Recently thought I had a fuel pump issue. I believe I ruled that out though. Runs fine sitting still. Zero issues starting hot or cold, revs up nice and quick. Stumbles under power, hot or cold. I wrapped the fuel line by the headers just in case of vapor lock. I have a small hill in front of the house, and it won't make it up it without stumbling. Tripple checked the timing. 12 at idle and 32 all in around 2900 rpm with vacuum hose removed. Moved it around some and made no difference on the stumble. Drained fuel tank and its as clean as it gets. Suspected the fuel is why I drained it. This all started after filling up with ethanol free fuel couple weeks ago. Also, I was headed up a mountain and this happened maybe another 1000' in elevation. Last summer I drove it up on the Blue Ridge parkway which is double the elevation where I live and normally drive it around with no issue here or higher elevations. It's a sealed system so ran without fuel cap, no difference. Installed a fuel pressure gauge on windshield. Sits around 5 psi all the time. As a side note at first i thought it was a failing fuel pump because it showed zero pressure, but I pulled the little bleed pin, and it showed pressure again. Replaced fuel pump anyway since I had it. Edelbrock street performer. Took the intake manifold off, why i don't know, but everything looked fine so new gaskets on it and carb gasket. It has a Holley 750 with mechanical secondaries and manual choke. Choke it wired open, no chance for it to move. Carb was purchased new last summer and maybe 1000 miles on it. Pulled it apart and went through it. Clean. Floats seem to be adjusted ok via watching the fuel level through the float bowl glass. The hard fuel line is original. I chased it down as much as possible while it was running and can't find any issues. Fuel tank and rubber lines are new last summer and still appear fine. I replaced the coil, wires, plugs, cap and rotor. All MSD. Even replaced the MSD box last summer just because the original MSD box was really old. I installed an aftermarket temperature gauge and sending unit just to make sure it's not running hot. 180-190 max. I installed a new aluminum radiator last summer. Can't get the original gauge with a sender to read correctly. I just rebuilt this engine maybe 400-500 miles ago. Not burning any oil or leaking. No smoke. What am i missing here? Any help would be greatly appreciated.

 

Do you have the vacuum advance hooked up? How much timing is it adding, and at what level of mercury is it all in at?

 

It is stumbling with it hooked up. I assume 20. I have it at 12 when its at idle with vacuum advance hose disconnected and 32 all in at 2900 rpm. I have Lars timing adjustment papers but I'm not 100% I'm doing it right because I have MSD distributor. One heavy spring and one light silver. I have the MSD spring kit but not 100% sure about the limiters it has. What also has me not focused on the distributor is that this happened all of a sudden. I been running the same setup for 2000 miles. But it could very well be the issue. Maybe i need to dive deeper into it. I been looking for a smoking gun but have failed to find one so far.

 

Unless you’re experiencing pinging there’s no reason not to run 16 initial/36 total. Also 20 from the VA is far too much. You will want to limit it to 10-12, and make sure it’s fully in at 2 inches of mercury fewer than what your engine produces at idle.

Like you said, probably not your issue, but should help.

 

No pinging at all. There's part of my confusion. I'm just not really understanding how it gets 36 from 16 without the 20 coming from the vacuum advance. I don't fully understand that i guess.

 

If you currently have 12 degrees at idle and 32 at 2,900 then you have a 20 degree swing of mechanical advance. Making your timing 16 at idle should give you 36 total.

Your vacuum advance is more or less independent of that. You said yours is adding 20 degrees of timing, which happens to be the same number as your swing of mechanical advance, but the numbers are not related. If you set your timing to 16 at idle, limited the vacuum advance to say 10, then plugged it into a manifold (has vacuum at idle) source you should be reading 26 degrees with your timing light.

 

Either 12* or 16* initial could be ideal depending. What are your cam specs?

29* total at 3000 seems too conservative. 36* total if old school heads, 32* total if modern heads with modern chamber shapes & hi swirl. You basically only pull it back from these total numbers if you hear bad WOT detonation (pull throttle back instantly!) This can vary depending on your compression and gas octane. Tune it for 1 type of gas octane.

It should run OK without the vac can, but may need a big squirter shot to cover any bog.

With the vac can hooked up, to full intake manifold vacuum, it should only add 10-12* at idle with this setup.
And more and that can make it way over-advanced and give it a bog on transitions. Depending on your mufflers, you may not hear a rattle. It may need less squirter shot this way.

What is your vacuum at idle? The vac can must stay full advanced until vac drops and is 2" lower than that. Miss that by too much either way and you get a bog.

Holley carbs must have their shooter pumps dialed in just right. It must not dribble at idle, yet it should squirt immediately on the slightest throttle movement. Easy to eyeball check. Or a bog results. IIRC .015" clear at WOT is how you set them.

Double Pumpers tend to run very rich. And can go over-rich very easily if big squirt shots are used. Bog results.

 

I agree with Leigh1322!

Before you get too far along I would try a second (different) ignition coil to be sure that your ignition coil is not on the way out. Coils can fail like that sometimes, I have had one fail with very similar breaking up in the mid-upper range RPM's.

Be sure that you have a good ground strap connecting your engine to the chassis.

 

I have a spare coil I tried but that probably doesn't mean much since it could be junk too. Tested it with my meter and it appears ok but again... still might be junk. While engine was out I took ground strap off and cleaned up connections. Which weren't even dirty.

 

About 7 -8 years ago I had a random engine popping under hard acceleration at about 3000 - 3500 rpm. I bought a new MSD and installed it, car won't start. Put old coil back in, car starts. Had a new Accel that I installed and still no start. Bought a new Delco U505 coil. Fired right up and no popping at 3000 - 3500 rpm. So 2 "NEW" coils bad right out of the box.

 

So...... I just bought a new MSD coil this morning and fingers crossed.... it made it up my test hill about 10 times without an issue. Lugged the engine several times by starting out in 3rd gear and no hesitation like before. I now have three coils to choose from and they all seem to have conflicting ohm readings. Quick check and i do need to adjust the timing more but it started storming. When weather is better i'll run it further down the road. I live in a national forest on a mountain road so I'm weary of even going down the road as there's hardly anywhere to pull over if i have a problem.

 

Glad for you that the coil you've got is working. If ever my current Delco coil starts acting up, I have a Standard UC12 coil on the shelf. That UC12 coil is pretty popular with our C1 - C2 friends.