Carb setting help

[20975]

Hello all. I am looking for advice on my 1975 R90/6.
First some history. Over a decade ago a friend had this bike. He decided to do a major overhaul. He sent the engine to Bob's BMW where it was dual plugged. He lso had 9.5 pistons with ligt wrist pins installed. He put in a 336 cam, ss 40 mm exhasut valves, 42 mm intake valves and had the heads ported. The flywheel was lightened and everything was balanced.
Unfortunately before the bike was up and running he was killed in an accident. Now I have the bike and I have it back together. I added a dyna ignition rebuilt the carbs and have it running. The carbs are stock 32 mm Bings.
My question: The bike idles fine but under power feels sluggish. Can anyone suggest changes to the carbs that would help performance? I am wondering about jetting, needle settings.
Thanks in advane for your suggestions.
John

 

[20774]

John -

Sorry to hear about the previous owner...lots of work done on the bike. What are the carb numbers? The basic specs for the carbs are given here:

https://w6rec.com/wp-content/uploads/2017/07/Page2-2.jpg

So, it would be good as a first start to verify what sort of jets and other settings you're starting with. Also, before fiddling too much, you should go through a carb synch process to ensure that the idle speed, idle mixture, and cable tension gives balanced performance. You could do a plug chop where you find a safe spot on the road, run along for a few minutes at say 40-50 mph, then chop the throttle, pull in the clutch, and kill the engine. Check the plugs...if they are whitish, then you're running too lean.

All that said, if I had to guess, I'd think that maybe the throttle cables are pulling evenly or that in the mid range of throttle, the needle jet and jet needle need to be changed. One thing that could be done would be to raise the needle one notch. This is a big adjustment but it would let you see if things improve. Finer adjustments are made with changing the jet to larger sizes.

With all those changes, I'm wondering if the carbs are starving the engine. Seems like the modifications are going to demand more from the engine...hotter cam, larger valves, higher compression ratio. So maybe look into how the carbs are set, get a synch, and go from there.

 

[20975]

Thanks Kurt. The carbs and all the internals are stock per the Bing manual, which I have. I agree the mods are all perfotmance oriented. I am not well versed in matching fuel supply with these mods. That is why I am seeking advice and ideas.
John

 

[stevenrankin]

Start

You are best suited to start with the stock settings Bing suggests then work from there.

Not to throw rocks at the previous owner but he fell into the common myth that installing the 336 cam would help performance. He was correct, sort of, this cam produces performance at higher RPM than the stock cam. Great for a race track but perhaps not so on the street. It can be used on the street but unless the RPM on the bike is high, it is not optimum.

Start with the basics. First, do you have the valves properly adjusted? Second do you have points? Before you mess with the carbs, it is imperative you have point gap set as well as timing set. If the points and idle timing are set properly, does the timing advance properly as the RPM is increased? Are the advance weights sticking?

As for the carbs none of the modifications I see that have been done would in my experience, call for any kind of re jetting of the carbs or going to any settings beyond Bing recommendations.

My friend just had an expensive "cafe" R75/6 in his shop that did not run properly. Almost like what you are experiencing. The customer was frustrated that the builder was not able to get the performance issues sorted, so he brought it to my friend. The issue turned out to be the "builder" had no clue about airheads, had just stuck richer jets and moved the needles in the carbs to richer settings for more "performance". Glenn, set the jets back to stock Bing set. Moved the needle on setting leaner and solved the problem.

So I guess story aside, I would check the needles are at the third click or notch.

Last issue, are the chokes off?

Use the color of the spark plugs after a good long ride to aid you in the direction to go. Black is too rich, white too lean. Tan is the color. Good luck St.