<VV> Bucking / Stalling diagnostics

[Dale Dewald]

At 09:25 7/03/2008 -0400, Doug Mackintosh wrote:

 >>>See comment below...

>OK all you diagnostics wizards, I need help!
>?
>My daughter's Spyder has an intermittant bucking / stalling problem. This 
>has been going on, but progressively getting worse, over the last few 
>months. In addition to the bucking / stalling it had difficulty revving 
>over 4000 RPM and the tach needle would wiggle and drop when it was 
>bucking. I found a very sloppy distributor bushing and repaired that, 
>which seems to have cured those problems. The car ran like a champ after 
>that for 2-3 days.
>?
>But last night we went for a Corvair cruise out on the back roads and 
>after about 30 minutes of spirited driving it started acting up. Here are 
>the symptoms:
>?
>- It starts and drives fine for a while
>- It then starts to buck (jerk) intermittantly
>- The bucking increases in frequency and severity
>- It then stalls for a few seconds, then catches again and is OK
>- The stalling recurs, increasing in frequency and lasting longer each time
>- Eventually it stalls long enough that the car coasts to a stop. After 
>waiting a minute, it can be restarted and goes a short distance before it 
>happens again
>- Last night, we stalled at a convenient place and I let it sit a few 
>minutes, then had Leigh crank it while I checked for spark (coil wire held 
>near ground) - nice fat spark (but it may have already cured itself from 
>the resting time). We restarted and drove (at a leisurely pace) about 10 
>minutes back home.

These are the symptoms of a failing coil.

> >From the behavior I have seen I am suspicious that the problem is heat 
> related since it never stalls until we have been on the road awhile, and 
> it seems to cure itself by sitting briefly, and it seems the longer it 
> sits, the longer it stays "fixed".

Old coils often fail by the secondary winding shorting to ground.  When 
cold there is sufficient gap for them to work properly. As they heat up the 
gap closes, then there is weak or no spark.

>  Also it seems to get progressively worse and more frequent with driving 
> time. The only other possibility I can think of is a partially plugged 
> gas cap vent making it "run out of gas".

The car starts right up after sitting--right?  If it had run out of gas you 
would need to crank for a while with a mechanical fuel pump.

Dale Dewald
Hancock, MI