1982 R100RS first carb sync attempt, dubious compression test results, and a rat hole

[drj434343]

I have a 1982 R100RS with 60,000 miles. I purchased it last summer with 59,000 miles in good condition. I recently replaced the original ignition system with a Motorrad Elektrik kit, and replaced the throttle cables as well. In order to bring it back to base tune, I did a valve check while cold (0.006" intake, 0.008" exhaust), and adjusted the timing.

I read Snowbum's guide for syncing carbs as a starting point, then launched into making those adjustments today; using the vacuum gauge method. After taking the bike out for a ride and setting it up in front of fans, I proceeded to first adjust the idle screws, then the mixture screws, then sync the cables at off idle speeds. Somewhere in the process I got myself turned around because I was having a hard time either getting the engine to smooth out, getting the gauges to sync, or getting the RPM's to come back down to idle after a throttle blip to clear things (it would rest at 1500 then drop to 1000 after 5 seconds). To simplify things, I then reset both mixture screws by turning them 2 rotations out (per Snowbum) and starting fresh. However, at this point, idle adjustments would either yield a runaway RPM condition, or the bike would die. So, I started leaning both carbs out 1/4 turn at a time and attempting to get a decent idle each time with idle screw adjustments. However, I couldn't get the feel for it, or get a decent idle that the bike would return to using this technique.

During this time, I noticed the ventilation air flow temp coming off the left jug was noticeably hotter than the right (oriented as sitting on the bike). I got out my infrared thermometer and while running, measured about 340 F on the right jug right where the exhaust pipe exits the head, and 520 F on the left jug. I know exhaust temps are very affected by fuel mixture, but to make sure I wasn't going down a rat's hole, I decided to do a compression check.

It turns out, the left jug (the one at 520 F) showed 105 PSI, while the right jug (at 340 F) showed 85 PSI.

So, the question is, are these numbers low enough, or perhaps different enough that I need to fix this issue BEFORE continuing to sync the carbs, or are those exhaust temp differences I observed due to my poor tuning attempt and I should keep at the sync efforts and let the cylinders and compression numbers alone?

Cheers,
Jason

 

[20774]

The compression numbers seem low, but could be lowered by the way you did the compression check...you didn't say. Did you remove the carbs from the heads? Did you use a manual method to hold open the slides while spinning the engine? You can't just twist the throttle and expect that the slide will move out of the way on the CV carbs like you would on the slide carbs. Compression numbers are lower, I'm guessing 25% lower, if you don't do it right.

If I read the specs right, the '82 RS should have 9.5:1 compression...I actually thought it was quite a bit lower, more like 8.2:1. The range of compression for these two ratios runs from about 135 to 155 psi. If it's more like the lower value, then you're readings, if you didn't remove the carbs, might be OK.

 

[drj434343]

I was not aware of the carb removal issue until I started searching prompted by your comments. So no, I didn't take them off or move the slides up before testing. I'm a bit surprised this is an issue, I've never had to do it on any other carbed vehicles I've tested in the past. I would have assumed that there was at least some amount of opening when not running, even with the slide all the way closed. That small opening would just increase the number of strokes required to reach peak pressure.

That said, now I have a cold bike with a carb tune that makes it unridable, so if the PSI numbers may be OK, I have to ask if there is any other written guide about how to sync the carbs other than Snowbum's. I didn't do very well with his description of the vacuum gauge method. In case it matters, I ordered a Bing CV manual, so that should be here next week.

 

[20774]

Note that for CV carbs, you don't control the position of the slide...you only control the butterfly valve which in turn gets the slide to go up and down. For sure, you need to hold open the butterfly WOT. Sure there's a small opening under the slide, but you're restricting the ability of the cylinder to pull in all of the air it needs to compress. So, it will have an affect on the results. Maybe my 25% is high...that might be more for a cold engine. At any rate, to get more accurate numbers, you need to hold the throttle open and physically raise the slides.

I don't use the water filled vacuum tubes if that's what you ended up doing. I have a TwinMax (similar concept but a differential measurement device). I've also used the plug shorting method which Snowbum talks about. I don't recall reading anyone else's website where carb synching is talked about.

Any chance you can hook up with some other Airheads in the Portland area? It might be helpful to have another set of eyes on what you're doing and to provide additional advice.

I've never had the kind of issues you wrote about. I wonder if you got your cylinders a little too hot...how long were you fiddling with things? I'm usually done with a carb synch in 3-5 minutes.

 

[jad01]

There is a really good airhead group in Portland that has regular gatherings and tech days. I sent you a PM with the Portland air marshal's contact info.

 

[drj434343]

I've been doing compression tests on old Volvos with SU carbs for years. Those also have vacuum controlled slides, and the throttle only controls the butterfly. I've never heard of any SU guy talking about propping it open before a test. Maybe I've been skewing my results all these years. I'll pose that question to some local experts to see.

I haven't directly connected with any NW airheads yet, though I know of a group. Unfortunately they meet about an hour away from me. There's also a tech day coming up I believe, but again, about 2.5 hours away. My bike would be a great candidate for a tech session I think, though I'd have to actually get it there first.

I'm stuck at this point, and am going to need to get it tuned well enough to actually warm it back up and start the sync process over again, hopefully with a better understanding of the technique. I had a dual fan setup blowing on the jugs, and also have an oil temp gauge, so I don't think I got it too hot. I did probably run it for 5 min at a time, my 3 times in a row with some cool down in between. It sucks trying to tune these things while worrying about the temp and racing against time; especially when you're new to syncing.

 

[20774]

I understand the anxious situation of standing next to a hot running engine while making carb changes. I don't think I'll ever get used to doing that!! What I would do is the following, which should get you good enough to start and ride the bike to warm it up. If it doesn't run very well after this, then you have other issues within the carb or the valve clearances are not quite right.

- set idle mixture screws as suggested...a couple of turns out from lightly seated
- idle speed set so that as you turn the screw in and it first makes contact with the carb plate thingy (don't know what to call it!)...after first contact, go one more turn
- ensure sufficient slack in the throttle cables, approximately 1-2mm at the knurled adjusters where they enter the carb top
- go for a ride to warm the bike up and come back and try the synching process again

Be sure your chokes/enricheners are completely off. Hopefully, they aren't open such that it is such air. Also, be sure that the carb-to-head clamps are tight so there are no air leaks.

I see where someone said there is an Airhead group in Oregon...maybe that's the ticket. But try the above.

 

[jad01]

They (Portland ABC) are a pretty good group- I wouldn't be surprised if someone might come out and give you a hand. I built a homemade manometer to balance mine, but I found the starting point adjustments noted in the Clymer manual were pretty close.

 

[dennisdarrow]

The carbs are cleaned, rubber bits in good shape...........????????..............Glad you are smart enough to run cooling fans....You can't have enough so make sure you get LOTS of air going...........

For me, I separate "tuning" the carb into two different seperate procedures. First is the idle, and then the syncing of the cables.

First, I get the idle and then after I am happy with both carbs I work on the cables............

Begin with backing off the throttle cables to around 2mm of slack there at the carb.......

Adjust the idle screw where it is just touching the arm on the carb that it engages with. For me, I put a small piece of paper on the end of the screw and then when I get clearance, I KNOW IT...........Turn them in perhaps 1 full revolution and then be prepared to back them off........Whatever you do to one carb at this point do it to the other.........

Now, screw in the air screw til it bottoms out and then out perhaps 2 turns...............

Hook up the test device or get the plugs ready to short out........

Start the engine........back off the idle screws until you can see the RPM begin to drop

Now adjust the air screw, in or out, until each carb runs it's best.............

NOW........back off the idle screw and adjust the air screw..........

FINALLY you will get to the point where the idle screw is in as far as you can, without it NOT idling right.........No trick to this just go back and forth to each carb and then adjust idle screw and then the air screw for best running...........

By the way, if later, when decelerating or going down hill with throttle rolled off go get a bit of popping or backfiring.........go IN on the air screw perhaps 1/16 of a turn on the carb that is popping..........

For me, I like to idle just as the red/alternator light is going out or flickering........Some folks say, 1000 RPM or even 1200........For me though....I kinda like the chug chug of a steam engine...........

OK............let us know when you have this done and doing the cables is a snap.........God bless.......Dennis

 

[drj434343]

Dennis, thanks very much for taking the time to write up your procedure. I'm at work right now, so have time to armchair quarterback and ask some questions before tinkering again.

First, I have not personally pulled the carbs apart to inspect them. Bike was regularly maintained, and service records indicate some refreshing was done several years ago. That said, it is possible something is amiss. I'm not quite at the point where pulling them apart seems reasonable. If I keep at this and still can't get it, that's the next step.

1. When you say air screw, are you talking about the idle mixture screw right?

2. You talk about adjusting each air screw after the idle is set until the carb "runs its best". Can you describe that in more detail? Snowbum attempts it, but I still struggled when applying those descriptors to what I was actually hearing.

Yesterday, when "zeroing" things out, I started with the air screws 2 turns out and the idle screws one turn in from touching. In that condition, the bike started and ran, but had a very low chugging idle. Continuing to turn the idle screws in exposed a strange inflection point in the idle speed. It would either chug along at 800 RPM sounding terrible, or it would immediately jump to 2000 or 2500 RPM. I couldn't find any speed in between by changing the idle screws. That's when I started leaning the carbs out with the air screws thinking I was overly rich. The inflection point seemed to still be there in leaner conditions, but it was less pronounced. There was no "sweet spot" that I could find, which is why I started suspecting other things.

 

[20774]

I'll comment and let Dennis add later:

Yes, the air screw is the idle mixture screw. A small note is that this screw performs different functions on the slide carbs versus the CV carbs. On the slide carb, this screw controls an air circuit, so it is truly an air screw. So, turning it in will shut off air, thus making it richer. On the CV carbs, this screw controls a gas circuit, so screwing it in shuts off gas, thus is making things leaner.

The purpose of the idle adjustment is to find the center between when turning the screw in causing the engine to stumble and turning it out also causes it to stumble. So, ideally, at either extreme, the cylinder isn't running well, and you need to find the spot in the center...this is where you have to keep count of how many turns each direction. Once you're in the center, it is probably best to make the mixture slightly richer (richer is cooler for the valves) so turn it maybe an 1/8th turn towards the rich side (CV carbs, that would be CCW).

So, I would say once you the engine begins to change it's tune when turning CW, don't keep turning...just note where you are and then start counting turns (say 1/2 of a turn at a time) CCW until it stumbles. Divide the number of turns by 2 and turn back CW that amount...plus accounting for the slight rich setting at the end.

If you started at 2 turns out, then in 1 turn and the bike began to chug, time to stop, or back up and find that point at which it began to change.

Setting the idle speed requires either reading the manometer or the differential device or using the shorting method. The blipping of the throttle is necessary when you're doing the shorting method. Since you will have shorted a plug, the cylinder is still sucking in gas. Blipping the throttle helps clear that. After noting the RPM when each cylinder is shorted, you probably will want to increase the slow side until they are both even. Then if your idle speed is higher than you want, turn them both back a skoosh until you get the RPM you want...1000 to 1200. You might want to recheck by shorting and see if you're still getting the same drop in RPM.

Technically, you need to go back and recheck your idle mixture, but I usually don't.

The final step is the cable synch or cable tension. The best RPM to check this is just off idle...this is where the differences in throttle position is noticed the most. So, with the still 1-2mm of slack in the cable, open the throttle up to 1500 to 2000. Either have someone hold it or use the throttle stop screw on the bottom side of the perch. Proceed to check your vacuum or do the plug shorting. After plug shorting, let the cylinder run for a bit to clear itself. To make adjustments, I highly recommend that you slow down the faster cylinder rather than speed up the slower one. The reason is that if you speed up the slower one, you will be reducing your slack at idle. If you take up more than the 1-2mm of slack, you've now hosed your idle settings. Best to slow down the faster cylinder...you're adding slack. Of course too much slack is not a good thing, so if you feel you have too much, start off with closer to 1mm or maybe slightly less.

 

[30064]

Jason,

The last thing I want to do is complicate things any more than they are, or increase your frustration level. I'm more curious about the deviation of pumping power of your right side cylinder vs. your left. The numbers in your original post indicate a 20% deficiency in compression of your right side cylinder. Unless my memory has failed me the low end of acceptable compression is about 115-120 psi. Granted, that is with a fully charged battery, carbs removed, adjusted valves and warmed up engine. I know you had the carbs installed when you measured the compression which will give you a lower number. But if you have a weak cylinder - almost any effort of balancing the carbs may be futile or challenging at best. One side will not be working as efficiently as the other causing some rough running.

Before you follow Kurt and Dennis' detailed adjustment procedure, you may consider checking the valve adjustment and settings again. If they check out ok, try the carb adjustment and balancing as Kurt and Dennis has described.

If that doesn't give you any happiness I would look into doing a leak-down test. Leak-down tests are very comprehensive but require a special tool and air compressor. It would be interesting to see the results of the function of both sides of your intake valves vs. exhaust valves vs. piston rings. This test is also best performed and recommended with the carbs removed.

We're following your progress - with fingers crossed.

-Mike V.

 

[lkchris]

It is dirt simple to just remove the carbs from the intake tubes when doing a compression test.

If the compression varies by this much doing it that way, then the next step is a valve job, as it is simply futile to attempt carb synching on an engine in that state. It will never happen.

I know from experience that such a compression variation can result in runaway idle.

USA versions of the R100 have 8.2 compression in 1982. Rest of world versions are 9.5 and it's really simple to swap in the high compression pistons and then raise needle jets a notch. RS, RT, and all R100 engines are same in those years.

 

[dennisdarrow]

Jason........

First off you are getting some really good advice here.......Mike's thoughts and concerns are totally valid and if your compression really is as bad as indicated you will NOT get proper idle...........

For me, I find that the shorting method is great for checking and setting idle; but for the cables not so much. As with much of everything, each technique, shorting or gauges, has it's plus and minus side.........

When setting the idle, the air/mixture screw, if right on, and with a good ear, one can find that oh so sweet spot may be 1/4 to 1/2 turn of the screw. You will definitely hear the engine begin to stumble going either way. I hate it when I begin to go CCW and the durn engine totally speeds up.....means I have to reset the idle screws on both sides probably........Anyway, when you find that spot....you know it.....lolol......is there a "G" spot in there somehow??????????

For syncing, I find that I use the guages most often. I set it at idle, then again as Kurt says, just beginning to roll up out of idle, and then again up around 3K RPM or so. Some argue that the last is not needed; but for me, it's where I ride, not at idle...........

I hope this helps you; but am really concerned about your rubber O rings and diaphragms as they can deteriorate in one seasons riding with this fuel the gov is foisting upon us.........Good luck Bud........God bless.......Dennis

 

[20774]

and then again up around 3K RPM or so. Some argue that the last is not needed; but for me, it's where I ride, not at idle...........

True, but at the higher RPMs, the butterfly is pretty wide open and any small changes has little or reduced effect on the change in vacuum. At low RPMs, small changes have a bigger effect, so it's easier to hear/see what's going on. If your cable tension is changing that much, maybe it's time for new cables!!

 

[drj434343]

Update

I've had a chance to spend more time on this issue. Thanks to all for the excellent advice so far.

I reset everything before starting the bike cold. Idle mixture screws out 2 turns, and idle screws in 1 turn from first contact with rocker. Both throttle cables with ~3mm of play. I double checked that the choke cables also had play when the choke was off.

The bike started right up on the second choke setting (most rich). Stepped down to first setting after a few seconds and let it idle for a minute. Test drive was successful, the bike actually felt pretty normal.

Got back, removed carbs completely, disconnected ignition and coils, removed spark plugs.

Left cylinder, previously 105 PSI now shows 150 PSI
Right cylinder, previously 85 PSI, now shows 142 PSI

So, here's a good data point that not removing the carbs can affect the pressure reading by as much as 60%!

Furthermore, the new difference between my cylinders is ~6%. I consider this acceptable, but welcome comments here.

So now, the next step is to re-try the syncing instructions with a new trust in my compression.

Side question, is it valid to use exhaust temp differences as a sync indicator for mixture? On race cars, this is often the case by either using 1 pyrometer or 1 oxygen sensor per cylinder to understand the mixture differences. I don't see that talked about on BMW bikes. Does anyone use that information when syncing?

 

[20774]

Furthermore, the new difference between my cylinders is ~6%. I consider this acceptable, but welcome comments here.

...snip...

Side question, is it valid to use exhaust temp differences as a sync indicator for mixture? On race cars, this is often the case by either using 1 pyrometer or 1 oxygen sensor per cylinder to understand the mixture differences. I don't see that talked about on BMW bikes. Does anyone use that information when syncing?

I think the talked about spec on difference is 10% so you're well within that range.

As for using exhaust temp difference, typically that's not really mentioned. What I would do is finish the synch the preferred method so that the bike runs as best as you can that way. Then see what the exhaust temps are. If they're close in temp, while not proving the theory, it might be a quick way for you to monitor things in the future. If you feel that the carbs are off for some reason, first check the temps and see if they have drifted apart.

 

[dennisdarrow]

I really have never heard of taking temp readings to check out the mixture for a small type engine.........Can't see why it cannot be useful information. In the past, seldom heard anymore, was to take plug readings over a short distance to see how the mixture is doing........Whole nother thread here; but it is useful and was once "the way"

Goooood deal that you got the idle figured out........On your ride, when you are cruising along at 4k RPM at maybe 50 or 60 mph....do a roll on to full throttle........is there some hesitation there????????..........LOTS???????.......or just a really good reach down and you can feel that torque come on????..........I ask this because this is a really good indicator of how your diaphragms are. If it seems to bog down.....probably holes in the rubber.........

So.......get it idling like you want it..........

have perhaps 2mm of play there at the carb for the throttle cable ends.........The old way to do it is to eyeball one throttle arm and feel over the tank on the other.........roll on the throttle just a bit...take out the play with the throttle on the handlebar.......you should see or feel which carb is coming up off the seat first............For me, I adjust the "loose" one so they are pretty close.......

Now.......hook up your guages.....manometer...whatever.....lol.....my mercury one broke and I made one of the $4 ones that works just fine..........Anyway.......check out if they are pretty balanced at idle.........then just roll on.....barely take the engine up off of idle.....adjust the weak one up to the strong one.......or do the opposite...................For me, I hesitate to give the cables any more slack, so that comes into consideration...........

Now.......get them balanced.........and as mentioned earlier.......I.........NOT ALL FOLKS BUT SOME..........roll the RPM's up to 3K or so and check it there..........

NOW.........RIDE THAT PUPPY.............don't forget to put the screws back in the vacuum ports on the carbs.........HAVE FUN and God bless.....Dennis
PS...........before you do any carb adjusting........take it for a 10 mile or so ride to warm things up......otherwise YOU WILL....get lousy settings that mess up your day

 

[drj434343]

This is getting to be frustrating now. I can't find a sweet spot.

I should clarify first that I double checked my valves before starting. I also tested the full roll on throttle at speed and experienced no hesitation. It moved!

I should also ask for clarification. When you all say 2 turns out, do you mean 2 180 degree turns, or 2 360 degree turns? I was originally assuming 2 360 degree turns, but at that position, it seems to run pig rich, the idle never wants to come down. In fact, when experimenting, it runs best at 2 180 degree turns. Any more out then that, and there is a huge delay before the engine wants to come back down to idle. Sometimes it will hang at 1500 for a long while before going down. Anything around 2 180 degree turns and it seems to run lean. It will hit and idle where I have it set (around 1050 RPM), but it wanders around that at least 100 RPM in a haphazard way. Anything more turns in then that and I start getting popping during deceleration.

So, it seems there is overlap between low chugging wandering idle with some popping and hanging high RPM's coming off speed that never seem to come down. I can't seem to find a reasonable place in between. The frustrating part about this is that the bike was tuned and ridable before I started mucking with it, so I know there must be a way. It just seems like there is too much noise in the system (RPM fluctuations at idle, inexperience with listening for when one side "struggles"), that trying to tune in 1/4 or 1/8 turns on those idle mixture screws isn't helping me get a handle on how the bike responds.

I need to walk away for a while and think about this a bit more.

 

[20774]

One turn should mean 360 degrees. Popping on decel means it's too lean. Are you sure there are no air leaks between the carb and the head? Often times, an air leak will create problems tuning and running. Another thing...is the pulse-air system still installed on this bike? I wonder if that could be causing some issues and/or some of the connections are loose.