Hey Fellas, I've searched all through the forum looking at what jet sizes people are running in their 390 Holleys. Now I know 51 is the standard main jet size for the 390 and from what I've read everyone seems to run 51's to 54's. The thing is, my engine runs the best with 59 main jets, which is a fair jump from standard jetting, which concerns me a bit. I hope my situation is just an exception to the above quote from Holley because my engine runs so damn well as it is. Anything less than a 59 jet causes a lean flat spot. Up until last week I was using 57 jets but I've just added a 1/2" phenolic spacer and that leaned things out a bit causing a flat spot hence the 59's.
I've tried 53's, big flat spot, and 51's made it far too lean with a back fire out through the carb on acceleration.

Here is a quick cut and paste of my engine specs
225 +0.100"(238cid)
Rollmaster dual row timing chain
Head shaved 0.100" and ported and pollished with Ford 302W inlet and exhaust valves with heavy duty springs, retainers etc.
Cam specs .295"(cam lift)@ 280deg Exh .295"(cam lift)@ 270deg. I think .295" cam lift equates to about .445" valve lift(not 100% on that)
Block decked(a little bit)
390 4BBL Holley on a Cain aluminium intake
Genie headers

Oh yeah, I'm using a 45 power valve. Vac at idle in gear is 9". I've tried a 65 and an 85 PV and they just come in too soon, particularly the 85. All the jet sizes I've tried have been tested with the 65 PV as well as the 45.

Is this something I should be concerned about since the engine runs so well? or am I worried about nothing. Or does it sound like I need a bigger carb for my "application"

Any info is appreciated

Cheers, Jon

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

I have run mine with as big as 64's in both sides. (I have an 8007 that has has a metering block put on the secondary side)

Right now I have it at 58/60, but I think I am a little fat.

Does yours still have the plate in the back side or a metering block? Are you covering up a lean secondary with a rich primary jet?

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Official Cookie and Mater Tormentor.

Still using the secondary plate.
I locked out the secondarys with the black spring while playing with primary jets so I don't think it's a secondary lean spot.
I get a good puff of smoke when the secondarys open so maybe they're a bit rich, or they're coming in to early.
It's good to read that you've run as big as 64's slantzilla

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

G'day Jon,

I checked my Mopar reference, and it recommends a 64 jet with a 650 cfm Holley for racing using the Offy manifold. Don't know what difference the manifold makes, nor do I know how gas in Western Australia is formulated. In the US there are summer and winter blends of fuel that require different calibrations for peak performance.

FWIW, Mopar recommends 49/52 on the 350 & Offy combo for street use. Guys around here say it is too lean and recommend 52/55 on the Offy. I don't know what difference the 'cain will make. I'm set to go with either recommendation when I get around to installing mine.

Cheers

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You can't fix stupid. - Ron White

Dump the plate, if you are tuning a holley, it's best to make everything easier to dial in, which usually means an investment in the quick change secondary vacc. pod, and a secondary metering block.

When your flat spot occurs is it at launch or is it after it's moving and it just won't go any faster? What size shooter and pump cam are you running? I know that you may be looking at a bit bigger carb if your jets are that high, sadly Holley kind of stinks when it comes to 'mid range' carbs, it's either the 390 or you go to a 600... the "ford" 450 carb (0-1848) has the manners of the 390, but gets mileage like the 600...

Another thing to note, ignition problems can also act like carb problems occassionally. Have you pulled plugs after the flat spot or have a A/F ratio meter to kind of judge subjectively how things are going?

Good Luck, dialling in that last bit of stuff is fun,

-D.Idiot

Thanks for the reply 72 Demon.
I was just looking at the Holley carb numerical list to see what size secondary plate the 390 has. The list says it should have a 34R9716-59 plate which equates to a 56 jet. I'm not sure if I have this plate in my carb because it's a rebuild and the guys that did the job didn't build it to spec. 59 jets instead of 51's 85 PV instead of 65 so who knows what the secondary plate is?? they said they built it to suit my engine, and to be honest, besides the PV they pretty much got it right.
But.......is it wrong or not normal to run bigger jets in the primarys and smaller in the secondarys?
Another question I have is if I decrease the size of the primary jets can I up the size of the accel pump shooter to cover the lean spot, or doesn't it work that way??

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

Just saw your post DusterIdiot. I currently have no flat spot now, I've taken the phenolic spacer out for now and everything is back to normal. I've left the 59's in because it seems to pull harder with them.
Yeah a secondary metering block is the way to go, that might be my next step. I have installed a quick change secondary vac. pod, that was the first thing I did to the carb. I think they should be standard on all vac sec Holleys.
The flat spot was more of a bog really. Hard accelerating from part throttle at cruise would cause a big bog with the 57's and a very small slight bog with 59's. I also had a big bog from idle. Floor it and it would bog for a split second then take off. Both of these have gone now that the spacer is out.
Ignition is all good, Crane Hi-6 triggered by pertronix.
Shooter is a #25 and the pump cam is orange with the screw in position #1. Before all this mucking around with the spacer I played around with these sizes and this is what works well.
Oh and I use the purple vac spring on the street and short yellow spring on the strip.

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

IME, the primaries are set up with smaller jets than the secondaries. One of the reasons is to prevent a bog, where more fuel is being provided to the engine than it can handle at that particular moment. This is counter to your contention that the problem is leaning out and not being too rich.

I've never tried to use the accel pump to compensate for a lean spot. Don't think it would work.

You seem to be real close to getting this thing sorted out. It's frustrating at times and it can seem like you're losing ground when you're actually gaining. (You'll be ruling out what doesn't work.) Hang in there.

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You can't fix stupid. - Ron White

I hear you. I installed the phenolic spacer in hope of improving things, which in some instances it did, but on the whole it threw my carb tune totally out of wack, much more than I thought it would.
Like I said it's all running very well now with no bogs/flat spots.
I might take off the secondary bowl and find out exactly what secondary plate is in the carb. It may well be bigger than the standard plate for the 390.

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

The carb is supposed to have bigger jetting in the secondaries since you are demanding more from the engine and when they kick in you have lower vaccuum (i.e. less 'draw' through the carb).

The acc. pump's job is to cover the transition from one circuit to another (idle/transition circuit to the main jetting),and to make sure enough gas is available during low vaccuum/low draw events (i.e. going from cruise to WOT, 9" vaccuum to 0".

You can try putting your pump cam screw into the #2 slot and back the jets down 1 or2 and see if anything changes...what color cam are you using?

-D.Idiot

I've tried the screw in #2 hole and it made no difference. Pump cam is orange.

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

A spacer will tend to make the motor want more jet. When I had 64's in mine I had 1-1/2" of spacer and a 1/2" nitrous plate under the carb.

4 hole spacers will tend to want less jet than an open one.

For big fun you can get a Holley Trick Kit. It will have more parts in it to try than you know what to do with.

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Official Cookie and Mater Tormentor.

I've got another question for you guys.
If my car pulls hard with not even a hint of a bog when I give it full throttle from idle can I assume I have the right shooter size and pump cam setting?

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162

Hi

I run 55 primaries, 59 secondaries, 60PV, 466 cam I think it should be orange but is pinkish,in the 2 hole and she is fine -BUT your engine is bigger than mine...and has all the special bits - things like headers and so on leans out the mixture.

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Regards,
Fanie Gerber
It's never junk, it's just a part you're not currently using

http://www.valiant50.co.za
Just say I own a few Mopars

What size accel pump shooter are you using Fanie?

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62 Valiant, 225cid Slant 6/PB 904 with lots of good bits

http://www.moparmarket.com/readersrides ... user=SV162