Carburation

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Carburation

Postby MalcW » Wed Apr 24, 2019 9:17 am

Hi all. My 650 twin has recently taken to occasionally spitting back through the carb. It's slightly worse when cold, and only does it on very slight throttle openings. Tickover is fine. I'm therefore thinking that there's either an air leak in one of the manifold joints, the pilot jet is partially blocked, or it's the air screw adjustment. Fuel supply must be adequate, or it would be worse at higher throttle openings. Likewise, a valve problem would also be worse under higher throttle loads. An ignition problem (magneto) would also manifest itself more when under load, e.g. pulling up a hill. Does this seem reasonable? Have I missed anything?

Malc

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Re: Carburation

Postby Rob Harknett » Wed Apr 24, 2019 10:46 am

Do you have an AR on the mag, that is stuck on an advanced setting? If you have a lever, perhaps you are fully advancing it too early. Did you just go round the block, and the engine not really warmed up? If the bike was running OK do the usual, clean carb, clean points, check connections not forgetting the carbon brush on the mag pick up's which must move freely.

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Re: Carburation

Postby Mick D » Wed Apr 24, 2019 4:49 pm

Hi

If you're running a Monobloc it may be worth checking the pilot by pass is clear, (item 8 on the attached), this drilling passes fuel to compliment the pilot flow during the initial opening of the throttle.

Mono X Section.png


Regards Mick
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Re: Carburation

Postby bob121 » Wed Apr 24, 2019 6:10 pm

You could also be pulling in a small amount of air past the exhaust.

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Re: Carburation

Postby ajscomboman » Wed Apr 24, 2019 6:45 pm

I wouldn't rule out too tight an inlet valve if it's spitting back on a slight opening especially if it's a new issue.

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Re: Carburation

Postby MalcW » Mon Apr 29, 2019 8:27 am

So, I finally got a chance to check everything out. When I last put the carb on I nipped the nuts up but didn't go too tight to avoid potentially warping the flange and getting throttle problems, but they seemed looser than I remembered. Still, while I was there I removed and stripped the carb. Everything looked fine but I blew it all through with a compressor just in case, then reassembled and refitted it. I went out for a 40 mile test ride. At first it seemed better, but later it started again.

Thoughts so far:
Was the tank bag (I like to carry tools, lots of tools!) blocking the fuel filler cap vent. Apparently not.
Partially blocked fuel tap filter? Turning on the reserve as well didn't seem to help, and anyway I would have thought that would be a problem at higher throttle openings.
Tight valve, as previously suggested? I don't think so, as when it falters it does so, briefly, on both cylinders.

My next step is to ride it again, but then pull in somewhere once it's hot and go back through the carb air screw/throttle stop tuning procedure. If that fails to sort it, my thoughts will be turning to the magneto.

Anything else?

Malc

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Re: Carburation

Postby SPRIDDLER » Mon Apr 29, 2019 8:52 am

There's a hint that it's o.k. when cold and at tickover but the spitting occurs when warmed up. Purely from the experience that many carb problems turn out to be ignition related and vice versa it's worthwhile spending just a few minutes cleaning the mag slip ring and maybe fitting new plugs if you have a couple to hand. Presumably you don't have suppressed plug caps or carbon HT leads which could fail.

Just idle thoughts............. :?
I poke badgers with spoons.

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Re: Carburation

Postby MalcW » Mon Apr 29, 2019 9:25 am

Hi Spriddler

I have fitted new plugs, though they are 1 grade hotter than previous. The colour seems OK though. Plug caps are resistor free and the leads are copper.

The magneto was rebuilt about a year ago, but the sparks aren't enormous, in my opinon. Good idea about the slip ring; I'll clean that, and recheck the points. If nothing else it's a good excuse for another ride. It's not a good hot starter, though a modest bump start will see it fire up, so the mag is doing something when hot, though not necessarily enough at kicking speed. On that basis, when running I would have thought it should be OK - unless it can't keep up?

Malc

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Re: Carburation

Postby SPRIDDLER » Mon Apr 29, 2019 10:17 am

MalcW wrote: It's not a good hot starter, though a modest bump start will see it fire up, .......


'Bump start'? Cuh! :roll: You youngsters! Kick-starting knackers my joints. :cry:
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Re: Carburation

Postby raffles » Mon Apr 29, 2019 6:03 pm

If you have to bump start any thing you really should be looking at ignition not carb problems
Tony

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