Carb slide, bin or sandpaper?

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Jon K
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Carb slide, bin or sandpaper?

Postby Jon K » Mon May 21, 2018 4:06 pm

One of my carb slides is badly scratched.
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Should I try sanding it with wet n dry, or do I need to buy a new (used) carb?

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Re: Carb slide, bin or sandpaper?

Postby CaptAirhead » Mon May 21, 2018 4:46 pm

Others may disagree but my first course of action would be to smooth it down with fine wet and dry then give it a try. If it runs OK then job done. If it doesn't then think about expensive options.
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Re: Carb slide, bin or sandpaper?

Postby barryh » Mon May 21, 2018 5:42 pm

I agree, as long as the slide moves freely when you are done and there isn't too much leakage past the slide it should work OK as it's the diaphragm seal that affects slide lift. I guess it must have seized with corrosion at some time in the past so the carb body may also need attention with the wet and dry. If it had been a flat top it might have been more of a problem as they don't have the upper guide tube that yours has.

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Mon May 21, 2018 6:56 pm

Thanks, I was thinking the same - what have I got to lose?

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Carb slide, bin or sandpaper?

Postby Jon K » Mon May 21, 2018 7:14 pm

There may be another problem. Is the slide piston meant to rattle in the bore? Both mine do.

I sort of doubt it There is considerable play.

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Re: Carb slide, bin or sandpaper?

Postby Rob Frankhamr » Tue May 22, 2018 10:29 am

Hi Jon,

I rather fear that your carb is worn to the point where it is beyond complete revovery. I don't even think that fitting new slides would give a complete cure as there is/will be corresponding wear in the bore in the carb body.

In my opinion, you would be better off running it 'as is', provided the slide moves freely in the bore. Taking sandpaper to it will simply remove more material and make things worse. The area below the diaphragm is at atmospheric pressure (or more accurately, inlet tract pressure) and the area around the bottom of the piston is a partial vacuum so excessive leakage of air between the two is not a good thing.

I believe the engine should run reasonably with the carb reassembled as is and I would suggest that this is the best course of action for the time being.

I have heard of people using various plating techniques to recover badly worn parts like this but it is normally done for parts that are irreplaceable and is likely to be expensive. I would suggest that you should keep an eye on the various second hand suppliers and ebay to try and source a replacement carb.

In passing (and to keep up my tradition of being a Jonah) you might like to give some thought to why this part has worn so badly. Has the bike been run without an air cleaner (or with velocity stacks). If so, then you might want to consider examining pistons and bores. If the wear on the carb slide is caused by grit in the intake air, the cylinders will probably have suffered as well... Sorry...

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Carb slide, bin or sandpaper?

Postby Jon K » Tue May 22, 2018 6:27 pm

Oh dear!

Thanks Rob. I’ve had the bike over 10 years now and certainly never run it without an air filter or with velocity stacks. I had to google velocity stacks just to know what it means.

I’ll do as you say, but maybe just give it the very lightest rub with fine grade paper and a polish with something. Same in the body. Just to reduce any drag or hang up. It doesn’t feel as bad as it looks in the photo I think.

One thing I noticed when stripping the right carb, ie the one with the most slide piston damage, was that the main jet and it’s holder were were loose - so much so that the big washer slipped about freely and it unscrewed by hand. Actually, although irrelevant, the choke assembly screws were loose too. Maybe the piston rattling/vibrating loosened them?

I can’t see me stripping the bike down to the extent necessary to examine the head bores, but I’ll check the compression. There’s no excessive smoke that I’ve noticed. If all seems OK I’ll just see how she goes, for now.

Thanks to all respondents

Jon.


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Re: Carb slide, bin or sandpaper?

Postby krankshaft » Tue May 22, 2018 6:52 pm

Hi, Jon
This may be a bit out there, but it's something you might consider before buying expensive parts
and that is anodizing your old slides. There are lots of videos on youtube etc showing home anodizing techniques
and anodizing will increase the dimensions of your slide, maybe enough to take up any play.
Just a thought.
Ride safe
Jim

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Tue May 22, 2018 6:54 pm

Cheers Jim, YouTube here I come :)


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Re: Carb slide, bin or sandpaper?

Postby barryh » Tue May 22, 2018 9:24 pm

Loose enricher cover screws are not that uncommon. If you take the cover off it's worth lapping it on some fine wet and dry to ensure the mating surface is flat.

The loose main jet is much more unusual as they generally stay put even when only moderately tightened which is all they need. Don't be tempted to over tighten them on assembly. I use only a 1/4 drive socket on a screwdriver handle.

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Re: Carb slide, bin or sandpaper?

Postby Roy Gavin » Wed May 23, 2018 12:06 am

If the main jet and holder are loose chances are the needle and needle jet are worn too.
The piston looks like it has corroded at some time, and it is the corrosion that has caused the problem - I would just clean it up with metal polish, no popint in removing any more metal.
Anodizing thickness is usually measured in microns, 2/3 being a common figure, so probably wont make a lot of difference, and IMHO Utube is best viewed as a comedy/ spoof show, I have yet to see anything that isn't straight out farce!

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Sat May 26, 2018 1:20 pm

Just an update: my carbs are all back together. Now just need to refit and balance them.

Fingers crossed :)

Thanks for all the amazing help. As ever, I couldn’t have got this far without you.

Jon.


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Re: Carb slide, bin or sandpaper?

Postby Jon K » Sat May 26, 2018 1:22 pm

Image


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Re: Carb slide, bin or sandpaper?

Postby Jon K » Mon May 28, 2018 12:35 pm

Seems I posted the mending boy too soon.

In trying to balance my carbs I discovered the threads in the carb top that hold the throttle adjusters are stripped. Both sides. Whether or not this is my fault due to previous over-tightening isn’t important right now. New carb tops at £40 a throw from Motobins then. Damnit, they’re going to look all new and horrible!

I also wasn’t happy with the vacuum gauge readings, so before purchasing the above I really must do that compression test.

Ah the joys...


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Re: Carb slide, bin or sandpaper?

Postby Rob Frankhamr » Mon May 28, 2018 1:07 pm

I wouldn't be too worried about those threads...

As long as you are willing to make sure that there is a bit of the adjuster showing below the alloy, you can carefully take a section out of a spare nut so that it willo go over the cable then thread it onto the bottom of the adjuster. They don't need to be very tight.

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Mon May 28, 2018 1:29 pm

Thanks Rob, that’s a great idea. And I’ve probably still got my old cables somewhere from which I can cannibalise the nuts. Cutting them might be fun though. I imagine they’re fairly tough. I haven’t got a grinder (might be able to borrow one, but it’d be a good excuse to buy one ) - would a jigsaw do it?


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Re: Carb slide, bin or sandpaper?

Postby Mike D » Mon May 28, 2018 1:45 pm

I also wasn’t happy with the vacuum gauge readings, so before purchasing the above I really must do that compression test.
What is it about the vacuum reading that leads you to need to do a compression check?


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Re: Carb slide, bin or sandpaper?

Postby Jon K » Mon May 28, 2018 3:04 pm

I’m not sure. The readings were in a red section marked ‘late valve timing or leak at inlet manifold but I don’t know if that’s relevant to these bikes. And on increasing the revs one side didn’t really change. The vacuum gauges I have are a bit cheap. I was better off with the homemade yardstick ones I’ve made before. Might make another.

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Re: Carb slide, bin or sandpaper?

Postby barryh » Mon May 28, 2018 4:36 pm

Although bourdon tube gauge measuring absolute pressure have their uses, for balancing carbs I'd go with a homemade manometer which is an order of magnitude more sensitive.

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Re: Carb slide, bin or sandpaper?

Postby PhilO » Mon May 28, 2018 8:43 pm

The alloy metal used to make the carbs is relatively soft and if the threads for the cables are worn the hole can be tapped up a bit with a hammer to get a grip on the cable ferrule threads. In practice they will still work if there is no grip. The last resort is a nut on the underneath which may have to be split for the cable. I have repaired snapped off threaded cable seats on carb tops with metal putty from The Range. It is still on after 3 years but for how much longer?

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Mon May 28, 2018 8:51 pm

Cheers PhilO, but I cant see how they could work with no grip. The nut would just turn freely with vibrations and unbalance the carbs every few yards, surely?

I think cutting a nut is my best option, because if I start attacking those quite thin allow bits with a hammer they might break, and then the cut nut is no longer an option.

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Re: Carb slide, bin or sandpaper?

Postby boxerman » Mon May 28, 2018 11:00 pm

Whether or not the cut nut suggestion works depends very much on how much free play you have. Sometimes it works, sometimes it doesn't but it's well worth a try.
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Re: Carb slide, bin or sandpaper?

Postby Rob Frankhamr » Tue May 29, 2018 12:02 am

Thanks Rob, that’s a great idea. And I’ve probably still got my old cables somewhere from which I can cannibalise the nuts. Cutting them might be fun though. I imagine they’re fairly tough. I haven’t got a grinder (might be able to borrow one, but it’d be a good excuse to buy one Image) - would a jigsaw do it?


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They're ordinary mild steel nuts. A junior hacksaw will do the job quite easily. It's the sort of thing you should have in your toolbox... they're quite inexpensive.

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Re: Carb slide, bin or sandpaper?

Postby Jon K » Tue May 29, 2018 7:03 am

Thanks Rob, even I’ve got a junior hacksaw :)

Do you remember coming to see me in Brentwood once?


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Re: Carb slide, bin or sandpaper?

Postby Rob Frankhamr » Tue May 29, 2018 10:40 am

Yes, I remember it well... starter motor wasn't it...?

My apologies, didn't make the connection. Hope life is treating you well. As you can see, I'm a little further away now. Afraid an evening trip to Brentwood isn't on the cards any more :smile: . Still, there's much better roads up here.

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