High rpm misfire...SBC

General engine tech -- Drag Racing to Circle Track

Moderator: Team

User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

High rpm misfire...SBC

Post by BrazilianZ28Camaro »

Hello fellas

I'm having a problem with my "el cheapo" SBC with it involuntary rev limiter :lol:

It misfire and pop at exaust at 6500 rpm and above...these are the details that might explain the problem, I'd like somebody can point if yes or not :

The dizzy is an Accel single point converted by a mallory E-spark point eliminator kit grounded to the intake. The engine is carefully grounded to the battery - and body at both heads (intake to body measured resistence is .001 ohm)

The rest of the system are MSD SF wires, new AC delco .035 gapped plugs, new dist OEM style cap and Mallory Promaster 440 coil (working cool).

I have the factory rpm tach AND an Autogage connected to - coil. I asked to a eletronic's professional if would be a problem have two tachs connected to the coil, he said no. :roll: I just like to see the factory instruments working #-o

The distributor wires are new BUT engine have a oil pressure safety switch conected to coil + (If the oil pressure is under 7psi the switch cut the ignition ) ....Wondering if some kind of oil pressure fluctuation or air bubbles can be fooling the dizzy by the pressure switch, but the oil pressure is at 40 psi at idle and 55 psi at 2500 and above?

What do you fellas think? This problem is normal with a setup like mine??

Thanks for help!
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
cjperformance
Guru
Guru
Posts: 3661
Joined: Tue Aug 19, 2008 7:20 am
Location: South Australia

Re: High rpm misfire...SBC

Post by cjperformance »

Hi, I have had factory tach's give a similar problem, just dissconnect it and try, nothing to loose!
Try dropping plug gap to .030" just to see if you note a difference.
What valve springs do you run,?, your 292ci is going to rpm harder for a given cam/spring combo than say a 350ci will, if the springs are going a little soft they may be giving a little valve float which can give the popping too.
Craig.
Troy Patterson
Guru
Guru
Posts: 3416
Joined: Mon Sep 04, 2006 12:18 am
Location:

Re: High rpm misfire...SBC

Post by Troy Patterson »

Ya, just disconnect the factory tach and see what happens. Some factory tachs require a converter / adapter for everything to work correctly.

If it's not the factory tach, make sure the MSD is grounded properly (I'm sure it is, just got to say it). Otherwise, it's probably something else.

Troy Patterson TMPCarbs.net TMP Carbs
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

cjperformance wrote:Hi, I have had factory tach's give a similar problem, just dissconnect it and try, nothing to loose!
Try dropping plug gap to .030" just to see if you note a difference.
What valve springs do you run,?, your 292ci is going to rpm harder for a given cam/spring combo than say a 350ci will, if the springs are going a little soft they may be giving a little valve float which can give the popping too.
Ok. I will disconnect the factory tach and see what happens.

Yeah, the lil engine loves RPM even with a XE 274 H cam!
It have 800 miles old Lunati 1.26" single valve springs, 100lbs seat pressure and 320 lbs open.

I believe that is a ignition problem because the way the engine cut and I think the springs are plenty for 194 150 valves.

A few weeks ago the engine was revving 6800-7000 in drive :lol: with no problems... :?:

thanks for reply!
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

Troy Patterson wrote:Ya, just disconnect the factory tach and see what happens. Some factory tachs require a converter / adapter for everything to work correctly.

If it's not the factory tach, make sure the MSD is grounded properly (I'm sure it is, just got to say it). Otherwise, it's probably something else.

Troy Patterson TMPCarbs.net TMP Carbs
The car is a Z28 71, I don't know its need specific adapters etc...but work correctly...

I'll disconnect the factory tach an do a test.

I forgot to say, the el cheapo haven't a msd box and respective rev limiter :? #-o

Maybe I'm asking too much for the coil and dizzy alone??

Thanks for reply Troy!
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
raynorshine
Expert
Expert
Posts: 940
Joined: Tue Mar 06, 2007 1:59 pm
Location: BC, Canada

Re: High rpm misfire...SBC

Post by raynorshine »

do you have another coil to try? something compatible with the rest of your ignition system? :?: :?:

i chased a high speed misfire years ago, changed the coil and voila! :lol: of course, i changed a lot of other parts first :oops:
Use it up
Wear it out
Eat it all!

-the greatest..."Dale Armstrong"
barnym17
Guru
Guru
Posts: 1341
Joined: Fri May 12, 2006 4:37 pm
Location:

Re: High rpm misfire...SBC

Post by barnym17 »

Does your ignition still have the ballast resistor wire feeding the coil?If so when they go bad these are the symptoms, cuts voltage to the coil down to far causing high speed miss.
Winston Wolf
Member
Member
Posts: 111
Joined: Wed Apr 14, 2010 12:15 pm
Location:

Re: High rpm misfire...SBC

Post by Winston Wolf »

I just went through the same thing. Was using a Mallory Unilite conversion in a stock points distributor, MSD 6AL and blaster 2 coil.

I put in a MSD E-Curve, new coil, and kept the 6AL. Solved the problem (after a plug change too), but I changed it all at the same time. I would consider getting rid of the Mallory for something else. I ran it for 10+ years and the car always seemed to have a miss I couldn't figure out at the strip on hot days. I also do run a factory tach. Coils are cheap, so I would start there. Then maybe spend the money for a new distributor. The E-Curve has a tach output and a rev limiter built in, plus you don't need a MSD box to run it.

Test the voltage from the cars power wire to your distributor and make sure it reads battery voltage. Also might want to run a ground jumper wire from the alternator bracket to the head, on the same bolt as the negative cable on the bracket from the battery.
user-z68766209

Re: High rpm misfire...SBC

Post by user-z68766209 »

I have also seen this and it was the battery, it had a dead
cell and would start to mis in the higher rpm range..
volts would check good but if you hydro each cell one was
pretty much dead..

Brian B
Dakota Parts Warehouse
The Ferrea Valve Guy
Stocking Hylomar
Ask about our Fast Setting Silicone
it Fast....
877 235 2832
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

raynorshine wrote:do you have another coil to try? something compatible with the rest of your ignition system? :?: :?:

i chased a high speed misfire years ago, changed the coil and voila! :lol: of course, i changed a lot of other parts first :oops:
It had a Crane PS40 coil before, did the same thing... :x

With the new, stronger Mallory ProMaster coil the performance looks like the same.

Thanks for reply
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

barnym17 wrote:Does your ignition still have the ballast resistor wire feeding the coil?If so when they go bad these are the symptoms, cuts voltage to the coil down to far causing high speed miss.
Yep!
I installed a new resistor wire when bolt on the parts, but it have a bit lower resistence than indicated by Mallory :roll:

Thanks for reply
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

Winston Wolf wrote:I just went through the same thing. Was using a Mallory Unilite conversion in a stock points distributor, MSD 6AL and blaster 2 coil.

I put in a MSD E-Curve, new coil, and kept the 6AL. Solved the problem (after a plug change too), but I changed it all at the same time. I would consider getting rid of the Mallory for something else. I ran it for 10+ years and the car always seemed to have a miss I couldn't figure out at the strip on hot days. I also do run a factory tach. Coils are cheap, so I would start there. Then maybe spend the money for a new distributor. The E-Curve has a tach output and a rev limiter built in, plus you don't need a MSD box to run it.

Test the voltage from the cars power wire to your distributor and make sure it reads battery voltage. Also might want to run a ground jumper wire from the alternator bracket to the head, on the same bolt as the negative cable on the bracket from the battery.
Interesting...

Car have a 120A alternator, but it haven't a ground wire #-o I'll make one for sure.

Also I'll check the dizzy positive voltage.

The actual Mallory 440 coils is compatible with the E-Spark optical module...I'll keep one eye on them.

Thanks for reply!
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
User avatar
Baprace
HotPass
HotPass
Posts: 1905
Joined: Sun Aug 06, 2006 4:57 am
Location: Henrietta NY 14623

Re: High rpm misfire...SBC

Post by Baprace »

I'm leaning towards spark plugs giving you the high speed miss.
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

Brian B wrote:I have also seen this and it was the battery, it had a dead
cell and would start to mis in the higher rpm range..
volts would check good but if you hydro each cell one was
pretty much dead..

Brian B
Dakota Parts Warehouse
The Ferrea Valve Guy
Stocking Hylomar
Ask about our Fast Setting Silicone
it Fast....
877 235 2832
That's interesting Brian!

What you mean with hydro each cell??

My Car have a sealed battery, (seem pretty strong) and a 120A alternator.

I'm wondering IF this ignition problem might cost high rpm power before the misfire? :?

Thanks for reply
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
User avatar
BrazilianZ28Camaro
Guru
Guru
Posts: 3939
Joined: Mon Feb 09, 2009 10:52 pm
Location:

Re: High rpm misfire...SBC

Post by BrazilianZ28Camaro »

Baprace wrote:I'm leaning towards spark plugs giving you the high speed miss.

Possible, but they have only ten hours of use and are factory style, not performance plugs.

I have accels U-Grove plugs to try but they are a bit hotter than actual AC Delco plugs.

Thanks for reply
'71 Z28 street strip car
Pump gas All motor SBC 427
3308 lbs-29x10.5 Hoosiers
NEW BEST ET
1.38 60' / 4.05 330' / 6.32@111.25mph

https://www.youtube.com/watch?v=99p13UK ... ture=share
Post Reply