R90 Mixture?

[20975]

Hi, I have a 75 R90/6. It is dual plugged with a dyna ignition. I rebuilt the carbs recently and did the choke circuit at the same time. If memory serves me correctly the needle was n the #3 position prior to the rebuild. When i put things back together i consulted the Bing rebuild guide and set them at position #2.

The bike starts fine with choke and will idle, without choke when it is warm. The problem is on transition from idle to higher RPM it kind of coughs and sputters, and nearly dies, before finally catching and taking off. If I leave the choke on it transitions fine. That leads me to believe the bike wants a richer mixture. My question is: Does the idea it wants a richer mixture sound reasonable? If so do you think a return to the #3 needle position would fix the problem?

thanks in advance for any ideas and advice!

John

 

[20774]

John -

I take there was no real problem before the rebuild... Did you take the bufferflies off the shaft? If they don't get back on the proper way and/or are even around the throat, you might have problems off idle.

What did the plugs look like on #3? What sort of gas mileage did you get? If this was all good, then I'd be inclined to go with #3.

What have you done with timing now that you have the dual plugging? Could this be an issue? I've read a lot about ways to do this and at best it's a compromise. The most simple approach is to set static timing to near OT. There are issues with what happens to full advance...it would be moved consistent with moving static timing...you'd get to full advance earlier. IIRC, the best solution is to leave static at the S mark but limit the amount of advance range...most likely I've got that wrong. But be sure your timing is proper for dual plugging.

Does this problem happen at any throttle setting...is it only starting from idle? If it happened more from say 4000 RPM in 4th or 5th, then maybe you should go to a larger main jet. The main jet becomes more prominent in the response once you get to 1/2 throttle or beyond, out to WOT. If it's only on the transition from idle, would being gentler on throttle be better?

Where's you idle mixture settings? Maybe you need to make it slightly richer at idle. The idle mixture screw is a gas circuit, so moving it out a small amount would make it richer.

How did carb synching go? Typically one adjusts the cables on the carbs for just off idle, say 1500-2000 RPM. This is where the vacuum has the most affect. Could be the carbs aren't pulling evenly in this region...

 

[20975]

Hi, thanks for the response. The timing is not an issue, I have been religous in reading the material and setting it to the recommended specs. Carb balancing is no problem and the idle screws are set to obtain the best idle, starting from the pre-set, which if I recall is 3/4 turn out.

I balanced with my carb stiks and my home made balancer both at idle and at 2000RPM.

There is no problem when under way and under load. Throttle response is good at 4000 RPM.

My plugs used to run a light grey/brown and mileage was good. I have not been able to assess them since re-doing things. I am not concerned with gas mileage. I will re-try the idle screws and then I think I will try setting the needles at position 3. Failing that I may look at new main jets, but I was hoping to avoid that.

John

 

[20774]

...and the idle screws are set to obtain the best idle, starting from the pre-set, which if I recall is 3/4 turn out.

Sounds good...definitely try a slight enrichening of the idle mixture...it's easy to do and really is better for the engine, to run a little "cooler" with a richer mixer at idle.

 

[hoffmanswen]

The bike starts fine with choke and will idle, without choke when it is warm. The problem is on transition from idle to higher RPM it kind of coughs and sputters, and nearly dies, before finally catching and taking off. If I leave the choke on it transitions fine. That leads me to believe the bike wants a richer mixture. My question is: Does the idea it wants a richer mixture sound reasonable? If so do you think a return to the #3 needle position would fix the problem?

John

John,

Assuming that no other carburetor changes have been made (same-sized main and needle jets, same slide, etc.) and no substantial changes have been made to the ignition (timing, advance, plug heat, etc.) it would seem that your diagnosis is right on the money.
It sounds like another way to put this is that it wants a richer mixture from about 1/4 to 1/2 throttle. If you dig through the fine print about the jet needle/needle jet in the Bing manual, it will say that the two ways to achieve that are to use a shorter jet needle or to move the needle to a higher setting.
To paraphrase the manual:
Below about 1/4 throttle, there isn't enough airflow through the venturi (throat) to draw fuel through the main jet/needle jet stack. Above about 1/4 throttle (off-idle), the effect of the idle jet becomes trivial compared to the contribution by the needle jet/jet needle combination, so it effectively drops out of the picture. The needle jet runs the show until about 2/3 throttle when its open area begins to approach the area of the main jet. Then the needle jet is no longer the major flow restriction and the main jet is. Above about 3/4 throttle, the main jet, butterfly, and slide run the show.

Because the bike seems to idle well, your idle screw and mix screw seem ok. Because it runs well after you get well past the transition point, it seems that it's ok when the carburetor lets the engine have enough fuel. Try putting the needle back in the #3 slot and see how it goes.
Good luck. I hope this helps.

--slh

 

[sumran]

There are idle mixture adjustment screws as well as idle speed adjustment screws. How did you set your idle mixture screws?

 

[20975]

Thanks everyone. I got serious today, went for a good long ride then came home and fine tuned the idle circuit using my Bing manual. It took only two minute adustments of the mixture to get a beautiful transistion from idle to power under load. The bike is now a dream, especially since I put the carb stix on and readjusted the idle speed and high speed while I was at it. Now the bike idles perfectly at just over 1K and pulls strongly from a stop to full throttle. So i had to go for another ride. The afternoon was young and now I am back and grinning after a 150 miles of bliss!

John

 

[barryg]