idle diagnosis

[meyekj]

All,
I've been lurking for a while and got some great help as I started on my bike lsat year, so I need to start with a big THANKS for all of the great info on this site. The bike is running "OK" and I'm riding to work 2-3 times a week. I've slowing, but surely, been tuning everything in, but now I need a bit of help as I've hit a road block.

I'm having trouble with the sequence of events to have it start right up and idle well. When the bike is warm, it idles at 1200-1500 RPM just fine, but when cold it seems like I have to jump through some hoops to get it on the road. Here are a few things I've noticed and I'm hoping you guys can point me in the right direction...

The bike: '78 XS400 2E, so no electric start (photos in my profile)
- new plugs
- new-ish battery
- new plug caps (5K Ohm)
- original coils
- points ignition (timing is OK)
- carbs REALLY clean this year
- stock air filters
- aftermarket pipes (probably more free flowing than stock, no leaks that I know of)
- I can't remember the numbers on the pilot and main jets at the moment
- mix screws are turned out about 3 revolutions on each side...can't recall exactly
- When I check the plugs, they look OK...not overly lean or rich

Starting procedure
- I have to start the bike on full choke and it takes a good 5-10 kicks to get both cylinders to fire and idle at about 1500 rpm...it takes some work on the throttle to keep it running
- after about a minute of idling on its own, it revs up to about 3000 rpms and will stay there if I let it go.
- I adjust to half idle and the revs come back down to ~1500 rpms
- I let it idle for another minute or so, put it in gear and take off.
- As I work from first to second gear, the revs stay up around 3000 rpms so I have to take the choke all the way off.
- As I ride I have to blip the throttle at stop signs and lights to keep it running until I've been on the road for 5-10 minutes. After that it seems to idle ok
- the thing runs like a bat out of hell when I give it gas. The top power end is fine/great.

Things I've tried:
- Adjusting the mix screw in and out to lean and richen. I've read that if the revs race up, it can mean the mix is too lean
- played with the idle, but ultimately keep it where it is so it revs at 1500 when warm

Things I've considered
- carbs are still dirty (choke and pilot circuits)
- old/weak coils
- inadequate pilot jets
- I'm not letting it warm up enough

Please help because sometimes it acts like it's running lean, but other times it acts like it's running rich. I'm not sure what my next step should be. Ultimately, I want to be able to idle on it's own without supervision.

THANKS, again!
Kevin

 

[xschris]

Have you synced the carbs with a manometer? I would also check and make sure the idle mix screws have not broken off in the carbs. Check to see if there is a hole in one of the diaphragms. Of course it could be a dirty pilot circuit or you need to bump up the pilot jets to a 45.

 

[meyekj]

Thanks! The tips of the screws were good the last time I had them apart and I'm VERY careful with them so I'm 95% sure they're OK, but I'll check again. Same goes for the diaphragms. I haven't synced my carbs with a manometer so that may be my next step.

 

[xschris]

Carb syncing will make a big difference on how the bike starts/runs.

 

[meyekj]

Just out of curiosity, what is the conventional wisdom on the limit for backing the mix screw out before you should bump the pilot jet size up? 4 turns out?

 

[xschris]

Yes 4 turns. I prefer to run a larger pilot jet and keep the turns out to two or less.

 

[meyekj]

I turned the mix screws out to about 3.75+ turns and the bike starts and idles much better so I ordered 45 pilot jets. I have some 50's, but I'll try the 45's first. I'll post an update when I get the new jets in next week.

 

[xschris]

Sounds like that is what you need to do.