Alright, so the last time I posted I couldn't get my recently purchased bike running (1994 CBR600F2)... everyone was right though, it was absolutely the carbs being dirty, as it seems it usually is. All the problems I though it had cleared up once it got running. I was convinced the moron who sold it to me had ruined it, but as hard as he tried, it still lives!

So here's some history since then... I pulled and cleaned the carbs (one of my main jets was almost 100% clogged with crud), replaced the spark plugs, and it was running pretty well except for a stutter under 3000rpm (more on this in a sec). So I bought a Dynojet kit since it seemed like it needed a little more tuning with it's k&n filter and Yoshi exhaust. It was lacking pretty bad in it's mid rpm's and also popping pretty heavy on deceleration, I also figured I'd play with those fuel screws to fix my issue.

I pulled the carbs again, installed the kit, and turned all my fuel screws out about 2 turns (they had been all over the place)... Thought this would have solved my stutter, it didn't. Mid range is much better, top ends a little flat now... just gotta play with some different jets. I also synchronized the carbs, but that darn stutter won't go away. I have turned the screws out to 3 turns now and everywhere in between just to test, at 3 she has a little trouble starting back up when warm, but it doesn't seem to help or hurt the stutter. Back to 2 it is. I'm also still getting a bit of popping on deceleration when the rpms are pretty high. I thought the fuel screw adjustment was supposed to help that, but it didn't, and I don't seem to have any intake or exhaust leaks either fyi.

Now about this "stutter"... The motor makes a "blub lub lub lub" kinda sound under 3000 rpm, sounds like it's missing pretty badly or something. The bike runs as intended otherwise! As soon as it gets over 3000 it's smooth and takes off hard. The idle is fairly smooth and it revs fine if you crack the throttle open hard, no stumbles. It is very jarring under 3000 though, especially in first gear taking off from a light into a turn... and if the roads wet, you can probably see my white knuckles through my gloves, ha.

My charging system seems to be operating as intended from what I understand of motorcycle systems, 12.8v at idle up to about 14v when reved up. The R/R has been replaced at some point to one with cooling fins on it. So everything seems fine there.

If you made it this far, I'm sorry for the long winded post. I just wanted to be as thorough as possible, since I've seen many similar posts, yet they all seem to have some other symptom, like a rough idle or a bad stumble when the throttle is quickly opened. My theory is that I'm not getting a good enough spark below 3000, but I wanted to check with you guys to see what you thought, or if you've ever seen this before. Any input is much appreciated!! What's my next step?!

 

My first thought is that the bike might not be firing on all cylinders. It would be more noticable at lower rpm and once the revs pick up the engine would run better. You said you changed the spark plugs - I'd check they are all giving a spark, make sure they are fitted correctly and that the leads are attached firmly and in the correct position for firing order.

As far as I know, the popping on deceleration comes from air entering the exhaust system and mixing with unburned fuel, causing it to ignite. So, you may have a leak somewhere. If you have a cyclinder not firing then that would be pumping fuel into the exhaust and adding to the popping situation too.

 

Sounds like my bike, know that feeling, not fun at all. Have you tried cleaning
out the carbs with Seafoam? I'd check the HT leads too.

 

Do a compression test....

 

Optimus: I agree completely that it may not be firing on all four under 3000, but above that, it really sounds and runs fine... It seems it is clearly a transition from missing to firing properly. I have checked the order of the plug wires and they are all correct. I will pull my plugs tonight and make sure all of them are firing. Come to think of it, when I pulled #1 back before the bike was running and grounded it on the case, the spark seemed kinda weak and thin looking to me, but I ignored it since the bike hadn't run in a while. I wonder if I'm getting too weak of ignition under 3000.

Hojen: Yes, I have cleaned the carbs thoroughly and run some seafoam through with a few tanks of gas.

74demon: Do you know of a compression tester for a reasonable price that will work on my bike? I bought one from Auto Zone for around $30 and the fittings didn't fit... I could only get one of them threaded about one thread in, cranked the engine and it read about 100 but I could hear air escaping around the fitting. I've been looking at the craftsman one, but I've been hesitant to spend more money on kits that may not work.

Thanks for the suggestions guys! Keep 'em coming and I'll keep testing stuff!

 

74demon: Do you know of a compression tester for a reasonable price that will work on my bike?

Try a motorcycle store like cycle gear(if there is on by you) or get one online. Our spark plugs are different than what's covered in a standard automotive kit.

 

Alright, so I pulled each plug wire off individually and cranked her up to see if any one of them didn't change the way the engine ran... unfortunately and fortunately each one made a big difference that could clearly be heard and felt as a cylinder not firing. So it IS running on all 4, its just not running RIGHT on them. I will grab one of those compression testers and check that (thanks for the info Demon!), but it would seem to me that my bike would run bad in more than a few thousand rpms if it was a compression issue. Maybe I'm completely wrong about that.

So I shot a video just to help you guys get a better idea of how its running... At the beginning and around the 54 second mark it really stands out as I hold it around 2500rpm. It also doesn't sound like it really clears up until a little after 3000rpm in the video, but when running on the street that seems to be the magic spot. It really doesn't even sound that bad I guess, but it sure makes for a rough ride when you put a load on the engine.

CBR600F2 Stutter - YouTube

Sorry for the poor focus and the shakiness of the camera... expensive DSLR doesn't autofocus on video mode and is a little hard to hold one handed.

 

Sounds exactly like mine, except for the exhaust note. Have you tried adding
some fuel additive to the gas? Could be bad gas (too much ethanol) I would
check the ignition leads carefully too.

 

I have been having the exact same issue with mine between 2500-3000 rpms.
As soon as the rpms pass 3000 the bike takes off like a rocket.

New plugs, and have run 3 tanks of gas with seafoam and Startron Enzyme treatment to rule out any bad gas/gunk in the carbs problems.

I'm beginning to wonder if it may be time to replace the coils and wires.
-Matt

 

Hojen: I haven't run anything other than Seafoam in it. Maybe I'll give that a try, but if that's the case, I'll be selling the bike. I don't want to have to put fuel additive in it every time I fill up. I just feel like bad gas would show itself a little differently. I don't feel like it would have such an abrupt change over at 3000rpm. I've been wrong before though!

Matt: I'm sorry you're having this same issue. It sucks. But I'm feeling the same way about the coils and wires. I'm going to try to test them this weekend.

I do find it really interesting that this problem also coincides with the voltage climb through the rpm's that all bikes apparently have. I'm used to cars, where it sits at 14v constantly when running, so I thought it was very interesting that my bike sits at 12v ish at idle and doesn't reach 14v until around 3000rpm. Makes me wonder if the coils aren't working well below 14v for some reason. Could be completely unrelated... But I couldn't help but wonder since that voltage peaks at the same spot my bike starts running fine. Anyone else think the two could be related somehow?