Getting familiar with her personality - idle mixture, cold start, tinging noise - 1982 R80ST

[bmcdonou]

Hi all - Just put the first 30 miles on a new-to-me ST. I did all the fluids, head bolts, valve clearances, new tires, and rebuilt carbs. This past weekend I did some break-in rides and balanced the carbs. Now I am getting more familiar, I have some questions on things I've notices but don't know if they are typical or things to be concerned about.

Idle mixture - Is it normal to end up at 1 3/8 turns out when base setting is only 1/2? After rebuilding the carbs I set the idle mix to the base for my carb numbers, which was 1/2 turn. She ran okay like that on the first ride (was a quick one at night), but during the more proper warm up ride to balance the carbs, she ran like crap. Hanging idle, some backfires, stuttering under throttle. Warmed up I set the idle mix following what thought to be the right process - back out (richer) until RPMs pick up but stop improving beyond that, back in (leaner) until it RPMs drop, find that line, back out (richer) 1/8th. I don't recall where I ended up in total but it wasn't far from 1/2. After doing this the idle was great, but any throttle would result in backfires, and eventually the right cylinder died (I checked fuel was in bowl). Given the backfiring, I assumed she was way too lean. So I started over but this time at 1 full turn and repeated the process. I ended up at 1.25 and then added the extra 1/8th, so 1.375 total. My thoughts --> I assume it is not normal to end up so far from the base setting, but better to be rich and lean. She runs fine at this setting with no real issues (little cold to start - see below), I don't smell an overly rich mix when she run in the garage, etc. but I will check the plugs after 50 miles or so. Also - why does the idle mix impact performance under throttle? I thought that would have been the jets.

Cold to start - how long should it take for choke to no longer be needed? I recognize some bikes hate running cold, but I also know that carb sins can be overcome with extra idle screws / choke. I've read the threads where people have all sorts of experiences they seems okay with. So my real question is how does a properly tuned machine behave? With the carbs balanced and idle just north of 1k when fully warmed, she requires a little throttle to start, even under choke, and requires choke for a while as well as some throttle to hold her at 1k-1.5k until fully warmed. This takes several minutes or a few blocks if I get impatient and take off. Is this normal SOP for these BMW airheads? I read snowbums article on this and I believe he said it was? My only real comparison is my other bike, Honda cb550 that only requires about 10 seconds of choke if having not run in over a day. So just curious if this experience is normal, or indicative some thing else I may need to address.

Tinging noise while running (added video) - I recognize bikes can be noisy things and not hard to convince yourself that every little noise is something terribly wrong. For example, the Hondas valves are extremely clicky and there are many threads of people on a fools errands to solve them. So with that, I hear a 'tinging' noise, which I believe is verberating from the heads. I don't know enough about these machines to know if that is something bad or if it is just how vibrations and sound travel through those heads with the long/deep air channels. I don't know how much dampening gaskets provide, but I suspect mine are rock hard. They were for the value cover. They are not leaking so I decided for now just to leave them as they are. But perhaps a culprit, if there is one.

That's it for now. Appreciate the help in advance.

 

[bmcdonou]

Added video Here

 

[GTRider]

Valve clearances have been set cold to correct clearance? And when the valve clearances were being set, was the rocker end play checked or set? With each cylinder at TDC on compression stroke, the rockers should be able to move end-to-end only enough to cause a slight “ridge” of oil to appear at the end of the rocker when the rocker is squeezed against an end block.

Best,
DeVern

 

[olironrider]

… it is not a “choke”, restricting air flow to create a higher vacuum and drawing in more fuel. It is an enricher or better thought of as a Starting Carburetor. The starting carburetor has its own air & fuel intake, multi-ported metering device and, own outlet to the engine.

By “goosing” the hand throttle while enricher is set for richer starting, lever on airbox, you effectively shutdown the enricher as, the “main” carburetor is less restrictive than the Starting Carburetor (enricher) so… airflow goes through the main carb.

“Goosing” the throttle is a hold-over from some automotive carburetors having Accelerator Pumps within them; raw fuel is literally “squirted” into the carb Venturi and intake manifold(s). The BING constant velocity carburetors do not have accelerator pumps.

Let the hand throttle closed to its idle setting on the main carburetor(s); allow the starting carburetor(s) to do its work. The Idle Mixture Setting Screw is part of the main carburetor circuit, as is the Idle Speed Screw. Everything else is jetted, needled, drilled.

A’yup… it’s a “dance” you have to learn as, the Airheads are cold-blooded.

 

[stevenrankin]

I have pretty bad hearing so I am afraid your video didn't do much to help. I will say however that IF you have the valves adjusted properly, airheads are not quiet as a mouse. There is some valve noise. LOL, there is a sweet spot for noise, too quiet, valves too tight, too noisy, too loose. Usually when they get out of adjustment, idle goes off and the bike won't idle properly.
Carb screw adjustments are starting points, once you balance things they can be set differently than what is recommended.

I had two bikes, a R80RT and a R100RS. The RS warmed up quick and I could turn the choke off fairly soon, the RT takes a bit longer to warm up and the choke stays on longer, maybe a half mile down the road? Welcome to the world of airheads. St.

 

[Seaweed]

That idle mixture screw along with float level will be in control up to about one third throttle opening, then the needle/jet will take over. Idle mixture screw not just for idle.
IIRC, that screw is an "air screw" . Turn out to add air and lean the mix. In to enrichen the mix.

 

[20774]

That idle mixture screw along with float level will be in control up to about one third throttle opening, then the needle/jet will take over. Idle mixture screw not just for idle.
IIRC, that screw is an "air screw" . Turn out to add air and lean the mix. In to enrichen the mix.

On the 1970-on carbs, especially the CV type carbs, the screw controls the fuel mixed in at idle. The slide carbs and pre 1970 bikes, it was indeed an "air" screw.

 

[bmcdonou]

Very helpful. So I am not crazy with the idle mix screw being more than a 3/4 out from the base 1/2 turn setting. Cold throttle starts and choke/enricher running is also not abnormal nor indicative of something sinister. This is good to hear.

I did not specifically check the end play when I did the valve clearances. I wiggled and pulled to make sure nothing was loose, and don't recall feeling really any free play, but didn't look that closely. I went back to the manual and now see it called out as a potential for noise, and read snowbum's article on this - exactly same as you describe, DeVern. So I will go start here.

In revisiting the manual, I should also admit - in case it had any implications - that I did not loosen the head nuts before retorquing. Rather, I set the torque to 25 and just checked them as they were. Only one I recall moved slightly, and then I did the valve clearances, for which I brought the exhaust in just a tiny amount. Logic for this was that I didn't want to risk stripping threads or loosening a rod as I was not ready to dive into larger problems - just wanted to make sure they were not loose. Let me know if there are strong feelings on omitting this part of the procedure.