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770cfm Holley Too Rich?

9K views 45 replies 15 participants last post by  Rickles 
#1 ·
'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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#2 ·
I would cut the threaded area off the plug and look at the fuel ring before I leaned it out.
 
#6 ·
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 :D>:)
 
#10 ·
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
 
#9 ·
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.

I’ve got one of the old Holley jet-kits and was quite surprised that I’m 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 couldn’t possibly get me there.
 
#14 ·
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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#31 ·
Bill,
Bare with me. You showed them the picture of the missing hole, correct? (Got to love the modern age and being able to share pictures via email!) You said they had never "heard" of that before. I wonder if that's the pat answer they are supposed to give when presented with an obvious (courtesy of pictures) mistake on their part or whoever manufactures their carbs these days. :confused: They've made millions of carbs in their 60+ years and they've never missed a hole before??? Hard to believe!

Lesson: If your brand new carb has some running issues, GET familiar with where all the passages are supposed to be in that carb and check to make sure they are ALL THERE!
 
#16 ·
As Mobley said. If you're rich at cruise it's more than likely not a main jet problem. You probably have too much emulsion air. But you need to get your ignition right first.
 
#17 ·
Tom: 50mph in 5th (overdrive) is about 2,300rpm, 50mph in 4th (1:1) is about 1,500rpm.

I checked the timing today. Dist. vacuum line off and plugged, idle dropped to about 700rpm, initial turned out to be 16* or 17* and total about 36*, I think, at about 3,200rpm or so. The first time I checked the timing when I got the car in April, I couldn't quite get the right viewing angle with the Serpentine system in the way. I thought it was at 10*, but now that I found the correct viewing spot, it turns out I was looking at an angle and what I thought was 10*, was actually 16*, so it was at 16*-17* all along.

I adjusted the mixture screws with a vacuum guage connected inline with the distributor vacuum hose (the only line that fit with my guage). Turns out they also were set correctly; there are no mixture screws on the secondaries on this carb.

Vacuum, set up this way (inline with dist.), was about 350 at idle of about 800+- rpm.

Since I dropped the idle to 850 or so (from 1,000+), the tendency to run-on when I shut it off has disappeared.
 
#18 ·
Is this a Street Avenger carb?

350 vacuum at idle isn't a unit of measure that most of us are going to understand. Do you have that in inches of Mercury?

How do you have more RPM at the same speed in overdrive than in 4th?
 
#19 ·
Last car I tuned with a Holley 770 SA was on a roller cammed 454 HO crate engine.
I set the timing at 20* base and it had 38* total. Ran like a top and didn't need to touch the carb. It was fine out of the box.
Owner was very impressed and he said it smelled "cleaner" at idle
Car came to me with timing set at 4* BTDC

..and he was holding on for dear life when I drove it :D
 
#20 ·
If this is a SA then he has something wrong and it's pulling fuel from somewhere. Your results are what I would expect. The SA have a far milder transfer circuit and emulsion package. It should be pretty darn clean. Sounds like his timing is close.
 
#26 ·
Sorry, had that backwards: 50mph in 5th (overdrive) is about 1,500rpm, 50mph in 4th (1:1) is about 2,300rpm.

About 15 inches of mercury (in-Hg scale), I'm not real familiar with the vacuum guage scales, the 350 was millimeters of mercury.

Yes it is a Street Avenger; although, on the the Holley website, the carb. pictured has secondary mixture screws and mine does not. I realize, of course, there are lots of minor variations, and maybe Chevy speced no sec. screws to save money on the millions they bought. However, in the Holley specs. they say 70 primary jets and 75 for the secondaries (73, and 75 on the spec sheet that came with mine).


Edit: Came across another on Holley's site without the secondary mixture screws. Still lists 70's as the primary jets.
 
#27 ·
Do you have another carb to try real quick? Where are you located? I work in Paulsboro.

15" tells me the engine is good - depending on idle speed....
 
#28 ·
figured it was backward or upside down or something.

at 1500 RPM that engine is going to be running off the idle transfer circuit.not on the main jets at all. Might still be on the ITC at 2300. Does your carb have screw-in air bleeds?

look down in the air horn, engine off. see if there's 4 brass looking set screws along the backside of the choke horn.
 
#30 ·
These are the numbers stamped on the horn; the "7" is superscript. I'm pretty sure the air bleeds are pressed in, but Holley says never to mess with them.

19172225
80713 7
2492

Don't have another carb. The 15" was at 800-900 rpm with vacuum advance connected when I was adjusting the idle mixture screws.
 
#34 ·
Planning to do that Bill, base gaskets I ordered should be here Friday. I got the open type gasket because the Chevy manifold is a dual-plane with the cutout in the partition. I don't know if it's OK to reuse a base gasket, so figured better to get a new one.

Since I have 15 in. at idle, I guess the 6.5 Power Valve is correct.
 
#36 ·
No, Tom, I don't have any way to measure the 02, just looking at the plugs (not very reliable) and the black, sooty tailpipes. Wish there was an inexpensive unit you could just temporarily put in the tailpipe. I'm going to check the float level today (Holley suggestion for rich running on their support page FAQ's); although, I suspect they are fine.
 
#37 ·
I would use the Holley factory jetting as well but that will only change your cruise AFR a slight amount if at all. I feel like you're pulling fuel from somewhere else. Definitely verify float level. Has this carb ever been apart?
 
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