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Carburetor Problem?

Posted By charliemccraney 5 Years Ago
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charliemccraney
Posted 5 Years Ago
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I have a fuel cell.  I have 2 screen filters installed.  It's been a long time since I got them but I think they are 40 microns each.  One installed between the fuel cell and pump and the other between the pump and regulator.  I have removed the screens that came in the carburetor so there is nothing between the regulator and carb.  The regulator would have a diaphragm.  If that has failed, it is not what was in the float bowl.


Lawrenceville, GA
Tedster
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Charlie, what is the average engine manifold vacuum at idle?

I seem to recall it's got a cam in it right?? Maybe your carb & engine wants a 5.0" PV and not a 7.5" or a 6.5" ? Or even lower, at least for initial tuning purposes. I think even a cutout plug is available. Altitude especially & ignition timing play a role as you are undoubtedly aware.

I superglued one side of the PV gasket, to the PV itself. This holds up in gasoline pretty well. The PV itself takes a pretty good torque, heat cycles will cause them to loosen up a bit otherwise. The thing about powervalves is they have to be known good, and out of the cruise circuit, or tuning is impossible. The backfire issue I think is probably exaggerated.
charliemccraney
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I have what is suppose to be a repop of the '57 high lift cam.  It actually has about 17" hg at idle.  It took some tuning and intake mods to get it to that point, mostly by accident, a good consequence of other modifications and upgrades.  The valve was chosen when manifold vacuum was 15" which was over a year and a half ago at this point.  Since the 7.5 power valve has been in use for years and nothing else has been changed that would affect the vacuum spec required, the valve spec is not the problem.
I'm not having any problem with backfires.
At this point, I only have a 6.5 pv that tests good and was not installed when the problem started so I'll put that in and see what happens.


Lawrenceville, GA
Tedster
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That's what I figured. And it's tough to imagine 2 or 3 bad power valves in a row, though it's possible I guess. Does it cruise on level ground around 15" of vacuum?

Strictly speaking, how often is it necessary to pull a carb and clean it just on GP? I think once a year sounds good, though this soon seems to stretch to 2 years.
charliemccraney
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I don't know what the vacuum is at cruise.  I've never checked.


Lawrenceville, GA
charliemccraney
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I put the 6.5 pv which tested good in.  No change.  So at this point I'm going to say that it has nothing to do with the power valve.  This time it just didn't idle at all, unlike last time where it did idle for about 5 minutes after cleaning everything and replacing the gaskets and pv.

I also disconnected the vacuum advance and pcv valve and plugged the ports since I had not tried that yet.  No change.

The slowest I can get it to run is about 2000rpm, which should be into the primaries.  It accelerates well with a pump of the gas and I can confirm visually that the accelerator pump works while it's not running.  So it really seems like an idle circuit fuel problem.

I got a remote fuel supply so I will check with fresh fuel and see if anything changes.  If not, then I will put the new 670 on and see if that will idle.


Lawrenceville, GA
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You might check that the throttle plate to main body screws are tight.  There’s also the chance the gasket at that location could also need replacing.


Lorena, Texas (South of Waco)


charliemccraney
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I tried fresh fuel.  No change.
For those who may not know what a remote fuel supply is, it is a small tank that gravity feeds the carburetor.  You can get them at motorcycle and other powersports shops.

Tomorrow, the 570 comes off and the 670 goes on.  Hopefully that solves it because my car developed a cold idle problem this morning (fuel injected) so I need the truck back on the road.  I'll check the base plate screws when the 570 is off.

http://forums.y-blocksforever.com/uploads/images/b6cdb9ae-3c59-4f7a-81f5-2a82.jpg

http://forums.y-blocksforever.com/uploads/images/360f80cc-faf7-4d1e-aa6e-7459.jpg



Lawrenceville, GA
charliemccraney
Posted 5 Years Ago
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I did find the two base plate screws closest to the primary bowl to be a hair loose.  I'll estimate no more than 10 degrees, probably not that much.  I don't think it would be enough to cause any kind of leak but maybe.

I started getting the 670 ready.  It's an aluminum Street Avenger, part 0-85670.  It has 4 corner idle.  Holley's instruction manual says we know best, leave it where it is and don't question us.  I'm paraphrasing, of course.  They should know best but the baseline spec would be nice so it can be verified.  The primary screws have 1 1/8 turns and the secondary screws have 3/8 turn.  David Vizard suggests in his book to start 2 turns out on all four.  Should I leave it as Holley set it or would something else be better for the initial startup?

They also have the primaries way open but that may be to make it easier to get started initially.  I can see quite a bit of the transfer slot.
I did find one base plate screw a hair loose and all of the accelerator pump screws needed to be tightened.  The bowls are good.  I had them off when I got it to verify the calibration against their specs and install a 7.5 power valve so if anything is loose in them, it's most likely my fault.  I replaced the inlets with -6 fittings and left the screens in place for now.



Lawrenceville, GA
FORD DEARBORN
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Here's a little trick making it very easy to adjust the secondary throttle stop. This was on a 302 build and if I can remember correctly, the carb was a 570 Aevenger. http://forums.y-blocksforever.com/uploads/images/acc0961c-514c-4858-a000-139b.jpg


64F100 57FAIRLANE500


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