The old montesa carb dilema.
I've tried various options on my '99 mont, and I dont think the original carb is rubbish enough to throw away, but i tried the later '02 PHBL by borrowing it and the airbox rubber, and making a sleeve for the inlet side to fit the larger '99 intake manifold. It worked fine, small tweak on the mixture screw, and all was well. Improved slow speed pick-up (smoother and more controlled) but by only 10% ish. Not really enough to warrant a change, so then I fitted a new VSHT Dellorto jetted for the mont. Had to modify an air cleaner side rubber hose to fit the large bellmouth on the dellorto, inlet manifold rubber fits straight in. Improved slow speed pick-up and response from original carb, but over half throttle she really goes but in a much more controlled fashion than either of the normal slide carbs.
Flatslides have allways been prefered for higher revs, whereas normal carbs are better at slow speed stuff.
So now my VSHT flatslide Dellorto up for sale, complete with air filter side rubber hose, 3 trials from new. It's cost me over
