High rpm misfire...SBC

[Winston Wolf]

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

I've had plugs go bad for no reason in less time than that, AC Delco standard ones, so I would definitely change them.

[Lazy JW]

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

Thanks for reply

This ignition is INTENDED to use reduced voltage?

[BrazilianZ28Camaro]

This ignition is INTENDED to use reduced voltage?

Yes, Mallory advertise to run one resistor wire or ballast to avoid module self destruction

[barnym17]

Make a run using a ballast restor and jumper wire for ignition power see if that cures it.Wire may check fine at room temp but as engine compartment heat goes upresistance goes up, current flow goes down.Hardrunning also will heat the wire causing higher resistance.

[JoePorting]

I was thinking valve springs. 100 pounds on the seat is marginal, and if that was 800 miles ago they might be down to 80 or 90 pounds. I'd consider changing the springs to the Comp Beehive springs with around 120 to 130 on the seat. I think there open pressure should be around 300 pounds with that setup if I remember right. Shouldn't beat up your cam too much.

[Troy Patterson]

Mallory is very specific about what ballast resister to use with their distributor. There is possibility for problems there.

Troy Patterson TMPCarbs.net TMP Carbs

[BrazilianZ28Camaro]

I was thinking valve springs. 100 pounds on the seat is marginal, and if that was 800 miles ago they might be down to 80 or 90 pounds. I'd consider changing the springs to the Comp Beehive springs with around 120 to 130 on the seat. I think there open pressure should be around 300 pounds with that setup if I remember right. Shouldn't beat up your cam too much.

This make me remember a trouble a friend of mine passed...his mild camed SBC 350 floated the valves at near 6500 but he had some Isky dual springs good to 6800 rpm ...he removed the valve covers and surprise!

Engine builder forget to replace the inner spring after the cam break in ....turns that he had about 70 lbs at seat and 230 lbs open ...running about.510" lift the engine floated the valves at 6500.

Returning to the topic, the way the engine cut and pops is identical to how a rev limiter do on car equiped with ignition box, ...very different from valve float.

[BrazilianZ28Camaro]

Mallory is very specific about what ballast resister to use with their distributor. There is possibility for problems there.

Troy Patterson TMPCarbs.net TMP Carbs

Troy

The ignition key + wire to the coil have a resistor wire with a bit less resistence than the ballast resistor recomended by mallory.

There's no resistor on the + cable going to the E-spark module into the dizzy, accordingly to mallory instructions.

I'm missing something?

[Matt Gruber]

all the normal things have been covered.
maybe a chance fuel pressure is too low at 6500?
tape guage to windshield and test drive.

[BrazilianZ28Camaro]

all the normal things have been covered.
maybe a chance fuel pressure is too low at 6500?
tape gauge to windshield and test drive.

Well, its possible, but would the exaust "pop" due too low fuel pressure at every gear at same rpm?

Car have a 3/8" steel fuel line from the tank to the Carter 80ghp mechanical fuel pump. I'm aware this pump is not that good...

I'll keep one eye on it and do some other car tests soon . I'll keep this post updated.

Thanks for reply

[Troy Patterson]

A few other thoughts just to cover a wide range of possibilities:

1) air cleaner too tight over carb's ventu tubes?
2) valve springs binding?
3) fuel pressure / volume at WOT & 6500 RPM?
4) going lean up top?
5) going rich up top?
6) the ol' mouse get's tired at that speed because it didn't get it's Wheaties?

Troy Patterson TMPCarbs.net TMP Carbs

[BrazilianZ28Camaro]

A few other thoughts just to cover a wide range of possibilities:

1) air cleaner too tight over carb's ventu tubes?
2) valve springs binding?
3) fuel pressure / volume at WOT & 6500 RPM?
4) going lean up top?
5) going rich up top?
6) the ol' mouse get's tired at that speed because it didn't get it's Wheaties?

Troy Patterson TMPCarbs.net TMP Carbs

The way the engine cuts equals a built in rev limiter. My modest knowledge say is not carb related problem...

I did some full throtle passes on the backstreet to check the spark plugs reading...looks perfect..engine pulls strong up top till it cuts, best car speed 1/4 mile is 103 mph through half a$$ air and grip...for a 10 CR , lil 292 mouse pushing a 3500lbs car seem aceptable?

I haven't a AFR meter and fuel pressure gauge into the car, but teorically a 80ghp Carter Mechanical fuel pump should feed my engine correct?

Carb is a Holley 750 DP 4779, with 71-76 jets, 6.5pv and factory HSABs,IABs and IFRs.
I did a 12mm step under the down leg boosters, polished the venturis and removed the choke tower to improve flow.

The tests are done with no air cleaner. The vent tube is about 2 1/4" from the hood.

The carb works amazingly well with a Stealth air strike DP.I can wot at 2000 Rpm without problems.

Valve springs are good to .580" lift... the engine have only .523".

I'm wondering if this problem can be only a limitation of the E-spark module,rotor or cap...but the dizzy is in very good condition.

This is the carb:


Uploaded with ImageShack.us

Thanks for reply Troy

[Troy Patterson]

Pretty center section there BrazilianZ28Camaro.

Does it have a belt driven fan or an electric? I was tuning a car years ago with a regular fan, a really great fan. For convenience I didn't bother to put the air cleaner back on for each test drive. Well, the fan blew so much air, it changes the metering of the carb at high rpm. Just a thought.

You didn't say anything about the Wheaties. Maybe that's an American joke.

Troy Patterson TMPCarbs.net TMP Carbs

[BrazilianZ28Camaro]

Pretty center section there BrazilianZ28Camaro.

Does it have a belt driven fan or an electric? I was tuning a car years ago with a regular fan, a really great fan. For convenience I didn't bother to put the air cleaner back on for each test drive. Well, the fan blew so much air, it changes the metering of the carb at high rpm. Just a thought.

You didn't say anything about the Wheaties. Maybe that's an American joke.

Troy Patterson TMPCarbs.net TMP Carbs

Thanks Troy, two days hand polishing the carb...yes,I had time

Car have a 14.5" eletric fan ...I'm concerned with the air flowing above carb...should a good air filter improve it metering and ET on 1/8 mile??

Yeah, I know what "wheat" mean but didn't know about "Wheaties"...so I'm little off about the joke
I'm glad if you can explain it!

Strange as may seem, over here SBCs are scarce ...we have plenty of Chevys L6 ,SBM and SBF here...generally people don't know the SBC is nicknamed mouse too ...

Thanks a lot for the tips!

[SchmidtMotorWorks]

What heat plug are you running?