I'm working on my in-law's 1978 D100 with a 318 engine. It runs fine at idle or revved up with no load. As soon as I start off in drive or reverse, it starts hesitating and backfires through the carb. If I put it in drive with the brake on and give it any gas, it does the same thing. New plugs, wires, coil, cap, rotor, vacuum advance. Timing is good and vacuum/centrifugal advance works as it should. No erratic spark at any engine speed. Timing mark doesn't "jump around" at any speed.

Compression 130-140 each cyl. Carb is only a year old, but I took it off and cleaned it just for grins. Good accelerator pump. Good jet of fuel from acc pump and good atomization when engine is revved up. Bypassed fuel pump and tank with an electric unit to no effect. Float level is good and I even ran it with the top off the carter 2bbl to prove the float level to myself with the carb mounted.

Valve timing dead on (new gears and chain). Engine runs GREAT - until you put even a slight load on it. It does seem to start bogging down a bit at 2500-3000 RPM or so, almost as if it's not breathing well, but I cannot prove it. It's more of a feeling I've got and I might be wrong.

Disconnected exhaust at cat flange (outlet of Y pipe), so clogged cat/muffler is not the issue. Heat riser is free.

No vacuum leaks. I sprayed carb cleaner everywhere looking for an induction leak. Vacuum is 15-17 on idle, which seems a bit low. Rapid fluctuation between 15-17 with each revolution. Drops to 5 when engine revved. If I let off the gas abruptly, vacuum jumps to 22" then settles right back down to about 15-17 again.

Plugs all looked good when I changed them. It does look like it's running a bit lean, but not excessively so. The plugs have always looked this way when I change them (slightly white instead of ash brown).

This started out as a hesitation when starting out from a stop or on acceleration It has gotten worse. The truck is a daily driver for my 80/90 year old in-laws, who bought it new. I'm afraid to have them drive it, so they're using the '65 Coronet until we get this figured out.

I'm stumped. I've tried all the usual tricks, but I'm not that familiar with Mopar engines, so I'm tossing this one out to the experts. Any ideas? Are there any special considerations or secrets that I should know about?

I would appreciate any ideas on this.

Thanks,
Bob

I'm thinking vac leak: bottom side on one of the side intake gasket(s), carb base gasket, power brake booster and do you have another carb you could throw on there real quick. EDIT when it started doing this when taking off from a stop sign was that before or after the carb swap

late or to advanced timming

have you put a vac gauge on it to see how much vac its pulling (I see you did)turn dist while watching vac gauge move it to get highest vac reading and adjust air screw to highest reading

I always set my timing and idle/mixture screws with a vac gauge to get highest reading

the poping my be from late timming after its in gear

also may want to pull a valve cover to see that all the rockers are moving the same amount and you dont have a push rod bent or punch through a rocker

maybe a lobe going on the cam of lifter cupping

what is idle rpm?

how much rpm drop does it get when droped into gear?

where is the timming set at on the damper,mine runs good at 10*-14* advanced base timing

I had a 76 Trailduster Sport, rebuilt or replaced everything. Ran great at idle, around the neighbor hood, get it on the road, sputter, cough, finally die. Wait a minute, Ok, than back on the road same thing. Finaaly found a piece of rubber hose in feul line, moving back and forth, when there was demand, it would move up to carb, and start pluging. No demand when at idle.

" Rapid fluctuation between 15-17 with each revolution."

I believe you have a valve issue. Pull the valve covers and verify all rockers are moving the same distance. If I had to guess, I'd say you either have a worn cam lobe, broken valve spring, or some other valve train issue.

Mark

Just ran into this on my 1979 Dodge 318 trk
The ECU was bad back was starting to melt out. Check the grd and the ECU

I suspect it's a mechanic problem rather than a mechanical one.

I tried the suggestions and the one that seems to work best is to set the timing using a vacuum gauge instead of a timing light. I guess I don't understand the markings. The harmonic balancer has markings from 0-30 BTC and 0-20 ATC. there is a machined groove at the 0 mark. The timing cover has markings from about 0-20 BTC/ATC. I presumed (error) that I align the machined mark on the balancer with the timing cover mark that gives me the specified 2 degree BTC timing at idle.

Boy, was I wrong. I had to advance the timing more than 10 degrees to get the engine running smoothly again. I used a vacuum gauge to get it into the ballpark and now the vacuum is >20". I would never have thought of that myself. It has a LOT more power, but the timing marks don't seem to have anything to do with reality. My rebuild and tune-up books don't discuss this. They just say to "align the timing marks".

I would now like to set the timing exactly to specs. Is there a secret way to do this other than the "set it 'till it pings under load and then back it off" method?

I really appreciate all the help!

Bob

The problem is still there.

I thought I was on to something when I timed for maximum vacuum. If I advance the timing to about 25 degrees BTC the vacuum climbs from 18 to 22, the engine runs well, and it does have more power. Unfortunately, when I start off it still bogs right down under load and it pings like crazy. I set the timing back to where it belongs and ran a few more tests.

Valves - all valves open the same amount and all work. No broken springs that I could see.
Compression - confirmed at 130-140 per cylinder.
Vacuum - 18 to 19. Vacuum fluctuates between 18-19 at the manifold port and stays rock steady at the carb base.
Catalytic - pulled the exhaust off behind the Y pipe. No effect.
Spark - steady at all speeds. No timing fluctuation at any RPM (once it is at a given RPM the relative position stays steady). No loss of spark at any RPM.
Vacuum advance and centifugal advance - both work.
Vacuum leaks - none that I can find by ear with a stethoscope or by spraying carb cleaner at any potential leak point.

I did notice a lot of soot out the tail pipe, but the exhaust is open.

The engine purrs at idle and at RPM with no load. Under any sort of load the truck acts like it has a Briggs and Stratton under the hood.

I suspect the next step is to pull the intake, but I hate to do it wothout some sense that it might have a leak.

I could swap out the ECU, but since it runs fine under no load and since the spark is steady under all conditions, I'm not convinced I should spend another $50 there.

I bypassed the fuel system all the way to the carb bowl with an electric pump and a gas can and the probem is still there.

This one still has me baffled.

Make sure the wires are indexed and plugged on the ballast resistor correctly. If so try a known good ecu. You have an ignition breaking down under load. As the cylinder pressures rise the mixture becomes harder to ignite.

i just had the ame problem with my 318. someone mentioned the heat ports in the middle of the intake that run under the 2 barrel will get cloged with carbon and choke up the engine

If it were me, I would check 2 things.

1) Max (total) spark advance. You need a timing light with adjustable advance, but total timing could be an issue. Total timing should be around 36 to 38, right?

2) Try adding fuel under load. If the engine will hesitate on a snap throttle, do it that way. If not, add extra fuel via a vacuum line and propane or carb spray. I have had issues with carb emulsion tubes plug up with debris that looked fine on inspection.

You want to try to give the engine what it wants. It acts like it wants fuel (lean backfire). Give it some and see how that works. Note: do not use ether (starting fluid)! You need something that will burn similar to gas.

If you do find adding fuel helps, it means you have too little fuel metered for the correct amount of air ingested, or too much air for the right amount of fuel metered.

Mark

Do you have another Mopar that you could borrow the ECU out of for a few minutes to eliminate it? I only have 4 at the moment(I'm keeping things under control ) sort of

I will try squirting some fuel into the carb while revving it a bit under load. I agree no ether for safety reasons and to avoid washing the oil out of the cylinders. I'll keep the fire extinguisher handy in any event ;-)

Advancing or retarding the timing affects power as expected, and idle at the two extremes, but it doesn't seem to have much effect on the underlying difference in behavior under load/no-load conditions.

The only other Mopars I have are two 1965 Coronet 500s so no spare ECUs to try. AutoZone has an ECU for $30, so I may as well just throw parts at it and try a replacement. I'll pick one up tomorrow. The ballast resistor is good.

If those two actions fail, I will pull the intake off and check it for cracks, leaks, or a clogged crossover.

I appreciate all the ideas. This one has humbled me...

Thanks to all,
Bob

too bad your not closer as I have a new ECU that we could try. I hate throwing parts at a problem. If you had a SB point dist in your stash you could sub it in real quick & wire it up

I would swap in a points diast in a jiffy

use the one keyed hot wire thats on the coil and add the black points wire to the coil

been do every elec mopar I have had in my life time

that was when an 75yr old man fixed mine on the side of the road one day....

I got one on the bench with new points if ya need it

and I still think 18-19-20 on tha vac gauge is were its gonna want to run ones you get this grimlin fixed

thats where I start,swap to points and tune with a vac gauge..has never failed yet