Backfires when slowing down.

[Rays1969cud]

Getting exhaust backfire when letting up on the throttle in city driving. Like 25 to 40 mph then letting up quickly. 10% throttle to 0% this can be first or second gear. It is unburned fuel in the exhaust. Cruise af is 14.3. This even happens just going from idle to cruse mode. Timing is 15° @ 850. 28 ° @ 2800. My cam is 226° intake 230° exhaust @ .50" 110 lobe separation. .521" intake, .530 exhaust lift 1.5 ratio.

[BITE_ME]

What size is the motor, and what Calibration map are you using?

[Rays1969cud]

345 ci LA ported RPM Heads 10:1, 91-93 gas. Timing was 28°@ 2850. Dropping it to 2800 helped. Now have 33° @ 2400 RPM. Vacuum advance is 7.5°.

[BITE_ME]

What spark plugs are installed?

[Rays1969cud]

Using a set of E3.48. Like that the are definitely pre-gaped.

[Rays1969cud]

One of the big reason I bought the PF-4 was I had a MSD distributor that didn't have vacuum advance. I do take lots of screen shots. I also drive at least 20 miles after any changes. I only change one thing at a time too. I have been watching my Manifold numbers along with the long & short #'s. Yesterday @ 2526 rpm , InHg -16.9, coolant 178 , AFR SP 14.4 , AFR 14.4, Short 1, long FT -12. Injection 2.93. injection Duty 6. Spark adv 33, throttle 11.

[2700 Cast]

Using a set of E3.48. Like that the are definitely pre-gaped.

Ray,

Try a set of NGK or similar resistor plugs --Gap @ .035

Note: PF4 Electronics, dont play well with E-3 style plugs---due to Elect. noise.

Good Luck...

[Rays1969cud]

Ray,

Try a set of NGK or similar resistor plugs --Gap @ .035

Note: PF4 Electronics, dont play well with E-3 style plugs---due to Elect. noise.

Good Luck...

You were right. I left the E-3 plugs in until tonight. The car had run well when I was testing on a country road. I didn't have enough road to keep my foot into it for a 6200 shift from second to third. This weekend I went to a test and tune & my rev limiter was coming on at 4500, 5300 & just any random RPM. It fell on it's face and I played with timing & AF to no real satisfaction. The car had run 12.52 last year on the Carburetor. This time the best was a 13.43. But it was running 1.86 60 foot! The strange thing was on the highway it ran smoothly and for this car sipped gas.
. Tonight I reread your answer and switched back to Bosh resistance plugs. Took it for a drive on a road where I it felt like the fastest it has ever ran up to 6200 in a blink of an eye and banged second and third hard. So the noise was there, at higher RPM. With a 3.91 gear it ran nicely at 33° advance. I was running 36°, when I ran a carburetor with no vacuum advance. It is now at 29° @ 2300 with 4.50° vacuum advance.
. Thank you. I can't wait to get back to the track.

[Rays1969cud]

I just played back my screen recording. My shift light is set for 6200 but I I shifted at 5990? The light was on! But I might have anticipated it with the sound and a 3400 stall converter might have let the light come on as the transmission grabbed. Man it was fun and I will learn to get the most out of this 345 ci Edelbrock RPM ported headed machine with this PF-4.
. Chassie dyno later this week. See if they have any fine points..
. Once again Thank you. There is a lot of great information here.

[Rays1969cud]

Here is last nights test run with new plugs. I did see I never hit 100 % but I was on street tires & on a unprepared road. Any suggestions for improvements I will gladly take in.

[Rays1969cud]

I was reading another question on backfires and saw a guy had the wrong g fuel pressure. So I just checked mine.
I am running the 5016 _65 map. It calls for 58 lbs of fuel pressure. I have only been running 53 at idle. I am using the Edelbrock remote fuel pump. I will adjust it to 58. Don't know why I set it low. I had a fuel pump go bad on an S10 and it was just 8-10 low and it would barley run this being 5 lbs low is a big thing. So I will set it to 58 then clear the learning program.
. Lots of great information here. Just so many of us in can be hard to drill around and find everything. Learning to use the search page better and it is getting faster. Tha is for all the help.

[2700 Cast]

Ray,

Those spec's are engine running with vac hose removed & plugged.

Just F Y I.

Note: with vac hose connected at idle, there is normally a 5 psi drop or so.

[fleet man]

maybe change the decell fuel cut off . a couple post hear said it cleared up the problem on hear.

[Rays1969cud]

Thank you I am doing that now. It was set high to keep the motor running when the throttle was snapped close at over 4000 rpm.

[marc s]

Thank you I am doing that now. It was set high to keep the motor running when the throttle was snapped close at over 4000 rpm.

what did you set the decel fuel cutoff to? I have slight backfire when I decelerate through the gears.