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1980 Ford Bronco 302 v8 misfires under load

TTMotorsports

Red Skull Member
Joined
Jan 7, 2021
Member Number
3315
Messages
1,291
Loc
Lucerne Valley, CA
Ok so I have a 1989 bronco with 302 4 speed manual. It revs and runs fine in neutral sitting in my shop. When I drive it I get a misfire when I get fast enough to shift into 2nd gear and I hold steady throttle until 3000 rpm than runs smooth until I shift again. Does same thing misfires even worse in 3rd until above 3000 rpm. I can't drive in 4th because misfire is bad and I lose speed where I have to downshift to maintain speed. I have installed a new rebuilt motorcraft 2150 carb from gaurenteed carburetors. New cap rotor plugs and wires. Fuel tank dropped and cleaned spotless. New pickup tube and new fuel filters. Fresh tank of gas. What should I check to figure out why it does this and fix it so I can drive it on the highway, get it smogged and than sell it.
 
How do you check the coil?

Brand new mechanical fuel pump on the motor so fuel pressure shouldn't be an issue since it's not EFI
 
Do you have all the smog vacuum controlled stuff: EGR, air pump, evap, vac canister, thermal switches, etc?

Those 2150’s for smog applications need at least the EGR functioning correctly because they’re built to rely on them (jetting etc).
 
That motor was originally efi no?

What distributor does it have now?

what is base timing set at, how about full advance? When does advance come in?
 
The motor is completely stock. Has EGR valve and stock distributor with the pickup not points that came in the 80 bronco 302.

I've heard from a few people that the timing chain stretch would cause a similar issue. Anyone familiar with that OR know how much rotation should the crank go before rotor turns on distributor?
 
Timing is set to factory 10 deg at idle and it's fully advanced at 2200 rpm it appears by my light. Hard to test exact RPM doing all this Solo and not being able to see the tach AND check with timing light.
 
TTMotorsports said:
I've heard from a few people that the timing chain stretch would cause a similar issue.
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I bet that it has the plastic cam gear, you can rock the crank back and forth with the valve cover off and see how much play it has, but you need a similar engine to compare it to.
 
Yes nice good spark. No leaking oil from coil. Fresh fuel filter inline after tank before fuel line to front AND fresh fuel filter on intake before carb.
 
So it has duraspark not tfi? Because it is in fact 1980 not 1989?

Guessing Either timing chain stretch or bad icm
 
Checked timing chain but turning and seeing distributor rotation. It only rotates 2 degrees on the timing marks before rotor turns which doesn't seem excessive.

How the whole fuel system is new. Next to check is the distributor. It has good spark and runs fine BUT would that cause a misfire intermittently under load only? When it starts to misfire I'll push in clutch and it revs up and runs perfectly.
 
TTMotorsports said:
and not being able to see the tach AND check with timing light.
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Grab a tach and install it under the hood while troubleshooting. We have an old one in the garage just for that use. Only takes a min to wire up and zip-tie somewhere.


Wrong duraspark box? With the brown (which is friggin 3, not 2) grommet my fairmont ran like crap, but when i swapped to a blue one there were no more issues.
 
TTMotorsports said:
Timing is set to factory 10 deg at idle and it's fully advanced at 2200 rpm it appears by my light. Hard to test exact RPM doing all this Solo and not being able to see the tach AND check with timing light.
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I also wouldn't rule out a bad carburetor rebuild :homer:, do you have another to swap on?
 
I have swapped out the rebuilt 2 different carb for another China one from Amazon and same issue, which leads me to believe the carb isn't the issue.
 
arse_sidewards said:
Fuck that shit.
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Hurr durr let me add a few more butt connectors and random pieces of wire to my failing Duraspark wiring harness and hope it runs okay over 2000 RPM

Fuck that shit. Duraspark modules are hit or miss, the wiring is all fucked at this point, $60 buys you an HEI for your Ford
 
Did you ohm the new plug wires? I've gotten new plug wires that would miss under load, checked and sure enough, one or two had too much resistance. Now I take my meter with me to the parts store and on more than one occasion I asked for another box of wires and continue testing until I had a good set.
 
Thumping said:
Did you ohm the new plug wires? I've gotten new plug wires that would miss under load, checked and sure enough, one or two had too much resistance. Now I take my meter with me to the parts store and on more than one occasion I asked for another box of wires and continue testing until I had a good set.
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Think that would do the same in reverse? Went wheeling Saturday, truck misses at idle now, runs great at RPM. I need the check the wires, and probably pull plugs. How much resistance should the wires have?
 
posford said:
Think that would do the same in reverse? Went wheeling Saturday, truck misses at idle now, runs great at RPM. I need the check the wires, and probably pull plugs. How much resistance should the wires have?
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It should say on the box what the resistance should be. Normally there's 10 to 12k ohms of resistance per foot.

That's odd. I wouldn't think it would change but possibly. I've just had the issue with wires when under a load causing a miss.
 
Ok so I was told by a few Ford mechanic friends to check timing chain for stretch. It had a lot of degrees of rotation on crank before rotor would move. So I changed the timing chain and it's slightly better in 3rd gear but still does it in 4th and I can't get over 50 mph. I don't have box to check plug wire resistance but I'll do some math and make sure they're all the same resistance per ft. I also have old wires still so I can check those and see how it is. I'm not dumbfounded on what it will be since it has an entire new fuel system, new timing chain, cap rotor plugs and wires. Any other suggestions on what to check
 
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