Top speed about 60-ish 1968 CB250K0

Windmill John

Windmill John

Veteran Member
Joined
Jun 4, 2022
Total Posts
753
Total likes
329
Location
East Sussex, England
I’m just checking if there are any favourite culprits for this.

As above, my top speed is in the 60s, revving like a good un!

I will check my main jets, but plug chop didn’t look too lean. I didn’t make a note of jet sizes…
I will check needle positions.

Okay, timing spot on, points ignition. Any benefit running these slightly advanced?

I’ve stripped the carbs twice due to an idle issue. Bike runs sweet as a nut except no top end.
Diaphragms good, no inline filter, tank filter cleaned, fuel cap clear.

I’m running the float height at 21mm.

New clutch, adjusted.

Just ideas needed, but I possibly I need to check those jet sizes and needle positions.
 
I can't speak to the performance of the 250 version of the bike we only got in the 350 size, but it would seem to me that it should do at least 70+ mph. You mention new clutch so you're sure it isn't slipping? No excess engine revs without acceleration then? Because your phrase above "revving like a good un!" gives the impression the engine is revving like crazy but not attaining more speed as a result.
 
That was bad wording. No I was trying the ‘ancientdad’ approach of taking the revs up way past my normal 5000rpm.
So basically, after the engine was fully warmed, I accelerated pretty strongly in every gear.

The revs increased not the power. Sounds like clutch, but it is adjusted correctly.

Before this ride, I took the left points to .4mm, reset timing and it gave me a little extra. I’m still wondering about float height. Some literature said 23mm, but I think that’s 350. My manual says 21mm and I’ve seen 19mm somewhere….. I’m debating about trying 19mm. I’ve had the carbs off twice last month, so may leave it for a while and then get them off again.
 
Try accelerating in 1st to @6K, shift into 4th and quick release the clutch. Result should be the engine bogs down, if it revs then the clutch is slipping.
Note: only do this once as it's extremely hard on the frictions.
 
Back in the day, the CB250K0 was reputed to be a 100mph bike. Obvious wishful thinking, but a genuine 90 should have been possible.
 
I’ll be out for a ride in a bit, but something is crossing my mind.

When I had my 350 RE Bullet, the four speed, one method for increasing power was to fit a higher compression piston and retard the inlet cam by one tooth.
Wow, this was a great idea which I did and really noticed the difference.

Why mention this?….. I have a receipt with my 250. Back in 2012, one of the POs ordered pistons rings. I’m beginning to wonder whether when fitting these, the chain was fitted one tooth out…. The wrong way….. I don’t know the tolerance of these CBs and whether this can be done. If it can, it would explain the massive power drop.

If after my ride the clutch is not slipping, I may next week advance my timing slightly to see.

when I recently increased my points gap from .3 to .4 and readjusted the timing, the bike certainly had more oomph on acceleration up to 30.

Of course it could still be jets, needles…. I just have a feeling these haven’t been changed.
 
Richard Pitman said:
Back in the day, the CB250K0 was reputed to be a 100mph bike. Obvious wishful thinking, but a genuine 90 should have been possible.
Click to expand...

A slipping clutch prevents my 63 CA95 Benly 150 touring from breaking 70mph, full tuck, downhill, downwind, full refried bean thrusters.:biggrin:
 
Seriously, If you are overadvanced you will lose top end because the spark is fighting the piston too much. Use strobe light, only way. Not to mention seizure possibility and other damage.
 
I may be worrying about nothing. Just got back from a ride. I borrowed a GoPro, once I’ve played with it, I’ll post it hopefully.
But anyway, got to just over sixty. Trying to stay in the law, I rolled off the throttle. But then I tried it again, going over sixty, being a sit up bike I virtually laid on the tank and Dude went to over 70 pretty quickly.

All may be fine. ballbearian, maybe it is what it is, but judging by the difference on my Bullet 350, could the chain be out by one tooth on these bikes? I just don’t know.

I’d prefer plain speaking and someone just say all’s great. The bike runs great and maybe this is de riguer for bikes with high lift cams.
 
I was meaning, normal timing adjustment with the points plate. But being a whole tooth off on the chain, I got no clue. Sounds serious as a heart attack to me, I'd be scared to start it.
I know conclusively that I don't know nearly enough.

If you are a big guy, John, that makes a difference. I'm a peewee, 150lbs.
 
Though I've not intentionally tried it on any OHC Honda engine, typically one tooth off in cam timing means a bent valve or two. Every engine I've seen put together incorrectly like that has resulted in unintentional damage.

Full refried bean thrusters... LMAO (y)
 
Okay, tested. I think I'm worrying about nothing.

First bit of video, dropped to 1st gear, accelerated to 6000 rpm, up to 4th quickly. Bogged a bit, still think clutch is okay.

Next bit, just testing the acceleration. Went up to about 60mph. Bike didn't really climb after that, but I did back it off trying to stay legal. On the next bit, I opened it up, acceleration slowing just over 60, but when I dropped myself to the tank, it went to 75 pretty quickly. I think I'm happy.




John
 
Your bike seems to run well, and you're still not using its entire powerband. If you were to shift at 9000 to 10,000 in second and third gear, you'd get into a higher rpm range in 4th and would have already gotten beyond 60 mph at that point I'm sure. I don't think you have anything to worry about.
 
Thanks ancientdad. Considering the number of bikes I’ve had, including multis, traillies, sports tourers, tourers etc. I’ve never had one which needs to run so high for most of its power band.
I wouldn’t mind if it wasn’t so loud, it probably offends people.
But, you’ve put my mind rest, time to just enjoy the riding…. Which I do.
 
Actually, in the video it sounds like stock exhaust to me. Now my bike is loud, but that's because it has no silencers... and yes, it probably does offend some people but I'm as polite as possible in my neighborhood, and outside my neighborhood those who are offended won't be so for long as I'm zipping past and away from them. :)
 
As I discovered on that ride, my ‘new’ tacho got hesitant above 6 to 7000 rpm. The revs did go higher, they just didn’t always show.
The tacho can stay like that, it cost a ton!

Tomorrow, I’ll get a T shirt printed with ancientdad on the back in ginormous font and ride in lots of other neighbourhoods :)
 
In my experience, all Honda's (1970's through 80's) have clutch slip with original springs.
I always fit heavy duty springs to my bikes, way cheaper than big bore kits and I've also found it necessary when using synthetic or semi synthetic oil.

The 'problem' is the way friction plates are made.
Originally they were 'cork' but since around 1995 have been 'paper'
Aftermarket, you don't know as they usually say OEM spec
 
You do need to rev the nuts off these bikes, redline them through the gears to achieve highest top speeds.

As I may have mentioned once or twice (yawn) I had a brand new 250G5 back in the day. That was probably good for 80 mph, not much more. At the time, I had the opportunity for a ride on a well worn CB250K0. It was noticeably more powerful ( and much louder ) than my G5. No opportunity to try for top speed, but it accelerated much better than the G5. (Claimed 27bhp vs claimed 30 bhp).

A couple of years earlier, when I was still riding a C100 50, a local lad had a brand new K0, and rode it appropriately. It would lift the front wheel on hard acceleration in first gear, not clutching it up, just opening the throttle, an image that has stuck with me over the years. His bike was also quite loud, easily heard several miles distant in our then quiet rural environment. ( From Whitbourne, over the Downs to Bromyard, for anyone who knows the area).
 
Richard Pitman said:
( and much louder )
Click to expand...

No sh……………. Sugar!

Don't let ancientdad know as he loves it… it’s as loud as buggery! Had I known it was this loud and the power band was so peaky, I might not have bought him.

I had a 250K4 years ago and it was a pleasure to ride and to listen to. This might be an anathema to some, but I prefer quiet. Actually, thinking back, the K0 is as loud as the scariest bike I’ve ridden…

No, not the sport bikes I’ve ridden. No, not the enduro bikes I’ve ridden over rocks…

A TM125 Suzuki!! It’s power band was like the Tasmanian Devil!
 
Have you tried comparing your speedometer to GPS? On all my bikes and scooters, they are all off by a certain percentage. ie..My Piaggio always shows I'm traveling 5mph faster than the GPS.
 
Scootman said:
Have you tried comparing your speedometer to GPS? On all my bikes and scooters, they are all off by a certain percentage. ie..My Piaggio always shows I'm traveling 5mph faster than the GPS.
Click to expand...
I have yet to find an OEM motorcycle speedo that reads correct, they're always reading fast.
 
If you had the original handbook or a copy of it,you would see the phrase 'do not operate in red band for extended periods'
Honda intended it to be revved high.
As most 'new' engines designed at the time, (designed in 60's or early 70's) airflow was all about large ports without understanding you can easily go too big so lose low and mid range.
The 250 is really a 350 with small pistons for British market (250 'learner law' came in around 1965?)
Sixteen year olds on 100mph bikes had some serious consequences (if they could afford BSA 650, Bonneville, Norton 99, etc, that is)
It didn't take long for 250's get get some development
I had 'real' Royal Enfield Continental GT, first genuine British production 100mph 250, (originally it only made about 14bhp in 1955)
Mine only had 4 speed though as the 5 speed was too unreliable (gears too narrow so they could fit 5 in space for 4)
For comparison, 1965 Honda S90 made about 9 bhp before it got some 'modifications' and a 90mph top end.
Anyway, the point is CB250 needs to be revved in lower gears to reach max speed. If you fitted a new clutch, you did fit new springs?
Personally, I think I've only ever fitted new plates in one of my bikes and I was racing that.
Even my 1977 CB550 has original plates at about 106,000 miles (but I did fit heavy duty springs around 4000 miles due to clutch slip at higher speeds, 110mph +)
 
Might seem a waste, but each time I’ve changed a clutch I fit new springs.

I think I mentioned it earlier, but just never had a bike that needed so many beans to go; ignoring the TM125.
Love the bike though.
 
Has the engine been worked on?
Cam timing may be out by one tooth?
You can check by removing points cover, points plate and advance mechanism.
The locating pin should be straight up or straight down at TDC.
Did you do the oil transfer piece mod yet?
At least that's very easy on a 250/350 as you don't need to remove clutch cover
 
crazypj said:
Has the engine been worked on? ——- All I know is that in 2012, the PO had bought piston rings, but I don’t know what was done.
Cam timing may be out by one tooth?
You can check by removing points cover, points plate and advance mechanism.
The locating pin should be straight up or straight down at TDC. ——- I think…. It is about 5pm or 7pm, I cannot recall.
Did you do the oil transfer piece mod yet? —- I don’t know about this!
At least that's very easy on a 250/350 as you don't need to remove clutch cover
Click to expand...




John
 
You must log in or register to reply here.
Back
Top Bottom