Car Missing under full throttle

August 28, 2020, 06:12 AM

Hey guys, looking for suggestions here. I'm chasing a Wide open throttle miss on the car. It was doing it at higher RPM (5750-6500) right around gear change, Just a couple Ba da da's. But last past last night at test an tune it was missing from about 100' on and slowed down 4 #'s total. What could this be? The other car the motor was in did the same thing (minor miss around gear change, never the bad one).

This car has
different MSD (7AL2) known good
New Coil
Newer MagnaFuel 500 pump
New Magnafuel regulator
Older plug wires (changing to the previous ones today, but motor still had the miss with those)

It doesn't miss when I mat it. Only in the upper RPM range.

Any ideas?????

August 28, 2020, 06:37 AM

1320racer

What plug, brand and #

August 28, 2020, 06:39 AM

Holytown

They're NGK plugs, not 100% certain on plug # at the moment.

Gapped at .030

August 28, 2020, 06:46 AM

samchev

Valve springs?

August 28, 2020, 06:56 AM

"The Bender"

Engine info? Plug gap? Gas or alcohol?

272" Spitzer
540 Chevy
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3.69@199
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August 28, 2020, 07:18 AM

Holytown

Engine is 468 with Iron 990 chevelle heads on Alky. Plug gap is .030

Valve springs have 280 lbs of pressure. Thats how i bought it, broke one of those red comp rockers, so I Put ultra pro magnum rockers on it to deal with the spring pressure.

August 28, 2020, 07:21 AM

TonyB6255

quote:
Originally posted by Holytown:
Engine is 468 with Iron 990 chevelle heads on Alky. Plug gap is .030

Valve springs have 280 lbs of pressure. Thats how i bought it, broke one of those red comp rockers, so I Put ultra pro magnum rockers on it to deal with the spring pressure.

Have you actually checked spring pressure or just think that's where they should be?

August 28, 2020, 07:28 AM

N2Ofrog

Have been plugs been changed? Try disconnecting the power and tach wire that goes to MSD.

August 28, 2020, 07:30 AM

"The Bender"

Try closing up the plug gap to .020......

August 28, 2020, 07:31 AM

Holytown

quote:
Originally posted by TonyB6255:
quote:
Originally posted by Holytown:
Engine is 468 with Iron 990 chevelle heads on Alky. Plug gap is .030

Valve springs have 280 lbs of pressure. Thats how i bought it, broke one of those red comp rockers, so I Put ultra pro magnum rockers on it to deal with the spring pressure.

Have you actually checked spring pressure or just think that's where they should be?

They were checked.

[/QUOTE]
Have been plugs been changed? Try disconnecting the power and tach wire that goes to MSD.
[/QUOTE]

Old man is down there pulling the plugs right now. (He's retired, I gotta work lol). What would disconnecting the power and tach wire that goes to MSD do? I've heard of this but just wondering.

August 28, 2020, 07:52 AM

57 Vette

If it is on the rich side on alcohol it may cause it to misfire down track. Called a alcohol hiccup.
Try taking some fuel away from it,and see what happens.

IHRA ACDELCO Canadain Nationals
Top/et Iron man Champion

Don Davis 57 Corvette Super Gas roadster

SDPC Race Shop 582 Head Hunter on alcohol

August 28, 2020, 07:52 AM

Holytown

quote:
Originally posted by "The Bender":
Try closing up the plug gap to .020......

Just checked plug NGK #8 Think you were asking that earlier

August 28, 2020, 07:58 AM

Holytown

quote:
Originally posted by 57 Vette:
If it is on the rich side on alcohol it may cause it to misfire down track. Called a alcohol hiccup.
Try taking some fuel away from it,and see what happens.

IHRA ACDELCO Canadain Nationals
Top/et Iron man Champion
Don Davis 57 Corvette Super Gas roadster
SDPC Race Shop 582 Head Hunter on alcohol

I do still think its rich. Its not building heat going down track. Made 2 passes back to back yesterday with 168s. Car was 150 in burnout and after the 2nd pass was 160.

Changed jets to 156s I believe, and it built a little temp but still not much. 165 or so in burnout, 170 stage, maybe 173-175 at trailer.

Should i go down to 148s??? seems super small. Carb is a 950.

August 28, 2020, 08:01 AM

Holytown

quote:
Originally posted by Holytown:
quote:
Originally posted by 57 Vette:
If it is on the rich side on alcohol it may cause it to misfire down track. Called a alcohol hiccup.
Try taking some fuel away from it,and see what happens.

IHRA ACDELCO Canadain Nationals
Top/et Iron man Champion
Don Davis 57 Corvette Super Gas roadster
SDPC Race Shop 582 Head Hunter on alcohol

I do still think its rich. Its not building heat going down track. Made 2 passes back to back yesterday with 168s. Car was 150 in burnout and after the 2nd pass was 160.

Changed jets to 156s I believe, and it built a little temp but still not much. 165 or so in burnout, 170 stage, maybe 173-175 at trailer.

Should i go down to 148s??? seems super small. Carb is a 950.

Pulled #2 plug real quick while on a break at work and it was light tan, so looks good

August 28, 2020, 08:19 AM

Bob Payton

Might sound stupid but I missed it. If you have a diaper and are connecting the diaper to the headers with straps make sure the straps are not too close to the plugs. I had one on the back of my engine on my Nova cause a miss fire. It would arch thru the boot and miss. Brand new plugs and wires.

August 28, 2020, 08:46 AM

Holytown

Good to know. Didn't put the diaper on from the other car, but I was going to. Maybe ill check the relationship of the wires and the headers to see if it might be trying to arc

August 28, 2020, 09:30 AM

HS professor

quote:
Maybe ill check the relationship of the wires and the headers to see if it might be trying to arc

Just start it up and night and you'll see if you have a problem. Had a similar problem a few years back and it turned out to be a wire arcing on the header. Only found it by wiggling the wires at about 3k rpms at night. New wires too.

I've got a 950 on the truck and we ended up at 154's and it's still a bit fat

August 28, 2020, 10:07 AM

CURTIS REED

Did this just start or new combination and it does it? If just started was anything changed just before?

Small block or big block?

August 28, 2020, 10:13 AM

Holytown

quote:
Originally posted by CURTIS REED:
Did this just start or new combination and it does it? If just started was anything changed just before?

Small block or big block?

The miss has always been there slightly right before gear change, but got real bad last night last run of the night.

Has crank trigger
New coil
Different MSD (so i doubt thats the issue since it did it with the other one also)
New regulator
New fuel pump
Putting different wires on it right now.

Gotta be something in the carb in my opinion, just don't know what. I'm leaning towards the hiccup theory from being to rich that 57 Vette was talking about