Your thoughts on a carburation matter?

[John Orchard]

On a piston-port 250, bikes starts & idles fine but the idle adjustment screw is all the way in, having quite a high slide setting.

Any ideas why it needs a high slide setting at a low rpm idle?  Too rich needing more air?

[Slakewell]

Slide cutterway

[80-85 husky]

if the air screw does nothing... there may be an air leak in the motor

[sleepy]

If it starts and idle good it doesn't sound to bad. How does the bike ride? If it ride ok with no carburation problems you should be happy.
Have you looked inside the carb throat to see how high the slide is or are you just going on the screw position. I have seen some variations in slides with the stop notches at different heights.

[John Orchard]

Sorry for the confusion, the slide idle positioner is screwed all the way in, looking in the venturi yes the slide is high.

The idle mixture screw is set at 1 turn out.

The bike idles nice at the high slide setting, if l back the slide down the engine stops.

[sleepy]

I feel that a piston port engines slide would be higher compared to a similar reed engine due to the pulsing nature of the intake at idle. Quite a lot of the charge is pushed back out of the carb as the piston starts back down the bore which means a higher slide position is required to get enough air/ fuel for idle.

If there was a problem with the idle circuit of the carb being to rich or lean the high slide to get it on the needle would mean very high idle and poor starting.

You could stick in a bigger pilot jet as a test if it really bothers you or play in and out with the air bleed screw to see what effect that has.

[John Orchard]

I feel that a piston port engines slide would be higher compared to a similar reed engine due to the pulsing nature of the intake at idle. Quite a lot of the charge is pushed back out of the carb as the piston starts back down the bore which means a higher slide position is required to get enough air/ fuel for idle.

If there was a problem with the idle circuit of the carb being to rich or lean the high slide to get it on the needle would mean very high idle and poor starting.

You could stick in a bigger pilot jet as a test if it really bothers you or play in and out with the air bleed screw to see what effect that has.

Yeh l was thinking PP the reason, l'm going to try a piston with a 4mm longer skirt to see if that changes things.

[sleepy]

Have you measured the inlet port duration in degrees?

[fred99999au]

Is the pilot jet installed and clear?

[John Orchard]

Is the pilot jet installed and clear?

Yep all basic stuff is right. l have swapped up & down in pilot sizes, and air-screw settings.

[Momus]

Same idle issue on my RM370 John- case reeds and a replacement new VM38. Played around with the normal calibrations but it would not idle reliably. The speed screw was all the way in and graunching the spring. I ended up taking about 1.5 mm or so off from under the screw head- about 2 turns and voila. I will get around to turning up a longer screw with a decent finger grip at some stage, but in the mean time it is fine with another turn in.

I don't like letting it idle much anyway, just so it will run at the gate when something needs doing with the throttle hand. The original carb had no idle screw and there have been various theories offered for this. One of my own is that Suzuki realised their ignition system was on the weak side at low rpm/any rpm and long idling was a recipe for a plug foul.

[John Orchard]

The DT1 still idles fine, the bike starts first kick with choke circuit open when cold. lt's just that at a very slow idle, the slide is quite high, from lifes experiences l'd expect the bike to reving its tits off with such a high slide setting?

Yeh l have the bike idle just long enough to get my goggles on at the start gate, l need all the engine braking l can get when l decide to close the throttle, with a corner rapidly approaching :-)

l'll get to the bottom of this :-)

[TT5 Matt]

this is an interesting subject!so what happens if you introduce more air by say loosening a inlet manifold bolt/s?not that you would want that as a good way to stuff/dust a motor but to see what effect is has if any.are after market reed valves allow in the class your going to race it in?might be a way of calming down radical port timing.i remember old member Johney O say saying a reed valve kit helped his piston port cr250 which had an light switch power band

[John Orchard]

this is an interesting subject!so what happens if you introduce more air by say loosening a inlet manifold bolt/s?not that you would want that as a good way to stuff/dust a motor but to see what effect is has if any.are after market reed valves allow in the class your going to race it in?might be a way of calming down radical port timing.i remember old member Johney O say saying a reed valve kit helped his piston port cr250 which had an light switch power band

Funny you should say that Matt, the barrel is so wildy ported, the inlet port is so large, there is minimal flange area left around it to seal, l did have an air-leak the first time l started the bike, as you'd expect, the idle rpm revved scarily high :-)

Nah not allowed to run reeds but that's cool, they dealt with it back in the day, l want the 'full period experience' ;-)