Bing Jetting advise

[drpetemurray]

Subject matter 73 R75/5 has hesitation off idel with a slow twist of the throttle. Fast turn of the throttle notes slight hesitation.
Disclaimer the carbs have been fully rebuilt and balanced with the Carbmate, float levels are properly set, valves properly adjusted, static and advanced timing verified with a timing light. no air leaks.

Standard jetting with the needle in the third position from top.

The idle mixture screw was set for the smoothest/fastest running, final setting at 3/4 of a turn out after a 1/8 turn-in. At that setting the hesitation is present with a slow twist of the throttle.
When I adjust the idle mixture to 1/2 turn out there is no hesitation and great throttle response. Additionally the bike pulls stronger through the full RPM range at this setting. The only problem is the engine is not smooth at idle with this setting it tends to shake from side to side. I verified the carb balance at the 1/2 turn setting.

So should I go 1 step larger on the idle jet or raise the jet needle 1 notch?

Thanks

[srankin]

If I am reading correctly, you are having and issue with synchronization of the two carbs to work together for idle and pull off.

As for idle don't think that turning the mixture screws exactly the same for both carbs will ensure smooth idle. One carb may need a tiny bit more or less of a turn to smooth things out. Bing book recommendations are starting points. Tiny adjustments may be needed for finish.

I think Boxer two valve has a video, and perhaps Brook Reams, on how to balance carbs and synchronize them. St.

[r67boxer]

If you don't mind the odd f-bomb, Chris Harris puts out a couple of excellent videos:

https://www.youtube.com/watch?v=a_HyX1qLPbQ

or for the slide-type carb (/5):

https://www.youtube.com/watch?v=HShbaC9ORSo

Kevin

[drpetemurray]

If I am reading correctly, you are having and issue with synchronization of the two carbs to work together for idle and pull off.

As for idle don't think that turning the mixture screws exactly the same for both carbs will ensure smooth idle. One carb may need a tiny bit more or less of a turn to smooth things out. Bing book recommendations are starting points. Tiny adjustments may be needed for finish.

I think Boxer two valve has a video, and perhaps Brook Reams, on how to balance carbs and synchronize them. St.

Thanks for the response, syncing the carbs is not an issue. The issue is a slight hesitation upon turning the throttle when idling.
Mixture setting in my case both carbs are nearly exact when setting to engine RPM/speed.

So back to the original question. In order to remove the hesitation/flat spot/Bog off idle is the issue related to the need to go up to the next larger idle jet or should the needle be raised 1 notch?

[drpetemurray]

If you don't mind the odd f-bomb, Chris Harris puts out a couple of excellent videos:

https://www.youtube.com/watch?v=a_HyX1qLPbQ

or for the slide-type carb (/5):

https://www.youtube.com/watch?v=HShbaC9ORSo

Kevin

Thanks Kevin, I always enjoy his style.

[srankin]

When I adjust the idle mixture to 1/2 turn out there is no hesitation and great throttle response. Additionally the bike pulls stronger through the full RPM range at this setting. The only problem is the engine is not smooth at idle with this setting it tends to shake from side to side. I verified the carb balance at the 1/2 turn setting.

If this is the case, you don't need to go up a notch or a bigger idle jet. What is the color of your spark plugs after a good long run? If they are tan, then your carbs are set.

So if the bike rocks back and forth, I will again say it is a synchronization issue that could be resolved with minor adjustment of the idle screws. What are you setting your idle RPM at? The older /2 bikes could be set at a very low RPM and idle smooth, the newer bikes will idle smooth but not as.

St.

[drpetemurray]

When I adjust the idle mixture to 1/2 turn out there is no hesitation and great throttle response. Additionally the bike pulls stronger through the full RPM range at this setting. The only problem is the engine is not smooth at idle with this setting it tends to shake from side to side. I verified the carb balance at the 1/2 turn setting.

If this is the case, you don't need to go up a notch or a bigger idle jet. What is the color of your spark plugs after a good long run? If they are tan, then your carbs are set.

So if the bike rocks back and forth, I will again say it is a synchronization issue that could be resolved with minor adjustment of the idle screws. What are you setting your idle RPM at? The older /2 bikes could be set at a very low RPM and idle smooth, the newer bikes will idle smooth but not as.

St.

I understand what you are saying in the last paragraph.

When carbs are in sync with a smooth running engine the mixture is about 3/4 turn out both sides. With this setting the annoying hesitation is present.

Plug readings at 3/4 turnout appear slightly lean. The engine pulls good but not as strong as 1/2 turn out.

Idle RPM is about 1000 rpm not going by the Tac.

Plug reading at approximately 1/2 turn out yields a tan plug color with no hesitation and the engine pulls stronger throughout the RPM range. As noted before rough idle.

My goal is to maintain the performance of 1/2 turn setting with smooth idle and no hesitation. At this point testing needs to be performed with an idle jet change and or needle clip position.

Lastly, according to Brook R he states turning the idle mixture screw (IMS) IN decreases fuel flow through the idle jet and turning the IMS OUT increases fuel flow. In accordance with this statement by Brook my performance improves with a leaner IMS setting 1/2 out. This is contradictory due to the fact the plugs are tan at the 1/2 setting and are not tan at the 3/4 lean plug reading setting. Suggesting the IMS controls air flow not fuel flow.

Any other sources regarding which of the 2 the IMS meter either Fuel or AIR flow?

Thank You

[schrader7032]

You don't say what carbs you have on the bike, but if they are stock, they are Bing constant velocity (CV) carbs. As Brook has said, the idle mixture screw controls the amount of fuel flowing in the idle circuit. So, turning the screw in leans idle mixture out.

If you find a setting for the idle mixture and idle speed, then seems you should look then towards the needle jet or jet needle. Bing describes how the internal settings control (or influence) three separate regimes of throttle. The first 25-30% of throttle opening is influenced primarily by the idle circuit. The next 30-70% of throttle opening is the intermediate circuit which is the needle jet and jet needle. Finally after about 75% or so, is the wide open circuit and fuel flow is influenced by the main jet size. So as throttle opening gets bigger, the idle circuit is less and less involved.

Synching the carbs help to isolate these separate circuits. If you have hesitation during say throttle roll-ons while doing 30-40 mph, then you should be looking at the needle jet/jet needle. Clip positions on the needle are crude jumps in fuel flow. Obviously jet sizes can fine tune performance.

[srankin]

I give up, the bike runs strong, no hesitation, and proper mixture. What is the problem other than rocking at idle?

What is it you are looking for? IF you are looking for a smooth idle, either you are setting idle speed too low or idle mixture on one or the other carb needs to be tweaked not in 1/4 or 1/2 turns but minute increments. The recommended mixture settings are starting points.

I mean come on, if the bike is running strong, no hesitation, good fuel economy, proper plug color, ride it as long as it doesn't stall at idle and is fairly smooth. St.

[drpetemurray]

Bing describes how the internal settings control (or influence) three separate regimes of throttle. The first 25-30% of throttle opening is influenced primarily by the idle circuit. The next 30-70% of throttle opening is the intermediate circuit which is the needle jet and jet needle. Finally after about 75% or so, is the wide open circuit and fuel flow is influenced by the main jet size. So as throttle opening gets bigger, the idle circuit is less and less involved.

Synching the carbs help to isolate these separate circuits. If you have hesitation during say throttle roll-ons while doing 30-40 mph, then you should be looking at the needle jet/jet needle. Clip positions on the needle are crude jumps in fuel flow. Obviously jet sizes can fine tune performance.

Thank You Kurt!

This is exactly the information I was seeking in the thread starter post. I find it very interesting Bings account as to the percentage of influence the Idle Jet has 25-30 % of the first throttle opening.

With the mixture screw about 1/2 turm out(leaner setting) I no longer have hesitation with slow throttle roll-on and much stronger low to mid-range power . Conversely the engine rocks side to side at idle. With the idle screw set for fast smooth engine speed I am at about 3/4 turn out. At 3/4 turn out (richer setting) no rocking but the hesitation is present and the low to mid-range power is less. Plus at the leaner setting(1/2 turn out), my spark plug now has a tan color. At the richer idle screw setting (3/4 turn out) my spark plug has a lean reading. I would think at the rich setting the plug would have a darker color.
Perhaps the intermediate circuit needs attention, Needle Jet and or jet needle. I know the needle jet has a smaller incremental influence compared to the jet needle.