Engine misses when hot

J

Joe Immler

Well-known member
The engine on my '41 Ford misses when the engine temperature reaches 180 degrees. When the temperature reaches 190+ degrees the miss is severe. Now that Fall is here and the ambient temperature is cool it runs fine. Today it is 50 degrees and I went for a 20 mile drive city and highway. The engine temperature peaked at 140 degrees and it ran fine. I put a piece of cardboard in front of the radiator and the temperature reached 180 and 190 degrees and the miss returned. Here is what I have done this summer: The gas tank is clean,there is a complete new fuel line with an Airtex electric fuel pump wired to run as needed (it makes no difference),new flex line to fuel pump,new mechanical fuel pump,new carburetor,Skip Haney coil (2 tried),new NAPA condenser (3 tried), new distributor caps and spark plug wires as well as plugs,distributor rebuilt and set up on a KRW machine,all wiring related to ignition seems to be OK. I tried the ignition switch by-pass by running a wire from battery to coil.
I am at the end of my rope. HELP!!!!
 
have you done a vacuum check? cold? hot (180)? could be a sticking valve as temperature tends to make skip present--run a little MMO down the carb when it is sticking/missing
 
Alan I did do a compression test but have not tried vacuum. The compression read :
FRONT
105 95
98 105
90 105
92 100
The engine had a valve job last year and has 793 miles on it as of today. I will try your suggestion and use MMO
 
I recently had my engine rebuilt and started getting similar problems. I found all the exhaust valves were being held open when the engine got warmed up. Not sure how the clearances were lost yet, it's in the shop. Compression numbers were not as good as yours, but remember (I assume) that the test is done cold.
 
Ross Yes the compression was done cold. Let me know what the shop finds. Am going to do the vacuum tests tomorrow.
 
One other thing to look at is the mixture.
My 34 would get a slight miss when hot on the highway,took a LONG time to find it,main jets too big.
Lawrie
 
Since you have checked the ignition and the contacts and the engine doesn't skip when cold but at operating temp has an intermittent skip, My first approach is to take your ride for a test drive unit the problem develops. At that time take a plug check to check the fuel burn on the plugs. Most of the new ethanol fuels have a lower vaporization rate and in a flathead the fuel gets pretty hot in the manifold. This evaporation produces a lean mixture and the engine pops. The plugs will exhibit a very white clean burn. My choice is to richen the jetting up by 2 points on all ethanol fuel engines which richens the mixture up to prevent lean conditions and this popping.
 
Dick and Lawrie, I have checked these items. Plugs are the right color. This miss is very severe not just a slight miss. I have also put the old carb I had back on and it made no difference. The engine ran well with this carb before. The Ethanol crap is the reason I started out checking and replacing all the fuel related items. Thanks for your replies.
 
Yes I did richen it very slightly. I tried various settings throughout this ongoing problem with both carbs. Did not make any difference
 
No I dont mean adjusting the idle screw. Turning this screw will have no bearing on the performance after idle. you have to physically change the jet size.
 
Did you do a compression test right after the rebuild, that you can compare to? Also, what power valve are you using? (#)
 
DICK SPADARO said:
No I dont mean adjusting the idle screw. Turning this screw will have no bearing on the performance after idle. you have to physically change the jet size.
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This is not true. The idle screws still contribute plenty at around town and cruising speeds.
 
Flatjack not so, The idle screw adjustment is just what it is, idle to off idle fuel supply. Once the throttle is opened up the air pressure suction moves from below the throttle plate to above it. This pressure drop initiates the main flow of fuel from the main jet and almost eliminates the flow of fuel thru the idle circuit, at very low rpm ranges yes some fuel flows thru the idle circuit but the porting is so small that not enough will run thru it to make the engine run efficiently at low to high speed. Too many people think that just turning the idle adjustmet screw is the correct adjustment for fuel metering at driving speeds, its not. The original poster has a lean run issue I believe and is not addressing it correctly.
 
I would suggest the use of a vacuum gauge , compare the cold and hot readings .
This will isolate between a mechanical and electrical problem.....
 
I stand by my statement, although I shouldn't have used the word plenty. But around town and light load conditions the idle circuit is still functioning and contributing to the mixture.
 
flatjack9 said:
I stand by my statement, although I shouldn't have used the word plenty. But around town and light load conditions the idle circuit is still functioning and contributing to the mixture.
Click to expand...

I've seen this too; too lean at idle will cause some hesitation in the 30 - 40 mph cruising range.
 
ROSS The power valve I used was Mac's PN:78-9904 Holley 30338H
The new carb- I have no idea what is in it.
BUBBA I will do the tests in the next few day.Out of towners visiting for a few days so I am out of the Man Cave for awhile.
 
OK I did the vacuum tests and here is what I got:
Idle cold 18Hg
180degrees 16 Hg
190 degrees 16-17 Hg Rapid oscillation
210 degrees same result
190 degrees after driving same results
So...valves?
 
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