Engine dle speed too slow. Why?

calebacm

calebacm

Active Member
So I recently bought a cb400t and the idle speed is way too slow. It will stall out while braking if you're not twisting the throttle. Any ideas what could be causing this? And if so, what are the solutions? I plan on taking the carbs off tomorrow for a good cleaning as the bike has been sitting for about a year. Thanks!
 
the carburetor has an idle adjustment screw. All you have to do is turn it to set the lowest possible position for the carbs, this sets the engine idle speed
 
Welcome to the CM400 family. I recently had to adjust mine. They are on the underside of the carb. Tighten them up all the way, then let them back out 2 full rotations iirc (could be 2 1/2, but start at 2, idle should be at 1200rpm +/-100rpm when warm). That is where they should be for stock.

Also, do yourself a favor, download this

http://www.mediafire.com/?cnkmdznfjgz

I keep it on my phone for when I am working on my bike.
 
Thanks, I did download that. I got the idle to 1,200ish and it started acting up. It will randomly rev itself up to around 3k and start misfiring some. Could the carbs being out of sync or am I dealing with something much worse?
 
Yes, it's fine. I took off the carbs and cleaned them up, they weren't too bad but there was some gunk. I put them back together and back on the bike and now its really acting up. It idles fine until you touch the throttle and then it will jump to 3.5k or 4k and stay there for awhile and then come back down or it won't at all and I then turn the bike off. I took it around the block and it does the same thing while its in gear. Very poor throttle response and then BAM! 4k rpm. I can't get past 15mph without it acting up. Could this be a sync problem, did I potentially goof something up in the reassembly or what? Or both? Could it be one of the vacuum pistons is sticking open? I don't know. Any suggestions where to start would be great.
 
I'd check ignition timing and compression first. After you've verified those two things, take a look for air leaks and/or a carb sync issue.
 
Sonreir said:
I'd check ignition timing and compression first. After you've verified those two things, take a look for air leaks and/or a carb sync issue.
Click to expand...

All electric ignition system. Single coil driven by a CDI. Not much on adjusting the ignition timing unless the CDI took a crap.
 
calebacm said:
Yes, it's fine. I took off the carbs and cleaned them up, they weren't too bad but there was some gunk. I put them back together and back on the bike and now its really acting up. It idles fine until you touch the throttle and then it will jump to 3.5k or 4k and stay there for awhile and then come back down or it won't at all and I then turn the bike off. I took it around the block and it does the same thing while its in gear. Very poor throttle response and then BAM! 4k rpm. I can't get past 15mph without it acting up. Could this be a sync problem, did I potentially goof something up in the reassembly or what? Or both? Could it be one of the vacuum pistons is sticking open? I don't know. Any suggestions where to start would be great.
Click to expand...

Sounds like there is still gunk somewhere. Question though, how well does the throttle come back on it's own when you open it up? Like if you pull the throttle back closed manually, does it still like to hang up high in the RPM range?

Also, have you made any mods, like removal of the air box, removed the cross over exhaust chamber or anything like that?
 
rlhphotos said:
Sounds like there is still gunk somewhere. Question though, how well does the throttle come back on it's own when you open it up? Like if you pull the throttle back closed manually, does it still like to hang up high in the RPM range?

Also, have you made any mods, like removal of the air box, removed the cross over exhaust chamber or anything like that?
Click to expand...

The throttle itself snaps shut like its supposed to. It takes the bike a second or two to catch up with it. Or it doesn't at all and stays at the high RPMs. There are no mods, completely stock bike.
 
The throttle closes but the revs stick in the 4k range. It's almost like the carburetors get stuck open, even after the throttle "closes." I think it's a combination of the carbs still having some gunk in them and sync being off. I have some vacuum gauges coming in soon from DCC so I'll check it then. Until then, I'll take off the carbs again and re-clean them.
 
calebacm said:
The throttle closes but the revs stick in the 4k range. It's almost like the carburetors get stuck open, even after the throttle "closes." I think it's a combination of the carbs still having some gunk in them and sync being off. I have some vacuum gauges coming in soon from DCC so I'll check it then. Until then, I'll take off the carbs again and re-clean them.
Click to expand...

If you close the throttle, and it still hangs in an RPM range, that screams vacuum leak in the carbs to me.
 
Make sure you have enough free play in the throttle. If you do I would then make sure there are no vacuum leaks.
 
Update: I went out today and looked at the bike. I noticed that I had put the choke cable on too tight. It held it at half choke permanently, so I fixed that, now it's completely opened. I also noticed that I hadn't set the carbs quite right in place so I fixed that as well. It sounds better but still revs back and forth between 1,200 and 3,000 rpms pretty consistently while idling. It always starts at or around 1,200. Haven't tried to ride it since it attempted to throw me off the back yesterday. I'll check for vacuum leaks as well as synchronize them when the gauges show up next week. A vacuum leak would be the result of a bad gasket or bad carb boot right?
 
I'd say bad gaskets first. Especially if it is fluctuating. A bad boot would just cause one side to die out.
 
You can check for bad boots by spraying a little starting fluid around said boots. If the engine revs way high with a small shot of starting fluid, you've got a leak somewhere. In addition to rotted boots, there could be a leak around the screws used for syncing as well.
 
calebacm said:
Update: I went out today and looked at the bike. I noticed that I had put the choke cable on too tight. It held it at half choke permanently, so I fixed that, now it's completely opened. I also noticed that I hadn't set the carbs quite right in place so I fixed that as well. It sounds better but still revs back and forth between 1,200 and 3,000 rpms pretty consistently while idling. It always starts at or around 1,200. Haven't tried to ride it since it attempted to throw me off the back yesterday. I'll check for vacuum leaks as well as synchronize them when the gauges show up next week. A vacuum leak would be the result of a bad gasket or bad carb boot right?
Click to expand...

Are both cylinders always firing during idle? What I mean is, does the bike usually idle on one cylinder but on occasion the other gets a little gas and shoots the revs up?
 
fresh_c said:
Are both cylinders always firing during idle? What I mean is, does the bike usually idle on one cylinder but on occasion the other gets a little gas and shoots the revs up?
Click to expand...

Both of the cylinders are firing but it seems that the 2nd cylinder is the one that is causing the high revs. I let it run some again just a couple minutes ago and noticed some white smoke coming out of the 2nd cylinder exhaust. It'll start at 1,200 and then go to 3,000 and go back to 1,200 if I adjust the throttle stop screw a little. To answer an earlier question, the throttle itself seems to have plenty of play. I'm still planning to check for leaks and hook up some vacuum gauges to make sure that those aren't problems.
 
Those carbs can be pretty frustrating if they are not really clean and synced. You can bench sync them with a drill bit pretty easily to ensure both slides (edited to say butterflies, not slides) are set at the same height. If they are unsynced, the bench sync should take care of most all of the idling issues (if that is the cause).

Do you happen to know which cylinder you suspect is acting up? Right or left? The accelerator pump could be acting up at idle.

Are you sure there is nothing blocking the fuel line like a clogged petcock or fuel filter?

Are you sure all the tiny, little passageways are clear? You mentioned some gunk when cleaning and if you didn't use compressed air there could be blockage.

I know these are a lot of questions but there are a lot of possible things to be going wrong and we want to be sure you are covering all your grounds.
 
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