71scgc

Greetings,
This past Friday, I was out in the am driving my 1965 BB equipped car. I was taking off in 1st, and ran up the revs into the 5000-5500 range. This isn't uncommon, I do it often. anyway, the engine misfired 2-3 times in quick succession, so I shifted to 2nd, and backed off it a bit. It continued to misfire on acceleration. When giving it a bit of gas, it would not respond initially, but not really "bog down". Then it would seem to catch up and accelerate, but would misfire while doing so. I drove home, about 8-10 miles, and parked the car.

Yesterday I had a chance to look at things. Went through the timing. The only thing I could find wrong was the dwell was at about 12*, so I set that back to 30*. It didn't help much. The engine was still acting the same as when I parked it Friday. Additionally, when starting the car and warming it up, it wouldn't hold a steady idle. Giving it a little pedal to get the RPMs up to about 1000, the engine would rev up to 15-1700 RPM, then drop down to a little less than 1000, then rev up again. It would do this as long as a gave it a little pedal. If I gave it more pedal, it would behave the same, just at an increased RPM range.

I haven't dug into it any deeper as I wanted to run it by the collective wisdom here. I can't tell you much about the internals of this engine It is the "numbers matching" block. Currently running a points ignition system. Has about 11" of vacuum. The engine has been gone through, but I can't tell you how. What paper I have on it is in French, and hand written shop work order type paper work, as the car was originally sold in Quebec, and spent most all it's life there. I think it has a hydraulic cam in it, as it isn't very noisy, so I don't take it above 5000-5500 RPM. Carb is a 1966 dated 3124 List Holley, which was tuned/gone through by Lars before installation. Up to this point, the car has run very well.

Looking forward to any and all suggestions on how to proceed.

Carter

JF in MI

Just going through the same thing with a friend's '66 small block with a Holley. He's going to perform a final test today by bottoming out the idle mixture screws to see if the engine dies. If it doesn't then, what we suspect, is the power valve diaphragm has ruptured/cracked/rotted. I'm told this is a common result of a backfire on older Holley's without the anti-backfire check valve.

71scgc

JF,
I checked the idle mix screws on mine. Running them in, the motor responded as it should, vacuum dropped and idle slowed.
Was wondering about the power valve as well...

Carter

JF in MI

Did the engine die or just slow when they were bottomed?

71scgc

I never fully closed them, just twisted them in until I got the expected response, then returned them to give max vacuum.
Didn't see any reason to fully close them, as they acted normal.

Carter

leif.anderson93

When under hard acceleration, the combustion chamber may have let fly a small piece of carbon and it got stuck in a spark plug(s). Pretty simple check to pull all eight and check them out. Adjust gap while you have them out

JF in MI

This from the Holley site (IIRC);

Problems:
The incorrect size power valve, or a blown out power valve can cause problems such as poor fuel economy, black smoke emanating from your exhaust, dark or fouling spark plugs and a poor idle. If you suspect that your carburetor has a blown-out power valve, you can perform this simple test.

1) Check the manufacture date of your Holley carb.

Performance Holley carburetors come with a power valve blow-out check valve built in. It prevents damage to the power valve in case of backfire. Holley carbs older than 1992, however, may not have this check valve built in.

2) Test it using the idle mixture screws

If you still suspect the power valve is blown out, start your engine and allow it to idle and get to normal operating temperature. Then, turn the idle mixture screws all the way in. If the engine dies the power valve is not blown.

dcamick

Stock 427/390 or 427/400 in good condition pulls 16-17" of steady manifold vacuum at idle. A 427/435 pulls 15-16". Just thought I'd chime in.............

Good Luck!

Chuck Gongloff

When you changed the dwell...you changed the timing if you didn't reset it after the change. 18* on the dwell is a big change. Worth checking your timing again.

Powershift

We determined awhile back that timing will change a degree for every degree change in dwell. So in this case engine timing was RETARDED 18 degrees.

Larry

PS: Initial timing is typically set at curb idle (below 700 RPM) with the vacuum advance disconnected and vacuum hose plugged. If you can't get the idle speed down to a stable value, might need to set timing at the WOT point when it is all in and no longer advancing.......and set for 36-38 degrees BTDC.

Larry

GTOguy

My take: dwell usually goes from 30 to 35-50 as the rubbing block wears. It doesn't go to 12 degrees unless something is loose. Or defective. I would very carefully check the points breaker plate and the rest of the distributor, and if ok, install a NEW set of points and condenser. Fix what's broken before chasing ghosts in carburetion, etc. All due respect, Powershift is incorrect. A dwell of 12 degrees will advance the timing, not retard it. Bigger dwell angle = less /retarded timing.

Powershift

Jeff:

My RETARDED comment was the OP changing dwell setting from 12 degrees to 30 degrees. He increased the dwell..........so he reduced the timing advance.

I think we are saying the same thing..........but coming at it from different ways. I agree that if dwell was originally 30 degrees and somehow changed by itself to 12 degrees dwell, this change would advance the timing.



Larry

GTOguy

Thanks, Larry. I KNEW there had to be a mis-understanding....this is basic stuff here. I still think that since his dwell decreased, there is a mechanical issue....rubbing blocks don't grow with use! And if the spring is weak on the set-screw, it won't hold a setting either.

71scgc

Larry,
After I reset the dwell to 30*, I went through the timing. Have it at 36* advanced all-in, less the vacuum advance. If I'm remembering right, it was at 50* with the vacuum can connected.

To all who have responded, thanks mucho! I will go back and try the power valve check with the idle mix screws. I was not aware of this check, as I haven't explored any carburetor issues as of yet. From all I have read and learned on this forum, the timing should be sorted first. To loosely quote Lars Grimsrud, "Most carb problems are really ignition problems", or something to that effect.
I was only thinking of carb issues due to the idle going up and down when the revs were raised up to 1000ish RPM. It acts like a modern EFI car with a bad throttle position sensor, like its hunting around for an idle. I've never experienced a carburated car acting like this, revs rising and falling when held at a steady position. the curb idle is set at 700 RPM.

I haven't opened up the distributor or checked plugs, etc. yet. I wanted to come here and ask you guys first. Y'all are WAY smarter and more experienced with this stuff than me.
So please, if anyone else has any suggestions or insights, I would really like to hear it. This car is probably the only car on my dream car list that I will ever be able to afford to own, and therefore is my baby.
Unfortunately, 250 GTOs and real Ford GT-40s are a bit out of my price range.

Again, thanks all, and keep it coming!

Carter

71scgc

I went back through the timing, again. This is what I got: (Done with an Innova 5568 Timing Light)

- Dwell @ 30* angle.
- 13* Initial advance (@750 RPM)
- 36* "All In" advance (@3000 RPM)
- 50* "All In" with Vacuum Can connected.

The surging seems to be gone, but the car still runs the same, lots of missing, popping out the side exhaust. Kinda feels like it's dropping a cylinder or two when held at a steady speed, regardless of RPMs. Give it some gas, it seems to pick up until the speed steadies, then it goes back to that condition.

I ran the idle mix screw in on the right side of the carb until fully closed as suggested above. Engine died. Returned mix screw to 1 1/2 turns open, which is where it was. Didn't do this check on the left side mix screw. Didn't seem necessary.
So I guess it's time to dig deeper. Plan to open distributor and check the points. Probably run to NAPA first and get a set of points that SWC Duke recommended, just in case...

Thanks all, and if you think of anything, lemme know.

Carter

Powershift

Some additional ideas:

1. When does vacuum can start pulling in and when is it all pulled in. Measure in inHG and also check engine vacuum at the same time. The Vac can may be "dithering/flucuating" causing the surging.

2. Wires may be breaking down causing misfiring. Check resistance of wires .........especially if carbon core.

3 Coil could be breaking down under load.

4. Still might have carb issues, but try above suggestions first.

Larry

Pop Chevy

Pull the plugs !

GTOguy

So, you're opting to not check the reason your points opened up and caused an issue in the first place? And fiddle with the carburetor instead? Have fun.

I'm done here.

71scgc

I don't recall saying anything about fiddling with the carb...

Carter

426 Hemi

My 396 all of a sudden stopped idling. Found a broken valve spring.

Just a thought.