Carb/s not keeping up with engine

[MGST]

Bike: 1985 Honda GB 250 Clubman

Engine: 250cc twin cam ( 4 valve ) single cylinder, twin carbs ( 1 for each inlet valve ). Keihin twin carbs, only one operates under light throttle, if you wind it on they both work ( progressive? like a mechanical secondary on a Holley ). The carb pistons ( ? - I'm used to SU carbs on cars ) are about 20mm diameter, chrome ( steel ? ). I can provide more specs about them from the workshop manual when I get home if required.

Problem: When boosting it on the open road, sometimes it surges as if running out of fuel, and when doing traffic light drags etc, will not rev right out.

Back ground:I rebuilt the top end 18 months ago, blasted and cleaned up the head, recut valves / seats, hone, new rings. New gaskets / seals for engine and carbs. It has been running well every since, but has never quite been the same. When riding on the open road, and pull out to overtake, or come to a hill or even a rise, and I wind on a little ( just enough extra to try to maintain speed up the rise ), the bike starts to surge like it's running out of gas. It starts to slow down, so I ease up on the throttle, still doing it, change back a gear to 5th and try to keep road speed, still doing it, change back to 4th, and usually it will be ok, but by now I've lost a good 20 to 30 kph. It doesn't do it all the time, it seems to do it for a while, then it's ok, then it wil play up again ( all on the same ride, not one day it's ok the next its not ). My usual open road riding is 90-100kph where 100kph in 6th = 7000rpm ( max = 10500 rpm )

When increasing engine speed to red line through each gear ( traffic light drags etc ), it feels " out of breath " around 9000rpm and the rate of acceleration ceases. It will slowly just about make redline ( 10500rpm ) but it is quicker to change up a gear, and I never take it to redline now.

I have striped the carbs down several times and blown them out and made sure everything is clean, seating correctly, blah blah blah, to no avail.

It's to the point now where it is taking the enjoyment out of going on rides because it loses so much ground going up hills that I have to go flat out to the next hill to stay within reasonable sight of the group, only to have to do it again the next hill. I ride on the weekends for enjoyment at or just below the speed limit, I can't really be bothered fanging it ( geez I'm sounding like some old codger!! ).

Does anybody have any ideas? If I can sort this problem, and the front suspension, I'll keep the bike. Otherwise I'll sell it, it's pissing me off that much, but don't really want too.

Thanks in advance.

[F5 Dave]

erm, could be hard to trace, but for a starter best to tune it ie: new plug & plug cap. Check filter (if foam, is it genuine? real filter oil?,std airbox?).

ok is there a in line filter? Are the lines kinked or routed awkwardly? Has the tank ever been sealed? Check tank breather. Check tap for rust debris.

Carbs, so one is a CV type? Check diaphragm is not perished.

[fridayflash]

i know some people used to file the slot in the linkage between
the carbs on these rfvc engines when fitted to xr's and xl's
to aid starting,ie engine starts one one carb then 2nd one kicks
in as you open the throttle. just wondering if the linkage might be a bit flogged out making the problem worse?

[Tumbles]

Sounds to me like you need to adjust the float height. Asking more from engine, engine stutters, engine comes back up once power requirement is gone? Sound like the float tank in the carb is being sucked quicker than it can refill. Dunno much about Keihim carbs, but there should be a tab on the floats you can bend to make the floats sit higher. i.e. the valve letting fuel into the float tank will open earlier.

[MGST]

Right, some more info.

The carbs are Keihin, Type: piston valve, ID number: PH02B ( E, ED ) PH02C ( SW ), float level 18mm, Primary main jet #102, secondary main jet #105 ( richer I assume? ).


There is an inline filter, which now you mention it F5 Dave, I've never checked. No kinks in the lines. I got the old tank sealed at Reflections on the North Shore, but the holes maust have been too big cos it still leaked. Can that sealant stuff block the filter?

The linkages are all ok.

I had wondered about the float height too Tumbles. It runs ok around town on light throttle so the primary carb is obviously ok. Maybe the secondary carb float height is too low.

[Tumbles]

There is an inline filter, which now you mention it F5 Dave, I've never checked. No kinks in the lines. I got the old tank sealed at Reflections on the North Shore, but the holes maust have been too big cos it still leaked. Can that sealant stuff block the filter.

Without knowing what the sealant was, who knows. The fact is though that being a 1985 bike, there is every possibility that over time crap has built up in the lines and blocked the filter. Definitely worth looking at.


I had wondered about the float height too Tumbles. It runs ok around town on light throttle so the primary carb is obviously ok. Maybe the secondary carb float height is too low.

Also a valid guess. I would check whatever is easiest to get to first.

Also yes you are correct, the higher the number, the more fuel the jet will pass through it, making the mixture richer.

[kiwi cowboy]

Without knowing what the sealant was, who knows. The fact is though that being a 1985 bike, there is every possibility that over time crap has built up in the lines and blocked the filter. Definitely worth looking at.



Also a valid guess. I would check whatever is easiest to get to first.

Also yes you are correct, the higher the number, the more fuel the jet will pass through it, making the mixture leaner.

just a thought tumbles but to me the higher the jet number the ritcher the mixture would be wouldnt it?

[Tumbles]

just a thought tumbles but to me the higher the jet number the ritcher the mixture would be wouldnt it?

Sorry, typo!!!!!!! Post changed to RICHER :spanking:

[F5 Dave]

Bugger I had meant to add float height as well. I would raise it & see if it makes it better.

But now you have answered the other questions I'll say to do first:

Pull the tap out & check it isn't blocked or at least restricted. Also the fuel breather could have been blocked at bit by the sealing process, this is quite common, usually the cap in that era Honda I would think. Take it apart.

Fuel filters are notorious for either blocking, or more likely causing a kink in the fuel line that you can't see. Either take it out, you shouldn't need it once the tank has been sealed, or be super careful that the route the petrol flows is smooth & always downhill. You can buy right angled elbow filters that would have been just the caper for my old GS which had a tight fuel line run.

[MGST]

Took the filter and line off yesterday and blew the filter out backwards. Nothing came out that I could see, but I got a new filter and some new fuel line anyway and fitted them yesterday at work. Couldn't tell on the way home if better, but it wasn't worse!! Will have to find some time to pull the carbs one weekend and check the float levels.