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Classic Mustangs Tech Forum

Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 12-20-2009   #1 (permalink)
rpmcarter is offline GT Member

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Default 351 Cleveland back fires at low rpm

Upon my purchase I was aware I had tuning to do on the Cleveland!!
Launching into what works and doesn't.....I found a few things. The distributor and coil is Accel electronic not stock. 351 is suppose to have the rotor set toward the carb??? true? the Distributor cap should have number 1 toward the left side behind the distributor clip sorta as well??? true or myth?
I am getting backfire at low rpm and acceleration. Past about 1500- 2000 rpm's she perks right up runs and sounds very smooth. I set the timing mark and pull the cap. Rotor is dead on the number 1 plug at 180 out...but get this firing order on the cap is set closely to the 302 set up NOT the 351. I'm scratching my head thinking with a great engine ..WHY do that? could it be spec's on the cam require a different method for timing?

the spec's on the cam are
Isky
514 lift
224 duration at 50
282 advertised duration
108 degree lobe separation
2v heads and eldebrock performer intake HOlly 3310 carb "750"


Just before I decide to pull the distributor to reset it.....
so she's running smooth......then while idling ....the rpms increase by itself (never did this before) ...then die off. NOt die as in abrupt but as if it ran out of gas. A very slow run down, it does start back up no problem. Does it AGAIN.....I look at the gas gauge and it looks very near empty. I go buy some gas it starts back up.
Runs fine for a few minutes, then it does it again. THIS time it won't hit....spins as if it is not getting fire. The carb needs rebuild so I check and it is getting fuel.

OH and the plug wires etc are very new, vaccum hoses appear to be in good order etc..
Coil is my best guess...what say you????
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1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-20-2009   #2 (permalink)
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for all those curious....it is the coil.
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1967 Diamond Blue Convertible all original survivor
1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-20-2009   #3 (permalink)
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OH the question about the wiring vs a 351 is still open. the issue with dying...solved.
I just can't think of a reason that you intenionally set the time 180 off and wire up like a 302????
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1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-21-2009   #4 (permalink)
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Hello rpm carter,
The most likely reason is laziness on the part of a previous mechanic.
Someone dropped the dizzy in 180 out and rather than pull the distributor, they just re-wired the cap. This works but if you don't know this was done, it causes head-aches. So a lazy mechanic did this.

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Unread 12-21-2009   #5 (permalink)
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Write down what you have and then put it like its supposed to be for a 351C, if its some knind of wierd cam deal call the manufacturer, they will know. Changing firing order wont hurt anything except make it run terrible - if at all. good luck bro, keep us posted
Joe
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Unread 12-21-2009   #6 (permalink)
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Print Dad.
I agree....but just in case I slept through part of 351 class or cam class, I can't think of a reason except inept procedure professionals.

Silver....will do I've got to do some compression checks, vacuum leak test...replace the carb....and on to the resetting. I've marked everything "just in case" so I can go back IF necessary.

Print Dad, I spoke to the cam mfg today....he said he knew of no reason the cam would require a unique wiring for it to run efficiently.
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1967 Diamond Blue Convertible all original survivor
1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-22-2009   #7 (permalink)
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Sounds like you are covering your bases well, keep us posted
Joe
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Unread 12-22-2009   #8 (permalink)
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Thanks Guys...hoping to sneak away from work to get started ....next is the paint !!!!
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1967 Diamond Blue Convertible all original survivor
1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-24-2009   #9 (permalink)
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so here is the answer......
I went thru the typical squences vacuum, compression, timing etc. after a second look at the firing order....1 and 6 were correct....the rest were precisely switched 'opposite' of the required order which closely resembles the 302 firing order.!!!

She's as smooth as silk and increased HP by bout 300....LOL!!!:hihi:
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1967 Diamond Blue Convertible all original survivor
1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-25-2009   #10 (permalink)
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Great news bro, its nice when you cna find something definitive like that.
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Unread 12-26-2009   #11 (permalink)
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Thanks Silver, this has been fun finding just how many things they had out of squence. Heck part of their problem is they forgot to tighten down the distributor ...and it was rotating out of time. I wonder how many times they scratched their head going okay this ain't right.
I know I did and I only had it 2 weeks. Today she gets the new carb:laughlitt
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"After all ......looks ARE important"

1967 Diamond Blue Convertible all original survivor
1971 Mach I: 351c,Performer intake,Holley 750,Doug Thorley headers,duals (of course)Flowmasters, Accel dist/coil, MSD brain,Isky Cam .517/ADV Duration 280@ .050 Duration 232 LC 108, FMX with shift kit, 2800 stall, 3:89 gears:hihi:
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Unread 12-26-2009   #12 (permalink)
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Cool man let us know how she kicks now! I had an oem dist locking up before and I would set the timing and it would run good and then out of the blue - terrible. I found out the shaft was galling in the housing and as I was setting the timing (again) as soon as I let go of the dist it locked and went WHACK and retarded all the way - good thing I got my fingers off the dist before that happened. Went to am MSD and have not looked back. I am glad you are getting things figured out sir!
talk to you soon
Joe
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