Strange starting problem - 82 GS750

[vadimg]

Ok, so I'm trying to get this 750 running off and on now for several months, TSCC engine. Today I finally got it to start, but the symptoms are pretty bizarre, to me at least.

The carbs are completely 100% clean inside and out, I don't remember what the jets are, but the bike was running 6 months ago on the same carbs, so those probably wouldnt be preventing it from running. Here's what happened today:

The bike would start, and I could keep it running by holding open the throttle maybe 1/4 or 1/2, this would keep it at about 2500 rpm, opening the throttle 100% would get it to a maximum of about 4500 rpm, after which it would die. After 30 secs or a minute of running, it would slowly start dropping RPM regardless of throttle position and then die. Letting go of the throttle at any point would lead the engine to die. Choke prevents the engine from starting. (it's warm outside, about 60 degrees)

It would start up several times, would run for a while if I held the throttle open, then die. As time went on and the engine warmed up, it became more difficult to start, and after a point the engine would no longer catch. I did this for a total of maybe 5 minutes of running time.

The spark plugs are brand new, gapped to about 0.025. The coils are working, not sure if the spark is great, but both coils are making spark. I am running fuel in from an auxiliary plastic tank. There is fuel in all 4 float bowls, the float levels are close enough as far as I can tell.

What does this sound like a symptom of? Could it be valves? If the valves are too tight, would it cause the symptoms I'm experiencing? I want to pop the valve cover off tomorrow and take a look, check the clearances and adjust them to be a little looser. If I do this, is it vital to replace the gasket or can I leave the old one on? Because if I do need to replace it, I would have to wait for a new one to come in the mail and I'd rather take care of this sooner rather than later if possible.

Also, if it were the valves, could I have caused any damage by running it for about 5-10 minutes today?

Thank you very much, appreciate all the advice.

[BassCliff]

Greetings and Salutations!!

Hi Mr. vadimg,

I don't think you've done any damage yet. Your symptoms sound like a lean condition. Are you using the stock airbox? Pods? Anything? Here's my unofficial "mega-welcome". Pay particular attention to the "Top 10 Common Issues", the Carb Rebuild Series, and the Stator Papers. Now let me roll out the welcome mat for you...

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Thank you for your indulgence,

BassCliff

[almarconi]

The carbs are completely 100% clean inside and out

Statements like that scare me. If the bike has been sitting for 6 months there is good possibility the carbs are gummed up. When you cleaned them did you take them completely apart, soak them, and then reassemble them?

You have it hooked up to a aux tank, is the vacuum line to the #2 carb plugged?

When was the last time the valves were adjusted? This should be done before messing with the carbs. If you are careful, when you remove the valve cover you should be able to reuse the gasket or use some rtv sealant.

You may want to check the advance mechanism under the right engine cover. Make sure you can move it, if not clean it.

the float levels are close enough as far as I can tell

Close only counts with horseshoes and hand grenades. This should be set according to the factory spec for your bike. Small changes can make a big difference.

What is the maintenance history of the bike? Was it sitting in a barn for 20 years or ridden daily and well maintained?

[vadimg]

Statements like that scare me. If the bike has been sitting for 6 months there is good possibility the carbs are gummed up. When you cleaned them did you take them completely apart, soak them, and then reassemble them?

Yes, taken off the rack, soaked, cleaned, etc.

You have it hooked up to a aux tank, is the vacuum line to the #2 carb plugged?

Yes.

When was the last time the valves were adjusted? This should be done before messing with the carbs. If you are careful, when you remove the valve cover you should be able to reuse the gasket or use some rtv sealant.

I don't know when the valves were last done, which is why I was wondering if that could be causing my problems.

You may want to check the advance mechanism under the right engine cover. Make sure you can move it, if not clean it.

Do you mean, can I turn the engine freely? The answer is yes.

Close only counts with horseshoes and hand grenades. This should be set according to the factory spec for your bike. Small changes can make a big difference.

I don't have calipers, so I'm guessing it's close. I measured the float levels at 22mm using measuring tape, but like I said, I don't know how accurate that is.

What is the maintenance history of the bike? Was it sitting in a barn for 20 years or ridden daily and well maintained?

Not sure, I picked it up for $500 6 months ago, it was starting at the time, but with a stumbling idle. I figured the carbs needed a good cleaning, so I cleaned them, and now here is where I stand.

I am not sure that I am suffering from a lean condition, since giving the bike any choke causes it to die. There are individual pod filters on the bike, I don't remember what the jetting was. When I took the carbs apart, all of the air screws were set about one turn out, so that's what I set them back to. I have a Colortune so once I get the bike running for more than a few minutes at a time I will adjust the pilot mixture to what it needs to be.

[azr]

If you run the bike with the petcock set to prime does that change anything?

[vadimg]

If you run the bike with the petcock set to prime does that change anything?

I am running fresh fuel in from an external tank, it's filling all 4 bowls.

[vadimg]

I checked the valves today, they were all in spec except for two, which I set to .005. I also checked compression, I'm getting 145-150 on all 4. So I'm down to spark or fuel. Any easy way of testing to see if fuel is getting in? Would it be safe to spray half a second of so of carb cleaner in each spark plug hole? That way, if the engine catches and runs for a few rpms, I would know that the ignition is good enough.

[gearhead13]

are all your pipes(headers) hot when you run it, or are one or two cold? are the plugs wet with gas?

[bmac]

Questions?

Reading all your responses to what you did seems logical for getting these bikes to run after sitting undisturbed for a long period off time. The question I have is, air cleaner on and filter installed and sealed? Are all the hose clamps tight and are the boots and o'rings new? Leaking fuel out of the vents could be has mentioned a stuck needle in the seat. I would address that problem first, even if it means taking the rack off, practice makes perfect. Even if you didn't o'haul these carbs it is a good idea to get to know the inner workings of these bikes. It will save you alot of money and personal accomplishments will be rewarding to yourself. This source has many helpful people that have been there, so do not hesitate to ask questions. The answers that will be given will be helpful and honest and free. bmac

[vadimg]

It looks like it's a fuel/air issue. I tested the voltage at the coils, it was 12.4 on both. I put a few drops of fuel into the spark plug holes and it fired right up for a second.

I'm going to pull the carbs again today, look at everything again, and bench sync them as well as I'm able.

Last time I cleaned the carbs, I pulled them apart and replaced all the vent and fuel T O-rings, float valve o-rings, etc.

It looks like there's a 130 main jet in here, a 160 air jet, and I can't tell what the size of the pilot is, there are no markings on it.

Tomorrow I'm going to go out and get a set of calipers so I can make sure the float heights are accurate. What's a good place to get some?

[almarconi]

What's a good place to get some?

Cheapest is probably harbor freight.

When I took the carbs apart, all of the air screws were set about one turn out, so that's what I set them back to.

I am running basically the same engine with stock airbox and pipe, my screws are set 3 turns out. Thats with stock mains and pilots. Since you are running pods, which flow alot more air than the stock box, I can't see how the bike could idle with those settings unless the pilot jets have also been replaced. There should be a number on them (probably need a magnifying glass to see them). Stock should be 42.5.



Did you remove one of the carb diaphragms and look at the needle? Are they adjustable? I'm wondering if the PO installed a jet kit in the carbs.
The mains you have in there are not stock so some attempt has been made to rejet the carbs.

[TheCafeKid]

Yes, taken off the rack, soaked, cleaned, etc.



Yes.



I don't know when the valves were last done, which is why I was wondering if that could be causing my problems.



Do you mean, can I turn the engine freely? The answer is yes.



I don't have calipers, so I'm guessing it's close. I measured the float levels at 22mm using measuring tape, but like I said, I don't know how accurate that is.



Not sure, I picked it up for $500 6 months ago, it was starting at the time, but with a stumbling idle. I figured the carbs needed a good cleaning, so I cleaned them, and now here is where I stand.

I am not sure that I am suffering from a lean condition, since giving the bike any choke causes it to die. There are individual pod filters on the bike, I don't remember what the jetting was. When I took the carbs apart, all of the air screws were set about one turn out, so that's what I set them back to. I have a Colortune so once I get the bike running for more than a few minutes at a time I will adjust the pilot mixture to what it needs to be.

FIRST, those arent AIR screws...they're overall pilot adjustment screws...they adjust the overall amount of air/fuel premix to the pilot circuit... Open them 2 turns out to start with, maybe 2.5 in your case since you have the pods...that IS PART of your problem, if not all of it....130 is probably about right for pods and pipe on that bike...its where im at roughly on my 1100ES..translates to BS32s i suppose.. Try turning the screws out...bet you get somewhere...

[vadimg]

I figured out what the problem was today. I pulled the carbs to reclean everything and recheck the float levels, threw them back on. Also interestingly, one of the pilot jets was different from the others. The mains were all 130s, but none of the pilot jets had markings, so I don't know what they were.

I was able to get the bike started today, it was stumbling around 1000 rpm for several minutes, warming up. I noticed that as time went on, the revs went up to a maximum of about 3000. I gave it some throttle, it revved to about 6000 then tried to die. Kept running at about 3000, sounding not too bad. After a minute, started dropping revs, sputtering, I added more fuel. As the fuel filled the float bowls, the idle went up to 3000, then dropped back to 1000, stumbling. I pulled off the air filters, the bike kept running.

I'm going to recheck the float levels once I get a set of calipers, but they were pretty close to the spec of 22mm using a tape measure. So, basically, it looks like the jetting is catastrophically rich. Only part that doesn't really make sense is that the bike was running and idling pretty well 6 months ago, I rode it around until it stopped starting in the winter which prompted me to clean the carbs. I guess it's possible that the carbs were just so clogged that it made a normally super rich condition into just the right mixture.

The air filters are clean, so I'm pretty sure that's not the cause. Besides, it shouldnt keep running with the air filters off.

It would be nice to find a set of carbs or a set of jets somewhere cheap. Also the needle doesn't look like the Dynojet needle that I have on my GS850, so I'm thinking that the needle here might not match the jet size (130 would correspond to the Dynojet stage 3 kit for the 1982 750) Anyone have old GS parts lying around?

[reddirtrider]

You don't need caliphers to set the floats, just a cheapo metal ruler that is in mm's. They practically give those things away.

Do you have any air leaks? I didn't read the whole thread, but are the boots good, or the o-rings (if your bike has them).

[gearhead13]

I have a set of 32mm carbs off an '82 750. They have had the slides drilled and main air correctors installed. Just took them off the bike to install bigger ones. Pm me if you are onterested.