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High speed misfire

June 12 2017 at 4:23 PM
John  (Login Argess)

I posted about this a week ago and at that time thought it was only at light loading. My mistake... twas coincidently when I shiffe4d. Let the throttle go higher and major misfire.

As I could hear gas igniting in the left muffler, (possibly the right also, but didn't hear that)and sort of firing out the exhaust. So I'm ruling out fuel.

Happens just after I pass 4500 rpm. Problem gets worse as engine fully hearts up.

I thought I had a bullet proof ignition system. Pertronix replacement for points as well as their recommended coil. New Autolite 45s... well... all is almost new, only a few years old, but only about 2000 miles on it all. Accel 8 mm spiral wrap wires.

I only checked one sparkplug so far. Porcelain was off-white (not bone white as running too lean). Electrode was same white from about halfway up electrode (vertical) and all of (horizontal) part. Rest, and circle around base was back.

So that's easy enough... just throw enough money at it and replace wires, coil, plugs, cap and Pertronix unit.

I'll check inside the dist cap next, then inspect all plugs, and eventually remove valve overs. I did have an outer spring break from this set once. Not likely, but makes me nervous.

Any thoughts? I just find it weird it pulls so hard until about 4500 rpm.

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Chris McAlpine
(Login chris401)

Sounds Like The Exhuast Could Be Closing Too Late

June 12 2017, 7:07 PM 

A valve or two sluggish to close. I have always wondered how much cam deflection there was front to back on a high performance engine at rpm.

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(Login 64fdgal)

High speed miss

June 13 2017, 9:19 AM 

I had similar problems with my 445 build, after lot of head scratching and research I discovered that the secondary jets on my Holley were quickly starving for fuel, I installed jet extensions and a notched float, problem solved,may,or may not be your issue, but thought I would throw that out there for you, I was sure I had a ignition problem

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(Login Bad427stang)

Some thoughts

June 12 2017, 7:17 PM 

Valve float
Too wide of a gap causing a cross fire under the cap
Bad cap
Bad plug
bad wire cross firing
[linked image]
- 70 Fastback Mustang, 489 cid FE, Victor, SEFI, TKO-600 5 speed, 4.11 9 inch.
- 71 F100 shortbed 4x4, 445 cid FE, headers, RPM intake, 1000 HP series Holley, 4 speed

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(Login Argess)

Re: Some thoughts

June 13 2017, 1:42 AM 

Yes, ignition is likely the culprit. Must check rest of plugs. I also looked in the dark (at idle) but didn't see any leaking spark or corona so wires "may" be OK. Cap will be easy to check plus I have a spare.

What's interesting is you both mentioned what basically amounts to a valve being open when it shouldn't.

These are Crane dual springs with close to 8000 miles on them. When I first installed them, I had one break somewhere in the first 1000 miles. An outer one, however the inner one kept things together. I have no recollection of having it cause problems, but I would have been taking it easy on the engine back then due to break-in. I discovered it by accident when I went to adjust the valves.

I called Crane (this was prior to them going out of business) and they told me they have not heard of a spring failure since they started making them in the 1960s. I sent them the broken one for analysis but never heard back from them.

So if one could break, perhaps I have a bad batch and another has broken?

Another thing I just remembered. I had a similar problem last year. Car hadn't been used for at least two weeks and the weather was very humid. Car is kept in a garage. I drove the 5 miles to town without incident which was easy as I just toodle along until the oil temp comes up. I then tried to give it a little gas and it sputtered and lost all power. I didn't think much about it as the problem went away on its own after a while. So could be dampness.... distributor cap maybe....

Anyway, I'll do some more checking today. Will post if I find anything. May post a pic of plugs too. Thank-you for your responses.

This message has been edited by Argess on Jun 13, 2017 4:03 AM

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(Login Argess)


June 13 2017, 11:43 AM 

Managed to do a little more checking this afternoon. As I know I heard misfires out the left side (might also be on the right side, but the left is closer to my ears), I decided to work on that side first.

Pulled all plugs from left side. One is shown below in the picture (sorry it's a little blurry). They all look the same.

Cap looks fine as did the rotor button and the general internals of the distributor... oh darn... must go look again and make sure I didn't lose an advance spring.

Removed left hand side valve cover and looked for broken springs, anything loose, etc. Everything looked fine.

Looked at all the wires. The boot ends had a fair bit of oil contamination and felt sticky. I assume this was from hot oil with a bit of sticky gasket sealer dripping down from the valve cover gasket.

So... only thing at all suspicious was the wires. It's looking like as I can't find anything definite, I might as well install some new wires.

Odd though... how all works well until 4500 rpm........

[linked image]

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(Login Jetman1969)


June 14 2017, 7:11 PM 

Firing order old school tip. 7 and 8 must not be parallel to each other. They may only cross each other one time and be kept as far apart from each other as possible from distributor to plugs. You said hearing left side may have spark happening at strange time. Another thought is spark scatter with the higher RPM you mention. Stock distributor with Pertronix is still an old housing with shaft, bushings etc. possibly out of spec. Dyno runs of my old stock dist. with Pertronix gave spark scatter. New distributor no more concern. MSD in now, DUI Ford Duraspark owned too for the close as stock look with stellar performance. Full length bronze oil impregnated bushing gave me the reason to go with this unit. Black cap with request available.
Product Description
The Ford Duraspark we offer is a high performance unit for those who want a completely stock appearing or factory style electronic distributor. Each Duraspark Electronic Distributor is custom calibrated on a distributor machine and blueprinted for your Ford engine to provide maximum power, torque and instant throttle response. We tune the centrifugal advance using information about the vehicle and engine such as cam duration & lift, carburetor size, compression ratio, vehicle weight and octane of fuel. A super smooth advance curve assures excellent performance and mileage. The vacuum advance is calibrated for maximum fuel economy and light throttle ping is eliminated. The factory center bushing on our Duraspark Distributors is replaced with a full length bronze oil impregnated bushing to provide increased durability and cure top end timing fluctuation, which is a common problem in these distributors due to the weaker stock bushing. Our Duraspark Distributor can be used for a number of applications from everyday street use to all out racing. The distributor is capable of 10,000 rpms with our Dyna-Module and Inferno Coil kit. It will work with other modules and coils, but rpm ability is not guaranteed. Comes with a brass terminal cap and rotor in your choice of blue or red.
Sorry for the long post. DW
1969 Ford Mustang GT 428 CJ 4 speed hardtop. Since 1980 owner, technician, driver.

This message has been edited by Jetman1969 on Jun 14, 2017 7:36 PM

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