Couple vacuum/Power Valve related questions:
1. Where does the Power Valve receive its vacuum? I am suspecting through the carb base plate or inside the venturis, but please confirm.
2. Can the Power Valve be affected by a vacuum leak? I have a total of 5 vacuum hoses that hook up to my carb and manifold. Curious if one of those are leaking that it would interrupt the function of the Power Valve.

71 Vette, 350 motor, AT, PB

And this may come up later if I am correct in my assumptions. The problem I'm having is this. My carb is popping to high heaven; especially upon excessive acceleration. As the engine warms up, I can very slowly increase the rpms more and more (I've got it to 4,000 rpm and the engine sounds fine). But if at any time I hit the gas, POP POP POP out of the carb. With the air cleaner on the carb it sounds like an old fashioned popcorn machine. At idle there is no popping once the engine is warm. Also when I hit the gas really hard, I get lightly sprayed with fuel because it shoots up and then the cooling fan blows it back into my face. Not so much fuel that you can see it come out, but enough that I can feel it.

Soooo, I'm thinking my Power Valve is opening up too quickly or too soon shooting loads of fuel in the engine. I followed Holley's Power Valve test of closing the air/fuel mixtures at idle to see if the valve is blown. The car dies signifying it's not blown, but I'm thinking it is the amount of fuel coming through or the timing of when that fuel is coming in.

Additional Info: Timing does not appear to be an issue. I've replaced the Dist and Coil. I am 1,000% sure the plug wires are set up correctly (1-8-4-3-6-5-7-2 Clockwise firing order). No slack in the timing chain based on a very steady timing light reading.

Please help.

 

1. The PV receives its vacuum signal from a passage drilled down through the throttle plate - it is exposed to direct manifold vacuum at that point.

2. The PV would only be affected by a vacuum leak if the leak were so huge that manifold vacuum were decreased to the PV's opening point. A small leak, such as a loose vacuum hose, won't cause that large of an effect on overall vacuum.

3. Popping up through the carb is caused by a lean condition - not a rich condition. If the PV were opening prematurely, you would not get any popping. The only adverse symptom of a premature PV opening is black smoke out the tailpipe and sluggish acceleration.

Popping through the carb means you're running lean. This can be due to a vacuum leak or an incorrectly set up carb. You could also possibly have a valve problem, but this would normally occur regardless of throttle opening rate.

Lars

 

have u installed a new powervalve they make a blowproof valve for hollys,if yours is blown,and it only takes one backfire thru the carb on the old valves they will pop and give u fits

 

No black smoke anywhere but sluggish acceleration is definitely there. One more thing to add is all my plugs are fouled. I replaced one thinking I found the problem in the early stages of troubleshooting. Within 20 minutes of running the engine, the plug was black on one side and white on the other. Saturated with fuel. I've also taken off the front fuel bowl and everything is very clean there. The float moves freely.

Manifold vacuum is still above 15 psi, so I should be good there.

Oh yeah, I didn't mention this is a Holley Street Avenger 670 carb (PN# 80670). Thanks for the reply.

 

99.99999% positive this Holley was purchased after 1992, so I should already have the bulletproof PV. I still think it is putting in too much fuel, or possibly my squirters, or the float, and so on and so on....

I've also thought of a dead cam, but what kills me is one day the car is running like a bat out of he11, the next day I'm just cruising at 65 mph and after 20 miles the popping begins and has not stopped since.

 

By the way Lars. I love your picture. After my next troubleshooting session, I have a huge feeling I will have the same pic.

 

if powervalve does not fix prob,it could be cam/valve train issue.did u replace cam recently?

 

Engine was completely rebuilt about 10-15 years ago / not more than 15k miles on it though.

 

take valve covers off and have someone turn engine over watch rockers a lunched cam lob is real easy to see ps if u dont want engine to start remove coil plug wire

 

Is it possible that you could have a broken Intake valve spring. Once I had a broken exhaust valve spring and it was popping as you describe out the exhaust pipes. Intake spring that is broken will not hold the valve closed and you would get popping up through the carb. This could be the case if you have no irregular fuel delivery symptoms.

Maybe something to check out. Good luck to you.
Greg.

 

I definitely need to take off the valve covers. Likely my next step as a matter of fact.

The weird thing about this problem is the "popping" is literally that. I've heard backfiring through a carb and this just doesn't sound the same. Backfiring initiated from a cylinder seems to have a deeper tone to it (I'm thinking because the noise is travelling up the intake runner). This pop has a higher pitch and I can feel it right at the carb. My extrasensories point me to the primaries or their metering block which is probably why I'm so stuck on it being a power valve, so that's on the list as well and at least I'll be able to put that idea to bed. But now that I think about it, the secondaries are closed, so if it is backfiring from a cylinder, the pressure has nowhere to go but the primaries. Shoot, I may be eating crow here...

Greatly appreciate all the replies. I'll pop the valve covers next time I have enough beer in hand to handle another disappointment...

 

Bet you a dollar your cam is going (or has gone) flat on one or more lobes.

 

I did that once.....fired up a new engine, followed break in procedures...and it would back fire and act all crazy. I pulled the valve covers and noticed a rocker needed tightening...........needless to say, it didnt.........I wiped a cam lobe!

But big thumbs up to Edelbrock.....they replaced the camshaft for FREE! Thanks Edelbrock!

 

If it is a cam lobe, I'll just have to get that roller valvetrain I've been wanting to do forever...oh darn.

 

Well, I found the problem... It's not the power valve but I at least got one word right. My exhaust "valve" on #7 is stuck. Not only that, but the intake on same cylinder was extremely loose with several other valves mildly out of adjustment.

So that explains it. I did not want to believe it, but here I am. I'll be posting a new thread in the upcoming days cause I really will need help with my next steps.

I have the head off and it doesn't look pretty on any of the cylinders. The short block appears to be intact, so it may be time to do a full topend upgrade. Thanks for the help from all of you. I'd be dead without the members of this forum.