Have been trying to sort out my ailing M20 today. Following on from Ian and Ron's advice, I removed the jet block and cleaned it out. I couldn't actually find any muck in there, but it was worth checking. I checked the small air metering hole for the pilot screw too. No problems there.

What has happened on rebuild, however, is that the slide is now a very tight fit in the carburettor (Amal type 76 or 276). I should have checked the fit of the slide without the jet block, but stupidly I didn't.

However, it looks as if the refitted jet block is interfering with the slide (jamming it). I need to test this tomorrow (jet block out). The clearance between the jet block and carb look uneven. But here's the question: can the jet block be filed or cleaned up or otherwise reduced in diameter to allow the slide to travel freely past it? Or is this a big no-no?

The carb slide needs to be forced all the way down for the bike to tickover nicely. In the lowest position, I can start the bike just by prodding the kickstarter and it ticks over nicely. But touch the throttle, and (of course) it all goes to hell.

It could be that a new carb is needed, and if so I'm tempted to go for a concentric for useability. Lastly, can a carb slide be fettled to free it up slightly? I've always thought that when they start distorting (if that's the problem), they're dead. But some new thinking might be out there somewhere.

I'm sure these questions have been asked, but I must have missed them. Any thoughts anyone?

email (option): dannydefazio@sumpmagazine.com

Hi Danny,

If you put back all the same components that were there before,
Nothing should be stuck..
(And at the correct orientation:
There is a pin and grove for the jet block,
And a rail and pin for the slide)

Sometimes, if the bolts which fasten the carb to the barrel
Are fasten too tightly, or unevenly, it could cause the carb body to deform,
Or if in any step of the process the carb body was held in a vice?...

You could go for a concentric, but it takes some changes in the jetting
To make it idle nice and slow (Usually)

I have a concentric on a modified M20 I am working on right now.
This one actually idles and runs OK.
I can check the jetting there, if you like.

Noam.

email (option): noam10@gmail.com

Hi Noam, thanks for that. I'm not worried about finding the right jetting for a concentric. Not yet, anyway. And as far as I can see, I reassembled the carb correctly.

My feeling is that the jet block simply doesn't return square into the bore. It feels like the slide can't pass it smoothly. And when I look at the jet block in situ, it doesn't look concentric. So I was wondering if the jet block itself needs a little fettling or even reducing slightly. The pin on the jet block is correctly in the groove.

I can't see any other possible obstruction. The thing is, I'm getting a little tired of the Type 76 or 276. So a concentric might be on the way. But I just don't want to give up yet, especially when it might be my own stupidity causing the problem.

I don't know what the "rail and pin" for the slide is, by the way. Can you explain?

Finally, there's a retaining component for the jet block. I don't know what it's called. It's a circular housing with a cast/forged nut at the lower end. It sits under the jet block and holds it in place. The thing is. it seems that this component, if tightened properly prevents the slide from moving freely. But if left loose, it leaks fuel. Any thoughts?

email (option): dannydefazio@sumpmagazine.com

Danny. The rail that Noam refers to, is up the right hand side of the jet block, and corresponds with the cut out all down that side of your slide. The bottom union nut should be done up dead tight.
It sounds to me as though your carb body might be distorted. Again as Noam said. " over tightening the flange bolts or whilst working on it"?

I would remove the jet block again and try the slide in the body. It should go through the body with silky ease.

I've had the same distortion problem on my M20. Even with a brand new carb. Maybe my barrel flange is not dead flat? Anyway I ran a fine flap wheel in my cordless drill, through the body for a few seconds at a time until the slide was free. Definitely not a proper engineering solution, but it works fine now. I use loctite on the nuts and only nip them up. Next time I ever have to take my barrel off, I'll check the flange. Ron

email (option): ronpier@talk21.com

Danny,
I have had this problem on a 76/276 when the brass jet block is distorted, they can distort when you remove them if they are tight in the carb body. To check this remove it again and sit it on a flat surface and put a steel rule over the top of it. Looking at it from the side through the large hole and from 90 degrees from that you will see a difference. If this is the case the jet block is bent, this would jam the slide when the mixing chamber nut ( your large nut is tightened). This nut needs to be tight and with the fibre washer under it. Check that this washer is not blocking the small hole in the brass jet block on its inside diameter. If the brass jet block is distorted it can be bent back if your lucky.

Tim W

email (option): t.j.walker@btinternet.com

Hi Ron,

Thanks for "Translating" me..

I use English every day at work, as I work for an American company,
But sometimes instead of decide if I shoud say "Groove" or "Notch"
And so on, I go for "Rail" Hahah..

Anyway Danny, the slide itself has also a groove from top to bottom
On it's side, and there is a corresponding small bulge on the carb,
Inside the slide barrel, and they should corespond.

The easy way to get it right is to notice that you insert the slide
Whith the chock slide "windows" positioned on the intake/rear side of the carb.

This photo is of a 376, but it's very similar to the 276



Good luck!
Noam.

email (option): noam10@gmail.com

Hi Danny...Did you warm the carb body before removing the jet block?...and how tight was the block in the body?...

I use a close fitting piece of wooden dowel inserted down the body of the carb to remove the jet block...Once in place I tap the protruding end of the dowel on a hard surface whilst holding the carb body....

Care must be taken not to apply excessive force when removing the jet block as the two 'uprights' of the jet block that support the top of the block can distort/collapse...This will cause a tightness of the slide when refitted as you describe...

After removal and cleaning up of the jet block (initially) it shouldn't be too tight a fit back into the body....

When you remove the block again check the fit of the slide over the block...Any damage/distortion to the 'uprights' of the jet block should be visible to the naked eye...Be aware that in this case the application of additional force to get things to go together isn't the right strategy...Ian

email (option): ian@wright52.plus.com

I had this tool lathed, in order to remove the jet block..

It is a copy of a tool from an old BSA workshop.
I don't know if it was an original Amal tool..

A project for Robb?

Noam.

email (option): noam10@gmail.com

Many thanks everyone. That's helped clarify my thoughts and answered a few questions.

I've been particularly careful not to overtighten the carb flange nuts. I made this mistake many years ago and ruined a carb body.

I didn't heat the jet block. Never occured to me. It came out with a few solid taps and went back in the same way. But I'm sure that distortion is the problem. It feels like only very minor binding. But that, of course, is all it takes.

I'll strip it down shortly and will have another look.

Thanks again for all the tips.

email (option): dannydefazio@sumpmagazine.com

Danny, the mixing chamber doesn't need to be that hot to release its grip on the jet block. I stand them in an old saucepan and pour a kettleful of boiling water over them.

Rik (Not a trader, never has been, never will be, copmpulsive tinkerer and bungling amateur)...but most enthusiastic.

(Not a trader, never has been, never will be, compulsive tinkerer and bungling amateur)....

'Bulk orders taken for rivets...all sizes...'... ...Ian

email (option): ian@wright52.plus.com

That's a nice tool Noam. I've never seen one. I guess as long as those weak threads on the top of the carb are good, then it will work a treat!
I use a hammer shaft as a drift after warming the carb with boiling water or blow lamp.

Ron

email (option): ronpier@talk21.com

Thanks, Rik. Have now identified the 1st problem as a slightly wonky jet block. But it looks like I\'ve managed to realign it (gentle tapping on one side).

However, the slide has a tight spot somewhere near the venturi. Might yet be distortion in the carb body. Also looks like I\'ve picked up some grit that\'s scored the slide bore. Nothing major, but not ideal. I suspect that it was me that knocked the jet block askew by not warming the carb body and applying too much force on one side.

Stupid. But won\'t do that again.

Am now trying to get the slide moving smoothly. Thanks again all.

email (option): dannydefazio@sumpmagazine.com

Hi Danny,

Sorry if it is a "stupid" question.. (stupid from my side, I mean.. )
But did you take out the idle adjustment screw off of the carb body
Before forcing the jet block out?...

If not, that could be the reason for the distortions..

Noam.

email (option): noam10@gmail.com

Hi Ron,

just noticed your reply now..

Its a great tool to use.
I will try to take photo of the original one day and post it.

I think that there is a way to improvise a tool like that:
As the most important and tricky part is the top ring with the thread,
May be it can even be made from a bellmouth,
As it has the same thread of 1 1/4x24.
If you take a top cover securing ring or a bellmouth, and close the outer side,
Then a long bolt can go though it, with a nut from the inside of that cover,
As a stopper, then you can put a coin on the jet block, in order to protect it
And distribute the force, and you have made a tool from scratch !

Reading it, it does sounds complicated, ha?...

Well..
Just a thought..

email (option): noam10@gmail.com

Hi Noam, I didn't remove the throttle screw, but I backed it right off for clearance. Thanks for suggesting that. I can't see any damage on the jet block. And my intial problem was happening before the jet block was removed.

In other words, the bike wasn't running correctly.
I suspected a blockage in the jet block.
I removed the block and found no blockage.
Then I discovered the tightness of the slide on reassembly.
That's identified a little distortion on the jet block.
That's now sorted (I think).
But the slide is still binding. Hence possible distortion of the body.
Now I just have to solve that.

Appreciate your persistence in trying to help here.

email (option): dannydefazio@sumpmagazine.com

Thanks Noam. I fully understand your description and suggestions. A quick tool can be made from an old carb top. Again I think some heat added to the body, then the tool would give a more even and steady pressure to the block. Ron

email (option): ronpier@talk21.com

Noam.

email (option): noam10@gmail.com

Noam if you running a monobloc carb be careful when tickling it as the petrol will run down into the magneto my magneto packed up due to this problem

email (option): roger.beck@node6.com

A point worth noting for those who have leaky taps or like to depress the tickler until the petrol is flowing copiously...

I've had a number of mags in for overhaul that have been ruined by the ingress of petrol...Softened shellac on older armatures, petrol soaked more modern armatures and failed drive end bearings with all the lubricant washed off....Not just with Monobloc carbs either!....

A number of years back when I used to persist with leaky brass taps for their appearance I even had my own fail....Ian

email (option): ian@wright52.plus.com

If anyone's still interested in this thread, I sorted out my M20 today and took it for a ride. Perfect. For now.

I did what I ought to have done in the first place, and that's to go back to first principles.

This time I carefully stripped the carb, examined it under a big lens and (using a marker pen) identified a couple of high spots. Using some ultra fine wet and dry, I cleaned up the high spots (mostly just removing crud/residue from the carb than removing much aluminium). At worst, I might have taken off a thou or two.

I cleaned up a couple of very minor burrs from the jet block and checked it all stage by stage.

Anyway, it all went back easily enough. And quickly. The carb snaps shut like a crocodile. The bike starts even as I'm walking towards it and frowning. Ticks over like a gieger counter.

Thanks for the advice everyone.

email (option): dannydefazio@sumpmagazine.com

Good news Danny...Now you can get back to enjoying it rather than cursing it...

Odd that we don't do what we know we should...and when we do everything works out OK!...I guess we all hope for an easier solution ....

I just keep telling myself that each time I work through a problem I've learned a little more....Ian

email (option): ian@wright52.plus.com

Hi Ian. That's true. We look for a simple solution - which often ends up complicating things.

The other problem here is bloody amateurs like me. I don't do enough mechanical work to "get in the groove". And between bouts/spasms of maintenance I forget everything I thought I knew.

Were I working on bikes on a daily basis, perhaps as a professional, I'd be constantly on the ball. But I try to avoid spanners and stuff.

And if I leave it long enough, not only do I forget how to maintain the bike, I forget that it meeds maintaining. Until it breaks down.

Like I said, bloody amateurs...

email (option): dannydefazio@sumpmagazine.com

Not to worry Danny...I don't think many of us are pukka qualified motorcycle mechanics...

I'm fortunate enough to have an Engineering Apprenticeship, 27 years in that trade and a number of years doing it for a living as a back up....I guess I should have had enough practice by now... ...

Oh..and I have eaten, slept and drank bikes for 44 years without a break...

At the end of the day though, we are all just enthusiastic hobbyists really...

Even after all this time there's still plenty I don't know and plenty for me to learn...and I have a virtually endless supply of 'projects' in my head......

I've also spent decades trying to build the bike that I really believe is the best I can produce..haven't got that yet either...

I think if it wasn't like that I would have lost interest by now....Ian

email (option): ian@wright52.plus.com

Hi, just a another tip to get a jetblock save out.

I was trying to get the jetblock of a M21 carburettor out without distoring it, and remembered the nice tool from Noam in the above thread.

But this tool puts its force still on the top of the block with the risk of deforming the top. It may work fine for the stronger choke blocks but the M21 block has a choke of 1 1/16 which leaves very little material on the sides.

So I made this which relieves the top end and worked fine in combination with a little heat.



Also take care not the heat the body too much as the jetblock a made of 2 pieces soldered with tin.

Cheers, Michiel

email (option): m.wijbenga@hotmail.com