Forum bmwbike.com

A couple of years back, I bought a pair of new Bing sidefloat carbs for my R60 as the pilot screw threads of the originals were stripped.

Since then, no matter what I do, I have not been able to get the mixture right at tickover and idle speeds. I have experimented with float heights, plastic float needles, brass niton-tipped float needles as supplied with the new carbs, and so on. Yes...I have ensured that the pilot jets are screwed home. The mixture is either too rich or too lean. No happy medium. It goes from one extreme to the other. At cruising speed and above, if I cut the motor, freewheel to a halt and remove the plugs, they have a nice rich brown colour. But after town riding, they are sooty. I don't mind a bit of soot but I mean SOOTY.

It's not, as far as I can see, an ignition problem. The mag is fine. The gaps are within a couple of thous of each other and the contacts open as equally as I can get them to open on both the S and F marks when timed statically. Testing with a strobe shows everything is OK for a touring motor. The plugs it seems to like best for cruising and road-burning are NGK BH8s - the recommended Bosch plugs do not last long, as it seems to need harder plugs than normal, friends' bikes running on BH7s or even BH6s (or ES if the motors are later) - and the air filter is clean.

The rings are almost new and have been run in correctly. I run on 98 unleaded, which it likes better than 95, occasionally topping up with LRP and, whenever I find it here in Paris, leaded petrol. No problems with valve seats, valves or seating. In other words, the motor is quite healthy. So...what's the story with these carbs?

Are they, like the new Amal I recently bought for a Triumph, simply badly machined internally? Visits to the dealer have met with that Sphinx-like reaction that tempts one to kick his windows in while calls to Bing have elicited the usual "Nobody has complained of this before...there must be something wrong with your motorcycle" kind of response. I reckon there must be a machining error of some kind in the pilot system.

Comments? Anyone had similar problems?

Prosper Keating

"... 1958 R60... At cruising speed and above... rich brown colour. But after town riding.... SOOTY... The plugs... are NGK BH8... The rings are almost new and have been run in correctly..."

I think your idle mixture could be too rich, or the new rings have not broken in and your plug is too cold to burn the oil fouling.

Turn the idle mix screw out 1/4 to 1/2 turn and see if it will idle OK and not foul the plugs as much.
OR
Check that the idle jet is correct for your carb number. You don't give the carb number, but the original carbs probably used a 35 jet.
OR
The NKG B8HS is a cool plug for broken-in rings. If your rings should not be broken in, the hotter NKG B6HS or BH7S might burn cleaner, or the Bosch W260T1. After the sooting has stopped, then go back to the cool NKG B8HS or Bosch W240T1.

Thank you very much for your reply, Mr Atherton. I should have quoted the carb numbers. The pilot jets are 35s.

However, experimenting with the idling screws doesn't achieve much. The mixture appears to go from excessively rich to excessively lean with nothing in-between!

I can get the engine to tick over like a grandfather clock but it is too rich and I am worried about the eventual effects of this on bore wear.

It is consequently rather rough for trickling through traffic and also coming off the pilot when accelerating. Am I being too picky here? It's just that the old, worn carbs were great, except for the ruined idling screw threads in the crystallised monkey metal body. I fitted them with Bostik, adjusted them and let it dry. No more fine tuning but the engine worked as it should...which indicates that Bing's replicas must be faulty.

Maybe I should just refit those carbs. But the new Bings were expensive and I am fed up with buying new parts that turn out to be faulty.

Regards,

PK

Thanks for the advice, too, about plugs. I have tried the recommended Bosch plugs but have been through three pairs in a year as they develop internal shorts. The BH6s work but the motor sometimes stalls as I brake at traffic lights. BH7s appear to be unavailable in Paris. Champion L82s behave like the BH6s. Champion L86s as fitted to my 1949 Triumph Trophy work but soot up like the BH8s. The piston rings appear to have broken in. I was careful and did bust the glaze before refitting the barrels. The valve guides are OK. In other words, the motor doesn't smoke. Filter is clean. I take her out on the motorway and wring her neck regularly to clear things out after town riding.

Maybe I am being obsessive. I just have a real problem with plugs that aren't a nice tan or brown colour when I take them out. Perhaps it's modern fuels. Perhaps it's the crap the French run their cars on...

Perplexed in Paris...

Best regards,

PK

>

It is not clear to me how you know this transition. I can tell this transition by running each cylinder alone, using electrode extenders to short out each side. With one cylinder idling alone, it is easy to tell the tranistion between rich and lean. How do you do it?

>

If the engine is happy, it might not be too rich. I don't think a rich idle will harm the cylinders.

>

So you have leaned the idle mix to where the engine is no longer tractable. I would richen the idle.

I would try a hotter plug. I have never had a bad Bosch plug so I use them exclusively. Do not use Champion, ever.

Then I would tune the carbs to where the engine runs nicely. I think after that, you may find the color of the plugs will improve. But it is better that the engine runs and feels right, than to have perfect plug color.

[quote]It is not clear to me how you know this transition. I can tell this transition by running each cylinder alone, using electrode extenders to short out each side. With one cylinder idling alone, it is easy to tell the tranistion between rich and lean. How do you do it?[/quote]

I can tell as I slowly open the throttle. Blipping the throttle also produces a puff of black smoke though not every time. And then - don't laugh! - there's the smell of the exhaust fumes.

By placing the palm of the hand over the end of the silencers and then sniffing the hand, or inhaling the exhaust gases, one can often tell the difference between a cirrect idling mixture and one that is too rich. This is then confirmed by plug electrodes and insultators resembling the interior of a chimney after riding the motorbike at low speeds around the block a few times.

These are methods that work quite well in the cases of my Vincent-HRD Rapide, Triumphs and other British machines I own or have owned.

Thanks for the photo, by the way. How does one use this tool for identifying an excessively rich mixture? I use the old cut'n'coast method on a long empty straight. I would use this tool for setting up the idling speed, installing one on each side with a length of wire and a crocodile clip for shorting as required.

Anyway, it is now running beautifully. I replaced the pattern BMW silencers with a pair of Hoske replicas. She pulls like a train fairly cleanly throughout the rev range. The motor settles down to a metronomic tickover at every stop. She comes off tickover as smoothly as a sewing machine now and trickles along at 'convoy' speed in traffic on a whiff of throttle, responding without any flat spots when the grip is twisted to accelerate away.

As you say, if it feels right, it probably is right so I shall ride and enjoy myself now. Thanks for bearing with me and for the advice and the photo. I am going to make myself a couple of those this week. for my toolkit.

The whole thing is most odd. Incorrectly machined pilot airways in the carbs? Pattern silencers with overly-restrictive baffles? Who knows? Who cares? She's running very well now and sounding wonderful. That'll do for me.

Regards,

Prosper Keating

"...How does one use this tool for identifying an excessively rich mixture?"

An electrode extender is placed on each plug, and one side is shorted with a big screwdriver to allow the other side to run alone. The idle air screw is turned back and forth across the setting where the cylinders runs the fastest. That is the ideal running setting, but too lean for the load when starting off. About 1/4 turn inward (richer) slows the cylinder a bit, but produces a mixture that avoids hesitation under load.

The electrode extenders are also good for balancing the carbs. Twist the throttle to bring the slides up so the RPMs rise a little, and let go. Then alternately slide one screwdriver than the other into the exposed electrode. This will immediately show any difference in RPM between the cylinders, so you can adjust the most convenient cable.

"..."...How does one use this tool for identifying an excessively rich mixture?"

To answer the question more directly, when you install this tool, and find that the cylinder runs faster as you turn the idle air outward, then you know the mixture was excessively rich.

Allan,

I see! Of course, it's obvious. Right then...I am off to the workshop to put this into practice now. I'll buy you a beer one day. I presume you're in the UK. I come over to London a lot but often stray further north and west.

Prosper