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Intermittent sticky valve

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PotatoHead202...
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PostPosted: 23:27 - 04 Aug 2022    Post subject: Intermittent sticky valve Reply with quote

Newly rebuilt cbf500 cylinderhead. Valves were lapped in, new seals and shims adjusted. Bike starts very easily, revs to redline without issue, doesn't seem down on power. Brand new plugs.

However, particularly when cold I'm getting a puff through the intake on cylinder number 2. Told this suggests a sticking valve. I'll stress that it's intermittent, not consistent in time between it happening, and largely disappears when warm and higher in the revs.

Any idea why this could be happening? I'll admit the fuel was at least 3 months old. Could one valve clearance be too tight? Dirty valve stem?
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stevo123
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PostPosted: 21:15 - 05 Aug 2022    Post subject: Reply with quote

I would doubt a sticky valve given that a good run normally rids sticky valve of being, well, sticky!

Maybe a quick, non invasive check, moght be a compression check and go from there. Sorry I'm not much moe use to you mate.
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Nobby the Bastard
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PostPosted: 21:56 - 05 Aug 2022    Post subject: Reply with quote

In my experience sticky valves quickly turn into broken valves....
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PotatoHead202...
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PostPosted: 16:35 - 06 Aug 2022    Post subject: Reply with quote

Right so I did a comp test and both running around 185psi.

I plugged up the carb balance holes/pair take off and it ran FAR worse than with them open. Puffing MUCH more.

Unplugged the TPS and it's almost completely stopped the puffing.....
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jeffyjeff
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PostPosted: 00:47 - 07 Aug 2022    Post subject: Reply with quote

Is it fuel injected or carbureted? Since it has ABS (your signature line) I might assume that it is fuel injected. If not, why would it have a TPS? If it is indeed fuel injected, perhaps your issue is a fuel injector not atomizing 100%.
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xX-Alex-Xx
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PostPosted: 07:16 - 07 Aug 2022    Post subject: Reply with quote

jeffyjeff wrote:
Is it fuel injected or carbureted? Since it has ABS (your signature line) I might assume that it is fuel injected. If not, why would it have a TPS? If it is indeed fuel injected, perhaps your issue is a fuel injector not atomizing 100%.


Advance the timing.

edit: 2004 CBF500 for example:

https://www.motorcyclespareparts.eu/en/honda-parts/2004-cbf500-motorcycles/e-19-carburettor-assembly

Part 9.
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PotatoHead202...
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PostPosted: 12:56 - 07 Aug 2022    Post subject: Reply with quote

jeffyjeff wrote:
Is it fuel injected or carbureted? Since it has ABS (your signature line) I might assume that it is fuel injected. If not, why would it have a TPS? If it is indeed fuel injected, perhaps your issue is a fuel injector not atomizing 100%.


No carbed. Your guess is as good as mine lol.

I assume it's something to do with the PAIR system?

Gave it a run today. Idled fine to begin with. Mile up the road getting fuel it started again. Took the tps out and it was better but not solved.......

Doing my head in.
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jeffyjeff
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PostPosted: 17:24 - 07 Aug 2022    Post subject: Reply with quote

OK, so I did a quick google search:
https://forum.allaboutcircuits.com/threads/what-does-tps-on-a-carburetor-do.165842/
Apparently the TPS advances or retards ignition timing to better match spark timing to engine load. WOT = fully loaded, part throttle = part load. Maybe there is an adjustment on the TPS, or perhaps an open on the resistance slide. Maybe you can disassemble and clean it with electrical contact cleaner. At the very least, you can take it off and check it with an ohm meter across its range of motion.
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stinkwheel
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PostPosted: 21:34 - 07 Aug 2022    Post subject: Reply with quote

If you're seeing puffs in the inlet, I assume you are running it without the airbox and filter. Try putting them back on. Maybe that "puff" would normally become a resonance wave?
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PotatoHead202...
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PostPosted: 09:24 - 08 Aug 2022    Post subject: Reply with quote

stinkwheel wrote:
If you're seeing puffs in the inlet, I assume you are running it without the airbox and filter. Try putting them back on. Maybe that "puff" would normally become a resonance wave?


No, standard airbox and filter.
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WD Forte
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PostPosted: 12:59 - 08 Aug 2022    Post subject: Reply with quote

My lovely lovely scooty has a tps on the carb

It's connected to the spark unit to give it some advance
as the throttle opens in a roughly similar way that old cars had those vacuum advance devices on the distributor.
When I tested it, I didn't find the silky smooth resistance transition from closed to WOT I hoped for, but signs of a somewhat notchy track.
new ones are 2-300 quid as I recall so sod that, I'll live with it
and have plans for a diy one in store when/if it goes bad.

I reckon the puffing may be carb related, mebbe ignition but
dont like the sticky valve idea much
Poor seal on the seat causing blow by at low revs maybe,
It has just been improved hasn't it?
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PotatoHead202...
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PostPosted: 14:36 - 08 Aug 2022    Post subject: Reply with quote

Gave it a good run yesterday.

Started fine without the puffing issue (albeit trouble idling until it was warm). Got down the road to the petrol station and it started it again quite badly as i pulled in slowly. Filled up with V Power and pulled the TPS out. It then was fine until I'd slow down for roundabouts/junctions etc whereupon it'd puff once or twice coming to/at idle then every 40/60 seconds or so if i sat there.

It doesn't seem to do it (not that I can hear/feel anyway) when it's above 3k ish which is where iirc the main jet takes over?

I did a compression test and both cylinders were registering just under 190psi.

It's driving me absolutely mad though!
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xX-Alex-Xx
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PostPosted: 23:11 - 13 Aug 2022    Post subject: Reply with quote

Have you actually tested the TPS with a multimeter? Service manual should give the expected resistance readings for fully open/closed.
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PotatoHead202...
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PostPosted: 11:10 - 15 Aug 2022    Post subject: Reply with quote

Finally managed to get hold of the guy who originally advised it could be a sticky valve. Took it to him and after searching for a while a can of WD40 with a nozzle extension found a slight air leak at the back of the carbs - seems to have sorted it. Originally tried updated coils which vastly improved it but not 100% so I think it was a combination of this and the air leak.....
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