Bike that has an aversion to the choke

[MagicalDave]

Hey,

Bike: Honda XBR 500 1987 (yes, another ).

This bike will start in any temperature without the choke, the other bike requires choke to start in anything below 20. Also smells rich when running.

Popped the carb off, cleaned the main and pilot jets (which were dirty, emulsion tube also off). No split hoses, and the scottoiler removed from the air cut-off valve hose. The rubbers are all in good nick and still flexible. The mixture screw was at 4 turns out (I wonder if they were making up for the jets being blocked). The choke cable appears to be an OEM honda and the choke plunger + needle slide freely into its opening.

The air filter is slightly dusty but otherwise looks new. I believe the fuel level is correct in the carb - none is leaking out of the overflow and no varnish above the top of the bowl.

Reset the mixture screw to 1 1/2 turns out and refitted everything, but the bike will still start without any choke - and if you apply even a hair it'll die very rapidly.

I'm a bit at a loss - I couldn't actually get the carb out to fully clean it, so managed to get at the jets with it rotated in the frame... it needs to come fully out doesn't it chaps? Any ideas on where to look first?

Just to check too, the mixture screw is on the engine side of the carb, which makes it a fuel screw rather than an air screw?

[jeffyjeff]

Hey MajicalDave. It doesn't sound to me like you have much of a problem here. Your bike starts consistently and predictably every time; what is not to like about that? My YBR starts like that. It used to require choke on initial cold start even at 30 degrees C, but now starts easily without choke after I cleaned it out and blew out the jets with compressed air. I love it!

A spark plug reading will positively confirm if it's running rich or lean. If the insulator is tan to eggshell in color after a decent ride, I'd be happy with it. Don't go looking for reasons to fret about your bike's performance; you'll probably find what you are looking for.
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History shows again and again how nature points out the folly of men - BOC

[RhynoCZ]

If it's a mixture issue, the spark plug condition is a telling sign, as Jeffyjeff says. By the way, try to replace the air filter. Slightly dusty might mean fully clogged and that would result in a rich mixture. The holes in the paper filter are very tiny, you know.

Also put valve clearances check on the list. Usually poor cold starts mean the clearances are too tight. 1987 was long ago and who knows when was the last time someone checked the valves.

When I cold start my XBR, I use the kickstarter. A few kicks with the ignition off and the choke on, then ignition on, still using the choke, and with a firm brisk kick the engine starts, even with older fuel.

You could also do the cylinder compression check test, but for that you need a specialized tool. Thinking of it, the XBR does have automated decompression valve, check that first, just to make sure.
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'87 Honda XBR 500, '96 Kawasaki ZX7R P1, '90 Honda CB-1, '88 Kawasaki GPz550, MZ 150 ETZ
'95 Mercedes-Benz w202 C200 CGI, '98 Mercedes-Benz w210 E200 Kompressor

[stinkwheel]

Might be worth just re-brassing the carb, it's hell of an old. The fact the mixture screw was so far out suggests someone was trying to compensate for some problem. Also probably only a matter of time before ethanol buggers up the needle valve. You aren't going to see an ovalled slow running jet with the naked eye.

You can get rebuild kits for fairly cheap or just get the jets. Google says it's a kehin VE10 so you can still get genuine kehin jets for them.

I'll second just replacing the air filter.

I'd also verify the float height, someone thought it was ok to have the pilot screw that far out suggesting they were prepared to mess with the carb but didn't really know what they were doing.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[twowheelwonde...]

The no choke needed bit and the rich smell are the same clue. With the pilot screw four turns out it is running like the choke is half on all the time, so cold starts feel easy. Sort the richness first, float height and pilot setting, then a plug chop. Bet it wants a little choke after.

[MagicalDave]

Hi all, sorry for the late reply, life gets in the way!

This bike is actually an 86, it's got a cable operated decompressor (so only on the kickstart). The valves were checked recently along with the plug, which was black and we changed it.

Managed to get hold of a rebuild kit, so rebuilt the carb (pilot screw assembly, main and pilot jet, needle guide (inside the throat of the carb), main needle, fuel needle and bowl gasket. Now a very slight bit of choke will raise the revs but then kill the engine. Cleaned all the passageways and blown through with cleaner and are all clear.

Air filter changed for a new one. No change.

Mixture screw does very little, and it'll happily idle even with the screw almost fully closed. It does run though, and run quite happily with lots of power (heh) when riding normally. That to me suggests the main fuel/air circuits are ok (or, at least, the fault is so small that it gets drowned out by the quantity of air/fuel when off idle).

The float height is not adjustable on this carb, the float is completely plastic... but it's not overflowing out of the overflow. The old needle valve was shot, very hard, and a ring of "crap" round the end. No wonder it used to leak every so often.

Checked the choke cable, fully disconnected it to ensure it wasn't being held open, also checked the end with the pin. The rubber seems soft still and not hard/damaged.

What I did notice, is that the air-cut diaphragm looks too small and was extremely rusty. It won't sit in the ridges around the edge of the seat, and it doesn't feel like it actually opens the valve... it also looks different to the ones that you find online - mine has no groove at the end. Maybe it's snapped off? I didn't find any bits!

Just currently trying to source one, and hoping I can rotate the carb in the bike to get to it without having to get the bloody thing out again. Resisting the urge to take the one off the working XBR to put on the non-working one to prove a point without firing the parts cannon...

Thanks for all the advice, it's very much appreciated!