Gonna Pump It Up

[Joe C.]

I dunno, but maybe I am just spoiled by today's seamless (when they ain't broke) fuel injection systems, but here is what I am experiencing with my 1965 327/365. OCCASIONALLY, I feel a slight hesitation (stumble) on acceleration, which varies in severity from a slight hiccup (when at operating temp), to a pop thru the carb (during warmup). I suspect a slightly lean condition, but my idle jets are set (both are backed out 1 1/2 turns) for optimum idle vac/RPM, and I would prefer not to richen them. My pump shot is adjusted as per Holley spec, which calls for .015" clearance @ WOT with pump diaphragm bottomed out.

My question is, what effect does tweaking the pump arm clearance have on the pump shot. Does it change the volume delivered? Does it change the duration? Does it change the timing of the shot? Also, what will be the effect of widening or narrowing the clearance.

The only time I get an occasional stumble @ op temp, is either while cruising at 2500 RPM or less, and getting into the throttle sharply, or in starting off in first gear, and not slipping the clutch enough. Is this acceptable for an L76??

Thanks in advance,
Joe

[Clem Z.]

Re: Gonna Pump It Up

the stated .015 clearance is to be checked at wide open throttle and this is to make sure you do not stretch the pump diaphram and break it. i would check to see if the check valve needle is under the squirter. i also would check to see if you have the proper power valve rating in the carb. if you have a power valve with too low a vacuum rating it will open too later and cause a flat spot. post the LIST number on the choke housing and i will post the correct items for your carb.

[Joe C.]

Re: Gonna Pump It Up

Clem:

Thanks for the response. Yes, there is a ball check under the nozzle. The installed power valve is a Holley 65 (open @ 6.5 in-hg), which I believe is not ruptured because the motor never loads up. The pump diaphragm is OK because I get a strong pump shot. The carb is a Holley model 4500, LIST 2818-1.

Do you know what effect altering the pump clearance will have on the pump characteristics?

Joe

[Clem Z.]

Re: Gonna Pump It Up

as long as there is no clearence in the pump arm with the throttle closed you are ok. at wide open throttle is where you should have the .015,any less could cause the pump stroke to be short or cause the diaphram to break. the carb should have pri jet # 65,sec jet #76, power valve #65 and squirter #.025. are you sure you have enought timing in the engine at idle?

[Joe C.]

Re: Gonna Pump It Up

Clem:

So far nothing I don't already know. My initial timing is 14* BTDC, which is actually hotter than spec, which is 10* BTDC. I will AGAIN ask the question: "how will any modification of pump rod clearance affect pump performance". I realize that the CRITICAL dimension is .015 @ WOT, to prevent rupture of the diaphragm, and to leave a generous margin for thermal expansion. I am getting the impression that there is very little room to adjust the pump, and it will probably make imperceptible difference anyway, as long as it is set at .015" plus or minus maybe .005", anything less, I suspect, would be risking a tear, and anything more would be compensated for by the tensioning spring. From the lack of any adjustment information, I think I can safely conclude that the accel pump is operating correctly. I don't remember if the pump cam is at position "1", or "2", but wherever it is, I know that it is according to what Holley specifies.

With nothing more to go on, I can only assume that the motor is performing within design parameters.

Joe

[Duke W.]

Is your heat riser passage blocked? *NM*

[Joe C.]

Re: Is your heat riser passage blocked?

Duke:

No, the passage is not blocked. But, this is the second brand new heat riser that I have installed. The first one, made in Mexico (not sure if that really matters or not), only opened about 3/4 in the heat of summer. The one that I have now (made in Canada), has a slightly different configuration. The shaft has more offset, so that the valve blows open with exhaust pressure. What happens, is that at op temp, at idle , at 45* ambient, the valve is about 3/4 open, but easily opens wide when revved. This is how I remember the valves to work (on mid to late sixties Chevy's, Fords, and Mopars that I have worked on). The passage is open, and I did not install the restrictor plate in the manifold crossover when I assembled the engine.

Is it possible that your AFB might have slightly different characteristics than my Holley. I would be interested to know John H's thoughts on this.

Dwell is set to 30*, AC44 plugs are all evenly tanned, with no deposits or fouling. Maybe I am expecting too much, as I said, this only happens occasionally. Axle ratio is 3.70:1. Clutch is Borg Warner. Close M21.

Joe

[Clem Z.]

a couple of after thoughts

is the lever arm on the carb shaft tight on the shaft or has it been replaced incorrectly, because i have seen this problem caused by these items, it puts the cam out of phase with the butterflys opening,the butterflys open before the arm moves enought. also check to make sure that the idle trasfer slots are open at idle about .030/.040 because if they are not the fuel transfer happens too late.

[Duke W.]

Re: Is your heat riser passage blocked?

My '63 340 would stumble on starting from a dead stop with the heat riser
blocked until the engine had been running for a good 30 minutes, so I unblocked it. It's tough to get these SHP engines off the line smoothly because of the aggressive cams, and on mine it's especially tough with my CR trans and
3.08 axle.

Modern cars certainly spoil us with their perfect driveability under all conditions. I think AC 44s are a bit cold. I always had best success with 45s in street driving, but fouled plugs usually cause high rev breakup, not
transient hesitation or stumble.

Other than the problem with the blocked heat riser my AFB always seemed to work very well. I did have idle stabililty problems, but they went away when I installed the "236" vacuum can to keep the vacuum advance "cocked and locked" at full advance under idle conditions.

Duke

[Joe C.]

Re: Is your heat riser passage blocked?

Duke:

Allright, I think I am being a bit too "anal". I'll just make the beast happy, and dump the clutch at 4000 R's. Gonna have to use some wide rubber in the rear, except for "showtime". Kidding aside, I am happy with the performance, and barring any definative remedies, I will assume ( assumptions only make an ASS out of U and ME )that the engine/carb is AOK.

Am not sure of the plugs, but the 1965 glove box rag/owner's manual says something like this:

Spark Plugs:

43 for extended high speed driving
44 best all around use
45 stop and go traffic

Based on the above, I am assuming (there's that word again) that the car was fitted with AC 44's when new. My TIM&JG does not specify. Doesn't really matter, because I won't be using this car to go and buy butter and eggs at the corner grocery store.

Joe

[Joe C.]

Re: a couple of after thoughts

Clem:

Thanks for the continued interest. I would almost bet my life on these items being OK, but I'll double check anyway.

Joe

[Clem Z.]

more after thoughts

do you have the correct pri metering body because the power valve restrictions are different diameter and if too small you will be lean when the PV opens. i have seen these bodies restamped with too small a hole. yours should be .040 dia.

[Duke W.]

Re: Is your heat riser passage blocked?

Despite what the owner's manual says, the OEM AC 44s were just too cold and foulded easily. on my SHP SWC. I found by experience that the 45s were best for all around driving - city streets or freeway.

It only takes about 35 HP to propel a Corvette on level ground at freeway speed. The engine is loafing 99 percent of the time, and you just can operate it at high output long enough on the street to overheat the AC 45s.

Back it the sixties when I took my SHP SWC out to Kent for a day of hot lapping, I installed AC 43s.

Duke