Help with diagnosis - Engine stutter & backfire

[gregjung]

Hi, My R65 is acting funny and I'm hoping for a little help in diagnosing. :-? She starts and idles fine but when I put it in gear and put on the gas the engine starts to stutter and backfire a bit.   Once I get to speed it runs ok for the most part but every now and then, especially if I throttle up, it will stutter again. Feels almost like the engine is shutting down and then starting right back up again. Any ideas? Thanks in advance

[tvrla]

When giving symptoms - please give a time line and whether anything had been done just before the problem.

Did this start suddenly? How long has it been going on - miles, days?

Or did it come on gradually.

How long since the last tune up?

What year bike is this? Makes a difference since the early ones had points.

From the symptoms my first guess is water in the gas. Lower the float bowls and look for what appears to be a bubble at the bottom under the gas. If that's the case, be sure to drain the tank and dry it out completely!

[gregjung]

I ride an 82 R65, fifty mile a day daily commute. Last tune up about 3 months and 3000 miles ago. Was acting up a few weeks ago and then my started went so had that and the battery replaced (it was time) then it ran good for about a week before the stuttering started again.

[tvrla]

It had this exact same problem a few weeks ago, then quit with a new battery and starter, then started up again...

Very odd.

In that case, it doesn't sound like water in the gas.

Maybe some sort of electrical fault - something shorting against the tank? When working on the bike and the tank off, the wiring got moved enough it took a while to vibrate back into position?

I think you're going to have to take a close look at everything - poke around and play detective. Sounds more like an intermittent problem at this time, and intermittent electrical faults are THE worst!

[Bob_Roller]

If you can get around to it, check the rubber diaphragms under the round top part of the carbs, they get hardened with age and crack .

they will give you problems during acceleration, but act fibne under constant throttle .

[montmil]

Did you check the float bowls? Also confirm fuel flowing consistently by gently moving the carb floats up and down. Might be debris in the fuel lines. Easy to check them. Old floats can sink and mess up fuel delivery. backfire, you say.

Bob R. has a good idea regarding the carb diaphragms. My '81 acted quite similar. One diaphragm was bad. Tiny split hard to see. Replaced both.

Check as many electrical connections as possible. Could be something as simple as an intermittent fault, but those can be the devil to find.

Here's an old Airhead "rule of thumb"...

If you think it's the carbs, it's likely electrical. If you think it's electrical, it's the carbs.

More truth there than you might think.

[nhmaf]

Does it exhibit the same behavior when running the engine and revving it up with the bike sitting still and in neutral, or only when moving?

Is it worse on bumpy roads?  Going up hills?

[raymr]

Any resolution?

My bike started acting up today after weeks of flawless operation. It's hot, in the 90s, and 10 minutes into the ride I got a momentary backfire and sputter while going around 30MPH. The tach needle swung down suddenly. It didn't stall but the power loss lasted for a second or 2. Then it did it again but less noticeable. The gas tank is nearly full. Last repair was putting new heat sink paste on the control module a few months ago, due to the same kind of sputtering/stalling. Does it sound like I need to replace the control module?

[DgM]

Check the coil.

[tvrla]

Water in the float bowl?

[raymr]

I'm figuring since I'm losing complete power it's not just 1 carb acting up. I don't think both would cut out simultaneously.

With the tach dropping to 0, I'm also figuring its either the cutoff switch, bean can sensor, or ignition module. Unless the coil activates the tach(?). The bike has a later model coil in it, so now I'm checking to see if the coil and ignition module are a correct match. According to some other stuff I read, a newer lower ohm coil could fry an older ignition module.  

[Bob_Roller]

Check all of your electrical connections on the bike .

Had a similar experience with my '81 R65 18 years ago .

After being told to check and secure all of the electrical connections on the bike, hasn't happened since that was done .

When you have the electrical connectors apart, check to make sure the pins are locked into the body of the connector and won't push out when the connector is put back together .

[raymr]

The blade connectors on the coil had some play in them, so I soldered and heat-shrinked new ones on - nice & tight now. I also ordered a new ignition module from Rocky Point Cycle, since the installed one is of undetermined vintage. The first time I took it off a few months ago there was brittle shellac-like substance that had to be chipped off the back. New heat sink paste seemed to fix the stutter for a while.

The current module only has the Bosch number stamped on it, no BMW part number, so I'm guessing maybe it had been replaced before?  

[montmil]

... The current module only has the Bosch number stamped on it, no BMW part number, so I'm guessing maybe it had been replaced before?

Likely not a concern looking for a BMW part number on the ICU module. I have one exactly like it. An OEM Bosch Ignition Control Unit factory installed in my 1986 Volkswagen Cabriolet. Bosch supplies many, many vehicle manufacturers with similar or identical parts. Most are available at your local FLAPS.

[RSMike]

Check the voltage across the battery at normal to high revs, as far as I remember should be about 13.9V, If your regulator is blown it will shoot up to about 17V.
Until I diagnosed this problem on my bike I was getting similar poor running issues to those described. Another symptom of blown regulator will be electrolyte in your battery dropping fairly rapidly as it gets "boiled" off by the high charging voltage..

High charging voltages like this will also cause points sparking and so they will go off quicker than normal. Also it can possibly damage coils, I ended up replacing my coils as an extra precaution.

good luck, hope you get to the bottom of it.