79 R65 - Anemic performance after 3K RPM

[bonesteel]

Hi All.

I'm still new to my 79 R65 and have been slowly working on tuning it. Have seemed to hit a bit of a wall on it and would like some ideas of what to check.

Symptoms - Starts and idles fine. Acceleration up to about 3K rpm is also fine and smooth. Carbs seem synced as the free play is the same and I connected the vacuum (Twinmax) and it looked good. When I try to accelerate beyond 3K or so there's not much response at all and it feels as if there is a load on the engine - or either fuel starvation or excess, I can't tell which. As long as I stay in a narrow power band it seems OK, but it won't give me much above 3K.

Tried - Removed the float bowls and cleaned. All seemed fine. Did not check or disassemble the rest of the carbs. Spark plugs look the right colour, per Clymer. Valves and points OK.

Have not checked timing - Is this my next thing to look at or is this a carb/mixture issue?

Anyone seen this and know what the silver bullet is?

I am a bit of a newbie, so feel free to smack me down with the obvious. TIA, Jon

[Bob_Roller]

Has the sound, of carb diaphragms, holes in them .

[mrclubike]

How many miles on it
The Needle jets do wear and very little wear can cause a over rich condition in the upper end of the throttle range

I would completely rebuild the carburetors
New floats and diaphragms needles and needle jets
Adjust the valves
Do a complete tune up
And then balance it
It is just normal maintenance on a bike from that time   

[bonesteel]

Thanks all, have a carb kit on the way. The bike has 11K miles on it, but old. I suspect the diaphrams.

JB

[Barry]

Thanks all, have a carb kit on the way. The bike has 11K miles on it, but old. I suspect the diaphrams.

I think that's probably correct.

Unless affected by corosion none of the jets should need replacing at 11K. You might need floats though if they have asorbed fuel and become heavy. If you have acess to an accurate scale new floats are 12 grams and anything up to 14 grams is perfectly serviceable with some compensation on the adjustment.

[DadsR65]

I had this, but at slightly higher revs.  After chasing fueling issues, two carb rebuilds, advance mechanism, it turned out to be the points gapping.

[mrclubike]

A scale for weighting the float is cheap

[georgesgiralt]

Your bike is a '79. So you should have the flat toped Bings.
Be cautious when trying to remove the plastic ring holding the diaphragms.
It can/will broke if too ham fisted. So search for the proper way to do it and proceed with care (mainly it involve heating the top of the piston in hot water to ease things).
BMW says you have to buy a new set of pistons with new diaphragms fitted on these engines. This is costly. So do not break them ;-)

[bonesteel]

Thank you, I have seen this mentioned before, so will be very careful. Yes, I have the flat top Bings. Will update the thread with photos and resolution once I get to them. Still waiting on my eBay delivery and it's a busy day ahead. JB

[bonesteel]

Update - the diaphragms both look good - no holes. Going to weigh the floats next to check them. Then assuming they are fine, open the front cover for the first time (after removing battery cable) and getting in to the bean can. New territory.