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Topic-icon R51/3 starting issues when warm

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3 weeks 5 days ago #6382 by CortlandBoxer
R51/3 starting issues when warm was created by CortlandBoxer
Help! I rebuilt a rebuilt a R51/3 Engine, and is starts perfectly when cold, and runs great as long as it's running. If I shut it off, and kick start it immediately, it will start right up.
But, if I shut it off for 10 - 15 minutes it will not start, and won't start again until the bike has cooled. I have already replaced the coil.

Appreciate any thoughts! Thanks
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3 weeks 4 days ago - 3 weeks 4 days ago #6384 by Wobbly
Replied by Wobbly on topic R51/3 starting issues when warm
You've got to check the "Big 3": fuel, fire, compression.

A lot of these issues come down to modern fuel. The liquid is being delivered quite nicely, but the engine doesn't crank on liquid gasoline. What the engine cranks on is the "volitiles" that waft off the liquid. Your friends at the EPA have made sure to reduce these "fumes" to improve the air quality. That's really great for everyone with fuel injection, but us carb guys have to suffer.

You can't see or measure these volitiles, so this is going to be difficult. I'm not familiar with your model or carb either, so this is going to be short on specifics. These issues are usually solved by insuring the carb is 100% sealed to the head. If the carb bolts to the head, the flange may be warped. If you tighten the bolts, the problem only gets worse. If the carb uses a rubber hose, then the rubber may be old and have microscopic holes. The slide may have freeplay due to wear. Butterfly shafts may need o-rings. Your air filter may not have enough restriction. The fuel level may even need to be raised. If this is your issue, then you have a million miniscule possibilities to check.

PS. I love the photo. Congrats.

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 3 weeks 4 days ago by Wobbly.
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3 weeks 4 days ago #6385 by CortlandBoxer
Replied by CortlandBoxer on topic R51/3 starting issues when warm
Thanks Man!! It's funny, as I lay awake last night thinking about this, cause that's what I do instead of sleeping, I got to thinking that I may be flooding the cylinders, but wasn't sure what to do about it. So, you gave me several thinks to think about here.
Due to the time delay, is it possible the carbs are seeping gas which is flooding the heads? When I start it cold, I always open the petcocks and start the process from there. Hold the ticklers for 4 seconds, close the damper, cold kick it to introduce some gas into the heads, then turn on the ignition, crack open the damper and throttle, then kick start it. It always starts, first or second kick.
So, thinking further, if I shut the petcocks after use, and the bike sits for a long time, if there is gas on the heads, will it drain out so that cold starts are always providing a proper condition for volitiles?
Thanks again for your thoughts.

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3 weeks 4 days ago #6387 by CortlandBoxer
Replied by CortlandBoxer on topic R51/3 starting issues when warm
Another thought, I suppose I can test the theory of seeping by closing the petcock after shutting of the engine, wait 15 minutes, then try starting it again? If it start, then I would know part of the problem and begin the search for the reason for the seeping?

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3 weeks 3 days ago - 3 weeks 3 days ago #6389 by Wobbly
Replied by Wobbly on topic R51/3 starting issues when warm
ANYTIME you have the fuel (and its associated weight) above the carb, you have to worry about petcock seepage. Thankfully, BMW still offers the petcock packing, which I highly suggest everyone with a pre-2000 year model replace. Take your time and lap the mating metal face on a very flat surface using 600 grit emery.

• If there is seepage, the problem isn't "starting". The problem is that the fuel very easily flows down the intake, past the (open or closed) intake valve, past the piston rings, and into the engine sump oil. It dilutes the oil and can cause major engine damage. I highly suggest you turn your fuel OFF every time the engine is turned OFF.

• Your ideas are great, but I took time to look at the parts fiche for the 51/3. Those type carbs have 2 major wear areas.
1) The 2-bolt flange is always over-tightened, which warps the flange and allows an air leak. A straight edge on the flange will prove this. Warping the flange also warps the bore for the slide, so it's not enough to file the flange flat again. And you'll need to renew the mounting gasket (13 11 0 042 146) and add a fuel-proof sealer to either side. I like to use brand new split-ring lock washers. When the lock washer goes flat... STOP tightening the nut. That's all the torque that's required.

[ The whole idea of the rubber hose mounting of carb bodies stems from the inherent warping of carb bodies when attached to engines with threaded fasteners. People simply can't understand the concept that "more is NOT better". ]

2) The slide-to-bore fit is also critical. You might have ridden the slide that loose in the 60's, but with the new low-volatile fuels it no longer works that way. A tight slide fit insures all the air is sucked across the venturi and jet to pull fuel from the "pool".

3) And as with any old carb, the needle has been hammering the needle jet.... and both those parts will need replacing.


In short, best cranking is going to be achieved with 2 new carbs.

Hope this helps.

Don't hide 'em, Ride 'em !!
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Last edit: 3 weeks 3 days ago by Wobbly.

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3 weeks 9 hours ago #6404 by CortlandBoxer
Replied by CortlandBoxer on topic R51/3 starting issues when warm
Hey Wobbly,

Finally got a chance to test some of the theories.

Got the bike going, went for a long ride, got home and turned the petcock off and shut the bike off for 15 minutes.

Turned the petcock on, kicked it once, and it started right up!

Repeated that for 30 with same result,

At 45 minutes, took two kicks

At 60 minutes had to go back to cold start process, and it started right up!

Thanks man, you got me thinking about the flooding issue.

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