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16H Rebore

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Hello there,

I'm going to send my barrel to a workshop to rebore, this time the value are really off limits.

I've got an +0.40 piston NOS from Wellworthy.

But i'm not sure of the piston to wall clearance on those old engine.( specifications on maintenance Manual says : Top of skirt : .007 and Botom of skirt : .004)

Have you  any suggestions, add more clearance or not? Should i take care of anything?

I also change valves and guides to brand new ones.

When i dismantled the engine i've notice that my exhaust valve was white on the top and full of carbon inside the manifold. The spark plug is full of carbon too.

I think i'm running a little bit weak but due to a high oil compsuption it's create a high carbon deposit inside the head.

I've got an Amal 276 with brand new repair kit from Amal with their specifications for 16H WD.

 

Thanks for your answers and tips.

 

 

 

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The oil burning should be solved by the rebore,I don't think I would vary the piston clearance much from the manual. Richen up the mixture a bit otherwise you will seize that new piston. Raise the needle a notch?, fit a slide with a smaller cutaway?.

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Main jet according to handbook is 170, but my WD16H seems a bit happier with 180. So I guess minimum is 170.

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Few old bikes are ridden on the main jet for any length of time.  Most of the running is on the needle and slide. Many machines have never had the correct main jet fitted but have given good service for decades. Newcomers do tend to focus on the main jet when their time would be better spent on the progression. 

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I always tend to err on the generous side as I don't want to spend the rest of my life running in. I would probably go for a .005" clearance (but then that's just me - but also Edgar Franks in his book). The bore wear limits in the same book are .007" (ie total clearance of .012") so that should allow a fair life in service.

I agree about main jets - people tend to get hung up on these. I for one very rarely use full throttle for any length of time. I always go for a size or two up on recommendation as too rich is better than too weak.

As a side issue, I've never understood why the Amal tuning sequence starts with the main jet. I always leave that till last.

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Maybe because finding th correct mainjet gives a good starting point for the rest of the tuning of the carburettor. Robert is correct in that newcomers often don't bother with progression, probably because they are satisfied with a correct main jet and a decent tickover.

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as long as the main jet is big enough it doesn't affect anything else, which is why I leave it till last. Too big makes no difference to idling or part-throttle running, which is where the vast majority of riding is done.

I like to spend a lot of time getting that progression right as it makes the bike so much nicer to ride.

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Many of the engine components are often well past the life span that the makers  expected. Orriginal parts that fit well are becoming scarce. A short burst at full throttle is about as far as I want to go now. Just occasionally I forget myself ( an afternoon blast round the south coast resorts in company with nephew on his fireblade left the 99 sizzling and me feeling guilty). A rod letting go would destroy the motor. 

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Problem with getting progression right is the cost of trial and error when changing the slide!  On my Dommie using non standard (concentric) carb, I started with a relatively small slide cut away and filed a bit more off in stages. When I was happy  I then bought a pair of chrome plated slides to match and put the second carb on. Trial and error, especially with two carbs, could cost a lot of money.

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I had some problems sorting the 99's  monoblock.   An occasional spitback coming off tickover would normally be cured with a smaller cutaway on the slide ,but did not respond to this. The new chrome slide seemed a good fit so I started looking at the bypass drilling (on the motor side of the slide) .I thought it filled  in between pilot and  cutaway. So me being me drilled it out a fraction thinking it would supply more fuel. It went weaker! . Apparently its function changes depending on the pressure differences .So not as simple as me!. The problem  spitback is caused by wear in the jet block which a new slide does not correct.

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... the cutaway can be tricky. The "correct" one for my 1952 ES2 is a No 4 which made it spit back quite badly. A 3 1/2 made it much nicer to ride.

As said though, elderly components all at different stages of wear can make chasing that grandfather clock idle and punchy takeoff rather elusive. My ES2 seems to change its mind dependent on the weather!

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Thanks a lot for allez those answers ans tips. I' ll call back my workshop for the correct piston clearance.

I'll check my carb for a fine tuning and set it richer.

Thanks you all !

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Wellworthy were original equipment and seem to be the best machined and most consistent of 16H pistons. I would see no reason to deviate if you're lucky enough to have found one.

 

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