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Beemer54
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VBMWMO #8832
Joined: 09/07/2014
Posts: 61

I've almost finished rebuilding a 1938 R12, which was a barn find. It last ran in 1965 and is the single carb version. Over the years it had 'lost' several parts - including the Sum carb.
In doing the rebuild I bought a new carb from RRC-Tuning. It has a 35 tickover jet, a 65 intermediary and a 70 mainjet. I've checked against info the BMW Club U.K. holds on the R12 and these seem to be the standard jets for this model.

The engine usually fires first or second kick when cold and always fires first kick when hot. It runs very well at tickover and never misfires. Acceleration is good. Everything is perfect - until the throttle is about 75% open. Then the engine dies completely. If I keep the throttle at over 75% open it will not start again, but as soon as I close the throttle to 50% then it starts immediately and runs well. It would run all day without misfire at 50 -60% open throttle, but then starts to lose power and dies completely at 75% open. It feels as though it is completely starved of fuel.

I thought it might be a blocked jet but all 3 jets are clean.

I have also checked that air is coming through the cap into the tank, so there is no vaccuum preventing fuel going to the carburettor.

This one's got me stumped but there are a lot more knowledgeable guys than me on this forum. Anyone got any ideas what might be causing the problem?

Many thanks,
Geoff

jrapose
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VBMWMO #8617
Joined: 02/12/2007
Posts: 263
Timing Light

If you have checked everything you can think of, I think I would attach a timing light to the bike just to make sure the problem is carburation and not spark related..... makes it real easy to see if the spark goes away at certain speeds.

Joel

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bmwmyplace
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Australia
Joined: 02/26/2007
Posts: 168
Geoff I had a set of standard

Geoff I had a set of standard jets and had similar trouble......If I recall I had to fiddle with the last jet and drill it out a little you can buy jet or number drills or metric drills If I recall the 70 equates to 0.7mm this can be checked with a 0.7 drill . take it out to say an extra 0.05mm and see how this goes and so on ... It would be good if you could get and old jet to play with, this way you could soft solder up the hole then drill it to suit if you over drill it you can re solder it till you have it right the when your happy drill out the good jet cheers Peter

Beemer54
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VBMWMO #8832
Joined: 09/07/2014
Posts: 61
R12 problem

Thanks for the input, both of you. Tried the strobe test but couldn't get it to mis-fire at all on the stand, no matter what revs I took her to. Further road tests revealed that she only really cuts out when under load. I can be running smoothly at about 50mph on the flat and she's fine, but come to a slight hill and she cuts out completely. I suspected that this was evidence of electrics breaking down under load.

I decided that if it were a mag problem then the best thing to do would be to run it on a much longer run than the 10 miles or so testing routes I've been doing. I wanted to make the fault get much worse - hopefully causing a complete breakdown. If something's broken completely it's much easier to repair than intermittentfaults like I've been having. So I added a couple of gallons as I knew I was low on gas and rode off, asking my wife to come out and collect me when the bike finally broke.

And it did. 35 miles down the road it cut out on me completely and would not restart. Great - now I could at least see if the mag had completely died. But the spark was there, as good as ever. Wolfgang had restored the mag and it always produces a HUGE spark. So why had the bike died? No gas, that's why. She's drinking about 15 mpg!

The float was set at it's highest possible setting. I'll drop this down to the lowest setting tomorrow and see how she runs. If it's no better then I'll start ordering smaller jets.

Or is there anything else that could be causing the fuel consumption? Yes, I have always made sure the cold start knob was in its proper, running position.

Ian R11
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VBMWMO #8148
Yorkshire uk
Joined: 10/22/2011
Posts: 195
Is this a repro carb or

Is this a repro carb or original? If original I can tell you the jet hole sizes mine runs well at, .45mm, .65mm, .80mm. Mine would not run with a new 35 fitted, I had to drill it to .45mm. I get 50+ mpg.
Which part of UK are you in?

Beemer54
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VBMWMO #8832
Joined: 09/07/2014
Posts: 61
R12

This is a new repro carb supplied by Wolfgang at RRC Tuning. I emailed him for advice a couple of days ago but he's not got back to me yet.

I've just dropped the float to the lowest of its two settings but it doesn't seem to have made any difference.

I'm not a million miles away from you. I live near Lincoln.

Ian R11
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VBMWMO #8148
Yorkshire uk
Joined: 10/22/2011
Posts: 195
I don’t have any experience

I don’t have any experience with repro carbs. But I have two SUM carbs, an early one and a 1941 OZ74 (better) one. They appear identical but perform different even with the same jets swapped. Probably any reference to original carb settings are a waste of time as I guess the repro carb will also have some minor differences. Is anybody out there that has fitted one?
See what Wolfgang says, but if you think it would help you can come here with the bike and bolt my SUM on it to try. It may give an answer.
Just a thought, is the cold start plunger seating correctly?

Bigsieuk
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VBMWMO #8791
Joined: 02/27/2012
Posts: 80
Bring one with me?

Shall I bring one of my carbs up for you to try?

Bigsie

Beemer54
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VBMWMO #8832
Joined: 09/07/2014
Posts: 61
Sorted

thanks for your input everyone. The bike is now pulling smoothly throughout the range. It was a plug problem. I've used my supplier of plugs for vintage bikes for years and never had a problem. He didn't have the Bosch plugs I asked for but supplied me with a pair of NGK's which he said were the direct alternative. I fitted them to the bike without a thought.

Last night, as I seemed to be running out of solutions, I decided to check the plugs out for myself. I discovered that the NGK's he'd supplied were nothing like the correct alternative.

I fitted the correct plugs this afternoon and have just done a twenty mile run, often at full throttle, without a single misfire.

Thanks again,
Geoff

Ian R11
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VBMWMO #8148
Yorkshire uk
Joined: 10/22/2011
Posts: 195
Sounds like you have it

Sounds like you have it sorted. Mine seems ok with NGK B6HS and B7HS. Contact me if you are in the area with the bike.

Beemer54
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Joined: 09/07/2014
Posts: 61
R12

Same for you, Ian. Yorkshire's a big place - whereabouts are you? Are a member of the BMW club? It would be good if we could meet up sometime.

Ian R11
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VBMWMO #8148
Yorkshire uk
Joined: 10/22/2011
Posts: 195
Guiseley, about 7 miles north

Guiseley, about 7 miles north west of Leeds. I am a member of the club but don't attend any meetings. I should really, they meet on sundays at several nice locations throughout the month.

Try contact me through the link and will email you my contact number.

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