Carb slides barely moving, very weak power mid rpm's and above

claytonia

claytonia

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Joined
May 16, 2013
Messages
82
Location
Olney, MD
Bike
1993 ST1100
I've never seen this before, but seems to me there must be only a couple things to check (or is it never that simple?):

At rest, with the airbox off, the bike runs fairly well, and seems to rev cleanly up towards red line OK. But none of the carb slides are moving up more than a few mm's. All 4 cylinders appear to be firing OK - checked exhaust manifolds at idle with an IR thermometer and all are within maybe 10 degree (F) of each other.

I cleaned all the carbs last year and replaced all the intake boots. I checked carefully when putting them back to be sure they all seated properly before tightening the clamps. If this were an isolated carb I could think of a few things to check, but all 4?! Not surprisingly, there is little power past mid revs .
 
claytonia said:
At rest, with the airbox off, the bike runs fairly well, and seems to rev cleanly up towards red line OK. But none of the carb slides are moving up more than a few mm's.
Click to expand...
The slides won't move much without engine load; it doesn't take much power to rev an unloaded engine.

Not surprisingly, there is little power past mid revs .
Click to expand...
If you mean without the airbox and filter, that's normal with CV carbs, which depend on intake restriction.
 
Larry Fine said:
The slides won't move much without engine load; it doesn't take much power to rev an unloaded engine.

Thanks - this I obviously didn't know.

If you mean without the airbox and filter, that's normal with CV carbs, which depend on intake restriction.
Click to expand...

Correct. So I guess that means running under load (riding it) with the airbox off will not make much difference.
 
You could have one or more perished slide diaphragms in the carbs. It may not be all four of them, as noted above it would be difficult to tell with no load on the engine. One or two faulty diaphragms would certainly cause the power loss. Also check the auto fuel valve diaphragm for holes or splits.
 
Can't the diaphragms be checked just by pulling up on the slide - if it drops right down then it's bad? That used to work.

I've bypassed the fuel valve, at least temporarily - trying to eliminate areas to narrow it down.
 
claytonia said:
Can't the diaphragms be checked just by pulling up on the slide - if it drops right down then it's bad? That used to work.

I've bypassed the fuel valve, at least temporarily - trying to eliminate areas to narrow it down.
Click to expand...

Visual inspection with a strong backlight would be more definitive and it’s not hard to do. You should be able to remove the tops of the carbs and lift the slide/diaphragm assembly out with the carbs still installed.
 
Thanks for the suggestion - certainly relatively easy enough to pull them. I suppose just one leaking could be enough.
I'm going to sell it, but I want it to be in better running condition than this. Willing to try just about anything within reason.
 
How did you determine that the slides were rising only a few mm? Did you remove the snorkels?
 
Larry Fine said:
The slides won't move much without engine load; it doesn't take much power to rev an unloaded engine.


If you mean without the airbox and filter, that's normal with CV carbs, which depend on intake restriction.
Click to expand...
At full throttle though, the slides should (I think) rise the same height as when under load since the butterfly is wide open.
 
Thanks! Right, this is with the airbox off (really the only way to see them). Like this, at rest, I've revved it up near redline, and none of the slides moved more than a couple mm.

I'm going to try 3 things right now, in this order (if one works it's always nice to know which one):

1. Pull the diaphragms and check for leaks
2. Replace the fuel line and filter, temporarily (already bypassed the valve, but I had to clamp the hose for several days as fuel was slowly leaking - turned out to be the hose clamp nearest the carbs - but I read elsewhere that it could be distorted from the clamping and restricting fuel flow)
3. Add Sea Foam to the gas, run it a while, and let it sit for a day or two (also read this somewhere else). I've only used the stuff once - anyone have good/bad results with it?
 
jrp said:
At full throttle though, the slides should (I think) rise the same height as when under load since the butterfly is wide open.
Click to expand...

They won't as they're not directly connected to the throttle cables, they depend on air intake demand from the engine, which is very weak when there's no load, even at high RPM. You can hit redline RPMs with very small throttle openings under no load conditions. You may not even have to open the butterfly valve far enough to expose the slide vacuum port to the intake vacuum, which would explain why they don't move much or at all.
 
wjbertrand said:
They won't as they're not directly connected to the throttle cables, they depend on air intake demand from the engine, which is very weak when there's no load, even at high RPM. You can hit redline RPMs with very small throttle openings under no load conditions. You may not even have to open the butterfly valve far enough to expose the slide vacuum port to the intake vacuum, which would explain why they don't move much or at all.
Click to expand...

Obviously I need some edumacation.

For a given RPM, say 8000, why would the vacuum pulled by the pistons (at point-in-time 8000 RPM exactly) be any different whether it was standing still or going 125mph? The pistons are moving at the same speed.
 
claytonia said:
I've never seen this before, but seems to me there must be only a couple things to check (or is it never that simple?):

At rest, with the airbox off, the bike runs fairly well, and seems to rev cleanly up towards red line OK. But none of the carb slides are moving up more than a few mm's. All 4 cylinders appear to be firing OK - checked exhaust manifolds at idle with an IR thermometer and all are within maybe 10 degree (F) of each other.

I cleaned all the carbs last year and replaced all the intake boots. I checked carefully when putting them back to be sure they all seated properly before tightening the clamps. If this were an isolated carb I could think of a few things to check, but all 4?! Not surprisingly, there is little power past mid revs .
Click to expand...
Did you sync the carbs? Are you loking for a problem or do you have one?
 
jrp said:
Obviously I need some edumacation.

For a given RPM, say 8000, why would the vacuum pulled by the pistons (at point-in-time 8000 RPM exactly) be any different whether it was standing still or going 125mph? The pistons are moving at the same speed.
Click to expand...

Because the throttle butterflies in the unloaded situation are barely open. The piston speed may be the same but the air flow certainly is not. If you're hitting 8,000 RPM on the road the throttle valves are almost completely, if not completely, open. Air flow will be magnitudes higher and the vacuum port to the diaphragm will be fully exposed to the intake in the loaded on-road condition. Air is compressible and expandable, so you can't think of an engine like positive displacement fluid pump. RPM and pumped volume are variable and not a constant relationship.
 
Actually the carbs will have to be pulled to check the diaphragms, as the frame blocks the lower screws - so I'm shifting over to the 2nd action on my liost (unlewss I get a wa


1. xx Pull the diaphragms and check for leaks - OK I take it back - the carbs need to be pulled to accurately check the child...0-00
2. Replace the fuel line and filter, temporarily (already bypassed the valve, but I had to clamp the hose for several days as fuel was slowly leaking - turned out to be the hose clamp nearest the carbs - but I read elsewhere that it could be distorted from the clamping and restricting fuel flow)
3. Add Sea Foam to the gas, run it a while, and let it sit for a day or two (also read this somewhere else). I've only used the stuff once - anyone have good/bad results with it?
 
So current plan (since I'd rather not pull the carbs again to check the diaphragms):
1. Rebuild fuel valve and put back (have a rebuild kit) - I thought I should use it, just on general principles...+ add new fuel lines
2. Run some Seafoam, let it sit a while, then start up and see how it runs.
3. Sync carbs (did this once, probably over-due).
4. Pull carbs to check diaphragms (last resort).
 
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