36MX4 engine questions.

[Brian Thompson]

Australian GRA member Geoff Lemar has been in touch with me about a very original 36MX4 he is in the process of recommisioning. He has a couple of questions that I thought I would pass on to our experts;

Hi Brian, Happy Easter to you and your family. I took the bike out for a ride today getting a pinging noise or the piston is hitting the head but only near full revs; other that it’s great. Do they have a head gasket as I can see a gap between the head and the barrel. I’m not sure but I don’t like the noise, but when you rev it up not under load you don’t get the noise !! very strange can you help??

Any ideas chaps?

Thanks!

I'll remind him to register for the forum too. It will be a great resource for our overseas members.

[riflegreen]

Brian ,

No head gasket , sounds like the ignition is slightly advanced .

There is a measurement for setting the piston height in the barrel to get correct compression by adding base gaskets , usually two gaskets sufficed .

Could be comression is slightly high and may need a third base gasket .

What the piston height measurement is , after 40 odd years , I have forgotten .

Chris

[Brian Thompson]

Thanks Chris!

I'll pass this on to Geoff, and hopefully he'll be on the forum himself soon.

[Rob]

According to the parts list, no head gasket is fitted. This is the same on the Griffon. (Looked that up before I saw Chris's post!)

The most likely cause of the ‘pinging’ is pre-ignition. A bike like this really needs to run on 50/50 avgas / premium unleaded (optimax or whatever it is called now – V Power?). It is no use expecting it to run on ordinary unleaded. The compression ratio could also be checked but if the engine is assembly correctly, this should be OK. I would still measure it though, in case some previous owner has done a bit of ‘tuning’. I would also use a fully-synthetic oil at 30:1 or if the bike is going to be raced frequently, a ‘castor R’ oil at around 20:1.

If you are worried about the piston hitting the head (squish clearance), then this can be measured by putting a piece of soft solder on top of the piston, turning the engine over slowly and then measuring the resultant thickness.

As Chris says, it is also worth checking the ignition timing is correct.

Hope this helps,

Rob

[Brian Thompson]

Thanks Rob, yer a diamond! I'll forward this on to Geoff via email, I'm sure it will be a big help.

I hope Geoff will be in touch soon to register on the forum himself, and I'll ask him to post a few pics of his 36MX4 when he does. He was really lucky to find an almost completely original machine that had done very little work, and it's lovely (and quite rare these days) to see such an unmolested example, still with all the original paint etc largly intact and with that fantastic 'used but cared for' patina.

Geoff has a really interesting collection of bikes, and all in extremely good fettle, with one or two other rare dirt bikes as well tucked away (though not Greeves!) I'll encourage him to post a few pics as I'm sure they'd be appreciated.

Thanks again Rob (and Chris too of course.)

Brian.

[geoff lemar]

36MX4 engine pinging

Thank you for all the information it has been a great help I run the bike on BP ultimate 98 octane with Castrol 474 racing oil at 25/1 I will check everything that has been mentioned and let you know the results soon. getting the gear box look at as it leaks oil as fast as you put it in and the main bearing behind the clutch has a lot of movement in it so after that is fixed I will look at the pinging problem, again thanks again to all that have helped.

[Brian Thompson]

A great pleasure Geoff, and that's part of what the forum is for of course.

The more questions like yours we can get the better, as this will help build up the information available for everyone to reference.

Thanks to Chris and Rob for sharing their knowledge and experience on this one, and the best of luck with the project Geoff.

Brian.

[Colin Sparrow]

getting the gear box look at as it leaks oil as fast as you put it in and the main bearing behind the clutch has a lot of movement in it .

Hi Geoff, welcome aboard! If you need strip-down instructions for the MX4 'box I can let you have it.

Colin

[geoff lemar]

gear box

Thanks Colin is it the same as the one in the 24MX parts list as it looks the same and I have already a copy of that if not yes I would appreciate that information regards Geoff.

[Colin Sparrow]

That's the one. C.

[Rob]

Colin - do have instructions on putting the MX4 box back together? I'm just having a bit of trouble with setting the selector when putting on the outer case.
Thanks, Rob

[Colin Sparrow]

Rob:

I wrote this up in Leading Link for (as I remember) the MX3 and MX4 boxes. The articles are called, appropriately enough, "Gearbox Strip - An Idiots Guide".

I was sorting out a couple of 'boxes and found the factory guidance decidedly unhelpful, so I put together a step-by step guide of my own. It works.

Have a look at LL86 and LL87. I think MX4 is in LL87, but I can't check at the moment. The parts list drawing is there as well.

Colin

[Colin Sparrow]

Found it! Here you go:


Four Speed "Round End" Albion Cam Barrel Gearbox - Reassembly

Locate mainshaft pinion into bearing
Place layshaft complete with two pinions into bush
Place cam barrel into bush
Place in selector forks together complete with two pinions (selector forks go onto cam
barrel, larger, nearer pinion onto layshaft)
Insert mainshaft complete with two pinions through last pinion fitted into mainshaft
pinion already installed in bearing
Rotate cam barrel back and forth to check operation of selectors
Place remaining large pinion on layshaft
Rotate cam barrel anticlockwise to stop. This will engage first gear.
Locate kick-start shaft in inner cover. Rotate anticlockwise (seen from inside) to stop
Check position of cam barrel operator assembly on inner cover. If necessary line up
previously made mark for first gear
Install inner cover and fit and tighten six bolts. Pinion on operator assembly has to
mesh with pinion on cam barrel
Replace oil thrower and lock washer. Note inner tab locates into groove on oil
thrower.
Fit nut (Left hand thread) and tighten
Bend down outer lock washer tabs
Replace cover plate to mainshaft bearing and fit and tighten two bolts
Fit control lever pivot
Fit return spring over control lever pivot and locate over stop
Fit control lever shaft, hooking picker arm onto cam barrel operator and locating inside
control spring. Fasten with domed screw
Temporarily fit footchange lever and change box up and down through the gears to
check proper operation. Indentations on operator assembly can be seen lining up with
previously made mark. (As a double check plunger assembly on front of gearbox can
be removed and the grooves in the indent shaft observed). Remove footchange lever.
Note control lever stop at bottom of case. This is eccentric and can be adjusted to
prevent cam barrel being rotated past next plunger groove. If the gearchange was
functioning properly before dismantling this should not normally be adjusted.
Also check pinions and mainshaft are rotating freely
Fit kick-start return spring
Fit outer cover and fit and tighten five screws
Insert clutch pushrod from clutch end, longest section first
Fit clutch cable
Fit footchange and kick-start levers
Fit rear drive sprocket and rear drive chain
Fit lock washer and nut and tighten. Bend down lock washer
Refit chaincase, clutch etc.
Adjust clutch
Fit cover to clutch adjuster

[geoff lemar]

gear box

Thanks Colin I see you have posted the reassemble guide for someone else I took a copy of that you can never have enough info!! On another subject I see I'm only a basic user and in my rules it states I cannot attach attachments how do I get upgraded so I can set up a photo album because in the FAQ it says just click on the album and pictures icon but it's no where to be found in my user CP and I see my HTML is also off ??? do you have any suggestions ? regards Geoff.

[Colin Sparrow]

If you click on FAQ and then on Registration it will tell you all about it.

You will need to respond to an email which will arrive in your normal email inbox after you register. Seems to be catch one or two out, but it's the only quick way of establishing bona fides.

Just to keep us on topic: would you like me to post the stripping down guide for the MX4 box?