Carb Issues... maybe... Please help!

[Lluraeden]

Hi everyone! I'm having issues with my GS and can't figure out for the life of me what is wrong. Any and all input appreciated! Also I have been looking around these forums as well as others, and while I've found posts with similar symptoms the solutions given there have not worked for me yet.

So here goes, and sorry about the lengthy post. Bought the bike last spring all stock with 8k on the odo. I did 0 modifications, rode all summer into the fall before I put her away for the winter, clocking almost exactly 4000 miles with no issues. Over the winter I did: new stock replacement air filter (not K&N), inline fuel filter, and not that it's relevant but new brake pads, r6 shock and sonic springs in the forks.

When I put the bike back together this spring and started to ride, I noticed this issue: Above about 50mph, in any gear and at any rpm (excluding 1st and 2nd gears) when I roll onto the throttle at just about maintenance throttle and slightly above, the engine feels like it's running out of gas, sometimes popping/backfiring. This happens regardless of the speed at which I open the throttle. It is much more pronounced as speeds and gears increase, and is not noticeable below 40-50mph in 1st and 2nd. Sometimes it catches power but usually it just decelerates until I open the throttle further/WOT.

Full throttle has power, top speed is normal (I had to test it, you know, for science ), idle is set to ~1500 and it idles fine, it revs in neutral smoothly through the range, I start it on choke but it can warm up on it's own after less than a minute. Running with the petcock on prime has no effect. Removing the fuel filter had no effect.

Yesterday I tried to isolate the issue and gather data. On the highway at about 60mph it will only accelerate or engine brake, there is no way to smoothly maintain speed. I tried riding in this "dead range" for a few seconds, heard a few exhaust pops, pulled in the clutch let the engine drop to idle (took a bit longer than usual to drop below 3k to idle) then when I tried to rev it (still with the clutch in) it had no/minimal response and wouldn't rev until i held WOT for maybe 1.5 seconds. When it did finally rev, response was quick and normal even after letting it drop to idle and revving it up again. Not sure what this means, but it might be relevant.

One other thing I saw is that when revving in neutral with the airbox off to see into the carbs, gas gets sprayed back out into where the airbox should be. I don't know if this is normal, but it happens evenly on both carbs.

Obviously the first and second thing I tried was cleaning the carbs. They are clean, the slides appear to operate correctly from what I could see, all the o rings are intact etc. I added one washer on each side to lift the needles up thinking maybe it was just running lean in the mids, but no effect (and I shouldn't have to considering it ran smoothly last year). New spark plugs and oil/filter. I keep forgetting to check float height, but will update this when I know. I have not checked valve clearances or compression/leakdown, but I can't see why sitting for 4-5 months would affect those in any way or cause this issue. No noticeable vacuum leaks. I am out of ideas 


About me: I am not a professional bike mechanic, but I'm also not stupid. I grew up fixing my cars (they had a lot of issues...) and have helped rebuild an old CB750 as well as doing maintenance and low-key mods to my GS. I will take it to a shop as a last resort but would prefer not to.

About the bike: 1989 GS500E, 12000 miles, all stock- airbox and filter, carbs, jets (one washer on needles), exhaust, engine (to my knowledge). NOTE: I want to keep this stock and just get it back to how it was!

SUMMARY: My baby is sick and it makes me sad please help!

[mr72]

First, understand the way the CV carb works, basically.

At low revs, small throttle openings, the slides do not come up because there is insufficient vacuum to pull them up. There's enough of an opening between the bottom of the slide and the throat of the carb to supply air. All fuel comes from the pilot jet. Running problems at low revs, low throttle, and idle are usually pilot circuit. Vacuum is not involved. Very little fuel is required. Float level can be real low and it will still work. Main jet fuel passages can be clogged and it will still work. Slides can be stuck down and it'll work fine. This is actually a majority of the time the bike is running for most of us.

At bigger throttle openings and higher revs, the engine vacuum is sufficient (difference in air pressure between the two ends of the carb) to lift the slides using the rubber diaphragm. This pulls the main needle out of the main jet and allows not only bigger air opening (by virtue of the slide moving out of the way) but also fuel can flow through the main jet in accordance with the needle-to-jet gap. In these cases, low float level will cause fuel starvation, vacuum leaks or torn diaphragm will cause the slides to not lift and this keeps the main jet closed and again causes fuel starvation. Your delayed throttle response (1.5 sec at WOT before it revs) sounds very much TO ME like a vacuum leak or torn diaphragm causing the slides to not come up, but it could also be low float level and fuel starvation.

Things I would check if I were you:

1. vacuum leaks. Actually I would proactively replace all o-rings in the carbs, plus the o-rings on the intake boots. It costs under $20 for all of the parts and can fix a ton of very hard to diagnose issues. Then once this is done, verify there are no vacuum leaks anywhere. Don't spray carb cleaner spray water on the places where you suspect a vacuum leak to help diagnose.

2. Float levels and float needle valves... actually truth is the float seat o-ring or the float needle seat o-ring may be had, causing your problem. But you may have a sticking (closed) float needle valve causing fuel starvation intermittently which you only notice when you need a lot of fuel. It can stick/unstick on its own often enough to keep fuel in the bowls to supply low-power needs but get stuck and you deplete the fuel in the bowls quickly when hitting the throttle under load. Low float levels could also cause a similar symptom. Again, once you have it apart, doing the o-rings like I suggested , then you might as well replace the float needle valves and set the float level.

3. torn/damaged rubber diaphragm(s). If you ever sprayed carb cleaner in the carbs while the rubber diaphragms were still installed, it's a nearly sure bet they are damaged. Carb cleaner basically melts the rubber.

4. clogged pilot jet. Again these are cheap and easy to replace when the carbs are apart. Plus you probably want to move to a 40 anyway. So I'd say while the carbs are apart, replace the pilot jets. Just to rule it out. The transition from pilot to main could be affected by a clogged pilot jet. You might have adjusted the pilot mixture way rich to compensate for a clogged jet but once you get very far off idle the pilot can't deliver enough fuel so you have a big gap between pilot and main.

Anyway I think prevention is way easier than diagnosis on these carbs. I would just do the float needles, ALL o-rings (including the pilot needle "mixture adjustment" o-rings!), new pilot jets, intake boot o-rings, maybe this is close to a carb rebuild, but do that then adjust and ensure there is no crack in a vacuum hose or missing "little o rings" from the top caps or tear or hole in the diaphragms. Start from a known good state. Problem may not go away but at least you rule out a bunch of things and make it a lot easier to diagnose.

[rscottlow]

3. torn/damaged rubber diaphragm(s). If you ever sprayed carb cleaner in the carbs while the rubber diaphragms were still installed, it's a nearly sure bet they are damaged. Carb cleaner basically melts the rubber.

This was going to be my first suggestion. I don't have nearly the wealth of knowledge that others around here have, but your issue sounds similar to one I had previously, which was the result of a damaged diaphragm. They're easy to check, so pull them out and make sure they're intact.

[qcbaker]

Other users here have had problem with "fuel starvation" at higher speeds and the issue turned out to be that air was forcing its way in when it shouldn't, leaning out the mixture. I'm pretty sure (and someone more knowledgeable please correct me if I'm wrong) that it can be that the airbox boots are not sealing properly and/or that the carb breather hose is routed improperly.  So, maybe check those things.

[Lluraeden]

Thanks for the replies guys, when I get out of work I'm going to check the float heights, then I'll go back into the carbs and replace all the o rings like mr72 said. The slide diaphragms looked fine visually last time I opened them up but I will look again and update.

[gsJack]

Problem could be as simple as the carb bowl vent hose isn't probably routed.  It comes off the top T between the carbs and should be run straight back over the air box and and hang down between the air box and battery where it's open end is not affected by air pressure or vacuum caused at the bikes higher speeds.

http://img.photobucket.com/albums/v443/jcp8832/top%20view_zpsdgdjolxi.jpg

[mr72]

I'm pretty sure (and someone more knowledgeable please correct me if I'm wrong) that it can be that the airbox boots are not sealing properly

Nope. This won't have any noticeable effect.

It's a truly tiny amount of air that will leak in on the filter side of the carbs in this case. It's not like the air filter is restrictive. Worst case is you might suck in dirt which would prematurely wear the rings etc.

[grader]

pull the fuel filter and try it, its the only thing you did that could affect fuel.

[qcbaker]

pull the fuel filter and try it, its the only thing you did that could affect fuel.

If it were the fuel filter, wouldn't the bike experience fuel starvation above a certain RPM regardless of speed?

[mr72]

pull the fuel filter and try it, its the only thing you did that could affect fuel.

If it were the fuel filter, wouldn't the bike experience fuel starvation above a certain RPM regardless of speed?

Not really. Fuel requirements are based on load, which equates to power requirements. You need more rpms at high speeds because the bike makes enough power at high rpms to meet the demand. You notice in neutral to hold say 10K rpm only requires a very small throttle opening ... it's doing 10K rpm while still on pilot and using almost no fuel. To get it to open the main jet and demand a lot of fuel requires load. The highest power requirement is at high speeds because wind resistance is the king.

The OP already said the fuel filter didn't make a difference, presumably it's been tested.

[J_Walker]

Sounds like my bike when I was running really lean. ever notice a faint glow down the headers at night? can you light a firecracker off of the middle of the H on the headers? if so, you're running lean. 

[Lluraeden]

Problem could be as simple as the carb bowl vent hose isn't probably routed.  It comes off the top T between the carbs and should be run straight back over the air box and and hang down between the air box and battery where it's open end is not affected by air pressure or vacuum caused at the bikes higher speeds.

http://img.photobucket.com/albums/v443/jcp8832/top%20view_zpsdgdjolxi.jpg

This appears to be the problem! If you're ever in the Hartford CT area I'll buy you a beer for figuring this out  I had thought that the breather hose was an overflow drain, so had routed it out the bottom of the bike not behind the airbox as it should be. I moved the hose, and the problem has completely gone away from what I can tell  Thanks so much everyone, I was going crazy not being able to ride I'll definitely be back here when the next thing comes up!