Airhead R75/5 poor idling and carb synchronization issue

[htwashere]

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.

Hi Kurt, yes, I have. I made sure that when the choke is off, that I have a bit of free play in the choke cable. Mind you, all things I have said here, I am doing them as a newbie, it may not represent what the reality is

 

[ebeeby]

Beginning to suspect those fuel filters....

 

[htwashere]

Beginning to suspect those fuel filters....

One of these days, I am going to remove those fuel filters and try again...damn, running out gas line. More shopping to do.

 

[20774]

Hi Kurt, yes, I have. I made sure that when the choke is off, that I have a bit of free play in the choke cable. Mind you, all things I have said here, I am doing them as a newbie, it may not represent what the reality is

I would also support that by actually looking at the stops on the inside of the carbs. Be sure that the lever arm, to which the cable is attached, is actually touching the stop...I believe it should be all the way down. You could have slack in the cable but the lever didn't actually find the stop.

 

[20774]

One of these days, I am going to remove those fuel filters and try again...damn, running out gas line. More shopping to do.

Have you done some fuel flow tests? Get something to catch the fuel (preferably something that is graduated like an old science lab beaker) and try removing the fuel line input to each carb, initially below the filters. Then turn on one petcock, measure the flow to each carb...of course, have the opposite side stopped up with something. Then do the other petcock. Do it with both petcocks open. Do it with the gas cap removed. Should be at least 200cc per minute for each carb.

 

[htwashere]

Have you done some fuel flow tests? Get something to catch the fuel (preferably something that is graduated like an old science lab beaker) and try removing the fuel line input to each carb, initially below the filters. Then turn on one petcock, measure the flow to each carb...of course, have the opposite side stopped up with something. Then do the other petcock. Do it with both petcocks open. Do it with the gas cap removed. Should be at least 200cc per minute for each carb.

Thanks Kurt, will add that into the to-do list.

 

[ccolwell]

Have you done some fuel flow tests? Get something to catch the fuel (preferably something that is graduated like an old science lab beaker) and try removing the fuel line input to each carb, initially below the filters. Then turn on one petcock, measure the flow to each carb...of course, have the opposite side stopped up with something. Then do the other petcock. Do it with both petcocks open. Do it with the gas cap removed. Should be at least 200cc per minute for each carb.

Better yet do it with the float bowls removed. Shouldn't make a difference with the carbs rebuilt but you'll know for sure. If there is reduced flow, then work backward, i.e., check the way Kurt recommends, then from the petcock itself.

 

[ccolwell]

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:

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?

Best.
Henry

IIRC the bubbles are normal. Still, see my note above re fuel flow.

 

[dennisdarrow]

I would also support that by actually looking at the stops on the inside of the carbs. Be sure that the lever arm, to which the cable is attached, is actually touching the stop...I believe it should be all the way down. You could have slack in the cable but the lever didn't actually find the stop.

Actually, once upon a time, the owner's manual suggested putting a piece of paper between the stop and the cable as a "test" device for when JUST ENOUGH SLACK WAS IN THE CABLE...………..

 

[htwashere]

Dear all, this time I hope I solved the mystery. As many of you suggested for me, it was indeed the choke. This time after a while riding, my idling was very rough (or sudden high idling) again at full stop. I had a screwdriver with me so I decided to push the left choke cable down manually (as far as I can, only about 1mm or so), and then magic appears that's all it takes to get the idling back in order. After that, I was able to maintain that idling at all full stops - mind you I have only found this in the last 10-15 minutes of my ride back home, so I don't know yet if this will last.

Another topic: once returned home, I decided to do the plug shorting again. The left side is as expected. The right side is still not shorting very well (does not seem to want to stall, very slow to come down on RPM). I'm beginning to suspect the coil or the sparkplug wire - I will check into that in the future.

For the choke adjustment, I'll try to upload a video here for other's refence since this seems to be a very common problem, and it only takes a fraction of the choke to upset the balance. Thank you all MOA'ers, you have been a great support resource.

PS: here is the video link: https://drive.google.com/open?id=1LHX0vsAqFRv4sp7xX1BPE66noQehnUBI

Henry

 

[pauls1150]

That the rpm doesn't come down when shorting one side sort-of indicates that the coil + cable + cap + plug are prob'ly ok... Need a close look at the advance mechanism: first check the actual timing on the "good" cylinder, then compare that to the "bad" side, at both static and running/advanced.

A few things can happen down there: the end of the cam (or the bearing it rides in) may be wobbly (see if the timing isn't "pretty darn close" to each other); the advance mechanism (and therefore the points cam) that the points' "finger" rides on may be sticking (verify smoothness, no burrs, and apply just a little high-temp grease); the springs themselves may be stretched by age, so the timing can't immediately return to "less advance" at lower rpm.

Anybody: What is the quick & easy way to verify that the points aren't the fake Chinese version?

 

[20774]

Anybody: What is the quick & easy way to verify that the points aren't the fake Chinese version?

Past post on the difference in the points:

https://forums.bmwmoa.org/showthrea...e-Points-Alert&p=478957&viewfull=1#post478957

Pretty obvious. Plus, with the Chinese points, I don't think it was even possible to get any where near the correct timing...the rubbing block was so tall.

 

[htwashere]

That the rpm doesn't come down when shorting one side sort-of indicates that the coil + cable + cap + plug are prob'ly ok... Need a close look at the advance mechanism: first check the actual timing on the "good" cylinder, then compare that to the "bad" side, at both static and running/advanced.

A few things can happen down there: the end of the cam (or the bearing it rides in) may be wobbly (see if the timing isn't "pretty darn close" to each other); the advance mechanism (and therefore the points cam) that the points' "finger" rides on may be sticking (verify smoothness, no burrs, and apply just a little high-temp grease); the springs themselves may be stretched by age, so the timing can't immediately return to "less advance" at lower rpm.

Anybody: What is the quick & easy way to verify that the points aren't the fake Chinese version?

Thanks @Pauls1150. A noobie question: how do I check timing on "good" versus "bad" cylinder? do you mean by shorting one side and look into the timing hole to see if the timing mark is still present? if yes, when shorting the "good" side, would my engine not stall quickly at static idling? RE: points: mine is supposed to be the "noris" points, if that makes any bonus point.

 

[pauls1150]

Assuming that your Static timing (engine off and turned over by hand) is correct (or pretty close) -

The "easy" way would be - You want everything hooked up normally, no shorting here. Use a standard old-fashioned timing light; a clip-on type (inductive pickup) is best, and it's even better if you have an extra spare battery to power it so there's no extra current draw from the bike.

With the engine running, at LOW idle you should see the "S" (static) mark when shining the timing light thru the window onto the flywheel, it's ok to be a little off here. Note that timing WILL be affected by the points' gap - so be quite sure that the gap is correct or just one RCH wider (to allow for wear of the rubbing block over time).
Switch the timing light's pickup to the other side, and you "Should" see the same picture in there.
Slowly open the throttle, and the mark will move out of view. Go to about 2000-2500 rpm, 3000 rpm on later models, depending on which advance mechanism the factory (or somebody else) installed. Now the timing should be fully advanced, and you should see either an "F" and a dot, or a "Z" and a line, again depending on what the factory put in there (which flywheel). This mark should be fairly centered in the hole; some holes have a little notch to line up to as an attempt to reduce the visual parallax ("out of alignment with the user's eye").
Switch the pickup again, and you should get the same picture.

A difference in these views - or seeing that the mark does not seem to be stationary - indicates a wobble on the end of the cam where the advance mechanism is mounted; this may be caused by somebody cranking down too hard on that little bolt in the center of the advance unit, there's not a lot of meat there.

It's usually more important to get the advanced setting correct, since at higher rpm, wrong ignition timing is more likely to cause some damage. On the other hand, the low-rpm timing will directly affect starting and idling, so a compromise may have to be found. Again, note that the points gap affects this, at ALL rpm, so if you change that, re-verify the timing.

Hope this is reasonably clear; somebody else may explain it more simply, or correct me if my memory is foggy again...

Paul S

 

[htwashere]

Assuming that your Static timing (engine off and turned over by hand) is correct (or pretty close) -

The "easy" way would be - You want everything hooked up normally, no shorting here. Use a standard old-fashioned timing light; a clip-on type (inductive pickup) is best, and it's even better if you have an extra spare battery to power it so there's no extra current draw from the bike.

With the engine running, at LOW idle you should see the "S" (static) mark when shining the timing light thru the window onto the flywheel, it's ok to be a little off here. Note that timing WILL be affected by the points' gap - so be quite sure that the gap is correct or just one RCH wider (to allow for wear of the rubbing block over time).
Switch the timing light's pickup to the other side, and you "Should" see the same picture in there.
Slowly open the throttle, and the mark will move out of view. Go to about 2000-2500 rpm, 3000 rpm on later models, depending on which advance mechanism the factory (or somebody else) installed. Now the timing should be fully advanced, and you should see either an "F" and a dot, or a "Z" and a line, again depending on what the factory put in there (which flywheel). This mark should be fairly centered in the hole; some holes have a little notch to line up to as an attempt to reduce the visual parallax ("out of alignment with the user's eye").
Switch the pickup again, and you should get the same picture.

A difference in these views - or seeing that the mark does not seem to be stationary - indicates a wobble on the end of the cam where the advance mechanism is mounted; this may be caused by somebody cranking down too hard on that little bolt in the center of the advance unit, there's not a lot of meat there.

It's usually more important to get the advanced setting correct, since at higher rpm, wrong ignition timing is more likely to cause some damage. On the other hand, the low-rpm timing will directly affect starting and idling, so a compromise may have to be found. Again, note that the points gap affects this, at ALL rpm, so if you change that, re-verify the timing.

Hope this is reasonably clear; somebody else may explain it more simply, or correct me if my memory is foggy again...

Paul S

Thank you Paul, very insightful. I will give it a try.

 

[pauls1150]

One more thing occurred to me - Note the brightness of the strobe. If the light on one side is dimmer than the light on the other side, or is not as steady and regular, then the dim side has an ignition problem (bad coil or cable or cap or plug).

 

[happy wanderer]

One more thing occurred to me - Note the brightness of the strobe. If the light on one side is dimmer than the light on the other side, or is not as steady and regular, then the dim side has an ignition problem (bad coil or cable or cap or plug).

What type of timing light does that? My timing light is powered from the battery using two clips and uses an inductive pickup. If I understand it correctly the power for the light comes from the battery and the timing of the strobe comes from the inductive pickup. I don't see the relationship between the intensity of the strobe and the timing signal.

Can you explain how that works?

 

[pglaves]

What type of timing light does that? My timing light is powered from the battery using two clips and uses an inductive pickup. If I understand it correctly the power for the light comes from the battery and the timing of the strobe comes from the inductive pickup. I don't see the relationship between the intensity of the strobe and the timing signal.

Can you explain how that works?

Years ago I had a timing light that was placed in series with the spark plug. One lead connected to the plug, the other to the cap on the wire. It was rendered obsolete by the much brighter inductive type you described.

 

[happy wanderer]

Years ago I had a timing light that was placed in series with the spark plug. One lead connected to the plug, the other to the cap on the wire. It was rendered obsolete by the much brighter inductive type you described.

Yeah, I still have one of those (bulb burned out after 30 years!) and a newer inductive one. Some of the really old ones had a neon bulb in them that worked in series. They _may_ have varied bulb intensity based on the spark energy but I do not recall that they did. But since the power for the tool is coming from the battery I don't see how the inductive trigger switch would have anything to do with the bulb intensity.

 

[pauls1150]

The power for the light can come from anywhere, it does NOT have to be the bike. A separate battery eases the load on the /5 electrics and doesn't introduce another variable.

I have seen first-hand a couple of times where the strobe was not as constant or regular as it should have been - which may have caused the effect of "not as bright" - and that did point directly to the issue in those cases. This was with a Craftsman inductive light, which I still have in my garage but haven't needed for a while...