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78 GS550E Carby Problem

  • Thread starter Thread starter Anonymous
  • Start date Start date
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Anonymous

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Hi,

I have been rebuilding a 78 GS550 for a couple of months now and have a little carburation problem (I think).

The top end of the engine has been totally rebuilt and I have replaced one coil, the points and condensors. The problem I have is that the bike is running only on two cylinders, it seems that the fuel is not getting through to the other two. The manual I have is not as specific or detailed as I would like, perhaps someone here knows the answer.

I can easily adjust the air/fuel mixture screws on the carbs, but this seems to make no difference to the cylinders that aren't getting fuel. Do I need to adjust the pilot jet at the bottom of the carbs? The manual said nothing about it but I'm sure they must be useful for something?

Also, one of the carby's that isn't letting fuel through seems to be flooding. I thought this might have something to do with the float level but I have adjusted this a number of times and it still seems to flood. It is as if the only way for the fuel to go is through the drain, and I have found no blackages in the carb itself.

If anyone could be of assistance it would be much appreciated!

Cheers,
Paul
 
Which two cylinders are NOT running?
Are you saying all the carbs have plenty of gas in their bowls, but after cranking, the two non-running cylinders are not getting gas on the sparkplugs?
 
78 GS550 Carbs

78 GS550 Carbs

Hi Keith,

Cylinders 1 & 2 are not running. Yes, you are right, that is whats happening. I thought there might be a blockage but this does not seem to be the case as the carbs have been cleaned out and rebuilt with new parts.

I have tried adjusting the air/fuel mixture and still has no effect.

Any ideas?

Cheers,
Paul
 
You may want to do the cleaning thing again. I took the carbs off my 81 550T and dipped, dunked, soaked and blew out the carbs and after I put them back together it wwould only fire on the two outside cyls. I took them off again and looked and from God knows where... there was crap in them again and they were plugged. So, I cleaned them out again and now they are fine.
 
I agree. The carbs need to be cleaned better and blown out with compressed air.
If you've adjusted the float levels and have the correct level of fuel in the bowls, then the only way the two cylinders can be starved for fuel is because of clogged jets and passages, and a very poor carb synch/lack of vacuum.
The flooding problem is because of a poor sealing float valve, or a float sticking or out of adjustment.
For the flooding, check the valve seat/fuel passage for any dirt, etc. Check the valve for any burrs or a weak spring. The little spring tip should always be under tension. If you push it in LIGHTLY and then let go, it should come all the way out on its own. If you can pull out any slack, it's weak. The gasket under the valve seat must be in good condition too. Check the floats for smooth operation. Make sure both sides of each float measure the same. They are commonly tweaked and uneven.
For the fuel starvation problem, I would clean each passage out and verify it's clear by blowing through each passage as you go. Same for all jets/bleed pipe. Adjust your pilot fuel screws (underneath and engine side of bowls) about 3/4 turns out if you have the stock air box with lid still on. DO NOT seat these sharp tipped screws tight, just lightly. You'll have to fine tune these screws later, but 3/4 turn out will not cause a fuel starvation problem.
Now turn the side air screws out to 1 1/2 turns. They should be adjusted using the "highest rpm" method, but 1 1/2 turns will be fine for now.
If any o-rings inside are hard/brittle, replace them. I suggest replacing the o-rings in the carb manifolds. If they're bad, you'll have poor vacuum. These o-rings should be replaced whenever it's been awhile since the carbs were last removed. Apply some hi- temp' bearing grease to help them last and torque to 6 ft/lb. If you notice the manifolds cracking or hardening, the manifolds must be replaced too.
Now you have to do a mechanical synch of your slides. This mechanical synch must be followed by a vacuum tool synch to avoid mixture problems. Be sure all rubber boot/manifold clamps are tight.
If you do all this correctly, the bike should get fuel to all the cylinders and start up OK. You can then fine tune the side air screws and get plug reads to tell you what else needs adjusting.
If you still have fuel starvation, I suggest a compression test.
 
One simple thing to check -- the plug wires- left coils feed cyl 1&4 Rt coils feeds 2&3
 
Sorted!

Sorted!

Hi All,

Thanks for your friendly advice and tips! After once again dismantling the two problem carbys, dipping, cleaning and getting compressed air through them (again), they seem to be working.

What a tedious job that was! I'll be happy if I never have to touch a set of carbys again!

Now it is a matter of fine tuning them and getting back on the road.

Once again, thanks for your input. Much appreciated.

Kind Regards,
Paul
 
Yes the carbs on the 550 are nighmarishly tight! I never want to touch them again either!
 
Follow up on same/similar issue

Follow up on same/similar issue

I have a similar issue with my 78 gs550 - I think it's running on only 2 of 4 cylinders however as I am a novice with bikes and the mechanics of same, I need some additional help. How do I identify which of the cylinders are not firing up? I have checked all plugs for sparks and all well and there is fuel in the float chambers. Are there some other things I can do which would prove that it's a carb issue before I go about stripping it down as this is challenging for me?
Thanks
 
Re: Follow up on same/similar issue

Re: Follow up on same/similar issue

seanm said:
I have a similar issue with my 78 gs550 - I think it's running on only 2 of 4 cylinders however as I am a novice with bikes and the mechanics of same, I need some additional help. How do I identify which of the cylinders are not firing up? I have checked all plugs for sparks and all well and there is fuel in the float chambers. Are there some other things I can do which would prove that it's a carb issue before I go about stripping it down as this is challenging for me?
Thanks
Click to expand...

Wet rag on the exhaust pipe as soon as you start it. You'll find the cold pipes is/are not working . fuel must be drawn into the engine, it is not enough to have fuel in the carb. If the carb is dirty that little tiny engine is not strong enough to suck fuel through a dirty jet. Motorcyles are more sensitive than cars to dirty carbs.

look at the bike history. Sitting around for years? no fuel filter? dirt visible in the gas tank? Dirty air filter? Adds up to carb problems. You might get away with some gumout spray onto the air jets and all that but it sounds like you need to tear em down.
 
Really appreciate your reply and tips. I will try the wet rag trick tonight . Bike was sitting outside since last summer (~8 months) until I bought it last week. Tank fuel was hideous and the air filter had disintegrated into dust. Cleanout out tank, valve and screens, made an oil filter from some foam I had lying around, changed the spark plugs and cleaned the points and put in new oil and filter. Suspect your right re it being blocked but would really like to try the 'gumout'. Is this a real product and where exaclty do you spray it and with or without the bike running?
 
Carb cleaner spray comes in may different cans. I prefer Berkeble 2+2. I would get access to the carb throats and try and clean the air passages and all that out. Start the bike and get it running warm first.

You might get lucky
 
seanm said:
made an oil filter from some foam I had lying around,
Click to expand...
Do you mean air filter? The foam has to be a certain kind that can be oiled and stand up to fuel vapors, etc, and be able to pass/filter air adequately. If you just used "regular" foam, it's going to deteriorate and cause more carb problems and allow dirt into the engine.
 
Speaking of foam, I saw some foam pods that need to be oiled in the Dennis Kirk catalog. They looked funny and I didn't have time to read about them. It's an interesting idea though. The convenience of pods without the jetting hassles.
 
Keith Wrote:
Do you mean air filter? The foam has to be a certain kind that can be oiled and stand up to fuel vapors, etc, and be able to pass/filter air adequately. If you just used "regular" foam, it's going to deteriorate and cause more carb problems and allow dirt into the engine.


I guessed that the foam needs to be of a certain type but I had some new chair seat foam which I soaked in oil and squeezed out to get me going - I was afraid to run the bike without anything in there as I read elsewhere in this forum that the bike could overheat. The foam density actually might be too great so I am orderging a specified filter (someone elses foam seat!). Thanks
 
Bite the bullet and get a proper foam filter. Using a different kind of foam will cause problems. It needs to flow enough air for the engine to run properly. Oil breaks down some foams, I cannot remember what kind is oil resistant but whatever the dealer sells for the airbox is probably the right kind.
 
UNI sells a foam filter replacement for about 9 bucks...a perfect fit.
 
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