Carb Rebuild Planned For This Weekend

[kmisterk]

First things first. Start with the basics. Are you sure your gauge is accurate?
Testing Compression (the same and balanced on both sides, ~75 psi)
Insufficient compression.
Or improperly executed compression test.
You need to have the carb butterfly open and the slide jammed upwards to get an accurate reading.(or remove the carb from the head altogether) Ground the spark plugs to avoid coil damage.
If you're battery isn't providing adequate cranking power, boost it with your car.

If it is actually only 75psi, and the valves have some clearance, you're due for a top end rebuild...

Hmm. I probably didn’t test them correctly, but I did test them consistently, and was more hoping to see the *same* number.

Is there a step-by-step somewhere or a “why it is tested this way” document/article somewhere? I’d like to revisit compression and put that concept out of the possibilities of failure.

[gspd]

Is there a step-by-step somewhere or a “why it is tested this way” document/article somewhere?

Ya, here it is.
1- Strong battery and starter
2- Good compression gauge.
3- No intake or exhaust restrictions.
That's all.
Any carb will cause a restriction (lower readings) if the butterflies are closed.
CV carbs compound this if the slide is not also held wide open.
Exhaust restrictions are never really an issue, unless maybe a header pipe is crushed to shit from an impact or a catalytic converter is solidly blocked.

Airheads should be in the 120 to 160psi, depending on compression ratio of the particular model.
Under 100psi might run, but never properly.
My PD was 125psi with 8.5 pistons when new.
Now it's 155psi with new 9.5 pistons.

[kmisterk]

Is there a step-by-step somewhere or a “why it is tested this way” document/article somewhere?

Ya, here it is.
1- Strong battery and starter
2- Good compression gauge.
3- No intake or exhaust restrictions.
That's all.
Any carb will cause a restriction (lower readings) if the butterflies are closed.
CV carbs compound this if the slide is not also held wide open.
Exhaust restrictions are never really an issue unless maybe a header pipe is crushed to shit from an impact or a catalytic converter is solidly blocked.

Airheads should be in the 120 to 160psi, depending on the compression ratio of the particular model.
Under 100psi might run, but never properly.
My PD was 125psi with 8.5 pistons when new.
Now it's 155psi with new 9.5 pistons.

I'm hesitant to think compression is an issue, then, and I would say my testing method was flawed but consistent.

The bike runs great, when brand new spark plugs are installed, and then, over the course of the next 25 miles or about a half-hour of riding, symptoms of the mucked-up spark plug surface again, with sputtering, misfiring, backfiring, and the spark plug showing signs of running rich (Black with a wet look, smelling like fuel).

But then...I suppose I have yet to ride a "guaranteed properly running/working" airhead, so I only have its own performance in regard to the history of me owning the bike to compare against when it comes to how I *feel* it is performing.

Hard to say for sure, without someone more accustomed to how these *should* perform getting on it and riding it.

Even still, I think I will still continue to put a proper Compression Test on the agenda at some point during the weekend as I attempt to rebuild the carbs. There really is no point not to, especially since the carbs will be completely removed from the heads. You stated it was important for there to be no intake restriction, and having *no* carb installed would literally be as little "intake" restriction as possible, as it were.

[gspd]

.....as I attempt to rebuild the carbs. There really is no point not to....

Your putting the cart ahead of the horse.
Decent compression is THE FIRST THING to establish.
75 psi is indecent. Might start and run. Will never perform well.
Check it properly before investing a dime (and time) in other stuff.
Although not ideal, as low as 100psi will 'work' OK.

[kmisterk]

.....as I attempt to rebuild the carbs. There really is no point not to....

Your putting the cart ahead of the horse.
Decent compression is THE FIRST THING to establish.
75 psi is indecent. Might start and run. Will never perform well.
Check it properly before investing a dime (and time) in other stuff.
Although not ideal, as low as 100psi will 'work' OK.

Hmm...

I see your point. You're saying that despite whether or not I wanted to do a rebuild of the carbs anyway, testing the compression is an issue to remediate before fudging with the carbs, anyway?

Man, I really wish I had an acquaintance with experience riding these older airheads so they could tell me how they think it's running compared to a known-properly-running airhead of similar age.

[Kurt in S.A.]

Clearly you have an issue if you're fouling plugs so fast. Do a proper compression test and see what that tells. You might need to consider a leak down test if compression is still low...that will tell you where the problem is.

I usually rely on general miles per gallon as my indicator for the way I ride. I've been getting 40 mpg on my /7 for years, so any deviations from that will tell me something is wrong.

Beyond that, I'm constantly listening to the engine. I can easily pick out the "sewing machine" noises of the properly set valves ticking away. If that sound ever changes, then something is wrong.

You should keep your eyes open...the Airheads Beemer Club dAirectory shows two Airhead riders in town. Quite a few more BMW riders in the MOA Anonymous. Can't be hard too find with only 40K people there!!

Kurt in S.A.

[kmisterk]

Can't be hard too find with only 40K people there!!

Kurt in S.A.

Hmm. I was able to find a members list, but couldn’t see how to isolate it down by city. Only state. Is there a different directory (link? Or is it an offline directory?) that narrows down to smaller regions than state?

Also, I performed a few minor maintenance tasks on the bike per the replies here and from another forum I also asked, and have a bit of an update to post on this topic In a bit. Dinner is waiting, but I wanted to reply to
You first about the airhead directory you were referring to.

[kmisterk]

Okay, gonna do a quick followup with what went down today!

Thanks to all who helped with recommendations, I feel like the bike is in a *much* better place now, even if she still needs a bit of carb work.

Whelp, here we are:

1. Adjusted the floats.

as many have suggested both here and elsewhere, the floats were not closing the fuel inlet when they should, allowing for float bowl overfilling and leaking when sitting. I'm not 100% sure if/how this would affect the symptoms above, but hey. I was able to fix it after carefully bending the float arms to an angle that was, and I quote "Parallel to the bottom of the carb where the float bowl connects."

2. Tested the float levels.

Running the engine with the petcocks off for a while let me drain the carbs. Then, with the engine off once more, I let the carbs fill back up again. I didn't have any measuring device with a millimeter scale on it, so I just visually compared the two on a level surface, and as best I could tell, they were the same.

3. Enricher Lever and Cable inspection

It was recommended that I ensure that the enricher/choke cable, lever, and controls were all actually performing their tasks as designed. I had to adjust a bit of slack out of the cable and tighten down the lock nut, but even before doing this, the levers on both carbs were actuating as expected per the relevant position of the adjuster control on the handlebar.

4. Airbox and Air Filter Inspection (shudder)

This one was a doozy. I actually managed to grab a couple of pictures of this.

Check out the album here then come back to finish the rest.

Basically, the filter itself seemed fine, if only a bit dirty. Nothing worth replacing yet, but I'm exceptionally glad I checked it; The entire airbox below the filter was covered in a good amount of slime of some kind. Liquid, gross, consistency of oil, but colored weird. I couldn't tell what it was, but my best bet was that it was a mixture of oil and ... who knows what else. Is there somehow a way that oil can leak into the airbox like this? Also, what is the red/black tube cutting through the airbox? is it at all responsible for the liquid build-up?

5. Throttle Cable Rerouting and Adjustment

Lastly, inspired by the recommendation for checking the cables and function of the enricher, I performed similar inspections and adjustments on the throttle. I noticed both throttle cables were not properly seated in the rubber mounting fixture that hugs the main tube on the frame, so I remedied that, and then proceeded to adjust the slack/play on the cables. Eliminating all but a slight amount of slack/play in the cable, I tightened the lock nuts down, which were not at all tightened before, and freely rotating on the spindle.

After all this, the bike is now idling better, and, despite still sputtering a bit while revving (will probably be worse under a load), it is definitively running better than it was when I started.

A quick video to demonstrate its most recent running state can be seen below. Any hints as to what may still be up? I plan to replace the spark plugs once more when I can tomorrow (Friday the 18th) and assess once more.

[Rob Frankham]

Well, that explains the fouled plug then...

The 'slime' in the bottom of the air box is gearbox oil. The bolt loose in the box should be securing the airbox to the top of the gearbox (transmission on your side of the pond... ). The threaded holes in the top of the gearbox is open to the inside so, without the bolt in place, gear oil can be thrown through the bolt hole to where it will get ingested by the air tubes. Gear oil is a very good medium for fouling plugs!

Replace the bolt and I'll wager that problem (at least) will be solved.

Rob

[Kurt in S.A.]

Hmm. I was able to find a members list, but couldn’t see how to isolate it down by city. Only state. Is there a different directory (link? Or is it an offline directory?) that narrows down to smaller regions than state?

The ABC and MOA lists are available to paid members. You might need to spend more time near BMW dealers and see who walks in the door! Or talk to the parts guy and/or leave your contact info.

I notice in the ABC Airmail that a TechDay is coming up on August 21 in Tacoma at the Boxer Barn. Not sure how much wrenching will be going on, but might be worth a stop to get to know people...maybe even join!

Kurt in S.A.