302 misfire and no acceleration under load.

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Author Topic: 302 misfire and no acceleration under load.  (Read 1894 times)

Offline Slugger

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302 misfire and no acceleration under load.
« on: January 14, 2020, 07:41:46 PM »
Hi brains trust,

I'm going crazy with this one. Car was running great, decided to install a wide band o2 sensor in the exhaust, removed plugs, header welded in the bung and blanked until I wire it up, I figured whilst I have half a bank out, I'd adjust my ticky valves. Adjusted intake when exhaust open, exhaust when intake closed. 3/4 turn.

Ran like crap. Sounded like a massive misfire and wouldn't accelerate in gear on the road. Re-adjusted valves to 1/2 turn. Same problem. Readjusted the valves again, when exhaust starts to move, adjust intake, when intake starts to close, adjust exhaust. 1/2 turn. Same problem.

So far I've checked every plug for spark, all good, replaced all spark plugs, checked resistance of msd 6al leads, all in spec, changed carb, checked fuel filters. All clean. The only thing I've noticed today is msd blaster 2 coil has 5.7ohm resistance, not the prescribed 4.7. I've spent 3 whole days chasing this and it's really getting to me.

Timimg 12 BTC, with vac advance disconnected, 30ish connected, 36 total. Timing is always spot on. Exactly the same every time. I can't recall how many times I've checked the firing order. 15426378, 7 and 8 are at least an inch from each other.

Im at a loss. Ready to start the fire anytime soon.

302, roller cam and rockers, afr heads, holley 670 Street avenger, msd 6al.

I'll post a vid or link when I get to the computer.

Online GEOFF289

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Re: 302 misfire and no acceleration under load.
« Reply #1 on: January 14, 2020, 08:03:56 PM »
302 roller cam as in mid '80's onwards 5.0? If so, that's the wrong firing order.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #2 on: January 14, 2020, 08:09:18 PM »
68 302, comp cams retrofit roller cam.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #3 on: January 14, 2020, 08:17:31 PM »

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #4 on: January 14, 2020, 10:58:32 PM »
So adjust the re-adjusted valves lash back to where they were before this issue and if it runs OK, but still has the ticking then you may have a lifter that is not pumping up. What make are your lifters?

Offline flatchat

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Re: 302 misfire and no acceleration under load.
« Reply #5 on: January 14, 2020, 11:13:12 PM »
Go check that the plug leads are in the right allocation

Offline flatchat

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Re: 302 misfire and no acceleration under load.
« Reply #6 on: January 15, 2020, 12:06:55 AM »
.......and do the valve lash procedure again --  at top dead #1 E,I back the adjust nut off then tighten till push rod slack is taken up then tighten 1/2 to 3/4 turn - lock the nuts  - Rotate crank 90deg clock wise and do # 5 E,I  --same for the rest in the firing order
Hope that helps ?
« Last Edit: January 15, 2020, 12:08:38 AM by flatchat »

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #7 on: January 16, 2020, 04:24:02 PM »
Go check that the plug leads are in the right allocation

they are comp cams roller lifters and trick flow roller 1.6 rockers.

I'm sure i had them adjusted at 1/2 turn when i first assembled the engine. I've done it twice now at 1/2 turn, still no good.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #8 on: January 16, 2020, 04:33:41 PM »
.......and do the valve lash procedure again --  at top dead #1 E,I back the adjust nut off then tighten till push rod slack is taken up then tighten 1/2 to 3/4 turn - lock the nuts  - Rotate crank 90deg clock wise and do # 5 E,I  --same for the rest in the firing order
Hope that helps ?

Haha, I've checked the leads and no 1 tdc at least 10 times. always good. I might mark my balancer at every tdc just for fun and games, then i can put timing light on  any lead.

i'll try the TDC valve lash, but before i do, i'll run a leak down tester over it all. if ones hanging open, that should find it.

I was also thinking, if i take off the carb and blank it off, run the leakdown tester and open up an intake valve, that should give an indication of any vacuum leaks?

thought i found the solution today. i pulled every lead of from the dizzy whilst running, not all at once, and there was no change in idle, so if it's every spark plug maybe the coil? the MSD blaster 2 had 5.7 ohm instead of the advertised 4.7, went and bought a Bosch un-resisted coil. same problem. inside drinking away now. it's really got to me.

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #9 on: January 16, 2020, 05:25:44 PM »
they are comp cams roller lifters and trick flow roller 1.6 rockers.

I'm sure i had them adjusted at 1/2 turn when i first assembled the engine. I've done it twice now at 1/2 turn, still no good.

I have heard of many people having issues with com lifters, that's why I asked. Get the valve lash adjusted first, you don't want to burn a valve from over adjusted valve lash and then try to ID which valve is ticking.

Offline flatchat

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Re: 302 misfire and no acceleration under load.
« Reply #10 on: January 18, 2020, 07:49:58 AM »
Purge all fuel and try some fresh fuel from a reputable supplier

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #11 on: January 19, 2020, 05:57:51 PM »
I was hoping it would be that simple, drained all the fuel, put some v-power 98, same problem, changed the dizzy to an old electronic points one I ran before the msd, can't even get it to run now.  :ouch:

I've checked the rods, none are bent at all. Leak down test was good. Minimal to no leaking on any valves, I then turned down the pressure, took carb off, blocked off the intake and opened the intake valve, almost 100% positive there are no vacuum leak on the intake manifold either.

I'm getting to the point where I'll pull the intake and heads. Just getting cranky now.

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #12 on: January 19, 2020, 08:55:27 PM »
Disconnect the distributor centre lead that comes from the coil and hold the bare centre about 5mm from a good earth should be a decent strong blue spark if its a weak yellow then you have an issue with your coil.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #13 on: January 19, 2020, 11:07:58 PM »
I did that with each plug the other day. It was definitely not blue. I did change out the msd blaster 2 with a bosch coil. Same problem. I'll try on the coil tomorrow. Cheers
« Last Edit: January 19, 2020, 11:12:22 PM by Slugger »

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #14 on: January 20, 2020, 12:06:23 AM »
The other issue you may have is timing, do you have a generic harmonic balancer? If so you may need a piston stop to determine the correct top dead center.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #15 on: January 20, 2020, 06:01:31 AM »
I have a summit balancer. I've taken no1 plug out, used the pressure to make sure it's on compression stroke, then watched the piston.

Offline 66FBK

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Re: 302 misfire and no acceleration under load.
« Reply #16 on: January 20, 2020, 09:03:36 AM »
I sounds like an ignition problem. Does it idle OK. If you have a 5L roller cam firing order and used a 302  not the 351 firing order for the plug wires , it will idle like a bastard or not even run. Little things like the engine to chassis earth lead not connected can cause havoc also. Good luck.
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Offline GLENN 70

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Re: 302 misfire and no acceleration under load.
« Reply #17 on: January 20, 2020, 10:50:39 AM »
Back off all the rockers so it rattles . Then set rockers with the engine running with 1 valve cover off at a time and a piece of cardboard sitting in the head down the side . Messy but give it a go . If you have an old valve cover around cut the top out and bolt that on so you can get to the rockers . Sounds like the rockers are too tight .  When you do a rocker put it down till it stops making a noise then give it a half a turn down .
« Last Edit: January 20, 2020, 10:52:22 AM by GLENN 70 »

Offline flatchat

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Re: 302 misfire and no acceleration under load.
« Reply #18 on: January 20, 2020, 10:18:21 PM »
What Glen said --"ole skool is best
another thought -- if you're running points in the dissy  then its more than likely the condenser that's carked it
over and out  :ouch:

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #19 on: January 21, 2020, 12:44:41 PM »
Msd 6a, msd pro billet distributor. Can't get it to Run now to adjust valves. Ordered a new rotor and cap, carbon button was worn down a fair bit. Then I'll hook it all back up and go old school on the valves.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #20 on: January 22, 2020, 03:04:04 PM »
New dissy cap and rotor arrived, so I slapped it in, gave the key a turn, and voila, same crap. Decided to take valve covers off and release pressure on the lifters whilst I go away for the weekend, but I did rotate the motor by hand, and every rod had some rotation to it when the corresponding valve was on base. So it couldn't be to tight. Right? And all rods are as straight as an arrow. I know because I wanted to throw them at something. Haha.

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #21 on: January 22, 2020, 04:34:33 PM »
So is it starting now or not? If it starts you may also have a vacuum leak.

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #22 on: January 22, 2020, 04:55:17 PM »
Starts but won't run at all. Guess I'll pull the intake next week.

Offline unilec5544

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Re: 302 misfire and no acceleration under load.
« Reply #23 on: January 22, 2020, 05:30:18 PM »
Starts but won't run at all. Guess I'll pull the intake next week.

Just make sure its not an issue with the carb first. Also the MSD may have a problem as well.
« Last Edit: January 22, 2020, 05:34:25 PM by unilec5544 »

Offline Slugger

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Re: 302 misfire and no acceleration under load.
« Reply #24 on: January 22, 2020, 06:39:42 PM »
Tried a mates qft, same problem, ran through all the tests for msd, everything good. Coil was a little high on secondary , got a bosch one still same problems. I've got spare intake gaskets here, and also slightly thinner head gaskets to get rid of the mls ones. May just be the time to do it.