Carb swap does the trick - Chevelle Tech
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post #1 of 8 (permalink) Old Oct 31st, 19, 7:05 PM Thread Starter
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Carb swap does the trick

So Iíve been running a 750 Holley 4150 on the 468 BB in my 67 SS for a while. Iíve always had issues with rich idle, fast idle when warm and dieseling on shutoff. I had the carb rebuilt and tuned by an old school local ďcarb guruĒ. Car ran better, but I still had those same three issues.

Recently a friend who has an engine shop handed me a lightly used Holley 650 HP 4150 (no choke) and said ďtry this, might work.Ē I slapped it on and wow, what a difference. It is just how it came out of the box, never been modified. I think 6.5pv, 69 primaries and 75 secondaries. Now Iíve got clean and stable idle, with no dieseling.

So whatís up here? Is the 750 bad? Is it too much for this motor? I donít have much info on the engine internals. I have dyno sheets from the builder showing 501 hp, but I donít have the build sheet. Previous owner ďstated 10.5:1 compression with mild camĒ. I know the 750 carb has a 4.5pv, 72 primaries and 80 secondaries. Obviously the POV and the primaries were changed at some point. Could this be the issue?

Iíll keep the 750 for now. Maybe it just needs a little tweaking. If I keep this 650 Iíll have to pay him something for it. I donít like that it has no choke, but I live in So Cal so itís a minor inconvenience.


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post #2 of 8 (permalink) Old Oct 31st, 19, 9:02 PM
 
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Re: Carb swap does the trick

Hello: I had the same issues,I switched to a Holley 850 with a 4 corner idle and a leaner setting on my electric
choke. This setup works great in cold start Michigan mornings.
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post #3 of 8 (permalink) Old Oct 31st, 19, 10:27 PM
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Re: Carb swap does the trick

BTDT. I took my car to a so-called "Carb Guru" who didn't know sh!t. This guy is a "former racer," etc., etc., BS, BS. What a waste of time and money. He probably had one of his minions mess it up.

I retuned the Holley according to the factory and some stuff I saw on YouTube (I couldn't do any worse than the so-called expert), and now it runs great.

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post #4 of 8 (permalink) Old Oct 31st, 19, 10:44 PM
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Re: Carb swap does the trick

If the 750 is a 4779 it's about perfect for your engine. Sounds like it just needs some love. What's your ignition setup?

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post #5 of 8 (permalink) Old Oct 31st, 19, 11:14 PM Thread Starter
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Re: Carb swap does the trick

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Originally Posted by Tommy the Cat View Post
If the 750 is a 4779 it's about perfect for your engine. Sounds like it just needs some love. What's your ignition setup?

Yep, itís a 4779. Maybe Iíll try putting it back to off the shelf specs (6.5 pv and 70 primaries) and go from there. I have the exact same carb on my 383 small block 69 Camaro and it works perfectly.


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post #6 of 8 (permalink) Old Nov 1st, 19, 11:44 AM
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Re: Carb swap does the trick

The dieseling on shutoff is caused by too much throttle opening.
Many ways to cure that.
More base timing is one .. this will make the engine idle higher then you can reduce idle speed by closing the throttle more.
Too rich at idle.
Was this verified with AFR readings or just going by smell?
lean conditions will smell pretty bad also..especially if it gets into a lean miss.

I live in central Oklahoma and do not have a choke on any of the carbs I run.
In the 30's recently and not much issue after about 45 seconds of run time.
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post #7 of 8 (permalink) Old Nov 1st, 19, 11:57 AM
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Re: Carb swap does the trick

and the trouble with an older unknown origin carb is you never know if someone drilled out some airbleeds or something. Also, you will always get better low speed throttle response with a smaller carb.

I would return the 750 to near stock specs. Do you know how much idle vacuum the engine shows? I prefer fat primary PVs like 085-105. It lets you sometimes get a bit leaner on the jetting, like 68/9 in this case to keep the cruise AFR high enough.

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post #8 of 8 (permalink) Old Nov 1st, 19, 11:57 AM
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Re: Carb swap does the trick

reset the throttle blades, that power valve sounds too small to me

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