I recently bought a 1990 cbr 600, it's my first bike, ran a little rough when I bought it, the guy said it was from sitting, (I just heard it idle, I didn't ride it) thought just needed carb cleaning and possibly sync. I cleaned the carbs and it seemed ok until I rode it, it is completely guttless. The bike has no power, it is working really hard just maintain highway speeds. I think it is just running on cylinders 1 and 2 because if I cover up the carbs on cylinders 1 or 2 when the bike is running it will want to kill right away. If I cover up 3 or 4 it has very little affect and there isn't much suction at idle. I gave up on it and took it in to a mechanic. He cleaned the carbs, adjusted the valve clearance, checked the compression (compression was fine), and checked the spark which was fine. He's not real sure what it could be, and I have no idea. Also, I put a set of carbs off of an 87 on there because my other set was leaking pretty bad after I cleaned them, but it ran the same. If anyone has any ideas I would appreciate it a lot, I have no idea what it could be. Thanks.

 

I would start by NOT trusting the mechanic. Then I would check the spark plugs and actually make sure that all of them have a good spark. If cylynders 2&3 are not firing, your right ignition coil might be busted. Easiest way to check without taking anything off the bike is by starting it, let it warm up a little and check the #2,3 exaust headers, see if theyget as hot as #1,4.

BTW, I'm still learning too, but this is would be my diagnostic, following the symptoms you described.

 

1. are you sure all carbs are delivering fuel?

2. have you checked plugs for weak spark?

3. does it act the same way when you just rev the engine at standstill? or does it only happen when moving?

4. do you have cali model or 49 state model bike?

5. any vacuum leaks?

 

Thanks for the reply. I did check for spark on cylinders 3 and 4 and it looked fine as far as I could tell. Also I did see some fuel on spark plug 4 at one point when I took it out. Plus, after I cleaned the carbs, they leaked, instead of buying all new o-rings/gaskets I ordered a used set of carbs off of an 87. I put them on and the bike ran the same. I would assume the problem isn't carb related because of that.
I assume I have the 49 state model, as I live in WI, I guess I didn't look into that close but I didn't see a charcoal cannister or those extra vacuum lines.
The bike seems to idle, rev and run smooth, but it seriously lacks power-I'm talking top speed of maybe 75mph, and quite a wait in getting there. Also at idle, as I said I can cover up the carbs on cylinders 3 and 4 and it has no affect on how the bike is running, if I do it on cylinders 1 or 2, it will kill almost immediately.
Also I forgot to mention, if I rev it I can see all the vacuum pistons opening together on all the carbs, so I assume it has decent compression/airflow, plus the mechanic said he ran a compression test and it was fine.
There is definately something funny going on with just cylinders 3 and 4, I'm not going to get off easy with just a new coil. I have no clue would would cause just 3 and 4 not to work properly. What I thought was air is getting in those chambers someplace other than the carbs, but with all the vaccume pistons opening together that seems an unlikely issue.
As far as vacuum leaks, I've heard that can be a problem, but I don't know where to look for them, plus assuming I have a 49 state model there aren't a lot of places for vacuum leaks are there?
Well thanks in advance for any ideas whatsoever, I am getting quite frustrated, and I suspect I already have a pretty large repair bill waiting for me at the shop...

 

I'm having the same issue with my Hurricane I got this winter. We've had the carbs apart a few times now, changed out the spark plugs, fixed up the fuel pump, etc... Runs smooth, sounds great and there's crap for power. Same top speed as you.

Whoever figures it out first can let the other know. We're taking one last go at it on Thursday and if we don't get it going properly it's going into a mechanic next week that used to race these things.

 

My mechanic has a pretty good guess what the problem could be. He's figuring a mouse or something built a nest in the headers, and won't blow out. That would cause a lot of backpressure, and the chambers wouldn't be exhasted properly. I havn't found out if that is the prob or not yet, but if I were you I would disconnect the headers and run it w/o exhaust to see if it is something with the exhaust. I can't think of anything else it could possibly be, but then again there's plenty I don't know.
It's worth a try before u take it in, I'm sure your mechanic is going to want to go through your carbs again even though you already have, and it isn't cheap. Hate to sink too much into this old of a bike. Good luck.

 

mouse nest, huh? have you tried to rev the bike and see if anything came out aside from exhaust. and i don't mean a low rev, i mean give the throttle a good yank.

now to more plausible possibilities...

1. ignition timing - check spark unit or pulse generator

2. clutch slipping?

3. valve clearance - adjust valves or worn valve seat

4. cylinder compression

5. spark plugs - fouled? incorrect heat range or need new plugs

6. remove cylinder head cover and inspect lubrication - if not lubricating properly clogged oil passage, clogged oil control oriface, contaminated oil, faulty oil pump

there is more but the above seems to be most likely.

the original problem is no suction so maybe valves need adjusting. and misfire so could be timing issue.

 

Thanks for the reply, but...
The compression and spark has been tested and is fine.
I'm quite certain it isn't the clutch slipping.
The valve clearance has been adjusted. Worn valve seats wouldn't account for that kind of power loss, It very significant, barely enough to get up hills.

Also a slipping clutch or valve issue wouldn't account for a huge lack of suction unless maybe a valve got whacked and bent, but again, compression is fine

Again, Something is quite screwed up with the lack of suction in cylinders 3 and 4. That has to be connected to the lack of power problem, I don't think either cylinder actually ignites. I have seen cylinder 3 backfire out the carb during start up however. I would have to say it is a problem with air/suction not ignition.

The only plausible senario's I can think of would be a huge vacuum leak, or exhaust not escaping. There aren't many places to look for a vacuum leak on this thing. There is some suction when it is revved, and the vacuum pistons on the carbs open together so I'm not real confident about the mouse nest theory either. I can't think of any other possiblity however.
If anyone has any other ideas I would appreciate it. The guy I bought it from (might be lying) said it was fine, but sat for a long time, when he rode it again it didn't run very good.

 

So my buddy opened up the cams last night (and I'm going to owe him big for staying up till 4am working on it) and guess what? The moronic idiot who owned it last had set them wrong. I'm just going to cut and paste in whathe emailed me...hopefully it helps...

"looks like both cams where out a tooth. what a f**ker of a job. but starts like a dream, idles for 20 minutes steady as a rock no more hunting around and readjusting the idle speed all the time rev it and it drops right back to 1500 and rev like a jet instead of the steady chug of a freight train. if you follow the instructions in that crappy manual you can be out a tooth and it almost looks right. but on the other side of the cams is a notch that you can slide a verner caliper into and level with the head that sets the cams perfectly. figured that out after f**kin around for half an hour trying to take a good measument with a razor blade off the head to the valve adjustment notches on the cam gears like the bulls**t manual says. dropped the razor blade down the chain slot and had to fishy it out with a magnet wonder if the guys who owned it thought they had it right and just traded it off cause they gave up thinkin the motor was just hatched LOL. cause it hasnt run right since they would have out it back together... looks like it might be a wrecker motor. head has paintpen and so do cam bearing retainers. need a few things. couple hose clamps that are weak and leakin when the thing comes up to temp. should haul ***** now i think."

 

This statement is interesting but worrysome at the same time. Having similar symptoms I wonder if I got the timing right. Your buddy is right in that the manual(s) can be confusing.

I was referencing both the Clymer and the Haynes manuals only to get more confused. Both were showing the timing marks on the cam sprockets to be in different positions at TDC. Don't remember exactly which, but one manual was showing all four marks to align with the head surface while the other had only the exterior marks aligning with the head while the interior ones were not. Since I could only align the exterior ones with the head surface level, I left it at that.

Is there any way your buddy could be more precise about how to make sure the timing is correct?