There have been many questions over the years concerning carburation problems on 4 strokes (mainly cubs and B 40s) in which riders complain of a flat spot or hesitation on opening the throttle relatively quickly from idle. There seem to be two approaches to tuning a 4 stroke, namely fit the engine with soft cams and a high compression piston or with sporty cams and a low compression piston. I wonder to what extent the perceived carburation problem is also a function of valve timing. I would expect the sporty cams would be more difficult to setup at the low end and perhaps aggravate the problem rather than the soft cam approach. Does anyone have any opinion or direct experience with the two? Do Serco recommend the soft or sporty cam setup? Also the modern 4 strokes, Montesa etc seem to use over square engines with relatively high compressions, does anyone know what valve timing is used on these bikes and how much advance and retard is built into their ignition systems?

P.S. I have no intention of building a 12:1 compression B40, I am just curious.

Forget valve timing, bore/stroke ratios or fancy electronics, it seems to me that the way Montesa get round the problem is simply to run the tickover at 1700rpm, ie above the point where spitback would occur. I'm sure it would work on a Cub but would not be very pleasant to ride.

the "old" serco used to recommend the R cam, low compression but usually in conjunction with a 230 (big bore) or 250 (big bore and longer stroke) conversion. Strangely enough the local club changed the rules to allow "other the british manufactured carburettors " on cubs to try to over come this problem.

The main reason that modern Four strokes dont cough and spit back is not so much to do with cam profiles or bore to stroke relationship It is that they all use an excellerator pump, and an air cut valve, built in to the carb.

hey,

the spitback problem on 4 strokes is mainly related to carburation, air filter, and exhaust.

my honda tlr had the same problem but a new carb and proper setup solved this problem, also using the stock exhaust, on the wes exhaust my bike didn't run good.

my sherco 4stroke is perfect, no spitback perfect starting again thanks to a new carb and good setup

my b40 runs on a amal (new one) and never had this problem

i guess bikes who have a spitback problem have a carburator thats used or set up incorrect (assuming the ignition is ok and valve clearance is ok)

after 20 years a carb is just worn, a new needle and jets will not solve the problem.

regards bob