gs550 no start when hot. trying to jet carbs.

[Peterk97]

looking for a little advice. resto-modding an 1982 gs550. went thru entire bike adding new parts or rebuilding. Got the bike running with K&n dual filter set up.seems to run fine after putting new main jets 2 sizes up from stock. Except will not start when hot just sputters a bit till she cools off then she will start again. I have not synched the carbs yet but I did do a bench synch. Could the motor still be starving for fuel?

 

[Steve]

If the only thing you have done to the carbs is go up two sizes on the mains, yes, it's starving for fuel.

Stock mains are 92.5. You saying that you installed 97.5 jets?

What exhaust system? Stock or aftermarket?

With K&N filters and a stock exhaust, I would expect at least 102.5 mains.
With an aftermarket system, maybe up to 105 or 107.5.

Did you CLEAN the carbs? If not, why not? If you did, HOW?

Did you do anything to the needles?

What is the setting on your mixture (pilot) screws?

A bench sync is good for starting the bike, but really won't affect its running temperature to the point that it won't start hot.

.

 

[Nessism]

How long since your last valve adjustment?

 

[Buffalo Bill]

Old GS coils stop working when hot. Learned that the hard way.

 

[Peterk97]

I was thinking she was lean just didn't have any idea of how far I should jet it .I did rebuilt the carbs. dipped carbs in carb clean by gunk. clean out and blew out all passages,New float needles and all new o-rings. The exhaust is stock and I just upped the mains to 100.seems to be runing better but still no hot start. I have about the same spark size and color spark with the engine hot or cold. I just ordered 102.5 and 105. I am hopping one of these will work.I have not changed the needle yet and I have the mixture screws at 2 full turns. I did bench synch the carbs and recheck float levels when I put in the last set of mains.
I am not new to jetting or synching carbs on cars with multiple carbs . just new to bikes. The valves are in spec. the only left to tackle is a weak charging system when I get the engine purring

 

[Big T]

Changing the main jet size has zero effect on start up

 

[Steve]

How long did you dip the carbs? GUNK brand cleaner is pretty much the same as Berryman's, which is the local favorite, but it also requires pretty much a full day, not the 15-30 minutes suggested on the can.

As Big T mentions, the main jet has no effect on starting the bike. You could have mains as small as 50 or remove them completely, it won't matter. What you need to address is the pilot screw, also called the mixture screw. What is your pilot screw setting?

.

 

[Peterk97]

I let them soak for around an hour or so. the passages are all free and clear. I have the mixture screws backed off 2 full turns from the seated postion. what I dont undertsand is why she starts right up when cold or warm and not at operating temp. I will be working on her this weekend. installing the bigger jets and hopefully resolving the hot start issue. I So the you guys think it is an issue with idle circuit? whish the carbs had accelerator pumps and this wouldn't happen..

 

[cowboyup3371]

Soaking for an hour isn't going to dissolve everything. As Steve said, it would be better to let them sit for several hours

 

[Steve]

An accelerator pump is just another failure point that won't really solve your starting issue.

These carbs were tuned rather lean to meet EPA regulations. With the gas that is available today (compared to when the bikes were new), there is less actual gasoline in the mix, so they need to be set a bit richer. Try turning your mixture screws out to 3 full turns to see if that helps. Three turns will likely be too rich, so some fine-tuning will be necessary after that, but in the meantime, that richer mixture acts a bit like a choke to provide a better starting mixture. Also note that each carb needs to be adjusted individually, as there are minor differences from carb to carb. One carb might end up at 2 1/4, another carb at 2 1/2, etc.

.

 

[Buffalo Bill]

Duh, you did try opening the throttle fully wide before cranking?

 

[londonboards]

Tight of badly seating valves, when the engine is hot, will reduce compression and create hot starting problems.

Do a compression check on a hot engine. Follow the instructions of the compression tester carefully ie keep throttle wide open, remove other plugs etc

 

[Steve]

Tight of badly seating valves, when the engine is hot, will reduce compression and create hot starting problems.

I don't disagree with your statement, but I have found that hard starting due to tight valves usually shows up with a cold engine first. :-k

Especially if it is the intake valve that is not seated. As the piston is coming up on the compression stroke, some of the air will leak past the intake valve and go back through the carb. That "air" has already gone through the carbs, so it has fuel added in a somewhat correct mixture. The jets in the carb don't really care which way the air is moving. If enough air goes past the jets, it will suck up some fuel. On the next intake stroke, that fuel-laden air goes past the jets again, picking up more fuel. By now, you have a mixture that is WAY too rich to burn properly, let alone in a low-compression setting due to a leaky valve.

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[Peterk97]

I did check the compression of one cylinder when I pulled a plug while checking if I fouled one. I will check the compression when I get it back together again. finishing some wiring up. And yes I opened the throttles. I did not try squirting gas into throats though. will do some further checks when I get it back running again. Btw does anyone have a wiring diagram that is readable for my 1982 GS550L?

 

[adalah]

never experienced it. but this is a very interesting discussion to follow

 

[Peterk97]

just an update on my issue. Last week I pulled the carbs off again rechecked the bench carb synch and float levels all ok. set the mixture screw to 2.5 turns and installed 105 main jets.
Bike starts and runs well both hot and cold now. I did not touch anything else on the bike.
This weekend I did install new dyna green coils and new wires. Once I get the tank painted I will be onto her maiden voyage in over 20 years.

Thanks for all the help