Bronco Forum - Full Size Ford Bronco Forum banner

1 - 6 of 6 Posts

·
Registered
Joined
·
126 Posts
Discussion Starter #1
I've got an issue with my 79 Bronco. It has a 400 built up to the classic PJB build (Badger flat tops at 9:1, Comp Cams 255DEH, etc).

I just recently got a brand new Holley Truck Avenger 770 CFM carb. I know that this is probably more carb than I need, but theoretically you should be able to make any carb work if the AF ratio is being maintained.

Anyway, the issue is that I can't get the vehicle above 1500 RPM. At all. Well, maybe if I was going down a steep hill, but haven't tried that yet.

I did the old paperclip trick on the vacuum secondary rod, and sure enough, the secondaries are opening, possibly when I have the hesitation. Basically if I push down on the accelerator midway, it will run fine up to 1500 RPM and then almost seems to stall. If I let off the throttle, it regulates itself back to idle, but I just CAN'T get it above 1500 RPM.

Does this sound like the vacuum secondaries are opening too soon, or possibly another issue? I'm running the full port vacuum.

Thanks for any advice! The truck basically feels like there are major drivetrain issues at 1500 RPM (like my transmission is bucking badly). I hope it's something as simple as the secondaries.

Anyone experienced this before with Holley vacuum secondaries? Is that what it feels like?

-James
 

·
Registered
Joined
·
527 Posts
1500 rpm huh? This actually sounds like a float problem. How much fuel pressure do you have and does it change when you depress the throttle?
Is the symptom related to load or does this vehicle do this when it is not in gear driving down the road?
Fuel delivery can be a problem here. It is also possible that you have the rear float set too high, and it is flooding the engine from the back boosters.

How does this rig idle?

How is the A/F ratio at idle? Smell gas or unburned fuel at idle?

Have you verified the timing and does it advance? Stick a quality light on the engine to verify the timing curve? If the timing does not advance, it will run like a pig and never rev.

Several variables which can cause this problem, but before you start yanking the spring for the secondary side of this carb, lets be sure that other problems are eliminated.

You should be able to rev like crazy with just the primary side functional, and even if the secondary does open pre maturely, iy would just bog, but still rev. and eventually you could achieve WOT. It sounds like this is not possible, which leads me to believe that there is a problem elsewhere.
I would start with the basics and timing would be the most basic.
 

·
Registered
Joined
·
126 Posts
Discussion Starter #3
Thanks! To answer some of your questions:

Yes, idles like a kitten.

Yes, when parked, I can rev throughout the range. SOMETIMES, if I depress the accelerator too quickly, it bogs down, but that might be because I need to mess around a little with the accelerator pump nozzle/cam/screw.

I don't smell gas at idle.

This only happens when under load, and it happens in 1st, 2nd and 3rd.

I think you are right at that timing could be an issue. I will check with my timing light tomorrow.
 

·
Registered
Joined
·
126 Posts
Discussion Starter #4
So I messed around with the timing today (went from 6 degrees BTDC to up to 16). No difference. I've always been told that there's not TOO much difference between having the vacuum advance connected to the full vacuum or ported vacuum, but I switched from the full vacuum (down low on the Holley Truck Avenger) to the ported/timed vacuum up high on the side of the carb. Ran like crap, so I rechecked the timing (had been set to 10 degrees BTDC) and it was at something like 4 degrees ATDC! Tested at the exact same conditions (idle), yet there is a difference of almost 14 degrees between full and ported vacuum at idle?!? Anyway, reset the timing to 10 degrees, and it runs like a champ now. No hesitation anywhere.

Thanks for the advice! I appreciate someone telling me to hold up and check the simple things, at it saves me money :) Your check's on the way...
 

·
Registered
Joined
·
527 Posts
So glad to hear that a little troubleshooting did the job. Congrats.
Save the check for the truck, after all, you fixed it, I only threw out a few suggestions.

Help another down the road, or assist a fellow off roader when they are in a similar position. Oh, and if you are ever wheeling in the So Ca, area, I do enjoy a cold adult beverage once in a while. You could buy me one of those.:beer

Enjoy the good running truck.....
 

·
Registered
Joined
·
207 Posts
if i understood you right did you say you have the dist. hooked up to full vacuum? the vacuum advance should have no vacuum at idle. it works off a accelerated vacuum. as rpm increases so will the vacuum and that makes the advance work.
 
1 - 6 of 6 Posts
Top