Surging idle (video) (1992 MK2)

[sbc]

Hello,

My father and I have succeeded in putting my CRM250 1992 Mk2 back together after a lower and upper engine rebuild.

It's got a surging idle, which it had before the rebuild. It's no better but seems okay with the idle turned up a bit:
https://www.youtube.com/watch?v=5myvU5z93ho

I Googled around and there is some suggestion that adjusting the air screw might help. Which one is that?? The idle is easy to find but the air screw?

If adjusting the air screw doesn't help, where would you start? Could it be a worn pilot or wrong jetting?

Thank you.

[gregorious77]

What parts did you replace in the rebuild? you could try screwing the air screw inwards (richer) but it wont have a huge effect. You may have a partially blocked pilot jet. If the air screw doesn't help then buy a standard pilot jet and maybe the next size up and try that.

[fallenmikethebike]

What parts did you replace in the rebuild? you could try screwing the air screw inwards (richer) but it wont have a huge effect. You may have a partially blocked pilot jet. If the air screw doesn't help then buy a standard pilot jet and maybe the next size up and try that.

+1
Also check the rubber air intake boot for any perishing, or splits, this would allow excess air in.
Can you hear or feel the throttle slide bottoming out when the twist grip is allowed to close briskly?
Mike

[gregorious77]

Hopefully you did the crank seals?

[sbc]

Hopefully you did the crank seals?

My father did replace the seals - he ordered everything, a kit I think, from Leisure Trail.

So I followed your advice, no obvious perishing from the rubber boot. However the air screw was all the way out!

So I closed it in quite a bit and then went for a ride to get the bike warm and spent 10 minutes riding round dialling the adjustment in.

It works much better now! There is room for improvement though as something strange happens:
https://www.youtube.com/watch?v=XN1RbHO3FM4

As the video shops, when I stop the bike and there's a good strong idle, then it dips down a bit but doesn't stall. Next I lean the bike over and put it on its side stand and the idle spontaneously recovers. Why would the idle recover being on the side? Could this be a sign I need a new pilot jet?

The idle sound at the start of the video sounds different to the end as well??

One thing to note is that I still need to fit a gasket where the engine meets the downpipe as it smokes when it firsts starts there, and I know this can effect back pressure which is vital in 2 strokes - but wouldn't thought this would be sufficient to cause an idle issue? Especially as it seems to idle find when leaning but sometimes lumpy when upright.

[gregorious77]

could be float height?

[gregorious77]

where are you located in UK?

[sbc]

where are you located in UK?

Float height? That's interesting. We didn't adjust anything in the carb - we just put it back how it was. I'll never know if it was running 100% before the rebuild though tbh.

I am near Brighton in sunny Sussex!

[gregorious77]

Nice I am near Portsmouth in sunny Hampshire. welcome to come ride the lanes round here anytime. Whats it like for lanes over that way?

[pudzie83]

Could be throttle cable not routed correctly

[mikeelwb]

can i have your number please greg im supposed to be meeting someone soon to ride round butser hill once iv finished repairing my bike after crashing into a tree at salisbury lol i could give you a call and meet up

[sbc]

It doesn't appear to be throttle cable related. I took the bike out again today for 4 miles to warm it up thoroughly. It idles better when warm (like most 2 strokes I've had), but won't sustain an idle for very long.

Aside from that, it's working great! It suggest to me that the carb pilot jet is probably needs replacing.

The carb seems to be a bugger to remove insitu. Is it just the case of slackening off the subframe to have enough wiggle to get it?

[mikeelwb]

theres a knack to get it in usual involves losing skin on the knuckles lol

[gregorious77]

I took top bolt out of subframe to aid removal. Putting it back in is worse.

[sbc]

Guys! I followed your advice and looked to the carb. I realised I didn't have to take it all the way out to access the jets - I just loosened the two clips off, took the needle out and could rotate the carb to get access to the bottom.

The carb was clean but I took the pilot just out anyway. There was no obvious debris and it was passing light through but I cleaned it anyway.

The exercise made no different to the bike idling at all. However, I discovered once the bike was warm that if I tightened the air screw all the way in I get a completely stable idle!

The bike works lovely now, but clearly something isn't quite right as it's default is normally 1.25 out.

So, if you were me, would you just be happy it works fine and get on with enjoying the bike, or would you troubleshoot further?

The previous owner was using the fuel pump however we disabled that when rebuilding the engine and run on 40:1, which I appreciate might also make a difference.

Should I look at buying a larger pilot jet? If so, does anyone know what size I should look at buying?

Thank you.