How to tune the carburetor when you have Pamco High Output Electronic Ignition Kit w

[fiftynineab]

Hello,
I Have a standard 1979 xs650 With the pamco High Output Electronic Ignition Kit with Electronic Advancer. I am trying to understand how to adjust the carbs (vm34mm mikuni) with out using the dead cylinder technique. If I pull one of the plug wires while the bike is running you lose the ground and cook the controller. Any thoughts?

 

[azman857]

Useing a mannometer would be best. You need vacuum ports. Search this sight on how to make the manometer. No vac ports? Hand at the end of exhaust pipes. Just make sure both have about the same pressure comming out. Before doing anything, valve adjustment and ignition timeing is spot on. Do the ball bearing sync on the bench small ball bearing in the mouth of the both carbs) and roll the throttle. Both should drop the bearings at the same time. Reinstall carbs(without the ball bearings!) without changing the settings should be a good start.

 

[5twins]

Syncing dual cable carbs amounts to nothing more than matching the freeplay in the two cables to one another. If one cable is tighter than the other, that slide will open first. You must set your idle speed first because that changes the slide height and cable freeplay slightly. Yes, vacuum gauges or a manometer are the most accurate way, but you can get them very close with a cable play match.

 

[fiftynineab]

Thanks for the info and the reply. I think I got it. Adjust the valve, I have electronic ignition and electronic advance. I have vacuum ports and a manometer. first bench sync the carbs. I was going to use the smooth side of a 5/32 in drill bit. set the air fuel mixture crews to 3/4 turns. next put the carbs on the bike (Put the bike back together.) attach the manometer to the vacuum ports. start the bike. This next part is the part i am a little unclear about.
adjust the mixture screw on the right carb out in 1/2 turn increments listening to the rpm's rise until the bike starts to run bad. count the turns, make a note. then turn the screw inward in 1/2 turn increments till the bike stars to run bad. make a note. proper air fuel mixture is someplace in-between the two points where the bike runs the best. airing on the side of rich rather than lean.

Next repeat the process on the other side (left hand side in this case). Use the manometer to equalize the carbs.

Next readjust the slide valves or throttle cable to lower the rpm because it should have risen because the bike is better adjusted. Target rpm's 1000 to 1200

Next repeat the process on the right side then on the left side. and check the rpm

Does this sound correct?

 

[5twins]

With carbs like the VMs that have individual throttle cables, you actually have to do two syncs, one for idle speed and one at a higher speed with the throttle open some. The idle speed sync is pretty self explanatory. You want both carbs idling at the same speed. You must set this first as I mentioned earlier. Your idle speed is controlled by a tapered screw bearing into an angled ramp cut into the side of the slide. Turning the screw raises or lowers the slide and that increases or decreases the idle speed. Doing this also adds or removes slack in the throttle cable. That's why this setting or sync must be done first, before the "at speed" sync. Make sure you have adequate freeplay in the throttle cables or the slides may not drop all the way, a too tight cable holding them open.

For the "at speed" sync, roll the throttle open slightly so both slides have opened some. Sync by adjusting the cable freeplay on one carb or the other. I usually get the idle tune and sync squared away then set one carb's cable freeplay where I want it (about 1/8" to 3/16" freeplay) then lock it down. I then do all the sync adjusting on the other carb's cable, matching it to my properly set first cable. It's not always possible to get a perfect sync at all speeds with individually cabled carbs, especially if the cables are old. Cables stretch with age and can do so unevenly. In a case like that, try to get your best sync in the 3K to 4K RPM range, the area you'll be running and cruising in mostly.

The mix screws on the VMs are air regulating screws, not fuel regulating screws like on the stock CV carbs. This means they act opposite to the CV mix screws. Turning them out adds more air and makes your mix leaner. As you may be aware, too lean of an idle setting can raise RPMs and even cause the engine to race. Keep this in mind when adjusting the air screws. If the idle speed keeps rising and rising, and you're getting to like 2 to 3 turns out, you've probably entered this "too lean" territory. Instructions for setting the air screws on BMW Bing slide carbs tell you to turn the screw back in 1/8 to 1/4 turn, richening the mix back up some, once you enter this increased idle speed area.

 

[fiftynineab]

5twins
Thanks for taking the time to explain. It good info and a big help. The first two parts make a lot of since now that you explained it. I am still a little hung up on the third part. the part about setting the mixture screws.

lets assume the carbs jetted correctly. I have completed the first two parts and the carbs are synced at idle and at 3000 to 4000 rpm's. now I back the mixture screws out (leaning the mixture) until I hear and see the idle speed begin to rise. then turn it inward (richening the mixture)1/8 to 1/4 turn. and its set. do the same on the ether carb.

I apologize for my thick headedness.

 

[5twins]

I should have mentioned, do your mix screw adjusting along with the idle speed syncing operation. Get the idle all squared away (sync and mix screw settings) before moving on to the "at speed" sync. If you did the two syncs first and then the mix screw adjustments required tweaking the idle speed settings, that's going to throw off your "at speed" sync. Any time you adjust the idle speed, it can throw the sync off with these two cable carbs. For that reason, if you do need to adjust your idle, adjust the idle speed screw on each carb the same amount. That will keep changing the sync to a minimum.

Yes, you have the mix screw adjustment pretty much figured out. Do keep going on the screw adjustment once the speed starts increasing just to test and see how far you can go. You may just be entering the good running range where the screw should be set. That constantly increasing idle speed for several or more turns area usually comes after this. Also keep in mind the spec setting. I think it's something like 1.5 turns out for your VMs (but don't hold me to that, look it up). If you need to go way out past or way in to near closed for good running, that usually indicates the pilot jet needs changing.

 

[fiftynineab]

5twin
Thanks I think I got it.