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Unread 06-19-2013, 11:31 PM   #1
xnodesirex
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distributor and vacuum advance

Help me oh jeep gods.

Finally got my 304 with a 470 Holley T/A timed correctly at initial (~6 BTDC) using a timing light, with vacuum advance disconnected and plugged. Runs a little high at idle (~850 RPMs), and this produces around 16g of vacuum.

now, when connecting vacuum, it stumbles like a drunk and then dies quickly. Just for kicks, I backed out the idle mixture screws, which SLIGHTLY helped but eventually brought about backfiring out the exhaust.

Tried the ported spark on the side of the carb - and it's driving vacuum at idle (~7-9g). So that's a little weird on its own, but same exact problem. I know the consensus is to use manifold, but why on earth is it causing my engine to die within seconds of turning over. Without vacuum its totally fine to idle.

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Unread 06-20-2013, 04:27 AM   #2
keith460
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Sounds like you may have an internal vacuum leak inside the distributor vacuum Advance Unit. If the vacuum advance line is disconnected and "plugged" and it runs and idles fine, but when connected back up it stumbles, it could be leaking internally. Another cause may be that the Advance Unit may be adjusted incorrectly or the springs for the centrifugal advance plate are so weak and worn or missing.
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Unread 06-20-2013, 05:10 AM   #3
Matt1981CJ7
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What cam are you running?

If it's stock, you should be drawing more than 16" at idle. 18-20" is more like it at your elevation.

You shouldn't be drawing any ported vacuum at idle. Throttle plate positioning is usually the culprit.

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Unread 06-20-2013, 05:45 AM   #4
SuperSportCJ
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I'd have to agree with Keith. Try unplugging the vacuum advance line but do not hook it immediately up to the distributor allowing it to leak vacuum and see it it idles and dies the same as when it's hooked up.

I have the same engine and carb timed around 6 BTDC as well with a little larger comp cam and I have 16" vacuum at idle. My throttle plates are barely off the stop to idle at about 750 rpms.
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Unread 06-20-2013, 08:09 AM   #5
swatson454
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What distributor?


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Unread 06-20-2013, 11:36 AM   #6
xnodesirex
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Working on this engine, despite simple parts, has me right about to the point of lighting it on fire or junking it. Replaced EVERYTHING, and it still is nothing but problems.

Brand new re-manufactured distributor from autozone. Old one definitely had warn springs, and a very weak vaccum advance (you could move it by hardly sucking).

No idea what the cam is. PO made a few changes, and I'm not quite sure if he changed the cam or not. New intake, new valve covers, etc. Said it was "rebuilt" but that is a questionable comment. Not sure how it was running when I got it as it was timing at over 60 at idle (with vac advance plugged in).

Checked the new one before installing, and it took quite a lot to move the advance - damn near saw some stars.

No leak in the vaccuum lines. I can get to 18hg, but at a much higher level of idle.

Not sure about the spark vacuum - I'll check the throttle plates, but it's basically stock from Holley out of the box. Only thing thats been adjusted is the floats and idle screws. Their whole "pre-adjusted" mantra is sheer bull****.
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Unread 06-21-2013, 06:53 AM   #7
GlennLever
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I agree with Keith, it is easy to check, put a hose on the vacuum advance on the distributor, suck on it and put your tongue over the hole in the hose. The "sucked" vacuum should not leak off, if it does replace the vacuum canister on the distributor, they are cheap.
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Unread 06-27-2013, 12:04 AM   #8
xnodesirex
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Didn't even get that far as Glenn suggested.

Now back to square... 0.

Replaced the carb to intake studs to make sure the carb was torqued to spec. One of the old ones was stripped, so there was a possible vaccuum leak (though starter fluid didn't show any acceleration).

After doing that... engine idled HIGH (1300 RPM). So... now trying to figure out why the timing is off, idle is too high (idle screws are basically bottomed out), and how much more it will take until i start losing hair.


Engine idles super high now 1300 RPM. Adjusting the timing by rotating the distributor gets it down to about 1000. Engine SOUNDS closer to normal, but still remarkably high in RPM. Idle mixture screws can't go any lower, so I'm kind of stuck. I do know TDC is correct, and the distributor was installed correctly, but I'm miffed as to what's going on NOW.

Any suggestions?
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Unread 06-27-2013, 01:06 PM   #9
SCSEAJAYSEVEN
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Pull the carb off and check your throttle butterfly plates. They may be inserted through the throttle shaft backwards or not fully seated in the shaft. Also check for sloppiness of the throttle shaft where it pivots in the carb. Could be that the holes or bushings are worn so bad causing a vacuum leak.

There should be a small screw that secures the throttle plate(s) to the shaft. They maybe missing, causing the throttle plates to move slightly and not allow the plates to close in the right position.

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Unread 06-29-2013, 12:14 AM   #10
xnodesirex
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Quote:
Originally Posted by SCSEAJAYSEVEN View Post
Pull the carb off and check your throttle butterfly plates. They may be inserted through the throttle shaft backwards or not fully seated in the shaft. Also check for sloppiness of the throttle shaft where it pivots in the carb. Could be that the holes or bushings are worn so bad causing a vacuum leak.

There should be a small screw that secures the throttle plate(s) to the shaft. They maybe missing, causing the throttle plates to move slightly and not allow the plates to close in the right position.

SCSEAJAYSEVEN
I'll give it a check.

If that's the case, I'm going to be severely pissed at Holley. This thing has seen ZERO miles on the road thus far....
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Unread 06-30-2013, 10:46 PM   #11
xnodesirex
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updates!

- replaced the gasket under the carb, reinstalled the carb and retorqued to spec.
- While the carb was off, I also checked the intake manifold to ensure that it was torqued to spec (it was mostly).
- While the carb was off, I backed out the curb idle screw, and the fast idle screw so they wouldn't influence anything. I noticed a slight/minor gap between the throttle butterfly and the walls. If I pushed slightly on the throttle level the wrong direction, this gap closed.
- Installed an extra spring to hopefully help close this gap.
- Adjusted the float level to the proper position (may have been set too high).

Net - Engine idles MUCH better and sounds more like a V-8, timing is still at 6-8 BTDC, but still idles high (950 RPM measured on an old external tach meter) and vacuum is at 15. There is a minor wiggle in the needle which has me a hair concerned. I'm at less than 1000 feet elevation, so this seems TOO low. Idle mixture screws don't bring this down before the engine dies - so something else is going on? Additional leaks?

Second, I tried Glenn's great suggestion and it held the vacuum against my tongue. Granted it wasn't a perfect measurement, but it worked well enough. When attached to the manifold vacuum - there is a strange added sound. Like ball bearings rubbing or something, that comes from the distributor. Definitely doesn't seem right to me. This causes the engine to idle higher (1050-1100 RPM) and I believe it causes the vacuum to drop lower (memory is a bit fuzzy on that last part).

Any ideas on all of this. I'm beyond stumped at this point.
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Unread 07-02-2013, 12:25 PM   #12
xnodesirex
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Anyone??
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Unread 07-02-2013, 06:42 PM   #13
fourbtgait
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Your vacuum is to low at idle for your elevation. Vacuum leak someplace?

The idle speed will increase naturally when hooked to manifold so just adjust it down on the carb.

Set your timing independant of your idle speed. Two separate things.
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Unread 07-03-2013, 09:48 AM   #14
xnodesirex
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Quote:
Originally Posted by fourbtgait View Post
Your vacuum is to low at idle for your elevation. Vacuum leak someplace?

The idle speed will increase naturally when hooked to manifold so just adjust it down on the carb.

Set your timing independant of your idle speed. Two separate things.
Thanks four.


Idle doesnt change (much) at the screws until it cuts off - so fuel is leaking somewhere, though I can't find where. The throttle plates are fully closed though.

Timing is slightly independent - if the idle is too high, trying to confirm timing isn't going to make sense. Since I can't get it down much below 1000 RPM, the timing is definitely coming in too high at this point.
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