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post #1 of 19 (permalink) Old Feb 23rd, 08, 07:57 AM Thread Starter
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Holley power valve plugging

So my new motor is running very low intake vacuum. 2" wc if I am lucky. It was extreamly rich on start up and I can see why with a 6.5" PV installed. So I quickly installed a plug in it. This helped. But its still on the fat side.
Holley doesnt make anything smaller than a 2.5" PV. So.... I will still idle on my idle circuit but I suppose my transition is going to be lean?? When I reach a load and the power valve would open to add more fuel yet it is not there anymore will I now be pulling straight from the main well?

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post #2 of 19 (permalink) Old Feb 23rd, 08, 08:20 AM
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Re: Holley power valve plugging

Your motor is very similar to my 355 but you've got more cubic inch, you can get it to idle. Give it more initial timing (24 degrees) and use a 2.5 power valve. Have you used your carb before or is it brand new out of the box?

My 355 had 11:1 compression, same heads as you and the cam was 264/274 @ .050" with .630" lift. It would idle out of gear with about 5.5" vac at 1,200 rpm. In gear it would have about 3.5" at 900 rpm.

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post #3 of 19 (permalink) Old Feb 23rd, 08, 09:40 AM Thread Starter
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Re: Holley power valve plugging

Thanks for the reply Steve
It actually idles fine and is very responsive. Just a bit on the fat side. Timing is locked in at 35* I also changed the idle air bleeds (larger) to get more air and this helped some but is still fat. I can try to bigger here. Mixture screws are backed out 1 turn on all corners.
I built this carb. It started life as a 850. I bought a billet baseplate, and a Holley HP 950 center section. Notched rear float with jet extensions. Has 84/86 jets. Set the transfer slots properly, floats, ect. Has the 850 metering blocks and I am told they are the same as a 950? Its a nice carb and I am amazed at how responsive it is. No stumble what so ever. I've played with holleys since I was 16 and have always been able to get them close. Just never had such low intake vac and had to plug a primary PV. I thought about getting some billet metering blocks so I could play with the IFR's. Idle feed restrictors. I guess now that the power valve isnt in the circuit cruise conditions will be hard to get perfect. The best way to set this all up i presume would be a wide band 02 sensor. So you can actually see what the air fuel mixture is.

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post #4 of 19 (permalink) Old Feb 23rd, 08, 10:39 AM
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Re: Holley power valve plugging

How high is it idling?
Could the throttle plates be open too much? Maybe your carb is a candidate for idle holes in the butterflys.

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post #5 of 19 (permalink) Old Feb 23rd, 08, 10:55 AM
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Re: Holley power valve plugging

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Originally Posted by sixd8rs View Post
Thanks for the reply Steve
It actually idles fine and is very responsive. Just a bit on the fat side. Timing is locked in at 35* I also changed the idle air bleeds (larger) to get more air and this helped some but is still fat. I can try to bigger here. Mixture screws are backed out 1 turn on all corners.
I built this carb. It started life as a 850. I bought a billet baseplate, and a Holley HP 950 center section. Notched rear float with jet extensions. Has 84/86 jets. Set the transfer slots properly, floats, ect. Has the 850 metering blocks and I am told they are the same as a 950? Its a nice carb and I am amazed at how responsive it is. No stumble what so ever. I've played with holleys since I was 16 and have always been able to get them close. Just never had such low intake vac and had to plug a primary PV. I thought about getting some billet metering blocks so I could play with the IFR's. Idle feed restrictors. I guess now that the power valve isnt in the circuit cruise conditions will be hard to get perfect. The best way to set this all up i presume would be a wide band 02 sensor. So you can actually see what the air fuel mixture is.
You're welcome! I would definately change the metering blocks so you can change the IFR. I have an 825 Race Demon on my car and I had to change the IFR's to get it to idle nice. Out of the box my idle mixture screws needed to be only 3/4 turn out and it still didn't idle that clean. I changed the IFR's and now it idles cleanly with the screws 2 turns out.

I would hold off on drilling the throttle blades. Mine aren't drilled and it runs great.

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post #6 of 19 (permalink) Old Feb 23rd, 08, 01:10 PM
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Re: Holley power valve plugging

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Originally Posted by sixd8rs View Post
So my new motor is running very low intake vacuum. 2" wc if I am lucky.
2in/Hg?? That is just not enough vacuum. I'd be looking at finding out the reason why your vacuum is so low before adjusting the carb.

If you plug a power valve, you need to richen the main jets about 3 or 4 numbers from memory, otherwise you are going to be lean when attempting to make power. Power valves are there for a reason.

What cam (duration @ 0.050"), and compression ratio do you have?

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post #7 of 19 (permalink) Old Feb 23rd, 08, 01:20 PM
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Re: Holley power valve plugging

I got my PV from these guys they have them down to 1.5 http://cgi.ebay.com/ebaymotors/Holle...3756.m20.l1116

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post #8 of 19 (permalink) Old Feb 23rd, 08, 02:17 PM Thread Starter
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Re: Holley power valve plugging

Thanks for the help!
I thought about drilling those holes but wanted to investigate before I drilled them. I also thought I would lose more vac. if they were drilled too. So I havent tried this yet. It idles at 850-900. Throttle blades are set at the transfer slots at .020
My specs are in my signature below. For optimum street performance I feel it should have a power valve. No question about it. At this time, I need to find a way to make it work it there is a way. Actually the roads here are covered with ice so its no rush..lol

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post #9 of 19 (permalink) Old Feb 23rd, 08, 05:44 PM
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Re: Holley power valve plugging

I would think with a camshaft this large, idle rpm would be above 1000-1200 rpm min. With a long duration, vacuum will be low.

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post #10 of 19 (permalink) Old Feb 23rd, 08, 06:16 PM
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Re: Holley power valve plugging

just from experience.... watch that low idle with a big mech. roller, that's when the lifters go- not enough oil going around. Your combo is very similar to the 358 I took out of my '68 and am using for my '69; idle was 1000-1200 even though it could go as low as 800-900; also you might be over-carb'd for the street; I always had better vac and street manners with a modified 750 carb for normal use; I would pick up a bit at the track with a 950 though. -Dan

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post #11 of 19 (permalink) Old Feb 23rd, 08, 07:38 PM Thread Starter
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Re: Holley power valve plugging

Good info!
I didnt give the thought about the low RPM and the lifters. I did get the Crower cutaways with hippo oiling and would hate to ruin them. I'll bring that rpm up some just to be safe. I had given some thought tonight about trying a friends Holley 830 HP.
Thanks again for the responses!
Those 1.5 power valves could be worth a try too!

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post #12 of 19 (permalink) Old Feb 23rd, 08, 08:10 PM
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Re: Holley power valve plugging

CC #12-822-14 or #13-908-9 or #?

Eitherway, this cam should idle at much more than 850-900 and make more than 2in/Hg of vacuum.
I doubt they'd sell a cam that makes such lousy vacuum.
Why not contact CompCams and ask them.

Perhaps try increasing your idle, re-setting your timing, putting your carb back to more factory settings and checking for vacuum leaks.

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post #13 of 19 (permalink) Old Feb 24th, 08, 08:51 AM Thread Starter
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Re: Holley power valve plugging

Blackout,
CC-12-908-9
630/630-264/270@50 on a 106*
Idle is now at 1100-1200
Timing is still at 35*

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post #14 of 19 (permalink) Old Feb 24th, 08, 10:04 AM Thread Starter
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Re: Holley power valve plugging

Just an update
I borrowed my neighbors vacuum guage and my old one is now in the trash. It almost went thru the window in the garage. Good to see that I do have some vac anyhow! I was able to get it up to 6". My concern is now that I have exposed so much of the transfer slot to get the idle up to 1100 its still extreamly rich. As "ldrisner" above mentioned can it be a candidate for holes in the butterfys? More air should help lean it out and allow me to close up the transfer slots some. Will my Vac drop if this is done?

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post #15 of 19 (permalink) Old Feb 24th, 08, 10:59 AM
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Re: Holley power valve plugging

A 1/8" hole in each of the primary butterflies is a Holley recomendation.

Out of interest, does this carb have a 4 corner idle? (Idle circuit in the secondaries as well as primaries.)

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