Airhead R75/5 poor idling and carb synchronization issue

[htwashere]

Dear fellow MOA'ers, I have some puzzling issues with my R75/5 carb idling and synchronizing. I have read as many journals and how-tos on carb synching and adjustment before attempting and yet, I still have issues, hoping you can help shed some light:

A bit of background, mine is a 1971 R75/5 that i had purchased a couple years ago in hope to restore to good running condition. The bike had been sitting for many years by previous owner and when I took over, I worked on it here and there and managed to get it working and is ridable but it has inconsistent rough idling. I decided to have my carbs professionally rebuilt by Bing USA. This weekend I gave the bike an overall tune up treatment by changing out the plugs, new contact point, adjusted all the valves, timing etc. For the idle mixture screw (located at the bottom of the carb), I had it set to one full turn CCW from full right as per recommended setting. The bike started instantly which I was happy. But then I noticed a few things:

1) During cold start, the bike feels really smooth and nice engine sound. After 10 min warmup ride, I came back and tried out the shorting method, the left side shorts out as expected, but when I tried to short the right side, it almost seems like the shorting never happen - what I mean is, the engine reduces RPM ever super slowly and it feels almost unnoticeable, like I did not short the right side spark plug at all - I cannot imagine that it is normal? Can you tell me why that is ?

2) after fidling with the synchronization - honestly I don't think I did any better for the engine; I went back out for the test ride (low to moderate speed only). The bike rides well but my idling is super low at full stop, in fact, the engine would stall if I do not blip the throttle up. So I proceeded to adjust the top screw idle stop but then sometimes, the idle would bounce up high - All to say that it is very sporadic idling

3) When I got back and checked the spark plugs, the plugs looked really black with soot so I think my fuel mixture is way too rich on both sides .

What do you think I have done wrong? or my next step should be?

Your help is appreciated.
Henry

 

[20774]

Henry -

How are the valve clearances? Poor idling can be attributed to clearances being too tight.

What are your carb numbers? I believe owners of the early R75/5 have complained that the early carbs presented problems and difficulty in working with. Seems that some have sought out later model carbs which helped tremendously.

 

[htwashere]

Henry -

How are the valve clearances? Poor idling can be attributed to clearances being too tight.

What are your carb numbers? I believe owners of the early R75/5 have complained that the early carbs presented problems and difficulty in working with. Seems that some have sought out later model carbs which helped tremendously.

Hi Kurt, thanks for quick reply. For valves, I set at .006" for Intake and .008" for Exhaust, as per the BMW manual. For carb number, I'm told mine is a 64/32, if that makes any sense? the carbs are in good condition and now that they are rebuilt, I can only hope that they are working as good as new.

Henry

 

[20774]

Is there a number after 64/32? Maybe 1 on one side and 2 on the other? Here's a couple of other threads on early R75/5 carbs:

https://forums.bmwmoa.org/showthread.php?94196-Carb-Replacement-72%92-75-5

https://forums.bmwmoa.org/showthread.php?89676-1971-BMW-R75-5-idle-baseline-setting

 

[htwashere]

Is there a number after 64/32? Maybe 1 on one side and 2 on the other? Here's a couple of other threads on early R75/5 carbs:

https://forums.bmwmoa.org/showthread.php?94196-Carb-Replacement-72%92-75-5

https://forums.bmwmoa.org/showthread.php?89676-1971-BMW-R75-5-idle-baseline-setting

Hi Kurt, I found it, it says 64/32/313 on the left side; 64/32/314 on the right side. I'm signing off for tonight and if you're writing me again, I will respond tomorrow. Cheers,
Henry.

 

[natepetersen85]

Make sure your float levels are correct. If the seats leak or if they are set too high she'll be rich. Especially coming to a stop the fuel will surge forward in the bowls and itll want to drop the idle and stall, the harder you brake the nore noticable it will be.

Sent from my SM-G960U using Tapatalk

 

[20774]

Good call on checking float levels. With the numbers 313/314, the carbs are clearly not the earlier type which presented problems.

 

[htwashere]

Good call on checking float levels. With the numbers 313/314, the carbs are clearly not the earlier type which presented problems.

Thank you Kurt and @natepetersen85, I will look into that and report back in next few days. Btw, I had installed a fuel filter on each side, and i noticed that with the tight clearance, the filter and the petcock and the T-joint are very tightly joined together, I wonder if that could restrict the fuel flow somehow and cause the bike to stall at braking to a full stop? I feel it should not be the issue because I have ridden the bike on highway with the fuel filters installed and I don't feel any fuel starvation before.

The other thing is that since the spark plugs are so black so quickly, I'm pretty sure I need to re-calibrate the fuel mix and retry.

Henry

 

[pauls1150]

If shorting one side has little or no effect, that indicates an ignition problem - coil, sparkplug cable, sparkplug cap, or the plug itself. Coils, cables, and caps are common issues with older airheads; if you have "resistance cable" and/or caps, it makes a LOT of difference which parts are in there. Wet plugs and/or rough idling are more symptoms of this, as is the problem surfacing after warm-up. Not recognizing this will sometimes cause the owner to adjust the "good" side to compensate, thus masking the issue.
You mentioned that you have swapped out the points; how about the condenser? And be careful not to pinch that little wire near where it passes through the ignition cavity wall when you reinstall the front cover. Were the old points in decent shape (except for "typical wear"), or did one side have a pit and the other a build-up of metal? That indicates a bad condenser. They can be intermittent, along with outright failure (open circuit or shorted).
And to expand on what Kurt said earlier - my habit is to always set the valves just a hair on the loose side.

 

[htwashere]

If shorting one side has little or no effect, that indicates an ignition problem - coil, sparkplug cable, sparkplug cap, or the plug itself. Coils, cables, and caps are common issues with older airheads; if you have "resistance cable" and/or caps, it makes a LOT of difference which parts are in there. Wet plugs and/or rough idling are more symptoms of this, as is the problem surfacing after warm-up. Not recognizing this will sometimes cause the owner to adjust the "good" side to compensate, thus masking the issue.
You mentioned that you have swapped out the points; how about the condenser? And be careful not to pinch that little wire near where it passes through the ignition cavity wall when you reinstall the front cover. Were the old points in decent shape (except for "typical wear"), or did one side have a pit and the other a build-up of metal? That indicates a bad condenser. They can be intermittent, along with outright failure (open circuit or shorted).
And to expand on what Kurt said earlier - my habit is to always set the valves just a hair on the loose side.

Hi Pauls1150, yes, I thought exactly of the same concern too (the shorting). I realize my idling is bad and needs to be fixed but the shorting issue definitely baffled me. And I do think because of that, I am over compensating the adjustment on one side like you said. I did suspect the coils or cables but didn't look over them yet. I had put in new plugs so I will for now eliminate that as a suspect. As for the old point, it looked pretty clean, just a tiny bump and I would attribute that to typical wear. I proceeded to replace them anyway since I was tuning up the bike. For the valves, I thought I did set them a bit loose as well.

Once I get a chance to go back to wrenching, I will re-examine all of the above ...will keep you posted.

H.

 

[1074]

Setting carbs

When you initially set the idle mixture to one turn out you are getting a base line to get the bike to start. Once it is running then you have to "tune the carbs." run the engine up to operating temperature which will take about 15 miles or so. Go to one side and turn the idle mixture screw in until the engine stumbles. Then turn the screw out slowly until it reaches maximum speed. You'll go out far enough to get the engine to slow down and then screw it back in to get a final adjustment.
Do the same for the other side. Now adjust the idle speed to even them out. You should of course, have slack in the cables when you do all of this!
Now that you have the idle speed where you want it, go back and readjust the mixture screws as you did before to get the highest idle speed. You may have to do this more than once. When you are happy with the idle speed and evenness of both sides go back and back out the mixture screws about 10 degrees. This will make the mixture just a bit rich and allow a smoother transition from the idle jet to the needle jet.
Now all you have to do is adjust the cables to make them even. As long as you have slack in the cables and there is roughness in the sync then you can hold one or the other cable housing tight to see which direction you have to go to smooth everything out. For example, if you hold up the cable housing on the right side and the engine accelerates smoother, then you have to take up more slack on the cable on that side. Once you've done this a few hundred times it gets to be easy!

 

[htwashere]

When you initially set the idle mixture to one turn out you are getting a base line to get the bike to start. Once it is running then you have to "tune the carbs." run the engine up to operating temperature which will take about 15 miles or so. Go to one side and turn the idle mixture screw in until the engine stumbles. Then turn the screw out slowly until it reaches maximum speed. You'll go out far enough to get the engine to slow down and then screw it back in to get a final adjustment.
Do the same for the other side. Now adjust the idle speed to even them out. You should of course, have slack in the cables when you do all of this!
Now that you have the idle speed where you want it, go back and readjust the mixture screws as you did before to get the highest idle speed. You may have to do this more than once. When you are happy with the idle speed and evenness of both sides go back and back out the mixture screws about 10 degrees. This will make the mixture just a bit rich and allow a smoother transition from the idle jet to the needle jet.
Now all you have to do is adjust the cables to make them even. As long as you have slack in the cables and there is roughness in the sync then you can hold one or the other cable housing tight to see which direction you have to go to smooth everything out. For example, if you hold up the cable housing on the right side and the engine accelerates smoother, then you have to take up more slack on the cable on that side. Once you've done this a few hundred times it gets to be easy!

Thank you @1074 BoxerBruce, it will be my 5th trial coming up, will be a long way counting to a few hundred times Cheers.
H.

 

[htwashere]

Thank you @1074 BoxerBruce, it will be my 5th trial coming up, will be a long way counting to a few hundred times Cheers.
H.

Greetings @1074 BoxderBruce and others, I would like to provide an interim report to get your expert opinions. So last night after work I took all your suggestions and re-checked all my settings. The point gap seem to be closed in more so I readjusted. Also the valves on the left side engine seem tighter so I also re-adjusted. I checked the fuel bowl and it is approx 22-23mm (left side seems a bit lower). Then I proceeded with the instructions provided by BoxerBruce to get the baseline setting for the idle jet mix screw (one full turn out). I also check the spark plug spark and there is a nice bright spark when cranked.

When the engine was cold, the bike starts instantly with choke, purrs and runs ever so smoothly. During my 15 minute warm up ride, the bike no longer stalls at full stop but the idle RPM is still very low. After 15 minutes warmup, I proceeded to adjust idling and this is my finding, I'll quote the step by step instructions from BoxerBruce:

1. Turn the idle mixture screw in until the engine stumbles: when I do this, instead of engine stumbling, mine speeds up even more?
2. Then turn the screw out slowly until it reaches maximum speed: once I have turned the screw in and start to back out, it seems very difficult to achieve another max speed
3. You'll go out far enough to get the engine to slow down and then screw it back in to get a final adjustment: my issue is when the bike is stationary at this point, the idling is very high, I cannot seem to get it slow down, it is running at least over 2000RPM. If I blip the throttle several times, then some time the idling would come back down but then it is as if it is very rough
4. I also want to point out that the exhaust fume is very smoky and foul smelling - especially on the right side, I'm going to assume that it is burning oil or my fuel mix is too rich?

By the time I completed this, it was closed to 11pm so I did not go back out for a test ride. I will do so another day when I get a chance.

Cheers,
Henry

 

[1074]

It's possible that the advance unit is hanging up.

 

[ccolwell]

I'm sure you already checked but make sure the sleeves between the carbs and heads are leak free.

 

[htwashere]

I'm sure you already checked but make sure the sleeves between the carbs and heads are leak free.

Thanks @ccolwell; actually I have not yet, I should the next time I look into this. The sleeves are pretty worn out currently. Thanks for this heads up.

 

[htwashere]

Thanks @ccolwell; actually I have not yet, I should the next time I look into this. The sleeves are pretty worn out currently. Thanks for this heads up.

Hello @ccolwell and others, here is another update. The sleeves are good, I did not find any leakage. This evening I took her out for a longer ride include a highway run What I noticed is this: when coming to a complete stop, the bike wants to stall due to low RPM idling that I talked about previously, then in about 15 seconds or so, it starts to rise up to 2500+RPM. This has been consistent at every traffic stop light, low RPM then slowly rise up to 2500RPM. When coming back at home, I pulled the spark plugs and this time they look pretty clean, it looks like the bike is actually running on the lean side because the colour of the plug head is dark'ish gray (closed to black). Any other ideas I can try to correct the idling issue?

Thanks. Henry.

 

[ccolwell]

I don't have much. 1) How did you check the sleeves? I'd spray WD40, or carb cleaner or water, at the sleeve connections and the sleeves themselves. I'd do this both cold and then fully warmed up. If the idle speed changes, replace the sleeves and the clamps. My R90S needed the clamps tightened more than seemed reasonable to eliminate leaking. 2) you say you warm it up for 15 minutes; all my airheads took 20 min. of riding before they idled correctly. 3) Rereading the original post I can't tell whether what you did initially made it better or worse. If worse, recheck all the mechanical work before readjusting the carbs. 4) aside from the shorting issue, it does sound more like the carbs to me. Nonetheless, on a bike this old I'd install new a new coil and plug wires. 5) check fuel flow. If original, replace the fuel lines, fuel filters and pull the petcocks to check the screens inside the tank. Unless the look is important to you, get some Motion Pro tygon fuel line. I don't think many people agree with me on this, but I think the ethanol does bad things to old style fuel lines.

 

[htwashere]

I don't have much. 1) How did you check the sleeves? I'd spray WD40, or carb cleaner or water, at the sleeve connections and the sleeves themselves. I'd do this both cold and then fully warmed up. If the idle speed changes, replace the sleeves and the clamps. My R90S needed the clamps tightened more than seemed reasonable to eliminate leaking. 2) you say you warm it up for 15 minutes; all my airheads took 20 min. of riding before they idled correctly. 3) Rereading the original post I can't tell whether what you did initially made it better or worse. If worse, recheck all the mechanical work before readjusting the carbs. 4) aside from the shorting issue, it does sound more like the carbs to me. Nonetheless, on a bike this old I'd install new a new coil and plug wires. 5) check fuel flow. If original, replace the fuel lines, fuel filters and pull the petcocks to check the screens inside the tank. Unless the look is important to you, get some Motion Pro tygon fuel line. I don't think many people agree with me on this, but I think the ethanol does bad things to old style fuel lines.

Dear ccolwell, thanks for more ideas. Here are replies to you, overall I think I have taken pretty much the same approach as you have listed. I'll answer them here in sequence:
1) check sleeves: I sprayed brake cleaner around the gaps, looking to see if there is any draft (and there is none), and the RPM did not fluctuate. I only did this during warm up, so I'll do it again next time after another riding.
2) warmup ride: actually I did more than 30 mins, I just generalized my description as 15 mins, I'm pretty sure the engine is warmed up. By the way, I actually think when the engine is cold, it sounds so much better than it is hot. When the engine is hot, it just seems like it is huffing and puffing (and the smoke smells very harsh).
3) Has it improved? yesterday was my 3rd trial, I must say it has improved, it was probably my best ride, hence I did not want to play with the jet mixture again - but the idling is still bothering me quite a bit. I never went back to the shorting exercise yet.
4) fuel lines: all new, I even changed out the line that connects both sides (tucked inside the air filter area). Speaking of fuel flow, I wonder if this sounds suspicious to you: I mentioned that I installed some fuel filter situated right under the petcock. Because of tight clearance, the filter ends are sandwitched tightly between the petcock and the T-joint with really no free play. When I come to a stop, I looked at the filter, and I noticed a surge (air bubble) in the filter as if, the engine is sucking gas faster than the fuel flow can provide, then the bubbles slowly go away and the filter is full of gas again - now, this can be totally normal as expected? I only look at it because I am looking at every nook and cranny currently - would you think that there is a fuel starvation issue going here? but if that is the case, why would the bike go smoothly at full throttle?
5) yes, some of the things I have not changed out are: the coils, the spark plug wiring. I will do that next - I am also going to replace those two springs that control the timing advance.
6) anything else I should change out while I make up the wish list?

Oh well, she is nearly 50-years old gal after all, so no miracles expected here. I will keep looking and tuning, will report back again.

Best.
Henry

 

[20774]

Have you confirmed that the choke is completely off when you turn it off? Running well when cold might be helped by a slightly open or functioning enrichener but then when the engine is warm, a partially open enrichener could be adding more fuel to the mixture, making it run rough.

The carbs were rebuilt by Bing, but you had to make the connections to the controls.