I figured I would make a new thread about this.

I bought a 700 CFM Double Pumper through ebay. Here is the Description of what the guy did and said.

Holley double pumper 700 cfm square bore carb dual feed with the center hung floats. I removed the choke flap as it is nothing but a restriction. I tuned this for my 306 cid ford. I ended up with 69 jets on the primary side and 78's on the secondary. Both ends are using the red pump cams and 30 cc pumps on both ends and a 8" power valve since my vacuum at idle was 10.5" as recommended. This carb was phenominal and responce is as good as the best injection setup. It takes a lot of tuning to get it to this point. My setup was as follows:

306 cid ford
10.5-1 (zero deck)
"B" cam (10" vac @idle)
performer rpm
long tubes
1" phenolic spacer
If your running anything close this carb will rock! I painted the float bowls a gray color to match the rest of the car. The primary side has the teflon "O" rings so they won't get damaged changing jets as well as the good blue non stick gaskets on both to make tuning a not so miserable experience. The pictures show the insides as flawless and I will even include the aluminum and braided hose dual feed line.

So anyways, I installed the badboy, and if I try to rev it high it will bog and backfire through the carb. If I gradually do the accelerating it will run better. This doesn't happen only WOT since I cannot even get that far without bogging and backfiring.

I jacked up the fuel a bit to 6 PSI while running, and tiny bit better not much.

Soon as I romp on the pedal the car wants to die and backfire etc.


I need this car drivable ASAP. Any help is appreciated, I know pretty much nothing about Holley Carbs.

-Cale

 

Start by reading through this http://www.holley.com/data/TechServi...ech%20Info.pdf

To supplement:
Fuel pressure has nothing to do with all of this. All fuel pressure does is push fuel to the fuel bowl, which is basically a pool from which the carb sucks fuel into the air stream. If the pressure is so high that it affect the float setting (by pushing the needle off of the seat), that's as much as you can say fuel pressure affects the way the car runs.

What you need to look at first is the accelerator pump. It should squirt a stream of fuel into each venturi when you open the throttle. Check that without the engine running.

After that, the power valve may be causing issues. An 8.5 PV should be high enough to avoid problems in a street engine.

Most likely you've got a problem with the accelerator pump. Check that first.

 

backfiring sounds like timing not the carb. Float level? idle mixture screws set? Idle vacuum in gear while the motor is hot? What do the plugs look like?

 

My plan was to rejet the carb using stock jets as the baseline and not what that dude had.

Five7, I read that site, which was very good, but it does not tell me how to trouble shoot the carb or how to replace the pump.

I need this car drivable tomorrow so my wife can have my truck. I sold all my spare carbs so this is what I have. I will go to the shop today and shoot a light on the timing. I will adjust the timing to see if it gets better.

I hope it is minor problems and nothing I cannot get at autozone or O-reillys

 

If you didn't change the timing from the Edelbrock to the Holley, it should be fine with either carb.

Backfiring and bog are typically lean conditions. Timing can contribute, but if it ran with the other carb and timing wasn't touched, not likely.

Also check for any possible vacuum leaks - wrong carb mount gasket, vacuum port holding the carb off of the intake, vacuum ports left open, etc.

Check for that squirt of fuel from the accelerator pump(s). If you don't get anything, check the linkage (I saw a carb at the track a week ago that didn't have the pump pivot arm properly attached). If that's all okay, replacing the pump diaphragm is fairly simple (need to remove the carb or the fuel bowl since it bolts to the bottom of the fuel bowl).

 

You know what... I have my nitrous plate on, underneath the carb. The solenoids are all attached and whatnot, but they aren't hooked up. I will remove the plate, then try the carb, then check timing, and venturies.

I will check back with you guys later on.

Thanks

 

I started the car and the fuel bowls were leaking like crazy. Found out they weren't very tight. Removed the bowls reseated the gaskets (they were the blue reusable kind) and it stop leaking.

Took the nitrous plate off... no joy still bogs.

I did not have chance to find my timing light.

No vacuum leaks. I am at wits end.

The venturies all shot fuel perfectly

I bought the haynes repair manual for Holleys, and bought a rebuild kit as a last resort, which is about what I am going to do.

I messaged the guy from ebay, he called me. Said he just rebuilt the thing and it worked fine. He did not know what to say or do for me.

He said that the power valve he used was 8.0 and thinks I should use a 10?

So anymore ideas?

 

I am wondering, if instead of rebuilding the entire carb if I should just replace the power valves and diaphrams for the accelerator pumps.

 

If you're getting a good healthy squirt into each barrel from the accelerator pump squirters, just replacing the accelerator pump diaphragms isn't going to help any.

Ditto with the power valve (the numbers always have .5 in them, by the way - as in 8.5, 10.5, etc.). Stock is 6.5 and works for a huge majority of streetable engines out there.

If you're going to use anything out of the kit, do the whole thing.

Are the accelerator pumps adjusted per the Holley instructions?

 

Are you running a vacuum advance distributor? What port do you have the vacuum line plugged into?

 

Yes. The one on the upper passenger side. The one above the choke.

 

I run mine on the bottom port for mainfold vacuum. Passenger side.

 

put it on the bottom port and see how the motor reacts, i'm still thinking the timing may be off.

 

I really do not think it matters, but I will check. I have seen weirder things.

I will check it out tonight.
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Well I sure hope you are right, I have duty tonight I will not be able to get on TGO but the base is where my car is. I will shoot a light on it and make adjustments as needed.

I will put it on the bottom port too, incase the vacuum port on the top does not work as well as the bottom.

 

the vacuum port on top is ported vacuum not manifold vacuum. They are different. One has vacuum all the time and one has vacuum only on the main circuit.

 

No kidding, It actually fixed my problem. Who would have thought?

So I mean, what the heck would you use the other vacuum port for?

 

Timed (ported) vacuum is the "normal" configuration for a whole lot of applications. The factory started it in the early days of emissions controls.

My MSD distributor says to use ported vacuum. The reason for that is performance cams often don't have enough vacuum to consistently apply vacuum advance at idle. The result is a searching idle. Unfortunately, my Proform main body isn't drilled for time vacuum, so if I want vacuum advance, I'm stuck with full manifold.

If switching to full manifold vacuum "fixed" your problem, you probably don't have adequate advance.

 

1 beer please Also this means your advance curve is too large and you need to limit the mech. advance by welding some of the slots closed in the dizzy so the rotor can't move back and forth as much, 24 initial 36 total or so would do it.

If your cam doesn't make that much vacuum at idle then you probably could just run it locked out or skip vacuum advance at all on it. Everything I put a big cam into just gets the timing locked out or a very short curve if the starter can't turn the motor over, usually 25 initial 36 total or the like with medium springs because the light springs cause rpm fluctuations. If it's street driven mileage will suffer, but with a big cam, mileage is the last of my worries.

 

Fuel consumption is already high enough as it is with E85. I typically only drive it to and from the track, about 15 miles round trip, but I buy fuel in 5 gal batches. It isn't a big deal, using the full manifold vacuum source doesn't cause enough problems to worry about.

Being able to tailor the mechanical advance curve is a big plus. Love the MSD distributor for that. I'm using the biggest bushing, which keeps idle advance high enough to help it idle well, and low enough to prevent starter kick-back.

 

I run my 383 with an XR282 off manifold vacuum, since I don't really like the tip-in feeling on ported. I have the advance rate set quick enough that it's fully advanced below the normal idle vacuum of about 12 inHg so there are no hunting issues. Even still, I run about 24 degrees of base timing (retarded down to 9 degrees for starting), and the additional vacuum advance brings it up to about 43 degrees. I could probably just get rid of the vacuum advance entirely and lock the centrifugal out at 34 degrees, since teeny little primaries are never opened up enough for the vacuum to drop far enough affect the advance under normal driving with a 3000 RPM stall.

It's just one of those things you have to play with sometimes to see what works, and sometimes there's more than one way to skin the cat. Having a fully adjustable distributor makes experimenting a lot easier.

 

Car runs fine, however, I must replace the diaphragm for the accelerator pump anyways for the secondary side.

It is leaking fuel from the seal now. I tightened the four bolts, but it is still leaking some. I cannot live with some.

Also, anyone recommend a great set of fuel bowl gaskets that are reusable? I have blue reusable ones on it now and they get hard easy.

 

spray them with WD-40 before installing them and they shouldn't get hard.