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Finally got everything, I thought, with my 1951 Dodge Dump fixed & decided to make a couple trips to the rock quarry.  No issues, made (2) trips.  After that, I took an hour break.  Decided to make (1) more trip.  On the way back it started to miss.  I had to let off the gas, coast awhile amd them resume.  Could not give it full throttle w/o it bucking & chucking.  I just installed a SS heat shield, hoping that would do the trick, but not the case.  could this be a timing issue?  I recently converted to 12 volts & installed Pertronix kit.  I have had the mising issue in the past, before converting to 12 volts.

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If letting off the gas for a bit temporarily cures the problem, it sounds like a fuel delivery issue.  The carburetor may be running dry, and then letting off the pedal lets it build up a little more fuel in the bowl to run again. Replacing or rebuilding the fuel pump couldn't do any harm. Often they become marginal...sometimes working well enough , sometimes not. Along the same line, you might check any fuel filter that you have, as well as a restricted fuel line. The short flexible line that connects the pump with the steel line to the tank can fail. It's under suction, so it can collapse internally.

Your pump should be good enough to send a steady stream of fuel to the carburetor with just the starter motor turning over the engine. Disconnect the line at the carb and see if the pump is getting gas there. Don't be fooled just because you see gas in the glass sediment bowl. It can be full even though the carburetor is dry.

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At what temperatures is this happening?  Did you possibly change fuel caps and now you have one that’s not venting correctly?  Sounds a lot like the issue I was having awhile back

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  I'm pretty sure it is a 251 or 265 cu. in.  (#T318-8488)  (B3J)  & it does this at normal operating temp, 180 Degrees.  Do not know what timing is supposed to be set @.  Can someone help me out there?  I did buy a replacement fuel cap from DCM, but it does it no matter which cap I use.

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My idea is purely basic fuel delivery.... lets clear that first.

I did a bad thing and I added fuel to my original tank .... it ran great for awhile. Then the floaters would plug the lines and stop fuel flow .... Let it sit for awhile and the floaters would settle and clear line again. Then it would run out of gas again as the fuel line got clogged.

 

This describes exactly what you are dealing with ... Could also be other issues. Start with the basics, what shape is your fuel system in?

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As I suggested in an earlier post, disconnect the fuel line at the carburetor. Turning the engine over with the starter, there should be a good stream of fuel being pumped. If there isn't, you have a fuel problem between there and the tank. Basic troubleshooting is quicker and easier than speculating. Checking for fuel supply is a good first step.   

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Have done that & ruled it out.  I have good supply of fuel to the bowl.  I rebuilt the carb as well.  Thought that might be the issue.  No such luck.  I have had this problem the whole 25 years I have owned it & have just put up with it.  Tired of settling. Want to be able to go get a load of dirt or gravel & not worry about making it back to the shop or not.  I did the nasty & converted it to 12 volts, but it starts a heck of a lot easier. 

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Maybe check that wire that goes from coil to distributor ground, easy enough to replace with a fresh conductor...also, I have found that as battery cables age, outward appearance may be OK but corrosion will form between the conductors inside the insulation, driving impedance sky high especially with heat soaking...if replacing that ground cable, a performance upgrade is replacing a starter bolt with a stud, then routing the ground cable to that stud.  Another area of interest is any excessive play in the distributor shaft could affect ignition timing, as that shaft would be kinda bouncing around instead of rotating accurately...I'm assuming that the ignition coil is new and matched with the pertronix unit...

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If it stops the misfire when load is reduced but immediately misses when the throttle is opened it could be ignition.  Hard to 'feel' it from here, but a fuel delivery and spark problem feel completely different.  Ignition issues generally change very quickly when the throttle is opened or closed.  Fuel delivery is slower to react. 

 

When I was working on the older stuff a lot, spark plug wires, condenser or coil issues would almost always show when loading the engine heavily, wide open throttle, high gear and 40MPH or so and was intermittent and jerky..   Fuel delivery was usually an issue above that and appeared as a loss power but not a sharp jerk or miss.

If you have any doubts about the ignition, I's recommend a full tuneup, wires, plugs, check points, except the condenser unless you know it's bad.  I've installed more bad new ones than removed bad old ones.  My plan is a change to a slant six modded  distributor, electronic ignition and no condenser.

 

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3 hours ago, 51 Dodge Dump said:

  I'm pretty sure it is a 251 or 265 cu. in.  (#T318-8488)  (B3J)  & it does this at normal operating temp, 180 Degrees.  Do not know what timing is supposed to be set @.  Can someone help me out there?  I did buy a replacement fuel cap from DCM, but it does it no matter which cap I use.

 

timing for the truck 251, with no vacuum advance, only mechanical, is 2* after TDC, not before TDC.  when i recently installed a 251 into my 1.5 ton to replace the blown 237, i initially set the timing at 2* before TDC.  started decent, idled excellent, and had good power on flat ground, but would have no power on hills.  it also would miss, and get hot pulling those hills.  when i checked the timing, i discovered my mistake, and set it to 2* after TDC.  runs cooler, and has power for the hills.

Edited by wallytoo
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My thought -

 

carry a volt meter with you and this issue comes up, stop and check voltage to the coil while still running. Low volts can cause this.

 

It may be getting very low due to heat in wiring, ign. switch, other issues?

 

DJ

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Just because a carb has been rebuilt and is getting fuel does not mean it is not the problem. I worked and worked on my original carb and my truck ran. But it was never quite right no matter what I tried or did. I ended up finding a NOS replacement and put a fresh kit in it and no more problems.

The point I am trying to make is that on going issues like this generally have a "part" that we assume is OK.

Jeff

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