Was browsing around looking for some good advice when I came upon this site. What I have is an '85 F150 short step side I'm rebuilding with some modes added in. 300 l6 bored 20 over, new crankshaft, melling street torque cam, oil pump, D.U.I. H.E.I. Headman headers, holly 390cfm, offenhauser dp intake. The engine idles smooth as can be, full throttle response is awesome but to ease into the throttle and it bucks and misses something fierce. I havn't been able to identify the problem. Any help would be greatly appreciated

 

Welcome to FTE! The Panhandle, huh? Where, exactly?

Your engine sounds like it is lean. Put a vacuum gauge on it and see if it stumbles until it reaches a specific vacuum, like 6". If so that says it is lean until the power valve opens. And that probably means your accelerator pump isn't set right. However, since you say it runs well at WOT, it sounds like the accelerator pump is working when you floor it, so it must be working.

Anyway, do some testing and let us know.

 

I live in Borger, north and a bit east of Amarillo


Will run a vacuum test on it today, will let you know the results, And thanks for a
quick reply

 

I know where that is. As for the engine, let me explain further: ease into the throttle while watching the vacuum and observing the engine's performance. My guess is that it will stumble until you get enough throttle to open the power valve, which happens at a fixed vacuum level - maybe 6".

 

I got the following info from the Clifford web sit a few years ago:

JETTING YOUR NEW HOLLEY 390 CFM 4 BRL CARB.

292-300 cu. in. 8.5 power valve .035 (tube type) discharge nozzle .052 mains.
May not be perfect, but a great place to start tuning your new Holley.

Jim

 

Welcome to the forum, sledrock52!

 

I think I found the problem!!! Ran a vacuum check and everything was fairly with in range, had 13" of vac. I put the timing light back on and it was so far out I couldn't even see the timing mark on the balancer. Turned out the mechanical advance was plenty enough for the distributer. The vacuum advanced it way to far. (Davis Unified Ignition) Runs beautiful at any rpm range, road tested really nice. Couldn't go very far since it's only the cab with out doors. In the process of painting all the sheet metal. will up load some pics when I figure out how. Again thanks to everyone for their time and input.

 

Somethings still not right. You shouldn't have so much mechanical that you don't need vacuum. And, the mechanical shouldn't be coming in at idle.

Over the weekend I worked on a 1986 F150 w/the six that is getting excellent MPG. Check out this thread to see the timing.

 

When I hook the vacuum line up to the distributer and give it fuel it sputters bad, take the vacuum back off and plug it then everything lines out. I know why it runs better with it off. Maybe I'm doing the timing process wrong. Vacuum line off and plugged, timing set to 12-14 BTC then hook vacuum back up.

 

I mean don't know why

 

Yes, that's how you should set the timing - with the vacuum off. Have you confirmed that the vacuum advance is good and isn't blown. If blown you will have a large vacuum leak that could cause a stumble.

 

The distributer is new out of the box, how would I check to see if it's blown

 

Put a piece of hose on the vacuum advance and suck on it. You should be able to pull a vacuum and have it stay if you put your tongue over the end of the hose.

 

Manifold vacuum or ported vacuum?

 

If you have ported vacuum use it. It will give a more stable idle than manifold vacuum.