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-   -   Misfire Under Load (http://www.clubcobra.com/forums/fe-talk/141036-misfire-under-load.html)

cycleguy55 06-08-2018 03:08 PM
Quote:
Originally Posted by Spyglass (Post 1445909)
I recently bought an ERA with a 428, it's 5 years old and has approx 3,000 total miles on it, I understand the engine build is:

Believed to be a 1968 block
Scat 4.250 rotating assembly, 462 ci
Mahle forged Pistons
Edelbrock Performer RPM heads
9.5 to 1 CR
Edelbrock intake
Holley 750
MSD distributor
Howard hydraulic roller cam
Harland Sharp rocker assembly
Canton road race oil pan

I've driven the car approx 500 miles and experienced a progressively worsening oil leak(s) and misfiring under load. The oil leak(s) appear to be coming from the upper front drivers side of the engine, I've replaced the fuel pump gasket and managed to get 1/4 turn by hand on the remote oil filter, both of which appeared to be culprits. I haven't driven it since to know if this has cured the leaks.

The misfire is of greater concern, the engine starts, idles and runs fine at light throttle applications. However when I accelerate harder, say 1/2 throttle and 2500 to 3000 rpm the engine starts to misfire/stumble, it will continue to pull through the misfire. On the few occasions I've accelerated at full throttle, it again misfires/stumbles, then appears to clear itself and when I look in the rear view mirror there is a large blue cloud of smoke in the road. I don't think this is a constant trail of smoke, rather a single cloud that's blown out.

Apologies for the very long post, if you've managed to read this far, do you have any ideas or suggestions as to what's wrong?

Thanks in advance for any assistance.
Regards
Kevin

I had a similar problem when I replaced my old MSD analog box with a new digital box from Summit Racing (supposedly built by MSD). It would cruise fine and full throttle wasn't a problem, but partial throttle acceleration was a bad misfire. It seems the part-throttle acceleration was giving me both vacuum and centrifugal advance - too much in a relatively light car. Full throttle dropped the vacuum and eliminated the effect of the vacuum advance, hence the reason the engine ran fine there.



Perhaps the problem didn't show up on the old analog ignition because it was 'slower' in responding to ignition signals than the newer digital box, but that's merely conjecture on my part.



Solution was to eliminate the vacuum advance and go strictly centrifugal.

Anthony 06-08-2018 03:32 PM
I've had more problems with MSD than anything else, corrosion, malfunction, etc. Won't buy another. You need to drill the cap to vent the ozone to prevent corrosion, misfires.

Spyglass 06-11-2018 11:49 AM
Guys, thanks for all of the replies and suggestions.

Unfortunately I had a busy weekend so not much time dedicated to the Cobra. The replacement fuel pump gasket and oil filter seal appears to have cured the oil leak. I removed the PCV valve, cleaned with WD40, manually manipulated it and reinstalled. Removed the plugs and they looked OK, but difficult to really judge as I'd run the engine to move the car around the garage earlier in the week.

I went for a quick drive on Sunday evening (it's currently too hot here to drive it during the day!) and whilst traffic prevented a full test, the misfire appears to have gone. I won't declare this resolved yet, but it certainly seems to running better. Does it make sense that the PCV valve could have cause this?

Stay tuned and thanks for the contributions.

Kevin

patrickt 06-11-2018 11:57 AM
Quote:
Originally Posted by Spyglass (Post 1446125)
Does it make sense that the PCV valve could have cause this?
Not really. But any time you start pulling hoses, pulling plugs, and the like you stand the chance of "fixing" a poor connection, shorting plug wire, or the like. On the bright side, if you can get the problem to go away, even for just a short while, by effectively "fiddling" with stuff, then the problem is not a bad one... it just has to be found. It could all be as simple as a temperamental coil or plug wire that works fine when it's in one position, but is intermittently crappy in another. Back in the old days, if you just had plug wires 7 and 8 resting too close to one another on an FE you could get a misfire.

FWB 06-11-2018 05:03 PM
Quote:
Originally Posted by patrickt (Post 1446126)
Not really. But any time you start pulling hoses, pulling plugs, and the like you stand the chance of "fixing" a poor connection, shorting plug wire, or the like. On the bright side, if you can get the problem to go away, even for just a short while, by effectively "fiddling" with stuff, then the problem is not a bad one... it just has to be found. It could all be as simple as a temperamental coil or plug wire that works fine when it's in one position, but is intermittently crappy in another. Back in the old days, if you just had plug wires 7 and 8 resting too close to one another on an FE you could get a misfire.



Damn it i hate agreeing with Patrick........but i must

Spyglass 06-12-2018 08:46 AM
Quote:
Originally Posted by spdbrake (Post 1445950)
Curious if you have vac advance on your distributor?
You mentioned easing into the throttle (vac advance would be pulling max advance still) and it runs fine.
Then you mentioned when throttle up aggressively you get the misfire (vac advance will not be helping the the timing will be retarded quite a bit).

You may have phasing issue in the dizzy where your too much advance or too much timing retard (your rotor is in between spark plug posts). This is a problem more common on small diameter distributors and can cause scatter and crossfire.

https://www.youtube.com/watch?v=aWMlNwGW0tM

Bad phasing and crossfire
https://www.youtube.com/watch?v=zulDzQ5q9TM

Everything You Wanted to Know About Vacuum Advance and Ignition Timing
Good info, thanks. Not sure whether I have a vacuum advance on the distributor, I'll check. Since the issue may now have moved to the dreaded intermittent category, this may not be the issue. Appreciate your input.

Texasdoc 06-12-2018 02:35 PM
Hope it's gone. But another thing to check with a misfire over 3000 is to check your valve springs. Ask me how I know.

LG57 06-14-2018 03:35 PM
Next time you're under the distributor cap, check for corrosion. Changing the cap cleared up a lot of hard starting/backfire issues on mine.

LG57 06-14-2018 03:37 PM
Where in Katy? I'm in Grayson Lakes.

Spyglass 06-14-2018 04:28 PM
Quote:
Originally Posted by LG57 (Post 1446334)
Next time you're under the distributor cap, check for corrosion. Changing the cap cleared up a lot of hard starting/backfire issues on mine.

Thanks, I'll add it to the list, hopefully this weekend I 'll have a chance to spend more time on the Cobra. I've still got a few of odds and ends to do on the house following the post-flood reconstruction. You just know which one my wife thinks should take priority :LOL:

Spyglass 06-14-2018 04:34 PM
Quote:
Originally Posted by LG57 (Post 1446335)
Where in Katy? I'm in Grayson Lakes.
We're pretty close, I ride my bike past your sub-division every Saturday morning. I'm at South Fry and Westgreen.

How long have you had your ERA? We should get together sometime.

Danr55 06-19-2018 09:17 AM
I was going to "kind of" agree with Patrick. Have you tried setting up in a dark garage and accelerating to see if you were sparking between plug wires or between plug wires and valve covers, block, etc?

Spyglass 08-15-2018 01:19 PM
After a lot of traveling I'm back on the Cobra oil burning and miss fire investigation.

It still starts, idles and runs fine below 2,500 rpm and low throttle applications. However as soon as you get into the throttle it exhibits a variety of behaviors, all of which can all be experience in one drive. These can be:

1. 1/4 to 1/2 throttle acceleration, engine stumbles/misses but no obvious smoke from exhaust. I can't be 100% sure about this as it's difficult to see.

2. 1/2 to full throttle acceleration, engine stumbles/misses (more pronounced), seems to clear and blows out a huge continuous trail of blue smoke from both exhausts that is clearly visible in the mirror - it literally fills the road behind.

3. 1/2 to full throttle acceleration, engine stumbles/misses (more pronounced), blows out a single huge burp of blue smoke that's clearly visible in the mirror, seems to clear and car accelerates hard.

4. 1/2 to full throttle acceleration, slight engine stumbles/misses, car accelerates hard (as you'd expect), no obvious smoke visible in the mirror.

It doesn't seem consistent, or perhaps I'm not being very scientific in my testing, which is entirely possible.

I removed all eight plugs, they all had similar oil and carbon residue, see the attached photo. Between my oil leaks and oil burning, I'm using about a quart of oil every 300 - 400 miles.

Any ideas or recommendations for an FE specialist in the Houston areas I can take the car to?

Thanks
Kevin

https://i.imgur.com/S1U3edhl.jpg?1

patrickt 08-15-2018 04:25 PM
Quote:
Originally Posted by Spyglass (Post 1449640)
Between my oil leaks and oil burning, I'm using about a quart of oil every 300 - 400 miles.
That's a lot of oil.%/ If it were me, I would disconnect the PCV, and cap it off so you don't have a vacuum leak, then bolt on a known-good carb and take her for another spin. I know that's not as easy as it sounds, but usually someone in your local Cobra club has a known-to-be-decent Holley 750 sitting up on his shelf to loan out.

FWB 08-15-2018 04:46 PM
Quote:
Originally Posted by Danr55 (Post 1446765)
I was going to "kind of" agree with Patrick. Have you tried setting up in a dark garage and accelerating to see if you were sparking between plug wires or between plug wires and valve covers, block, etc?

will that keep me from agreeing with Patrick?....:LOL:

FWB 08-15-2018 04:50 PM
Quote:
Originally Posted by Spyglass (Post 1449640)
After a lot of traveling I'm back on the Cobra oil burning and miss fire investigation.

It still starts, idles and runs fine below 2,500 rpm and low throttle applications. However as soon as you get into the throttle it exhibits a variety of behaviors, all of which can all be experience in one drive. These can be:

1. 1/4 to 1/2 throttle acceleration, engine stumbles/misses but no obvious smoke from exhaust. I can't be 100% sure about this as it's difficult to see.

2. 1/2 to full throttle acceleration, engine stumbles/misses (more pronounced), seems to clear and blows out a huge continuous trail of blue smoke from both exhausts that is clearly visible in the mirror - it literally fills the road behind.

3. 1/2 to full throttle acceleration, engine stumbles/misses (more pronounced), blows out a single huge burp of blue smoke that's clearly visible in the mirror, seems to clear and car accelerates hard.

4. 1/2 to full throttle acceleration, slight engine stumbles/misses, car accelerates hard (as you'd expect), no obvious smoke visible in the mirror.

It doesn't seem consistent, or perhaps I'm not being very scientific in my testing, which is entirely possible.

I removed all eight plugs, they all had similar oil and carbon residue, see the attached photo. Between my oil leaks and oil burning, I'm using about a quart of oil every 300 - 400 miles.

Any ideas or recommendations for an FE specialist in the Houston areas I can take the car to?

Thanks
Kevin

https://i.imgur.com/S1U3edhl.jpg?1

that much oil consumption i would guess the intake is leaking. Can't help you with a recommendation for someone in Houston, i can recommend someone in San Antonio area.

studebaker53 08-15-2018 10:32 PM
Does your MSD have a vacuum advance hose? Disconnect it and drive it.. Sounds like your distributor advance is not working properly.

Gaz64 08-15-2018 11:36 PM
AS patrick has said, disconnect the PCV hose from the base of the carb, plug the carb nipple, replace the spark plugs with a fresh set.

If all the plugs look like that, I would be very concerned about the engine build quality.

Danr55 08-16-2018 05:57 AM
Have you done a compression test on the motor?

patrickt 08-16-2018 10:17 AM
Yeah, maybe start out really simple by just disconnecting the PCV and put in a fresh set of plugs, forget the carb switch for now. That's a lot of oil going somewhere and, if you're really lucky, you've just got a big glob of it coming in the base of the carb through the PCV, then it hits the plugs, causes a misfire, then it burns it all off, scares the people behind you that you're on fire or something, and then it runs good for a while before it does it again. Maybe.:cool:


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