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Flathead tuning and other issues.

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Old 07-18-2015, 01:51 PM
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Flathead tuning and other issues.

I'm having some issues with my flathead, and I'm turning to the smartest group I know for advice--you guys.

Here's what's happening currently:
The truck starts and idles fine. (At around 600 rpm). After I get it up out of my driveway and drive about half a mile, I have to stop on an incline before I can turn. Once I hit that incline, it starts to sputter, and will die if I don't immediately hit the gas. It does the same thing when I back down the driveway after returning home.


Here's what was going on before I started messing with stuff:

  • Carb was leaking fuel, and has been for some time now.
  • Motor started missing/running rough. Not often, but the last couple of times I had taken it out (before I started working on things, that is) it ran rough until it had warmed up, and then seemed be fine.

Here's what I've done
:
  • I rebuilt the carb the other day, to fix the fuel leak issue. This is the first time I've rebuilt one.
  • I emptied the sediment bowl on the fuel pump, as it had a fair amount of crap in it.
  • After rebuilding and reinstalling the carb, I made some adjustments to the idle rpm, dwell, and to the spark plug gaps based on the specs I've read here: Flathead Tuneup Specifications for 1949-53 V8 (239 & 255).
Here are some questions I have:
  • Where do you think I should be looking?
  • From the MacVP link, it stated that the idle rpm should be 475. Is that where I need to be? It's at 600 now, and was around 750 (or higher) for the past year. If I take it any lower than it is now, it barely runs.
  • What should the spark plug gap be set to? I've found some conflicts between my Motors manual and the MacsVP page. I've set them to .030
  • Same question for the dwell. I adjusted the point gap (it was around .010) to .016, and the dwell fell to 30, so I readjusted until it reached 28. I don't know what the gap is set to now, but it's certainly above .016.
  • Who's idea was it to let me work on a truck anyway?
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Old 07-18-2015, 01:58 PM
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Stock fuel pump?
Stock coil/ignition?
Is the incline up or down (front lower or higher than rear)?

38Coupe recommends setting the float level on Holleys quite a bit lower than spec to help with ethanol related issues. If the problem is on inclines, that sounds like spillover from too high a level.

With stock ignition, .025 on plugs. No real benefit IMO to go higher, but a stronger hotter spark may mask an overrich condition. How do the plugs look?

You're OK on dwell. Did you re-time after setting dwell? You need to. Are you getting vacuum advance?

475 RPM may be spec, but most usually idle better around 600. Have you adjusted the idle mix screws? Put them lean (screw in) until it starts to sag, then out only enough to reduce missing.
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Old 07-18-2015, 02:11 PM
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Quote:
Originally Posted by ALBUQ F-1 View Post
Stock fuel pump?
Stock coil/ignition?
Is the incline up or down (front lower or higher than rear)?

38Coupe recommends setting the float level on Holleys quite a bit lower than spec to help with ethanol related issues. If the problem is on inclines, that sounds like spillover from too high a level.

With stock ignition, .025 on plugs. No real benefit IMO to go higher, but a stronger hotter spark may mask an overrich condition. How do the plugs look?

You're OK on dwell. Did you re-time after setting dwell? You need to. Are you getting vacuum advance?

475 RPM may be spec, but most usually idle better around 600. Have you adjusted the idle mix screws? Put them lean (screw in) until it starts to sag, then out only enough to reduce missing.
Stock fuel pump.

Coil/ignition, I don't know for sure. PO converted the truck to 12v. Do those get changed for 12v conversion?

Up. Front end is on the higher elevation.

I'll take a look at the float and readjust it. Thanks.

All the plugs were coated with carbon. Pretty consistent across all of them. I should have mentioned that earlier.

I did not re-time, as I don't have a timing light. (yet). I'll pick one up today and I'll learn how to set timing.

I did adjust the idle mix screws, and that's how I settled on around 600 rpm.

Thanks!
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Old 07-18-2015, 02:27 PM
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The coil probably got changed as part of the 12v conversion (plus added ballast?), but what I was after is that you don't have a high energy system (pertronix or MSD).

Definitely sounds like sloshing. There is a vent tube from the fuel bowl to the throat of the carb going front to back. You really have to have a high level to get it coming out of there, tho.


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Old 07-18-2015, 03:17 PM
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How is your gas tank?.........crud in the fuel pump bowl does not sound good......I played with mine for years before I put in a new tank and problems went away
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