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240 misfire

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Joined: Mon Mar 11, 2019 6:09 pm

240 misfire

Post #1 by Andyman54 » Mon Mar 11, 2019 8:05 pm

Hello all I have been working on a 1972 f100 for a little bit slowly getting it together. And so I started with a new carb. Started it up and it worked good at idle but under a load it starts missing all the time and I can barely go anywhere with it.

I have had similar issues with a inline 4 boat motor and so i decided to check the hydraulic lifters, opened the cover up and took one out i could not push it in turns out all the lifters were frozen so i replaced them all with new. The bottoms had no major pitting or anything so i do not believe any damage was done.

Fixed that issue nothing changed but it did run a bit better.

Next change spark plugs put a set of new plugs in, and new wires! No change

Also I changed my fuel pump out that was dead in the very beginning so fuel pressure is good as well.

Check timing/ Change distributor .

I bought a new HEI Distributor with a coil built in. Set cylinder number 1 at TDC pointed the distributor in the right direction and it fired right up like a champ runs great when not in gear! Checked it with a timing light everything seems to be correct. The engine is running better than it ever has.

THAT still did not fix the issue... I looked further and started spraying around the carb with carb cleaner to see if i had a vacuum leak somewhere. I did have one so I fixed that issue but no dice. I have also riched the carb out and leaned the carb in it doesnt matter which way i set the screw it will continue to poppopopop through the carb when i try to accelerate.

anyone run into this issue?

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Re: 240 misfire

Post #2 by MechRick » Mon Mar 11, 2019 9:32 pm

Sounds like an exhaust valve not opening. Might want to pull the valve cover, disable the ignition and crank the engine over while looking at the valve train...
1994 F150, 4.9L/ZF 5 speed, C-Vic police driveshaft
EFI head w/mild port work, 3 angle valve job
1996 long block, stock pistons, ARP rod bolts
Stock cam, aluminum cam gear
Hedman header, full mandrel bent duals, crossover, super turbos
Bronco II with a 2.3L swap
1988 F250 2x4, 460 ZF 5 speed.

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Re: 240 misfire

Post #3 by 68Flareside240 » Mon Mar 11, 2019 10:26 pm

I had a similar issue. I rebuilt the carb, set timing, etc. Ended up being the new wire set was not making good contact with the distributor cap. The rubber boots were too far down, next to the terminal. It would idle fine, but under load it would miss and spit. Got them seated and no problem.

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Re: 240 misfire

Post #4 by Blairsville Ed » Tue Mar 12, 2019 8:39 am

A good starting point is to check the mechanical condition of the motor.
Do you have reasonable compression on all the cylinders.
Is the valve timing correct.
Is the Valve lash or lifter pre load correct.
Is the transmission working correctly.
Is the oil pressure within specs.

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