I ordered a Holley 4555 rebuild kit from Zip and rebuilt the carb on my 1970 LT-1. I had the following part left over. What is it used on?


Thank you!
James

 

The part you have left over is the front accelerator pump check valve used on a newer Holley. Old Holleys use a steel ball. There is a measurement/adjustment for the travel of that ball. If the travel is too great, you could get a hesitation in the accelerator pump circuit.
I was chasing a hesitation in throttle response, and that was one of the things I checked. Mine had too much travel. A slight tap on the retainer closed up the gap.
It didn't fix my hesitation.

 

Barry,
Thank you for the help! Yes, I have the old style and I checked the travel of the ball.
Just curious, did you find the cause of your hesitation?

James

 

James,
After much investigation & frustration, 100,000+ miles, my throttle shafts had worn the throttle plate holes. The rear had more slop than the front shaft.
The shafts were fine. I installed brass bushings in the throttle plate, front and rear. The shafts then were a bit on the snug side. A slight hone on a couple bushings fixed that.
What a huge difference the new bushing made in the throttle response.

Barry

 

Barry,

Did you re-bush the shaft bores yourself? Where did you get the bushings? Did it require align-boring for the bushings?
I don't think this is something that I need to do immediately, but I'm just curious how you did it.

Thanks again!
James

 

Since you are asking about hesitation or bog i will add this. If you are having a hesitation or a bog when you hit the accelerator it can be a number of issues with a Holley. Without knowingbif its a lean or rich bog i will explain 3 common easy to fix problems. Of which are an improper cam, improper squirter nozzle (accelerator pump nozzle) or just improper pump arm adjustment. The gap called for in the instructions is not with the pump at reat, it is at full stroke and that gap is what your spring should allow at full stroke. So you slip a feeler gaugenl in there at full stroke or just visually confirm there is spring gap left in the coils. If you leave a gap between the cam and the arm then you are openning the throttles with no additional fuel. Holleys directions are misleading and they have tried at times to correct it in stories, addendum, and updates but not in the printed instructions. You really have to hunt to learn that

With that adjusted then you would have to determine if it's rich or lean on hard acceleration. If you can accelerate very slowly through your idle, idle transition and cruise circuits without it bogging then it's linked to the accelerator pump. If you are running a stock mid hp motor I would try a red cam or the blue cam. See how it performs. Then if you are at a 21 nozzle I would go to a 25 and see if its better, then a 30. If you get a bog but also a puff of black soot then it's too rich, go smaller with the cams. Having an AFR gauge makes this process so much easier

If your bog happens under an extremely slow acceleration then it's your transition circuit and your idle set screw is in too far....thats another post worth of info and involve either improper timing or another issue like vacuum

 

Thanks for the info. That is very helpful!

James

 

I hope it helps you out

 

On your idle screw mixture adjustments, make sure you can get both a lean roll, and a rich roll, and adjust them for high vacuum.
Then see if they are both turned-out the same.
Mine were 3/4 of a turn different!!!
Now that's a long story.
Took a major rebuild by Holley to fix my 70, way back in the 80s!

 

ERIC JACKSON VintageMuscleCarParts.com

 

Yes, I re-bushed the shaft bores myself. Purchased the kit from Amazon. It was a relatively easy and quick process, it went well.

You just use the reamer from the opposite end. Insert the shaft of the reamer in the bore first, this aligns the reamer, then chuck your drill on the end of the reamer, (on the reaming end). You do the reaming from the shaft end of the reamer.
This is for a Q-jet, however the same procedure for a Holley.

The only thing I did different was to use a drop on Loctite on the bushing before inserting.

Note, identify your throttle plates before removal and install to their original bores and carefully align the plates when reassembling. Don't forget to peen the plate screws after assembly.

Barry

 

That's a great writeup on hesitation. In my case I tried everything you mentioned except changing the pump cam, it was to be my last change. I left the pump cam to the end of the process using the logic that it worked correctly for 50+ years, why not now. So, the final fussing with the carburetor I discovered the worn shaft bores. It was amazing how well the carb worked with the new shaft bushings.
As the shaft bores wore over the years, you adjust around the worn bores to compensate, until there is no compensating.
Barry

 

I'll bet that constant vacuum leak messes with the carbs AFR calibrations.
And then it gets altered when you move the throttle too!

 

Exactly, it was getting to be a pig to drive. As I mentioned before, it got to a place where you could not tune around the problem. Just starting away from a light required much more care than it should.
After the shaft bushing, the carb needed to be "adjusted" again. Now easy/pleasurable to drive.