Back for more help *eye roll*

[Guest]

Hi all, back at it. I'm going to try to keep this short (as short as possible)... About a year ago, my brother rode my bike, stalled, then put it down on the ground (idk why, it has TWO kick stands).

Got it home, the pin in the carb was stuck in the venturi part so I took apart the carbs. They were gross.. cleaned them (ultrasonic in carb cleaner) re assembled with new rubber.

Doing some PM, I installed new points, regapped them, checked the timing (perfect!) And put it all together.

Checked the valves, perfect clearance, good pressure on each side (150 PSI).

Bike will start okay when cold. Runs decent (other than hesitation at 3-4k) but solid in the other RPM ranges. Once the engine warms up it dies, and is tough to restart.

My plugs keep carbon fouling. If I let it cool down and clean the plugs it will run again until again it warms up.

This is the loop I'm stuck in. My research leads me to believe I have a float height issue. It has been tough to find the actual float height for BS34 carbs, but what I am set at is 27mm from the gasket surface.

Please help, this bike means the world to me, I need to ride it again! Thank you

 

[Rob S.]

What bike (year, model)?

 

[Rich82GS750TZ]

There are factory service manuals for many GS years and models on BikeCliff’s Website. The info you seek will be in there.You don’t mention your year/model. That’s the kind of thing that’s good to put in a signature. Yes measure from the gasket surface.

 

[Guest]

Excuse me! I keep forgetting to add that to my signature. 1977 GS400

 

[Gorminrider]

The Clymer manual is probably at the link above. It's not the best manual and a bit odd about float heights: it says 25.6 to 27.6mm for "all bikes" in it's purview EXCEPTING the 450 which it says are 21.4- 23.4..small diff suggesting the older bikes like it a little richer. Likely these decimal mm are converted from inches...but a float level just below the gasket on LEVEL carbs (not the bike!) should work well enough.

It's very noteable that bikes with the same problems all show up together. Nowadays, it's "bikes that die when warm" so the Mass Brain should have this nailed by now.Look at some recent threads in other mechanical sections...aka ignition, petcocks and all that.

I expect we are nearly ready to troubleshoot SKYNET at this point...

 

[1978GS750E]

This may help.......

 

[Rich82GS750TZ]

This may help more:

This is from the Carb Specs/Cleaning file link found on BikeCliff's Website. I did not compile this data. If I recall correctly, there have been errors found in the table.

 

[Guest]

This may help.......

I actually found this chart when looking for more information today... the 27mm I referenced before was from a Clymer's guide and It had called out for 25.6 to 27.6. I went on the high side, my exact height was 27.2.. If I refer to the table I should be at 26.3 +/- 1. I will set them at 26.3 this afternoon and try again!

Does my diagnosis / conditions sound like a float height is likely to blame...?

 

[zed1015]

Sounds like wet fouling due to an over rich mixture.
Could be too high fuel level ,float valves sticking , blocked pilot air bleed passage, blocked main air jet passage etc.
You should also check that the choke plungers are seating fully when closed.

 

[Guest]

Sounds like wet fouling due to an over rich mixture.
Could be too high fuel level ,float valves sticking , blocked pilot air bleed passage, blocked main air jet passage etc.
You should also check that the choke plungers are seating fully when closed.

It definitely seems over rich. I just finished reassembling with a float height at 26.2 on each side, and cleaned everything while I was in there. Same deal, runs decent initially, run it up and down the driveway a few times and same deal. I will say that the 3-4 k issue seemed.. better .. as in less stuttering until I stalled it in the room range and it was all over .. if I wanted to lean it out with the float height I would go large correct? So instead of 26.2 I go to the max of 27.6? Or should I go the other way considering that the max is similar to where I was before and it didn't work? I guess I just don't know what to try next

 

[Guest]

I feel confident that the floats are set as they should be, so now I feel like I need to focus elsewhere such as mentioned above: float valves sticking , blocked pilot air bleed passage, blocked main air jet passage.

I did try my darndest to clean all those locations which carb cleaner and compressed air... The float valve sticking seems like it may be a culprit. I noticed during bench testing of moving the floats up and down occasionally one of the valves would stick up, I kinda figured with any sort of gas flowing it would drop back down as it did when I blew air into the fuel inlet but perhaps not?

 

[rphillips]

You haven't raised, or changed, the needles, in any way have you? Whatever is wrong with one carb has got to be wrong with both. Doesn't seem to make sense that both floats are sticking, both air jets stopped up, both choke plungers, naw it don't have a choke cable.... Just thinking out loud.

 

[zed1015]

both choke plungers, naw it don't have a choke cable.... Just thinking out loud.

It does have two choke plungers operated by a single pull bar which if not returning will soot up both plugs when the bike has warmed up.
Also with it being both cylinders suffering the issue it's possibly an ignition issue , weak spark , ignition coil breaking down etc.

 

[Gorminrider]

Once the engine warms up it dies, and is tough to restart.

My plugs keep carbon fouling. If I let it cool down and clean the plugs it will run again until again it warms up.

...I wouldn't think a float level within a millimeter would cause this problem....to be plainer: possibly because the ignition is failing while the gas is continually being sucked in.... or less likely something else like fuel supply failing...

 

[rphillips]

Just thinking the manual choke would be manually pushed completely closed as opposed to a cable that may not be adjusted correctly to push completely tight... I dun'no.

 

[Guest]

...I wouldn't think a float level within a millimeter would cause this problem....to be plainer: possibly because the ignition is failing while the gas is continually being sucked in.... or less likely something else like fuel supply failing...

Some of you guys have now mentioned my ignition system or specifically the ignition coil and I have been talking to a buddy of mine who is really into model a Fords and he said in his experience this sounds like an ignition coil issue where it runs fine until it heats up.

I feel like I can eliminate a fuel supply issue as well because I have disassembled, and rebuilt the petcock and it operates as it should, but also as a worst case scenario it operates just fine on the prime setting ensuring optimal flow of fuel to the carburetor and as you mentioned above if my float height is close in theory I should be getting good fuel through the carburetor.

I guess the next thing I have to check would be my ignition system but that I am not familiar with at all. I'll break out the clymers manual and take a look, but where would you recommend I begin when troubleshooting the ignition system / ignition coil

 

[Gorminrider]

I'd start with replacing the condenser, checking the points and the centrifigal advance mechanism (behind the points plate is it?) thereafter, i'd run the bike until it stops and check the spark at the plugs exactly then while still hot... You may need new coils but check everything else that's fixable and cheaper (including wire harness, connections, spark plug caps and KILL switch) first. rare things too, like chafe or cracked rubber on hi-tension leads...

I'd recommend you get familiar with how ignition works- it's a very common system you have especially with "points" as your bike came with? It surely does not need yet another explanation where the internet has so many...AND You can learn from manuals of other bikes of the same vintage that have exactly the same system-POINTS-it's very common-it's NOT "cdi" so look away from anything about that...

 

[Guest]

I'd start with replacing the condenser, checking the points and the centrifugal advance mechanism (behind the points plate is it?) thereafter, i'd run the bike until it stops and check the spark at the plugs exactly then while still hot... You may need new coils but check everything else that's fixable and cheaper (including wire harness, connections, spark plug caps and KILL switch) first. rare things too, like chafe or cracked rubber on hi-tension leads...

I'd recommend you get familiar with how ignition works- it's a very common system you have especially with "points" as your bike came with? It surely does not need yet another explanation where the internet has so many...AND You can learn from manuals of other bikes of the same vintage that have exactly the same system-POINTS-it's very common-it's NOT "cdi" so look away from anything about that...

Thanks for the list of things to check! I will speak to some things I tried below:

I have checked the points (both old ones and new ones I have) and confirmed those are good, and my gaps have been set properly.

The advance mechanism appears to work fine (not sure how to check it other than make sure it moves freely?).

The plug shows signs of definite carbon foaling. A soft black sooty deposit that can easily be wiped off.

I have thoroughly checked the wire harness and it is flawless.

I had an issue with a kill switch in the past and it has been replaced and double checked after replacement.

I am going to buy some new condensers, I found some online reasonably priced. Based on the above, it appears I am inching closer to the problem. Thank you all so far! I will comment once again when the condensers arrive.

 

[Gorminrider]

yes, no rust and free motion is Pretty ok on the advance unless you have a strobe light...it's just something to bear in mind as part of the system as your issue doesn't seem to be timing...
If it is the coils, in the last resort, at least you will have a good chance they won't go bad again with every thing else in good nick...or maybe they've just aged out or previously abused.
Do be sure it's them-you might even find similar used ones from a generic any-brand motorcycle to put in-don't be fixated on "Suzuki" because all this stuff is really from "nippon-Denso" or the like. The mounts to fit them in the space IS important though. be sure of that, Suzuki or not. Pictures online can be unreliable...
If you can, buy decent condensers...Modern ignition doesn't use them anymore and the cheapos can be pretty badly constructed so maybe something from Bosch? for a vintage BMW? would be a good bet...if it fits in the space any condensor should do.

Internet BMW Riders - Substitutes for BMW Parts

 

[Guest]

Okay I'm back, finally got around to this once more and I'm stumped again. New condensers installed, out everything else back together aaaaaaand... Nothing. Well to be specific it starts just runs very rough... Doesn't idle, needs the have the throttle open about half way to stay running but is only at like 2000-3000 RPMs... Idk what to try next...