315 Carburetor

[geoffersgra]

Hi All. Does anyone have any photos of the carb for the 315. Mine is a 1999 model fitted with a dellorto PHBH26CS. I have an exploded view in a parts book but need to know what tubes fit where. At present my bike only has 1 tube fitted on the left hand side of the carb (sitting on the bike) connecting to a breather unit next to the rear brake fluid resivoir. My parts book says this is "tube, main air inlet" There are a couple of pipe nipples on the right hand side of the carb with no pipes fitted. The book shows a u shaped pipe that I dont have called "tube,air vent".

I bought the bike back last year & am only just getting ready for my first outing. The bike is very difficult to start but I did eventually get it running.

The bike was left to idle for approx 10 mins (fan switching on & off so up to temp) when it suddenly revved up-to max. revs as if the throttle was pinned, I was on the other side of the garage at the time. Very strange. Hit the kill switch & restarted and off she went again up-to max. revs. I removed the air box & the Carb slide is free to move as the throttle is opened. Was wondering if the lack on vent tubes may have some bearing on this or perhaps I have a ghost rider in my garage with a heavy throttle hand. Any help or advice or pics would be greatly appreciated. Dont fancy my first outing with the a mystery throttle problem

[tt5th]

Hi,

The air inlet pipe should be on one of the nipples on the right, then the vent pipe with the slash in it goes between the remaining nipples, one on the right, one on the left. I can't remember exactly which one the air inlet pipe should be on, i think its fairly obvious but I'll have a check later.

Check you haven't pressed on the throttle cable when replacing the tank. If its not that check your fuel supply, make sure nothings blocked or pipes kinked, lack of fuel will make it run lean which will cause it to rev lots.

Edit: Just had a look on the parts diagram, the inlet pipe goes on the nipple you can't see on the diagram!

Edited January 25, 2010 by tt5th

[wherry]

Hi there. The tube you've got connected is to the sub air filter mounted, as you say, near the rear brake reservoir. Pretty sure this should go to the 'odd' connector on the exhaust side, with the U-shaped tube connecting the 2 similar connectors at the front of the carb. See HERE. The 2 connectors for the U-shaped tube are just breathers to allow atmospheric pressure to enter the carb. I've never really considered what the sub air filter is for I'm afraid. Sudden increase in revs is probably due to the mixture weakening for some reason - maybe a jet blocking somewhere. If you take out the jets to clean them look through them very carefully to make sure they're clear - they have extremely small holes in them. Also remember that in the PHBH there is a 2nd slow speed jet down the hole once you've taken the obvious one out of the top - very easy to miss. Mixture screw should be 1.5 turns out from fully in +/- 0.5 turns depending on atmospheric conditions. Having said that in the recent cold weather mine is only about 0.5 turn out from fully in.

315s should be 2nd, 3rd kick to start. If yours is not there is something not quite right somewhere.

[geoffersgra]

Hi All

Thanks for the info. I will strip & clean the carb & connect the pipes as suggested.

Thanks & I will keep you posted

Geoff

[fastducs]

If memory serves me right, the sub-cleaner feeds air to the Main Air Jet, which is necessary to emulsify the fuel as it travels up the needle jet. On most carburetors the MAJ is fed by a small opening drilled in the carb bellmouth lip, but this particular Dell'Orto is different. Drilled-bellmouth-lip type is subject to pressure pulses in the airbox, so apparently for trials bikes it is advantageous to eliminate the pressure pulse effect by feeding MAJ externally.