I have 75 f100 w/ 390. I have a holley street avenger 670 that I have been having issues with tuning. My problem now is that when flooring it from a dead stop it sputters or chugs then clears up. Its not really a bog or complete flat spot. I have the 50cc accel pump on as i could never get it to not backfire through carb with the 30cc. On a different subject how long should fuel pressure hold after shuting off engine. My fuel pressure holds at 6 psi for about 10 seconds then drops very quickly.

Thanks

 

It sounds like you might be getting too much fuel when you stab the throttle. Perhaps changing to the 50cc pump wasn't the correct fix for the carb backfires.

I managed to remove a stumble by increasing my initial ignition timing (while maintaining my max timing at 38).

Could you elaborate a little more on that carb backfire problem?

I have a fuel pressure gauge on my twin Holley set-up (although it's pretty useless, as the low psi gauge suffers from engine heat and is inaccurate), and the pressure can drop right away, or stay up for a minute or two, or anything in between.

I think part of it is where the float is when you shut off the engine, and that's a random thing. I do believe that the one-way valves in the fuel pump don't always seal 100% when the pump isn't operating. Sometimes fuel just flows back and the pressure drops.

More or less normal, so I wouldn't worry about the pressure thing too much, with one exception:

You may have some sort of internal carb leak, thus explaining the dropping pressure and a rich mixture when you accelerate from idle.

For example, if you have a leaking needle/seat (or even set so high the float can't close it properly), your float may be too high, your mixture too rich, and your fuel pressure may drop quickly once the engine is shut off.

So you may have two problems: 1/ A carb problem, such as an internal leak, too large a accel pump, and 2/ Something else wrong causing the backfire, such as wrong cam timing, wrong ignition timing, bad valve, bad lifter....not really sure of all the things that can cause a carb backfire, but others may have more to say.

Wish I knew the exact answer, but those are my thoughts.

 

I agree with Argess.

 

Carb backfire was only on intial acceleration which holley told me was accelerater pump problem. i bought the pump cam kit, went up and down on nozzle size and tried every adjustment i could but the problem only got better when changing to 50cc pump. The cam was set with specs from comp when installed. The timing is set at 12* intial with 36* total in by 3200 rpm. I have duraspark distributor with MSd ignition. I have had a lot of problems with this carb and thats the reason im leaning toward it as the problem but im open to any help or suggestions.

 

Holley is full of crap on this. That is a timing issue. Where is the vacuum advance connected to?

 

Advance is connected to ported vacuum on metering block. Another thing i forgot to mention is that i get the best idle and vacuum 14" with the mixture screws at 3 turns out. If i turn them in any it pops through the exhaust while just at idle.

 

Sounds like a blown power valve.

 

Well I finally got all the problems sorted out. First the built in throotle return spring in throttle shaft by the accelerator pump cam was causing the pump arm to not contact the cam and therefor creating a lean spot. I removed that spring and went back to 30cc pump with green cam and it runs great. I then found that the vacuum advance diaphram was torn so no vacuum advance. The tear also cause a vacuum leak which was why i had the higher then normal idle mixture screws. Replaced it and the motor runs better then ever. Thanks for all the suggestions

 

If the vacuum advance was connected to the ported nipple on the carb, there is no vacuum on it at idle so, NO it was not causing a vacuum leak at idle.

 

Yes it is on the ported nipple on the metering block. it would create a vacuum leak at slight throttle and crusing speeds and from everything i have read on here and in my holley book mixture needles affect idle and part throttle which would be why i needed the mixture needles so rich, with needles at 2 turns out it would surge and chuge at part throttle.