I just bought a 1979 f150 with a 460. The motor seems fresh, it has an rv cam, headers, and a holley 750 double pumper.It starts right up and purrs like a kitten. Revs fine in the driveway. While driving, however, I can only give it about a quarter throttle. Any more then thst, and it stumbles, and backfires out if the carb. I have adjusted the timing from 2 degrees btc ub to about 25 btc. It seems to run best at 25 degrees btc. It was jetted for sea level,(71 and 80) I went down two steps to 67 and 76. I pulled the timing cover, thinking it may have jumped time, but it was all in tip-top shape. The motor also has 100 psi compression in all 8. What is wrong with this truck?
Mike

 

check the timimg with a timing light first, If the holley backfired through the carb you'll probably need to replace the power valve, I'd also look for any vacuum leaks that could be causing it to run choppy.

 

#67 primary jets sounds a little lean to me, you might have to jet it up a little. Backfiring through the carb can indicate a lean condition. As stated above, a vacuum leak will also cause a lean condition and backfiring can damage the power valve. Is your fuel pump OK? Fuel filter?

 

Howdy!

Well, first thing that popped up in front of my eyes was where you set your TIMING! 25*btdc is WAY too advanced! (as well as the 2* btdc is rather over-retarded). Set that timing between 8* and 12*. I believe factory suggests 8*. You better throw a timing light on there and confirm if your vacuum AND mechanical advance is working properly too!

I currently have mine set at about 13-14* and it's just fine, really responsive...but I ONLY run super unleaded fuel. My motor is primarily stock except for the 3" exhaust to Flowmaster 40. Oh, and a K&N Filter.....for now.

I bet your poppin' and pinging are from your timing.

Now about that Holley! There are a laundry list of details to consider with your carb...I suggest you go get the Holley Tuning book...it has helped me quite a bit since I bought it 20 years ago!

Since you've already changed your jets, did you take a look at your power valve? What rating is it? What is your manifold vacuum, at idle?...under normal load???
You could possibly have a blown power valve. They are very easy to change once you take the float bowl and metering block off, but again, I recommend you research the book for installing the best 'fit' for your engines specs.

I have been been from front to back of my Holley book (probably twice or thrice over!) for the past couple days now. I just put on my new (re-manufact. at Holley) 750 (w/vacuum secondary) yesterday. I took off my old 600 cfm that I had on my Bronco 17 years ago! That old dog was still working pretty good!

I've got a few lil' bugs to work out, but I'm sure it'll be screaming by the end of the day. I drove it for about 60 miles yesterday and it's yREALLY strong...just a little bogging on initial HARD acceleration, but I'll get that smoothed out no problemo! I absolutey smoked a 90's Camaro last night when he pulled up to me and started hot-doggin' me and scratchin his tires! That dang little 305 motor w/fart-bomb exhaust had no chance against my big block!

Best of luck to ya! Give us all a reply when you get some of the puzzle figured out!



OH! The books to look for are:

HOLLEY-Rebuilding & Modifying by Jeff Williams
ISBN 0-931472-27-X

or

Super Tuning and Modifying HOLLEY CARBURETORS by
Dave Emanuel
ISBN ?

 

Thanks for the responses!!
I set the timing with a light. I have run it at many different timing settings, with a light. It will only run well with the timng between 24 and 28 dergrees btc. That seems a bit extreme to me, but I do live at 5500 feet altitude. I did a pretty thorough sweep of the vacuum system, found very little leakage. The lines are all old and dry, but I coldnt find many leaks. The manifold didnt seem to have any vacuum leaks at all. New plugs, cap, rotor, fuel filter, air filter, carb rebuild, etc. When I changed the jets, i had to back the fuel/air screws out 2 3/4 turns to get it to smooth out. I believed I was leaning it out. Was I wrong? Is tightening them leaning them out? Also, When I did the carb, there was only a power valve in the front bowl. The rear only had a plug in the hole. I left it as it was. Could this be my culprit?

 

For hpproron:
Nice truck! The photo you have with your username is exactly like my truck! Its a 79 and so am I. That was my 30th birthday gift to myself. A truck my age. Its a great truck, I know there is just something I'm overlooking. I will take the bowls off today, and swap the powrevalve, and replace the plug in the rear metering plate with a powervalve. I'm at high fairly altitude, so I think I'll stay with the jets I have maybe, one step up.
Thanks for your input, I really appreciate it.
Mike

 

10-4 Good Buddy!

Keep us all posted!

 

I think you went the wrong way with your jets. The air at 5000ft is less dense than the air at sea level, it needs more air / gas to run.

When you screw the fuel/air screws out, it makes it richer.

I would go up 3 steps from where it was originally and try it again.

My 460 with Stealth intake and Holley 650 (mech. secondary) runs better at 4* of timing. I think your right in hunting around for what works best in the timing department, now just jet the carb up.

 

I too went with a 67 in my 4150 and got the same outcome. Went back to a 71 and she ran great. Too lean.