Help- I am almost there.

[bogie1]

I have been doing a bulid up on my original owner 1986 GN over the last year.
I have done all the work my self except the trans swap.
My combo is this:
Stock block and heads, valve covers never been off.
CPT65
50# inj
LT-1 MAF
precision front mount
Aluminum RAD w/ ramcharger fans
reds XP pump + adj FPR
Trans +
Extender or sometimes Turtbo tweak Alchy chips
Razor's progressive ALKY KIT
New timing set
PTS Extreme Strip/Street Trans
9X11 3000 converter.
autolite 24 plugs gapped at 30 (10 miles on them)
New GM Coli pack and Ignition module ( installed today)
JW Wheel SFI lightwheight Flexplate
RJC Billlet Pully set.

Here is my last hurddle and I have had a few.
I took the car to the track Sunday and Ran 7.7 's @ 92.5 MPH backfiring in the exhaust down the track with 25# of boost.
The back fire is consistant with anything over 20 lbs of boost.
Under 20 # no backfire.
I replaced the fuel filter, plugs, coil pack and ignition module (both GM) today trying to figure out what is going on.
Same problem. BOOM! sounds like a stick of dynamite under the car and black smoke bomb too.
I am running out of ideas and I am so close to having a damn fine running GN that all of us can be proud of.
I need your help.
Thanks,
Rich

 

[getchasum]

Rich,
When I had this problem, mine was any easy fix, original negative battery cable was corroded on the inside of the casing, didn’t find it until we cut it into the ground wire that bolts to the fender. It was so bad that we decided to check the rest of the cable...same thing. Replaced it w/ new cable and it was fixed.

Not so easy....A good friend of mine chase a problem that sounds identical to yours, anything over 20psi and it bucked and back fired. Turn out to be a wiped cam and worn lifters.

 

[bruce]

Originally posted by bogie1

autolite 24 plugs gapped at 30 (10 miles on them)

Try cutting the gap down even more, say .025, and maybe using 23s.

Make sure the Fuel Pressure is raising with the amount of boost your running.

While rare, a bad TPS can do that.

 

[bogie1]

Thanks Guys.
I will check the neg terminal
I would think that a wiped cam lope would show up on a Vacum gauge and my vacum look fine.
As far as the fuel pressure, Hmmmmmmm. you may have something there.
Although I have a less than 2 year old Reds XP FP and adj FPR my fuel pressue gauge is not telling me good news.
At 15 # boost it will run up to 53-54# FP. (static Vac/on is 38)
But at 25# of boost it still reads 55#.
I chalked it up to a bad fuel gauge because my O2's are ok and I have no knock.
I thought that a muffler back fire was caused by unburnt fuel exploding in the exhaust. I didn't realize low fuel pressure can also cause this.
Can you help me understand this? Pardon my ignorance.

 

[bogie1]

Bruce mentioned that low fuel pressure can cause a exhaust backfire. Can some one explain how that happens? It seems like it would take raw fuel after-burning to do this. I also see a boost spike when this happens. My Electronic boost controller has recorded spikes up to 40# of boost at the time of the BOOM!

 

[bogie1]

I figured out how to redirect the backfire and turn it into an afterburner.
I ran a 5.9 @149MPH. on 89 octane, street tires and 5# of boost.
The secret is to run massive timing (50*)