I put it on the KK's motor, and right away it smoked at idle (her dad's backup carb does not do that on this motor so I know it's a carb issue). It was a over rich condition causing it. I changed powervalves in the front (plug in the back), thinking that might be causing it. It wasn't. I pulled it off, and while looking at it I found the vacum cap that was on the big port in the baseplate in back was cracked badly. Thinking it had a bad vacum leak, I replaced it. Didn't help. I decided to change all the vacum caps (one did have a small hole in it). No change. Then I decided to fool with the idle richness screws (4 corner idle). The more I turned them in, the faster it idled. I had all of them originally turned out 1 1/2 turns. Now they are only out 1/2 turn, except for the back one on the passenger side and it;s out a full turn, and I had to lower the idle almost 1000 rpm. And, it appears it has stopped smoking at idle. I figured it must still have a vacum leak, so I took a can of carb cleaner and sprayed all around the base of the carb. No change. The only time the idle changed at all was when I sprayed it directly on the vacum port on the front metering block, then the idle slowed down.

I would like to use this carb as I feel it will be faster than her dad's carb (even smoking going down the track it currently is only about .06 slower now), but I am stumped as to what is causing this. Is it possible the gaskets under the 4 corner idle screws have dried out?

Any ideas would be greatly appreciated.

(edit I have checked both float levels and they are fine.

It sounds more like the idle circuitry is too fat for the signal the carb is receiving. You might try removing the metering plate(s) and inserting a small diameter piece(s) of wire to lean out the idle circuit.

Does it have a pcv system hooked up? If not, you might try adding one to give the carb more air underneath the idle transfer slots to lean it out.

Not sure about this, but is there a chance someone put the wrong metering plate gaskets on the carb? 2 circuit metering gasket when it needs a 3 circuit or vise versa.

quote:

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Originally posted by kid vishus:
so I took a can of carb cleaner and sprayed all around the base of the carb. No change. The only time the idle changed at all was when I sprayed it directly on the vacum port on the front metering block, then the idle slowed down.

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If it's a small leak that is 'sucking air' it can cause a faster idle. Sometimes a spray will close off that leak temporarily and slow the idle speed.

You might want to take the whole carb apart and clean it really good... especially if it's been sitting on the shelf for a while.

Good Luck!

I just replaced all the gaskets on the richness screws. Now, it doesn's seem to matter where they are turned except for one of them. And if it's turned in too far it kills the motor. The rest of them don't change a thing.

It has been sitting awhile, but it was in a plastic bag in the Holley box. but I suppose it's possible something got in it.

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-Paul

70 Mach1 351C
79 Ranchero GT
97 Expedition XLT 5.4
00 SVT f150 Lightning 4149 of 4966 Born on: 06/26/2000

The first time we tried to use my carb it was so rich at idle it fouled a plug. That was the first time I had ever fouled a plug when running an MSD set up. When I changed the plugs in it that day, they were sooted up badly.

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-Paul

70 Mach1 351C
79 Ranchero GT
97 Expedition XLT 5.4
00 SVT f150 Lightning 4149 of 4966 Born on: 06/26/2000

Since the idle screws have no effect it tells me it's running on the main circuit.
If it worked fine on another engine I bet the other engine had more cam.

Mien had the same problem when it was still a street car and had a milder cam than what I ended up with. The idle mixture screws had zero effect. It was pig rich at idle. You could barely sit in it at idle with the windows down.

I cured mine be drilling the throttle blades on the primary side. I had a small hole in each blade. About 3/32" I think.
Later,
David Cole

If your idle mixture screws do not respond to adjustments, you may have several different problems. The most common is using the wrong carburetor for the application. Smog carbs with reverse mixture screws (these are
the ones with the little decals, telling you to turn the screws in for richer mixtures) only have a small band of adjustment built in. Smog carbs work well on stock engines only, and you can't expect them to work well on a modified engine, especially one with a lopey cam.

Another common cause of non-responsive idle mixture screws is having too much (more than .040 inch) of the idle transfer circuit exposed. The idle circuit allows a very small amount of finely metered fuel into the engine. By exposing the transfer circuit too much, the gross feed of fuel coming out of the transfer circuit overpowers the fine amount coming out of the idle ports. This is akin to adding a squirt gun's flow into a garden hose's flow...the fine amount of the squirt gun is so insignificant compared to the garden hose, that there is no way that adjusting the flow of the squirt gun will make any difference. There are several ways to fix this problem:

1. If your engine is worn, or has a big vacuum leak, you should fix those problems before trying to crutch your carb. Sometimes, with worn engines or one with a vacuum leak, the carb must be opened quite a bit just to get the engine to run.

2. Buy the right sized carb! If you're trying to use a 600 cfm carb on a lopey-cammed 460, then you must open the primaries too far into the transfer slots just to get enough air into the engine to get it to idle. A larger carb on this example would have a larger throttle plate, which would need to opened less to allow the engine to idle.

3. Open the secondaries a bit, and close the primaries a like amount. This will allow more air in, without exposing quite so much of either primary or secondary transfer slots.

4. If opening the secondaries doesn't work, then you may have to drill small holes (1 per plate) in the primary throttle plates to allow air in while the plates cover the transfer slots. This is a trial and error procedure, so start small, about 1/16 inch. You should not have to go much larger than 3/16 inch. Drill on the side opposite of the
transfer slots, to help keep this added airflow away from the idle ports and transfer slots.

Maybe this will help some....

or this picture from holley....

Bigger pic here

I'm starting to think it needs the secondary blades opened up a bit as it is probably running on the main circuitry at idle.

quote:

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Originally posted by Fordwiser:
First thing I would try is shoot some carb cleaner down the air bleeds let it set a few minutes, then blow them out with compressed air.
Roger

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That was one of the first things I did do. It didn't help at all.
Since it decided to rain tonight, we'll have to wait till saturday to give it a try. Maybe I'll crack the secondaries open a bit first now that I have a couple of days before we go.

quote:

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Originally posted by Fordwiser:
First thing I would try is shoot some carb cleaner down the air bleeds let it set a few minutes, then blow them out with compressed air.
Roger

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That sure sounds like it... might take the metering block(s) off while you do this, just to make sure your getting a clean shot through the air bleeds.

btw, what size engine is this?

The annular boosters are very sensitive to how much vac it's pulling and lg CID engines.

------------------
Alex Denysenko
Co-Administrator and Moderator

NHRA/IHRA/SRA member and licensed Superstock driver
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The Barry of BarrysGrrl

My 750 HP was blowing soot at the track once and I found out that the primary bowl was dumping fuel into the boosters under a hard pull. Turned out to be a problem with the float assy. I added new needle & seat and changed primary float and the problem was instantly cured.

How much vacuum does it have at idle?
A 1/2 to 1 turn on the mixture screws is not that unusual for a 750 performance carb on a mild SBF. It depends on the idle rpms and amount of vacuum it pulls at idle. My 331 has about 12.5" at 900 rpms and it only uses about 3/4 turn on all 4 corners on the street. At the track I richen the idle mixture up about 1/4 turn more for better reaction with the stick shift.

Hope you figure it out. Let us know.

Good luck at the races!

We made the first pass with the 750 annular carb since it wasn't smoking at idle anymore. Her first pass with it was a touch slower than what the car ran last week (8.09 as opposed to 8.06) and the mph was down about 1 full mph (85.6 as opposed to 86.4). Knowing it was slower than last week with her dad's carb, and the since she was only getting 2 time runs, we decided to put her dad's carb back on to try it. It was .02 slower than my carb but the mph was better. It appears as if my carb is working "right" now. But we really need to get to a test n tune so she can make several passes with each carb and then we can compare times.

I think I'll fatten my carb up a tad before we go again.