the Dune Buggy Archives
DBA Forum - Not logged in
Previous Next Up Topic Engines / Corvair Power / Covair 390 on a 140 need some help. (26537 hits)
By John L 1232 Date 2008-10-19 23:53
Hi Guys.. I have a covair powered engine that I converted to the 390 on a 140 engine. I have been haveing some problems trying to get the engine to idle. The carb is new and I did make some changes on the primary side when to 2.5 hg power valve, 50cc pump, and 49 jets. Still cant get the thing to idle down low with out stalling. I was looking at one of the pictures on your forum  and it looks like someone has the 390 installed and the vacuum not used as well as the PCV line blocked off? I could use some help on any suggestions first time using the 390 but have over 15 years of experience with corvairs and engines. But when it comes to the 390 Holley not that much. It is very simple to adjust and but it seems like I can get it down to idle even when I adjust the fast idle screw.?


Wakefield, MA
By Steve L 858 Date 2008-10-20 18:15

I have a Corvair in my Manx with a 390cfm Holley.  Make sure all your gaskets are good at each of the runners to the heads as well as the base of the carb.  You may have a vacuum leak somewhere.  Also, try getting to idle with the vacuum adv. plugged just to verify nothing going on with dist.  I don't have a PCV on my motor but rather a draft tube so not using the ports of my carb except the V.A.  I'm running about 18 deg. BTDC and mine idles smooth at about 900rpm. 

By John L 1232 Date 2008-10-21 00:00
Hi Steve;

Thanks for responding, I am going to try blocking off the engine PCV line that is at the bottom of the carb. I think that allot of the extra vacuum is pulling from the PCV line from the engine. I will probably put in a filter on top for internal pressure.I could go with a PCV valve as well.

I appreciate you response.

By Bob E #19 Date 2008-10-21 15:47
John in Wakefield.

I'm using the 390, but still learning and adjusting things, 1st time Corvair.  I've had problems with the idle also and currently have it smooth and running around 1,200, , but would like to get lower. 

Currently using the 110 dizzy with vacuum advance (that helped), timing is at 14, but response is not what I'd like and plan to try more advance, 18 like others and see how that works. 

I also use the early down draft tube and have all other carb ports blocked off (2 or 3 of them)
and the two on the intakes.

I'm almost a neighbor of your, located in Hopedale, MA.

Boston Bob E.

Boston Bob E - -
By Bill K 96 Date 2008-10-21 15:57
I believe the Corvair had a metered opening for the PCV system.  I don't recall the size but this may be the issue you all are running into.  I'll do some research on the subject and get back with you.
By John L 1232 Date 2008-10-23 01:31
Thanks for getting back with me. What I am going to try to do is remove the PCV line into the engine and block it off at the based of the carb with a nipple cover. This is at the intake cross over manifold at the Holley base. Then tie the line into the air cleaner from the engine Pcv orfice or directly to the engine, dont knwo which one yet. There is a Pvc line that originally came with the engine has three connections, one side went to the air cleaner the other went to the balance tube and the other had a built in Pcv orifice. I am not going to use the vac advance at this time, I am using a dual point from mallory and dont have the provisions for one on it. So I think that the PCV line is sucking to much air at the base of Holley and not enough throught the ventorie or idle circuit.? Well see?

Wakefield, MA
By John L 1232 Date 2008-10-23 21:23
Hi All:

Well I did some fiddleing with the PVC line and blocked it off at the base of the carb. I am able to get the car to start and run for about a couple of seconds but then it seems to die and it wont comeback if I give it some gas. I am thinking that I could have an electrical problem somewhere...>? Sometimes  I think that it could be the starter soliniod kicking out. When I turn on the ingnition key I get voltage to the points when they are open but I am not sure what is going on after the fact. I am also thinking that it not be getting enough gas as well. I do have a flow gage after the pump and I can see gas at the inline filter before the pump. I have to pump the carb before starting it and it will start up but just for a moment then it dies out? Lost for words?

Wakefield, MA
By Bill K 96 Date 2008-10-24 15:23
John,  The metered PVC orefice for naturally aspirated Corvairs is .062.  Turbos are .089.  Looking at my 110 the orefice is located on the balance tube running between the two sides of the engine.  A hose runs from the balance tube and tees into another larger tube that goes into the aircleaner  and vents the crankcase.

Since you switched to a common manifold you have connected both sides of the engine and done away with the need for a balance tube.

The turbo uses a manifold that connects both sides of the engine too.  The turbo uses a hardline with one end tapped into the intake side of the turbo and the other end restricted to .089 then tee'd  into the crankcase breather line.  The one end of the breather line is connected to the aircleaner just like the N/A engines.

I would fashion a system similar to what the turbo has.  Run your crankcase tube up to the base of your air cleaner and tee that into the base of your carb's PVC port with a restricted orefice.  This will keep any crankcase pressures at bay(less leaks) and lower the contamination of your oil.

Hope this helps!   Bill
By John L 1232 Date 2008-10-25 14:54
So what you are saying is remove the balance tube all together. Block off at the base of the carb pads at the heads and then plug the bottom base of the intake runner just at the holley. Currently I have the balance tube hookeded up to the bottom of the runner tube. Remove this line and place a cap on the end of the port on the bottom based of the holley carb. I did cap off the PCV intake on the Holley base and will run a direct line from the engine crankcase to the air cleaner.

I am able to get the engine to start but still having a problem with the carb getting gas to the engine. It is Not sucking any gas through the ventories. If I give it some gas at the begining it will start up but not run after a couple of seconds. When the choke is on it will stay running but if the choke opens it dies out.

I am thinking that I have a blockage somewhere in the carb what do you think.

John Luscinski
Wakefield, MA
By Jeff GS Date 2008-10-25 18:37 Edited 2008-10-25 18:40
It does sound a bit like blockage John.  Does the accelerator pump work?  That should squirt every time the throttle is moved open, running or not.  Sounds a bit like the float bowls are running dry after a short bit of running?  That would be a fuel feed issue, fuel just not getting into the bowls.  When it quits running - immediately try to work the throttle and look for fuel coming out of the accel pump jets.  If fuel does come out as normal - then you've got fuel in the bowl, if not the bowl has run dry (try at least 2-3 squirts with the accel pump - should be a full squirt each time, not petering out).

The basic operation of the PCV system is to vent the crankcase vapors, while not creating a vacuum leak in the intake system.  Flow generall starts from a filtered source (intake air filter, or remote) - then goes into the crankcase volume somewhere - generally into a valve cover (or both covers?).  Travel path will then be from the heads, down into the crankcase and back out, through some kind of closed tube, through a PVC valve and into the intake under the carb.  Pretty simple really.  On a V engine it generally runs from one valve cover, through the crankcase and out the other valve cover - then into the intake system.  The PCV valve is the key, as it seals up tight under high vacuum (idle conditions), but opens up under low vacuum and/or increased crankcase pressure.  Using a PCV system really helps to keep the engine clean inside as it removes all the water vapor that condenses after the engine is shut down.
My Subaru works like that and I gained a better understand of it when I re-worked it for a more compact and cleaner looking system.  The turbo complicates it, but otherwise it's pretty simple and could be copied on any boxer engine!
By Jeff GS Date 2008-10-25 18:53
Hmmm... re-reading your earlier post John, it sounds like your running issue may be clogged idle jets?  That would be the case if, as you said you pump the gas a couple times and it starts up (from the fuel from the accelerator pumps) but then nearly immediately stops running.  You "should" be able to keep it running by continuing to work the throttle (keeping the accel pumps working) for as long as you want, but when you let it idle it'll die if it's not getting any fuel from the idle jets.  Tear down, clean the jets, blow out all the carb orifices with carb cleaner and put back together.
I'm presuming it's not flooding causing it to quit?  Fuel would be coming out of the bowl vents and that would be fairly obvious!
I recall the Holleys run a power valve.  IIRC, it uses engine vacuum to hold it shut, but allows extra fuel to flow at WOT.  I guess if it wasn't sealing/operating properly it might cause an internal vacuum leak, affecting idle operation?
If you don't know for sure, I'd pull that carb down and check everything out and throw in a rebuild kit with new gaskets and seals.  If it's old, could be a multitude of issues lurking inside!

By Bill K 96 Date 2008-10-26 03:00
John,  Reading back to your original post you stated that you made some mods to a new carb.  The 50cc pump was right on but your power valve at 2.5 seems off to me.  This means that there won't be any additional fuel available(high speed) until the the vacuum drops below 2.5.  The valve stays shut until the vacuum drops then opens to deliver additional fuel to take over when the fuel delivered by the accelerator pump has been used up. To figure out the correct valve you need to get the engine to operating temperature, take a vacuum reading, then divide that in half to get your proper size valve.

Now Jeff may be onto something also.  Did this run engine run okay before you switched to the Holley?  Is the fuel pump delivering 4-6 psi?  Is the old VW gas tank full of crud and now your new carb?

You also mentioned a possible electrical problem....Are you running a resistor?  Are you running points or have you switched over to electronic?

I just went out to look at a junk 140 engine I have and the hard line that comes from the top shroud up into the air cleaner also has the tee with the metered orefice.  So all you need to do to is run the vacuum hose from the base of the Holley(PVC port) to the tee.  Check to make sure yours has the small orefice.  Again, there should be no need to run the crossover tube any longer since both sides of your engine are now connected.
By Jeff GS Date 2008-10-26 13:05
Sorry about that previous post - I went back and re-read from the beginning - NEW carb....  The old foot-n-mouth kick'n in again!

If the carb bowls aren't staying full, it could be from crap from old and dirty fuel system.  Doesn't the Holley come with a small sintered fuel filter at the carb inlet fitting?  Could it be clogged?

My way of thinking is to work from the "known's" - beginning at the source.  Know the fuel coming out of the tank is clean, know the pump is pumping it properly (observing flow out a disconnected fuel line and into a clean container - also looking for crap coming out), all filters are fresh and clean, carb settings are correct - float level, jet sizes, etc., no blocked orifices inside the carb (even for a new carb).  That can really help to narrow down the unknowns for sloving the problem!

By Jeff GS Date 2008-10-27 16:47 Edited 2008-10-27 20:58
More on PCV systems:
Here's the link (changed to a pic!) of a Subaru 2.5L normally aspirated PCV system diagram.
Fairly simple layout and while likely not identical to original 'Vair layout, I don't see why it couldn't be adapted to any boxer layout engine.

By John L 1232 Date 2008-10-29 02:36
Hi All:

Thank you for repling to my post. I will give you the run down of what I have been doing to clearify things
1. Tank has been cleaned and filled with gas.
2. Has inline filter to the pump. See through type. Is Clean
3. Pump is working which has inline holley fuel flow indicator.
4. Shows above 4 psi to 7psi.
5. Strainer is clean at carb connection.
6. Has 50cc accelerator pump.
7. 49 jets.
8. 2.5 hg power valve. Will change back to 6.5
8.5 Vaccum port at base of Carb has been blocked off so no connection between the carb or engine is connected. The connection will be at the air cleaner.
Vaccum advance has been blocked off.
Using Mallory dual point with no vacuum advance.  RESISTOR BALLAST  is there but no being used  it is disconnected. Wire from solinoid is connected to + of coil, Coil is new points and cap and rotor are new.
9.The carb has been cleaned twice with both bowls off and new gaskets.
10. It does squirt gas into manifold.
11. When choke is closed engine starts and stays running but Runs high rpm, when the choke opens it dies out.
11.5 When the choke is open and I give it some gas squirted into the carb before start up it will start but only for couple of seconds as soon as I let go of the key in the ignition. If I keep my foot on the pedal it still wants to die out.
12. I have not changed or removed the balance tube yet.

I am going to read your post that all of have sent me again but I think that I have solved the problem with PCV system. I think that I have a carb problem more than anything else. I have gas in the bowls but I dont know how much. I think that I need to have a baseline on the correct setting for the needled and seat. I think that I need to establish that first.

What is that setting I dont know? If I take out the bowls and readjust what am I looking to for to make that adjustment. I guess this means opening up the bowl plugs on the sides and adjust but I cant get the car to stay running? I am still confused... It has to be something simple?

John Luscinski
Wakefield, MA
By John L 1232 Date 2008-11-03 02:19
Hi All:

I just wanted to give you all some feedback. I had very good luck this weekend. I was able to get the car going with great success. All the things that you mentioned to me was looked into. I did open up all bowls cleaned out all the ports and holes with compressed air adjusted the floats midway changed out the 2.5 and back to the 6.5 power valve, took out the balance tube rearange and located the engine vent to the carb base. Adjusted the idle speed and mixture. Engine idled down to about 18 BTDC. Changed the oil, smelled very strong. New filter as well. Very good response to engine speed but no street driving yet. I think that my next project is a clutch job.... been there and have done that. I feel like I have made allot of succeses this weeked.

Wakefield, MA
By Jeff GS Date 2008-11-03 21:43
Sounds great John!  Now to figure out which thing you did really fixed it!  LOL

If you ever need any expertise on tuning it further, I'd shhot off an email to Steve of Rear Engine Specialists:

Steve's been around hot-rod Corvairs probably a looooong time.  He can give you the low-down on how to get it running in top form.

By John L 1232 Date 2008-11-05 17:47
Hi Jeff:

I beleive that it had to be the fuel bowl level and replacing the gaskets. The gaskets were in good shape but replaced them all. I then blew out some of the venturie port and holes with some compressed air as well. I put everything back together made some of the mods that we have talked about before such as remving the balance tube and pluging up some unneeded ports. A couple of turns on the engine had the choke working and walla!. I do have to change the choke settings at idle starts way to high then it idles off nicely. Never had to touch the secondarie screw at all. From looking at the PSI gauge for gas delievery I am think that I am running high at about 7-8psi on the output at the gas pump. Maybe to high.. ? What do you thing about this?

By Bill K 96 Date 2008-11-05 21:02
John,  Glad to hear you got it!  The Holley carbs I have run in the past run best between 5-7 psi.  Weber's typically like no more than 4.5 psi. 

By John L 1232 Date 2008-11-06 00:54
I am thinking that I might be getting to much fuel in the bowls but who is to say that the gauge is right.
By Bill K 96 Date 2008-11-06 22:54
John,  If you have fuel coming up out of the vent tubes then you have too much fuel pressure or a stuck needle and seat assembly.  Alot of store bought gauges are far from accurate!

By the 'Maniac Date 2009-07-25 04:44
I know its been awhile since this post was active, but here's some info to be shared.

On a Holley 390 cfm 4bbl carb on a 140hp Corvair engine with tube headers, the primary jets should be #56's, the secondary metering plate should be a #57. Use a 31cc fuel pump & a 7.5 hg power valve, though a 6.5 hg may work fine.
Any vacuum ports not in use need to be plugged.
Previous Next Up Topic Engines / Corvair Power / Covair 390 on a 140 need some help. (26537 hits)

Powered by mwForum 2.10.2 © 1999-2007 Markus Wichitill