Hesitation and backfire on acceleration

[apostle228]

Ok all, I am having an issue with the 318 in my Satellite... Yesterday, I replaced the spark plugs, cap, rotor, wires, reluctor and pickup in the distributor, as well as a new coil, ignition module, and voltage regulator.

Today, it starts right up... Better than it ever has. Idles very smooth, and I can rev it up with no issues, very smooth across the entire power band.

Then, I take it around the block... When it is under load, it sputters and hesitates, and even occasionally backfires through the intake. None of this happens when revving the engine in the driveway, it only happens when its under load and accelerating. Once I am up to speed and hold my speed, it stops hesitating and backfireing. Any ideas on what could be causing this? Where should I start on my troubleshooting?

 

[Meep-Meep]

Is the timing set properly? Mechanical advance working? You can also check for accelerator pump shot in the carb (not with the engine running). Move the throttle and you should see a nice steady stream of fuel crashing on both booster venturies.

 

[Coronet 14]

Vacuum leak. My 72 Coronet with 318 had exact same problem. Drove me crazy The intake manifold had vacuum leak in the front. Also the distributor shaft would move up and the rotor would get torn up inside the distributor cap.

 

[apostle228]

The timing is correct, and its not the accelerator pump I dont think. Very nice squirt of fuel there when I slam down the throttle. It has vacuum advance, and I went ahead and sucked on the vacuum line for it and I can draw in air freely AND blow through it!... so I think that diaphragm in there may be shot, what do you think?

 

[khryslerkid]

If your vacuum advance dosent hold a vacuum it's definitely bad and that would explain your hesitation. Not that hard to replace...

 

[apostle228]

I went down to the local o'reily's, they ordered one for me and it will be in tomorrow. I'll let ya'll know what the outcome is

 

[bigaadams]

after replacing the distributor pickup..is it properly gapped...I have seen a couple aftermarket units that when it advances it actually changes the gap as the unit closes the gap on rotation...

 

[Meep-Meep]

Yep, the vac advance is shot. Taking into account the lack of advance and the vacuum leak at tip in may be your problem but not guaranteed. It all depends on how you drive it when the problem arises. If you stomp on it you may not see the backfire issue, and that might indicate the mech advance is working as well as the accel pump.

Good idea to check your mechanical advance anyway. Give the distributor rotor a twist and see if it snaps back. If not pull the rotor and the little felt sponge in the center of the shaft and put a few drops of oil in there. Pull up and down and twist to get the oil worked in until it snaps back freely. WD40 to get the oil to capillary and loosen up the crud then motor oil so it can hang in there.

 

[apostle228]

I'll do as you guys suggested and double check my gapping on the pickup and oil up that mechanical advance too while I am at it. Good suggestions guys, thanks for the input!

 

[apostle228]

Ok, I got the new advance installed, oiled up the mechanical advance, double checked my gapping and put it all back together. The hesitation is gone but it is still backfiring through the intake. What is my timing supposed to be at at idle? The decal under the hood says TDC @ 700rpm, but when I put it at 0* it still backfires through the intake... Any ideas?

 

[moparmarks]

The 2 things that cause a backfire out the intake is a lean condition or spark while a intake valve is still open.
Triple check your firing order beings that you did plugs and wires.
Check for vacuum leaks. With the motor at idle put your hand over the carb and try to choke it out. If it chokes then probably not a big vacuum leak. If the rpm increases then you have a vacuum leak.
Timing TDC to 10* advanced.

 

[apostle228]

I am almost positive it is a timing issue, because when I make very minor tweaks to the timing it stops backfiring ALMOST alltogether but still does it a little... at 8 or 10* advanced is when it does it the least, but it still does it. Before it would backfire non-stop while accellerating, over and over again, now it will just pop once then continue accelerating... Think I might just be off by a degree or two on the timing?

 

[bigaadams]

another reason for backfiring through the carb under hard acceleration is a worn exhaust cam lobe..

 

[apostle228]

I think I may have left out an important factor... It didnt do this before I changed everything listed above. No pooping or backfires before, replaced all that, now it backfires...

 

[bigaadams]

possibility of a crossfire?...you did verify firing order...

 

[coloradodave]

Worn exhaust lobe on cam? I need an explanation on that... Anyway, keep advancing the timing a couple of degrees at a time until you hear detonation (pinging) and then retard it a bit. Have you disconnected the vacuum can, plugged the hose and then run the RPM up with a timing light to see what your total mechanical advance is and at what RPM? Total usually falls between 34*-38* between 2500-3000 RPM. Also not a bad idea to see where initial timing is and then plug in the vacuum advance to see how much advance the can is adding. Should be +-10-12*. I've seen some that added twice that. Also, pull the plugs and look for a fouled one and check the resistance of each of your new wires. Look for +- 5,000 ohms per foot. It's going to be something simple...good luck.

 

[moparmarks]

Ya I would need some help with that too. I drove my 64 Chevy for years with a worn E-lobe and never had any backfire. Can't see how and E-valve can cause an intake backfire. Exhaust backfire, yes.

 

[apostle228]

Heres another interesting thing... I dont think that its crossfire or wired wrong (I did verify), but I dont think it has to do with spark really and heres why... Its not consistant. I can drive the car around the block 3 or 4 times, and when I do, on time around it may not pop at all, then the next time it will pop a couple times, then it wont again, etc... It isnt consistent, its intermittent now. I am pretty sure the timing is set up all right, but heres another question... is there any way idle could have anything to do with it? I tried playing with it a little, but its also doing this... When its in park, it idles nice, almost kinda high, and no popping. When I put it in drive or reverse, it drops down REAL low... Like, almost stalling low, and it even did stall out on me a couple times. Could that have anything to do with it?

 

[moparmarks]

You need to find the baseline. Its TDC at 800 rpm. Your not on the idle circuit. Probably can turn fuel mixture screw all the way in with no change to idle rpm. You need to increase the timing and back off the idle screw.

 

[apostle228]

Ok. i'll play with that idle and timing a bit more