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Re: [T3] slightly jerky acceleration - update, plus my original mail with the details


Chris,

Thanks for the re-post. I don't think the type 3 list never got to see this 
whole description.

I just noticed that the 2 posts I just wrote in response to yours had gotten 
addressed directly back to you. This means that you've set the "Reply to:" 
field in your email software. There's nothing inherently wrong with this, but 
most of the time it's unnecessary, and it overrides the email list's efforts to 
get responses directed back to the list. This may explain what happened to your 
posts: They may have gotten diverted into personal exchanges.

Unless you really need this for other reasons, it's best to leave the "Reply 
to:" field in your email software blank. I had to change mine once I realized 
what the effect was.

On 26 Sep 2005 at 10:02, chris wright wrote:  

> You can imagine it best by imagining accelerating on a very bumpy road so
> that the car jerks over the bumps.

To me this sounds more like ignition "miss" than anything else. I would be 
looking at ignition problems.

Under acceleration, the throttle is open more so more air/gas gets in the 
cylinders; this leads to higher chamber pressure when the spark occurs. Higher 
pressure air/gas is a better insulator, so higher voltages are needed to break 
down the spark gap. At higher voltages, there is a greater chance that the 
spark will jump over somewhere else first.

Things to check:

Measure the resistance of each of the spark plug wires. These may have been 
resistor wires originally, but you'd be best off now with the Bosch SP 
connectors that have the 1000 Ohm resistor in them. Make sure they are used 
with copper core wire and that the resistors haven't gone open. Measure each 
wire from end to end. Measure the coil wire, too; it should measure zero, 
unless it has one of the coil end resistors in it. Individual connectors which 
have gone open can be replaced; they are cheap.

Around '71, Bosch/VW came out with some nice red silicone rubber covers for the 
ends of those connectors. They seal the SP insulators from dirt and water and 
make the engines much easier to start in damp weather. They are a worthwhile 
investment. DON'T buy the earlier (~'67) version which is just a cap on the 
connector; these don't work as well and usually come off on the SP.

Same with the dist rotor. I believe they should be 5000 Ohms, but some are zero 
Ohms. Just make sure that it isn't open.

Check the SP wires. They should be copper core with no cracks in the 
insulation.

Check the spark plugs and make sure they haven't worn so that the gaps are 
excessively large. The gap should be .028". The center electrode should have 
nice sharp square corners.

Check the cap, inside and out, for dirt, carbon tracks, and moisture.

Your dwell should be 45-55 degrees and should be stable regardless of rpm.

I've also seen startup ignition problems in rainy weather when water collects 
in the air intake system and gets sprayed over the cylinders and heads at 
startup. The SP connector covers mentioned above help that a lot, but another 
thing to do is to make sure that the drain hole at the bottom center of the fan 
housing is clear. They often get clogged with dirt; clean it out with a small 
stick.

Where are you located? Has your weather been rainy/damp lately? A bit of 
condensed moisture can certainly cause problems like this, and the moisture 
will be driven off by engine heat once the engine warms up.

-- 
Jim Adney
jadney@vwtype3.org
Madison, WI 53711-3054
USA

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