Engine (carb?) trouble

[Brown_Muscle]

ok heres the problem,, i have a 5.8L with the Variable Venturi. If you give the engine a little gas it will rev up and down rhythmically (1800 down to ???? the tach doesn't show it goes down to 0),

however if you give it a lot of gas suddenly it will rev up normally.

The fuel filter is good, as is the coil...just filled it with about 4 gallons of gas....

I'm not sure what direction to go from here, i will try some carb cleaner spray soon... any suggestions?

[Tiggie]

So at part throttle, the engine won't settle on an RPM? But at most-to-full throttle, it's ok?

Check to make sure those venturi valves on top aren't sticking. Does it choke out, stumble, or miss?

[gadget73]

did you pull the fuel filter to see if theres any water in it? Not sure if those have an inline filter between the pump and carb or if its right in the side of the carb.

[Brown_Muscle]

it got better after some revving... not prefect though...thinking it may be the gas, i will get some better gas from a different station and see what happens.

No inline filter just one between the carb and the line,

venturi's act normally.

[gadget73]

Any water come out when you pulled the filter though? usually it will bead up on the rag or on the manifold. Might want to throw some drygas in there just for S&G, but it sounds like something got into the carb. If one drop of water made its way into the bowl, it can lay in the jet and basically shut half of the carb off. The carb jets won't pass water beads. Normally the filter won't either but I dont know what sort of filter these have. If its not a paper filter or the dense brass mesh style, then the water will very likely pass.

[86VickyLX]

Any water come out when you pulled the filter though? usually it will bead up on the rag or on the manifold. Might want to throw some drygas in there just for S&G, but it sounds like something got into the carb. If one drop of water made its way into the bowl, it can lay in the jet and basically shut half of the carb off. The carb jets won't pass water beads. Normally the filter won't either but I dont know what sort of filter these have. If its not a paper filter or the dense brass mesh style, then the water will very likely pass.

The filter reminded me of the orifice tube in the ac system. That mesh is what it was made out of. After revving it a while, it kinda snapped out of it, now it has a little stumble, but nowhere near as bad as before.

[gadget73]

probably wouldn't be the worst idea to add a filter ahead of the carb

[johnunit]

Are you getting any other symptoms? Vacuum leaks do some funky things to those carbs.

[Brown_Muscle]

we put a vacuum gauge on it and it seems decently normal....maybe a little low at 15....

I put gas treatment in the tank and filled up with mid-grade gas from a different station and sprayed carb cleaner in the carb....

Took it for a ride, it bucks at light acceleration but when given 1/2 to full throttle responds well... It no longer rhythmically bounces up and down when revved, however occasion seems to miss...and im not sure if its a miss so much as the whole engine just dying momentarily, the tach reads 0 and then it comes back to life magically...

any more ideas?

I'd drain the bowls but this is no lawn mower...

[johnunit]

how fast is the bucking? every half second? every couple seconds?

[Brown_Muscle]

id say almost every second when it does it... decently fast

[Brown_Muscle]

FOR ANYONE WHO HAS WORKED ON VV CARBS!!

How do i drain the fuel from the bowl(s)? I noticed in the front of the carb below where the fuel line goes in there is a plug with an alan key or 4 side socket drive in the middle of it, do i simple remove this to drain the fuel?

THanks

[johnunit]

http://www.surplussales.com/Automobile/Carburetors/pdf/ato-77-336.pdf

This may or may not help. I've never noticed an obvious way to just drain the bowls. You could take the top off, but you better have a new gasket ready, and those are made of near unobtaniaum.


edit: do you have a 7200 or 2700 model?

[Brown_Muscle]

7200, that PDF is nice...i saved it for future reference

[johnunit]

yeah, I've referenced it a few times. It can be a bit confusing/frustrating because it doesn't make it clear what is 2700 only, and a lot of the adjustments can only be done with the carb pretty much totally apart.