390 won't handle the 750hp double pumper

[rcodecj]

Runs like a banshee, except from a dead stop, it runs rich, that's right rich, you would think it would be lean, but no hesitation or stalling like when it's lean, it just sounds like it's missing until it clears up. I tried squirters and idle air bleeds. I put an 800 Edelbrock on it and it runs fine.
I also put the 750hp on my other car and it runs fine.
I need to do some things to make just a bit more power, either that or change the converter for more stall.
It's real close to running right, but I feel it will be worse at the track with better traction and more load on the engine at takeoff.

My combo:
1968 390 2V 68k standard bore
Cam Research 226/232 .540/.565 110 lsa cam
Survival Motorsports stage X heads with 2.2” intake and 1.66 exhaust valves
PI intake manifold
428 cj exhaust manifolds
750 HP double pumper carburetor
Faron recurved distributor

C6 transmission
ATI 2700 rpm torque converter
3.50 traction lock rear gears, 28" tall tires
Magnaflow 2.5” mandrel bent exhaust with modified stock exhaust tips

In a 69 mustang

One thing I know is wrong is that I used a Mr Gasket set that included .060 head gaskets, I feel it lowered my compression only a little but I'm sure I've got lousy quench which would account for some lack of torque. The pistons are around .020 in the hole so even with a .040 gasket it still will be bad. I'm not going to deck the block, too much work/money. The converter is stalling more like around 2400, I ordered it to be 2700, but I think the lack of torque is affecting the stall speed. The thing feels strong until I drive my 67 with the very mild, worse compression 460, then 

I've got some ideas that I will be posting in the future, meanwhile I'm running out of time for cool dragstrip nights around here.

[jayb]

How about some more details?  For example, from an idle, when you stab it does it stutter somewhat before it clears up and goes?  How about from a roll at around 1500 RPM?

I'm thinking you may be getting extra fuel from the power valve, and you need to go down to a lower power valve number.  Do you know what number the power valve is that is in the carb?

[rcodecj]

How about some more details?  For example, from an idle, when you stab it does it stutter somewhat before it clears up and goes?  How about from a roll at around 1500 RPM?

I'm thinking you may be getting extra fuel from the power valve, and you need to go down to a lower power valve number.  Do you know what number the power valve is that is in the carb?

From an idle and dead stop when you stab it, it starts missing right after it takes off, and misses for quite a while, and eventually clears up. It clears up faster if you let up a bit, if you keep it down the missing continues longer.
From a 1500 rpm roll it runs fine.
The power valve is 6.5.
Here's the weird part, the engine only has around 7.5 in vac in park, 5.5 in drive.
I have checked with 3 vac gauges, but it doesn't make sense, because the power brakes work fine.
I have checked from the carb and from the intake.
I cannot find any vacuum leaks and it runs fine, has no indications of an intake leak, I have complete control with my idle screws, and Edelbrock narrow band a/f shows idle mixture the same as my other car. I can idle it way down if I want to but currently have it at 950 rpm's. I know narrow bands are not very accurate but it's all I have. I figured when you punch it from a dead stop, vacuum will be pretty much zero, so any powervalve would be open, but I have not watched the vacuum on takeoff. Still think powervalve? Thanks.

[jayb]

I do still think power valve.  You might be loading up at idle because the power valve is bleeding in some fuel, and you end up rich when you slam open the throttle.  At 1500 from a roll you are burning some of that power valve fuel more effectively, so the engine isn't loading up as much.  I'd put in a 3.5 and see if that doesn't help.

If it doesn't, I'd jet down on the primaries.  When you slam it open from an idle you are probably getting the main circuit involved right away, and there may just be too much fuel for the engine to use at that point.

Let us know what happens...

[rcodecj]

I do still think power valve.  You might be loading up at idle because the power valve is bleeding in some fuel, and you end up rich when you slam open the throttle.  At 1500 from a roll you are burning some of that power valve fuel more effectively, so the engine isn't loading up as much.  I'd put in a 3.5 and see if that doesn't help.

If it doesn't, I'd jet down on the primaries.  When you slam it open from an idle you are probably getting the main circuit involved right away, and there may just be too much fuel for the engine to use at that point.

Let us know what happens...

I don't think it's loading up at idle because I would see that on the a/f monitor. Besides the power valve feeds through a pair of small passages that extend from the pv opening to the main wells and I don't see how that could be activated at idle. Sure it has a different fuel path, but the fuel should not be there until the main circuit is activated. I do see your point about the pv affecting takeoff though. If my converter is stalling around 2400, it could be activating the mains, dumping in fuel, and then the engine rpm's go down and it can't use the fuel.
A 3.5 power valve might delay or even stop the extra fuel until the engine can catch up with the load.
This doesn't happen if the car spins from a dead stop, only if it gets pretty good traction. Also it doesn't help I only have 3.50 gears, 28" tall tires, low converter stall, a cam that's a bit big for the compression, poor quench....
It may be when the converter flashes, the power valve opens, but then the load on the engine pulls the rpms down and it can't use the fuel. I think that's what you're saying and I think the power valve is worth a try.
According to Holley web site it's not sized right for sure. 3.5 should still be high enough to still work on the topend.
It will affect though crusing up hills where you need the extra fuel because of load and throttle position. I don't see that being a problem with this car though.
I think I'll order a power valve. I'll let you know. Thanks for the help.

[bear mccoy]

what kind of intake gaskets are you using.  the felpros leaked on my 390 ended up us victor reinz.  I would still change to a 4.5 or lower power valve.  might want to block the back ones and up the jet sizes in the secondarys.

[rcodecj]

what kind of intake gaskets are you using.  the felpros leaked on my 390 ended up us victor reinz.  I would still change to a 4.5 or lower power valve.  might want to block the back ones and up the jet sizes in the secondarys.

I'm using Mr Gasket intake gasket that came with the engine rebuild set. The carb does not have a rear power valve.

[billballinger]

what kind of intake gaskets are you using.  the felpros leaked on my 390 ended up us victor reinz.  I would still change to a 4.5 or lower power valve.  might want to block the back ones and up the jet sizes in the secondarys.

I'm using Mr Gasket intake gasket that came with the engine rebuild set. The carb does not have a rear power valve.

Your power valve is way to close to your idle vacuum, I would bet that it is seeping and dithering a lot down low cruising, even if it seems responsive.  That will wreck a set of plugs, and the test you are using will show that, sooty plugs show up at a dead stop stomp.  I think a 3.5 PV  and a new set of plugs will do the trick.

[rcodecj]

what kind of intake gaskets are you using.  the felpros leaked on my 390 ended up us victor reinz.  I would still change to a 4.5 or lower power valve.  might want to block the back ones and up the jet sizes in the secondarys.

I'm using Mr Gasket intake gasket that came with the engine rebuild set. The carb does not have a rear power valve.

Your power valve is way to close to your idle vacuum, I would bet that it is seeping and dithering a lot down low cruising, even if it seems responsive.  That will wreck a set of plugs, and the test you are using will show that, sooty plugs show up at a dead stop stomp.  I think a 3.5 PV  and a new set of plugs will do the trick.

The plugs look good so far, each time I have checked them. I just ordered a 3.5 Holley power valve. I'm hoping it does the trick. Thanks guys.

[rcodecj]

Man, opened up the carb today and found  while it did have the fiber reusable gaskets, it did not have the  buna rubber ones.
One of the "reusable gaskets" tore, so I ordered a set of the blue buna ones.
Another delay.   

[machoneman]

Something didn't ring true about your very low idle/drive vacuum. Once you get it back together and running, try these:  

1-You may have a leaking intake gasket causing a vacuum leak. Know you mentioned this and checked it but.....it could still be leaking externally (pulling in air from the atmosphere) or it could be leaking internally (pulling in air from the crankcase). To test for external leak, use an unlit propane torch and play the gas around the intake, any intake runner plugs or ports and the throttle shafts of the carb and see if the idle changes. Don't point the gas into the carb's venturis though. If so, you have found your leak.
2-To test for an internal leak, block off all vacuum sources such as brake booster, pcv, etc. and block all vents such as valve cover breathers, oil cap, brake booster vaccum line etc . Duct tape is your buddy here! The crankcase needs to be fully sealed from the outside atmosphere. Pull the dipstick and attach a vacuum gauge to the dipstick tube with a length of hose and clamp if need be. If you get a vacuum reading with the engine running, you have an internal vacuum leak and most likely it is a sucked in intake gasket. Could also be a cracked intake. Most likely an intake gasket though.
3-If power brake equipped, pull and plug the vacuum line to the booster and recheck vacuum.

Keep in mind the goal here is to eliminate variables. If vacuum is still low after all checks out, I'd also check the curve on your ditzy as that cam would like lots of initial advance.  

[My427stang]

What is your initial timing set at?

The follow on question is, do you run vac advance and is it on a ported circuit or manifold?

If it is indeed rich, a bad PV, as opposed to an early one could make it pretty rich and not need the boosters to have airflow over them.  However, your low vacuum makes me think that you have a timing problem

Ross

[rcodecj]

What is your initial timing set at?

The follow on question is, do you run vac advance and is it on a ported circuit or manifold?

If it is indeed rich, a bad PV, as opposed to an early one could make it pretty rich and not need the boosters to have airflow over them.  However, your low vacuum makes me think that you have a timing problem

Ross

Initial timing is at 18 degrees advanced. I don't currently have any vacuum advance on it.
Sorry guys, I have not put the power valve in it, as I won't be able to work on it for some time.
I won't be around the computer much either, but will try to check in once in a while.
Thanks