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CB250 K4 carb settings [message #300713] Tue, 14 May 2019 11:22 Go to next message
jeffers100 is currently offline  jeffers100   United Kingdom
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Good morning everybody,

I'm looking for some advice on getting my CB250 K4 running sweetly.
I have recently had electronic ignition fitted, and have serviced the carbs with new O rings, jets needles etc.

The problem I have is that every now and then (10 - 20 miles) the bike hesitates and sometimes cuts out, as if I've run out of fuel. After waiting two minutes the bike re-starts and runs normally until the next time.

Is this a float height issue? As I said, all jets and needles are new and has new O rings throughout. The bike runs fine until it hesitates/stutters, usually when going uphill or is under load (3/4 to full throttle). If I slow down it sometimes picks up again.

Any ideas?

Jeffers100
Re: CB250 K4 carb settings [message #300715 is a reply to message #300713] Tue, 14 May 2019 12:19 Go to previous messageGo to next message
Kermit is currently offline  Kermit   United Kingdom
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(22) Giacomo Agostini
Only 2 things I can think off looking at

1) Float heights - Are the jets/needles you've fitted standard/stock items or aftermarket? MIght be the float hieght on aftermarket kits needs adjusting also but iirc kits usually state whats needed if thats the case
2) Easier than #1, is there any chance you kinked the fuel line when putting the fuel tank back into place after the carb service as a slight kink restricting fuel flow just a bit (as opposed to fully trapped and no running) might well be fine 90% of the time and only exhibit itself after prolonged running at higher revs.

[Updated on: Tue, 14 May 2019 12:20]

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Re: CB250 K4 carb settings [message #300718 is a reply to message #300715] Tue, 14 May 2019 16:13 Go to previous messageGo to next message
spagiratwink is currently offline  spagiratwink   United Kingdom
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(6) Moto1 GP Racer
Hi , could it be your Electronic Ignition, is not installed correctly
Re: CB250 K4 carb settings [message #300719 is a reply to message #300713] Tue, 14 May 2019 17:09 Go to previous messageGo to next message
Kakugo is currently offline  Kakugo   Italy
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(18) Geoff Duke
An engine hesitating or dying under load is a typical symptom of fueling being far too lean. Usually you can correct that by increasing float height. Remember that K4 bikes have a float height of 25mm (Haynes says 19mm, which is correct for earlier models but on the lean side for the K4).

But remember... those old Keihin carburetor have a rubber membrane which Honda calls "throttle valve" and some oldtimers call "diaphragm" or something similar. That membrane will go brittle and crack with age causing the bike to run lean (the condition is called an air leak) regardless of jetting and float height. These membranes are always the last thing we replace because they are so expensive, but when everytthing else fails...

Just hit everything with a big enough hammer. Wink
Re: CB250 K4 carb settings [message #300725 is a reply to message #300719] Wed, 15 May 2019 07:38 Go to previous messageGo to next message
jeffers100 is currently offline  jeffers100   United Kingdom
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Hi Kermit, Spagiratwink, Kaguro,

Thanks for responding.

The refurb kits are aftermarket, supplied by David Silver Spares so I assume they are "proper". the diaphragms were in good order, no cracks or nicks, the electronic ignition system was installed by my local m/c dealer and I am pretty confident in them.
The float heights have been set at 23mm (with the carb upside down, floatbowl removed, measured from carb body to float) which I think means there's plenty of petrol in the bowl.

The plugs are showing quite black, which to me means they're running, if anything, a bit rich.

Its all very confusing....
Re: CB250 K4 carb settings [message #300726 is a reply to message #300725] Wed, 15 May 2019 08:03 Go to previous messageGo to next message
spagiratwink is currently offline  spagiratwink   United Kingdom
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(6) Moto1 GP Racer
/forum/index.php?t=getfile&id=22426&private=0Hi maybe pilot screws adjustment, here's a setting sheet
Re: CB250 K4 carb settings [message #300730 is a reply to message #300726] Wed, 15 May 2019 19:13 Go to previous messageGo to next message
jeffers100 is currently offline  jeffers100   United Kingdom
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(1) Club Racer

Thanks spagiratwink for that.

It quotes a float height of 21 +/- 0.5

So how is that measured, if not how I did it?

Jeffers100
Re: CB250 K4 carb settings [message #300731 is a reply to message #300730] Wed, 15 May 2019 19:35 Go to previous messageGo to next message
spagiratwink is currently offline  spagiratwink   United Kingdom
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(6) Moto1 GP Racer
Here it is take no notice of the measurement use yours:)
/forum/index.php?t=getfile&id=22427&private=0
Re: CB250 K4 carb settings [message #300734 is a reply to message #300731] Thu, 16 May 2019 08:29 Go to previous messageGo to next message
Gearbox is currently offline  Gearbox   United Kingdom
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(15) Kenny Roberts
Another thought, does the K4 have a vacuum fuel tap? Had a similar problem with a suzuki and the tap was the problem. Assume the vent in the fuel cap is clear also.

You may get other contributions from the honda sohc forum. ( www.sohc.co.uk )
Re: CB250 K4 carb settings [message #300738 is a reply to message #300734] Fri, 17 May 2019 18:43 Go to previous message
Kakugo is currently offline  Kakugo   Italy
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(18) Geoff Duke
Just a quick thought: do you ever smell fuel and/or does your exhaust tip look sooty?
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