Hugo Piche writes:
> A while ago I posted a message regarding some problems I was having
> with my recently bought Gas Gas GT25 Contact '92.
> 
> Well, I'm still experiencing some problems... it's all about jetting
> I guess.
> 
> When I bought the bike, it was running 1/2 regular gas 1/2 aviation gas
> with 80:1 Gas/oil mix (Clutz oil). The pilot jet was also changed for

That's 'Klotz' oil. 

Clutz oil is what the points fairies spray on my bike before events
to make me drop points on things that I can clean in practice...

> a slightly larger one (an upgrade of '5' was apparently done, but the
> owner could not confirm that....)
> 
> I decided to run the bike with a different setup. Richer ratio,
> something near 64:1, using regular gas only and Golden Spectro oil.

Ok.  Remenber that more oil == less gas.  So 64:1 is a richer oil ratio
but a (slightly) leaner fuel-air ratio.  If you do the math, it's:

1:80 == 1.23 % oil
1:64 == 1.54 % oil

So the difference is .29%  Not large.


> Many people recommended me not to use that aviation gas because of
> the lead it contains. Finding it here would have been a problem
> anyway so I really want to run the bike on straight gas. Also, the guy
> was oiling his air filter with his usual Clotz two-stroke oil... I
> decided to re-oil the filter with an appropriate foam filter oil.
> 
> All theses changes made the bike run richer, and it is the reason I
> assume why it keep on fouling plugs. After not even one tank used, I
> have to stop... the plug comes very black. (with the guy's old setup, it
> was very rarely fouling plugs, from what he said).

My guess is that you have over-oiled the filter.  The rich running from
the over-oiled filter and the larger pilot, combined with the
extra oil is causing you to oil-foul your plugs.

If you're a beginner you're probably spending a lot of time idling around
while practicing.  For that, 64:1 is too much oil.  80:1 would be better.
If there's oil driping out the pipe then you are running too much oil.
As a comparison, the top US riders run very lean oil ratios, like 100:1
or 120:1.  Of course they get new rings for free, and they don't want to
take a chance on the bike loading up in a section...

The filter needs only enough oil to wet it.  I dribble a little
oil on the filter then squish it around to distribute the oil.
I use about 20-30ccs of filter oil.

BTW, AvGas is not good but race gas is.  Half or 1/3 race gas
and the rest pump premium is a good way to make sure that your
bike won't start detonating.  Trials bikes don't use all that
much fuel so it's not all that expensive.  I buy race gas
at one local bike shop that carries it, you might have to
look around to find it. 


> I tried just to give the carburetor more air with the air screw, without
> any signifiant change.

No change in running, or no change in fouling?
If there's no change in running that's a sign that the pilot is
not right.

> Does the pilot jet has really something to
> do with my problem? Maybe the changes that I've made makes it
> necessary to put back the stock pilot jet? (which I don't have...)  I
> disassembled the carb and it has a #33 pilot jet right now.

I'd try going back to the stock pilot.  You'll know if it's too
lean- the engine won't want to come back to idle after you blip the throttle
and it'll take a long time to go off the choke.


> Does someone knows the stock settings/jet sizes for this Dellorto PHBL26
> carb? Also, some people tells me about reducing the plug's gap to
> prevent plug fouling. Some told me about a 0.02 mm gap. Other tells me
> about 0.06mm.  Well... I wish I could find a shop manual for the bike,
> but it seems that this item does not exists...

.06mm is right, .02mm is very small.
Smaller gaps in general make the plug more resistant to fouling
but .02mm is too small.  A smaller gap means that there's less
area of fuel-air to be ignited.  That's ok if the f/a in the gap
when the plug sparks happens to be the correct 14:1 ratio.  But
engines don't provide perfect fuel atomization- some parts of
the f/a mix will be richer than others.  The larger the gap is, the
more likely that there'll be f/a at a burnable ratio in the spark path.
This is why modern engines use very powerful ignitions and large gaps.
Unfortunatly off-road bikes haven't followed suit, so they
need to run smaller gaps.


If you make all the changes above (making sure that there's not too
much oil on the filter, going back to 80:1 and using a leaner pilot)
and it's still fouling plugs, then you will need to check for other
things, like poor connections in the ignition (plug cap to coil wire
is often a problem) or leaky crank seals.


-- 
Eric Murray  Chief Security Scientist  N*Able Technologies  www.nabletech.com
(email:  ericm  at  lne.com   or   nabletech.com)          PGP keyid:E03F65E5