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Discussion Starter · #1 · (Edited)
I'm in need of troubleshooting suggestions to be able to compete at this weekends points race due to a mysterious backfire of my 496/Vortech V-7 on a '70 chevelle. The backfire occurs only on each run down the track- not during street use. I get a single backfire either just before or just after the 1-2 shift on the turbo 400. I can't tell if its out the exhaust or back through the Holley carb and the rest of each run is the same as before. I've run this set-up for 3 years.

I replaced the HEI distributer with an MSD 8361 billet distributer and Blaster SS coil, and the backfire remains. The vehicle already had the 6AL BTM ignition system. I do have another regular 6AL box I could borrow from another car. I replaced the large port iron heads with new Edelbrock aluminum 60559 heads in the offseason, and had picked up .5 seconds, but have now fallen off from 11.1 to 11.9 but care more about protecting the engine and the race season. MPH drops from 122 to 115.

The distributer was changed because the timing was erratic and I likely had too much timing - maybe even 50 degrees due to the wavering timing signal and a bad induction timing light. Just hope there isn't interal damage. Will go back to the basics of compression test, etc. My A/F ratio runs at 11.8 and the plugs are Autolite AR3935 with dark tan coloring. Timing is now 34 degrees, all in at 3500 RPM. I replaced the large stiff silver mechanical advance springs with the light blue, but had the same backfire with the 15 year old HEI so the ignition might be eliminated as a cause. Couldn't make the vacuum advance work as it gave too much timing - so just have initial advance of 13 degrees and mechanical of 21 degrees more. Boost level is a conservative 7 to 9 lbs and I have a bypass valve - one step smaller than the Vortech Mondo bypass valve. Fuel is currently 100 octane plus Lucas octane booster. I live in Denver at 5800 ft. track elevation.

I went to the semifinals on the first weekend of the points season, but this might change alot of that. Thanks for your consideration.
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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I'd guess it's either leaning out on top or has a valve spring/valve float issue. Did you add a little valve spring pressure to compensate for the boost in the port? Checked the valve springs lately?

What is your fuel system like? Got a quality fuel pressure gauge tapped in near the carb and visible from the seat? If not, it's the first thing I'd do.
 

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Discussion Starter · #3 ·
The valve springs are all new as part of the Edelbrock aluminum head installation, and the spring pressures were very similiar to the Comp Cams 924 springs before, even a little tighter. I also added a fuel sump and entire Aeromotive A1000 fuel system this winter, with a guage at the regulator which shows pressure is holding constant and is read under the cowl hood from the drivers seat.

I don't understand why it would backfire around the 1-2 shift, and not further down the track. Wouldn't it have the same problem at the 2-3 shift or towards the finish stripe also as the valves would float again or run lean if thats the issue?
 

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Do You Have A Battrey In The Trunk
Friend Of Mine Same Issue
Drove Fine Reved Fine In The Pits
Back Fire Bad
Replaced Every Thing
From Dist. To Box
End Up Just Enough Flex Of The Chassis And Boom
Backfire
Battery Cable Was Grounding Out
 

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if you have an afr gauge, are you able to see it when the backfire occures-might be a lean condition-is the carb setup for a blowthru? the rate of rpm climb is greater on the 1-2 shift, things have settled down a little on the 2-3
are you running jet ext. in the rear jets? are the plug wires good, not bundled together
i prefer to see around 18 int timing, all in around 1800 rpm, to whatever total it likes
what hi speed a/b's are you running
 

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I'd guess it's either leaning out on top or has a valve spring/valve float issue. Did you add a little valve spring pressure to compensate for the boost in the port? Checked the valve springs lately?
I am with Tom... you may be hitting your highest rpm on the 1-2 shift and also reaching the highest boost there during the whole run. just enough to hang an intake valve open and allow the backfire. What kind of seat pressure do you have?
 

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Discussion Starter · #7 ·
I've managed to outdo myself again. The problem was a plugged primary fuel filter again - and had just replaced it a few weeks ago. The welder who did my large Aeromotive fuel sump is a much better welder than a housekeeper - the filter was plugged a second time with shiny metal filings from the installation. The first time was 10 miles after leaving the welding shop.

Had no idea that running out of fuel creates a backfire - and by the time I got my eyes back to the fuel gauge pressure had returned after reaching second gear where the rate of acceleration is slower and the fuel system demands lower. But I'll take the hit to my troubleshooting pride instead of having to delve into valve spring/exhaust valve issues.

This may be the zillionth problem this forum has helped people solve.
 
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