Off idle, full throttle.... is it running still???? [Archive] - Nova Tech

: Off idle, full throttle.... is it running still????

Oct 1st, 06, 09:43 PM
I have a problem and I haven't put much thought into a fix for it, so.... this might be simple. I have a 350 .030 over, 520\544 cam, 308\318 duration. 2.02\1.60 valves, pro-topline heads (220cc intake runners,63cc combustion chambers), Flat top speedpro pistons (forged aluminum), Edelbrock performer rpm air gap intake, 1inch aluminum carb spacer, holley street avenger 770cfm carb, msd pro-billet dist., msd 6al, accell supercoil (the big yellow one),small tube headers and a way too open exhaust (3" pipe and flowmaster super 40's). Problem is if i stomp the ol'e gas pedal to the metal, the engine sounds as if it simply cut off for about half a second, maybe more. Then it unleashes itself and runs like it should, but that slight moment of silence, almost as if to say "Please forgive me im about to do something wrong", is really kinda crappy when i want to run....... answers? answers? suggestions? random stuff? Thanks for any help you can give!!!

Oct 1st, 06, 10:24 PM
90% of the time this kind of problem is either a lean spot in the idle transition circuit (which you can fix by adding bigger accelerator pump nozzles...bigger than the size 28 that come in most carbs) or not enough initial timing at idle.

Sometimes the accelerator pump cam needs to be adjusted. Some carbs come with the white cam and the motor may like the pink one.

What size accelerator pump nozzles do you have in your 770?
What's your initial timing? Are you running a vaccum advance on your distributor hooked up to full manifold vaccum?
What color pump cam do you have installed?

Mike Feudo
Oct 2nd, 06, 12:34 PM
Like was said acc. pump problem. More than likely on the sec side. It needs more fuel.

71 bow tie
Oct 2nd, 06, 03:41 PM
I had the same problem and I got the Acc. pump cam kit from summit and experimented and I got rid of the problem with a cam that squirts the gas sooner and faster.

Doug G
Oct 2nd, 06, 04:33 PM
Flat spot or bog as its called can be a number of things and most of the time its the cam/s and/or squirters.

Accelerator pump cams come in various sizes and are color coded and number coded by Holley. The cams have different shaped ramps that the arm from the accelerator pump rides on. By changing the size and shape of the arc on the cam, the pump shot can be tailored to start early or later as you go from off idle to full throttle. Changing the cams can have an effect on the way a vehicle leaves the start line in a drag race. If you leave the line off idle or at a higher RPM (while foot braking or when using a trannie brake or when using a clutch with a manual transmission) experimenting with the pump cams can help. There is no set rule for use, you just have to experiment with the different cams and the different cam positioning holes in the throttle linkage of the carb. Holley sells individual cams or you can buy their kit which includes an assortment of cams to choose from. Pump shooters are another area of experimentation. Holley carbs come with a standard shooter size which differs by carb list#. If you are experiencing a bog or hesitation off idle, you can try a larger, higher # shooter size. The bog or hesitation may be caused by a momentary lean condition when the carb goes from the idle throttle postion to the main metering system. The shooters help richen this momentary condition and eliminate the stumble. Keep going up in shooter size until a puff of black smoke comes out the exhaust, then go back one or two sizes. Playing with the shooter sizes is particularly helpful, when you have an intake with a large plenum area, such as a large open plenum or a tunnel ram. Keep in mind that as you increase the shooter size, you may also need a "hollow" screw to hold the shooters in the carb. At shooter sizes over .039, Holley recommends that you use the "hollow" screw (PN-26-12) which allows more fuel to flow to the shooters.

Oct 2nd, 06, 09:03 PM
I know very little about carburetors, its a holley street avenger 770, has the medium spring for the vacuum secondary, how difficult is it to change that cam? location? pics? I have no idea of the timing, right now the car is finally at the garage getting the 4speed swapped into it, been waiting 2-3 months, but back to the subject, I have no idea what size accelerator pump nozzles, what color pump cam, it does have a vacuum advance, not sure if its connected to the correct vacuum port or not, and i dont truly know what the initial timing is on it, i have never actually put it on a light. it started right up when i first put it together, I only had to turn the distributor ever so slightly to get it to idle and respond decent to the throttle. The garage that has it at the moment is going to try to tune it some for me, im horrid at tuning. My carb came with different vacuum secondary spring rates, would putting a lighter spring in it help?? Well if this gives any more insight into the problem please let me know.... Thank you everyone for your help so far.

Oct 2nd, 06, 09:30 PM
The type of spring could have a lot to do with the bog also. Try experimenting with the other springs. The lighter the spring the quicker the secondaries open. The stiffer the spring the latter they open.

Oct 4th, 06, 12:09 AM
I have tried swapping the secondary springs before, but did not see much difference in this problem...

Oct 8th, 06, 03:38 PM
Get your timing right, put A light on it make sure it advances as your rev motor then start playing with carb.

Oct 8th, 06, 04:53 PM
Well i gotta throw my 2 cents in here and i ask that you dont take offense because this is constructive criticism that im going to give based on my personal expierence and knowlege.

I think there are a few things you need to concider with your 350. Youre running a 220cc intake runner, a very large runner for a 350, that when ran on a 350 is going to make power in the upper RPM ranges probably 5,000-8,000 rpm but lag on the bottom because of a lack of port velocity. You have aprox. 10-10.5:1 compression wich will allow you to run a variety of cams from mild to wild. Your cam has a very long seat duration {BTW what are the .050 specs and youre l/s?} Im guessing its way too big. All of this says to me that you want to wind it up high maybe to 7,500 rpm wich is fine. But then you have a vacum secondary carb wich isnt really ideal on a wild motor and a dual plane intake that is going to die at 6,500 RPM and is counteractiing the large cam and heads. I think you need to sort out your combo a bit. With your heads being as big as they are i think a mechenical secondary 750 cfm mighty demon carb would be in line with a port matched victor Jr intake and possibly dropping back on cam quite a bit would work out very well and make alot more power AS WELL as be much more streetable. If you are at 10.5:1 i would run somthing very close to 290 advertised duration, lower-mid 250's @.050 and in the mid .500's for lift. probably ion a 108 l/s. I dont tell you this to be a jerk, im trying to save you time by not trying to sort somthing out that isnt really ideal. I just went through some of this with my 350 and im running much better now after dropping back from a 268/278 @.050 cam to a 258/264 @.050 cam and i wish i would have gone a hair smaller yet. Trust me on this one....let my high dollar mistakes be your advantage.

Oct 26th, 06, 01:48 PM
I thought too big o' cam, too. I agree with Nova guy about his suggestions. Also about how to tune the exsisting carb. What your problem is, is you have a super lean condition off idle. Part of that is combination, and part is tuning.

Marv D
Oct 28th, 06, 11:47 AM
90% of the time this kind of problem is either a lean spot in the idle transition circuit (which you can fix by adding bigger accelerator pump nozzles...bigger than the size 28 that come in most carbs) ..........

I'll see your 90% and raise you 99.9%
I couldn't agree more. This sounds like the typical 'tip in' lean condition caused by not enough fuel as the butterflies are snapped open. As many other have said,, the combination appears to have been chosen with the attitude of "bigger is better",,, but even so, it should run smoothly once the fuel curve is corrected.

I'd vote for a more aggressive accelerator pump cam, then investigate larger squirters. I don't know what the 770 avenger comes with, but the pump cams typically have 3 positions. Go to the Holley site and look up the screw position that makes what you have deliver the largest pump shot. Then make SURE the spring is adjusted so that the slightest movement of the throttle lever moves the pump lever and delivers fuel. Some guys think the 0.015" pump lever clearance Holley talks about is at idle,, NOT SO,, you want 0.015" clearance when the throttle lever is at full OPEN. The clearance is to prevent ripping the pump diaphram at WOT. If you have clearance at idle,, that could be 99% of the problem right there.

Also,,, I'd loose the 1" spacer under the carb. Adding more plenium volume is moving the wrong direction IMO. You have such poor port volocity with the small cubes, big cam and large cc intake port that I really think your shooting yourself in the foot with a spacer. Carb spacers are one of those things that you try on the dyno or on the track to see if there are any gains or not. Starting with one 'just because', well... I sure wouldn't.
Just a BTW,,,, we added a 1" SuperSucker spacer under the carb on the dyno and it cost almost 4 ft lb of peak torque, but gained 3HP in the 7200-7600rpm range on a 800HP small block. Actually,,, 797HP small block. We were looking HARD for that illusive 3HP to make 800 ;) I'll take more 'average torque', over a few 'peak HP' any day!

Tom Mobley
Oct 31st, 06, 07:51 PM
over-headed, over-cammed 350. You may need to investigate a higher stall speed torque converter and lower gears in the rear end. You'll probably want to go to a stiffer secondary spring, try to build some velocity on the primaries.

But, you're wasting your time fiddling with the carb when the engine hasn't been timed. What you will want is very aggressive initial timing, up to 20* or so. Normally this will cause way too much total timing when the mechanical advance comes in. The distributor advance curve needs to be adjusted to limit the advance to 16* or so. IOW, you need somebody who is a competent hi-perf tune-up guy. Not many of these left anymore.