High throttle miss - Page 5

lippy · #81 (**permalink**) 07-02-2016, 05:45 PM

If you are getting a new ignition box anyway, get a 6530. You can lock the dizzy and program whatever advance curve you want. You also can use a MAP sensor to give you more advance under light loads. An added benefit when you use the MAP with a wideband is that you can log vacuum, RPM, and AFR at the same time and really dial in your PV. MSD 6530 with MAP sensor

Texasdoc · #82 (**permalink**) 07-06-2016, 08:24 PM

Well, no dice on the new box. I have a strong spark but still falls apart above 4500 RPMs. I installed the borrowed distributor (thanks Brent!) tonight and will try it tomorrow.

Texasdoc · #83 (**permalink**) 07-07-2016, 07:17 AM

No luck with the new distributor...

I'll send it back to you Brent. Thanks anyway.

I'm not sure where to go from here. I guess I'll just find an old-school mechanic nearby and let them hunt it down.

Jac Mac · #84 (**permalink**) 07-07-2016, 01:34 PM

1. Have you tried the timing light suggestion I made a while back to help rule out ignition or fuel problem yet.
2. I note in your posts that you were disconnecting power leads etc during tests, is there any chance during these procedures that you have inadvertently reversed polarity of any of those connections.... along the lines of the well known MSD polarity reversal when racers substitute waterproof connecters and get the loom colours confused.

blykins · #85 (**permalink**) 07-07-2016, 05:30 PM

Quote:

Originally Posted by Texasdoc

No luck with the new distributor...

I'll send it back to you Brent. Thanks anyway.

I'm not sure where to go from here. I guess I'll just find an old-school mechanic nearby and let them hunt it down.

Go ahead and send the carb to me as well. I'm dynoing next week and I can throw it on and give her a spin.

Texasdoc · #86 (**permalink**) 07-07-2016, 07:43 PM

Quote:

Originally Posted by blykins

Go ahead and send the carb to me as well. I'm dynoing next week and I can throw it on and give her a spin.

I've got one or two more things to try. I already sent the distributor. If it doesn't work, I'll send the carb back to you to test. Thanks again for the test distributor.

I think it is spark rather than carb too.

Texasdoc · #87 (**permalink**) 07-07-2016, 07:50 PM

Quote:

Originally Posted by Jac Mac

1. Have you tried the timing light suggestion I made a while back to help rule out ignition or fuel problem yet.
2. I note in your posts that you were disconnecting power leads etc during tests, is there any chance during these procedures that you have inadvertently reversed polarity of any of those connections.... along the lines of the well known MSD polarity reversal when racers substitute waterproof connecters and get the loom colours confused.

I haven't done the timing light but I do think it is ignition related at this point. It will even break up at 4500 if approached slowly, not at WOT.

I replaced my MSD box with a new digital one that says it senses reverse polarity and won't fire if wired backward. I verified the wire stripe colors and the instructions to both the box and the distributor and have the polarity correct.

Texasdoc · #88 (**permalink**) 07-07-2016, 11:34 PM

Question: Would a cam that is incorrectly degree'd cause a high speed miss? I degree'd it when I installed it, but if I made a mistake then, would it cause a high RPM miss? Just grasping at straws here. I don't think this is the problem as it ran great with the EFI (whenever the O2 sensor decided it wanted to work that day). I don't remember what I ended up setting it on but I think it was advanced.

I also had to use a shorter timing chain due to excess slack from having the block bored/honed. I doubt a shorter timing chain would affect the cam to crank position enough to cause problems.

blykins · #89 (**permalink**) 07-08-2016, 04:39 AM

Camshaft timing would not cause this.

olddog · #90 (**permalink**) 07-08-2016, 12:06 PM

I have not read all of this thread. Skimming back, it looks like you tried a different (Known good distributor). If you have ruled out all of the ignition parts and are stumped, pull the valve covers and check your valve springs carefully. I have seen broken valve springs cause what you would swear is an ignition miss.

My son's Lincoln dropped the valve into the cylinder for a spectacular ending before he found his miss.

marklotus · #91 (**permalink**) 07-08-2016, 12:57 PM

My Cobra did not want to rev over 4,000 RPM. It was fine at low RPM but under load at higher RPM's it would sputter and break up. I had the car at a high performance Ford shop and they tried everything...battery, carb, coil, plugs, valve adjustment, valve springs, ignitor and other stuff that I forgot. They tried everything and nothing seemed to work. Eventually they found that someone had tapped into the coil wire for the electric choke. At higher RPM's the voltage wasn't there for what the coil needed to fire properly.
A very simple fix but a difficult to find problem.
I hope this saves someone a lot of frustration.

Texasdoc · #92 (**permalink**) 07-08-2016, 03:49 PM

Quote:

Originally Posted by marklotus

My Cobra did not want to rev over 4,000 RPM. It was fine at low RPM but under load at higher RPM's it would sputter and break up. I had the car at a high performance Ford shop and they tried everything...battery, carb, coil, plugs, valve adjustment, valve springs, ignitor and other stuff that I forgot. They tried everything and nothing seemed to work. Eventually they found that someone had tapped into the coil wire for the electric choke. At higher RPM's the voltage wasn't there for what the coil needed to fire properly.
A very simple fix but a difficult to find problem.
I hope this saves someone a lot of frustration.

No choke

The only two wires going to the coil are straight from the new MSD box.

I am borrowing a carb tomorrow to see if that helps. It is an Edelbrock 500, which is a little small, but should at least tell me if my Quick Fuel is the source of the miss.

I rerouted the plug wires further away from the alternator, twisted the magnetic pickup wires to the distributor (I have the shielded extension cable), twisted the coil wires, made sure the coil wires were not anywhere close to other wires - all in an attempt to shield from EMI. Coil wires are on the opposite side of the motor - away from the pickup wires. Plug wires 1&3 are separated, as well as wires 5&6. I ran it with the fan on & off. Air cleaner on & off. Still misses.

Another problem is this heat and humidity. I only have about two hours in the am before it gets too hot to work on it. I'm getting too old to be working in the 100 degree shade. Is there a pulling my hair out emoji?

Texasdoc · #93 (**permalink**) 07-08-2016, 04:02 PM

Quote:

Originally Posted by olddog

I have not read all of this thread. Skimming back, it looks like you tried a different (Known good distributor). If you have ruled out all of the ignition parts and are stumped, pull the valve covers and check your valve springs carefully. I have seen broken valve springs cause what you would swear is an ignition miss.

One thought previously mentioned was a failing spring causing valve float. I've looked at the springs, even rolled the motor over a few times with the starter while the valve covers were off. I have a spring compressor coming from summit so I can disassemble the valve train to closer inspect the springs. Will a damaged but not broken spring be readily apparent or is there an easy way to test them?

Gaz64 · #94 (**permalink**) 07-08-2016, 07:06 PM

You could also buy a valve spring pressure tool, to measure spring pressure in a vice.

Fordzilla · #95 (**permalink**) 07-09-2016, 08:45 AM

Don't know if you ruled this out (didn't read through all the posts) but is good info: I had a break-up of ignition in my FE starting at about 4000 rpm. Visual inspection of cap, rotor, magnetic trigger showed nothing wrong. I ordered new parts & when taking out the magnetic trigger discovered that the magnets were actually 2 magnets, one in front of the other. The magnet you could not see was cracked in half. all there but cracked so when you removed it, you had 2 separate pieces. I replaced the magnetic trigger & no more high rpm breakup.

Fordzilla · #96 (**permalink**) 07-09-2016, 08:48 AM

OK, now that I have read the thread, I see you tried a different distributor. Maybe my post helps someone else someday. Hope you find the cause of your trouble soon, I know how frustrating it can be.

olddog · #97 (**permalink**) 07-09-2016, 05:25 PM

Quote:

Originally Posted by Texasdoc

One thought previously mentioned was a failing spring causing valve float. I've looked at the springs, even rolled the motor over a few times with the starter while the valve covers were off. I have a spring compressor coming from summit so I can disassemble the valve train to closer inspect the springs. Will a damaged but not broken spring be readily apparent or is there an easy way to test them?

As mentioned you can measure the spring pressure. I was thinking more in the line of a broken spring, which you could visually see. This is assuming that the engine at one time ran perfect and this is a new sudden problem.

As I said I did not read the thread so you may have already ruled out things and you may have already explained things that I do not know.

If I removed a spring, I would run the piston to TDC and measure the valve clearance with the piston, especially if this is an engine you never saw run correctly. A valve lightly kissing a piston at high rpm may give strange results.

The valve spring and clearance may not be the issue, but I didn't want you to over look it because these can totally kill an engine.

On the ignition miss list, I had an issue that on hot days my engine would break up bad. I sounded like someone was turning the key off and on rapidly. In the beginning, it might do it once and not again for another year. It took me 4-5 years to track it down, because it would never do it when I had any tools to troubleshoot with me, and I could not make it do it. In the end, it was the ignition key switch. Yes someone was turning the key off and on rapidly. My dangling key chain.

Good luck

CHANMADD · #98 (**permalink**) 07-10-2016, 07:08 AM

All I can say is......Chuck the msd....All of the components.I know you guys don't want to hear this.......buy an HE I.....with a 65K coil and module.......which they just come with.....New wires ..tweak the timing and your problems will be over.........I just chased the same thing .....now pulls to 7000 rpm clean......this is the most expensive one out there ....I've had one of these cheap dizzies for a long time.....
https://m.summitracing.com/parts/sum-850025

DWRAT · #99 (**permalink**) 07-10-2016, 07:39 AM

How did the different carb do?
The HEI would be my next step if the carb didn't help out.

olddog · #**100** (**permalink**) 07-10-2016, 09:12 AM

If you can find an older mechanic that still has the old scopes they used in the late 1970's and 1980's, they could pin point your problem quite quickly, if it is ignition.

Also if you have an older school electrical guru friend, with an oscilloscope, you could track it down.

I have to admit, I have always been quite fond the the GM HEI distributor. If you do go to a different distributor, make sure the gear material is compatible with your cam. Many horror stories here.