Acceleration stumble

[bgilmore]

I'm to get a little direction from some of you IH experts with more experience than I. I have a Holley 2300 on a 345. About 1/4 thru acceleration while in gear the engine runs very rough and will cough out of the carb and sometimes die. Idling and upping the RPM in park is fine. While accelerating, when it gets to "that spot" in the curve (prob 1500 RPM), if I pump the accelerator about 3 times I can get thru this rough spot and move into the upper RPM ranges where it seems to run ok. Here's the weird part. If I set the manual choke to full even when hot, it runs a whole lot better. No real stumble and smooth acceleration. However, whenever I set the full choke, the RPMs increase as well at idle or at a dead stop (which I think shouldn't happen).

Here's what I've done so far:
I've rebuilt the carb and put in a new power valve. Checked for vac leaks with carb cleaner. Looks good. Also have observed a functional acc. pump before and after the rebuild and set the float as instructed. Changed the plugs (which were very black) and the wires. Still the same problem.

What I need to do:
Set the timing with a timing light, but like I said, it runs fine but only with full choke, so I think I'm at least close here.

At Idle, I get about 20" of vacuum from the vac adv port at the bottom front of the carb. Revving the engine drops it to 0 and then back up to 20 but it seems to keep pretty steady for the most part.

After reading about anything I can find, I'm thinking I may need to look at the mechanical advance or the fuel pressure.
Do either of these sound like possible culprits? One more likely than the other? What else am I not thinking about? Any ideas are appreciated.

Thanks.

[ScoutmanSV]

Definately check the timing (0 to 5 degrees) & see if the mechanical & vac adv are also working. It kinda sounds like you are advancing too far & might have the vac adv hooked up to manifold vac instead of ported vac off the carb. Try disconnecting it, set the timing, leave vac advance off (drive it) & then connect it (drive it) & see what happens each time.

Having it run good w/the choke on sounds like a vac leak. It could be the EGR or even the power brake booster. You can disconnect each & 'trial & error' that way too. Be careful as the brake pedal will be real hard & you might wanna find a deserted road/parking lot to test in. All you're really testing is the initail start (where it stumbles) anyway, right?

20psi vac at idle does sound great (no vac leaks & in time) but it is obvious something is wrong. If the dist vac advance is manifold (20psi & advancing @ idle) & then it retards when you step on the gas & lose vac > that *could* be it. Ported vac on a 2300 is on the pass side of the carb metering plate, IIRC.

HTH,

[ScoutmanSV]

Shoulda mentioned > when you set the timing: disconnect the vac adv line to the dist. These time on #8 cylinder, btw, NOT on #1!

[Paul 3]

I'm with ScoutmanSV check the timing for sure. If your plugs are black, but your getting a very strong 20" of vacuum, it sounds like your timing is off. And as ScoutmanSV said you may want to check the EGR and power brake booster for a leak. BTDT only spent a frustrating week looking for a leak before realizing it was the power brake booster.

[bgilmore]

He fellas, thanks so much for the tips. I bit the bullet and went out and bought myself a brand spanking new timing light. Turn out I was pretty far on the other side of TDC. I set timing at 5* BTDC using the #8 plug wire. Then I set the idle mixture to where my vac Hg lifted to 20. I took it out for a spin and wallah. It's like magic.

First time I ran without the vac adv. hooked up the 2nd time I did hook it up. Seemed like it actually ran a bit better unhooked. I was thinking that I could watch the timing increase by hooking up the advance but the timing stayed right at 5* BTDC with or without it connected at idle. My Holley has a single vac line on it which is coming out the front middle of the throttle body. This one doesn't seem to be ported because it pegs at 20Hg at idle and drops close to 0 when the engine if revved. Could it be that my vac advance is toast? I 'er sucked on the hose connected to the vac adv. thinking that I would feel some pull back but it seems that air flowed very freely thru the hose no matter how hard I inhaled (yes, I did inhale). Does this indicate anything (about the vac adv. not which team I play for)?

Thanks BG