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  #1 (permalink)  
Old 08-04-2005, 12:47 AM
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backfire

Last night my car backfired and then stumbled when I accelerated hard. I got home and noticed I had a bad spark plug wire. Changed the wire and it backfired once again tonight. But only backfired once and I drove about 20 miles. Also seems to be a little rough in the upper rpms. I am running straight 110 VP race gas for this weekends Hot August Nights in Reno, Nevada. Unitl last night I had no issues with backfireing or lack of performance. Been running 110 straight for one week....

I have a SBC 355
.600 lift roller cam
Mighty Demon 750
MSD 6A
Mallory Comp 9000 Distributor


Everything was good until last night. Dang ol Hot August Nights Demon paying me a visit......

I am thinking plugs or another wire. Or possibly I need to go with a hotter plug with straight 110?

Thanks in advance for any suggestions....

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Old 08-04-2005, 07:55 AM
Technical Support Barry Grant
 
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Backfiring?

Black 66,

A couple of questions; Is the engine backfiring through the carburetor, or is it an afterfire out of the exhaust? When is it happening? How much compression does the engine have? Is this the fuel you normally run, and how old is it?
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Old 08-04-2005, 09:04 AM
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VP110 will net you less power then pump gas if your compression is lower than 12:1. If your compression is to high for pump gas I would still be looking at something near 100 octane. You will also have to readjust your floats and fuel mix jet's due to the specific gravity differances. VP110 just burns to slow in a low comp engine to make good power. Even if you over advance the timing.
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Old 08-04-2005, 05:58 PM
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Race gas

Ok, I will get some regular unleaded in there to get it down to 102 octaine. The pop is coming from the exhaust.....

Also what is the heat range on a stock sbc spark plug?

Thanks...
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Old 08-04-2005, 08:29 PM
Technical Support Barry Grant
 
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Backfiring out of the exhaust is indicating that you're too rich.

What compression ratio is the engine, and what spark plugs are you using?
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Old 08-04-2005, 10:38 PM
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Demon Carb

I am running a 750 Mighty Demon. I jetted it down three jets from stock for my altitude. 5500 feet. My compression is 11.7. My plugs are accell shorty plugs. And if it was poping from the carb that would mean what???
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Old 08-05-2005, 06:54 AM
Technical Support Barry Grant
 
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You're running way too much octane for the type of compression you're running. Lower octane should cure most of your problems. A backfire or pop through the carburetor would indicate you were too lean. Which heat range shorty plug did you install?
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Old 08-05-2005, 08:25 AM
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You 110 octane burns so slow it's still burning while the piston pushes it out the exhaust. A header manifold leak can do the same. Improperly tuned idle mix screws will do it to.
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Old 08-06-2005, 02:15 AM
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I am not sure on the heat range for my plugs. Tonight I drove my car and it was off and on with the popping from the exhaust. I had a 1/4 tank of 110 left so I put in 10 gallons of super unleaded. The popping still comes and goes. A few times the performance was excellent. Other times it seemed to be very sluggish. I will wait to see how it runs as the tank get closer to empty. I checked a few of my plugs and they where a nice light brown on the electrode.

Also I have a lot of white smoke when I launch hard. I have jetted the main and primarys down three jets sizes. And I went down one size on both squirters. BG is your jetting down 1 jet for every 1500 feet usually close?

Here is a run down on my car...

355 SBC
11.2 Hyper Tech Piston with 64 cc heads
Camel Hump 1.95 head turned into 2.02's
Comp Roller Cam. 245 duration with .600 lift exhaust and intake.
Torker II Intake
Mighty Demon 750
Mallory Comp 9000 Unlite
MSD 6A

I know this setup might not be the best. But the motor was built completely free. And I enjoy the motor a lot. Changing to 195 CC runner heads and different intake the first of September.

Thanks a lot guys....
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Old 08-07-2005, 01:06 PM
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backfire

I was able to pin point some of my problem. The timing has moved down to 13 BTDC. I put it back up to 20 BTDC and the car runs great. But I still get a lot of white smoke when I launch hard. Is the smoke a direct indication of still being to rich? And if so, do I change the jets or squirters? Or both? I have already jetted down two jets sizes on both the P and S jets....
Thanks.....
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Old 08-08-2005, 10:42 AM
Technical Support Barry Grant
 
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Quote:
Originally Posted by black66
I was able to pin point some of my problem. The timing has moved down to 13 BTDC. I put it back up to 20 BTDC and the car runs great. But I still get a lot of white smoke when I launch hard. Is the smoke a direct indication of still being to rich? And if so, do I change the jets or squirters? Or both? I have already jetted down two jets sizes on both the P and S jets....
Thanks.....

Normally if the engine is too rich the smoke would be black.

White smoke can be condensation in the exhaust system.
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Old 08-08-2005, 11:25 PM
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smoke

Another thing BG Tech, when I start my car up it always has black residue spitting out of the pipes.

For jetting the demon BG calls for one jet down for every 1500 feet. Is that just a starting point? Or is that usually right on the money??

Thanks......
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Old 08-09-2005, 06:59 AM
Technical Support Barry Grant
 
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Quote:
Originally Posted by black66
Another thing BG Tech, when I start my car up it always has black residue spitting out of the pipes.

For jetting the demon BG calls for one jet down for every 1500 feet. Is that just a starting point? Or is that usually right on the money??

Thanks......

As rich as you've been running, burning the fuel in the exhaust that wouldn't suprise me until you get all of the sout burnt out.

1 jet for every 1500' is a safe starting point, but every engine is going to be a little different.
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Old 08-10-2005, 12:28 AM
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carb

So BG Tech do I jet down the P and S jets at the same time? And I also jetted down on size on the squirters. Any thoughts on what I should do next?? Thanks....
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Old 08-10-2005, 07:08 AM
Technical Support Barry Grant
 
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Jetting for altitude.

Normally you'll want to go down on both the primary, and secondary jetting the same amount when you're adjusting for altitude. As far as the squirter nozzles go, as long as the engine responds properly you should be OK.
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