1974 CB360 Carburetor Tuning Help!!!

[Amcn44]

Okay guys, sorry for the long rant here but I am at my wits end with this bike. I have a 1974 CB360 I got from a buddy that hadn't ran in a few years. I cleaned it up, cleaned the carbs, adjusted the cam chain, adjusted the valves, and put new spark plugs in. It has a Charlies Place electronic ignition and the carburetors have been swapped out to Mikuni VM-30s, so I'm following the Mikuni Tuning and Jetting Guide from The Vintage Bike Builder. I can get this thing idling at around 1400RPM pretty consistently, although sometimes when I start it up it runs super high (like 2000) and other times it runs right at 1200-1400 like I want. I'm getting to the section about tuning the air mixture screw and that's where it all falls apart. I ground the spark plug on the right cylinder and the left cylinder/carburetor idles low (which I would expect) and the air mixture screw does what it should (i.e. turning it out increases the idle speed to a certain point and then it starts to decrease). I feel confident that everything in that circuit works correctly. However, when I ground the spark plug on the left cylinder the right cylinder/carburetor idles super super high (like 2000RPM high), and turning the air mixture screw out just increases RPM with no stop in sight (I can unscrew the whole thing and it wont start to decrease the RPMs at all). I confirmed I don't have any vacuum leaks using the carburetor cleaner method sprayed around the boots. Do you guys have any clue at all what is happening? I feel like I'm about to go insane.

 

[ancientdad]

First, I wouldn't use the idle drop method disconnecting one plug at a time. With the air filters removed, use a small drill bit to get both slides adjusted to the same height using the idle speed screws and then adjust them equally each time you have to lower or raise the idle speed. Once you get them reasonably synced, then do the mixture adjustments until each cylinder has the highest idle speed and readjust the speed afterward to 1200 rpm. Then with the engine off, check and adjust both cables at the carb tops so both slides lift at the exactly same time using a finger one one slide and watching the other as you twist the throttle.

The spark advance can also contribute to slow idle return if the return springs on the weights are a bit loose, and you can watch that happen using a strobe timing light with the rotor cover removed. Full advance timing should be checked anyway after adjusting the static timing first, so that will help identify if the advancer is functioning correctly.

 

[Amcn44]

Just to make sure I understand - with the carbs synced at idle, I adjust the mixture screw on a carburetor (with both plugs attached like normal) until I see a rise in RPMs from that cylinder, and then I move to the other cylinder and repeat the process. Once thats done I adjust the idle screws again to bring it back down to 1200RPM idle speed? Should I be concerned at all about one cylinder running the engine twice as fast as the other with the carbs idle synced?

 

[ancientdad]

Just to make sure I understand - with the carbs synced at idle, I adjust the mixture screw on a carburetor (with both plugs attached like normal) until I see a rise in RPMs from that cylinder, and then I move to the other cylinder and repeat the process. Once thats done I adjust the idle screws again to bring it back down to 1200RPM idle speed? Should I be concerned at all about one cylinder running the engine twice as fast as the other with the carbs idle synced?

You're dealing with aged, used parts and the engine is not as equally balanced as it was when new. Unless you've done compression and leakdown tests you do not know how equal the cylinders are, and the engine was not intended to be run on one cylinder anyway. Here is the proper procedure written out.

Mixture adjustments

Many riders and wrenchers have asked about mixture adjustments. Many have misunderstood the purpose and function of the mixture adjustments. Screws located on the engine side of the carb (between the engine and the throttle plate) are generally fuel screws. Turning them in (clockwise) makes...

www.vintagehondatwins.com