tune the carbs - in fact, I noticed the top of the engine (valve cover) was cooler to touch then the coils were, so it's very unlikely the coils were heating due to residual heat off of the engine.

TwoTired · « **Reply #19 on:** May 12, 2014, 12:46:25 pm »

PD carbs do do have (easily) replaceable float valve seats. With some prowess, it may be possible to polish the seat to return proper needle/seat seal. Perhaps you just found out why the carbs "were relatively cheap"? Unclear.

Really, you are going to tell us that the coils overheat without stating what ignition system or brand of coils or ignition trigger you have?

The stock coils are ~ 4.9 Ω. Which means when provided 12V they dissipate 30 Watts. If the aftermarket coils are 3Ω, they dissipate 48 Watts each.
The coils will heat whenever the points are closed (190 degrees of crank rotation). A Dyna-s (if you have one) leaves them both on all the time (except for about 30 degrees of crank rotation).

Ever hold on to a 30 watt incandescent bulb? The stock ignition, while running, only heats each coil with about 15W. Stop the engine, and leave the ign on and one coil will heat, (as well as one of the point sets). If you have a Dyna-s, both coils will heat up with the engine stopped, ign on, and only slightly less so with the engine running.
This would have been less wordy with some relevant information included with the questions.

If you take the tank off the bike, does it still get fuel to the carbs? If not, do you still wonder why it ran like it "ran out of gas"?

If you connected a fuel line between the tank and carbs, did you account for gravity to feed the carbs?

petersan · « **Reply #20 on:** May 12, 2014, 01:32:04 pm »

Thanks TT - I'll take them in order.

Regarding float valve seats - I do have a spare set of carbs now (with good seats), so if I knew how to replace, I'd do it. I did try thinking of a way to polish the bad one, but I'm not sure I have the tools (or imagination) to do it. Can a pick with a small hook, grab the seat from behind to pull it out? Is it press fit too tight for that - can you describe in brief how to remove/install?

I'm not sure the make of the coils - they look similar to those I remember seeing on the nighthawks (they are not accel or dynas). I didn't measure the resistance in the coil, so not sure if they're closer to 5 or 3 ohm. The bike does have a dyna-S ignition.

I've never held onto a 30 watt incandescent bulb.

When balancing carbs, I set the tank on a close workbench that puts the tank in a higher gravitational potential compared to where it sits on the bike, with about an extra 18" of fuel line - I don't have problem getting gas to the carbs when I've done this on other bikes.

TwoTired · « **Reply #21 on:** May 12, 2014, 02:25:55 pm »

I Don't believe the PD carb valve seats can be removed without destruction (drilled out or milled out), as they are pressed or swagged in with an interference fit and a good deal of force. Which means you would need a source of new replacements, which I've never known to be available. It will be quite the feat to replace them successfully, possibly even needing an oven to raise the carb body temperature.

If you feel you must replace the valve seat, then the entire carb body is probably going to need replacement. Alternately, a machinist might be able to recondition the seat while in place.

As mentioned, the DYNA-S will provide full power to the coils whenever the switch is on. Undoubtedly why the coils became hot, with or without the engine running. They won't be much cooler when it is running. The Dyna-s is a rather poor design, imo, and will tax the Cb550's charging system, being the power hog that it is.
I think the nighthawks have 2.4 Ω coils, don't they? They will certainly be toasty driven with a Dyna-s.

Have you checked the battery voltage when it "runs badly"?

petersan · « **Reply #22 on:** May 13, 2014, 12:51:10 pm »

Ok - I'm hitting up harisuluv about carb body replacements for the fuel leak issue. I'll try to polish it a bit (if I can figure out a way to get to it) and try different fuel valves to see if I can stem the leak.

I'm not sure if the coils are actually from the nighthawks, just know that it looks similar to the coils that those bikes had - I'll try to get into the garage and check the resistance in those coils this week.

Thanks again TT

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Author Topic: UPDATED - 78 CB550 problems (Read 3293 times)