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Fuel pump causing rich idle?


Farmall Cub
Hello, I recently joined this forum after picking up a 67 Scout 800 with a 266 V8 that has sat for many years. I'm currently having a problem with what seems to be a rich condition at idle. Runs great at other speeds, but sitting at idle will burn your eyes from the fumes. I rebuild the Holley 2300, adjusted the timing, new ignition parts, fresh gas, etc. I have searched the forum and found lots of info regarding power valves, base gaskets and the like, but someone said that a fuel pump could cause a rich condition at idle. Could someone elaborate on that?

Vacuum gauge shows 14" with a 2-3" pulse at idle. Mid throttle holds a steady 20-22". Baseplate idle mixture screws can be turned all the way in with no difference in running, and if I remove one, the idle increases. I'll be happy to give any other info, thanks for your help!

Chad Stretz
Columbia, MO

Gary Billings

Dreams of Cub Cadets
The fuel pump cannot cause it to run rich. If it's pushing that much fuel through it, it will overflow out the vent or flood out and kill the engine or puke gas all over the intake.

If the idle mixture screws don't do anything, you've either got a vacuum leak or most likely the power valve isn't seating all the way or you're missing a gasket, or something like that. Do you hear any hissing coming from behind the dash?



Farmall Cub
No sir, no hissing. I've cupped my hands above the intake and can pretty much kill the engine, and I've sprayed carb cleaner all over the intake/base of carb. No change. I'll look at the exploded again to see if I missed anything. I replaced the power valve with the carb kit the first time, and then I put another one in when I thought I might have blown it.


Binder Driver
Your throttle butterflys may be open too much at idle exposing too much of the transfer slots. This may account for low vacuum, no response from idle adjust screws, rich running, etc. It is impossible to adjust idle in this condition. What is your idle speed set at


Farmall Cub

The idle speed was about 350-400 rpm when I timed it. I'm probably closer to 600-800 rpm now.

Jay Tabor

700 is a better idle speed, 400 is way to low, so adjust idle speed at 700 then adjust mix.
i think its your power valve. easy way to tell is pull carb off engine
pull off fuel bowl, and metering block, when metering block is off if you see fuel on/in the vacuum chamber for pv, at front of carb body, that prooves its leaking.
you might have put the wrong metering block gaskets on, as well.
too high a float/fuel level will cause bleed over into venturi cluster giving rich idle.
and or thru the fuel bowl vent..
get holley pv! none of them after market rebuild kits have good powervalves in them..

Bill Bennett

High Wheeler
After the carb rebuild, did you re set the float/fuel level in the carb? The setting is adjusted with the engine running and the vehicle sitting on a level surface. Remove the sight plug = brass screw on the passenger side. Adjust the float so no fuel runs out of the hole. The fuel should be just below the bottom of the hole. Maybe 1/32". Reinstall the screw and gasket.
The fuelpump pressure should be 5 to 5 1/2 pounds?
If the main body and throttle plate don't seal, usually due to 'warping', you will have a vacuum leak into the power valve chamber. Caused by overtightening the screws that hold the 2 parts together. This is a 'internal' vacuum leak and can't be found from the outside. Acts like a blown power valve = runs rich at idle and can't be controlled by the idle jets. Engine develops a slow 'lope' = speeds up, slows down, rich fuel mix.
If the problem is due to warpage, it can be repaired and will become just another part of the carb rebuild process. Does not require new parts.


Farmall Cub
Per Jay, I will pull the float bowl/metering block off again and check for moisture around the power valve. I don't remember seeing any before, but hey...
I did replace the power valve with an after market 7.5. (What the book at the auto parts store said) It was not a Holley part.

Bill, I did set the float level with the engine running, and have since lowered it below spec to see if it would help. I have not checked the fuel pump pressure. How does one repair an internal power valve vacuum leak?


Dave Peters

Farmall Cub
Not sure on a 2300, but I had a warp of the base of my 2210. I placed it on a sheet of glass, found the high corner and sanded the other corners and also toward the centre of the base until it was flat all the way around. Used 220 grit for the rough sand and 440 to shine it up. Took my caliper back to keep an eye on how it was coming along- laid it on edge across the base from bolt hole to bolt hole. Then I checked it on glass to check my progress- took a while but it's fine now. When it was remounted I sprayed carb cleaner on all mated surfaces while the engine was running and listened for any RPM increase which would indicate a vacuum leak.


Farmall Cub
Pulled the carb again tonight. I carefully removed the float bowl and metering block, and the vacuum chamber for the power valve was bone dry. I measured fuel pressure and it is just shy of 5 lbs. I checked all my gaskets and reassembled carefully. No change. Runs great at speed, burns my eyes at idle. Idle screws are there for decoration. Leaves nice black spots on my driveway from exhaust.

Sound like warpage? I have a sheet of glass around here somewhere...

Bill Bennett

High Wheeler
Bill, I did set the float level with the engine running, and have since lowered it below spec to see if it would help. I have not checked the fuel pump pressure. How does one repair an internal power valve vacuum leak?


The base of the main body and the throttle base need to be surfaced so when they are joined, there is a good seal between them. The problem seems to be with the 'ears' on the main body becoming bent down from overtightening after many carb refresh sessions. Because of the metal the main body is made from, don't try to bend the ears back up into position. They may break off and a replacement body is the only alternative after that.
You will need a medium metal flat file about 1" wide. I use a piece of highly polished 'flat' floor tile with a 'square' edge and 150 and 220 grit sandpaper. Not the wet-dry paper.
With the carb disassembled, lay the file across the base of the main body on the 'ears'. Most likely, the ears will be the high spots and will need to be filed flat to match the surface of the rest of the base. The file work on the 'ears' is all the file is used for. The carb metal is fairly soft so it doesn't take much effort to remove those 'high' spots.
Place the 150 grit paper on the smooth side of the floor tile and drag the bottom surface of the carb main body across the paper. One side of the main body base has a ridge that will prevent the base from sitting flat on the paper and tile. By positioning the paper even with the edge of the tile and placing the carb base with the ridge off the edge of the tile, you will be able to sand the entiire base of the carb to get the flat surface. It won't take long before you will see the metal that is not being touched by the paper. When the area is close to flat, change to the fine 220 grit paper. It doesn't need to be perfect across the entire surface. The gasket will seal the opening into the power valve chamber with a reasonable amount of sanding effort. Correcting the 'ears' will make the most improvement. The sanding of the base surface only needs a couple thousands of metal removed to seal the vacuum leak. Not a big project!
The attached image shows the base of a 2300 main body with the opening marked for the vacuum port into the power valve chamber. That port, if not sealed by the gasket, is where the vacuum will be lost = internal vacuum leak. With low manifold vacuum to the power valve, the power valve sees the lack of vacuum as like opening the throttle and lowering the intake manifold vacuum below the value needed to open the power valve and it will supply additional fuel to the airflow through the carb.
After getting the main body corrected, take a few passes to the throttle plate. Turn the 'butterflys' so they aren't touched by the sanding. Blow some carb cleaner through the passages in the carb base and throttle base.
If this sanding is done every time the carb is refreshed, (just a couple passes over the 220 sandpaper) you won't have internal vacuum leaks.
Good Luck


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Jay Tabor

YOULL NEED SOME CLEANING SOLVENT AND COMPRESSED air, the sanding puts all sorts of gritin/on/around the carb body.
you need to make sure all of it is removed before reassembly.

Doc Stewart

Content Team
Staff member
It helps on the Holly carbs to use a flat washer under the mounting bolt heads rather than a lock washer. The lock washer on the aluminum creates uneven pressure on the mounting ears and contributes to the warping of the base. Also, most carb bolts are horrendously over tightened. 15 foot pounds will adequately hold those 5/16" mounting bolts.

Jay Tabor

and the real reason for bent warped ears/ on any and all carb base body, is- is--------------- the 1/4" thick base gasket - old and compressed and the spacers in the base gasket aint there no more . ..
the metal cant bend unless what its mounted to isnt perfectly flat .
so dont forget a new base gasket!