‘Gummy’ throttle slide

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Martin K
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‘Gummy’ throttle slide

Post by Martin K »

I’ve been enjoying some leisurely and entertaining miles on my 56 Huntmaster. Stopped a few oil seeps, sorted the brakes (they now work!), and generally relaxing into the whole thing as the engines appears not to want to stop u expectedly 20miles from home. One of the chrome flutes on the tank flew off at about 50mph, but I found it in a hedge - now more strongly refitted.

Current - and so far only - real issue is a worrying sticking throttle. As there is no positive ‘shut’ mechanism, if it jams open, lots of fun can be had stopping the bike before it a) self destructs at full revs b) hits anything.

It DID shut smoothly to begin with, but now very slowly returns to idle if it was open any more than 1/8. I took it out and cleaned it all, nothing obviously sticky in there - but…

Could it be evaporated fuel residue gumming the slide? (after only 100miles or so that’d be odd). Cable is free and not trapped in any way, twistgrip is too. In fact if I shut the throttle hoping to slow down, the cable inner stays extended (because the slide is staying open) and the outer flops around - so (at worst!) the little brass fitment in the twistgrip drops out giving even more problems slowing the engine down. Clearly can’t ride it safely till I’ve sorted out a carb slider that responds to commands. Thanks…
will_curry
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Re: ‘Gummy’ throttle slide

Post by will_curry »

Monobloc carbs, particularly early ones, distort very easily if the fixings
are over-tightened and especially if a thick gasket is used between
the carb body and the cylinder head flange. This distortion will cause
the slide to stick open.

Look at the carb body from the engine side and see just how oval the
opening is.

A new slide in an old body can also stick.
Mick D
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Re: ‘Gummy’ throttle slide

Post by Mick D »

Hi Martin

I don't think gumming could be your issue if you use the bike fairly frequently as the fuel vapour circulating around the slide will dissolve any residue.

A few thoughts:

Is your slide return spring up to spec? Maybe give it a bit of a stretch to see if that improves things.

Could the choke mechanism be interfering with the slide? - try running with the mechanism removed, (remember to blank off the cable entry in the top cover with a bit of tape).

Does the slide exhibit any shiny signs of contact ? look on the inside as well as outside and make sure there's no burr where the throttle stop contacts it.

Is the needle fouling on its jet, (slightly bent)? again check if removing the needle cures the stickiness.

Regards Mick
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Re: ‘Gummy’ throttle slide

Post by Roger Gwynn »

This problem can occur as an old tank lining material slowly dissolves and enters the fuel system leaving a sticky residue on the slide. Poke around with a screwdriver to ascertain if the tank has been lined with a resin coating that is now soft on the surface. This was a plot by the government to take old vehicles off the road by introducing ethanol fuel that caused throttle slides to stick open on vehicles that had resin in the tank. If this is the case you will have to remove the old lining and replace with a new version that is impervious to ethanol. https://draganfly.co.uk/shop/87481/slos ... ml/#OI-PEC
Roger Gwynn, Membership Secretary, curator of the Machine Register and the works drawings. Director of Draganfly Motorcycles, Craven Equipment and Supreme Motorcycles mostly retired.
Martin K
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Re: ‘Gummy’ throttle slide

Post by Martin K »

Thank you gents. Distorting oval shape of the carb is a worry. Will check for that - but purposefully didn’t supper-tighten the two carb/head nuts on reassembly so hopefully haven’t done that!

Tank was never lined - just a bit rusty inside, so I did indeed invest in some Drags slosh and lined it. So all should be good there.

Bent needle - now there’s a possibility. Mick, I’ll dismantle and do as you suggest and see what’s going on.

Does anyone make a stronger return spring if needed (recognise that’ll make the throttle harder, but might be worth it!)
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Re: ‘Gummy’ throttle slide

Post by Mick D »

Hi Martin

My suggestion to try stretching the spring was based on not knowing what condition yours is in, a previous owner may have shortened it or the spring may have lost some of it's elasticity over time. If no other symptoms are evident and stretching the spring works I'd suggest fitting a new one.

Regards Mick
Martin K
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Re: ‘Gummy’ throttle slide

Post by Martin K »

Thanks Mick

Getting a little more mysterious. I’ve taken the carb off and pulled out the carb slide and needle (with a lot of sticky effort) to find the barrel, slide and main ‘throat’ from air filter through to the head all very very sticky.

Needle isn’t sticky, float not sticky, fuel tap and pipe not sticky. I’m fact nothing else sticky….

Brasso has dissolved it all so I’m back to a nice slidy slider, but where is it coming from if not the fuel? The liner inside the tank is brand new and was allowed to cure for Days before petrol went anywhere near it. Could that be just the liner ‘settling in’ or is it likely to be just not working and gradually dissolving into the fuel? Wouldn’t that make the needle, float and everything else gum up too? (Note: gum is only front to back from air filter through to the head, and on the slide and barrel).

Is it vaguely possible that heat is causing the rubber air filter hose (from 1956….) to be decomposing and that rubber residue being sucked through the carb?

Or am I just trying to avoid the obvious - the tank liner was a bad move?
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Re: ‘Gummy’ throttle slide

Post by will_curry »

If you're worried about the air cleaner rubber pop it off for a while, It shouldn't
hurt and I doubt you'll even notice.

Petrol-air mixture does go quite a long way the 'wrong' way so I wouldn't rule
out the gum being in the petrol just yet.

Worth trying perhaps running out a small portion of fuel from the carb float
bowl and letting it evaporate in a container to see if it deposits anything.
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Re: ‘Gummy’ throttle slide

Post by Mick D »

Hi Martin

From your further observations it does sound like the contamination is fuel borne. I'd agree with the suggestion to decant some into a glass jar and see if it leaves a gummy residue when it evaporates, if it does I'd put it down to the tank liner leaching into the fuel. I don't line my tanks after cleaning for this very reason and don't suffer any issues with gumming or water, (keeping the tank near to full whenever possible).

Photos of my FH carb and ex tank liner during restoration:
Crud.png
Crud.png (288.98 KiB) Viewed 372 times
Tank Sealer.png
Regards Mick
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Re: ‘Gummy’ throttle slide

Post by simon.holyfield »

I guess you used the chain instead of a collection of nuts & bolts? Good idea!
cheers

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