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Re: [T3] Hesitation


On 15 Oct 2001, at 10:38, Herman Haustein wrote:

> A while back I wrote about having a problem with hesitation on
> acceleration, under load. MY car is a Carb'ed '69 T34. 

Are they the OE carbs and the OE dist? If not, what are they?  

> Well, after checking the carb for vacuum at different rev's and
> checking the distributor for advance at different vacuums I've
> reached the conclusion that I'm getting to much advance, to soon.
> My carbs are new and are giving 200mmHg vacuum at 1500r.p.m and
> 300mmHg at 2000r.p.m. - according to the Bentley for a similar
> distributor this vacuum should only be reached at 3000r.p.m
> (crankshaft). 

You have to be careful when making comparisons between curves 
of different carbs/dists. The Bentley only covers the FI dists. Also, 
the Bentley graphs are taken straight from the Bosch literature, but 
they are NOT intended for you to make translations from vacuum to 
RPM. They are only written that way to allow the graphs to be 
compact and still allow you to read the mechanical advance as a 
function of the RPM and the vacuum advance as a function of the 
vacuum.   

> This is confirmed by the car performing very well,  on uphills
> (under load) it runs well at about a 1/3 pedal, but lousy above
> that, worse, the more I put my foot down - I hear a Brrrr... sound
> from the back - I guess this is pinging?! 

Probably not pinging. I'm guessing that it's more likely that you just 
don't have the right advance at those RPMs. It's possible that you 
have a mismatched dist and carbs and that they don't work 
correctly together.  

You might find it instructive to put a Tee in your vacuum advance 
line and run a gauge into the car. That way you can see what the 
pressure REALLY is when things are running well and running 
poorly. I suspect that you will be surprised.  

Carbs that are made to work with a vacuum advance have special 
vacuum drillings that are made to provide an appropriate vacuum 
signal to the vacuum advance. Then the dist has to be provided 
with the proper advance can to match that carb. Most aftermarket 
carbs do not have this drilling, so they tend to only work well with 
mechanical advance.  

My guess is that you have most of your advance at idle and that it 
actually goes away as you put your foot into it and the RPMs rise; 
exactly the opposite of what you want, and what you were 
expecting. That's just a guess, however.  

> So, what is the solution, less static advance i.e. set advance to
> 3 deg. BTCD instead of the current 7.5 BTCD ? 

Is 7.5 BTDC the correct setting for your car, or did you come to 
this conclusion based on some info that you used to come up with 
a timing scheme for your non-stock setup?  

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

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