Author Topic: 81 Cb650C won't rev past 4500 rpm (Read 3414 times)

Jbarth13 · « **on:** June 02, 2014, 05:48:08 AM »

Hi Folks

Newbie here, both to CBs and motorcycling in general. Just got my new-to-me bike on the road, and found it won't rev past 4500 or so rpm. Sounds a little like a rev limiter in a car. I don't know much about carbs, so help me out here. Any thoughts?

Thanks so much!

cb650 · « **Reply #1 on:** June 02, 2014, 06:07:22 AM »

Assuming the bike is mostly stock. Could be many things and I wont make it more complicated than it needs to be. First try the air filter out. Second try new plugs. Then try giving it choke. Post results and condition of the bike and the brainiacs will tell you how to stop time.

Jbarth13 · « **Reply #2 on:** June 02, 2014, 08:11:26 AM »

Ok, so more info about the bike. It has a stock airbox, and a V&H exhaust. Just rebuilt the carbs. I will go pull the filter and do plugs then post my results.

iron_worker · « **Reply #3 on:** June 02, 2014, 08:35:51 AM »

Hard to say from your description but it sounds like it's way rich or way lean. The color of your plugs will likely tell you more about that.

To know if it's rich or lean you have to listen and feel how the bike recovers from the hesitation. If the bike "burbles" and coughs back to life after hesitating then it's likely a rich condition (as the cylinders are full of excess fuel and take a few cycles to clear completely). If the bike cleanly recovers from the hesitation as the throttle is rolled back then this would indicate a lean condition (since there is a lack of fuel in the cylinders and once the mixture is returned to normal it will burn normally again immediately).

IW

Jbarth13 · « **Reply #4 on:** June 02, 2014, 01:37:55 PM »

Alright, plugs are all good, no real hesitation revving off idle once its warm. Ran it without the air filter and cover, and it will go to 6k before bogging, but I want the full 9

It also has no real get up and go right now, feels pretty similar to the 250 Nighthawk I rode at the MSF course, maybe even a bit slower. Thoughts?

iron_worker · « **Reply #5 on:** June 02, 2014, 01:52:28 PM »

If running without the air filter improved it then you are over-rich.

Do you mean to say it won't rev past that RPM in neutral (or clutch in) or do you mean when on the road in a certain gear?

Is your choke somehow stuck on or something?

IW

Jbarth13 · « **Reply #6 on:** June 02, 2014, 02:10:55 PM »

I can lean it out another 1/4 turn or so. What I mean to say is that I won't rev past 6k when I'm riding it. It will when parked. Just doesn't have much power and hits a wall at that rpm and boggs.

cb650 · « **Reply #7 on:** June 02, 2014, 03:02:40 PM »

Turning the mixture on helps the idle. Out will make it richer anyway. What jets where in it when you cleaned the carbs?
Try new plugs before going crazy too. I bet with the V&H someone put to big of jets on it. Post pics of the pipes. and bike.

Jbarth13 · « **Reply #8 on:** June 03, 2014, 05:54:28 AM »

Not actually sure what the pilot jets are. The main jets are 120s. I've done the plugs with no real changes. Reading some other threads, maybe the ignition advancer needs to be cleaned? The engine sounds pretty smooth as is. I suppose I could try and pull the heat wrap off the pipes and take an IR thermometer to it for tuning purposes, but the plugs also looked good.

cb650 · « **Reply #9 on:** June 03, 2014, 06:36:50 AM »

120 is stock so should be ok there. Advance unit is a good place to work on too.

fmctm1sw · « **Reply #10 on:** June 03, 2014, 07:30:19 AM »

I have two CB650s and I've always had issues going to full throttle on the road. cb650 has provided me a lot of advice in the past about the air filter and what not but nothing I seem to do makes much difference. The CB650C I have (down for repairs now) topped out at about 75ish and the CB650 standard I just got back on the road tops out even less than that. It hasn't bothered me much, I'm no racer and I don't ride the interstates with it but it does bother me knowing there's more there. I have three sets of carbs, one on each bike and a spare set, that I'm going to work on and see if I can figure it out. I didn't think about the spark advancer though. Right now I have swingarm issues to deal with first but yeah, you have company...

Jbarth13 · « **Reply #11 on:** June 03, 2014, 07:57:46 AM »

Any advice on working on the advancer unit? I'm worried about screwing up timing. Looking for some step by step directions for cleaning, ideally with some pictures. I'm not super confident in my mechanical skills and want to get this right.

fmctm1sw, right now mine tops out at about 50, which is unacceptable given the commute I want to make on it. It seriously feels like one of the 250s they had us riding at the MSF course. Good to hear I've got good company though!

RJ CB450 · « **Reply #12 on:** June 03, 2014, 09:37:47 AM »

K, you said it sounds smooth, but dies out?

Assuming properly rebuilt carbs and smooth so they are sync, I need to say, check your choke cable. Lever for my 82 at open is perpendicular to bars. Full choke is parallel.

This is assuming that it is like when I left on partial choke and one hit a certain throttle and rpm, it seemed to cut out like rev limiter. Just a smooth vrrr.... And power cuts out. Rich or lean cutout, would be getting nasty popping and sputter/surge as you increased revs.

Choke is easiest, start there.

If that is fine, check ignition timing. 650 can be mechanically timed for ignition very easily.

After that, try carbs a bit, but I suspect it could be cam timing as well if the rocker assembly has ever been off. Isnt hard to have it off by a chain tooth...

Jbarth13 · « **Reply #13 on:** June 03, 2014, 10:26:30 AM »

Already checked the choke, butterflies are horizontal as can be. No worries there. Looking for directions on taking apart and cleaning the ignition advancer next. I'm a bit of a newbie, could do with some pics of where to mark and what to remove. REALLY don't want to screw up the timing on the bike. If someone could just pull off those two little bolts for the cover, mark theirs, and take pics, that would be huge.

RJ CB450 · « **Reply #14 on:** June 03, 2014, 10:47:07 AM »

There are marks for all the ignition timing bits on it from factory. I dont havr it handy, but there is a posted honda service manual on line. Search cb650 service pdf. Just need socket to turn crank by hand, and screwdriver for adjustment screw. Is under that two bolt cover, right side. No gaskets, takes about five minutes. Hardest part can be seeing marks on plate so a bright white flashlight or service light is handy.

cb650 · « **Reply #15 on:** June 03, 2014, 02:58:51 PM »

All you need to do is take the cover off and start it and see if the weights move. If you have to remove the pick ups just mark where they are at. The gap is more critical on the "CDI" than the timing.

Jbarth13 · « **Reply #16 on:** June 03, 2014, 05:49:51 PM »

Boy I feel like an idiot. My apologies to all of you who spent good time typing and helping me solve what turned out to be a really, really dumb problem. I traced the wires coming from the pulse generator and found that one of them was loose. Fixed that up, reset mixture to stock, got it idling properly, and it runs like a top. Plenty of power, maybe a bit wanting in top end but a huge improvement. Thank you all for your support!

RJ CB450 · « **Reply #17 on:** June 03, 2014, 08:05:59 PM »

Ha ha! Dont apologize. Half the time is usually something minor like that. Two hour diagnostic for a two minute fix. I dunno about the top end lacking. I find it smooth and aerodynamics are really the weak link. I did a tune run on a stretch of open road (no dynos here) and geared out...

cb650 · « **Reply #18 on:** June 03, 2014, 08:08:53 PM »

Well nobody has ever dont that before.

Jbarth13 · « **Reply #19 on:** June 18, 2014, 05:14:56 PM »

Never mind, so I lied. The bike has plenty of low end torque up to about 5.5k, but stutters at 5.5 to 7k (goes to 7 in loqer gears). Once it starts to stutter, applying more throttle makes it stutter more, and easing off makes it come back. Timing advancer works fine, charging is fine, and applying choke doesnt have much effect even at WOT. Carbs are stock inside as far as I can tell. Suggestions? Thanks!

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Author Topic: 81 Cb650C won't rev past 4500 rpm (Read 3414 times)