Misfires when warm
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Joined: Feb 2007
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Misfires when warm
Help!! I have a 74 F250 highboy with a 360ci engine. The truck runs great for about 5 minutes then it starts missing so bad that it dies. It has good compression 150 across all cylinders. The fuel tank has been purged and new fuel and filters have been added. It is timed at 10 before top dead center.
The engine has points and they appear to be in good condition (no pitting )
I am at a lost as to why it runs so smoothly for 5 minutes then misses so badly that you can't keep it running. Any help would be greatly appriciated. Frustrated in CA.
The engine has points and they appear to be in good condition (no pitting )
I am at a lost as to why it runs so smoothly for 5 minutes then misses so badly that you can't keep it running. Any help would be greatly appriciated. Frustrated in CA.
Cargo Master
Joined: Dec 2004
Posts: 2,232
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From: NE Texas
I FULLY expect that the problem lies in the fuel system. Are you SURE that the fuel line screen in the tank is clean and clear? If not, this will cause exactly what you describe. Also a fuel tank cap with no vent hole could cause what you describe after several minutes of high fuel demand driving, but I don't think it would cause trouble after five minutes of idling.
Hope this helps,
Hope this helps,
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Wow! Fast responce. The plug wires, plugs, cap and rotor are all new. The coil is not. The coil is an aftermarket (Accel) and I don't know the condition of it. They are not that expensive so, no problem replacing it just to eliminate it as a possible culpret. The Fuel cap is a new OE bought from LMC and the tank is vented to the charcol canister. But the fuel screen is something I know nothing about. So this screen you are describing is on the end of the pick-up tube inside the tank??? Gary
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Joined: Jan 2008
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Coils do fail after years of service.
Here is the test for it.
http://www.fluke.com/application_not...AGID=1&SID=103
Here is the test for it.
http://www.fluke.com/application_not...AGID=1&SID=103
Fleet Mechanic
Joined: Aug 2007
Posts: 1,806
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From: Piney Woods of East Texas
Is this an automatic Choke?
It takes about that long before it starts opening up. Other wise I'm with the other on the coil and fuel screen.
It takes about that long before it starts opening up. Other wise I'm with the other on the coil and fuel screen.
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It is an automatic choke. If that is the problem I should be able to force it open and it should smooth out, correct??? The cold coil resistance on the primary is 1.6 ohms the secondary is 7900 ohms. I think that is ok. The test will be when its hot, I will run it tomorrow and then check it and report back. I took off the fuel line for the tank. there is no screen in the tank, at least not one I can see. Great responce, thanks for all of your help. ( I NEED IT. )
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From: Emmett, ID
I would have to agree with blue68f100, i actually got my ford because it was doing the exact same thing...i took off the stock carb bec. i didn't like motorcrap and put on my eddies carb and it has never done it since.
just curious, is auto choke the same as electric choke???
just curious, is auto choke the same as electric choke???
Fleet Mechanic
Joined: Aug 2007
Posts: 1,806
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From: Piney Woods of East Texas
I do not recall when the electric chokes came in. Some of the older one (60's) used a heat tube and these would burn off and plug up. They were made of steel. I use to replace them with copper. It used a very small vacuum to pull the heat from the manifold to the carb.
If electric, the heater element can fail and not open the choke up. As a engine heats up it would flood an engine and kill it. If you were going down the road there was a lever that forced the choke open so it would not flood.
Just have the breather off and look at the top of the carb and see of the choke is closed. Then start the engine and watch it. The choke heater is on the rt side of the carb. It will have a wire running to it. And some had a ground to connect to.
Auto check is any choke that open on its own. Yes, there were several types. My 68 has a manual, controled by cable inside the cab.
If electric, the heater element can fail and not open the choke up. As a engine heats up it would flood an engine and kill it. If you were going down the road there was a lever that forced the choke open so it would not flood.
Just have the breather off and look at the top of the carb and see of the choke is closed. Then start the engine and watch it. The choke heater is on the rt side of the carb. It will have a wire running to it. And some had a ground to connect to.
Auto check is any choke that open on its own. Yes, there were several types. My 68 has a manual, controled by cable inside the cab.
Last edited by blue68f100; Aug 20, 2007 at 08:37 AM.
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Misfires when warm.
Well I ran truck this morning, the coil is fine, the ohm reading was equal to the cold reading. 1.6 primary and 8000 secondary. The choke opened up after a couple minutes, so it is working. The vaccume is 17psig, I think that is ok, its not jumping, stays right at 17. One thing though, I was looking in the carb while the truck was running and it appeared there was quite a bit of fuel in there (puddled in the bottom). Shouldn't the fuel be atomized, even while idleing???
Still frustrated in CA.
Still frustrated in CA.
Fleet Mechanic
Joined: Aug 2007
Posts: 1,806
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From: Piney Woods of East Texas
The 2100 is a 2 barrel carb. You should not have any fuel above the butterfly. I have seen water condense if the conditions were right.
With the vacuum at 17 which is low, recheck the timing. Run a compression test, may have valve leaking.
Take the distributer cap off and look inside. You may have some carbon tracking. Carbon tracking allow spark to goes to adjacent plug wires. Move the rotor and see if it springs back.
With the vacuum at 17 which is low, recheck the timing. Run a compression test, may have valve leaking.
Take the distributer cap off and look inside. You may have some carbon tracking. Carbon tracking allow spark to goes to adjacent plug wires. Move the rotor and see if it springs back.