Sputter at 5400 RPM - Pontiac GTO Forum
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post #1 of 10 (permalink) Old 02-19-2018, 06:30 AM Thread Starter
 
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Sputter at 5400 RPM

Hi

I just put a new Edelbrock 750 cfm on my 64 GTO, 389. The engine is a fresh rebuld. We had the original AFB, but it just didn't run quite right.
NOW is pulls like a beast and idles perfect. Needs a bit more dialing but runs great up to 5400 RPM.
I know Pontiacs are not high reveing engines but why would it sputter and lose power at 5400?


It has the stock top end, ported stock heads, mild cam. New distributer and coil, but orignal style.

I am wondering if either
1. Fuel is not flowing fast enough
2. Spark is not keeping up.

Any suggestions welcomed.
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post #2 of 10 (permalink) Old 02-19-2018, 08:52 AM
 
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Here are a few possible causes.

(1) Not enuff gas for that many rpm. My guess.

(2) Ignition problems, such as point bounce. I used Accel points with the stronger spring. They'd go to 6000 rpm with no bounce.

(3) Your "mild" cam might be up to it's higher rpm limit.

For example: Crower rates this 60240 cam with a 5300 rpm redline.

https://www.crower.com/pontiac-287-4...m-270-hdp.html

(4) If you have cheap lifters, they could possibly be up to their rpm limit, with your cam/spring combo.

(5) If your valve springs are too weak for 5400 rpm, with the cam you have, that could cause the problem.
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Last edited by bigD; 02-19-2018 at 09:06 AM.
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post #3 of 10 (permalink) Old 02-19-2018, 10:23 AM
 
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bigD covered it all very well. Best of luck with it!!
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post #4 of 10 (permalink) Old 02-19-2018, 05:24 PM
 
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I would side with a fuel issue as being the first thing to check. If you have the smaller 5/16" fuel line and not the 3/8", then that could be your problem - running the carb dry.
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post #5 of 10 (permalink) Old 02-20-2018, 08:39 AM Thread Starter
 
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Thanks Guys. What do do you recommend for aftermarket ignition?
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post #6 of 10 (permalink) Old 02-20-2018, 11:58 AM
 
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Originally Posted by goatman1964 View Post
...What do do you recommend for aftermarket ignition?

If you have $300 to spend on it, go with a DUI brand HEI.

https://www.summitracing.com/parts/d...w/make/pontiac

The Pertronix & Taylor are $235.

https://www.summitracing.com/parts/p...w/make/pontiac

https://www.summitracing.com/parts/t...w/make/pontiac

There are cheaper ones, right on down to the cheapest Ebay Chinese models, for under $50. Some have reported that the cheap models worked OK, if they swapped out the module for a better quality one. Some say they are complete junk.

https://www.summitracing.com/parts/p...w/make/pontiac

https://www.summitracing.com/parts/s...w/make/pontiac

Hey, this one even has a 100% money back guarantee, FWIW.

https://www.ebay.com/itm/For-Pontiac....c100005.m1851

If you wanna keep the stock look, some recommend a Pertronix Igniter III set-up to replace the points, in a points type dist. These also have a rev limiter feature.

https://www.ebay.com/itm/Pertronix-7...daTi19&vxp=mtr

But, there are still some who prefer to run points. The reasoning is that they are less likely to quit suddenly, leaving you stranded in a bad situation. I used points for many years, street & strip. Never left me stranded. Always showed warning signs before quitting completely. Could always make it to a safe stop, where I could either change or adjust points. Not saying they provide max engine performance. But, they have proven very reliable, in many millions of vehicles, for many years.
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post #7 of 10 (permalink) Old 02-21-2018, 10:18 AM Thread Starter
 
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Great info. Thanks. I have never used pure mechanical advance. Is there an advantage? This is just a street cruiser. The flamethrower for $235 looks like a good option.
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post #8 of 10 (permalink) Old 02-21-2018, 10:35 AM
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Quote:
Originally Posted by goatman1964 View Post
Great info. Thanks. I have never used pure mechanical advance. Is there an advantage? This is just a street cruiser. The flamethrower for $235 looks like a good option.
No, definitely not. In fact it's a DISadvantage. The purpose of the vacuum advance is to add timing under part throttle, light load conditions. Doing that is more economical on fuel and also good for engine cooling. You tend to find mechanical only systems on race cars because 1) no one cares about part-throttle, light load operation in a race car which spends most of its time at wide open throttle (which means the vacuum advance isn't going to be operating anyway) and 2) removing or "locking out" the vacuum advance removes a component that can get "loose" and make it more difficult to precisely control the ignition timing advance curve.

Bear

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post #9 of 10 (permalink) Old 02-21-2018, 12:44 PM
 
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"...I have never used pure mechanical advance. Is there an advantage?..."


Street cars need the vacuum advance. All the dist I linked do have the vac advance. One of the Summit pics doesn't show it. But the description clearly says it has both vac & mechanical advance.

https://www.summitracing.com/parts/d...w/make/pontiac
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post #10 of 10 (permalink) Old 02-22-2018, 06:01 PM Thread Starter
 
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Thanks
I went with the flamethrower. Putting it in this weekend. Will let you know how it goes.
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