250 with 2 Barrel Holley Won't Run
 

I built a 1970 C-10 for my son a few years ago. I rebuilt the original 250 in line 6 and kept the Rochester Monojet on it. My son has had a lot of issues with the monojet and decided to upgrade to the Holley 2 barrel using a adapter. As soon as the 2 barrel was installed the trouble started. The mechanic said it would idle great, but coming off idle it would stumble and flashback through the carb. It also had little to no power. He worked on it a while and finally sent it to a dyno shop for tuning. They're having similar problems and are now looking to "rotate" the carb to get better flow into the manifold. I've never tuned a Holley so I got a couple of the SA type books on rebuilding and tuning Holleys, and I've watched a ton of videos including from Holley. Honestly, I don't think this is a tremendously complicated problem (I might be naive here). I am on the east coast and the truck in on the west coast so I can't get my hands on it. Has anyone out there had a similar problem where a 2 barrel on a 6 just didn't seem to work? I'm not sure of everything that has been tried. Based on my research, I would be sure the accelerator pump linkage is adjusted properly, ensure the squirter check is present, up the squirter nozzles about three sizes, test it and adjust from there. I would also swap out the power valve based on engine vacuum at idle and under load. I just don't understand why this is so difficult. Thoughts/comments/help???

Re: 250 with 2 Barrel Holley Won't Run
 

Maybe it’s not the carb!
Could be timing.
What’s the initial timing set at?
What distributor does it have?

And which Holley carb?

Re: 250 with 2 Barrel Holley Won't Run
 

The original mechanic messed with the timing a lot. I think it is around 6 to 8 BTDC. Would have to confirm this. Vacuum source is ported versus manifold, so no vacuum advance at idle. It has an HEI ignition system. I had originally installed an HEI ignition from the mid 70's. It was replaced with one from the 80's. Both systems were new at installation. Mechanic also replaced plugs and wires. Compression was tested and was acceptable, indicating engine was basically in good condition. It had been running with the Monojet, but poorly.
I believe the carb is the 2bbl Holley 350 cfm Street Avenger. I have confirmed that the carb is the Holley 0-80350 - Holley Street Avenger Model 2300 Carburetor

Re: 250 with 2 Barrel Holley Won't Run
 

Likely could bump initial up to 12 degrees.
Ask him if the heat riser valve (the flap in the exhaust outlet) is there and working.
Could be the carb and extended plenum is too cold causing lean running.
Lean and retarded timing make backfires thru the carb.

Re: 250 with 2 Barrel Holley Won't Run
 

2 carbs, similar problems. 2nd the thought it might not be the carb.

Check for vacuum leaks. If it has power brakes, it is very possible to have a performance altering vacuum leak and still have functional boost.

Verify the vacuum pot on the distributor is working. If it isn't, you are missing needed timing advance when the throttle comes off idle on a ported setup. As an experiment, move the vacuum advance from ported to manifold. Should see a higher idle speed if the pot is working.

Check the pressure out of the fuel pump. Could be low.

I have never used a Holley but check the float level. One of the usual suspects in carb issues.

Re: 250 with 2 Barrel Holley Won't Run
 

Couple of things. The intake/exhaust manifolds are sandwiched together just under the carb. That was to provide heat to the intake bottom to help with fuel vaporization. When I asked, the first mechanic said he blocked that connection, so I expect a longer warm up time for the intake now. Not sure why he did that. I do recall installing the heat riser valve on a new exhaust manifold. It was working when I gave it to my son. Not sure of it's condition now and will have to check.
As far as vacuum leaks go, the first mechanic did a pretty thorough job of checking and eliminating leaks, even to the point of having the manifold to head gasket surface faced at a machine shop. New intake gasket installed after that.
Don't know if he checked for power brake booster vacuum line leaks.
Mechanic did check float level, which is externally adjustable on the Holley.
I suggested there might be a fuel pump problem (installed new stock pump at the rebuild and drove it from PA to Oregon towing a trailer, so it was working at that point). I'm not sure if the first mechanic checked pump flow or pressure.
I talked to the dyno shop yesterday and asked specifically what work was done/tried on the accelerator pump circuit. Didn't get to talk to the tech, just the "Advisor" who didn't provide a lot of technical details.
The saga continues.....
More to come......

Re: 250 with 2 Barrel Holley Won't Run
 

Considering the blocked off heat source being eliminated I’d suspect it’s the fuel not staying vaporized. The six banger intakes in stock form ran cool to cold most of the time.
The liquid fuel puddles in the plenum and what vapor that does reach the chamber is too lean to fire.
A couple of cures are richer jetting that increases the amount of vaporized fuel and more initial timing.
The added length of cold steel of the adaptor makes even a stock intake have problems.
Ultimate cure is learn how to make a mono jet work.
Or an offy or Clifford intake with plenum heat added.