Hey everyone, thanks for taking the time to look at my question. I have a 1985 F-250 4x4 with a 460. I have been having a problem. The engine slightly hesitates when going from complete stop. If I try to give it more gas when it stumbles, sometimes it feels like it will cut out, but picks back up eventually. However, if I let off the gas completely, it idles very strong. It runs very well at high rpms and idles very strong. I sometimes feel the same hesitation when revving the engine. It gets worse the warmer the engine is. It started after I put in a new Holley 770 cfm carb. I put on a new intake, new MSD ignition system, new holley fuel pump/fuel pressure regulator. I have tried adjusting the carb, timing, fuel pressure. I have gone from 5-6, 5-5, finally a 5-3 power valve. The power valve seems to have helped slighlty but not enough where I would call it significant. Is this a vacuum problem? I and several mechanics have been stumped. This is really getting frustrating. Any suggestions would be appreciated.

 

Make sure the accelerator pump is working. You should have two strong streams of gas pouring into the engine if you look down the carb with the engine off. If it looks ok, you can get larger nozzles to squirt more fuel into the engine when the gas pedal is pushed.

 

FWIW....
I had this same problem with the 460 Super Cobra-jet I got from Ford Racing. I started with a Holley 780 double pumper. My problem was that it was flooding on low RPM and starving at high RPM. My fuel pump is a Holley Red 7PSI as recommended by Ford Racing. I spent months trying different jets, power valves, looking for vacuum leaks, you name it. I finally installed a Summit Racing axillary vacuum canister and all the problems stopped.

 

Thank you for the suggestions. I will see what happens from here.

 

82T1507.5 / Shovelbum:

I have a 85 F-350 2x4 w/ 460 auto A/C. It is all factory w/ the exception of a new Holley 600cfm w/ elec choke I had installed about 7 years ago. I get that same hesitation. If in idle, if I step on the gas quickly, it almost sounds like for second, that it will stall, then it comes right back up. If I dpress the gas pedal slowly, does not seem to do it. If I am ay highway speeds cruising at say 40-50 and "step-on it", it hesitates for a brief moment, then takes off.

I tried sea foam, I think all that did was clean the carb vacuum lines and pisibly the intake. I did not notice any obvious vacuum leaks anywhere. I think this weekend I'll trace each vacuum line really doos, check the accelrator pump (per Franklin2).

82F1507.5, I seen those auxillary vacuum canisters you were talking about. How do you hook them up. They seem simple. What does it do? How do you think the extra vacuum cured your engine situation?

Thanks:

coneynew

PS. The truck has 145k miles and it an auto w/ A/C. It only gets driven less than 1k miles per year over the last 5 years. Last year, I maybe put 400 miles on it.

 

My guess is the power valve is not the right one for your application. They come in a wide variety of vacuum actuation levels. Your best bet is to contact the Holley Tech folks and give them all the pertinent info about your carburetor and your vehicle and ask for their suggestions on power valve number. Here's a link: http://www.holley.com/TechService/TechRequest.asp . I do know that a power valve that opens too late can cause hesitation.

 

Thanks for the link Piffery1!

Much apreciated. When I am tracing out all of the vacuum lines, I'll get the info off of the carb and use the link. Back when I had the carb installed 7 years ago, I believe, I had a vacuum leak and at the manifold. I also had the shop replace the intake manifold gasket when the put the new carb on, and that fixed the original problem of pulling out from stop and the truck would stall. Could timing contribute to this as well?

 

I doubt your problem is timing but would not eliminate it completely. It could be as simple as the vacuum advance linkage sticking or a small leak in the advance diaphragm. If you don't have a hand operated vacuum gun, it may be time to invest in one. The cheapo Mighty-Vac is only about $25 and comes in really handy for all types of jobs, brakes, transmission, etc... The vacuum advance (unit) should hold any vacuum you put on it until that vacuum source is released.

I used the axillary tank because my engine is a race engine that has normally low vacuum to begin with. The booster for the power brake robbed any vacuum I had and caused the defroster door to open blowing my A/C all over the windshield instead of through the dash vents.

I hooked the 3/8" intake vacuum (power brake) to the cannister, cannister to power brake booster. Used a stock intake port (plug) for A/C, transmission module, and cruise control.

FWIW...carburetor aspirated 460's are notorious for intake manifold leaks. My Ford built Racing engine developed a leak 3 months after I installed the engine. The gaskets were showing the blue seal tracks on the LT bank, something I never noticed when I installed the engine. Only specialty race suppliers carry SCJ intake gaskets, so that took me a few days to locate and about an hour to replace. So if anyone reading this needs them, Martel Brothers in Houston sell and ship them worldwide.

 

Thanks for the info 82F1507.5!

Your truck is nice! It would be nice to have an engine like that in my 85 F-350, but I pull a horse trailer - I can see them now, holding on for dear life .........

Any way I was planning to install a Vacuum guage in the truck, so I may go pick up a vacuum gun too. The vacuum diagram on the rad. support is very visible, so I'll start the tracing out each vacum line this weekend. I did have a new exhaust manifold gasket put on the pass side about 8 months ago, and it would backfire once in a while, not everytime, now it does not seem to ever back fire. Do not know if it backfired through the leak or the carb. I have been reading (after searching) that you can damage to power valve? in the carb if that happens. Hopefully, this weekend, I'll get the carb info off of the carb (Holley 4bbl 600cfm w/ elec choke), I do not remember the model#, etc. Can't complain too much, the truck has 145k miles on the stock 460 (the only major (what I consider major in terms of price) were new front brake calipers, large aluminum radiator, new carb, right and left exhaust maifold gaskets and new master cyclinder).