head gasket thickness how much is too much?

[crider]

Just fired the BBC in my dragster for the first time today, It acts like it had a giant vacuum leak. doesn't want to stay running I have closed the idle mixture screws completely and have the carb idle speed screws backed all the way out as well. I even tried taping the air bleeds off. It will stay running like that but is idling around 3,000 rpm with everything closed off? It's a 427 tall deck block. never decked. 1050 dominator on a victor tall deck intake, so no spacers between heads and intake.  I had to go with a .065 head gasket to compensate for a bit of positive deck height on the pistons. Is it possible that has raised the heads up enough that it could be sucking air around the intake manifold? or should I be looking at something else?

[Frontenginedragsters]

Hello Crider:
 Sounds like a vacuum leak to me.
If its a tall deck block are you using spacers or do you have a tall deck intake?
Spacers work well but have an increase chance of vacuum leak.
Two types of vacuum leak are usually easy to find.
Use some type of spray can cleaner like brake clean. First spray it down the carb to see the reaction your looking for.
Some sprays will burn well and the engine will rev up. Some will try and kill the engine. Spray all around the intake to cylinder head area.
Also spray the base of the carburator to the manifold. Look for the same reaction as when you sprayed down the carb.
 The other type of vacuum leak is internal. Blank off the valve cover vents until you only have one open hole venting the engine.
You should have a small amount of blow-by or pulsing. If you have Vacuum there then the intake gasket is leaking from inside, in the lifter valley.
Most of the time when its internal its also sucking oil in from the lifter valley. Look at your spark plugs.

Hope you get her fixed, summer is winding down.

Matt

[crider]

Yes, it's a tall deck manifold, so no spacers. I'm pretty sure it is a vacuum leak too. Hopefully I will get a chance to work on it some this week. I'm still a long way from being ready to make a pass down the track. But I have until early Dec. before the last of the semi-local tracks closes down for the winter. Hoping to get it to a point I can make a few passes so I know what needs changes so I can be ready next spring

[crider]

Ok, still struggling with this thing. What can act like a vacuum leak that isn't a vacuum leak? I have checked for leaks everywhere I can think of to look. I've sprayed starting fluid around the carb base and intake flanges with it running. No changes while being sprayed. I puled the intake off and sealed off then pulled vacuum on each intake runner again no leak found. Put the intake back on and sealed the carb flange off and hooked up a smoke machine to it and couldn't find any leak that way either. I am stumped. Is there anything in the carb that could be causing this? 1150 2 circuit dominator. I pulled it completely apart and cleaned every passage. Didn't find anything that was blocked.  Or any other ideas as to what may be causing the problem? I'm out of ideas

[BK]

Are your headers real short or real big? I watched an engine without headers, just open exhaust ports not idle. Put headers on ran normal.

[crider]

They are short zoomies, I'll slip some pieces of pipe on them as extension,s. I've tried about everything else it's worth a shot

[denverflatheader]

crider- since you’ve eliminated possible vacuum leaks, my guess would be the carb.  The first item I’d look at would be the power valve.  The power valve is an enrichment system with purpose to add additional fuel to the carbs main well when intake manifold vacuum declines either through high speed or heavy load.

There are two different styles of power valves, one with small rectangular holes and one with even smaller round holes located just above the threads.  The rectangular holes flow more so that is probably what’s in your dominator.  The two different styles of power valves use a specific gasket, make sure you have installed the correct gasket for your power valve or it could leak and cause a higher idle.  Secondly, if your power valve is bad and stuck in the open position, once you start the engine it will flow more gas instantly through the carb and cause higher idle, versus when it should open as it senses a decrease in intake vacuum, i.e. full throttle. 

Without starting the engine, just turn on your electric fuel pump and fill the bowls, then shut off the electric pump.  If your power valve has a bad gasket seal or if it’s bad and stuck open, gas will flow into the intake and slowly drain your float bowls.  For example, fill at night, check in the morning and if your float bowl has drained down, your power valve needs attention.  Alan

[hemidakota]

My friend Scott's dragster is N/A injected alky and with zoomies it idles 2500 rpm, swap to headers and engine will idle as low as you want and also changes tune up quite a bit. We have to pretty much close throttle blades to keep idle at 2500 rpm with the zoomies. No real answer as why except for headers create a little back pressure and do not scavenge maybe? Camshaft could have a lot to do with it? It works so we have not been too worried about it.

[Frontenginedragsters]

We run a S.B.Chevy with Enderle stacks on alcohol and zoomies.
Idles at 1500 RPM very smooth.

[Paul New]

Do you have to have the idle cranked up to far that you are no longer in the idle circuit of the carburetor? I know years ago running a carb and I had to drill holes in the throttle blades to get it to idle low.

[crider]

Paul, no it is idling that high with the butterflys completely closed. That is what had me thinking that I have a vacuum leak

[Paul New]

hmmm yes that is kind of tough, do you have another carb you could try? a known carb that is working on someone else's car would be best that would rule it out.

[crider]

No, i wish I did, but I've been away from racing for too long now. Don't really know anyone locally to borrow one from

[crider]

I'm glad to report that I finally got the engine to Idle down, thanks to Bk for the tip on the headers. I slipped some extensions on the end of the pipes this evening and it took my problem away. I can idle it down as low as need be now. Thanks to everyone that offered suggestions. I may get to run this thing before the snow flys this winter after all

[MikenMpls]

I would have never thought that carbs are that sensitive to issues past the intake valve but I guess it makes sense. When the intake valve is open its all one signal to the carb. Wow!