Misfiring/Backfiring on Low RPM's

[mfrias2nd]

I think I'm brave enough to give the spark test a shot. Thanks! I think first thing I was going to switch the leads and while I was at it hit the connections with some ECC and wire brushing....my tail light went out a couple weeks ago and I spent all day puzzled at what it could be, taking apart the light housing checking for broken wires etc. etc. I just had to take the wire brush to the connection on the fuse box.

That gives you an idea in what condition the connections are.

[Bob_Roller]

You may want to remove the main grounding point for the bikes electrical systems, it's located in the area with all of the relays near the 'backbone' tube of hte frame .

There's about 10-15 solid brown wires that are 'ganged' together and then secured with a single screw/bolt to a sheet metal bracket that's welded to the frame tube .

Wouldn't be surprised if it's corroded/rusted as well .

[mfrias2nd]

Update...

Got home and fired up the bike. After not firing in a couple days I let the carbs fill up pulled the choke and it fired right up no problems. I'm still getting a bad shake. I swapped the leads and I'm still getting a sooty left plug. So far I think I've ruled out the PLUGS and the LEADS. My dad (an experienced car mechanic) said he believes it may require a valve adjustment...however! while riding my tach was jumping around like crazy which lead me to believe it was electrical and it acted like it wasn't getting fuel or too much (possible flooding) could it be the carbs? Should I try a cleaning and rebuild before messing with the valves? Keep in mind the bike never had this trouble before I swapped the float stop needles. My spark is strong on both leads.

Let me know your thoughts.

[R65Singh]

How old are your diaphragms in the carbs?  Try holding them against some bright light to see any pin holes/ tear in them.  I recently had a bad diaphragm and the spark plug was always sooty.  On close inspection found a very tiny tear in one of the diaphragms which could be only seen against bright source of light, and new ones from Capital cycle cured the problem.  Good luck!!

[montmil]

I'm listening to your dad. When was the last time valve lash was checked and/or adjusted?

Be it a fueling issue or electrical, it's important that the valves are set correctly. Could be the exhaust clearances have closed up preventing the exhaust valves from closing completely. That issue would definitely cause operational problems.

Valve lash checks are simple to do.

[mfrias2nd]

Can't say I've ever done a valve check on the bike. I brought it back to life with the carb rebuild and it's been running fine. Can anyone give a link to instructions on how to do. I'm the least mechanically capable man in my family but I get by with simple to medium tasks. Dad and brother are experts.

I will check the carb diaphrams next. my kit didn't come with them so they are the ones that were in there.

[Barry]

This link is a bit wordy but you did ask.

http://bmwmotorcycletech.info/setvalves.htm

If you want pictures this procedure for /5  is concise. Only real difference is shape of valve overs.

http://www.bmwscotter.org/procedures/valve_adjustment/valve_adjustment.htm


In summary

- Remove valve covers and sparkplugs.

- Remove rubber plug from the inspection hole next to the oil dipstick.

- Turn the engine over by hand * until 'OT' mark (Top Dead Centre) shows in the inspection hole.

- One cylinder will have both valves closed. Adjust that side. Inlet to.004" and Exhaust to .008"

- To adjust the other cylinder rotate the crank one full turn until TDC mark shows again.


* Method of rotating crank is to put gearbox in 5th gear and turn rear wheel by hand or alternatively and vastly preferred by me, disconnect battery, remove front cover and use an Allen key in the alternator bolt.


There are other things you should know (read the link above) like how to check/adjust rocker arms for minimal play but that's probably not necessary for your current purpose of getting the thing running right.

Although you may hear this suggested as part of the regular valve adjustment I would not recommend torquing the cylinder head bolts unless you have done quite some research on the pitfalls.

[RSMike]

Hi, sorry for not replying sooner, work and bad weather means I have not being paying attention to the forum for a while now.

There was a bit more to my story that is not mentioned in the thread you referred to. My misfiring woes did return, and in the end I think the root of all my problems was a failed voltage regulator.

The too high charging voltage (up to 17v) going through the electrics was probably making my points go off early, as well as possibly not being too kind to the coils.

Since I replaced my voltage regulator and coils (with a dual coil), my bike has been running sweetly.

More details in this thread: http://www.bmwr65.org/cgi-bin/yabb2/YaBB.pl?num=1312539150/0

( I never got back to try the rooster booster again, but probably will one of these days)

[mfrias2nd]

Thanks RSMike,

very helpful since I'm everything in my gut has been telling me it's electrical, especially since my tach was jumping around like crazy. But since my plugs and wires came back negative I was leaning back to the carb or valves.

The only thing that has been getting me is while riding and letting off the fuel or stopped when I start its stumbling and almost wanting to turn off but when given hard throttle it would just cough and GO without problems. I just had to keep the RPM's up.

I'll check out the following this weekend...

Carb diaphrams (being replaced)
New Floats (being replaced)

I'll also take the time to do a valve check anyway and if I have no luck looks like i'll fork out for the electrical.

[JPSpen]

You CANNOT properly balance a BMW R type engine without FIRST doing a VALVE SETUP..... I'm very surprised no one has mentioned this very inportant fact...

Sorry.. But you need to set the valves first... Then you can balance the carbs... You might want to check for a leak around the exhaust gaskets as well as the other things mentioned here..

Valves first always.....

Then, Ten miles is not enough.. Half an hour minimum ride before doing the carb setup.... Do the rough setup.. Then ride for half an hour, Then do the final carb setup with a big fan blowing at the front of the engine... I have one of those big shop fans...

Good Luck...

[Barry]

..... I'm very surprised no one has mentioned this very inportant fact...

They did....his dad.

...Ten miles is not enough.. Half an hour minimum ride before doing the carb setup....  

If you really want to push the boat out on defining when is enough warming up to tune the carbs it should be when the oil temperature reaches a certain figure. I think BMW specify 80 Deg C. That could be 10 miles in a hot summer and in a very cold winter you might never get there with the bike in motion.

[mfrias2nd]

very interesting! these are all things to consider and I'm very humble for everyones advice and tips. with this problem a good thorough tune up is in the works. My carb rebuild kit didn't come with floats or new diaphrams so I will tear them down again install the new components and rebuild. I will also check the valves. Proceed with a 30 minute ride around my house (in case i break down) and then fine tune the carbs.

If, however, my stumbling is caused by an electrical component then it will be useless to fine tune until the problem is fixed. I hope the problem is in the carbs. I'll keep you all posted. Wish me luck.

[RSMike]

You have my sympathy, I know how frustrating this can be.

I agree that I have found setting valve clearance, points and timing to be more productive than any carb adjustments.

Personally unless something is obviously off with carbs, I would leave those adjustments to last, (though I have not always practiced what I preach).

Now I am biased, but because of my own experience, I would encourage everyone who has not done so, to get a voltmeter and measure the voltage across the battery terminals with the bike running at different revs, you should not be seeing anything above 14.5v.

I failed to do this simple check early and all my other efforts were in vain.

[tvrla]

There's a rule of thumb that 90% of carburetor problems are electrical. While it's a very simplified over-simplification, you get the idea.

Sometimes the only way to attack the problem is to start at one end, work towards the other, and make right everything you touch. Eventually you'll find it, or at least will have enough data to make an educated guess.

[mfrias2nd]

Barry, thank you for the simple instructions. I took off the valve covers, plugs, etc. checked for rocker play, NONE! The only adjustment required was the exhaust on the right side. Which isn't giving me problems. It was near touching. The other sides were spot on .004 inlet .008 exhaust. I had my friend double checking my work as I went.

On impulse i took apart my left carb (problem side) and rebuilt it and it fired up 100% better almost no problem. NO further time went into the bike.

Today I went on a 30 minute ride with my brother following on a GT185 and the bike was running a lot better. Problems are...I'm still getting a jumpy tach. This time it doesn't jump along with engine stumbles it was jumping even when the bike was running steady. More often I noticed it would jump when I left off the throttle and when i hit the throttle again it would try and even out. Other than that I'm still getting some popping here and there out of the exhaust.

Are my problems still electrical? What's going on with my tach what are the relations? I'm still going to rebuild carb with new diaphrams and floats when they get here if that doesn't solve the problem looks like i'll start replacing electrical components.

further thoughts please share.