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Old 05-22-2008, 06:30 AM   #16
kixtand
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Location: Lexington, KY
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Many thanks for the replies and insight. I am in the process of putting together a /5 race bike myself, so anything you can share is much appreciated.

What size valves and carbs are you running?

Thanks again--

kix
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Old 05-22-2008, 06:46 AM   #17
Stagehand
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Quote:
Originally Posted by pommie john
Kixstand is correct about the reason for raising the engine. Cornering clearance. I did try it with the engine in the normal position and it definately turned into corners better, but I kept hitting the rocker covers on the ground so we had to raise it.
I later got hold of some Mahle race pistons which are 16mm shorter than standard ones. You then machine 16mm of the cylinders and end up with an engine that is 32mm narrower. I never have clearance problems now, and could probably go back to the original engine position if I could be bothered.

Mahle piston
Photobucket
Sorry for the possibly stupid questions, but do you shorten the head studs, pushrods, and conrods, as well?

Does it change the angle of the pushrods?

I guess I am curious about your compression ratio

Do you run with a starter?
OR Is it a total loss electrical?
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Old 05-22-2008, 07:49 AM   #18
Alfons
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Congratulations, pommie john!

may I humbly ask which exhaust system you are running? To me that looks like Supertrapp silencers. If so, which type is that?

greetings from Germany

Alfons
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Old 05-22-2008, 05:59 PM   #19
pommie john OP
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Quote:
Originally Posted by Stagehand
Sorry for the possibly stupid questions, but do you shorten the head studs, pushrods, and conrods, as well?

Does it change the angle of the pushrods?

I guess I am curious about your compression ratio

Do you run with a starter?
OR Is it a total loss electrical?
Not stupid questions at all. We shorten the studs and pushrods but not the conrods.

Yes it does change the angle of the pushrods so the pushrod tubes have to be bent a little to make them line up with the cam follower tunnels.
The changed angle of the pushrods is a problem because they are not pushing in a straight line they seem to flex. I am thinking of getting some seibenrock pushrods to try.

Compression is not too radical at about 10.5:1
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Old 05-22-2008, 06:01 PM   #20
pommie john OP
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Quote:
Originally Posted by Alfons
Congratulations, pommie john!

may I humbly ask which exhaust system you are running? To me that looks like Supertrapp silencers. If so, which type is that?

greetings from Germany

Alfons
You're right they are Supertrapp. The downpipes were made specially for me, but the supertrapps are just the normal ones with a racing outlet. They are about 480mm long if that helps.
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Old 05-22-2008, 06:13 PM   #21
wildbillhatchbac
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awesome bike, i'd like a R75 project in the future . . . when my wallet is deeper i think
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Old 05-22-2008, 06:16 PM   #22
pommie john OP
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Quote:
Originally Posted by kixtand
Many thanks for the replies and insight. I am in the process of putting together a /5 race bike myself, so anything you can share is much appreciated.

What size valves and carbs are you running?

Thanks again--

kix

I'm glad to see another airhead is heading for the racetrack.
I'd have to say that a heck of a lot of work went into the engine.
It was prepared by Cray Engineering in the UK. They did the original work to the engine but I do the rebuilds when necessary.
The basic rule is "Use the latest bits that BMW make."
Use the latest flywheel and clutch, which means later gearbox input shaft etc.
Use the latest rockers, the ones with the plastic cushion and DON"T LIGHTEN OR POLISH them.
If you want to rev the hell out of it, enlarge the oil pick up from the sump so it doesn't cavitate.
If you run an early model crankcase ( before they modified the breather arrangement in '81 I think) they have trouble breathing. The pistons come together and go apart 8000 times per minute, so crankcase volume changes by one litre 8000 time per minute. We put on a sump extension and dropped the oil level to match ( longer pick-up tube too) which increased the crankcase volume. I also have the later reed valve breather not the rubber flappy thing.
Don't go too big on valves 44 or 45 is big enough and 38 or 40 on the exhaust.
carbs are 40mm delortos. Still got the pumps in them. Everyone tells me to take them out, but they work, so I've left them.
If I were to start again I probably wouldn't go for the shorter pistons. I think the alignment problems of the pushrods causes too much pushrod flex.

My bike was featured in Fast Classics magazine Aug/Sept issue 1996. I wrote the tech stuff and there's no BS in it. If you can find a copy it will get you headed in the right direction.
John

pommie john screwed with this post 05-22-2008 at 07:55 PM
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Old 08-15-2008, 08:16 PM   #23
pommie john OP
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The rebuild is finished.
I'm just waiting for a new set of Avons and I'll be out on the track again.

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Old 08-15-2008, 08:31 PM   #24
wirewrkr
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Since no one else has inquired, I feel compelled, what front end is that?
Robert
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Old 08-15-2008, 08:49 PM   #25
pommie john OP
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Quote:
Originally Posted by wirewrkr
Since no one else has inquired, I feel compelled, what front end is that?
Robert

Marzocchi forks with Brembo calipers running on a Honda 750/4 front wheel and discs! ( a few spacers had to made to order).
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Old 08-16-2008, 10:25 AM   #26
wirewrkr
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I recognized the calipers but couldn't place the forks.
SWEEET!
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Old 08-16-2008, 03:23 PM   #27
Bigger Al
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Quote:
Originally Posted by pommie john
The rebuild is finished.
I'm just waiting for a new set of Avons and I'll be out on the track again.


Oh Sweet Jeebus, that bike is nice!!
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Old 08-16-2008, 03:48 PM   #28
Bloodweiser
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holy shit.
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Old 08-17-2008, 02:15 AM   #29
Nick Makwell
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Looks fantastic John

When are you taking here out next - the Paperclip (Ipswich Motorway) or somewhere else?

Could you PM me or reply on this board when you are, I'd love to take a look (and hear as well)

Just sold my R1200GS, and my R65 project is coming along, I now have two K100 front ends to play with...and all the other crap.

again - looks great - what tank is that?
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Old 08-17-2008, 02:26 AM   #30
pommie john OP
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I'll be taking it to the paperclip to run it in the week after next. Then I have entered a race at Phillip Island. I haven't heard whether my entry has been accepted yet. I should hear this week.

The tank was made to measure by a guy called John Pearson in the UK. I took him the frame with the seat fitted and a sketch of what I wanted. It was cheap (150 in the mid 90s).

He made the pannier tanks they used on the Gus Kuhn racers, so he was interested in BMWs.

I'll send you a PM later.
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