I know of one owner who had a good result by swapping out for Mikuni carbs. Rocky Point Cycle used to sell a conversion kit. (this was on an R60/6) I'm not sure that your trouble is with the carburetors necessarily, but the Mikunis are arguably an improvement over the Type 53 Bings. What do the spark plugs look like? Golden brown is good, sooty is rich, whitish is lean. Before spending serious money while guessing about a solution, I might try a tank of premium, non-alcohol gasoline and/or different spark plug heat ranges. Maybe try changing the air fuel mixture by raising or dropping the needle setting by 1 notch. Y'know, small alterations that do not involve serious expense. If you want to open up the motor, look for carbon build-up on the pistons and cylinder heads. This might hold heat in the combustion chamber and contribute to a tendency to explode the fuel too early or make it burn out too fast. I would also recommend that you carefully read (Snowbum) Bob Fleischer's excellent web site for insight into this issue. http://bmwmotorcycletech.info/technical-articles-list.htm

Thanks for the comment. I always run chevron premium in the tank. I may try an octane booster. As for the spark plugs, I would have to check again since I last tuned the carbs. In the past they had been pretty sooty. Maybe it was leaning up the mixture that caused the preignition to start. I know from looking down the spark plug holes that the pistons have a considerable amounts of carbon build up and small white specks marring the surface which I was advised could actually be small pin holes in the piston.

I think pin holes in a piston would result in a serious loss of compression. If it were my bike, I would try a cooler plug next. For example, I normally run NGK, BP6ES spark plugs. BP7ES would be one range cooler for an Airhead. Check out this web page http://www.ngk.com.au/spark-plugs/technical-information/heat-range-explanation

I would not advise any major engine changes until you sort out your present issues. Your issues are most likely NOT tied to being 600cc displacement. Therefore, any "updates" will leave you with the daunting task of sorting out this issue and sorting out the needs of the new big bore kit simultaneously. That would be like getting a divorce because you have colon cancer. The original issue remains while you pile on more headaches, expense, and complications.

Pre-ignition has several causes, some of which have been touched on above. In no particular order they are...
► Fuel octane too low for the compression
► Ignition timing too far advanced OR ignition timing advancing too fast
► Fuel mixture too lean
► Running the engine RPM too slow for the load

1) You already told us about your fuel. Chevron high test should be high enough, unless you buy it at a station that does not sell a lot of high test fuel. The problem is the ethanol separates out during storage. You could be getting mostly ethanol, which has a very low octane. Try fuel from another, more popular gas station.

2) It is always best to strobe time a BMW at high RPM using the "F" mark. Your issue could be a slack timing chain that allows more ignition timing leeway when you static time on the "S" mark. Or, your advance mechanism may need new springs to restrain the advance curve. Weak or missing springs would allow the ignition cam to advance too fast for the engine RPM. You'll be able to see both these errors by using a strobe timing lamp.

3) It is quite common for ethanol fuels to clog the smaller jet(s) and coat the needle with mung that will effectively lean out the mid-range. Lots of people think that the pilot jet only controls the idle, but a clogged pilot will tend to lean out the entire running range. So you need to remove both carbs, and without intermixing parts, compare them side-by-side. Then set the idle and cable adjustments using a manometer to insure perfect balance.

4) Although the 600 will putter around at low RPM very nicely, it cannot be done under any type load (2 riders, heavy luggage, heading up hill, etc). IMHO, all serious road work should be attempted with engine speeds in excess of 3000 RPM.

Hope this helps. 🙂

If it were my bike, I would try a cooler plug next. For example, I normally run NGK, BP6ES spark plugs. BP7ES would be one range cooler for an Airhead.

This thought is based on the idea that the temperature of combustion is set by the temperature range of the spark plug. In essence, a person can cool the combustion temps by using a "cooler" heat range spark plug. In fact, this is the exact opposite of what is happening.

The spark plug merely samples the heat of combustion in an effort to keep itself clean. That is to say, the different spark plug heat ranges are available to merely enable the spark plug to keep themselves from fouling.

The heat of fuel combustion is set by the ignition timing, compression, and fuel mixture. So I highly doubt that going to a "cooler" spark plug heat range is going to help.

Thanks for the input. It was always my understanding that heat range is an indication of how the construction of the plug promotes dissipation of heat in the combustion chamber, or promotes retention of heat in the combustion chamber. I could be wrong about that. It never occurred to me that this is entirely relative to a spark plug's tendency to be self-cleaning. Hmmmm.....

The Wikipedia explanation under the Heat Range section explains it much better....

https://en.wikipedia.org/wiki/Spark_plug

Not to worry. I worked as a professional motorcycle mechanic for years thinking the same thing.
😆

Hey Wobbly,

Thanks! It did indeed help very much.

I had, for a while, solved the preignition but it came back the moment I took my girlfriend for a ride to show off my fixin' skills. It mainly only happens in first gear or if I try to speed up without downshifting on the freeway so your last comment definitely resonates with me. I have the four speed transmission if this is of any consequence. I'm going to order some new jets and a carb rebuild kit since she started backfiring when the engine gets warm anyway. I think my solution is probably carbs and cleaning the piston for now. I could use some new piston rings anyway. I know my timing chain is a little loose but the F is always in the hole with a timing light it just jumps around a little bit.

What is your thought on naming motorcycles? I keep naming my vehicles and they keep developing problems soon after. I'm not too superstitious but, maybe some correlation? Any body else believe this?

Hey Wobbly,

Thanks! It did indeed help very much.

I had, for a while, solved the preignition but it came back the moment I took my girlfriend for a ride to show off my fixin' skills. It mainly only happens in first gear or if I try to speed up without downshifting on the freeway so your last comment definitely resonates with me. I have the four speed transmission if this is of any consequence. I'm going to order some new jets and a carb rebuild kit since she started backfiring when the engine gets warm anyway. I think my solution is probably carbs and cleaning the piston for now. I could use some new piston rings anyway. I know my timing chain is a little loose but the F is always in the hole with a timing light it just jumps around a little bit.

What is your thought on naming motorcycles? I keep naming my vehicles and they keep developing problems soon after. I'm not too superstitious but, maybe some correlation? Any body else believe this?

I had, for a while, solved the preignition but it came back the moment I took my girlfriend for a ride to show off my fixin' skills. It mainly only happens in first gear or if I try to speed up without downshifting on the freeway so your last comment definitely resonates with me.

The trick is to keep you mind open to new ideas and not succumb to the cure that presents itself front and center. In that vein, we must also consider that the casue could be the girlfriend. I suggest you send her on over, I'll place her on the back of my R100 and we'll see if any mischief befalls my machine. All in the name of science, you know. Anything to help a friend. 😛

I'm going to order some new jets and a carb rebuild kit since she started backfiring when the engine gets warm anyway. I think my solution is probably carbs and cleaning the piston for now. I could use some new piston rings anyway. I know my timing chain is a little loose but the F is always in the hole with a timing light it just jumps around a little bit.

I assume the backfiring "she" is the motorcycle ? :whistle:

Worn rings will place a lot of heavy carbon deposits on the piston crown from oil, that could be adding to your dilemma as per Scott's input.

Main and pilot jets can be cleaned. The only jet that receives actual physical wear, and thereby warrants replacement, is the needle jet.

Check out the prices on carb and top end rebuild kits on EUBMW.

What is your thought on naming motorcycles? I keep naming my vehicles and they keep developing problems soon after. I'm not too superstitious but, maybe some correlation? Any body else believe this?

First step is to properly determine the gender of your motorcycle. Wouldn't want to end up with a "boy named Sue" scenario !!

B)

Well the bike is a she for sure, though I wouldn't be opposed to naming a boy bike Sue, would surely toughen it up and then he would never stop running. Now to bring up the issue of my girlfriend... I would be totally open to the idea but, to be honest, she's trouble man. I don't think you could handle her. LOL. So maybe she is the issue--all 120 lbs of her.

I had, for a while, solved the preignition but it came back the moment I took my girlfriend for a ride to show off my fixin' skills. It mainly only happens in first gear or if I try to speed up without downshifting on the freeway so your last comment definitely resonates with me.

You could still have auto advance issues. No lead fuel likes a slower advance than the fuel this bike was designed for. Any slack or looseness due to aging of the advance unit will result in the timing advancing too fast for the engine rpm.

Ah, I just had to look it up. A little new on terms and whatnot. I fiddled with that already and got the dwell to stay around 74. Is that also referred to as the points?

Dwell is the amount of time that the points are closed, expressed in degrees of rotation. Whether 74 is a good value or not, I don't know. The 'dwell" is kind of opposite of point gap in that the point gap affects "dwell". Dwell time is what allows the coil primary winding to build the magnetic field so that when the points open and the field collapses, the secondary coil windings are induced to a voltage/current surge that we see as a spark across the gap of the spark plug. Point gap also affects timing in that if the point gap is set too wide, the points will open sooner during the rotation of the point cam on the advance mechanism. Gap is set at the high point of the cam lobe. If the gap is too wide there, the points will open sooner on the ramp-up to full lift of the point cam. When the point gap is just right, you will have a range of adjustments for timing by rotating the point base plate vs. the clockwise rotation of the motor. That is why the holes for the retaining screws are sort of crescent moon shaped. I hope one of the more experienced mechanics (Wobbly?) here corrects me if I'm in error and maybe offer additional clarification.