<VV> WC: Curing my off idle "stumble"

Sat Apr 25 19:28:49 EDT 2009

Hi David,

Thanks for your input and everybody else who emailed me over the last few days!  Here’s the latest (It’s a bit long):

I advanced the timing - with the car running and the vacuum line connected - from about 16 degrees in 2 degree increments until I got to an estimated 30-32 degrees (it’s off the scale at that point). At each step I test drove the car under various load and speed conditions and at the 30-32 range about 90% of the stumble had gone. I'm running with the top down and there is no discernible pinging on premium gas even when deliberately lugging the engine. I'm not sure exactly what role the Clarks anti-knock unit is playing here but the lack of pinging suggests that it seems to be working but there is no way of knowing what the actual timing is under load of course. The car feels peppy and is not overheating and if anything it seems to be running a little cooler. I still have a small stumble transitioning through about 1500 RPM but a bit of "throttle management" minimises that. I ran it around the neighbourhood and then at 70 MPH on the highway and no problems showed up. BTW, I have the Clark knock eliminator not Ray’s Safeguard unit (but one is on order).

It starts fine and better than ever when stone cold with this setup. It was suggested that it might have difficulties starting with this much advance but so far that hasn’t been a problem. Now, one tap of the throttle to set the choke, about 5 seconds or so on the starter and she’s running

BTW, the carburetor was rebuilt by Grant Young and distributor was also a correct turbo Dale rebuilt that I got when I got the advance/retard unit. I have to say I didn’t actually check them given the source. I am running an Ignitor II with the correct ignition coil, new plugs and the wires are from Seth, The harmonic balancer is also a rebuilt so I don’t think I have a problem there in terms of it slipping. The engine has less than 3,000 miles since a complete rebuild.

A few hours ago, I ran the engine until warm and checked the timing. When I fist blip the throttle the timing retards briefly about 6-8 degrees to the 24 degree range and then settles back. I am not exactly sure why this blip happens but you can see the advance/retard arm move. >From 1000 to about 4000 rpm it stays at a steady 30-ish degrees(no load of course). I tried pinching off the vacuum line and sure enough that retard blip goes away. Incidentally, I just checked “How to Hot Rod Corvair Engines” and it mentioned 29.5 advance at 4800 RPM so I don’t think I am wildly out of line here. That’s before the stock boost-based retard cuts in of course (supposedly up to 12 degrees at 4.5 PSI).

I test drove the car with the vacuum line pinched off and it went fine. My guess is that I could run that way permanently and just rely on the knock eliminator to retard the timing under boost. For safety reasons I won’t do that – just in case the anti-knock unit fails.

As an side, during one of my many tests over the last week, the car was cold and the choke was initially on – et voila - zero stumble so I guess that might be an indicator of a slightly lean condition? I had been running very rich and I lowered the float so perhaps I went too far?

The bottom line is that the above setup has dramatically improved the driveability and I’m going to leave it that way until I get the SafeGuard unit installed.

Thanks for everybody’s help!

Regards,

Bob

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From: corvairs-owner at cybrus.net [mailto:corvairs-owner at cybrus.net] On Behalf Of David Stevens
Sent: April-23-09 11:38 PM
To: corvairs at cybrus.net
Subject: Re: WC: Curing my off idle "stumble"

Hi Bob, some thoughts on your stumbling retarded engine.

 Vacuum shouldn't be affecting the timing as the diaphragm arm is supposed to sit tight  against the tin housing, preventing it from advancing or retarding  until boost presssure retards the timing. If it does move , it would be toward the advance position & that would only occur if the  diaphragm spring is shot/weak and not sitting tight. 

Do you have the correct press/retard unit? -it should have a sealing lip on the tube where the pressure line attaches. The standard vacuum unit has no such "lip", just a straight tube. The arm on the press/retard should have # 218, 224,239.or 250 stamped on it. 

The only other way I know that could cause it to retard the timing when you "blip" the throttle is if the distributor centrifugal weights are installed upside down (easy to do) , To check, remove the points plate, hold the points cam with one hand while you pull the end of one weight outward (away from cam). If this trys to turn the cam clockwise (advancing  the spark) , then you're ok. If it turns it anti clockwise (retarding it), they're upside down & would account for your 8 degrees retarded timing . Flip the 2 wts. over which then switches the mounting pins. With the springs attached to the cam plate everything should be tight (no play or wiggle).  

Hope this helps-you may already know this stuff.

David in Cobble Hill

----- Original Message ----- 

From: Bob Gilbert <mailto:bgilbert at redshift.bc.ca>  

To: corvairs at cybrus.net 

Sent: Tuesday, April 21, 2009 9:17 AM

Subject: RE: WC: Curing my off idle "stumble"

Hi James,

Thanks for the offer but not now..  J

All joking aside though, while there is a certain “cool factor” with the turbo,  for pure driveability I still prefer the time I spent with 140’s.

Regards,

Bob

From: corvairs-owner at cybrus.net [mailto:corvairs-owner at cybrus.net] On Behalf Of James Hogarth
Sent: April-20-09 10:07 PM
To: corvairs at cybrus.net
Subject: RE: WC: Curing my off idle "stumble"

Time to dump the Turbo Bob.  Too much trouble.  I would be willing to trade down.  My 140 straight across for your turbo ;)

Cheers,

James Hogarth

CORSA Western Division Director

Res: 604-476-1434

Cel: 604-319-9487

From: corvairs-owner at cybrus.net [mailto:corvairs-owner at cybrus.net] On Behalf Of Matt Nall
Sent: April-20-09 9:29 PM
To: corvairs at cybrus.net
Subject: Re: WC: Curing my off idle "stumble"

You're not setting the timing while running?  with the P/R  connected?   If so  30-32 deg. at idle! 

VA  falls off on any acceleration.... on any engine...

  your engine will crank at 24 deg... then as it starts go to 30...and only come back in at Cruise conditions..

 So, you'll be running at 24  most of the time...

 When boosted.. No Vacuum!  and as pressure retards....timing backs off from 24...

Matt Nall
Charleston, Oregon

-----Original Message-----
From: Bob Gilbert <bgilbert at redshift.bc.ca>
To: virtualvairs at corvair.org; corvairs at cybrus.net
Sent: Mon, 20 Apr 2009 8:53 pm
Subject: WC: Curing my off idle "stumble"

Hi,

I’m still trying to track down an annoying off idle stumble.

The car is a 66 180 , engine rebuilt, runs fine otherwise. I have the correct turbo distributor and a Dale advance/retard unit and I’m running the Clarks knock eliminator with dual sensors

When I set the timing to 22-24 BTDC at idle, I can blip the throttle and watch the timing retard by about 8 degrees (exactly as reported on Ray Sedman’s American Pi site).  I think this is the cause of the stumble.

As an experiment I pinched off the vacuum line and sure enough the ignition doesn’t retard and the stumble is significantly reduced (and the car feels peppier)

Would running in this mode pose any problems in the long run or at highway speeds given that I have the knock eliminator?

Any other ideas or suggestions?

Thanks in advance.

Bob

.

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