Missing at full throttle.

[drolston]

Earlier post noted that stronger return springs on the secondaries (Offy 3x2) fixed the erratic idle. It also seems to have fixed the problem of engine stalling when coming to a stop. Having those secondaries at anything other than tightly closed at idle creates all kinds of problems. So, hurray! However,

https://www.fordbarn.com/forum/showt...light=drolston

Testing this fix involved getting on it hard enough to fully open the secondaries. The sound and fury is wonderful but it produces a minor but very noticeable miss. It is not an erratic or sputtering miss, but regular; like one or two cylinders are not firing.

The ignition is all new 12v Pertronix; it just does not feel like an ignition problem. But, as often repeated on this forum, "most fuel problems are ignition problems," and vice versa.

Schneider 245f cam with .350 lift is not radical, and valve springs are Lincoln with about 60-70 lb seat pressure. Happens at 3,000 - 4,000 rpm, so pretty sure it is not valve float.

Electric fuel pump feeds through mechanical pump and a regulator that holds steady at 2.5 PSI. It does not act like fuel starvation, which would be sputtering and erratic.

I suspect that one barrel of one of the secondaries is not feeding. The carbs are Speedway 9super7, which makes them automatically suspect. Is there any way to test for that in the garage? Pretty sure the floats are set right. Any other suggestions?

[ford38v8]

Cams and Petronix out of my league, but could it be your advance curve doesn't match the needs of your cam at higher revs?

[drolston]

Originally Posted by ford38v8

Cams and Petronix out of my league, but could it be your advance curve doesn't match the needs of your cam at higher revs?

The Schneider 248f is a pretty mild cam, and all but special purpose cams (Lakes, etc.) are designed to work with the stock distributor advance curve.

Any other ideas out there?

[JSeery]

Think the question is are you sure what the Petronix curve is? Or is it a pickup that is installed in an original distributor?

Charlie NY has posted on the Super97 having some issues (which can be corrected).

[Bill OH]

Try locking the outer carbs from opening and run on just the center carb. Narrow the problem to one of the outer carbs. Are you using copper core, spiral or carbon core wires? If using carbon core wires, I would run an ohm meter check on each wire.

[Norris McCarty]

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Have you synced all three carbs using a Unisyn at idle? Have you adjusted the idle mixture screws on all three carbs either by ear or with a vacuum gauge? Sounds to me like your carbs are out of sync.

[drolston]

Originally Posted by JSeery

Think the question is are you sure what the Petronix curve is? Or is it a pickup that is installed in an original distributor?

Charlie NY has posted on the Super97 having some issues (which can be corrected).

12v Pertronix module in otherwise stock crab distributor.
I will search through Charlie NY posts.
Thanks.

[drolston]

Originally Posted by Bill OH

Try locking the outer carbs from opening and run on just the center carb. Narrow the problem to one of the outer carbs. Are you using copper core, spiral or carbon core wires? If using carbon core wires, I would run an ohm meter check on each wire.

Good idea; I will disconnect the linkage to the secondaries and try it again.

Pertronix recommends spiral core ignition wires, which I have.

[drolston]

Originally Posted by Norris McCarty

Have you synced all three carbs using a Unisyn at idle? Have you adjusted the idle mixture screws on all three carbs either by ear or with a vacuum gauge? Sounds to me like your carbs are out of sync.

Secondaries for 9super7 carbs are built without idle circuits. The only sync necessary is to be double damn sure both secondaries go completely closed at idle. I spent hours with that manifold on the bench getting the throttle plates perfectly centered in the bores, with no binding, and the progressive linkage aligned so they both go closed exactly together. Not too hard to do on the 97s, because you can do it with the just the carb bases installed.

[JSeery]

Think Charlie said they have leak issues. I'm getting ready to send him a set to look at.

[Norris McCarty]

Sorry, didn’t realize the 9super7s were built like that.....

[Bill OH]

Running a Petronix, I asssume you are running 12v with plug gaps at .035. The Petronix is added to a stock crab, distributor cap and rotor, the latter being used on plugs set at .025.
Querie- cap leaking causing the miss fire from the increased voltage to fire a .035 gap?

[drolston]

Originally Posted by Bill OH

Running a Petronix, I asssume you are running 12v with plug gaps at .035. The Petronix is added to a stock crab, distributor cap and rotor, the latter being used on plugs set at .025.
Querie- cap leaking causing the miss fire from the increased voltage to fire a .035 gap?

Correct; 12v and .035 plug gap. That could be the problem. I will regap plugs down to .030 and see if that helps.

[drolston]

Originally Posted by JSeery

Think Charlie said they have leak issues. I'm getting ready to send him a set to look at.

By the time I got my set of Super97s in December 2014 they had fixed the problem of leaking plugs. They still had quality problems though. One barrel would not deliver any fuel. They replaced that carb.

The floats were all off a lot. One too low and two too high. And in adjusting the float level, pay attention to the warning to not hold the float when bending the tab. Any stress on the point where the tab joins the nitrile float and you will cause the float to leak and sink. Yep! Just glad I have an electric fuel pump to test for leaks without running the engine. It was a gusher.

[drolston]

Progress Report, or not.
Disconnected the progressive linkage from the secondaries, so that they stay closed at all times; but still missing at full throttle. So it is not the end carbs.

Re-gapped the plugs down from .035 to .030 to see if the large gap (recommended by Pertronix) to see if the larger gap was causing crossfire somewhere in spark plug cables. Made no difference. But I did notice that one plug was black and maybe wet.

Next move is to replace the Pertronix crab with a rebuilt stock points crab. I will report back on that. Any other suggestions?

[Bill OH]

Are you using Power Valves in the secondary carbs? I have dual 97s at 600 ft elevation and I had to change to 71 power valves in place of 65s. I am using stock mains.

[Bill OH]

On the wet plug, I would check the continuity of the spark plug cable with an ohmmeter and if that is okay, then I would try a new distributor cap. I have seen strange things take place because of a defective cap that could not be seen with the naked eye. I do think I would not dump the Petronix because of a misfiring plug - I think the problem lays in the secondary. JMO.

[drolston]

Originally Posted by Bill OH

Are you using Power Valves in the secondary carbs? I have dual 97s at 600 ft elevation and I had to change to 71 power valves in place of 65s. I am using stock mains.

I am at about 500 ft elevation. Changed all three carbs from .045 to .047 main jets because the plugs were whitish. Now they show a little brownish, which seems more normal to me. Not getting any hesitation on throttle slams, so did not change power valves.

[drolston]

Originally Posted by Bill OH

On the wet plug, I would check the continuity of the spark plug cable with an ohmmeter and if that is okay, then I would try a new distributor cap. I have seen strange things take place because of a defective cap that could not be seen with the naked eye. I do think I would not dump the Petronix because of a misfiring plug - I think the problem lays in the secondary. JMO.

Good suggestion. I will test the secondaries, and if they are okay, change the distributor cap.

[Bill OH]

On the power valves, the engine would bog down when the throttle open the power valves and with the accelerator pumps disconnect, the engine responded very well -no bog. Hence, put in #71 power valves, which is half the size of a #65pv. Try disconnecting the accelerator pump rods and see what happens on wide open throttle. You might have to leave the center carb accelerator pump connected. Remember, you do not have the air flow thru all three carbs that you would have with a single carb. In other words, the pvs may be dumping more fuel than the engine can use.