350 Carburettors

George 350 · Post by **George 350** » 27 Jun 2007 14:42

Hi All,
My 350 Sport has done 92000 miles from new, and despite new slides, I have come to the conclusion that the carb bodies are worn out. I have some 350 Dart carbs (VHBZ25HS) and wonder if anyone has any experience of using them on a 350 Sport (originally VHBZ25BS).
On the face of it, both are notionally 25mm carbs, but with substantially different jetting. Externally only the vent pipe for the float chamber is different (pipe fitting instead of drilled hole) but internally the 'venturi' where the slide is, appears smaller, with a pronounced 'step' in the bore. Anyone know if the venturi is indeed smaller, hence the need for smaller main jets?
Is it a simple matter of replacing slides, jets etc with the appropriate parts for a Sport, or am I about to open up a whole new can of worms?
Thanks in anticipation,
George.

EVguru · Post by **EVguru** » 27 Jun 2007 15:00

The reason for the smaller main jets on the Dart is mostly the 'Flower Pot' that restricts airflow through the filter (see my website).

I think you'll find that swapping things over will be fine (don't forget the atomisers), but you might like to try smaller idle jets since Morinis tend to run rich at the bottom end on modern fuel. My Sport is running 43 idle jets to cure a stuble pulling away and I could still go a little leaner.

Just how bad are your carbs? Mine are pretty worn, but work fine.

George 350 · Post by **George 350** » 29 Jun 2007 13:57

I deduced that the carbs are beyond economic repair as despite fitting as you did 42/43 size pilot jets (Shows how long ago I did this - Phil Smith sold me the jets) and very careful balancing, I get a bad flat spot around 1/2 throttle 3500-4500rpm. New atomisers, needles and slides failed to help. Fitting a lambda sensor to each tailpipe and going for a ride showed that in most areas the fueling was really good - 14-14.5:1, but when in the flat region, it would drop down to 10:1 or lower, ie very rich. Of course it could be a lot of back pulsation in the inlet tract causing the richness, but it never used to do it.
Float heights are spot on, measured either by holding carb vertical or by fitting my special jet blocks with a hose on to see where the fuel level is when installed. Valve timing, clearances, are ok, there are no air leaks, compression is good, air cleaners and exhaust are standard. Ignition is good, and advancing properly on both cylinders. Only bits left to change were the carbs thought I.