Running in the R26

Begonnen von BSA Bob, 19 Dezember 2022, 05:01:14

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My R26, named "Hildy" is now on the road after over 40 years.
I need advice for running in. I'm running it in at between 70 & 80 km/h in top & not revving it hard in the lower gears. This is on lightly trafficked roads.
Dynamic timing seems OK on medium throttle. I don't want to scream the engine on full throttle, but when I rev it fairly hard the timing seems to retard itself.
As yet there's not a real lot of compression. Can I expect the compression to increase with running in? Currently I've done about 250km.
Thanks for any opinions.



Now is the time to rev it hard. But in very short bursts! Let it rev down then for a few minutes so it can cool. 

But first make sure everything is healthy in your engine:

What piston did you use? New or NOS?  And how do you know the timing retards when revving?

What is the compression now and how did you meassure?


Morning Bob,

I assume you honed the cylinder and installed a new piston. I further assume correct piston and ring clearances.

There are different opinions on how to run in engines from following the suggested procedures in the manual to running in hard. My personal approach (after proper warm up) would be medium revs for the first few hundred kilometers and full throttle every now and then for a short while. After 500 to 600 km the time has come to make sure the engine will take full throttle at any time. To test this out you'll need a handful of jets sized around the size of the standard main jet. Unfortunately, now you'll have to test (always the same flat or inclined stretch), until you find the best main jet. Highest top speed, no screaming or stalling engine. Correct ignition timing will be the basis before you start playing with the carb settings.

Theoretically you might expect a slightly higher compression after running in. If your testing conditions cannot be reproduced, you'll not be able to measure this. Don't expect a change anymore after 250 km.

Good luck!
umuntu ngumuntu ngabantu


Don't miss the oil-service after the first 50 to 100 km and also retighten the Head bolts (torque-wrench to specs)
Then doublecheck the valve-clearance mandatory as it will decrease by retightening the bolts (the head-gasket sets).
Next oil-service and valve-clearance-check is due after ~500 and ~1000 km.
Avoid continuous revs - prefer frequent shifting and changing revs.
Always give time to cool down, after demanding power with short throttle bursts.
Dynamic timing has to be dead on, or slightly later / less advance. Say ~ 40° deg.
Best way to check is a strobe. The "F"-mark has to stop moving downwards in the check-hole if you rev up to more
than ~ 3000 rpm. It should then stay in the middle (42°) or at the bottom edge (40°) of the hole.
If the "S" (late) mark then is off during idle, it doesn't really matter. Only the "F"-mark is important !
I'd choose the main jet one or two numbers bigger for running in. Give's a bit more internal cooling
and you're more on the safe side for sacrificing 5 or 10 kph top speed.

Good luck
Think outside the box !


Zitatmain jet one or two numbers bigger for running in

That's true. Many manufacturers recommended running the engine on the rich side (2-strokes and 4-strokes). The same applies when you start testing the perfect main jet for your engine. I typically start on the rich side (less risk of damage under full throttle) and then move gradually towards the lean side. At one point you'll find a max speed or that the engine doesn't rev out freely. Recommendation: only one change of the setup at a time. Otherwise, you won't understand the impact and interpret it correctly.
umuntu ngumuntu ngabantu

Kurt in S.A.

I typically run the engine in by keeping the high revs down but varying from low revs to medium then back down to low, and try to avoid idling for long periods.  So accelerate for a bit, then close the throttle and let the engine compression pull the rev back down.  Probably not go to top gear but use the others.  Acceleration put pressure on the rings and deceleration under engine braking does it too but a little differently.  Frequent oil changes out to say 500km as well as checking the valve clearances.  Wouldn't hurt to check the head torque a couple of times as thing settle in.

Kurt in S.A.


Thanks for all these replies.
The piston is new along with the crank & supplied to the mechanic by Uli's.
I don't have spare jets to test with yet, I've raised the needle 1 notch.
The head & valves have been attended to.
At about half throttle the dynamic timing seems close. I don't want to scream the engine yet but as I increase the throttle the timing  mark begins to be in the upper part of the window.


Then there is something wrong with your centrifugal advance unit. It could be something simple like a broken limiter spring. But it could also be completely worn out. For now I would set the timing so that the F mark appears in the right spot, also at higher revs. The position of S at idle isn't nearly as important. 

Don't ride the bike with this timing as it is now.


Hi Kees. Thanks for the prompt reply.
Actually I can't ride the bike now because the spark plug insert blew clean out of the head this morning. Not happy.
Your idea about the centrifugal advance seems  right.
I think I need a few beers.



The cylinder head's been repaired, the damage around the spark plug hole welded up & a new hole created.
I fitted another auto advance unit which gives better dynamic timing & advance. The dynamic timing's set, I got lucky there. I set the static timing using the cigarette paper method to get things started & that gave correct dynamic.
The handling's excellent. The bike steers in a straight line & just loves corners. Unfortunately we've already broken a couple of light bulbs. I'm thinking of trying LEDs as I've heard they're more vibration resistant.
And yesterday we blew a fuse wired into the battery in the normal +ve position (naturally at the bottom of a steep hill). I couldn't find anything wrong & the bike started immediately with a replacement fuse.
Running the engine with the tappet covers off, oil doesn't emerge from the vent hole in the rear rocker although it comes out at either end. There seems enough oil around the mechanism. The front rocker's fine.
I've done about 600km now.


Don´t forget to use a correct spark plug with a longer tread now, because the shorter, original one will not fit any more.

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