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High rpm misfire...SBC

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High rpm misfire...SBC

Postby BrazilianZ28Camaro » Mon Jun 14, 2010 11:12 pm

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!
I'm just another Gearhead with a tool box, learning and helping.
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Re: High rpm misfire...SBC

Postby cjperformance » Mon Jun 14, 2010 11:34 pm

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.
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Re: High rpm misfire...SBC

Postby Troy Patterson » Mon Jun 14, 2010 11:40 pm

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.

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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Mon Jun 14, 2010 11:58 pm

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!
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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 12:24 am

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!
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Re: High rpm misfire...SBC

Postby raynorshine » Tue Jun 15, 2010 1:35 am

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:
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Re: High rpm misfire...SBC

Postby barnym17 » Tue Jun 15, 2010 5:40 am

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.
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Re: High rpm misfire...SBC

Postby Winston Wolf » Tue Jun 15, 2010 8:24 am

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.
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Re: High rpm misfire...SBC

Postby Brian B » Tue Jun 15, 2010 11:28 am

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..

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it Fast....
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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 11:31 am

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
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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 11:35 am

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:

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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 12:06 pm

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!
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Re: High rpm misfire...SBC

Postby Baprace » Tue Jun 15, 2010 12:08 pm

I'm leaning towards spark plugs giving you the high speed miss.
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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 12:13 pm

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
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Re: High rpm misfire...SBC

Postby BrazilianZ28Camaro » Tue Jun 15, 2010 12:25 pm

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.

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