Help! Fuel delivery problems.

[Jethro]

I did a search in this forum for carb problems but came back with too many results to sift through- it was making my head hurt! Here's what I have happening: The carbs have just been gone through and cleaned and was running well when I picked it up from the mechanic. After about 20 miles the bike would bog and seem to be running out of gas with about 1/4 throttle. It idles perfectly, and when I really open it up it moves good. The problem seems most evident when I am crusing along and not changing the throttle position much. My bike is popping like Jiffy-Pop on the stove, but my plugs look like they are the much coveted light tan color. Could I have a rich mixture? I have a pretty mellow aftermarket MAC exhaust pipe, and I don't think any rejetting has been done. One point of note- I recently put a new tank on the bike with a pingle fuel petcock. I believe I pluged the correct hose with info I got on another post. All I did was drive a screw of appropriate size in the tube. Is there an intense amount of suction on this hose? Do I need to melt and crimp it? This screw is in there good. I sprayed carb cleaner around the intake gaskets, all seemed air tight (no change in idle). I really don't want to take the bike back to the mechanic. Please help!

[Jethro]

I should add that the bike runs good enough to ride, all it seems to take is a grip on the clutch and a blip of the throttle and the problem is gone for another 2 minutes. It is *@%^$*@%^$*@%^$*@%^$ing me off though!

[mcycle-nut]

I'm having much the same problem. Between about 3500 and 4500 rpm it feels like it's starving or something, sort of like the kill switch is being turned on and off, but only in top gear! Carbs have just been cleaned, all is good, running pods, Jardine header, and Boyer Bransden ignition. Has run fine for years before this. Actually, the reason I first pulled the carbs was that #3 cylinder was fouling plugs. This problem has been fixed. Must have been a tiny piece of crud keeping the needle open, even though it looked fine. Synced carbs afterwards. Idles fine, wack the throttle and it f*$%s off. What bugs me is why now, and only in top gear? Filters and breather have been cleaned. Valve clearances checked in March, only about 2 or 3 thousand kms or so ago(if that). Tan colour on the plugs. I'm a scratchin' my noggin' on this one! :?

[Anonymous]

I'm having much the same problem. Between about 3500 and 4500 rpm it feels like it's starving or something, sort of like the kill switch is being turned on and off, but only in top gear! Carbs have just been cleaned, all is good, running pods, Jardine header, and Boyer Bransden ignition. Has run fine for years before this. Actually, the reason I first pulled the carbs was that #3 cylinder was fouling plugs. This problem has been fixed. Must have been a tiny piece of crud keeping the needle open, even though it looked fine. Synced carbs afterwards. Idles fine, wack the throttle and it f*$%s off. What bugs me is why now, and only in top gear? Filters and breather have been cleaned. Valve clearances checked in March, only about 2 or 3 thousand kms or so ago(if that). Tan colour on the plugs. I'm a scratchin' my noggin' on this one! :?

what size are the stock main jets and what size main jets are you running now

[saaz]

In top gear the load on the motor is highest, so is exposing the tuning problem. If it has been working ok, may be a little crud in the carbs or perhaps some wear on needle jet. CV carbs can develop problems with diaphrams as well, but I have no practical experince with that (I have a slide carb model)

[mcycle-nut]

Running 120 mains. Don't know what is stock, it has been years since I jetted it for the filters and pipes. These are slide carbs. Like I said, it ran fine for years before this. :?

[saaz]

This may sound strange, but some years ago I had a similar problem, turned out to be a wisker of cabon on the plugs!.

Otherwise, it sounds like a slide cutout/needle jet circuit problem. This could be due to the float levels changing from what they should be. This nmanifest itself as a problem that goes away for a bit if the floats allow as much fuel in as possible, but as soon as you give it a bit, if the level is too low it runs too lean.

This definitely affects the mixture through the rev range. If it has changed, due to crud or wear, this may be first manifesting itself in that particular part of the rev range with the pipe/filter combination.

[saaz]

Oh yea..jetho's problem may be the other way...floats too high for the jets etc, so it runs a bit on the rich side. Whoever tuned it first may have reset the float level as part of getting it right. Running it right on the proble area for a minute, then killing the engine and taking a plug reading may give a clue.

[Jethro]

How do I adjust the floats? Do I need to pull the carb assembaly?

[Anonymous]

How do I adjust the floats? Do I need to pull the carb assembaly?

The carbs must be inverted to adjust. It's the weight of the floats that the adjustment must be made by -- there is no way to apply upward lift on the floats to adjust them. It must be gravity weighing the floats' needles on the seats.

So, with neither bike does there seem to be a problem with hard acceleration? This problem only occurs at freeway speeds? At this gear and throttle position the bike is using the least amount of fuel -- it should be steady state. So I'd be thinking jetting in that range myself, or something that is hanging up. When I rebuild my carbs I neglected to clean the needle jets and being they were vacuum operated, they were traveling at different rates depending on how much friction they saw. Polished them up and that was that.

The bike that ran for years and started acting up... I'd think that perhaps it's just time to do the rebuild. Varnish build-up over time can change diameters of both jets and needles.

Roger Moore

[Jethro]

Yeah, I think I will have to rebuild again and pay special attention to the floats and jets. I am very confused about the whole jetting thing. What do I buy? Do I just buy one jet kit and hope it works? Is it a trial and error type of thing? How do I know if the bike has been jetted before? What if it isn't the jets? Is there a way to adjust the richness of the mixture without removing the carbs from the bike? I am thinking it will have to go to a dealership for evaluation because this is too confusing for me. That ought to set me back into the poor house again. Should have bought a new Bandit or ZRX.

[Anonymous]

This is I think not an uncommon feeling. It's very fuzzy. I know that there is a known flat spot in most, if not all, these bikes. And I know (because I read it here on the forum) that there is a known fix for it. I know I want to look that up again because I have this problem.

Typically what you do is determine what portion of the throttle is causing the problem and deal with that portion of the carb. There are about 5 or 6 "circuits" within the carb that operate through different ranges of throttle position (there's charts out there showing this relationship -- where? Help guys?) I just did a search on the web for them. If the throttle position is known , then it at least narrows the area of concern to at least one or two circuits (if they overlap). Then the rest is a trial and error thing... If it's a rich burn issue -- smaller jets, and lean burn -- larger jets. Or changing the needle jet one notch one way or the other.

As with any bike that has been modified from original, you'll deal with these things. To indicate that headers are installed (which can drastically change the flow charachteristics of the bike), but that it may not have been re-jetted to match this change is perhaps the entire issue. Popping is usually indicitive of a lean burn condition, so it might just be that at this position of throttle you are running lean.

One thing you did say that struck me after re-reading your original post. You state that if you pull in the clutch and a "plip" of the throttle makes it go away for a couple minutes, to me might indicate the needle jet sticking. Do you have the vacuum version or cable slides? I found that if I lifted mine (with the tip of my finger -- and the vacuum type), 2 of the four slides were very sluggish in coming back down. It ran really bad then too. I took these apart and polished the needles and slides and the problem went away. The only reason I neglected these when I rebuilt the carbs is because I couldn't dip them in the carb cleaner because of the vacuum diaphrams. So, maybe it's as simple as that.

Hope this helps somehow.

Good luck,

Roger

[Jethro]

Thanks Roger. I am going to pull the carbs again and check all these things. I am not sure if I have vacumn or cable slides- can anyone tell me if the stock 81 GS1100EX carbs are vac or cable? Speaking of vacumn, I recently replaced my gas tank with a very good condition one with a Pingle Fuel petcock. I had to seal off the vacumn fuel line. Could this change the way the carbs function?

[saaz]

Jethro, you have vacuum carbs. The pringle should not have affected things. With vacuum carbs have a careful look through them as suggested by Roger. They have diaphrams as well as all the other needles and jets

[Anonymous]

I had to seal off the vacumn fuel line. Could this change the way the carbs function?

No, not in theory. If there was now a vacuum leak at the infamous #2 (or 3? What did we all decide that carb was? It's the #2 from the driver's side of the bike? ) this could affect that cylinder only. I'm not familiar with the new petcock, but you're probably OK there.

I've got an 81 GS750EX, so I'd guess you have the same carbs, but maybe bigger? You can tell it's a vacuum version if it looks like the ones that are shown in the carb series here in the GS garage. Those are like mine exactly.

Again... good luck.

Roger