7 replies [Last post]
jambo's picture
VBMWMO #9208
Long Island
Joined: 11/23/2016
Posts: 72

Last week I took my R50/2 out and found that it had all of a sudden developed a high speed miss/backfire. The motor sputters a bit when it happens and I got a backfire out of it a couple times. Not a bad backfire, a small one.

I can make it do it in 1st gear, but only if a have a lot of throttle on it and the RPMs are fairly high. In second it does it if I have a lot of throttle and the RPMs are high and in third it happens when I pull it, says up a hill, with 1/2 or more throttle on it.

I parked it and I am wondering where to start. The bike was fully gone through and tuned up 230 miles (3 months) ago.

Some parameters:
1) The gas is 100% fresh 110 octane leaded Sunoco racing gas. I like the lead for the valve seats which is why I use this.
2) New coil from BenchMarkWorks 60 miles (3 weeks) ago.
3) New plug wires, spark plug caps, rebuilt carbs, rubber, cables, fuel hose, filters, etc etc 230 miles (3 months) ago.
4) Plugs new 230 miles ago. Looks a bit lean but the race gas tends to make them look like that.

Any help in where to start would be greatly appreciated!!!



  • r50plugs.jpg

James W.
1966 R50/2, 1963 R27 and 2018 R1200GS

schrader7032's picture
VBMWMO #7032
San Antonio, TX
Joined: 10/27/2006
Posts: 6540
James - Wow, 110 octane for

James -

Wow, 110 octane for that low compression bike! IMO that's not necessary. My understanding is that high octane leaded fuel has a built-in tendency to not ignite...clearly the lead additives were used to prevent pinging and detonation, so the tendency to ignite was suppressed somewhat. So, right there, that octane could be hurting you. Not to mention lead deposits...you're basically just throwing money out the tailpipe.

I don't see a condenser I your list. Did you replace that? What is the history on carb rebuild and synching? Could be the lean condition your seeing on the plugs.

Honestly, before doing anything else, I'd start by running a much lower octane and see if the problem goes away. Easy to do and no fiddling with parts!!


Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

bm68andreas's picture
Joined: 01/01/2015
Posts: 15
I think you have to check

I think you have to check your ignition key if it's sits properly in high RPM.........



312Icarus's picture
Joined: 07/15/2016
Posts: 332
I might lean toward a bit of

I might lean toward a bit of grit in the high speed jet(s), or a sticking float. Alternatively an ignition problem, like a bad plug wire or connection. I found mine would miss at mid throttle due to over rich, mis adjusted idle needles.


stagewex's picture
VBMWMO #8810
Joined: 10/26/2014
Posts: 464
Try the simplest first...

Try the simplest first... start using the lowest octane fuel and see if it does away after 2 tankfuls.
I had similar issues when I fist bought my bike and put 93-94 octane in it. Ran hotter, milage was not that great, minor backfiring open deceleration. My r60/2 is very similar to your r50. You certainly don't need Racing gas.
110 octane, that is very cool you can still get that. But not for an r50.


mike wex/stagewex
1969 BMW r60/2, US Model, 1995 BMW K75, 2006 Yamaha TW200, 2007 Ural Patrol

jambo's picture
VBMWMO #9208
Long Island
Joined: 11/23/2016
Posts: 72
Problem fixed for high speed miss


I figured i would update all of you as to the outcome of my high speed miss. It is fixed now and the R50 runs perfect!!!!

Before I removed the carbs, I changed the condenser at the urging of Richard at Benchmark and it did not fix the problem. From all I read Richard was correct in suggesting this but in my case it just was not the problem.

1) Took the carbs off and I noticed that the carb flange bolts were not very tight. Not loose, but not tight either. It could be that air was being sucked in between the gasket and the head. Not sure if this was the problem.

2) Disassembled the carbs and it looked like one of the main jets was blocked. Blew it out with carb cleaner and put it all back in. Everything else liked fine, but I did do a full cleaning on the carb and all the passageways.

3) Put new plugs in, although I do NOT think it needed it. Does not hurt....

I did not change gas, still running 110 Octane race gas.

The bike runs GREAT!!!!! Now I need to re-tune the carb again!

The timing was fine before I started, but since I changed the condenser Richard thinks I should check it again which I will do when I have more time.

Just wanted to document the fix in case it helps someone else!!!




James W.
1966 R50/2, 1963 R27 and 2018 R1200GS

Daves79x's picture
VBMWMO #9030
Joined: 08/09/2015
Posts: 304
I Think

I think the key was new plugs. Even though you installed new plugs recently, you can get a bad one. Other stuff didn't hurt to check though. Glad it's working now!




Danedg's picture
Joined: 12/25/2008
Posts: 42
I can heartily recommend

I can heartily recommend pulling the magneto off the timing case cover and ensuring that all mating surfaces in and on the mag are making perfect ground. Without it, your plugs, coil, wires, condenser are all working overtime (misfiring) and the bike won't start very well.
Where the heck do you even find Racing Gas?


'54 R51/3, '65 R60/2, '69 R60/US, '95 Mystic, 74 MG Eldorado

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