Idle/Running problems with a 1983 GS750E

[John Adams]

Wow, this looks like a Great Forum!

I've been searching all over the web and it seems like this is probably the best place to ask about my problem.

I've got a 1983 GS750E with around 30K miles and I can't figure out why it only seems to run when the choke is fully out. At first, I thought it was the carbs, so they got completely cleaned and rebuilt with new jets, new float valve/pin, new pilot screws, springs, washers, O-rings, etc. The pilot screws are set to 2 1/4" turns out from lightly bottoming them out.

The bike still has the exact same problem.. It will only start/run with the choke out.. I can get the motor to idle if I turn the pilot screws out to around 4 turns, but if I give it any throttle, it just stalls right out.

I also adjusted the valves to .004-.005" like the service manual suggested. The plugs are also new so they are not fowled.

I know the carbs and fuel system are fine and they can't be the problem so I'm wondering what else could give the following symptoms:

Only starts/runs with choke out
Stalls as soon as the throttle is opened.
Will idle if pilot screws are 4+ turns out but will stall if throttle is opened.
The valve clearance is .004-.005
The plugs are clean (new)
Stock air box
Stock jetting


Could any of the following issues be causing this?

-I have not checked the carb boots in between the carb and the engine yet.
-I have not checked the ignition module/system yet.
-I have spark on all 4 cyls, and the engine sounds good when running on full choke.
-I have not checked the coils.
-I have not checked the battery.
-I have not checked the regulator.

Any ideas guys? I'm puzzled...

TIA!

 

[moto_dan]

Could any of the following issues be causing this?

-I have not checked the carb boots in between the carb and the engine yet.
-I have not checked the ignition module/system yet.
-I have spark on all 4 cyls, and the engine sounds good when running on full choke.
-I have not checked the coils.
-I have not checked the battery.
-I have not checked the regulator.

Any ideas guys? I'm puzzled...

TIA!


Check the first item on your list....carb boots.
The symptoms definitely lean towards an intake leak somewhere or a lack of fuel flow.

On second thought I would first insure the vacuum line is connected to your petcock(fuel valve) and that the line isn't split.

Good luck.

 

[GSstiny]

I would bet on the intake boots. With the bike idling, spray some wd40 on and around each boot. If the idle changes, then youve got an intake leak. Also double check the needle valves and float heights. If they are wrong it could be loading up with gas at idle. Keep it simple, youll find it.

 

[John Adams]

Thanks for the replys!

I've got the petcock vacuum line plugged, and I'm using a small funnel with a fuel
hose attached going right to the carb. I've rebuilt the carbs and I know they are good, including the float height, etc. The float seats and pins are brand new along with new jets, etc (I rebuilt them with a set of carb kits). Everything in the carb should be ok, and every channel/circuit is throughly clean so I think I've already exhausted almost every possibility with the carbs them self..

I'll remove the carbs again tonight and try to remove the carb boots from the heads and inspect them.. I'm hoping those carb boots are leaking because I really don't think there is anything wrong with the carbs.

What about the ignition coils, or ignition system? Would a weak spark require a super rich condition to run? (I.E: I can only start/run the engine with full choke).

Thanks again!

 

[moto_dan]

Thanks for the replys!

I've got the petcock vacuum line plugged, and I'm using a small funnel with a fuel
hose attached going right to the carb. I've rebuilt the carbs and I know they are good, including the float height, etc. The float seats and pins are brand new along with new jets, etc (I rebuilt them with a set of carb kits). Everything in the carb should be ok, and every channel/circuit is throughly clean so I think I've already exhausted almost every possibility with the carbs them self..

I'll remove the carbs again tonight and try to remove the carb boots from the heads and inspect them.. I'm hoping those carb boots are leaking because I really don't think there is anything wrong with the carbs.

What about the ignition coils, or ignition system? Would a weak spark require a super rich condition to run? (I.E: I can only start/run the engine with full choke).

Thanks again!

Before removing your carbs try the procedure GSstiny described by spraying WD-40 around your intakes....any change in engine RPM would signify an intake leak. Always good practice IMO to have a fully charged battery when trouble shooting just to rule out that possibility. Good luck.

 

[QuaiChangKane]

I'd use the WD trick to check for boot leaks - check the O-rings between the boots and the head, too.


Also check the o-rings on the fuel coupler rails between the carbs - this was the sole cause of my slow-to-warm problem....

 

[Guest]

Are you trying to run this bike without an airbox?

 

[John Adams]

Are you trying to run this bike without an airbox?

I'm having these problems with the air box installed. I also tried without the air box and it didn't make any difference.

The problem seems to be that I'm not getting enough fuel through the idle circuit. I know the idle circuit and the carbs are good though. The carbs had this same problem and behaved exactly the same before I completely rebuilt them. Even after completely cleaning them and installing all new jets, new floats, etc. The act exactly the same. If I open up the Pilot screws 4-5 turns, I can get the engine to idle without the choke, but if I give it ANY throttle, it just dies immediately.. I do hear a loud air/vacuum sound when I do try and give it a little throttle so maybe my boots are really leaky badly.

I'll try turning the pilot screws out to 4-5 turns so I can get it to idle, then I'll try spraying the boots and back of the boots where the O-rings are and see what happens... Hopefully all I need is a new set of boots and O-rings. I'll post an update afterwords.

If it's not the carbs, and not the intake boots, is there anything with the ignition system that might be wrong causing the need for a really rich mixture? I'm wondering what the symptons are for weak ignition coils.. Does a weak spark need a really rich mixture maybe?

Thanks again!

 

[GSstiny]

4 turns out sounds way too lean for the pilots. When tuning your carbs the baseline is about 3/4 to 1 out from a light seat. With it only running on the choke, Im really thinking its an intake leak.

Did you bench synch the carbs when you reassembled them?

 

[John Adams]

The service manual said 2 1/4 turns out for default pilot screws, but I couldn't get ANY idle so I played with the pilot screws and found that it would only idle (poorly) at 4 turns out, but stalls immediately if I try to give it gas. Anything less than 4 turns and it won't idle at all.

For the WD-40 tests, I'll try and get it to idle again (~4 turns out on the pilot screws)... If I put the pilot screws anything below 4 turns out, I can't get any idle from the engine and it will only run if the Choke is engaged. I figured it's probably better to do the WD-40 tests when the RPM is low so I can easily tell if there is a difference in RPM when I spray it on the boots.. I'm leaving work soon so I;ll try it out in an hour or so..

I did not bench sync the carbs but I probably should have.. They looked so good I couldn't tell the difference between any of the butterfly positions so I figured it was good enough to at least start up. I do have a vacuum sync gauge that I'll eventually sync them with if I ever to that stage with this engine

Thanks again!

 

[Guest]

On CV carbs turning out the idle mixture is richening the pilot circuit. Either your float adjustment is off or you have a blocked pilot circuit passage. Check the pilot air jets. They are on the throat of the airbox side of the carbs. Do a bench sync. On the pilot circuit the gas is sucked through the main and is diverted through the pilot jet because the main is blocked by your jet needle.This is a very small passage and can get clogged easily especially if the bike sat for any peroid of time. You should do a carb dip. Do you have just a rubber boot for your intake or a bolt on rubber boot with o-rings behind them?

 

[tconroy]

Thanks for the replys!

I've got the petcock vacuum line plugged, and I'm using a small funnel with a fuel
hose attached going right to the carb. I've rebuilt the carbs and I know they are good, including the float height, etc. The float seats and pins are brand new along with new jets, etc (I rebuilt them with a set of carb kits). Everything in the carb should be ok, and every channel/circuit is throughly clean so I think I've already exhausted almost every possibility with the carbs them self..

I'll remove the carbs again tonight and try to remove the carb boots from the heads and inspect them.. I'm hoping those carb boots are leaking because I really don't think there is anything wrong with the carbs.

What about the ignition coils, or ignition system? Would a weak spark require a super rich condition to run? (I.E: I can only start/run the engine with full choke).

Thanks again!

did you sync the carbs?

 

[John Adams]

Well, I didn't find any problems with the carb boots.. I did notice that the carb clamps don't really clamp very good so maybe the carb is leaking on the boots.

I did notice something strange with the fuel line going to the carb. I'm just using a funnel, with an inline filter in between. When I removed the carb, then re-installed it and connected the fuel line, I noticed that fuel went through the filter, but didn't fill up the fuel line going into the carb...

Maybe the inline fuel filter I'm using is starving the bowls? Why wouldn't fuel from the funnel go right though the inline filter, and fill the fuel line completely up?

 

[Guest]

Unless the flow from the filter is 5/16 it will starve your carbs. Try it without the filter.

 

[John Adams]

I think that might be my problem! I'm using 1/4" fuel line, and a 1/4" inline fuel filter in between.... I'll try 5/16 straight fuel line from a funnel tomorrow and see if that's my problem! I was reading up on inline fuel filters here, and I did see several people mention 5/16 is needed.. Would my 1/4" fuel line with a 1/4" fuel filter explain my super lean carbs? (Won't run unless choke is out, or pilot screws are 4+ turns out)?

 

[GSstiny]

I think that might be my problem! I'm using 1/4" fuel line, and a 1/4" inline fuel filter in between.... I'll try 5/16 straight fuel line from a funnel tomorrow and see if that's my problem! I was reading up on inline fuel filters here, and I did see several people mention 5/16 is needed.. Would my 1/4" fuel line with a 1/4" fuel filter explain my super lean carbs? (Won't run unless choke is out, or pilot screws are 4+ turns out)?

That very well could be it. I had roughly the same problem with a 1/4" inline filter. I didnt mean to misinform on the pilot settings. I forgot you had BS carbs.

 

[Guest]

Hey, my carbs aren't BS

 

[John Adams]

I hope that 1/4" fuel line and 1/4" inline fuel filter is my problem.. [-o<

That would be great! \\/

I'm temped to go out in the dark right now and try straight 5/16" fuel line :-D, but I'll wait until tomorrow when it's light outside and the misquotes are not feeding! :shock:

Yup, I have BS carbs, and hopefully BS fuel lines and a BS fuel filter

Thanks again!

 

[frosty5011]

For just getting it to idle, the 1/4" fuel line won't make a difference, but the fuel filter might. Do try it without the filter.

Here's a quick stupid question, but you do have your idle adjust knob turned in far enough, right?

 

[John Adams]

For just getting it to idle, the 1/4" fuel line won't make a difference, but the fuel filter might. Do try it without the filter.

Here's a quick stupid question, but you do have your idle adjust knob turned in far enough, right?

Yup, I basically tried everything. I also tried to come off the choke into throttle but any throttle would just kill it. When I have the pilot screws 4+ turns outward, I can increase the idle RPM just a little bit with the idle adjust knob.. If I turn it in too far, it just stalls out..

When I was actually pouring fuel into the funnel, I watched it run down the first 1/4" fuel line, then fill up the filter, but it would not fill up the fuel line below the fuel filter so I'm puzzled why that would happen? I guess the inline fuel filter that I'm using must require pressure and gravity is probably not enough to pull full through it fast enough. I would expect that the open funnel would completely fill up both 1/4" fuel lines.