Sticky throttle too

[spyke]

Hi chaps

Have read the other current post and my situatuion is a bit different.

Carb is 376 mono which has been bored and slide sleeved in brass.
New cable oiled and no tight bend also some free movement inner to outer.
Paper gaskets between all faces new rubber o-ring.
Tried  3 different strength throttle springs, tried with and without air filter
You can hear the slide snap down on its seat when throttle is released
Nuts are no over tightened, flanges true.

The throttle never sticks open fully or even half fully but just hangs before revs drop to tickover
This sometimes does or doesnt happen never when cold sometimes when hot'
Stripped the carb only thing I can see is the groove that the clip goes in in the needle is worn so some vertical movement between clip and needle.(Im going to replace these this weekend)
I know it seems a simple thing but i have tried all i can think of and its the only thing that spoils the ride at the moment .
Any more Ideas?
Spyke

[t20racerman]

Can you see wear high spots on the slide? If it is sticking, there will be one or two areas that are clearly 'polished' more than other areas. If so, a light dressing here will probably solve the problem. This did the job on mine recently. :-)

[spyke]

The carb has only just been refurbed though

[spyke]

You were dead right Racerman a close inspection of the brass on the slide showed a very small (2mmx2mm ) shiney high spot dressed it down and hey presto.Question tho how do they get there and will they occur again ?
Cheers Spyke

[t20racerman]

Glad to hear that it is sorted now. 

It happens because carbs are made from very delicate alloy that is SO easy to distort - usually by doing them up too tight. By ''too tight'' I mean only just nipped up. If I tighten mine to what I would call just about tight enough, the throttle sticks..... I now use thread seal on the nuts to stop them coming loose having just given them enough of a nip to hold the carb on.
Your refurbished one should be fine now, but if you do find it happening again, try slackening the mounting bolts a little.

[Davmacb]

Have just joined the forum and have the same problem with a brand new monobloc on my A7SS.  Can only get the slide to close if I tighten up the manifold nuts just over finger tight, which obviously means that tightning up the nuts warps the carb body causing the slide to stick!  The solution seems to be fit the carb with good gaskets and sealant and fit with self locking nuts only just tight enough to stop the body warping.  Why does a brand new monobloc have this problem? 

[trevinoz]

I would make sure both mating surfaces are dead flat.
Presumably the carby is OK but is the inlet manifold?

  Trev.

[Davmacb]

Yes, I have checked the manifold and it is O.K.  The strange thing is that the previous carb was tightened up sufficiently without any such problems.  Perhaps with the previous carb having a worn slide it allowed some leeway.  Thanks.

[spyke]

Its back again! Exactly the same as before throttle just fails to shut down momentarally so i pulled apart and lo and behold another spot where its just "picking  up" . 5 mins with some wet n dry and its ok again the trouble being that as that is the 5th time its happened since the carb was resleeved and every time i take a bit off im heading back to where i started ie.a slack slide.How can it keep requiring more and more off once its working freely?
Spyke

[MG]

I am just wildly speculating now, and probably I'm going to open a BIIIIG can of worms, but:

Ethanol is known to be corrosive to aluminium, magnesium and also zinc (carb body!). The presence of brass will create a galvanic cell, making the effect on the zinc even worse (brass is less active than zinc). So maybe you are facing the formation of zinc oxide on the surface of the slide bore, with the according increase in volume?

okay, keep 'em bricks coming now *duckandhide*

[t20racerman]

I am just wildly speculating now, and probably I'm going to open a BIIIIG can of worms, but:

Ethanol is known to be corrosive to aluminium, magnesium and also zinc (carb body!). The presence of brass will create a galvanic cell, making the effect on the zinc even worse (brass is less active than zinc). So maybe you are facing the formation of zinc oxide on the surface of the slide bore, with the according increase in volume?

okay, keep 'em bricks coming now *duckandhide*

You mean petrol isn't what it was? Why didn't you mention this before? 

[MG]

yeah, and guess what, they also found out that smoking is dangerous to your health 

[Rocket Gold Star Bill]

I am going to try the self locking nyloc nuts good Idea, plus the RTV high temp silicone on edge of gasket.  Here is one for you guys, I used to get a huge assortment of fibre washers from Lowes Home Improvement Ware house. I say used to, these are not the same, they are too hard on their surface, so when you get the jet block tight enough so it wont leak fuel, ( AMAL 389 CARB) it has distorted the slide area enough to stick the slide. Do not use these washers.  Another Amal carb problem, (276) some one had wrong carb on my bike RGS when I bought it, a 276 model carb > well the flange thickness on the 276 compared to the 389 1 & 5/32nds carb is humongous, plus the 389 has buttress re-inforcing so flange will not bow when installed, just a little past finger tight, but now I am going to use nylock nuts if I can find right thread, and RTV high heat silicone  lightly around the soft Insulators I use, do not like the hard phenolic ones, just a personal preference, not sure the phenolic ones transfer more heat to fuel charge, but I guess they would by a few degrees any how ?? Input on this would be very welcomed, no one discusses these Insulators  ?

[chaterlea25]

Hi Bill,
Can you explain ""soft insulators""?
If you use a thick soft gasket between the carb and head it will add to distortion issues
I find the best insulator  material to use is "tufnol"
it is a hard material, a paper gasket on the head side or a smear of Dirko just the carb O ring on the other
No distortion issues 

HTH
John O R

[cyclobutch]

Obviously given the environment that it operates in lubing the slide is a waste of time. But ... I've heard some folks like to add a dash of 'R' to their combustible of choice to give it that ah Bisto moment, but maybe this is the way forwards for healthily sliding slides.

I guess if you pursue that out towards two stroke ratios it's going to get a bit smoky back there is all. Would any of the contributers here like to give this a go and report back? 

[Gavin]

Have appreciated reading the posts... While I have a 276 carby it was the following comment that has helped me greatly:

It happens because carbs are made from very delicate alloy that is SO easy to distort - usually by doing them up too tight. By ''too tight'' I mean only just nipped up. If I tighten mine to what I would call just about tight enough, the throttle sticks..... I now use thread seal on the nuts to stop them coming loose having just given them enough of a nip to hold the carb on.

so thanks t20racerman... the 51 A10 is really purring now with a new carby, and so responsive it's a delight to ride.

[Triton Thrasher]

The new slide will have a closer tolerance than the original. Possibly too close.

You did well to catch the worn needle before it broke.

[Stephen Foster]

Excuse the Gonzo qyestion "chaterlea25" but what is Dirko please ?

Steve...

[t20racerman]

Have appreciated reading the posts... While I have a 276 carby it was the following comment that has helped me greatly:

It happens because carbs are made from very delicate alloy that is SO easy to distort - usually by doing them up too tight. By ''too tight'' I mean only just nipped up. If I tighten mine to what I would call just about tight enough, the throttle sticks..... I now use thread seal on the nuts to stop them coming loose having just given them enough of a nip to hold the carb on.

so thanks t20racerman... the 51 A10 is really purring now with a new carby, and so responsive it's a delight to ride.

Glad to have been of assistance. :-)

I stand by what I said earlier, but I must add that my concentric carb slide keeps getting stiff too. I've done 3000 miles on my A10 since finishing the rebuild in the summer, yet still the throttle regularly goes stiff half way up at some mysterious tight spot that is impossible to find if you strip the carb. Most annoying, and I do wonder if the earlier suggestion about modern fuels is partly to blame? My carb is second hand and hardly tight, so no reason for it as far as I can see. I've found that a squirt of WD40 straight on to the slide 'cures' the problem for a decent amount of time.

[andy2565]

just spring washers under the nut is sufficient to stop the carb nipping up,which i have had experience of at full throttle on my triumph metisse,of which i had to part company with,going down hill at a scramble,over tightening will distort the body,brass slides always seem to have tight spots when you first fit them,be careful putting R in your fuel,it may smell nice but it can also sink straight to the bottom of the tank,and block the tap,its a bit difficult to pick ur bike up and shake it to mix it up,that was an old wifes tale,putting R into the fuel to lubricate the slide,of which i think comes from blokes going to the speedway and wanting their bikes to smell the same as the speedway bikes.