360 Backfires Under Load

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JetCity68Dart270

JetCity68Dart270

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Howdy from JetCity. It has been nice weather and just in time, I finally got my car together and out of the garage. Now I am trying to get it to run properly- and failing.

I have a 1976(?) 360 stock rebuild from a local machine shop. I bought a Lunati Voodoo 256/262 kit, MP springs, Mopar electronic ignition and a K&N filter. I added a Holley Street Avenger to my Edelbrock TM5 and a 2 inch exhaust with Flowmaster 40(?)s and an H pipe.

I mentioned in my welcome thread that I am new at this. I guess I may add that I am having a hard time understanding how everything works together.

I have had the car for three years. I want it to drive around running errands and to take it on the freeway off and on. I would like to get a trans and rearend and go racing.

Before I spun a bearing the car seemed to work well on I-405 and around town. I had a leak on the primary side of the carb and could not keep it in time with the advance on the dizzy. It would surge. But with the 318 and Holley 1850 on a TM5 and dual point distributor, factory manifolds (273(?) cast iron) and a single exhaust it was fine and fun.

After deciding not to put any money into the 318 I went with the 360. After a few hassles I got it going only to find pretty good leaks at the manifold flanges and a severe backfire when I accelerate(anything more than slight pressure, usually). I drove it to the gas station and filled it up, I drove it to the muffler shop and I drove it to work and back, and a little test and tune tonite. I retarded the timing way back and checked for misfires. I did not change my intermediate shaft during the rebuild. I have done nothing to the carb(NIB Holley 570 Street Avenger) but put on my air filter and choke, set the idle.

What is up? :sign3:

What is going on?
 
Welcome to FABO.First I would check your firing order(plug wires)to make sure all are in correct position and on the plugs.I,d then check your timing with a timing light,you might have advanced too much.Backfiring is a sign of bad timing.Good luck.
 
Carb could be lean also, might need to jet it up.
 
Firing order, initial timing and carburetor jetting....in that order.
 
Okay. Gonna verify firing order first.

On the timing- I have a Summit balancer, I have the stock timing tab- the tab seemed go in only one spot. When I tdc'd using a plastic stop(possible error zone) the mark I found was around 3/4" towards the oil pan from the hole on my timing tab. I have an old skool timing light that connects to the plug wire(not inductive) It seems to work ok.

When I started to get it going I had the distributor 180 out for a round of crankings maybe six times over two days. I had prelubed it with a shaft and hi volume pump. Put the trans on and found the oil leak from the un-modified pan/hi vol pump clearance issue, changed to a stock pump, resealed and relubed with Rotella. Cranked it three times- saw a spark called it good and bought a new Street Avenger to replace the 1850. Next cranked it three times found the 180 out dizzy. Got it started, set the timing by ear and vacuum gauge took it the gas station. It idled well and cruised ok on minor throttle- backfire on pedal pressure- did not floor it.

Wanted to believe the single exhaust was the culprit I took it to the muffler shop. Same symptoms. Went over the timng found it to be around 15 before on the stock pointer and around five in my pencil mark. I dialed it down to 3 before and it seemed to drive better.

I wanted to start richening the mixture by backing out the idle mixture screws.
 
Have you checked for vacuum leaks when it's running? It almost sounds like a lean condition, do you get a shot of gas in the carb instantly when you crack the throttle by hand, with the car not running of course?
 
sounds like a lean condition ....... and also a bad pick up coil will cause a miss and backfire at higher rpms . jimjim
 
Thanks for the replies. I put the timing light and vacuum gage on the small untimed port on the carb. Advanced the timing and went from 5 to 15, unplugged the vac advance hose at the metering plate and it dropped to 12, sort of bouncing around 10-12 pretty much around 11. Did not see much pattern or wild movement. The needle was not still, though. Dialed the timing back and went around the four idle mixture screws. screwing out the primary side dropped the vacuum. Seemed to like 3/4 out from bottom. I turned out the secondary screws and it seemed to pick up a bit. The all aluminum Avenger has four corner idle mixture.

I drove it and it seemed to do fine in first but the 1-2 shift brought on some backfiring. I backed it up and did the same thing a couple more times. Seemed ok in reverse downhill(LOL) and would be babied in first uphill.

More info- I put a BM kit in the trans using the mild settings. I have Hurst Promatic with a burned cable(F&%$) that has a hard time getting first. Drive is ok but the ratchet action seems too stiff to ratchet(shifter works fine without the cable on it, but I had a rough time finding a spot where my clevis would drop in at neutral. A lot of post talked about moving the lever to the rear of the trans and shifter in first but mine seemed backwards.) The Hurst info said do it in N. If I try and kep it in first it will upshift around 25.

It was hard starting first thing. After tuning it started without the choke. Oh and my old timing light died so I am off to get a new one and some transfluid.

:newb:
 
A flat exhaust cam lobe or other problem holding an exhaust valve shut (or nearly closed) will also cause backfire under load as will a bad plug wire, and a "stuttering" (non rythmic) backfire can be low spark voltage, IE bad coil, bad coil wire type of problems
 
Did you check the pumpshot? With the car not running look down in the carb and crack the throttle by hand and see if you get an instant shot of gas. If so then double check your timing. A good place to start is around where you had it at 15* with the vacuum advance unhooked and plugged. Once you have that right check your vacuum at idle using full manifold vacuum, rotate the distributor until you have the highest reading and back it off a notch as Crackedback said. Double check your timing to see what you end up with. You can then move onto adjusting the carb.

Timing then carb.
 
Thanks Badsport. I got an accell superstock chrome coil and a simple inductive timing light

I am going to try and getsome reliable rreadingsfrakkin android sorry for hemispellings from my tablet
 
The pump shot looks good- really squirts in there, left side is a little more pukey than the right. I bought a new Accel supers stock chrome coil and hooked it up with the ballast res that came with the MP ignition. Vacuum was up to 15 and stready, timing looked like 15 before the mark I came up with and about 25 before on the stock tab so I left it alone- though since the vacuum reading went up a few inches and steadied maybe the coil had cured it

drivability improved somewhat but the 1-2 shift causes backfire, have not run it over 35 mph

I read somewhere it was critical for engines to start quickly or it could damage the cam- I don't think it started quickly. I used the Rotella ;) but I had the dizzy 180 out. Went through a round of cranking(probably more than I would care to admit) changed the carb, another round but less cranking and I found the distributor problem. It started and ran but the driveability problems were present. I was really spooked it would not start so I checked everything(LOL) and I tried to keep the cranking/starting attempts short, for what it is worth and I had prelubed it.

Starts pretty easy, restarted when the backfire killed it driving and the idle sounds pretty good. The manifold flanges are leaking at the down pipes so there is always a racket.

Play with the timing more? Do I pull the valve covers and look for rockers not moving? :banghead:
 
From what you describe with startup problems, I'd be checking the cam. You can pull the covers and "rig" an indicator on individual valves to get a rough check for lift. If it's bad enough, yes, you will be able to see a rocker that is not "doin' much"
 
After eating lunch and a rest I went back and started from scratch. I wrote the firing order on my hand. I found 1, 8, 3 uh oh ****. I corrected the firing order and drove away.:eek:ops:

Thanks so much to the members who commented. Reminding me of the details- well fundamentals was key, and the encouragement from collaboration and cooperation helped keep me mindful of my goal instead of fixated on failure:thumleft:.
 
!!!DOH!!!

homer-simpson-doh.jpg
 
Damn. We've ALL done it. and if we say we haven't, we're lyin.......or we ain't done it yet.
 
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