How do bikes know when to break?

[MarkCR750]

I've done about 350 miles without issue , tomorrow I'm booked in for a dyno fuelling run, I start the bike tonight and it's running really rough, a quick feel of the pipes says No1 cylinder isn't firing, or hardly firing, all I've done in the last few days is take the carbs off and put them back on, and adjust the mixture screws, when I put the carbs back on No2 carb overflowed, I gave it a few taps with a screwdriver handle and it stopped overflowing, so I guess it was a sticking float/float valve, I had turned the mixture screws in a bit yesterday so just now I turned the No1 screw out half a turn, then tapped the No1 carb in case the float/valve was stuck and not letting fuel into the bowl, it ran rough for about 30 seconds then after giving it a few revs it's running fine and the No1 pipe has come up to the same temperature as the others , how does an inanimate object know to start playing up just when it shouldn't ??🙄, any thoughts on what the issue could have been, I'm thinking stuck float?.

[Johnwebley]

its the bikes way of telling whos the BOSS,

[Bryanj]

Murphy's fifth law!!

[MarkCR750]

Your both totally correct , and Murphy's law also explains the fine weather all week and heavy rain now I'm having to take it 15 miles to the dyno, behold the beauty of my temporary lash up front mudguard! 😀

[royhall]

Shame to get it dirty when you've just spent ages shining it up. Dyno results are going to be an interesting read though, as your expecting worse than standard. Its amazing what the standard airbox manages to achieve.  Don't forget to share the results. Good luck.

[Greebo]

I've done about 350 miles without issue , tomorrow I'm booked in for a dyno fuelling run, I start the bike tonight and it's running really rough, a quick feel of the pipes says No1 cylinder isn't firing, or hardly firing, all I've done in the last few days is take the carbs off and put them back on, and adjust the mixture screws, when I put the carbs back on No2 carb overflowed, I gave it a few taps with a screwdriver handle and it stopped overflowing, so I guess it was a sticking float/float valve, I had turned the mixture screws in a bit yesterday so just now I turned the No1 screw out half a turn, then tapped the No1 carb in case the float/valve was stuck and not letting fuel into the bowl, it ran rough for about 30 seconds then after giving it a few revs it's running fine and the No1 pipe has come up to the same temperature as the others , how does an inanimate object know to start playing up just when it shouldn't ??🙄, any thoughts on what the issue could have been, I'm thinking stuck float?.

Hi Mark
this is exactly the same as what I had trouble with after the winter lay-up, when I put her to bed last November it was running great, when I put her into winter storage I emptied the fuel tank & drained the float bowls...
This April filled the fuel tank opened the fuel tap & within 2 minutes petrol was p*ssing out of carb 1, like you i tapped the carb, & then sprayed carb cleaner into the air intake,& that did the trick, when I got the engine started cylinder 4 was not firing, looked at the plug which looked OK, just a tad brown[ which is a new iridium] so then tested it for sparking out of the cylinder & it seemed Ok, so then checked that the accelerator pump was pumping fuel into that cylinder that's was Fine...scratching head violently ,.... so I phoned a friend , he said put another new plug in, so I did, & bingo it fired up & all was well again...for a while
.....what very strange & temprimental animals these bikes are...

Re: Dyno I reckon you get around 57bhp ish...which at the rear wheel is quite good.
We await the verdict ...
just hoping you are not also a fisherman Mark...
I caught one this ********************************* Big ...

[MarkCR750]

I got 53bhp, he said the exhaust is too restrictive, I'm not changing it though cos it's quite loud enough!, I was thinking it would be closer to the original claimed 67bhp but did Honda give the power at the crank?, if that's the case take off their exageration and dial in the transmission loss and wear (plus tappets etc need setting) and it's not such a bad result, fuelling is bang on which is what I was most concerned about , here's a YouTube link, no that's not Guy Martin, it's world champ Sam Ermelenko , google him .

https://youtu.be/dzyp3A1TQBY

PS that result was with 3 plug caps pulled off, God knows what it would have made on all 4 cylinders! 😀

[royhall]

If its an F2 engine and carbs it should be 72hp.

[MarkCR750]

It's a K6 Roy, PD carbs though , do you know if Honda quote crank or rear wheel bhp in their specs ?, I've been googling but can't find anything, my guess is crank because they'd want the highest figure for marketing reasons, so they'd get the best engine possible, then run it in perfect conditions , then lie a bit as well 😀 , my T250 was marketed by Suzuki as a 100 mph 250, then years later they brought out the X7 and called it the worlds first 100mph 250, maybe they forgot they'd made the T250 ? 😏

[Trigger]

Had a look at the vid Mark and it doesn't look like he took it to full power, did he take it to 8500 RPM ? The figures look all blurry to me.

[MarkCR750]

I had asked him to keep the revs down, because it's an old engine that I had not done any internal work on, however curiosity got the better of me and I asked him to give it one more run to 9k after the print out, it didn't produce any more power to speak of though, which makes sense with the red line being just over 8k i.e. it's tuned to make max around there, I'm still trying to figure out if Hondas quote of 67bhp is crank bhp?, if that's the case then mid to high 50's is what you'd expect at the rear wheel after drive train losses, depends which site you read but the majority say that Honda quote crank BHP, the bike feels quite quick but that could be down to all the weight it has shed, I am interested to know if it's making a lot less power than it should due to the restrictive silencer or if we're getting confused with crank bhp, has anyone dynoed a stock bike?.

[Trigger]

A engine loses BHP the more wear the engine has. I thought I explained this when we met mate. I would say that the original BHP was done at the fly wheel, as Honda would pull engines from the production line for testing.

[MarkCR750]

A engine loses BHP the more wear the engine has. I thought I explained this when we met mate. I would say that the original BHP was done at the fly wheel, as Honda would pull engines from the production line for testing.

They do Trigger but not that much, provided there is no major wear and the piston rings are sealing etc,  I had my 136k mile Alfa GTV dynoed and it made 152bhp, I had de-catted it but that's only good for 10bhp, when new Alfa stated 150bhp so it had hardly lost anything in all those miles , I just found this from a guy in the states that builds and races CB's
"I have not got my 836 engine on the road yet so it tough for me to make any comparison between the two. In top stock form a cb750 turns about 56hp while I've read dyno reports that an 836 will turn close to 70hp at the rear wheel"
So I recon mine is making pretty much stock power , it ties in with Greebos 56bhp at the rear wheel when dynoed, I still won't be fitting a louder/freer flowing exhaust though, it might give another 5bhp? but I value my eardrums 😀
PS your right about Honda and other manufacturers quoting crank BHP, I have now read that the rules allow them to quote "maximum engine power" so they use crank dynoes, therefore the figures they quote are only useful as "comparative" figures and aren't "real world" as they ignore frictional losses in the drive train which typically are between 10 and 14 percent .

[royhall]

Most automotive companies use crank output so can't see Honda being any different as they would seriously loose out on the PR game. My CB1300 has a full titanium Akrapovick (how do you spell that?) system with power commander that has a custom map, and that only just managed stock output. So I just gained back the transmission losses proving they are crank rated.

[MarkCR750]

Yes I've since read that Honda do indeed use crank bhp, what was throwing me was my XJR1300SP which is quoted as 106 bhp by Yamaha , I had it dynoed and it came out at 104 pretty much bang on, so either yam are being strangely honest for once or CMC calculate back to crank HP, I'll have to ask them , I subsequently changed the intake rubbers for FJ1200 items and it made 112, total waste of £70 for the rubbers cos the bikes all about torque not BHP, but I was younger then (2 years ago 😀), Sam's Dyno is rear wheel BHP (thankfully), anyway it's running nicely that's the main thing, been out on it since 7am must go home some time today!.