770cfm Holley Too Rich? - Chevelle Tech
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post #1 of 46 (permalink) Old Sep 14th, 19, 6:16 PM Thread Starter
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770cfm Holley Too Rich?

'67 Chevelle - ZZ427/480 - Legend 5 speed

About 1,500. miles on the engine

Timing 10*/34*ish total
Prim Jets 73 (according to the Holley book that came with it, for carb #19172225 stamped on the horn)
Secd Jets 75
Pr Power Valve 6.5
Sc Power Valve Silver
Fuel pressure 6psi
Floats seem to be set OK
I'm at sea level (about five miles inland from the Atlantic)

It runs fine, easy to start, wanted to run-on a couple seconds until I dropped the idle from 1k to 850

Here's the dilema:

Strong over-rich smell from the tailpipes at idle and driving (friend driving behind me says it was overpowering), the tailpipes are sooty black. I pulled #1 and #2 plugs and the insulators were half tan and half black while the inside is sooty black (see the pic). Both plugs were exactly the same; I can't recall ever seeing that half-and-half coloration before. I drove about 5 miles at 70mph, then about 6 miles 40-50 mph, then about 2 miles lower speed and pulled the plugs at home when it cooled down a couple hours.

The car's builder said he never needed the choke to start it, but I asked him to hook it up, though, and it's opening and closing as it should.

So, I'm wondering if those jets are too rich? I'm sure Chevy has it jetted for general purposes. It runs strong with no surging, hesitation, stumbling, etc. Are the jets too rich? How many numbers leaner should I go? I would appreciate any suggestions.


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post #2 of 46 (permalink) Old Sep 14th, 19, 8:45 PM
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Re: 770cfm Holley Too Rich?

I would cut the threaded area off the plug and look at the fuel ring before I leaned it out.
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post #3 of 46 (permalink) Old Sep 14th, 19, 10:14 PM
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Re: 770cfm Holley Too Rich?

full vacuum to the advance canister??

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post #4 of 46 (permalink) Old Sep 14th, 19, 10:39 PM
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Re: 770cfm Holley Too Rich?

For starters you really need to get some more base timing in there.
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post #5 of 46 (permalink) Old Sep 14th, 19, 10:44 PM Thread Starter
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Re: 770cfm Holley Too Rich?

Leo, yes, full vacuum to the distributor.

Vince, what timing would you recommend?
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post #6 of 46 (permalink) Old Sep 15th, 19, 6:19 AM
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Re: 770cfm Holley Too Rich?

Quote:
Originally Posted by Rickles View Post
Leo, yes, full vacuum to the distributor.

Vince, what timing would you recommend?
Do you know what the total idle timing is (base and vac advance added in at idle)? If you dont have enough timing, you probably have the carb idle screw in too much to compensate, explaining the run-on when shutting down. 1000 rpm idle speed does sound high.

I'd shoot for around 16* base timing and whatever the vac adds on top of that. Hopefully you'll end up with around ~25* or so. If you like the way that idles/runs, you may have to shorten the mechanical curve so as not to exceed your total target advance. See Dave Ray about shortening mechanical curves
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post #7 of 46 (permalink) Old Sep 15th, 19, 7:35 AM
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Re: 770cfm Holley Too Rich?

Quote:
Originally Posted by 69-CHVL View Post
I'd shoot for around 16* base timing and whatever the vac adds on top of that. Hopefully you'll end up with around ~25* or so. If you like the way that idles/runs, you may have to shorten the mechanical curve so as not to exceed your total target advance. See Dave Ray about shortening mechanical curves
Yes, so if you have 10/34 total now, restrict the mechanical from it's present 24 degrees down to 18 or slightly less, all in by 3000 rpm by using lighter springs.

Then you can increase the initial to 16+ as Vince recommended, add a crane vacuum advance limiter plate and set it at 10 degrees, and you will be very close.

You can review our recent "lively" discussion about limiting mechanical advance here :
https://www.chevelles.com/forums/374...ce-10-a-4.html

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post #8 of 46 (permalink) Old Sep 15th, 19, 11:13 AM Thread Starter
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Re: 770cfm Holley Too Rich?

I don't quite understand; are you saying the low timing is causing the over-rich conditiion while driving and the black sooty tailpipes? And it has nothing to do with the carb.?

Also, this is an HEI dizzy.
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post #9 of 46 (permalink) Old Sep 15th, 19, 12:10 PM
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Re: 770cfm Holley Too Rich?

I recently put that same carburetor on a newly built 467ci BB. I installed an Innovate wideband AF meter and running around on the primaries was getting me readings on the 11 range. I dropped my primary jets four sizes and it now cruises in the high-12/low-13 range. I was a bit surprised that it was jetted so rich out of the box. Prior to re-jetting any choke application when cold would just kill the engine. Now half-choke when cold makes for a nice cold idle.

Iíve got one of the old Holley jet-kits and was quite surprised that Iím only two sizes from the smallest jets in the box. I first thought something might be wrong with my AF meter, but the car is running great with that four number reduction. I have always thought that I should at least be in the low-14 range, but the jets in the kit couldnít possibly get me there.
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post #10 of 46 (permalink) Old Sep 15th, 19, 12:16 PM
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Re: 770cfm Holley Too Rich?

Quote:
Originally Posted by Rickles View Post
I don't quite understand; are you saying the low timing is causing the over-rich conditiion while driving and the black sooty tailpipes? And it has nothing to do with the carb.?

Also, this is an HEI dizzy.

Just wondering if your timing is so low that it is causing your rich running due to incomplete combustion or something like that. I'd jack the timing up a bit to see if all these issues clear up before going crazy

*69 SS 502 EFI /224* HR/9.8:1/T56/3.90/ 4000lbs RW 11.5/124mph
*11 C6, 6.2, 6spd
*96 Stang 6.0 turbo 10.5/135mph
*2011 Mustang GT premium
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post #11 of 46 (permalink) Old Sep 15th, 19, 12:26 PM
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Re: 770cfm Holley Too Rich?

Oh: and I live at sea-level, so no thin-air issue causing a rich condition here.
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post #12 of 46 (permalink) Old Sep 15th, 19, 1:01 PM Thread Starter
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Re: 770cfm Holley Too Rich?

I'll try the timing first, since that's the easiest.

Tom, that does make sense; the builder thought it might be jetted rich.
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post #13 of 46 (permalink) Old Sep 15th, 19, 1:02 PM
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Re: 770cfm Holley Too Rich?

It is a 427 (3.76 inch stroke), as is, it needs 14 initial, add 8 vacuum advance degrees (pull pin travel .086 inch), and 20 degrees of mechanical advance. 14 INITIAL+ 8 vacuum = 22 IDLE timing, 14 INITIAL + 20 = 34 total.
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post #14 of 46 (permalink) Old Sep 15th, 19, 3:16 PM
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Re: 770cfm Holley Too Rich?

Rick,
Something you might want to check. I just looked at a Holley Street Avenger for a customer that had hardly no miles on it but had been sitting. He wanted me to just make sure there wasnt any gummed up gas in it etc. When I was putting it back together I did my usual routine of spraying brake clean through all of the holes and darned if the hole that feeds vacuum to the power valve had not been drilled all the way through so the power valve was not getting any vacuum to close it That would definitely make it run rich. I got in touch with Holley and they said they had never heard of that but they did send me a new base plate. I attached two pictures showing the hole in the new plate and the missing hole in the old one. I would definitely pull the carb and at least take a look
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post #15 of 46 (permalink) Old Sep 16th, 19, 2:34 AM
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Re: 770cfm Holley Too Rich?

Rick, is you car even running on the main system at cruise? what's the cruise RPM?

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