GS750L - fuel blockage

[Crake]

So in the process of resurrecting my '79 GS750L, I've gotten her to the point she'll run on full choke. Any attempt to close the choke kills the bike. Not even a fraction of a millimeter less than full choke will sustain idle. After the bike has run it will accept a tiny bit of throttle input from the idle adjustment screw, but beyond 2000 rpm she dies out.

Seeing as she runs so well on choke I figured the idle circuit is plugged somewhere. I cleaned all jets thoroughly, but have been unable to get a good diagram of the VM carb as to where the idle circuit passages in the carb body go. I soaked the carb bodies good and long and blew them out with compressed air, but the bike did sit for a long time so there could be a pretty stubborn blockage inside the body I can't see.

I've done a quick sketch of the carb as I suspect there's a blockage somewhere inside the pilot fuel screw passage (as the bike clearly isn't getting any gas via that circuit). The hole into the carb throat is not plugged. Does anyone know the orientation and location of the fuel passage above the pilot fuel screw's o-ring and below the carb throat inhole?

[tkent02]

There are two or three tiny transition ports going into the venturi just behind the one the fuel screw goes into, and a passage from an air intake to the pilot jet bore. I'm looking for a good diagram but not finding it yet… Usually if you spray carb cleaner into the different passages on four different carbs you can figure out where they are all supposed to go. Won't work if they are all clogged hard in exactly the same place.

[Steve]

There are two or three tiny transition ports going into the venturi just behind the one the fuel screw goes into, and a passage from an air intake to the pilot jet bore. I'm looking for a good diagram but not finding it yet.

Here ya go, Tom.

This picture was "enhanced" to show the rubber plug in a previous discussion, but works well here, too.

In this picture, the pilot fuel mixture comes up the dotted passageway above the red plug, then over the top of the throttle plate.
You can see a transition port on either side of the throttle butterfly, as well as the adjustable port under the pilot mixture screw.



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[tkent02]

Thanks, Steve, do you have a similar diagram for the VM carburetors?

[Crake]

Thanks! I'll take a closer look once I pull the carbs off again.

[Big T]

Thanks! I'll take a closer look once I pull the carbs off again.

Spraying carb cleaner in the various pilot holes with both needles removed should give you a good idea of where to look

Some people use a guitar string, or a piece of fine wire down those passage to poke out any obstructing gunk

[Steve]

Thanks, Steve, do you have a similar diagram for the VM carburetors?

Will this one work for you?


Based on this picture, I had to re-think how the pilot screws work on the VMs.

I had alwasy been under the impression that the fuel screw (#3 in the diagram) varied the amount of fuel that was in the mixture and the air screw (#1) fine-tuned the mixture. Apparently, the pilot fuel jet (#2) admits a fixed amount of fuel, the air screw (#1) does fine-tune the mixture, and the 'fuel' screw (#3) actually controls the mixture.

Spraying carb cleaner in the various pilot holes with both needles removed should give you a good idea of where to look

Some people use a guitar string, or a piece of fine wire down those passage to poke out any obstructing gunk

I use a single strand of copper from an electrical wire on the principle that the copper is harder than any gunk you are likely to encounter, but softer than the brass, so it will not damage it. I have two sections of wire that have different-sized strands. The larger one will do everything except the tips on the pilot fuel jets.

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[Crake]

Hrm...

Beautiful cross-section. Exactly what I was looking for...

Pulled the carbs last night, stripped out all the jets and inspected them. Everything appears to be spotless and clean. All jets and passages appear to be clear - Following the passages on this diagram, and blowing compressed air through all the ports (including the pilot fuel screw socket) I determined that air flows nicely and exits where it should.

Studying the diagram and making my brain think really hard, I came to the conclusion it simply has to be a fuel supply issue. There's no ignition problem (it starts and runs well on full choke), there's no air problem (new air filter installed, all passages clear, and again, it runs well on choke). It's gotta be fuel delivery. I figure it's got to be the pilot fuel screw being in too deep (as it appears to control the exit flow of fuel/air mix into the throat). I had initially set them to factory spec of 1 turn out (as recommended in the rebuild guide). Before reinstalling the carb I set them all to 2 turns out. We'll see if that improves matters when I attempt a start tonight. More to follow...

[Steve]

I had initially set them to factory spec of 1 turn out (as recommended in the rebuild guide). Before reinstalling the carb I set them all to 2 turns out. We'll see if that improves matters when I attempt a start tonight. More to follow...

Have you made any mods to your bike in the intake and exhaust departments?

If the bike still has the stock airbox and exhaust, 3/4 to 7/8 turn out on the fuel screws usually works well.
If you have pods and/or a header, 1 to 1/4 turn will usually work.

The air screw usually starts at about double the fuel screw setting, then will be tweaked for best idle.

Have you checked your pilot screws for broken tips? Quite common in the VM carbs, and will exhibit the exact symptoms you are describing.

Rather than blowing air through the ports, use carb cleaner spray, so you can SEE that there is flow. Air will manage to make its way around some obstructions and you will feel it coming out the port, but you can't really gauge how much there is, only that it's there. Using a fluid, you can see the amount of flow. Compare that to the other carbs, you will be able to tell if it is obstructed. Now, if ALL of them are obstructed, you won't know the difference.

Another clue that tells me "obstructed pilot ports" is the fact that you need to use full "choke". When the carbs are (finally) set up correctly, you might only need half "choke" to start, unless it is REALLY cold.

.

[Crake]

Thanks Steve, appreciate the input. I've kept everything stock on the bike, including the air filter. The screw tips are intact and have a good point on them.

I'll try the jet settings I have tonight, and if there's no dice I'll set them all to your suggestions and see what happens. If that don't work I'll pull the carbs and try the spray as well.

[Crake]

Getting a little closer to a workable idle here. She started quickly with fuel screws 2 turns out. Ran a little better with fuel 2 1/4 turns out. Air screws are currently at 2 turns out. Bike runs higher idle with choke in. I was able to close the choke a quarter and she still idled, albeit a bit lower RPM. First time I've been able to blip the choke in without the bike dying. Seems to be the fuel screw... more to follow...

[Crake]

Oh and after a bit of running I was actually able to crack the throttle a wee bit without killing the bike. This could be progress...

[tkent02]

Two turns out on the fuel screws just isn't right, should be less than one turn. Something still blocked in there somewhere.

[Crake]

I thought so as well, then someone else's comment on a different site twigged a thought in my mind I hadn't put together. I suspected the FA mix might be too lean (as the choked mixture seems to run good) but up to this point I thought it just needed more fuel (ie: fuel screw OUT). Then I read something that chimed - "the fuel and air screws each richen and lean the mixture - turning the air screw IN richens the mixture". Up to this point I'd ignored the air screw, thinking I only used that to fine-tune the mixture once I had it running off choke. I turned them in a full turn and all a sudden I had a quick fire and throttle response. Gonna fiddle with it some more this weekend as it's supposed to be mild out...

[tkent02]

Usually the air screw ends up about twice as far out as the fuel screw.