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1978 GS750E Rich Carb

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Hi Guys, I am new to the forum, I am restoring a 1978 GS750E UK spec as I live there! I am looking for help with setting up the pilot fuel screws, yes the ones under the carbs that the manual says don?t touch. i have just completed a major rebuild of my engine following a camshaft bearing failure. The carbs were stripped and cleaned in my ultrasonics cleaner, hence why I removed the pilot fuel screws. I did a complete overhaul with all new jets etc. I set the pilot fuel screw to 3/4 turn out following a recommendation from a UK magazine. The engine runs starts and idles very good from cold with clean plugs but as soon as engine hots up the idle becomes very lumpy as though it is running on 3 cylinders. Initially I thought it was electrical but everything checks out OK, the bike has new points, condensers, plugs and coils. On closer inspection of the spark plugs No.3 plug is sooty and No.1,2 and 4 are clean. The pilot air screws are all set at 1.5 turns out, adjusting them to 2 to lean the bike seams to make no difference. The plugs becomes so sooty that it clearly is misfiring. Reading through a few posts I see the recommendations are similar to what I have used. I plan to strip the carbs tomorrow to see if I can find any reason No.3 is running too rich. My only thought at present is to reduce the pilot fuel screw to 1/4 turn out. I am familiar with working lean carb issues out but never had one which is over rich. Any ideas appreciated.
 
Hi, and welcome, from another UK member. have you set the float heights correctly, to the spec in the workshop manual? if the float is sitting to high in the bowl it will cause a rich condition.
 
I agree with Agemax on checking float heights and fuel levels in the carb bowls.

Are you using the stock airbox and exhaust? If so, the usual advice on the forum is to set the fuel screws at 3/4 turns out (like you have done) and the air screws at 1.5 turns out (like you've also done). Once you've set the fuel screws, don't touch them anymore and make all adjustments with the air screws. If number 3 is running rich, turn out its air screw a bit to lean it out.

If you have pods and/or high flowing exhaust, you want to start with the fuel screws a little further out and the air screws twice as far out as the fuel screws. For example, I have pods with the stock exhaust so i started with the fuel screws at 7/8 turns out and the air screws at 1.75 turns out.

Another tip I learned is to feel each exhaust header as soon as you start the bike. The cold exhaust pipe belongs to the cylinder that's not running properly. It seems odd that bike would start up with 4 cylinders and one cylinder would drop out after its warm. But if one cylinder is so rich that it fouls the plug, that would make sense.
 
Thanks for the replies,

I am running standard airbox and standard headers, the mufflers are some aftermarket which look close to standard. The bike is running very good when cold but the No.3 plugs fouls when the engine gets hot causing the misfire. The plug fouling leads me to think the pilot jet is too big, adjusting the air screws has not made any noticeable improvement. I am using 1.5 turns for the air screws as the specs on the UK bike changed almost every year, I could not find the spec relating to my year but the nearest one was 1.5 turns out, there are marks on the carbs which appear to line up with 1 or 1.5 turns out. I will do some more investigation tomorrow I checked the carbs after my engine rebuild so I am confident they are clean and float heights are set correctly. I think there must be a carb issue I am missing the other 3 cylinders are running very clean. I may also do a plug swap just to make sure it is not plug related which I think is unlikely. The annoying thing is am a retired automotive engineer so usually good at solving problems!!
 
Hi Andy, could be the petcock diaphragm. Pull off the vacuum line that activates the petcock from carb #3 and apply vacuum. If you have a hand vacuum pump it will easily verify the diaphragm condition. You can also check it yourself if you don't have a pump. Just don't post anything about orally checking a petcock.
 
Agemax said:
what measurement did you set the float heights to?
Click to expand...

I would have used the spec in the manual which is 25 to 27mm, I usually go for middle so that would be 26mm. That would be with the gasket removed. It does not explain why only one cylinder as all float heights were set the same?
 
UKAndy said:
I would have used the spec in the manual which is 25 to 27mm, I usually go for middle so that would be 26mm. That would be with the gasket removed. It does not explain why only one cylinder as all float heights were set the same?
Click to expand...
bad float, bad float valve/needle? lots of variables.
 
+1 on the bad fuel tap, which draws vacuum from #3
Extra fuel runs down the vacuum line into the cylinder

Easy test is to pull the line from the petcock and plug it with a bolt

Then, run the bike on Prime and see if the problem goes away
 
Hmm, US spec on float height is 23mm + 1mm. Can US/UK spec be that different?
 
I would have to question that too since the level is supposed to be roughly 1mm below the bowl mating surface so it is not above the gasket. I don't suppose the carburetor bodies are different.
 
I will update you what I found as the story got a little muddled. Since fitting new coils I had only run the bike for 40 minutes when I noticed the misfire, I was assuming it was No.3 cylinder but before I stripped the carbs down I did another plug check and noticed that No1 and 4 plugs were also sooty. I checked the Petcock as you guys suggested, it was working OK, I had rebuilt recently so did not expect to find any issues. The one things we have not discuss is the air filter, I had done the usual servicing but it looks OK so may be causing too much restriction, I will replace it and recheck the plugs. The carbs all checked out OK, the floats are all in good condition and the heights were all set to 26mm with gasket removed (mid spec according to the Suzuki manual). I did an air test to see if there were any blockages but I did not find any issues. The most interesting thing and probably explains what my plugs were showing is that Carb No.2 pilot fuel screw was set an only 1/2 turn out the other carbs were all at 3/4 turn out. So as an experiment I set all four carbs to 1/2 turn out, I know this sounds very lean but as No.2 plug was clean I thought a test was worthwhile. I rebuilt the bike and it started first time after warm up I could get a response from the pilot air screws, I ended up at 1 turn out giving the best running. Not test ridden yet, hopefully tomorrow and I can do a proper plug chop. I ran the engine until it was hot and did not get any misfires and was obviously not too lean as no backfires. If the air filter is too restrictive that may be why 1/2 turn out on the pilot fuel screw works. As I said above I am going to order a new filter and check again. According to what I have read 1/8 turn on the pilot screw should make a big difference so it will be interesting what my extra 1/4 turn shows up.
 
Definitely check out the fuel vacuum hose plus could also be the wrong jet needles / needles in wrong position. I've seen several 750s fitted with 850 needles / set up and they run rich.
 
I rebuilt the carbs with all new parts and I checked the needles were correct. My issue is on idle so the needle should not be in play until the throttle is open. Once I do my road test tomorrow and do a plug chop I should get a better idea of what?s going on. My vacuum hose is new. I have tried running with the petcock on prime setting with the vacuum take off blanked and it makes no difference. I also use this set up for balancing the carbs with an auxiliary fuel tank so I can adjust the carbs.
 
Are these new parts all factory parts? Sorry Agemax we must have had the same thought and you were faster.
 
I used a Mikuni Kit, the OEM parts were made by Mikuni so I guess you could call them genuine.
 
i wasnt aware Mikuni made a "kit" for these carbs. where did you purchase these kits from please?
 
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