Fuel pump??

[Gary_gtx67]

Just wondering should I stick with a mechanical pump or go with electrical? Feels like a's sson a's I hit the secondaries It falls on it's face. Could be jeted to high is a possibility just wondering what your thaughts are on it?

 

[Endangered Species]

Just wondering should I stick with a mechanical pump or go with electrical? Feels like a's sson a's I hit the secondaries It falls on it's face. Could be jeted to high is a possibility just wondering what your thaughts are on it?

Need a little more information. How big of a motor, how big and how many carbs are you running. Is it a street car, are you racing it or both?

Kevin

 

[Gary_gtx67]

It's a 383 .30 over Keith black hyper utechtec flat tops 10-1-1 compression not 906 heads but had work done to equal them comp 280 cam edelbrock rpm performer intake with 3000 rpm stall and 391 gears. 1 carb 750 cfm double pumper. For both track and street. I was told by several people that this setup alone would put me in the mid 7's in a 1/8 mile and the way she is acting now feels more like a 9.5.

 

[Gary_gtx67]

Oh yes and a factory replacement fuel pump currently.

 

[Endangered Species]

What does the car do after the bog? Does it clean out and pull fine or does it want to nose over on top end?
If it runs fine on top end I wouldn't think it was a pump problem. I would say your bog might be a squirter issue or you need jet extensions.

I'm not convinced this is your problem but I would get rid of the stock pump and go with a high volume pump, this way you know you have plenty of fuel.

As for what kind of pump, that's up to you. They make high volume mechanical pumps that would work fine but mechanical pumps rob horsepower.

Electric pumps are pretty much standard now a days. They pump a lot more fuel.

What ever you do with the pump, make sure you increase the size of your fuel line from the tank all the way to the carb. I would go with #8 or #10 line from the tank to the regulator, then #6 to the carb.

 

[Southernman]

Carb bogging...

Gary,

carbs rarely bog because they are getting too much fuel. It's usually the opposite - the volume of air being dumped (mechanically) does not contain enough fuel to keep "the candles lit".

I would check things in the following sequence:

First, make sure your ignition system is performing up to par. You want the maximum advance and lead - just short of detonation. I have several friends with fast pump gas cars, but none use the vacuum advance (they're all dialed in and locked out), and their total timing is usually somewhere between 32-34 degrees. Also remember; if you're adamant that you must run the vacuum advance, optimal timing on todays pump gas will most likely result in detonation. If it doesn't rattle with the vacuum advance hooked up and full advance, then your cylinder pressure values may indicate a low compression ratio (probably somewhere around 8-8.5-1) with steel heads. If that's the case, a vacuum secondary carb will probably work much better than the double pumper on the car now, but you can check it:

Your secondary bog can be addressed two ways:

Adjust your secondary pump shot so it hits sooner. Usually, you can take a little clearance out of the cam adjustment to see if it improves (mark your settings so you can always come back later).

The other thing you can do is increase the secondary jets by a few sizes to see if that reduces the bog. If yes, then you're on the right track. Increase until your feel performance drop off, then back up to the last "good" jets.

On fuel pumps, the reason most guys run electric pumps is to prevent vapor lock (at least here in the south). Every one of my buddies street cars vapor locked (at higher speed and rpm - on hot days) running pump gas, and the solution was to use a good electric. I'm sure the lower vapor pressure of today's fuel is a contributor, as the older "cold" fuels (and today's race gas) didn't have problems with mechanical pumps and vapor lock like they do today.

Sorry for being long winded, but there's no "quick and easy" answer to your questions. I hope you find this info useful.

Southernman

 

[scotts74birds]

I agree, most carb problems re in the distributor! Then, go from there.

 

[Gary_gtx67]

I have a bad rattle at anything higher than 10 degrees advanced with vacuum advanced hooked up. So what your saying is to just do away with that line and close the port on the carb?

 

[Gary_gtx67]

Also I took my new carb apart to change power valves from 6.5 to 3.5 and the back bowls have a blank in it. Is this normal or should I go ahead and install one?......

 

[Meep-Meep]

I'm assuming you have a Holley 4779? It's typical for the secondaries to not have a power valve. Main jets back there should be around a 78. Primary jets with a power valve maybe 70-ish on that size carb.

There is no reason to run without a vac advance on the street and as a matter of fact it's not good practice. Basic timing settings should be 34-38 deg all in by 2200-2500. This includes about 12-15 deg initial and vac advance disconnected. Vac advance hooked up to the ported port should end up around 45 deg at part throttle, which will go away at full throttle.

Low compression with a big cam is the kiss of death. Check your cranking pressure (compression test). Hopefully you are not low 100's.

Most carbs are calibrated at the factory with 7.5 PSI fuel pressure. Having the correct pressure will maintain proper fuel levels, which keeps the overall mixture in check. I recently was working out a stumble in my secondaries on my mild 440 and thought fuel pressure might be the cause, so I did a test (carb is a 750 AFB). I temporarily hooked up a pressure gauge and drove the car. My new stock replacement pump had 2.5 PSI and when I floored it the pressure would drop a little but recover quickly. After installing a new Holley 7.5 PSI pump I repeated the test and had absolutely no difference in regards to the stumble. Pressure with the Holley pump would drop to maybe 5 PSI and also recover quiclkly. It's possible that if I held the throttle open for an extended period of time with the stock pump the fuel level might have been lower than ideal causing the engine to run lean but for the short burst there was no difference.

When you think about it all the fuel the engine gets flows through two needle and seat assemblies with a hole that is .100" or smaller so adding 1/2" fuel line on a street car won't do you much good either - unless you are making big power. It's a matter of what's going out vs. what's going in while maintaining a consistent level of fuel in the bowls.

Oversized carbs can cause mixture problems too. I've seen cases where someone has put abnormally big jets in a carb and says it runs better because it was too lean before. In that case I have to say the velocity is low enough to not provide the proper signal to the booster venturis, which in turn won't flow as much fuel with the standard jet so the main restriction had to be enlarged to allow more fuel to pass for the amount of air admitted. The opposite can be true for too small a carb. Too strong of a signal can cause over rich mixtures. The emulsion circuit is designed to give the proper mixture at different RPM levels and is not user adjustable. Bottom line is a carb should run reasonably well with the factory jetting and only minor adjustments will be necessary otherwise it's in a wrong application! In my case I bumped the secondaty jets 4% richer and the problem was solved. Another thing to consider with double pumpers is they are set up for WOT efficiency. A more street friendly carb will have larger power valve channel restrictions vs. the main jet size. Double pumpers are more the other way around.

 

[Gary_gtx67]

See here lies the big stumble and nogin scratcher. My friend has a 67 charger running just about the same app except a little lower compression factory aluminum intake with a 650 vac secondaires and has the same dang problem. I was told by tech support from Holley that in order to find the correct power valve is to take a vaccum reading while running in gear divide by 2 and add .5 and that's what it should be. It came standard with a 6.5 powervalve 71 jets primary and 80 secondary. I bumped the primes down to a 67 and that seem to help so I changed the power valve out and will see what happens in the morning.

 

[Gary_gtx67]

Ok so I've changed out power valve and it's way freaking worste! Also adjusted the back butterflies about a turn and a half wich allowed the secondaries to respond very well! But I still have 2 issues. One is that off idle if you were to rap it out it back fires and stumbles bad and two if you past that point and start to drive and engage to secondaries she falls on her face a's if you were to block off all air supply a lot of noise but no go! Wth??

 

[Gary_gtx67]

Problem SOLVED! Jeted up the carb and also come to find out the fuel pump is not supporting the carb enough. So I'm buying the Carter that flows 120gallons per hour and up to 9psi. Thanks!

 

[Meep-Meep]

Hmmm, 9 PSI might be too much for the needle and seat.

71 & 80 jets on a 650??

 

[Endangered Species]

MEEP-MEEP is right......You're probably going to want to regulate that pressure. Go buy a Holly regulator and a in line (NON liquid filled) guage. You'll want between 5-7 lbs pressure going to the carb.

 

[Gary_gtx67]

Will do thanks for the heads up.