Just a quickie. Yesterday I finished building an MC21 300 motor for a friend, but found a rather naughty problem when I came to leakdown testing it.

Filled it up with 10psi and as quick as I could pump it in, it was squealing out. The good old soapy water revealed that it was leaking really badly between the carb insulator and the reed valves. The reed gasket was new, as were the valves, so I knew they were ok, but when I pulled off the insulator and stuck a straight edge across it, it was bent like a banana. Tossed that one in the bin, grabbed another which showed to be reasonable straight with the rule, but that too leaked. Then tried another and that leaked.

Two main problems. First is that over time the insulators have been fitted by someone who didn't use a torque wrench, and they've over tightened it, which bent not only the insulator but also the reed valves. Secondly. this insulator doesn't normally use a gasket as there's a seal built into it, but closer inspection showed that the seal was actually below the clamping face so was basically useless.

I eventually found a good insulator, which held the 10psi for a solid 10 minutes. Job done.

As for the torque setting on the insulator, normal torque would be 12N.m for an M6 bolt, but I used just 7N.m to seal it. This makes sure that the gasket does it's job without overstressing the part. On the leaky insulator I started at 6N.m and worked up to 12N.m and it made absolutely no difference. In fact, over 10N.m it got worse as it started bending even more.

Well worth a check, but don't get too paranoid about it. If in doubt a smear of gas resistant sealant will sort it out if it's not too bad. Messy and I can't stand using that goo where you shouldn't need it, but sometimes necessary to save your engine, your cash and your arse if it should sieze.

Hey Matt,

I run into this on my RS every rebuild.

Here is how I manage it.

1. I use a VERY light layer of assembly grease to help produce a better seal.

2. I torque the bolts down to to below the toque settings, and then get the leak down on it letting it leak. I slowely by hand tighten the bolts in a star pattern until the seal is good.

Thanks
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Charles Gallant

Never had this problem on the RS125 or 250, and it's the worse I've ever encountered on the NSR.

Pretty much everything is put back together with grease or oil or whatever is best for the job, but I just thought it'd be worth mentioning as there maybe a siezed motor or two out there that could have been saved.

Yup, wise words.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

That's something that you normally just presume to be good so thanks! Any leak there and I would expect to find a slice or tear in the rubbers...
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NSR300R - Why did i ever have a 250...

For the Dumb Asess amongst us AKA RC46 is that the rubber looking bit between the carbs and the engine, should i mention this when i get the bike set up (new reeds and stuffers)? or will most good tunners do a leak test and check?

Matt@TYGA wrote:Never had this problem on the RS125 or 250, and it's the worse I've ever encountered on the NSR.

Pretty much everything is put back together with grease or oil or whatever is best for the job, but I just thought it'd be worth mentioning as there maybe a siezed motor or two out there that could have been saved.

Matt,

Can't agree with you on this any more. An air leak on the intake can definitly cause a top end sieze.

The hard part of this is I have seen this happen with brand new stuffers on the RS This is when I started setting the torque in them with a leak down tester already on them. I have even seen people use a sealant on these parts to make sure it's all good.

Also something important to remember when tightening this part down is the pattern in which you do the bolts. I think there is a diagram in the manual (possible even on this site) that has the pattern. Following this pattern, and not overtightening them down will help not bend the insulator.

Thanks
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Charles Gallant

rc46 wrote:For the Dumb Asess amongst us AKA RC46 is that the rubber looking bit between the carbs and the engine, should i mention this when i get the bike set up (new reeds and stuffers)? or will most good tunners do a leak test and check?

Make 100% sure the person does a leak down on the bike, dont just assume.

Thanks
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Charles Gallant

cgallant wrote:

rc46 wrote:For the Dumb Asess amongst us AKA RC46 is that the rubber looking bit between the carbs and the engine, should i mention this when i get the bike set up (new reeds and stuffers)? or will most good tunners do a leak test and check?

Make 100% sure the person does a leak down on the bike, dont just assume.

Thanks

I will definatly ask the question im sure if Nigel has recomended it, they should be a good croud, (cool to hear a 2 stroke in the back round when i called them again)

Charles, I wasn't asking for agreement or otherwise, and I can't recall saying that an airleak will not cause a sieze. Quite the opposite in fact. What I was saying is that it's possible that there's been motors built that weren't leakdown tested, and a dodgy (leaking) insulator was fitted which then caused a siezure further down the road. So we're both saying the same thing.

As for the RS's. Probably built over 100 when I worked for Harc-Pro. Never had a problem. Every engine was leak tested and almost all dyno'd.

After I read the discussion in this thread, I checked my reeds cages and the insulators.

Matt@TYGA wrote:
[...] The insulator doesn't normally use a gasket as there's a seal built into it, but closer inspection showed that the seal was actually below the clamping face so was basically useless.

I checked whether the seal on my insulator is also below the clamping face and yes, it was identical to Matt's (Arrows point at seal's lip):



On the other hand this seal must have worked in the past because one can clearly see an "imprint" on the reed cages:



Maybe the rubber on the insulator shrinks over time and creates leaks on ALL of our NSRs irrespective of how well they were assembled? (correct torque etc.)

Opposed to Matt I think the main issue is the seal that no longer seals. The insulator, the reed cages and the crank case mating surface are pretty solid and I cannot believe that if the bolts are tightened to 12 Nm, anything would warp irreversibly. But that is just my opinion.

Btw, other than suggested in the thread, there is NO tightening sequence in the manual; but just like with multi-cylinder cylinder heads I would start on the four inner bolts and tighten the bolts in a cross pattern.


I will probably follow Matt's advice and use some Three Bond 1107B (same as for crank case) to seal it once and forever (replacing isn't an option because it is no longer available).

I've recently bought an insulator so this warning for me is timely.

I know we call it rubber but isn't it neoprene, if so heating up in hot water should bring it back to its original shape and worth a try rather than binning it.

Cheers.

Thanks for sharing the info Matt. Top man.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

I had exactly the same thing on a recent rebuild, the air was leaking out as fast as I could pump it in! I reassembled mine with a bit of Honda three bond and it was fine.

As mentioned highlights why its important to torque load stuff and carry out a leakdown test after a rebuild. I cobbled together my test kit for less than £15.

paul ghello, you have grabbed my attention with your leak tester for less than £15, can you give some details as I don't have one and being retired the least expensive the better.

Getting a seal round the rubbers, a friend of mine with a 4 stroke uses that silicone sealant on the exhaust system between the down pipe and silencer, it seems to do the job and takes the heat.

Dux.