Honda CB250N Super Dream Carbs
A friend had recently bought a very tidy CB250N which ran quietly but only on one cylinder. With a little coaxing and allowing the revs to rise it eventually chimed in on both pots. As soon as the throttle was released and the revs dropped, it went back onto just the left hand cylinder. The left hand down-pipe was really tarnished - presumably single skinned (aftermarket) but the right hand one still looked new, so this fault had been there for some time. It appeared that the pilot circuit on that carb would be to blame as there was fuel in the bowl and obviously a spark at higher revs so it was decided to pull the carbs off and strip them both allowing a good clean in the ultrasonic bath and I could tidy them up and bright zinc plate the fittings whilst I was at it.
I attacked the offending right hand (number 2) carb first after separating the two by removing the tie plates and fuel transfer tube, taking care not to lose the light compression spring between the two throttle spindles. Many of the screws had been damaged by previous owners but using the correct JIS screwdrivers they were still removable - I'll tidy those up a bit and then re-plate them, we're not aiming for concours - just tidy, useable and presentable! There are a great many drill-ways and passages to clear and check; some are really difficult to be certain of, as they have blanking plugs pressed in after machining which are not really removable. Even the ultrasonic cleaning bath might not always unblock a very fine passage if the varnish and silt have built up to any great extent.
At least the castings on these two Kei-Hins are in a pretty good condition without broken lugs or damaged float hinge posts so should hopefully live to fight another day. Unfortunately there were no really obvious signs when it was stripped indicating why this carb had an idle circuit which seemed inoperative. I'll just have to hope that if there was a blockage somewhere, then the repeated treatments in the ultrasonic bath have cleared it. Squirting carb cleaner through the many passages seemed to indicate that all holes were clear, especially the three really tiny ones near the throttle plate by the more visible pilot hole.
Well that's the first one done and re-assembled - the other one has a few more springs to take notice of but most of it is quite straightforward. I'll be giving them back to the owner when finished for him to fit back on the bike and hope to report that everything is fine! Assuming that's the case then I'll balance them for him sometime; although I have a set of 4 vacuum gauges, I have more recently been using a single gauge with a switchable manifold which eliminates any error between gauges and have found that my 350 Four runs far more smoothly using that method.
The second one (left hand number 1) was done in exactly the same way and stripped to a bare body so that all passages could be cleaned and checked. As mentioned earlier, there are more springs to take account of on this one but it's not that difficult to tell which spring goes where, especially if you have taken plenty of good detailed photos. Whoever had been at these carbs before had bent a few of the choke lever arms but a few minutes with snipe nosed pliers soon had everything back in line, ready for zinc plating. Joining the two together again is a bit fiddly and takes a bit of concentration as you have to re-connect multiple parts all at the same time. The pilot screws were set at 2 turns out initially and the two throttle plates were adjusted so that the edges were the same distance from the bore of the Venturi. Final synchronisation will be carried out later when they are on the bike again.
Footnote:
Good news - Derry picked up the carbs and refitted them and it started up fine on both cylinders but it will need all other settings checking before it takes to the road again.