Stumbling R65 Ideas, please

[montmil]

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.

How might an ignition problem cause such a huge increase in fuel usage. Just doesn't compute. Your bike is passing way too much gas.

[georgesgiralt]

Like you, I'm puzzled.
But the jetting hasn't changed and the diaphragms are in perfect shape. As the air filter (OEM) is brand new, it can't be the culprit. (the previous one was far from dead but I changed it anyway as a possible cause).
I think there is something cutting the ignition at certain RPM which cause the engine to stumble and the gas to go un-burned causing the low mileage.
Finding this intermittent is the puzzle I face.
Honestly, I've checked and adjusted EVERYTHING I can think of. And still have the problem. So ignition, having not being thoroughly checked is/can be the culprit.

[skippyc]

Is the ignition advance working properly.

[georgesgiralt]

Will tell after I check it ;-)

[marcmax]

Just doesn't compute. Your bike is passing way too much gas.

I have been accused of the same thing from time to time.  

[montmil]

Just doesn't compute. Your bike is passing way too much gas.

I have been accused of the same thing from time to time.  

Was waiting to see whom, among the usual forum suspects, would rise to this bait. [smiley=1drink.gif]

[georgesgiralt]

Hello !
This afternoon, the timing lamp was out of it's drawer and served to see that the advance was set a little less than specified *. And I saw that the bob-weights are perfectly working and the advance develops between 1500 RPM and reach the Z maximal value around 3500 RPM.
I'll go ride her in a couple of hours and see what gives.

* :  the S mark is underlined. This underline is aligned with the lower part of a starter teeth. My advance was set at the other side of the top of the teeth. So not a huge difference !

The bike runs better. When hot, she runs quite without any stumble. So definitely an improvement.
I think I'll try to substitute the new Bosch coil by either the old Crak-O-Matic or a Fiat Punto double coil I stock in case ... (0,8 Ohm primary and 7K secondary so in between the new Bosch design and the old one ).
My problem seems to have a lot of little causes (venting the tank, small obstruction on the petcock filter, bad initial advance, maybe bad air filter, poorly gaped spark plugs ....).
Friday I do not work. If the weather is fine, I'll take the bike in a trip around 200 km to see how she goes. I'm delighted in town by the setting I get using Advrider Grok Harmonizer. She is very very sweet. So it is a pity that she refuses to go fast...



Once I've cleared this problem, I'll document it into my log book in order to speed up the process next time !




Edit : added information after test.

[georgesgiralt]

Update,
This morning I took the R65 to go to the local Airhead Guru shop.
100 km total including deserted small roads and a return by the toll highway. She is a sweet bike ! On the small roads, i was able to open the throttle fully and see the cars disappear in the mirrors. Then o the highway, going back home I was able to have her go towards the red line in fifth gear so definitely and improvement ! I think I passed a traffic enforcement radar with the bike at full speed... Hope I'm mistaken.

[georgesgiralt]

This morning I had an appointment at the doctor.
I went there driving slowly because the roads where busy. But on my way back, I did a extra and rode on back roads, making 3 times more km on the return journey than on the initial....
Boy, the Harmonizer is such a fine piece of gear ! The bike is as smooth as silk and run like she was new ! I'm more than happy !
How fun it was ! I'm waiting for the mileage figure to show to see if I also get the mileage I expect .... If yes, anuthing will be PeRfEcT !  

[BimR65]

Conclusions?

[georgesgiralt]

The stumbling was caused by :
(by order of discovery)
1) grunge around the emulsion tube
2) small chip at the petcock filter
3) corrosion at the coil tower (inside)
4) incorrect static advance (a little bit to few)
and maybe a slightly clogged air filter. Add to this a possible venting problem of the tank.  *
As I synchronized the carbs at every search and modification, I was sure about it every time I ran the bike after a session.  Every step showed an improvement, and total remission was when I corrected the static advance.
Hope this helps.

* : The tank is vented trough the cap by a diaphragm which become hard with age. The local guru says it sticks when weather is cold (this could explain why I experienced this problem in winter). The bike ran better with the tank partially open but still had problems so i pursued investigation.
Then I found that the stumbling was existent even with the tank open. So I closed it completely and that did not change a bit the bike behavior...  

[BimR65]

Thanks georgesgiralt for your comprehensive conclusions. I read your comment about the Harmonizer and wondered what it was. After checking some more, I ordered one last night!
It will be good as a back up to my 2ft length of garden hose.

[georgesgiralt]

Frankly, it has nothing to do with the garden hose.
I own a lot of sync devices. One is a 2m lo,g mercury double column I got from jet engine set up and tune-up. I also own a set of dial gauges I used for 25 years. Then I got the harmonizer. And it made a huge difference.
The main difference is that you can set the difference to be really null as the device displays the true numerical value. And it is fast enough to display the difference when you open the throttle. So you can see that there are differences at various RPM !!!  (this is due to difference in compression, valve lash etc... and this induce variations in efficiency so a different vacuum produced that the one on the other cylinder). So you can adjust for the best overall discrepancy or for the perfect balance at the RPM you use the most (or use art cruise speed).
This is the first time that I see a device apt to do such things. And it make a difference.
I've used it on an oilhead, which had been set-up using abig professional tool (made by Souriau) and I showed the mechanic that there was still a difference between the two cylinders. I set the balance perfectly for idle. The engine was purring like a cat afterwards... I was happy ;-)
I have no connection with Grok except a very satisfied customer. If he made a 4 cylinder version, it would be fantastic !