carb adjustments

[Rob Frankham]

Here are three BMW Service Bulletins that might be of assistance (or at least interest)...

Irritatingly, can only post three images per page and each image is two pages so the final three are on the next post...

Rob

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Rob

[Rob Frankham]

Here are the second three pages...

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Rob

[gspd]

Here are three BMW Service Bulletins that might be of assistance (or at least interest)...

I would not recommend this mod unless you are in a country that has strict periodic emission tests for motorcycles and you don't know how to (or just don't want to) fudge your way through them.

I remember that bulletin from waaay back in the day when a BMW emblem on your gas tank identified you as a connoisseur motorcyclist who actually knew how to ride. Unfortunately it has now become only a coveted status symbol. In the late '70's (and ever since) all vehicles had to be jetted extremely lean to pass emission regulations. This BMW service bulletin 'choke' mod was BMW's only cheap and easy way to get their damned bikes to start, run and have any semblance of throttle response when cold, without affecting 'guvmint' emission test results once they were warmed up.
Many manufacturers had (and still have) plugs over the mixture screws that must be illegally drilled out to permit adjustments.

The better solution, which btw also applies to every carburetted bike from that era and up to the present day, is to richen the mixture adjustment, raise the needle a notch and/or increase the main jet a size or two. We (BMW dealership) did this to tons and tons of bikes over the years whenever the customer had cold starting or cold running issues. Canada did not (and does not now) have any mandatory emission testing for motorcycles. These mods always resolve cold starting issues and always cure the cold-bloodedness of any carb bike, although in some circles this could be deemed as un-ecological and politically incorrect. It also has the fortunate bonus side-effect of improving overall performance once warm. I doubt a bunch of motorcycles running a tad richer will have a noticeable effect on global warming, and if so, will it be any worse than all the that extra gas wasted while warming up ?

I pity (and loathe) all those fools that have no choice but to warm their carb bikes up for 15 minutes outside MY motel room in the early morning hours just so the throttle will respond enough so they can ride away without stalling.

Fortunately fuel injection has made all these issues a thing of the past.
Just hit that start button and ride away.

[barryh]

That choke mod is specific to early R45/R65's. I've done it to mine and it transformed the choke operation to something that actually works progressively. I would highly recommend it for applicable bikes. I suspect enlarging the counter bores is as much to do with the positive results as drilling the 4th hole. Prior to the mod I had the distinct impression that it was possible for the disk to end up in between fuel holes and do nothing which the larger counter bores resolved. It's easy enough to do although you will have to find something that will hold a 0.6 mm drill bit which is a lot smaller than you might imagine. On the basis that drill bits can have a tendency to produce an oversize hole I drilled mine 0.5 mm and then checked it with the 0.6 mm bit.

[gspd]

That choke mod is specific to early R45/R65's.

There may have been another bulletin later to specifically cover other models, but the one posted here by Rob covered all models.

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I'm not 100% sure about this but I believe that sometime in the early to mid '80's this mod was done by BING (or BMW) during production and all new bikes had the enlarged holes rendering that bulletin obsolete for later bikes.

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There were no r45's sold in North America, and not that many r65's either (compared to other models).
I've never had any reason to check the size of my PD's choke holes to see if they were actually factory enlarged.

[pdx_r100s]

GSPD, thanks again for this info. That video was useful--that's essentially the overhaul that I did. I believe that I put it back together exactly that way, but now I'm going to double check that I aligned the teeth inside of the actuating lever correctly with those teeth that pull the cables.

Also good to know about the choke--I was not using it correctly.

Yesterday I adjusted the fuel screws out as you suggested--I started with one turn out (previously they were 1/2 turn, per the clymer manual). Then I balanced the carbs. Low and behold, the popping is completely gone! However, now the idle isn't as steady. When the bike is fully warmed, it surges. If I come to a stop, it will sit at 1100 for a bit, then maybe drop down to 1000 or 950, then go back up to 1100. Before, when the idle fuel mixture was leaner, it idled perfectly steady at 1050.

Is this a sign that it is too rich now?

Also, I tested to see if blocking the exhaust caused it to stall, and it did, so I don't think I have exhaust leaks.

[gspd]

Yesterday I adjusted the fuel screws out as you suggested--I started with one turn out (previously they were 1/2 turn, per the clymer manual).

Everything I mention here has to be done with the bike at operating temperature.

MIXTURE:
Forget about the 'amount of turns out' Clymer or anybody else suggests.
They are just guidelines to start the bike.
Settings vary from bike to bike and the ideal setting may be as much as 2.5 turns out.
First make sure turning each screw all the way 'in' stalls that cylinder, that will assure that your idle circuit is working properly.
Then back the mixture screw out 1/4 turn at a time until the rpm no longer goes up as you turn it out.
Keep dropping the rpm back down to 1000 with the throttle stop screw.

You'll be in the mixture 'sweet spot' when the bike is idling at around 1000rpm and turning either mixture screw in a quarter turn audibly drops the idle a bit, and turning either screw out a quarter turn does virtually nothing. (oopps! edit: I erroneously said half, but I meant a quarter)

The amount of turns 'out' is not necessarily exactly the same on both sides, but it's usually pretty close.

SYNCHRONIZATION: only after the idle mixture and speed is properly set...
Adjust your throttle cables to just the slightest amount of free play.
Now pull on just one throttle cable a bit and note how crappy the bike sounds.
Then pull on the other throttle cable a bit, same shitty sound, right?
Now simultaneously pull them both slightly and hear how smooth it sounds.

Adjust that free play slightly so twisting the throttle grip slightly pulls both cables evenly and it sounds smooth.
Trying to sync the carbs at higher rpm by cutting out one cylinder and then the other and matching their rpms is a total load of bullshit and does not work.
The only synchronization that matters is right off idle, precisely at that point that the cables pull the throttles off their stops.
P.S. Try to straighten that kinked cable you mentioned. It could be a concern here because the initial few millimeters of twistgrip motion may be simply unkinking the cable rather than actually opening the butterfly.

Let me know if any of this helps, I find it's way easier to actually do than it is to explain.

[pdx_r100s]

this explanation is helpful. It is similar to what I'd done before to synch. however, I had read a write up that took a different approach to getting the initial mixture correct. I had read to start with them way *out* and then slowly screw in until the RPM drops, then back out 1/8th turn. That's how I got them to my initial setting of a little more than 1/2 turns out.

When I'm using your method and I'm initially taking the screws in 1/4 turns out to find the sweet spot, and then adjusting the idle screws down to 1000 RPM, is this a back and forth process for both sides, or do I do one side at a time?

I was also using a homemade vacuum gauge that I'd used on a different motorcycle years ago with good results--simply a clear plastic tube with some 2-stoke oil in it, attached to both carbs at the port. This doesn't measure amount of vacuum but does show any difference between the two, which I thought is what matters. So once I got the mixture to where the engine sounded happy (max RPM), I adjusted the throttle stop screws down to ~1000 so that the vacuum was equal between the two sides. And then I repeated this with the throttle slightly open to get things synched for off throttle (~1400 rpm).

Is that not a good method?

[pdx_r100s]

Ok, so I just tried as you suggested, screwing in the fuel screws all the way. When I did this on the left, the engine died. However, on the right, the engine kept running. So I presume that my idle circuit isn't working properly on the right side. What do I need to check out? Is there something that tends to get clogged?

[gspd]

this explanation is helpful. It is similar to what I'd done before to synch. however, I had read a write up that took a different approach to getting the initial mixture correct. I had read to start with them way *out* and then slowly screw in until the RPM drops, then back out 1/8th turn. That's how I got them to my initial setting of a little more than 1/2 turns out.

As you mention (in your next post) one circuit is blocked.
This link posted earlier should clarify the idle circuit better than I can explain it to you.
They are the best illustrations I have come across for these circuits.
https://electricmotorglider.com/wp-cont ... -web-1.pdf
Those by-pass holes are tiny and can be blocked by a tiny grain of sand or crap.
Use brake or carb cleaner spray with a tube nozzle (or compressed air) and be sure they are ALL clear.
Adjustments are futile until these function properly.

When I'm using your method and I'm initially taking the screws in 1/4 turns out to find the sweet spot, and then adjusting the idle screws down to 1000 RPM, is this a back and forth process for both sides, or do I do one side at a time?

You can do it either way. You can shut down one side at a time by screwing in the mixture screw all the way and then adjusting the opposite side. Then precisely count the turns (so you can later put it back exactly where it was) while screwing it back in to shut off that cylinder and do the other side. Only screw them in until they seat very lightly, too tight can break off the tip. You can also take them out just to confirm that one of the tips isn't broken off and stuck in the hole, something I've seen more than once.

I was also using a homemade vacuum gauge that I'd used on a different motorcycle years ago with good results--simply a clear plastic tube with some 2-stoke oil in it, attached to both carbs at the port. This doesn't measure amount of vacuum but does show any difference between the two, which I thought is what matters. So once I got the mixture to where the engine sounded happy (max RPM), I adjusted the throttle stop screws down to ~1000 so that the vacuum was equal between the two sides. And then I repeated this with the throttle slightly open to get things synched for off throttle (~1400 rpm).
Is that not a good method?

That's perfect. Sometimes mercury works better than oil.
I've seen tons of people try to sync the carbs at high(er) rpm's which is a total exercise in futility.
The problem with big twins is that often the pulsating at low revs makes it hard to precisely establish the level of the fluid in the tubes. Theoretically the level in the tubes must be exactly even at idle, and stay even at just above idle, just exactly at that point when the throttles leave their stops. Theoretically the thinner the vacuum tubes are, the more accurate the result will be, but this is splitting hairs. With a bit of practice you'll discover that your ears can be just as accurate as a vacuum gauge.


I have a custom made adjuster at the throttle so I can fiddle with it anytime, just to keep it finely tuned. You only need to adjust one cable to sync two carbs. A quarter turn either way makes a huge difference at just off idle because the butterfly is barely open to begin with. Once rolling down the road, a full turn either way makes no noticeable difference because the butterflies are way more open.