360 FE idle vacuum issue

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01-10-2014, 06:36 PM

mgreen

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360 FE idle vacuum issue

Hi there folks. New member here. I recently purchased a 1968 F250 2x4 manual with a 360 2v FE. Pretty stock as near as I can tell except the HEI. One thing that has me perplexed is that the vacuum advance port on the carburetor will pull vacuum at idle unless I have the mixture screws way out, like six or more turns out. This seems crazy to me so I thought I would it run past you for some things to try or to check. The reason this is an issue is that the vacuum will advance my distributor at idle and sets up this chain reaction where I cannot get anything tuned or timed right. Frankly, other than the idle mixture screws at risk of actually falling out from vibration because they are so far out, I don’t really see a down side to this and it runs ok. There has to be a reason for this though.

Here is some detail on what I have done already.

1 - Set timing at about 12* @ ~800 rpm with vacuum advance removed and plugged.
2 - Turned mixture screws out until vacuum gauge read “0” at carburetor port.
3 -With vacuum advance still disconnected I moved the gauge to manifold vacuum port and ran the mixture screws in to see what effect they had on vacuum. As the screws went in idle fell off so I had to adjust the idle linkage screw (which dramatically increased carb "ported" vacuum).
4 - Rechecked carb port and found it had gone up again so I ran them back out until no vacuum reading on carburetor port and then even more until I was able to get the manifold vacuum to its peak of around 18 inches at idle.

All I can think of is either a manifold leak or crappy carburetor. The carb was rebuilt by some outfit in Canada but I disassembled it anyway and blasted with compressed air. Also I have sprayed practically a case of carburetor cleaner, WD40, butane and even starting fluid on this thing looking for leaks and found none with that method. I also plugged the PCV and brake booster hoses to test for leaks there and found none.

Any advice would be helpful.
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01-11-2014, 09:27 AM

lee

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Hopefully someone with more Carb knowledge will chime in, but in the mean time.....

Since you have already set the initial timing, now setup the Idle circuit.

1. Gently screw the Idle screws in all the way. I say gentily because you don't want to damage the "seats". Next back each screw out 2 1/2 turns. Crank the truck. You may have to adjust the Idle speed screw to get it to idle. Just make sure you get it to Idle below ~800rpm.
2. Next hook up yer Vac gauge to Manifold Vacuum.
3. Adjust each Idle screw (equally) for the highest manifold vacuum reading. You may have to reduce the Idle speed if idle gets to high while performing this.
4. When highest Vac to obtained. Set Idle speed.

There are many theories as to which Vac signal (Ported, Non-ported) should be used for Vac Advance. Try each one and see which is best for yer driv'un style.

Take care,
Lee

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01-12-2014, 11:05 AM

krewat

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lee has you covered on setting everything up right.

One thing I'd like to add is that when the ported vacuum-advance port has vacuum, that means the throttle plates are open too far.

Are you SURE the timing marks are correct on the dampener? Sometimes the ring slips, and what you think is 12 degrees isn't

When you say there is vacuum at the advance port, and you play with the mixture screws and it goes away, are you adjusting the idle at the same time? Or does the vacuum just go away by playing with the mix screws?

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01-12-2014, 12:57 PM

mgreen

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Krewat and Lee, thank you both for your replies.

Quote:

Originally Posted by Krewat

Are you SURE the timing marks are correct on the dampener? Sometimes the ring slips, and what you think is 12 degrees isn't

are you adjusting the idle at the same time? Or does the vacuum just go away by playing with the mix screws?

It is the case that as I run the mixture screws in I need to readjust the idle screw to restore idle speed. As you mention, when I run the idle screw in it opens the throttle plates and increases the vacuum on the port. So it is this combination of mixture screw and idle screw settings that allow me to get the ported vacuum to zero. It only works if the mixture screws are out 6 or so turns.

Regarding the timing marks slipping. I had wondered about that too but it just seemed to remote a possibility so it was not at the top of my list. I do intend to measure TDC of #1 once I get my hands on a dial indicator or something (not sure I could tell if I am in within 10* of TDC with my fingers by just feeling a pencil stuck in the plug hole?). I can say that at one point during my fiddling with this I set the initial timing (with vacuum advance connected) at like 22* and as I drove it around I never heard it knock. The thing must of been advancing to like 60* at the time. Perhaps the slipping damper theory has merit...

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01-12-2014, 11:47 PM

orich

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Your idle should be at (550) rpm for auto trans: And (650) rpm for standard trans:

you should have the carb vac. port plugged while setting the timing. while your rpm is at 550.
The vac gage goes to Manifold port vac only for carb air/fuel best setting.

Also make sure choke is not on any of the choke steps of the ladder adjustment.

You don't need to plug the dizzy vac advance just carb port.

If your guessing about your rpm idle while making adjustments then that maybe your problem...orich

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06-12-2014, 10:37 AM

mgreen

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Issue solved

Just thought I would update this post I started a while back.

After playing with every conceivable setting to no avail (timing, advance, idle, etc..) I ended up buying a rebuilt carburetor off Amazon (CDFDEPOT - I would recommend them) and I think that was the issue. The idle circuit screws are now set about 2.5 to 3 turns out so a much more normal setting. It fixed the flat spots and to some degree the loud hissing sound at certain throttle positions although some hiss is still there. Additionally I don't need to modulate the choke to get it to run smoothly when warm.

I drove the truck from Seattle to Santa Cruz pulling a 2,800 lb trailer plus a 800 lb Harley in the bed and got 11.1 mpg at an average speed of about 50 mph so I am calling it good enough.

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