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Author Topic: Who's a 2 stroke Cylinder head Guru ?  (Read 9756 times)

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Offline micko

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Who's a 2 stroke Cylinder head Guru ?
« on: August 23, 2015, 04:52:55 pm »
Can't stop the dreaded RM370 from Pinging/slight detonating, i have been told it needs the cylinder head chamber re shaped etc. Can't seam to find anyone in Perth that really wants to tackle it, who do you recommend. And yes I need it done asap.
All suggestions appreciated.
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Offline Rossvickicampbell

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #1 on: August 23, 2015, 05:17:57 pm »
Mick - have you spoken to Ray Easson?  I know he did some chamber work on my 79 CR250 for me.
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Offline Tomas

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #2 on: August 23, 2015, 06:45:45 pm »
It is not that hard to do. I have done a head from 79 RM 125 that was fitted to an experiment 80 RM 125 engine that is running 2000 RM 125 piston and rod kit. Have the combustion chamber tig welded first and than start with sqish. I went with sqish height of 0.5mm plus thickness of head gasket(1mm) if you have/using one. Than I machined combustion chamber. Start with depth first to make sure that spark plug is not sticking out to far into combustion chamber and than shape combustion chamber so it is roughly hemispherical shape. I managed to stay close to 50% of bore diamater sqish and 50% of the bore diameter combustion chamber. Once machining was roughly done I fited the head on and checked compression. My one was prety close to stock 80 rm 125 compression ratio so the head only needed bit more machining and edge between combustion chamber and sqish band was blended in a bit( just get rid of sharp edge with a bit of sand paper. I did not want to run more compression just wanted to adapt head to newer piston that has lower dome height than the original piston. The motor runs well on 93-95 octan fuel. You need at least a lathe to do this and also may have to make a jig if your head has non concentric spark plug hole. I have used face plate with suitable holes drilled to bolt the head on. Only get to do this kind of work now and than when I am visiting my dads house that has all the machinery I need to do this kind of work. Good place to get the welding done is D and Y Services on Cullen Rd in Bayswater. Dave is nice guy and expert welder.
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #3 on: August 23, 2015, 07:40:38 pm »
Ross, Ray wasn't keen on welding it himself with his old ally welder was worried about getting it to hot and warping it but did suggest to find someone else to weld it up and he is more than capable of doing the machine work etc.
I will get the guys Tomas has recommended to do the welding and see Ray about the rest. Just thought the cylinder head fairy might just make one for me.
Times running out ???? 2 weeks away.
Thanks for your help guys.
« Last Edit: August 23, 2015, 09:59:17 pm by micko »
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Offline Lozza

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #4 on: August 23, 2015, 08:45:57 pm »
Quote from: micko on August 23, 2015, 04:52:55 pm
Can't stop the dreaded RM370 from Pinging/slight detonating, i have been told it needs the cylinder head chamber re shaped etc. Can't seam to find anyone in Perth that really wants to tackle it, who do you recommend. And yes I need it done asap.
All suggestions appreciated.

I just asked a fellow who cast up some 400 heads to fix that very issue. Short term fix is to run avgas.
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #5 on: August 23, 2015, 09:05:25 pm »
Lozza, I tried Avgas today and yes it seemed to helped and felt very strong but it still had that annoying ping up top. Tried retarding the ignition but not much different. I
Note: I run a Maico 400 piston in it and it is not high compression.
Just not enjoyable to ride unless I short shift it.
Starting to hate big bore bikes.
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Offline oldyzman

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #6 on: August 23, 2015, 10:22:07 pm »
Micko, did you make the mixture richer for the avgas? I would start at 10% larger in mikuni jet size. Seen a few DT bikes blow up from being too lean when switching over to avgas or RF100 without richening mixture. Lozza would know more than me though...
Brett
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #7 on: August 23, 2015, 11:03:33 pm »
Brett, went up one size on the main. Going by plug colour definitely not lean. Hopefully I can get this head sorted this week and work from there. Running standard ignition at the moment and is a 2 kick starter.
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Offline Lozza

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #8 on: August 24, 2015, 09:56:03 am »
Can the 400 head fit on the 370 cylinder? Welding up old heads is a bit of a lottery. If it has deto on avgas that head is a real problem
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #9 on: August 24, 2015, 11:08:49 am »
Haha, tell me about it.
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Offline Rossvickicampbell

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #10 on: August 24, 2015, 11:44:24 am »
Mick - do you need it fixed before the Nats?
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #11 on: August 24, 2015, 02:54:57 pm »
That was the plan Ross
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Offline micko

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #12 on: August 24, 2015, 03:10:36 pm »
Lozza, 78 400 may do. 79/80 different.
Seems to hard to get anyone to weld it up here, I can borrow another std head and might try THAT special ignition I have.Maybe that might do the trick.
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Offline Lozza

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #13 on: August 24, 2015, 03:46:49 pm »
I can get a 78 head
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Offline Rossvickicampbell

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Re: Who's a 2 stroke Cylinder head Guru ?
« Reply #14 on: August 24, 2015, 05:01:20 pm »
OK - sorry Mick - thought you could borrow mine if needed after the Nats but my bike committed for the event
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1974 Yamaha YZ360B
1980 Honda CR250R - Moto X Fox Replica
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