Holley carb help needed

[davidlee]

HELP!!

I have a 351/360hp crate motor prox 300 miles on it.I have a BAD stumble at approx 2100 rpm in 1st gear and the same in second but not as pronounced. The carb is a Holley 750 dbl pumper with mechanical secondaries. I have checked the fuel pressure {6 lbs} checked the float levels{spills out when you shake the car} Increased the jets from the stock 71s to 74s in the front have not messed with the rears. I checked the vacuum 16.5-17.5 at idle prox 20 at on load running on the highway. I have not changed the power valve and assume it is 6.5 from the factory.
This is a consistant stumble and will happen every time I hit prox that rpm level in the lower gears. If I short shift{ stay below 2000} no problem. The timming is set at 38 total. The plugs look like the fuel mixture is ok.
This stumble is sooo bad that my young son has bumped his head on the rear deck several times
I have tried disconnecting the vacuum advance on the dist but it made no differance. The accelator pump is set to squirt fuel as soon as you start to move the throtle.
I need help, this problem is embarassing as people think I am trying to race them every time I take off.
David

[Mr.Fixit]

It is a stumble during steady RPM, or during acceleration?

[Dan Case]

Since you have checked float levels; sounds like the transition circuits may be fouled. Holley's have "transition slots" to provide fuel flow between leaving the "idle circuit" and getting into the "main circuit". I have found trash blocking one of the transition circuits to cause what you describe.
Good Luck. Dan

[mrmustang]

David,

First off, you have too much carb for the motor. After trying no less than four different carbs on my 351W/385hp Ford crate engine, I ended up with a dyno tuned Holley 600DP 04776C, a 1:1 Moroso secondary linkage kit, and an electric choke kit. 72's in the primaries, 68's in the secondaries............


Bill S.

[davidlee]

Mr fixit

The stumble is only when I accelerate and hit that prox 2000 rpm point. It does not seem to do it during rapid acceleration just when I "try" to drive normal, constant easy pressure,hit 2000 then stumble and launch.

Dan
Where is the transition slot located? I have taken the fuel bowl and the metering block out and cleaned them with carb cleaner.
Thanks for the replies
David

Oh when I hit the stumble the vac drops aprox 2 to3 inches. Hope the info will help

[Dan Case]

The slots are cut into the throttle bore in the base plate. The slots get exposed more and more as the throttle open. Was the carb new? I have also seen rebuilt carbs with the incorrect base to body gasket cause problems. Dan

[Mr.Fixit]

What size accel pump squirters are you running in the front and rear? Might want to try a smaller squirter on the rear to lengthen the accel pump shot.

Is it a stumble or more of a lean flat spot?

[RedBarchetta]

Originally posted by davidlee


[b]....BAD stumble at approx 2100 rpm.....I checked the vacuum 16.5-17.5 at idle prox 20 at on load running on the highway. I have not changed the power valve and assume it is 6.5 from the factory.

Aside from running too big a carb, you might not be running the correct power valve (i.e. the factory one is set to open at a much lower "load" vacuum level than your engine may be experiencing). Just something else to consider.....

[davidlee]

Dan,
This is a new carb.

MR fixit,
Last night I swaped the front to the rear squirter. The factory had a 28 in the front and a 32 in the rear. I noticed no change when I reversed them. All I can say about the "stumble" when I take off normally I hit prox 2000 rpm the power goes flat almost like you turned the switch off for a nano second then launches off.
Am I looking in the wrong place?? Could it be something other than carb? I have a Ford duraspark dist and a msd 6 ign box. The only two consistant things are carb adjustments made no difference and the problem always occurs at prox 2000rpm. Thanks for all your input.
David

[Mr.Fixit]

Sure, it could be something other than the carb, but is most likely fuel delivery. My best guesses, it could be one of the following, in no particular order:

Transition circuit not flowing fuel.
Vacuum leak between baseplate and main body.
Accel pump shot needs more duration.
Need one jet size larger on the primary side to cover the flat spot.

remember these are guesses, it's difficult to diagnaose a problem by way of description only. Is it a new carb? Was the carb rebuilt recently? Was the carb "good' then just sat on a shelf for a while?

[trularin]

A quick way to check all this mess is to change the carb as suggested. A new 600 to 660 will allow you to see if it is fuel delivery.

You do not say what happens after 2100 RPM. Does the car/engine run like a rapped ape? Is there clouds of black blowing out the back/sides? Is the this under load only? ( maybe you answered this question and I missed the answer ). Mr.Fixit makes a good point, this is hard to going by description alone.

Just a thought.

[davidlee]

I agree if I had another carb I would put it on but I don't and yes after 2200 the car does fine. I checked the plugs and they seem to show the carb is not too rich or too lean.
I appreciate all the input. I'll keep tinkering I'll let you know what I find. Any more input is welcome.

[trularin]

Suggestion:

Remove the 35CC accelorator pump and put in a 50CC unit.
Change the pump CAM profile to cover more of the throttle.
Put a 5.5 or 4.5 power valve in.

Just a suggestion.

[Mr.Fixit]

Originally posted by davidlee


I agree if I had another carb I would put it on but I don't and yes after 2200 the car does fine. I checked the plugs and they seem to show the carb is not too rich or too lean.
I appreciate all the input. I'll keep tinkering I'll let you know what I find. Any more input is welcome.

reading the plugs isn't going to tell you squat about whether it is too rich or lean during this problem. It won't tell you squat unless done under specific circumstances, with fresh plugs.

I am guessing it is too lean during this problem. You can cover it up with the accel pump(s) or go up a jet size on the primary side. Try that and see if it gets better or worse. You can only theorize about what the problem is for so long before you have to try something and see what happens. better/worse/same, then take that off the list and try the next thing on the list.

[JB34]

Davidlee, I had the same problem that you do. I have a 302 with a healthy cam, stage 3 porting on the heads, a Victor Jr. and too big a carb...Holley 750. I messed around with this thing until I finally broke down and called the Holley tech line. It was a simple fix and I felt like a chump for waiting so long to call.
1.) check your float levels; when correct,
2.) set your idle mixture at all 4 corners.
3.) use the proper power valve Don't assume. Take your vacuum pressure at idle divided by 2 ( your avg is 17 so use an 8.5 power valve)
4.) check the clearance on the accelerator pump lever to the adjusting screw. This needs to have a .015" clearance so that your pump diaphram is not stretched too far. There is an adjustment screw with a spring and lock nut.
5.) Go to a larger pump shot on the primary!!! AS it is stumbling this is the final answer. You will need to jump three sizes to see any difference. Try a 35 or 37 and I bet your problem will go away.

Mine did; hope it works for you.

[davidlee]

I know this is a late follow up to my problem but I just got it solved. I know it's been a long time.
The metering plate was bad, not leting any fuel through the transition circuit. I replaced the metering plate and the problem went away.
Thanks to all who replied.
David