396 BBC holley 750 HP upgrade -- dead spot now

My motor is a 396 chevy, 10.25:1, LS6 cam.

I had a holley 650 DP (4777-2) carb with 72 primary, 76 secondary jets, NO power valve. I decided to put an HP upgrade kit on it -- the 134-300C holley part # (which should make it basically a 750 HP). I decided to try the jets that came in it -- 72 primary, 84 secondary and also I decided to put a 4.5 power valve in it. My thoughts were that it should run about the same as it did until the secondaries and power valve kicked in, at which point it would receive more fuel and air than it was getting before.

Before the upgrade, it ran great -- no dead spots up to full throttle. Now it has a very large hesitation when I get on it. After that it is fine wide open (sounds great). Any thoughts on what needs tuning? Would it be getting too much fuel or not enough? I didn't adjust anything when I put it back together -- just the jet change and power valve addition. I suspect it might have something to do w/ the power valve but I don't know a lot about adjusting holleys...Any recommendations would be greatly appreciated.


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You may have to mess with the air bleeds a bit. The 650 carb metering blocks internal fuel metering calibration do not exactly match the 750 HP body.
The supplied air blededs are not exactly right, expecially if you se the meteringblocks from a 650 or 700DP. I made custom sized ones for mine,
based on tuning with a AFR/o2sensor meter. When you get it right, the throttle response is excellent.
Remove the carb and flip it over. Look at the throttle opening at idle.
Check the throttle blade/idle transfer slot exposure at idle. Should be even on both the primary and secondary. about .020-.030". If the backs are closed or the fronts are too far open at idle, you will get a flat spot, off idle.
The accelerator pump shooters and cams may need tuning. May need to bend the pump linkage arm a bit to get correct action.

unless you changed the main body of the carb its still a 650cfm. is the part you swaped a body?
try a the bigger 50cc pump shot kit and a couple of different colored cams. you used to be able to by a kit, with different shooters and cams. just experiment, you will find the right combo that the engine likes.

Is the flat spot when you wack it?? does it fall on it's face then go, or does it start to go then fall on it's face???
How is it if you ease into it???

What is your initial and total timing. What distributor is in it???

Keith

650Dp'ers are spreadbores, you can't make them into a squarebore 750 with a "kit".

What concerns me is that there was no powervalve before, someone had to modify it to run without one...I would look to see what was done to it. There is never a reason to eliminate the powervalve on a street engine, the modifications required to do it essentially ruin the carb for anything else (I.E. drilled air bleeds, elongated idle slots, drilled throttle blades etc.). Sounds like you have a custom modified carb for your application and now you have to tune it again to fit the new mods you did to the carb.

I would sell the carb and get a real 750 and tune it WITHOUT removing things like powervalves or drilling anything in the carb. Holley specifically recommends not to modify the carb to make it run on your engine combination and without exception I have been able to get them tuned without wrecking the carb with "old school race mods". Your engine is mild and should run well with a Holley carb with nothing more than an accelerator pump cam change and jetting tuning.

Hey Chuck, the OP gave the Holley list # (4777-2) of the carb he has modified. The 4777-2 is a squarebore 650 DP. Yes, Holley made a 650 that is a spreadbore(list # 6210 comes to mind), but all 650's are not automatically spreadbore's.

To the OP, if you increased airflow thru the carb by replacing the main body and you now have a dead spot at the immediate hit of the throttle the first thing I would look at would be the accelerator pump shooter size. I would expect the carb now to need a 28 front and 31 rear shooter as a starting point, and possibly bigger depending on engine mods. Basically now you have an 800DP by way of the main body swap(650-800 have the same throttle base plate).

Thanks for the tips everyone-- this carb used to be used on a 350 drag raced in a Nova. I have run it for the last 6-7 years, un-modified from that and it has run perfect. I don't know what other modifications, if any, were done to it other than the power valve being taken out. If I wack it, it almost dies. If I ease into it, its not as bad. I have a stock HEI distributor, couldn't tell you what the timing is at this time though I doubt that is a problem since it ran great before.

I guess I'll just have to play around with it. I will not be buying a new carburator...I like this one Might take me a while though, as it is in my v-drive boat and it takes a trip to the lake to try out any changes...

The power valve vacuum port may have been blocked when the valve was removed. You may want to look at that also

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Chet

Interesting -- where would I find that ?

Thanks !

G'day fire_hawk,

The power valve (PV) draws it's vacuum through a large port between the two sets of throttle blades right in the middle of the carb at the base. You will need to remove the carb and flip it over to see it. The power valve relies on vacuum to keep it closed, hence the rating of 4.5Hg in an earlier post. That means that when the manifold vacuum falls below 4.5Hg the power valve opens and richens up the secondaries until the vacuum comes back up above that setting, then it closes again.

Obviously your carb has been extensively modified if the PV was removed. Sorry can't offer any fixes on a custom carb like yours. I would suggest putting it back the way it was if it ran OK like that, or as "4 Jaw" said, buy another carb.

If you don't already have them, buy a "How to Tune Holley Carb's" book, a decent timing light and vacuum gauge and your set for life !.

Have fun.........Barry.

Thanks for the info Barry, I'm actually kind of glad it doesn't run exactly right because it is forcing me to learn how to tune a Holley. I have a vacuum gauge on the way ... slowly but surely...

Bumping because a year and a daughter later I have finally gotten some time to care about this. Haven't made it out to the lake a whole lot until now...

So nothing has really changed, I have done some reading up and adjusted my idle mixture screws (which were way off) but that didnt seem to help..floats seem ok. We got to looking at it with the air cleaner off and it sure seams like the fuel is just "dumping" in...to me it doesnt seem atomized enough at low rms when the dead spot is present. I have 3 short vids showing this:

throttle view:
http://www.youtube.com/watch?v=qRy19-q5SvA

carb view (low RPM):
http://www.youtube.com/watch?v=n-h3e3bNMno

carb view (gunning it) - runs fine:
http://www.youtube.com/watch?v=O-28Osixxzo

I think FBIRD is on to something with the high speed air bleeds? Does it seem like I should go up in size as to lean out the fuel a bit? And doesn't this delay the transition of the main metering circuit? I think the bleeds read "36" and they "look" smaller than the ones on my other body.

Or would going down in main jet size (72) be somewhere to start? (this doesnt seem like the answer). Also I think more of my idle transition slot is present than should be at idle (more "rectangular" than "square") but I don't think that has anything to do w/ this issue ?

Thanks for any suggestions !

Bumpity. Anyone have any thoughts ?

Thanks !

I would put the carb back the way it was and call it good.

you said "Before the upgrade, it ran great -- no dead spots up to full throttle"

so why fix it if it wasn't broken?

If you want more power then get another carb. tuning a carb and "up-grading" a carb is two different things, as up-grading can be very tricky and should be left to the carb experts.

If you just "wack" it and it almost dies sounds like you need to look at tuning the accelerator pump circut. You went from a std 650dp to HP main body so your cfm potential has increased (more air) there are different shooters (squirters) and cam profiles that help to get the right combination to cover your bog by increasing shooter size or amount and delivery thru the cams. Here is a vid that might help. http://www.youtube.com/watch?v=k-Nx...feature=related