Backfiring on Deceleration...

Idhomustang · 07-24-2000, 09:18 AM

A total of 40 degree is tooooo much advance. 34-36 degrees is where your normal Fords like there timing.
Back if off to that level you should still have great out of the hole performance. try 14-18 degrees of intail timing and the rest should kick in around 3000. Use a timing light that has the advance knob and level the vacuum hose hooked up. You should be able to get both a total advance, and intail at idle figured out it may take a little math, but you can figure it out remember no more than 36 degrees. If you go to far you well cause detonation, your engines worst enemy..
The power valve you are using isn't causing any problems if your plugs are clean. If the power valve was bad your car wouldn't be putting out lots of black smoke and wouldn't idle well because the idle circut would be loading up with fuel.
There is another reason, for this back firing to occur. Try the timing and make sure your plug wires are on correctly. It is relatively easy to get on crossed.

------------------
393,830 Holley HP,Pro Billet MSD,MSD 6AL,Xtr-Com cam,et streets, 3:73,Eagle Long rods
3000stall covertor,Airflow Research, Hooker long tubes 3" exhaust. S&W 6 point roll cage.
12. at 110 mph. 2nd time out.

70 Pony · 08-22-2000, 09:46 AM

I'm a new guy so be gentle....Running a 351 W in my 70 Sportroof. 30 over truck block, TFS heads< Crane Roller rockers, Comp extreme energy cam 268 506 512 lift, EBrock Perf RPM intake, Holley 650 DP, Fed Mogul Hyperutectic Flat Top Pistons. Mallory Dist, MSD Blaster ignition and coil.
My exhaust is Hedman Headers, leading into 2 1/2 " pipes with Dyno Mac Race Mag Bullet Muffs, 4" X 9", dumping out in front of rear wheels....Now I've only been to the track once and ran a 13.44 at 107 MPH...the 3.55 Posi kills my launch and that was on New street tires BFG TA's as my grag radials were showing steel and I knew I wouldn't pass tech....
OK Now the question...On acceleration the car runs great, no misses or backfiring....but when I release the accelerator I get loud (almost gunshot) backfiring from my Pass Side exhaust..had a cross pipe welded on...same thing...Now I'm probably running a max of 6' of exhaust tubing from the collectors to the tips, and no that this can cause unburnt fuel igniting as it hits the hot pipes and cool air..BUT it's only doing it on the passenger side...went to one step colder plugs...no help...the only thing that lessens it but does not eliminate it is advancing my timing. My timing (initial)is near 27 without pinging or sluggish starting even when hot...But I know this is way too high...but if I back it down the backfiring gets worse...Seems to only do this when I suddenly decelerate at over 4 Grand..I can wind it to 6 grand and then the back firing gets pretty loud...maybe about 4 or 5 gunshots is it.....
Bored you long enough...any suggestions...
By the way..the 650 DP is jetted at 78 and 69 with 37 squirters. THANKS>>>

------------------
Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

steve351w · 08-23-2000, 12:07 PM

Eddie,

Check for intake manifold leaks. And maybe try a different carb. This is weird!

Anyone else have any ideas?

------------------
Steve
66 coupe w/351w

Nos351C · 08-23-2000, 12:40 PM

Sheesh guys,

a home away from home?
Eddie, are you sure you don't have an exhaust leak at the collector?

------------------
Marcus
68 Mustang giggling 351C coming soon

70 Pony · 08-23-2000, 04:50 PM

I'm going back to my installer....a good ford guy and hot rodder.....next week
Got a nice steady idle, but maybe a vacuum leak somewhere....
re torqued my collector bolts, and have copper gaskets. But who knows..was just looking for fresh thoughts and ended up with 2 M&M'ers...my luck!!!!!

------------------
Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

Gearhead999 · 08-23-2000, 08:08 PM

If it is backfiring in the exhaust. You must have fuel in the pipe to ignite. The timing could be way off too. As you stated it was way advanced intial, right. Check the balancer and make sure that it hasn't slipped. You can check it with a piston stop and see if TDC is correct.

Check power valve for leak.

How are the plugs running??
You maybe alittle too rich, causing the raw fuel in pipe that EXPLODES. Float level to high too.

Idhomustang · 08-23-2000, 11:56 PM

I agree with the the timing, check it and if needed you can make an inexpensive piston stop out of an old sparkplug by removing the insulator and tapping it to take a small bolt or brass rod.
Check the tdc of the piston against the tdc mark on the harmonic balancer, they should be close.
Plug wires in the correct order, you didn't cross wire two of them did you? What firing order are you using? Hopefully you tried both 137 and 154 to see which worked best? Your cam card should have given the correct firing order just guessing here?

------------------
393,830 Holley HP,Pro Billet MSD,MSD 6AL,Xtr-Com cam,et streets, 3:73,Eagle Long rods
3000stall covertor,Airflow Research, Hooker long tubes 3" exhaust. S&W 6 point roll cage.
12. at 110 mph. 2nd time out.

Rev · 08-24-2000, 06:18 PM

70, with 27 degrees initial advance, something is definitely off. Could be you're 1 tooth off on the distributer, but I don't think so. What I really think is that your centrifigal advance mechanism in the distributor is failing to give you any advance at RPM, leaving you with 27 degrees at all RPM's. This would leave you with an inadequate advance at the higher RPM's resulting in the phenomenon that you describe. Put a timing light on it at higher RPM's and see where you're at. I may be totally off my gourd on this, but it's worth a look.

Rev

------------------
'66 Coupe, 306, windage
tray, TRW flat tops, 351W
heads,pocket ported, 268
CC hydraulic cam, R tip
R's, Edelbrock Performer,
600 CFM Holley vac. secs.,
Mallory Unilite, coil, fuel
pump, Tri-Y headers, 2 1/2 "
custom pipes, 3 chamber
Flows, stock C-4, 3.25
Currie built 8" rear,
March under drive pulleys,
13.97 e.t., 100.1 mph
1/4 mi.

70 Pony · 08-24-2000, 07:51 PM

Thanks All: let me try to answer some of these questions: My plugs look great, slightly grey, no carbon,or speckles...I am using the 351 firing order...my cam is for the 351 W...I checked TDC about a week ago...and the best I coud see it was right on with that front pass side piston right on the top. My total advance is kicking in but again high..using a Mallory Unilite with Vacuum advance at 25 initial it was 45 at 3000 revs....I capped off the vacuum nozzle and the carb vacuum port and brought the total down to 40 at 3000 RPM's...somewhere something is not burning that fuel or I'm getting too much raw fuel...using a 6.5 power valve, brand new with the engins install in July..I'll recheck the float levels, but the last time I checked the primary bowl was a little high and I backed it down....honestly it's driving me nuts...but I could learn to live with it if I was sure it wasn't doing any damage and was not affectig performance...but it must be at least affecting performance somewhat, hopoefully that's all...

------------------
Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

Rev · 08-24-2000, 09:14 PM

Eddie D, I use a 3.5 power valve mainly just for economy at less than full throttle. Doubt if this causes your back firing, but it would shut off the the power valve enrichment a little earlier on decel. Just a thought.

Rev

StangFlyer · 08-27-2000, 12:58 PM

Just replying to fix the date on this post.

------------------
Dan McClain, Editor
The Mustang Works Magazine
1991 Mustang GT - NOVI Supercharged 377 Stroker
1999 Ford Lightning SVT - Supercharged 5.4L Triton

thunderbolt · 08-27-2000, 09:59 PM

Just a simple test but I haven't seen it yet in here. Try a different distributor cap. Cracked caps often give backfire problems and maybe you have a bad one. Doubt that is the soln, but worth checking in my opinion.

SA · 08-31-2000, 01:59 AM

Sounds like a bad timing chain to me...

08-22-2000, 09:46 AM	#2
70 Pony Registered Member Join Date: Aug 2000 Location: Glenmoore, PA, USA Posts: 8	Backfiring on Deceleration... I'm a new guy so be gentle....Running a 351 W in my 70 Sportroof. 30 over truck block, TFS heads< Crane Roller rockers, Comp extreme energy cam 268 506 512 lift, EBrock Perf RPM intake, Holley 650 DP, Fed Mogul Hyperutectic Flat Top Pistons. Mallory Dist, MSD Blaster ignition and coil. My exhaust is Hedman Headers, leading into 2 1/2 " pipes with Dyno Mac Race Mag Bullet Muffs, 4" X 9", dumping out in front of rear wheels....Now I've only been to the track once and ran a 13.44 at 107 MPH...the 3.55 Posi kills my launch and that was on New street tires BFG TA's as my grag radials were showing steel and I knew I wouldn't pass tech.... OK Now the question...On acceleration the car runs great, no misses or backfiring....but when I release the accelerator I get loud (almost gunshot) backfiring from my Pass Side exhaust..had a cross pipe welded on...same thing...Now I'm probably running a max of 6' of exhaust tubing from the collectors to the tips, and no that this can cause unburnt fuel igniting as it hits the hot pipes and cool air..BUT it's only doing it on the passenger side...went to one step colder plugs...no help...the only thing that lessens it but does not eliminate it is advancing my timing. My timing (initial)is near 27 without pinging or sluggish starting even when hot...But I know this is way too high...but if I back it down the backfiring gets worse...Seems to only do this when I suddenly decelerate at over 4 Grand..I can wind it to 6 grand and then the back firing gets pretty loud...maybe about 4 or 5 gunshots is it..... Bored you long enough...any suggestions... By the way..the 650 DP is jetted at 78 and 69 with 37 squirters. THANKS>>> ------------------ Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

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07-24-2000, 09:18 AM	#1
Idhomustang Registered Member Join Date: Sep 1999 Location: boise,idaho Posts: 220	A total of 40 degree is tooooo much advance. 34-36 degrees is where your normal Fords like there timing. Back if off to that level you should still have great out of the hole performance. try 14-18 degrees of intail timing and the rest should kick in around 3000. Use a timing light that has the advance knob and level the vacuum hose hooked up. You should be able to get both a total advance, and intail at idle figured out it may take a little math, but you can figure it out remember no more than 36 degrees. If you go to far you well cause detonation, your engines worst enemy.. The power valve you are using isn't causing any problems if your plugs are clean. If the power valve was bad your car wouldn't be putting out lots of black smoke and wouldn't idle well because the idle circut would be loading up with fuel. There is another reason, for this back firing to occur. Try the timing and make sure your plug wires are on correctly. It is relatively easy to get on crossed. ------------------ 393,830 Holley HP,Pro Billet MSD,MSD 6AL,Xtr-Com cam,et streets, 3:73,Eagle Long rods 3000stall covertor,Airflow Research, Hooker long tubes 3" exhaust. S&W 6 point roll cage. 12. at 110 mph. 2nd time out.

08-23-2000, 12:07 PM	#3
steve351w Registered Member Join Date: Aug 2000 Location: Northern, CA Posts: 11	Eddie, Check for intake manifold leaks. And maybe try a different carb. This is weird! Anyone else have any ideas? ------------------ Steve 66 coupe w/351w

08-23-2000, 12:40 PM	#4
Nos351C Registered Member Join Date: Jun 2000 Location: Bay Area CA Posts: 61	Sheesh guys, a home away from home? Eddie, are you sure you don't have an exhaust leak at the collector? ------------------ Marcus 68 Mustang giggling 351C coming soon

08-23-2000, 04:50 PM	#5
70 Pony Registered Member Join Date: Aug 2000 Location: Glenmoore, PA, USA Posts: 8	I'm going back to my installer....a good ford guy and hot rodder.....next week Got a nice steady idle, but maybe a vacuum leak somewhere.... re torqued my collector bolts, and have copper gaskets. But who knows..was just looking for fresh thoughts and ended up with 2 M&M'ers...my luck!!!!! ------------------ Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

08-23-2000, 08:08 PM	#6
Gearhead999 Registered Member Join Date: Sep 1998 Location: Oviedo, FL Posts: 992	If it is backfiring in the exhaust. You must have fuel in the pipe to ignite. The timing could be way off too. As you stated it was way advanced intial, right. Check the balancer and make sure that it hasn't slipped. You can check it with a piston stop and see if TDC is correct. Check power valve for leak. How are the plugs running?? You maybe alittle too rich, causing the raw fuel in pipe that EXPLODES. Float level to high too.

08-23-2000, 11:56 PM	#7
Idhomustang Registered Member Join Date: Sep 1999 Location: boise,idaho Posts: 220	I agree with the the timing, check it and if needed you can make an inexpensive piston stop out of an old sparkplug by removing the insulator and tapping it to take a small bolt or brass rod. Check the tdc of the piston against the tdc mark on the harmonic balancer, they should be close. Plug wires in the correct order, you didn't cross wire two of them did you? What firing order are you using? Hopefully you tried both 137 and 154 to see which worked best? Your cam card should have given the correct firing order just guessing here? ------------------ 393,830 Holley HP,Pro Billet MSD,MSD 6AL,Xtr-Com cam,et streets, 3:73,Eagle Long rods 3000stall covertor,Airflow Research, Hooker long tubes 3" exhaust. S&W 6 point roll cage. 12. at 110 mph. 2nd time out.

08-24-2000, 06:18 PM	#8
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	70, with 27 degrees initial advance, something is definitely off. Could be you're 1 tooth off on the distributer, but I don't think so. What I really think is that your centrifigal advance mechanism in the distributor is failing to give you any advance at RPM, leaving you with 27 degrees at all RPM's. This would leave you with an inadequate advance at the higher RPM's resulting in the phenomenon that you describe. Put a timing light on it at higher RPM's and see where you're at. I may be totally off my gourd on this, but it's worth a look. Rev ------------------ '66 Coupe, 306, windage tray, TRW flat tops, 351W heads,pocket ported, 268 CC hydraulic cam, R tip R's, Edelbrock Performer, 600 CFM Holley vac. secs., Mallory Unilite, coil, fuel pump, Tri-Y headers, 2 1/2 " custom pipes, 3 chamber Flows, stock C-4, 3.25 Currie built 8" rear, March under drive pulleys, 13.97 e.t., 100.1 mph 1/4 mi.

08-24-2000, 07:51 PM	#9
70 Pony Registered Member Join Date: Aug 2000 Location: Glenmoore, PA, USA Posts: 8	Thanks All: let me try to answer some of these questions: My plugs look great, slightly grey, no carbon,or speckles...I am using the 351 firing order...my cam is for the 351 W...I checked TDC about a week ago...and the best I coud see it was right on with that front pass side piston right on the top. My total advance is kicking in but again high..using a Mallory Unilite with Vacuum advance at 25 initial it was 45 at 3000 revs....I capped off the vacuum nozzle and the carb vacuum port and brought the total down to 40 at 3000 RPM's...somewhere something is not burning that fuel or I'm getting too much raw fuel...using a 6.5 power valve, brand new with the engins install in July..I'll recheck the float levels, but the last time I checked the primary bowl was a little high and I backed it down....honestly it's driving me nuts...but I could learn to live with it if I was sure it wasn't doing any damage and was not affectig performance...but it must be at least affecting performance somewhat, hopoefully that's all... ------------------ Eddie D, 70 RestoMod, Fastback, 351W, Arrest Me Red.

08-24-2000, 09:14 PM	#10
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	Eddie D, I use a 3.5 power valve mainly just for economy at less than full throttle. Doubt if this causes your back firing, but it would shut off the the power valve enrichment a little earlier on decel. Just a thought. Rev

08-27-2000, 12:58 PM	#11
StangFlyer Founder Join Date: Jun 1995 Location: Michigan Posts: 19,326	Just replying to fix the date on this post. ------------------ Dan McClain, Editor The Mustang Works Magazine 1991 Mustang GT - NOVI Supercharged 377 Stroker 1999 Ford Lightning SVT - Supercharged 5.4L Triton

08-27-2000, 09:59 PM	#12
thunderbolt Registered Member Join Date: Oct 1998 Location: Houston, TX, USA Posts: 380	Just a simple test but I haven't seen it yet in here. Try a different distributor cap. Cracked caps often give backfire problems and maybe you have a bad one. Doubt that is the soln, but worth checking in my opinion.

08-31-2000, 01:59 AM	#13
SA Registered Member Join Date: Apr 1999 Posts: 14	Sounds like a bad timing chain to me...