High throttle miss

[Gaz64]

.040 is a good compromise gap for a street strip car.

I would perform a fuel volume test at the last point prior to the carb, into a bucket.

Run the pump for 30 seconds and record the volume the collected.

If your 3/8 lines are aluminium, they may be restrictive due to their wall thickness.

Gary

[Texasdoc]

No Joy....

Increased fuel pump pressure. Increased the secondary jets from 82 to 86. Still starts missing after 4500 rpm. After letting off the throttle when it started missing, it backfired thru the pipe.

I think I'm going to either take the heads off and get them checked, or just take it in and let someone else fix it. Anyone know a good "hotrod" mechanic with a dyno in DFW?

[blykins]

Make sure you don't have a leaky header gasket....

Broken valve spring should be easy to spot, otherwise, I can't think of anything else that would warrant pulling the heads.

[bingo2]

This has been an interesting thread to follow with the many comments and suggestions regarding the fuel supply. Clearly, you don't have a fuel related issue. It has to be ignition or mechanical related. I'll make one more suggestion; I think you mentioned that you have a new coil installed, even new ones can be defective. Before you start to disassemble things, if you have a known good one lying around, swap it out and see what happens. It only takes a few minutes.

[blykins]

Very good point....I had a new Mallory coil take a dump on me once. Symptoms were a lot worse, but anything can happen.

I would try that before you tried anything else.

[Texasdoc]

I am using a TFI style (square) coil left over from when I had the EFI. The one I am currently using is a new MSD 5527. I had a Mallory coil fail on me and the MSD 8227 gets terrible reviews. I have a used Ford TFI coil out of an old mustang that I can try.

Does it matter that I am using a TFI style coil instead of a round style coil with the carburetor? I wouldn't think so since my Mallory HiFire box instructions said to use the Mallory square coil in the first place.

[fastd]

did you regap the plugs?

[Texasdoc]

I haven't yet. I will tonight while I'm changing the coil. I was at .040 for months and it was still missing then.

I'll also put the secondary jets back to 82. No exhaust leaks - just redid those with Percy's aluminum gaskets on the headers and new collector gaskets.

[CHANMADD]

.. I have been having the same problem ..changed .wires and just waiting for new coil,cap,rotor,and module

[Texasdoc]

Re-gapped the plugs - all now at 40 again. Went back to 82 on the secondary jets. Still at 6.5 on the Primary PV. Changed the coil. Tested the resistance of the wires - all uniform. Took off the valve covers (again), disconnected the coil lead, and spun the motor with the starter a few revolutions. All rockers/valves going up and down without any problems or apparent errant noises - so I guess no broken springs.

Now it's midnight in Suburbia so I couldn't test it. We will see what tomorrow brings. Still haven't been able to install the WBO2 gauge. Maybe next week if it doesn't run with the replacement coil.

Question still open - does having the square (TFI) type coil matter?

[fastd]

I did a search for "high rpm miss" - a lot of people with similar issue. Several reasons - seems like most were problem with distributor.

[blykins]

Originally Posted by Texasdoc

Re-gapped the plugs - all now at 40 again. Went back to 82 on the secondary jets. Still at 6.5 on the Primary PV. Changed the coil. Tested the resistance of the wires - all uniform. Took off the valve covers (again), disconnected the coil lead, and spun the motor with the starter a few revolutions. All rockers/valves going up and down without any problems or apparent errant noises - so I guess no broken springs.

Now it's midnight in Suburbia so I couldn't test it. We will see what tomorrow brings. Still haven't been able to install the WBO2 gauge. Maybe next week if it doesn't run with the replacement coil.

Question still open - does having the square (TFI) type coil matter?

Just as an aside....

A broken spring can still let a valve go up and down. If it's cracked or broken, it will let the valve float because the spring pressure is greatly reduced.

I'm of the mind that it's ignition too, but I'm trying to cover all bases.

[Jac Mac]

A quick check to eliminate the ignition... Hook your timing light up to the HT Coil to Distributor lead, tape the trigger/button to the on position and zip tie it in a position where you or passenger can observe when you run it up thru the misfire zone , strobe light should be virtually continuous, any uneven gaps in the strobe flashs show an ignition fault, if thats all good then you can concentrate on the jetting etc.

[Texasdoc]

New gaps, new coil, return to old jets, same old problem.

Will try the timing light and/or see if I can find a rental distributor.

[cgsincc]

Originally Posted by Texasdoc

New gaps, new coil, return to old jets, same old problem.

Will try the timing light and/or see if I can find a rental distributor.

What kind of distributor do you need?

[Texasdoc]

Originally Posted by cgsincc

What kind of distributor do you need?

Anything that I can use to put in this 408. Basically a 351W distributor with a steel gear. I have the 6AL box but that can be temporarily disabled. Right now I have MSD 84781 without the vacuum hooked up.

[blykins]

I'm ordering parts for a Windsor build. Email/text me your address again and I'll have an MSD distributor drop shipped to you. When you're done with it, just ship it to me. It will have a steel gear on it.

[blykins]

Ok, it left today from the warehouse.

[Gaz64]

Originally Posted by blykins

I'm ordering parts for a Windsor build. Email/text me your address again and I'll have an MSD distributor drop shipped to you. When you're done with it, just ship it to me. It will have a steel gear on it.

That's some beautiful service.

[Texasdoc]

Maybe.... Still trying to hunt down the cause of this. Waiting for the dist Brent sent.

While waiting, I decided to go back thru and check everything again. Even though everything has been checked, why not? Plugs gapped. Spark lead ohms good. Fuel pressure, timing, etc.

I pulled out the old/destroyed MSD cap that I used to set the rotor phase to check it again. Hooked it up and started the motor. Everything looks "in phase" but the spark is barely visible. Hmmm, weak spark.

Put the new coil back in (had replaced it with an old one for testing) and it still had weak spark. Since the plugs are good, wires good, coil good, lets check the Mallory Hi Fire box (6AL copy).

It gets 13 volts on the big red wire (I also have the MSD capacitor on this line to prevent spikes). 13 volts on the small red wire. Ground is good.

Somebody make sure I did this right:
1. Disconnected the coil lead at the distributor, put it 1/4 inch from ground.
2. Disconnected the Green/Purple wires at the distributor.
3. Ignition On, Engine off
4. Short the Green/Purple wires with a paperclip. NOTHING.
5. Disconnect the Green/Purple wires at the connection at the box (I have the MSD shielded cable as an extension). Short these wires - NOTHING.
6. Cut the connector off the Green/Purple wires so I only have a few inches of wires going right into the box. Short these - NOTHING.
7. Touched the white (points) wire to gound - BUZZ. Big spark. Nice loud buzzing.

So, it appears the magnetic pickup input to the Mallory box is dying/dead. Did I do anything wrong in the testing? There is not a switch to tell the box magnetic vs points.

Any other thoughts before I go buy a new ignition box?