25 T won't run when warm

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Model T Ford Forum: Forum 2012: 25 T won't run when warm
Top of pagePrevious messageNext messageBottom of page Link to this message  By Nate koetje on Saturday, September 22, 2012 - 01:26 pm:

Have a 25 T that ran great all last summer. This spring it started up and ran until engine gets hot. It starts backfiring, running rough until it dies. If I let it sit for a couple hours it will start and run great until its warm again. I have had carb rebuilt, flushed radiator and engine block (it doesn't seem to be over heating). I also checked compression cold and hot and remains constant. Anyone have any ideas?


Top of pagePrevious messageNext messageBottom of page Link to this message  By john kuehn on Saturday, September 22, 2012 - 02:19 pm:

Could it be its vapor locking? Starving for gas?
Shorting out in the coil box? A crack in the valve area in the block? PLugs? Those are things I would check.
Its probably something simple.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Saturday, September 22, 2012 - 03:04 pm:

Are you running the coils, timer, and magneto? If you are running a distributor, it could be a bad ignition coil. The coil could go open when it is heated. Less likely with coils and timer because you have 4 coils and even if one went bad it would still run on 3 cylinders. If it doesn't run on magneto when hot but does run on battery when hot, it could be a magneto problem.

From your list of symptoms I would suspect fuel starvation. Somewhere in the fuel system between tank and carburetor there could be an obstruction. Does it happen with a full tank or just when it is low? If it happens just when low, it is related to pressure in the line. Vapor lock is caused by vaporized fuel or air in the line. Vapor is compressable so if you have a high place in the line between the tank and the carburetor the vapor rises to the highest point and compresses. The fix is to have the line go uphill from the lowest point to both ends. Uphill to the tank and uphill to the carburetor with no high points between. One other problem is a grose jet. This is a ball valve fuel inlet float valve into the carburetor. The cure here is to replace it with a needle valve.
Good luck. I hope you find the solution.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Nate koetje on Saturday, September 22, 2012 - 03:24 pm:

Thanks for input. I thought it may be a fuel vaporizing issue also. I ran a exilirary tank in the cab through the fire wall directly to carb and got same results as using the actual fuel tank and line. Sorry I should have put in original post. I am still using still using coils and timer set up. Maybe I will pull head off and look for cracks in block


Top of pagePrevious messageNext messageBottom of page Link to this message  By John T. Tannehill on Sunday, September 23, 2012 - 11:35 am:

1-Sounds Like it could be a cracked valve. Do you have SS valves? I had a Fordor that had old two piece valves, Did the same thing you described tried everything except removing the head to look at the valves and sure enough number 4 had a crack in the valve. It didn't start running rough until the engine got hot. Back then I was more of a novice than I am now and didn't know about checking the compression in the cylinder after it was warm that would have pointed to a bad valve 2-Do you use a carburetor air heater? I live in the south and I found with another T model that it caused a similar effect on my engine. I removed it and no more issues. Hope my limited experience/knowledge helps some.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Sunday, September 23, 2012 - 11:41 am:

It almost has to be a carburetor problem or an ignition problem. A cracked valve, for example, would only make one cylinder run poorly. Same goes for a cracked block. Any T will run on 3 cylinders, or even two.

I think you need to find out for sure what you don't have when the engine won't start. My bet is fuel. Have you tried a different carburetor? When the carburetor was rebuilt, was it with a Grose Jet?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Andrew Deckman, Ogden Utah on Sunday, September 23, 2012 - 01:38 pm:

check the whole fuel system from the sediment bowl, the fuel line, and input to the carb.


Top of pagePrevious messageNext messageBottom of page Link to this message  By keith g barrier on Sunday, September 23, 2012 - 01:48 pm:

Nate, try draining your sediment bulb at the tank and the carb bowl, I have had some problem with this in the past and would find water in both. KB


Top of pagePrevious messageNext messageBottom of page Link to this message  By Erich Bruckner, Vancouver, WA on Sunday, September 23, 2012 - 03:57 pm:

How long is the run time? I am liking Royce's idea and it seems to fit if the fuel inlet valve (float valve) is not kosher the carburetor bowl will start out full. Engine will run till fuel level gets to low from blocked or slow filling, and it sputters and dies. Waiting a bit lets the bowl slowly seep back to full, repeating the cycle.


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