Intermittent throttle loss? Where to start...

[disco51]

Need some help on where to start on the diagnosing of a new problem.  Bike starts fine, runs great through town.  When I get on the interstate, specifically 5th gear, things go fine for 2-3 minutes, and then I will lose throttle / acceleration.  Will slow down and move over without any acceleration.  Engine is still running, though.  No gen or oil lights come on.  After about 10-20 seconds, downshifting to 2nd or 3rd by now, the problem seems to correct itself and then will run normal with no residual problems.  It's happened now about 3-4 times, but twice on the way to work today.  
Things I've done recently: adjusted valve clearance, changed timing, and balanced the carbs with the fluid in the hose method.  Not sure if these have anything to do with the problem.  It does have an aftermarket Boyer Brandson electrical ignition on it, even though it's a 79.
I'm guessing electrical.  I don't think it's a fuel problem, but will drain and clean the tank today as it was on my checklist of maintanence things to do on this "new to me" R65 anyways.
Where to I go from here?  Start checking grounds?
Thanks

[steve hawkins]

There are several unusual things to check whilst looking at the obvious.

Check you have no water in your tank, and therefore, float bowls.  Water will not be drawn into the carbs until sustained high speeds are in progress and the float bowl levels are lower, when it does, the engine will bog down but not necessarily stop.

Fuel cap breather blockage or partial blockage -  vacuum or partial vacuum in the tank stops the fuel from getting to carbs, takes time to come on but if it is bad can stop the bike.  But it will start again after a rest.

Crap in your filters/tank float bowl - by crap I mean anything that should not be there.

It really does sound like fuel though.  

Go for a ride in the dark and make sure your bike is not lighting up like a christmas tree when it bogs down : )

The experts will chime in for other suggestions though

[montmil]

My '81 R65 did the exact same thing. Spooky running along at 60+ mph in highway traffic, then feel the sag in power and speed reduction.

All the other suspects were eliminated. As Steve Hawkins says, "It really does sound like fuel though." And I believe he's correct.

One of the Bing carb diaphragms has a minute hole or tear. At some point, the leaky rubber can no longer hold up the slide so down she goes to a vacuum the rubber can support. Replace 'em both. Easy job. Use BMW OEM parts.

[disco51]

Thanks for the tips.
I'll start the hunt tonight.  Are those diaphragms easy to inspect before I order the part?
On an aside, I knew it was going to be a crappy day when the bike acted up and I couldn't get on post with the bike.  For the record, I know the Army requires ankle coverage boots (and a reflective vest, And gloves, AND MSF training for active duty), just wasn't my day.  Couldn't even sweet talk the guard.  

[montmil]

 Are those diaphragms easy to inspect before I order the part?

Kinda sorta. The concern is damaging the thin rubber diaphragms as you remove them and attempt to stretch 'em enough to locate the tiny hole.

Civilian or active Army, you probably knew you were not going to get a pass without the spec'd gear.

[disco51]

Monte - Active Army - at least for another 2 months.  Going to get out after 11 years.  Tired of getting deployed.  
You'd be suprised what the civilian guards will let through the gate sometimes - I've left my ID in my computer a couple of times and received the free pass / don't do this again wave on.  I took my medicine and turned home today.

[montmil]

... Active Army - at least for another 2 months. Going to get out after 11 years.  Tired of getting deployed.

Thank You for your service Delta-Five-One. That's a lot of sacrifices and not enough pay.

My youngest son has about 17 years in USArmy traces. Airborne. Five deployments, so far, to the Nasty Zones. Will spend some quality time with him this weekend at the RedBull MotoGP in Austin. He occasionally rides his R65 or Yammie crotch rocket on post... acid yellow-green vest

[disco51]

Update - no significant water or debris in the float bowls.  I emptied the tank as well.
Went on to the diaphragm issue that Monte mentioned.  It seems I have 2 different carbs.  The one that does not make sense to me is the flat top carb (as the Clymer manual doesn't help).  It appears that the previous owner did some sort of JB welb job to secure the nylon type washer to the diaphragm.  I'll attach a pic to show what I'm talking about.  Any suggestions for proceeding?
I didn't notice any small holes, so not sure whether to order the $70 replacement kit (both sides) or look for something else.
Thanks again in advance.

[montmil]

JB Weld is good for many things but I don't think this application is one of 'em.

Manifold pressure / vacuum is what lifts the carb slides. Just the added weight of the epoxy is going to play heck with the slide action and throttle. I might think that at a steady rate cruise, the additional weight imposed on that slide, plus the effect it has on the thin rubber diaphragm, may be the source of your problems.

I recently read on either this forum or Boxerworks or the Airhead List -I've slept since then- that the nylon 'rings' are NLA as separate items. Suggest you check around on several Airhead web stores or BMW dealerships to see what you might find. Again, I encourage the use of OEM Bing diaphragms. Often Bing parts are more economical through dealerships than from the Bing Mothership herownself. Go figure.

I still believe it's a diaphragm issue.

[steve hawkins]

The description is of a transient issue that comes and goes.  Surely a diaphram issue would be there all the time?

One reason that I mentioned water is that it is sucked up by the carb and passes through the engine and often disappears.  Although there is normally some still in the float bowl.  But you've check this one.

You mention having two different carbs, does that mean you have one flat-top and one later domed top carb?  I would be looking to source another matching carb down the line.  
But they obviously work  (ish).

Do the sliders in the carb move up and down smoothly?  With no stiction?  just thinking you might have one carb that is sticting a bit in 'cruise' mode.

Rev Light

[montmil]

The description is of a transient issue that comes and goes. Surely a diaphram issue would be there all the time?

Not necessarily so.

The higher up the slides are pulled, the greater stretch on the diaphragms -minimal as it is. The issue with my '81 occurred after a fairly good runs at 60+ mph.

With town riding and short work commuting, there was never an issue. The hole in one rubber was not visible until after an intense inspection of the removed part. No other alterations were done other than replacing the diaphragms so as not to wonder what the actual 'fix' was.

Add the imbalance weight of the JB'ed component plus any hidden leaks around the "mod" and you've got a valid enough reason to replace the parts.
 [smiley=2cents.gif]

[disco51]

Any chance my problem could be from loss of the screw that seals the vacuum port on one of my carbs?  It must have fallen off after I balanced them recently.  Any idea what size it is?  The bike definitely idles differently if I have the hole plugged with my finger.
I also replaced the inline Napa fuel filter and cleaned out the petcock.  Lots of junk.  Never found a tiny hole in the diaphragms, so put them back on.  We'll see if I get to the bottom of this.
Rode for 30 minutes today and couldn't get the problem to reproduce, but that was WITH the screw off on the vacuum port.
THanks.

[Bob_Roller]

Those ports should be closed off, if you can't find a screw, put a vacuum cap on it .

[Luca]

FWIW

My experience with the vacuum caps is that they quickly crack around the flare of the brass vacuum port fitting.  I use a rubber vacuum cap over a screw-plugged port as a locking device for my vacuum port screws.  Gives me a little better peace of mind than just putting a screw in by itself or with a washer.

Disco, the screws are a pretty odd size.  From the lord Snowbum himself:

BMW #13-11-1-259-869.  You do not need these screws, if you leave the interconnection vacuum hoses intact, which works just fine (plug the unused T port facing rearward, as above described). These screws are hard to find in hardware stores.   They are 3.5 x 0.6 mm, and about 5 or 6 mm long.   Try to find steel ones.

I got some at the local dealership.  They were inexpensive

[flybot]

If it only happens on the highway after a few miles, it sounds like poor fuel flow. Remove the the petcock valve and check the screen. Probably clogged up.

If both cylinders are winding down evenly, like you are letting off the gas, then its even more likely a fuel issue between the carbs and the tank. There is a min flow rate you should get from the tank. I think the test is described on Snowbums site. You disconnect the fuel line from one of the carbs and time how long it takes to drain X amont of fuel into a measuring cup. Pretty easy to check. Sorry, I dont remember the numbers tho. I would be willing to bet you have a low fuel flow from the tank.