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Author Topic: M91 Refurbishment  (Read 1151 times)

Hurric123

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M91 Refurbishment
« on: April 27, 2020, 12:05:51 pm »
Hi Guys, my M91 has been running well for the last few months, however it decided not to restart after I had been running it for 15 minutes.
There was no compression for the first time ever. after a number of checks I decided it must need a new piston O ring. I fitted the replacement ring on the lower groove which i remember reading here somewhere. While i was stripping the engine I decided to replace the crank needle bearings with oil filled bush's AM.0812-8.PL . These are a great fit and all that slop and rattle is gone now.
However after reassembly I cant get the engine to fire? Good spark, good compression . I have tried adjusting the ignition and the valve timing but no luck?. Can somebody on here, please let me have the ignition firing point BTDC, and also the point the exhaust valve opens .
I have put the engine aside for the moment as I am pretty frustrated with an engine that is not over complicated.

yozhek

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Re: M91 Refurbishment
« Reply #1 on: April 27, 2020, 01:57:35 pm »
Hi Hurric,

I suggest to set the ignition timing so it sparks at TDC or very slightly before.

And the valve timing i quote from Gil who posted this in an earlier thread;
"Try a valve lash of .031". That will help some. The engine will run good with the exhaust beginning to open 10-20 degrees before BDC and closing at or very slightly before TDC and firing at TDC. (fuel: Coleman or other pure fuel or white gas mixed 15:1 with Marvel Mystery Oil)"

Hope you get it running :-)

Ade


Hurric123

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Re: M91 Refurbishment
« Reply #2 on: April 30, 2020, 03:43:54 am »
Thanks for the info Ade
I have put it aside for a while too many projects but i will let you know how I get on.
One other thing that confused me is what controls the end float of the crankshaft. At the moment I can slide it across and back and the magnet can foul the ignition pick up. If I tighten the 6 screws more, that hold the casing together it stops the crank sliding but then its tight to turn?.

yozhek

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Re: M91 Refurbishment
« Reply #3 on: April 30, 2020, 05:57:04 am »
Hi Hurric,

The crank endfloat was controlled by the needle bearings. The bearings are positioned in the crankcase half prior to assy. to give the desired crank position and endfloat (the bearing cases contolling endfloat) then when the mating half of the crankcase is attached it holds everything in place.

You can do the same with your bushings, but as you say tightening the crankcase makes the crank bind then i would suspect the bores in the crankcase are either out of line slightly or just crushing the bush a little (bushes are much more sensitive to alignment etc. than needle bearings because of the closer tolerence).  Maybe put the bushes in the crankcase and tighten, then try to insert the crank into one bush at a time from the outside, if the crank fits and turns the issue is probably alignment.  Both problems can be fixed with a suitable reamer or you may be able to 'bed it in' with some moly grease and an electric drill to spin the crank.

Hope this helps and is not insuling your intellegence (no offence intended)

Ade


Hurric123

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Re: M91 Refurbishment
« Reply #4 on: May 18, 2020, 10:16:40 am »
Hi, and thanks for the previous information Ade.

I got the M91 down of the shelf yesterday, and stripped it down again. I was most concerned about the crank having lots of endfloat and binding up with the new bush's instead of the needle rollers. I centralised the crank webs in the crankcase and then moved both bush's in towards the crank web leaving a few thou each side for end float. I then used locktight between the bush outer surface and the top and bottom crankcase.
this gave the right sort of end-float to the crank although the crank was still tight to turn so I left the bolts a bit loose.
Next I double checked my ignition and exhaust timing and away the engine went . It runs pretty quite now without the clatter of the old bearings. After about 30 minutes running I was able to tighten up the crankcase halves and the crank is nice and free. So pretty happy with how she runs for now.

yozhek

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Re: M91 Refurbishment
« Reply #5 on: May 18, 2020, 10:33:11 am »
Great stuff, well done  :D

Hurric123

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Re: M91 Refurbishment
« Reply #6 on: May 27, 2020, 04:51:46 am »
Hi all
I had a nice session running the M91 in the garden yesterday. HoweverIi mustn't have tightened the screws holding on the governor weights properly. One of the weights became detached and started whipping around in an ever increasing circle of stretching springs .
The weight and screw shot off across the garden cutting my finger slightly in the process. I found the weight in the grass but no screw and the springs are now totally useless.
I do remember reading that somebody found a source of lighter spring on the forum. Does anybody have some of these spring spare or the details of how to get hold of a pair?

Oh what fun. Regards.

yozhek

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Re: M91 Refurbishment
« Reply #7 on: May 27, 2020, 05:26:04 am »
Hi Hurric,

If you are in the UK i have a pair of slightly stretched springs and a screw you would be welcome too.  They look fine and give a lower running speed but when i added a generator i needed the original running speed so these are surplus.



Trevor aka EEC1910 has got springs direct from Jin so i'm sure he could help out how to reach Jin with a request for new parts.

Hope this helps
Ade

Hurric123

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Re: M91 Refurbishment
« Reply #8 on: May 27, 2020, 02:09:52 pm »
Hi Ade,

That will help loads thank you.
I am in UK, in South Bucks , Windsor, Gerrards Cross , Slough area.

I will drop you a message and get the cost and send my full address

eec1910

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Re: M91 Refurbishment
« Reply #9 on: May 28, 2020, 03:18:09 am »
Hi Hurric,

If you pm me your address I will send you a couple of springs,they are the same size and work fine but not from Jin.

Cheers Trevor (eec1910).

Hurric123

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Re: M91 Refurbishment
« Reply #10 on: May 28, 2020, 03:11:34 pm »
Thanks Trevor have sent you a message

Brian