Choke question

Post your problems / information on the smaller GT Models here
User avatar
BATTS
Posts: 441
Joined: Sun Jul 15, 2012 6:47 pm
Location: Lancashire

Re: Choke question

Post by BATTS »

There's not a great deal of difference in the necessity of choke use on a 4 or 2 stroke engine, the real difference is that an air cooled engine will typically come up to operating temperature much more quickly. My 550 (when it was running) you could knock the choke off by the time I'd got my gloves and helmet on, my Kettle needs choke until at least I've pulled out of the junction at the end of the street otherwise it tends to bog down.

Engine temperature is important in the atomisation of the fuel, cold engines mean poor atomisation, which is where the "choke" comes in by significantly increasing the fuel/air ratio enough to get the engine to fire.

As Barney says most two stroke bike engines don't have a choke in the traditional sense of an air restrictor valve, but use an enrichment circuit fed directly from the float bowl, certainly true of the VM and BS carbs we use, this relies on lower air pressure on the engine side of the carb throttle valve, look where the small hole is on the throttle bores, once you start to open the throttle the relative loss of vacuum, will start to negate the choke enrichment effect, it becomes easier for fuel to be drawn through the main jet and pilot circuit, certainly at approaching full throttle the carb will be operating at an almost normal fuel/air mixture. Which is why with good clean carbs the engine should start more easily from cold with the choke on and throttle closed.
Chris
76 GT750, 78 GT550, BMW K1600gtl
barney01
Posts: 3118
Joined: Wed Sep 26, 2012 7:33 pm
Location: Prestatyn.Wales

Re: Choke question

Post by barney01 »

Just to clarify a point when the engine is cold and the choke is on the throttle is best closed as it becomes ineffective if the throttle is opened by just 25%
NO SMOKE .......NO POKE
Post Reply