Monday, April 11, 2016

Learning to clean Fiona's Carburetors

I have always a bit hesitant to mess with the insides of carburetors, limiting myself to perhaps changing a main jet for a different altitude, or spraying what I could of one with carburetor cleaner.

Most of my interactions had been more of the "set the idle and balance them" variety, going through the process of first using oil filled manometers, to the spark plug shorting method to what has worked best for me, the Harmonizer digital vacuum balancer.

So with Fiona, I would go through what I knew.  She had ran pretty rough at first and slowly I made progress but the achievement of a steady idle and smooth sound eluded me till today.

This past weekend, I took the plunge and basically took apart everything on the carburetors but the mechanical linkages.  The only thing I did there was to unhook both the throttle and choke cables from their respective anchor points on each carburetor so I could remove the top of the carburetors to inspect things.

Disconnecting the cables from their anchor points on the side of the carburetor

I found a small tear on the diaphragm for the right carburetor and while the local consensus was that it probably was sealing OK, I went ahead and replaced it and while I was at it, the diaphragm for the left carburetor as well.  These diaphragms are pricey things too, I got two from Dick Paschen, the CO Air Marshall for the Airheads MC club and ordered two replacements.

Old diaphragm on the left, it looks worse that it actually was, they
just tend to distend once unmounted from the carburetor.

I had removed the main jets on both carburetors and sprayed down into their mounting holes, noting at that point that the o-ring on both were dry rotted.  But I'd not gone and also removed the needle jet and atomizer before, once I got those out, a thorough cleaning ensued.

I'd loosened the pilot jets but had not been able to extract them until I found some forceps.  Then I was able to pull them out, ideally, you should be able to just turn the carburetor over and it drops out.  The pilot jet in the right carburetor was in a sad state:

Right carburetor's pilot jet
The one in the left carburetor was much better but still
had a dry rotted o-ring, though not as bad as above!

I'd not given the air mixture screw much thought, concentrating on the pilot and main jets, but on Sunday after the tech day, decided to remove them to check.

Air mixture screw for the right carburetor.  Yikes!
The left carburetor's mixture screw was OK but
again, with a dry rotted o-ring.

It is no wonder I could never get the idle to be steady and consistent!  Fuel and air were leaking past these points and causing intermittent surges.

Oh, and I also found that the throttle return spring on the right carburetor had become stretched and distended, so that was replaced as well.

right side carburetor carburetor, old on the left

left side carburetor diaphragms, old on the left

The rest of Sunday was spent in thoroughly, this time, cleaning out the carburetor bodies.  Made sure to clean out the mounting holes for the jets and mixture screws.  Using compressed air, and with no danger to the diaphragms now, I blew out what grit I could find in the carburetor passages.

After cleaning things, I borrowed the main, pilot jets and mixture screw from Brigitta, put them in the right carburetor on Fiona and the results were quite promising!

This afternoon I procured some o-rings from Clem C, former owner of BMW of Denver and local airhead Guru and replaced the o-rings on the main and pilot jets, and the mixture screw on both carburetors.

Put the carburetors back together, hooked them, mounted them and fired up the engine.

The engine ran much, much smoother and I was able to get a nice and steady idle.  Took Fiona for a test ride and she did feel better at first.  After a twelve mile test ride though, she started stumbling slightly at low speeds.  

So, some progress made, her engine is still not as smooth as Brigitta's R80 engine, but we're getting there.   The spark plug on the right cylinder shows signs of too much fuel in the carburetor mixture.

Clem is finishing the refurbishment of two carburetors that fit Fiona and I'll be exchanging Fiona's current ones with those sometime around this coming weekend hopefully.  I hope this carburetor swap will result in same running conditions as I get from Brigitta, my '87 R80 Airhead.

So, got over my nervousness of working with the innards of carburetors.  Not to the point of completely taking one apart yet mind you, but cleaning out the inside of one is not a big deal for me now.

Basically, everything to the left of the red line above is what I was able
to take apart and work on.  The mechanical linkages on the right half of the 
diagram I didn't touch except to unhook the throttle and choke cables.

Above diagram and listing are from the Clymer Manual for BMW Airheads

Update: 12APR16.  Found green corrosion in the plug connector on the right side coil, cleaned that up, checked the left side coil, it was fine.  These are 6 volt coils wired together inline.

Swapped the spark plug cables to see if the sooty plug condition followed the plug, a ten mile test ride later, doesn't seem to.  Kind of still points to the carburetor still.


Unknown said...

You are a brave man. When I get settled up near Fairbanks this Fall, I will attempt the same. I'll have all winter to get the R50 carbs done with Bob and Richard's help. (I'm leaving the R50 in Ketchikan and taking the R100 and Ural up North).

redlegsrides said...

Don't know about brave, Bruce....since the right cylinder is still running rich, polished up the hole that the fuel needle seals when the floats are in the proper position. It was filthy as well. Tomorrow's test ride will bear out whether that helped or I made things worse.

RichardM said...

In these days of electronic everything, working on carbs will soon be a lost skill. I guess if I was faced with those sort of pieces, I would just completely disassemble them (one at a time) and soak everything in solvent or carb cleaner. Then blow out all of the passages and replace every gasket and o-ring before even trying to adjust them. The CV carbs are pretty straight forward after you've taken a few of them apart.

redlegsrides said...

As with most things RichardM, I'm learning the hard way. But yes, these are pretty straightforward.

David D'Angelo said...

Hi there - so bummed I missed the tech day! Trying to get a hold of Clem - would you have his email address by chance. I've got a 720 number for him

redlegsrides said...


send me an email info AT