Stumbling R65 Ideas, please

[georgesgiralt]

Hello Lads,
As you may know I own an R65 made  April 1982 which I bought new in 1984. Since then I've done around 190 000 km. She stand unused roughly from 2000 up to 2012 when I decided to ride her again after a thorough update. I started front, then progressed to the rear. Including frame powder coating...
Last winter, she stumbled between 4000 and 5000 RPM. i tried about everything and exchanged the carbs diaphragms  in January. This seemed to solve the problem. This winter, again she was playing dumb with me.
I triple checked everything I could think of, finding nothing wrong. A local Guru told me that the tank is vented through the cap with a diaphragm keeping the vapors in. He says that this membrane becomes stiff in winter and the tank is not properly vented. So I rode the bike with the tank partially open. This vastly reduced the problem but did not solve it totally.
As when I was young I was told that "when you think it is carburetor related, it is not. It is an ignition problem. And when you think it is ignition it is a carburetor problem..." I'm ready to put back the "Crack-O-Matic" coil to see if it changes something...
So here I am seeking for a ray of light in this situation...
Help, please    :-?

[montmil]

By loosening the fuel cap, the tank no longer created a vacuum that prevented full flow of fuel. I believe that's a strong clue as to where your search should continue. With only one petcock on the R65 tanks, I'd be suspicious of a clogged or partially collapsed fuel line, a blocked mesh filter over the petcock straws, clogged petcock or even the carb's stub at the fuel line connection.

At the noted 4-5K sustained revs, reduced fuel flow would cause the engine to stumble.

You may be the first person to actually want to go back to that nasty Bosch "CrackO". For a short test maybe not so bad, but how will you know if the stumble continues? You do know the Crack-O-Matic is first cousin to Lord Lucas, The Prince of Darkness.

[georgesgiralt]

Montmil,
My gray Crack-O-Matic albeit heavily creaked did not cause ignition trouble.  
I played chicken and put a new coil in place because it looked so weird.... But I bet the old coil is still good..
Once my tank empties a bit, I'll remove the petcock and see what the problem is.

[Barry]

Can't fault Monte's logic - if increasing the fuel flow improved things then it needs increasing some more.

The restricted fuel flow must have resulted in a low fuel level in the carbs which will have been the direct cause of the weak mixture stumble. So another way of looking at it is to say the fuel level needs to be increased.  Whether you need to increase the level by improving fuel flow or whether it's just too low regardless of fuel flow is less certain.

By way of experiment you could shut off the fuel tap when the stumble occurs and quickly remove the bowls to observe what the actual fuel level is.   If it's lower than normal then there is still something restricting fuel flow. If it's no different than normal the either the normal level is too low or the fault lies else where.

[georgesgiralt]

Yes Barry, the logic is  good. But how come the bike only shows in winter ?
Last but not least, when I open the throttle fully, the bike seems to have too much fuel to swallow. If I back up the throttle a little, she seems to will accelerate ...
I hope that tomorrow I can empty the tank and have a look at the petcock.

[Barry]

Yes Barry, the logic is good. But how come the bike only shows in winter ?  

One possibility is that because the air is colder and denser in winter a carb will run slightly leaner.

Last but not least, when I open the throttle fully, the bike seems to have too much fuel to swallow. If I back up the throttle a little, she seems to will accelerate ...

This sounds like the old main jet test where you get the engine pulling hard on full throttle and then suddenly back off to 7/8 throttle.  The theory is that this creates a momentary richer mixture so that if the engine suddenly picks up when you roll the throttle off then then it was running too lean.


Seems to me that both the winter only aspect  of the stumble and the roll off main jet test point towards you having a marginally weak mixture.

[Bob_Roller]

Where do you have the needles set at .

Long before the main jets meter fuel, the needles are in control .

I operate my R65's from 19 F, -7 C, to 122 F, 50 C and never see issues like this on stock factory setting jets, needles, etc .... .

[georgesgiralt]

Bob,
Full stock.
And all new "copper" in the carbs  from top to bottom.

[montmil]

Bob,
Full stock.
And all new "copper" in the carbs  from top to bottom.

Likely brass.  You mention winter operations. Do you have a different gasoline blend sold in winter months? Not uncommon here.

[georgesgiralt]

I do not think so but it could be. Last winter was cold, this one wasn't. So the densiry difference is not as large as it was last year.

[Barry]

Almost certainly a winter blend.  It's a matter of legislation in the UK that the fuel companies have to deliver a winter blend between certain dates and it has approx 3% lower calorific value due to the more volatile fractions - which means 3 % weaker.  

There is no question that winter air temperatures and winter fuel will produce a slightly weaker mixture in an uncompensated carburettor.  That wouldn't  matter unless the the mixture was already marginally weak even in the summer.  This is my whole point that the symptoms point to a weak mixture probably due to low float level, the only thing to determine is the cause.

[georgesgiralt]

So,
Yesterday, the tank was quite empty. Spend some time removing the petcock and cleaning it. Had a look at the little mesh filter  at the base and found a very small chip of paint. Removed it.
Put a lot of air pressure in the fuel tubing after having removed the floats and the little pin. No obstruction at all and nothing comes out of the tube. Did it in reverse also just to be absolutely sure.
Today, I took the bike for a ride after having topped up the tank with Super 98/88 fuel which may contain up to 5% ethanol and added a dose of lead substitute.
Alas, on the highway, the bike still stumble between 4K~ 5K RPM.  And this whatever gear in use.
If I open the tank to cope with a possible venting problem, nothing changes except I smell gas  
I'm completely at loss here.

[georgesgiralt]

Hello !
Through this week end, I've verified all item related to carbs. From the tank and petcock to diaphragms and brass in the carbs. Still no joy.
As I had to fill the tank, I discovered that the gas consumption is huge. It is at 7.0 l/100km. A lot above what I measure normally (around 5.5 to 6 l/100km)
As requested, I've measured the compression, with an hot engine and carbs removed. the figures are : Left 11,0 bar and Right : 10.2 bars. So in the "good" range.
Next point is to do the same to the ignition system and see what it gives.
I'm unnerved a bit ....  

[montmil]

I discovered that the gas consumption is huge. It is at 7.0 l/100km. A lot above what I measure normally (around 5.5 to 6 l/100km)

Could be running a bit rich, eh? Time to dig into the carbs as something has come adrift.

[georgesgiralt]

Montmil,
Spent two days dismantling, cleaning, adjusting verifying the two carbs. Nothing wrong. Everything perfect as I did renew them less than two years ago and did not driver the bike that much since.
I'm beginning to suspect an ignition problem.