Vacuum Leak

I checked for vacuum leak just about everywhere and can not find one. I checked intake manifold, PCV, carb gasket, valve cover, but nothing but when I plug the carb it revs up. At idle it’s at 960-1100 rpm. I have a 225 oddfire-offy 360 intake w/4 bbl Holley 390 cfm carb. I adjusted the throttle screw to where it’s not touching anything and 1 turn out on fuel mix screws.. any ideas??

 

And old trick is to spray carb cleaner at possible leak areas to see if the engine revs up. Another good thing to do is unplug any vacuum accessories you may have and block off the carb/manifold ports to see if things improve. Vacuum advance, heater controls, PCV, power brake booster if you have one. Any of those things can leak. If it runs well with everything unplugged, start plugging things in one by one until you find the culprit.

 

A vacuum gauge might help.
What's your vacuum reading at idle ?
Does it hold steady or fluctuate greatly ?
Internal carburetor leak; wrong gasket ?

 

Power Brakes?

 

propane torch works too . "No Flame"
if it revs up . you found your leak . works in crankcase too

 

Everyone has covered the usual suspects that you need to check for. I would add where's your timing as too much initial advance can increase your rpm.
Having the idle screw backed all the way out to close the butterflies is not ideal as it will mess you up in regard to the transition slots and give you other issues.
These things can be pesky and require determination and persistent checking.

 

I sprayed and done propane and nothing. My vacuum at manifold was 16 @idle with a little bouncing. My timing with a 4bbl and HEI dist. is about 10-12 BTDC. Power brakes. When the rpm moves up and down I see the timing moves as well.

 

I will have to try to do the isolation of the carb. So just to make sure I’m doing this right. I unplug all vacuum lines. Plug the port and manifold ad PVC vacuum ports? What do I do with power brake line, plug that as well?

 

How about with the vacuum to the distributor advance removed and plugged? Is the dwell constant or is it possible your distributor is wore out and wandering?

 

16" a little on the low side. Not great not terrible. I'd bring your timing to 8-10 with the vacuum advance plugged. a few inches of wandering ok on an odd fire, not known to hold a steady pointer always. I'd remove and plug all vacuum, ensure no carb base leaks/ worn throttle shafts. Adjust timing to baseline (8 degrees) with vacuum gauge slowly adjust idle screws for highest vacuum. I would expect you to get high teens/ low 20's on the gauge with an engine without significant issue.

 

Yep, unplug everything and plug the ports at the source on the carb or manifold to see if it makes a difference. If it does, one of those items is leaking. Plug them in one-by-one to see which it is.

 

I had it at 16-18 with advanced plugged and gauge in manifold port. It did move a bit, but expect that with an oddy. Wasn’t jumping I will unplug everything this week and try to see what it does

 

I plugged all vacuum ports started it up let it run a bit and it was still at 1000 rpm. I covered the carb to see if it would die and it rev’d up. At idle on port side I get 0 on vacuum gauge. I rev it up a bit and it goes to about 10-20. By touching throttle

 

The engine should die either from
1) closing the idle mixture screws at the base of the carburetor all the way. The carb only runs on the idle circuit at idle speeds, and closing the idle circuit should kill the engine.
2) closing the throttle flaps completely. There is an idle stop screw on the throttle side that allows the flaps to close completely in the bores. There is also a fast idle cam on the choke side that should be completely clear of the fast idle screw when at operating temperature.

Air and fuel. If you cut either, the engine should die.

Sounds like you have a big vacuum leak. Most common leak is at the carburetor base gasket. You could also have some other port, like the power brake port, open to the air.

 

I plugged the power brake line. I also sprayed the base of the carb and nothing rev’d up. Only thing I found was on the throttle arm. Sprayed that and it rev’d up. But that seems to be around the throttle arm. Sprayed a little around meteoring device on seals, and it rev’d up a bit but I also just changes the power valve and gaskets. Tightened down 4 bolts on bowl cover and not too tight.
Also if I close mixing screws it does stop the engine. The throttle screw is set to have just the small square available on butterfly and that is with the throttle screw all the way out. Butterfly valves do not completely close with throttle screw all the way backed out.

 

There should be no leakage around the throttle shaft. If there is, that indicates that the carburetor body is worn. A carburetor shop can fix that by fitting an oversize shaft or putting bushings in the carburetor body.

The flaps are not held open on the choke side?

Another possibility is that the flaps in the shaft were taken out and put back so they no longer fit tightly. My understanding is the flaps should fit tightly in the bores when fully closed - I think that's pretty much universal for carburetors. IMO it's not a good idea for someone rebuilding the carburetor to remove the flaps, because of this sort of problem. The manufacturer stakes over the screws through the shaft for this reason. If this is the problem, I'd again think repair is a job for a carb shop.

Maybe somebody with better Holley cred than me should look at it. I know some but not a lot.

Is this Jeep or carburetor setup new to you?

 

Sounds like you have a relatively significant vac leak at either the throttle shaft or metering block, or both. If you had no leak or even a small leak, blocking airflow down the throat would cause the engine to die.

 

I would think that leakage around the metering block could be localized. If the OP says that there is no leakage around the base gasket, but there is around the throttle shaft, that's as much localization as would be needed to detect a leak around the metering block. The gaskets for the metering block are right there, between the primary bowl and the carb body. I recall there is only one gasket between the bowl and the body on the secondary side with this vacuum operated secondary.

Maybe obvious, but the secondary must close too. Anything holding open the secondary would be a problem.

 

I had the jeep running for 1 yr when I got it running I didn’t pay attention to idle more so then getting it moving. It is a 4160 390 cfm Holley so it has two gaskets around metering block one on each side. Changed when the PV was changed even put chapstick around gasket from some old timer who works on hot rods. So if it’s coming from throttle rod, I assume I need to get it rebuilt or can I replace the base plate?

 

I would think the base plate would be most of the cost of replacement of the carburetor. Fixing the throttle leak is a common thing, if you can find an olde tyme shop that works on carbs today.