Puzzling low power/performance issue *unless* timing is over-advanced

i’d love to get experienced perspectives on this puzzler!

short version (background to follow):

1991 mitsubishi mighty max (same as dodge ram 50) 2.4l inline 4 cyl. when the timing is set to 7 degrees BTDC, per spec, the vehicle hesitates to accelerate (even with no load, just revving in neutral), and backfires quite a lot. driving, it struggles to get up hills in 1st or 2nd. however, if the distributor is turned to max possible advance (about 30 BTDC, but beyond the markings), it performs better – less hesitation, and pretty much fine acceleration even up hills. however, it appears to run too rich and leave carbon on the spark plugs, etc.

what could be causing this?
how would i test to confirm?

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You know the part you look at w/the timing light while checking the timing? That part may be faulty, and has rotated w/respect to the orientation it should really be. That means that the timing you think you are setting isn’t really the timing you are getting. On my Corolla that part is called the crankshaft pulley, but on some vehicles it is called the damper or something similar. Harmonic balancer I think is another term. It has a rubber component, and since yours is pretty old, perhaps the rubber has deteriorated.


I’d first check if the vacuum advance and the centrifugal advance were working, and there’s not a vacuum leak.

here’s some background:

the truck was purchased last year off craigslist. the PO did not tell us about several problems, including failed coolant temp sensor, and the fact that the distributor had been recently replaced but had been set at the super-advanced timing above, in order to appear to run well.

the truck was driven for quite a while by people who weren’t (ahem) paying much attention to the sounds it was making. at some point the exhaust manifold cracked, causing massive loss of power etc.

at this point i starting trying to troubleshoot. have a parts truck – a 1996 mighty max that has most parts same. swapped the exhaust manifold and first part of exhaust/cat converter, which solved most of the problems, but still had a low power issue.

then redid the timing belt, which was off a tooth (common issue on this truck apparently). this improved it to where it is now, but in redoing the timing on the distributor discovered that the base timing had been set way off. with the base timing correct per timing light (and also in a more sensible place near the mark on the housing), the above situation w/ backfires etc.

most recently, i swapped the distributor from the parts truck to see if it was the crank position sensor. no dice.

this distributor apparently has neither mechanical nor vacuum advance – just the crank position sensor. one symptom is that when the ECU is jumpered out of the timing equation for setting base timing, the timing light is perfectly steady. but when the computer is setting the timing, it jumps around randomly on every rotation by up to 6 or 7 degrees. same with both distributors.

thanks for your thoughts and help!

thanks! there’s no vacuum advance (or mechanical advance, as far as i can tell) on this distributor.

That’s probably normal. Same thing happens on my Corolla. In normal running mode I think the software purposely dithers the timing a little to improve mpg/performance.

As George_San_Jose noted, it could be your harmonic balance has slipped a bit, so the timing mark is off.

One quick test is to remove the #1 spark plug, and manually rotate the engine so the #1 piston is at the top, or TDC for Top Dead Center. You may need to take several attempts to get it to TDC.

Once you get that piston to TDC, see if the timing mark on the harmonic balance is also at 0 degrees. It should be.

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I can’t tell you what the problem is without truck in hand. However, I can make a couple of comments.

You might check for a clogged converter. Testing with a vacuum gauge is the easiest and best method IMO.
You say it had a cracked exhaust manifold. A plugged up converter can cause severe overheating and even turn the manifold red. It can also cause rich running and sooted up spark plugs.

As for the timing, I suggest you do not run that much advance until something is sorted out. Excessive advance can easily destroy the engine; especially at sustained highway speeds.
I’ve seen more than a couple of engines wiped out because of something like that.

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(ps: that was my thought too, and people on a mighty max forum thought there was mechanical advance – as there is in many of the trucks. but not this year/engine, as far as i can tell from trying to twist it, and looking at manuals!)

this is a fascinating idea, and something i had no idea was a possibility! when removing/installing for doing the timing, it seemed like a solid metal pulley – was i not paying close enough attention?

this is a useful thought. how do i know i know it is at TDC for sure? using the old chopstick stuck down the cylinder trick?

another thing i could do is swap it with the one in the parts truck, that worked before i started pulling parts (but had low compression and chewed oil)

Beats me, no personal experience w/that problem myself. But it isn’t an uncommon problem noted here on older cars.

Distributors don’t necessarily need a mechanical advance mechanism if the engine uses electronic ignition. Electronics can replace that function. I don’t think my Corolla’s distributor has a mechanical advance. (My Corolla is of similar vintage to your car.)

that’s really helpful! i’ve asked and asked but not heard from others that they’ve seen this on their working vehicles. seems strange to me – obviously the engine moves the advance slowly with acceleration etc., but what is changing fast enough that it jitters on every cycle? :thinking:

anyway, thanks for helping me get over being fixated on this!

Given you’re looking for upwards of 30 degrees, then yes the “old chopstick stuck down the cylinder” would let you know if you’re off.

Years ago I had to time several engines starting with this approach, and you’d be surprised how close this method can get you. (Of course a dial-indicator gauge is preferred.)

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agreed! this may well have been (part of?) the original problem.

however, i’ve swapped out the cat with the parts vehicle, as part of the first section of exhaust. that truck was known-running before i started stripping it. so i’m assuming this is not an issue anymore.

also agreed! since i started working on it the vehicle has only been driven on test drives, a couple (painful) miles at a time.

thanks for these comments!

ok, i’m going to go give it a shot! will report back.

Does the shaft in your distributor have any play in it?
If you remove the rotor and wiggle the shaft, do you feel any play?

Tune it by ear, retard until it starts to bog down, then 1/4 turn advance. Just a hillbilly suggestion. Dewy Cheatem and how are not responsible, nor am I if you do this.

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darn. well, it seems the TDC is correct (at least within a degree or two). the chopstick went up, then kind of went very slowly for about 5-10 degrees, then down. 0 mark was directly in the middle of this. it was slightly tricky because the spark plug entrance is at an angle to the piston (i think), but seemed pretty clear.

yet another brilliant likely-seeming idea (apparently) bites the dust! (there’ve been so many …)

no play in the shaft. the distributor that was there when we got the truck appears brand new. earlier today i swapped in the known-working one from the parts car, and still have precisely the same behavior.

also, no jitter whatsoever when the ECU is jumpered out of the loop for setting base timing.

so i don’t think it’s that… maybe it’s just normal? :man_shrugging: