CB500 running

[masonmart]

Having finished my Dresda 500 I'm trying to get it running well. Have experienced a problem whereby the bike starts on choke, In warming, it initially runs on no choke but after a while coughs and then stops running. Leave it for a while and this repeats. We hooked up an exhaust gas mixture probe and what happens is that on choke it has the correct mixture for normal running, is too lean when the choke is opened but then the exhaust just gets leaner and leaner and eventually gets so lean it doesn't run. The fuel supply is from an elevated tank and the fuel lines run fine.

This happens with the engine on tickover but it seems not to happen when the throttle is opened into the area where it is running on the main jet not the slow running jet. On my brit bikes I'd look at checking the slow running jets and perhaps raising the needle a notch but I'm not sure with these carbs but I have them off the bike now and will check the jets. Any thoughts on this lean issue, especially why it would just keep on leaning out and stopping. The obvious thought is fuel starvation but the supply seems good.

[Johnwebley]

I assume you have checked float height.


What air cleaners/airbox do you have?




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[masonmart]

I assume you have checked float height.


What air cleaners/airbox do you have?


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Hi John, checked the float heights when I assembled the bike and they were fine. Air cleaners are aftermarket foam jobs.

[deltarider]

Inspect the tiny O-rings main jets seal well.

[K2-K6]

If everything is in order regarding float height etc and idle circuits are verified,  then you've more or less described the characteristic of running foam/individual filters and the effect they have on these engines.

Ordinarily you'll eventually end up with an unconventional setup to re-establish better compromise in getting the mixture right.

To be more specific,  the vacuum imparted is changing the draw on the idle fuel metering compared to std intake system,  and hence pulls less fuel in for the same rpm and air volume.  So you get your weak running condition.  The choke applied simply lifts the vacuum then working on the idle circuit.

Initially you can try and use 2mm raised float level along with airscrew set at 1/2 the way out to normal book  values.  These should both push the mixture upward at idle rpm without much affect higher up when onto mainjet territory.

It's desirable to approach this way as just going up in jet sizes will usually get you too rich at higher throttle openings,  so worthwhile an evaluation as above to judge if it gets anywhere near sensible fuelling wise.

[masonmart]

Thank you all for the comments.