Honda-SOHC
SOHC.co.uk Forums => SOHC Singles & Twins => Topic started by: rbt1548 on April 09, 2025, 11:49:53 AM
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I got a 1976 CB250, (UK G5), model, in nice condition which the previous owner left for over a year but didn't bother draining the tank or the carbs before laying it up. It did start but would only run on the left cylinder, the right plug was bone dry indicating to me that the carb was bunged up.
I cleaned the tank and had it lined, cleaned both carbs to within an inch of their lives, refitted them and both cyls fired, pleased with that but noticed when revved it would hang off idle before slowly settling back down. As there were a few things I wasn't happy with I decided to go through the bike, starting at cam chain adjustment then valve gaps, points and timing, the auto advance unit was filthy so that was stripped, cleaned, and lubricated, everything put back together and the hanging revs problem was gone, good chance it was the auto advance unit sticking a bit.
I realised the only thing I hadn't done was the float heights and the question is with regards to the float height, I have searched for a definitive answer and read info in various places and there seems to be differing opinions on float height and how the carbs should be sitting whilst checking the height, FSM says 18.5mm, carbs vertical with tang just touching float needle and choke side at the top, others give a range of heights from 19mm-23mm with the carb slightly tilted, others stating because of the newer fuels E10 etc heights should be increased slightly.
I would be grateful if someone could tell me which would be the best setting and way to do it please?
Thanks
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The FSM method is correct, so the float height is probably correct too. You're measuring the float height without any compression of the float valve spring.
Ignore any so-called experts who suggest changing the float height, shouldn't be using E10 fuel on these bikes anyway!
Always use E5 fuel which has zero or low amounts of ethanol. If not available, use minimal amount required of E10 and fill up with E5 ASAP.
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Thank you!
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I passed my G5 book on to a member on here and whilst 360 that bitvisxprobably same, hopefully he will notice your post and reply
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Thanks!!
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I have the FSM from Bryan, I’ve had trouble gaining access to the site so have only just seen this. I am away at the moment but will check as soon as I am back home.
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https://www.dropbox.com/s/mcug2pb5eaj6ydq/CB360_Searchable_Manual.pdf?dl=0
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Thank you for info, much appreciated.
As from Craizeehair,
I have the FSM from Bryan, I’ve had trouble gaining access to the site so have only just seen this. I am away at the moment but will check as soon as I am back home.
I too have been having difficulty gaining access to the site.
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In case it’s still needed?
(https://uploads.tapatalk-cdn.com/20250505/11977504189dfa13dfa22547b52b6543.jpg)
(https://uploads.tapatalk-cdn.com/20250505/aa6071610bffc7b678462e0ac0621135.jpg)
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Thanks for that clear picture, some info I had read showed the carb slightly titled back allowing the tang just to kiss the float needle, going by that picture and instructions the carb is vertical.
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After being unable to get to work on the bike for a wee while I have managed to get back to it.
Before measuring the float heights I noticed one of the float was not moving as freely as the other so I removed the pin and noticed it was slightly bent with a couple of gouges on it. I tried the float with the other pin inserted and it bounces about no problem. I understand pins are not available on their own, but I am able to get stainless steel dowels the correct length and dia. 28x2.5mm, would they suffice? I notice other carbs use stainless float pivot pins so I hope they would.
One set of floats was wildly out at 26mm the other 23mm which had me doubting I had measured them at the correct place, the highest point of the float as indicated by the arrow in the picture, could someone please let me know if I am measuring them at the correct part of the float?
Thanks.
(https://i.postimg.cc/MZLFYwtV/float.jpg) (https://postimg.cc/K4nfZd68)
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I tend to do my float measuring slightly different. To me it's not the height the float is set at, more the height it actually shuts off the fuel. Ideally these should be the same as the spec figure, however in reality they usually aren't. Sometimes the needle seat wears or the tip of the float needle wears, causing a leak past. As soon as that happens your float height doesn't matter anymore, it's higher than it should be.
So I tend to connect a remote tank up to the carb with petrol in it, then measure the height where the fuel shuts off, to do this you need to position the carb upright over a bowl to collect the split fuel. If it shuts off at the spec figure great, if it doesn't alter the float tang till it does. If for some reason the fuel just won't stop dribbling past no matter how high you hold the float then the needle and seat need either replacing or some attention until they do stop the fuel.
The other way is to do the height as you are now and then test it using the above method so ensure it actually shuts the fuel off. You'd be amazing how many don't shut the fuel off when they should.
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Thanks for that, on thinking about it when it shuts off the fuel seems logical as there will be wear on the needle and/or the seat after a while in use.
I have a remote reservoir so will give it a go.
Any thoughts on using a S/S dowel as a float pin?
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You can buy some brass rod on Amazon, right size as well.
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Some more info required from you kind people please.
I had set the floats to the heights to the measurements specified in the manual and pic in post without much hassle, however when I turned the carbs the right way up I noticed that one set of floats were hanging down a good few mm than the other set, they are ok when on their sides on the bench for measuring but one hangs a bit lower when turned vertical.
I had a good look at the floats and noticed a second tang on each set of floats which came into contact to on of the cast pillars that the float spindle goes through, one was slightly more bent on one set of floats than the other causing the float to hang down further when the carbs are vertical.
Obviously, both should hang down the same, but one set of floats is lower due to the bend in one of the tangs hitting the pillar first, is there a distance they should be set to or is it just a case allowing a bit of free movement?, as I mentioned when measured on their sides they are spot on.
Don't you just love 50 year old bikes that have gone through a few pairs of hands!
Thanks
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The amount they hang down is of no matter as long as they shut off the fuel at the same level.
I can only assume that the tang to stop them hanging down too far is to make it easier to fit the float bowls, cant think of any other reason
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I think it’s there to stop the float needle falling out if the float was able to fall as far as it liked.
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Thanks, I'm just going to set them both at the same drop when vertical after ensuring that they both are the correct measurement when done the service manual way.