Another carb question, final tune.

[hotrdd]

1976 XS650 with open exhaust and intakes. I have rebuilt the Carbs twice and they are in mint condition. I have use the manometer and the Carbs are almost dead level. I took the manometer off and am trying to use the dead cylinder method but I must be missing something. I pulled oneof the plug wires and took an extra spark plug and hooked it up to the spark plug cable, leaving the dead plug in the cylinder. I was able to dial the throttle stop screw to idle around 500 rpm. But when I adjust the fuel mix screw I don't get any rpm change. Even if I go 1/2 a turn in either direction. Same problem on the other carb except in orderto get it toidle at 700 rpm the idle stop had to be turned all the way in. What am I missing? Is there another way todo this?

 

[5twins]

Well, there's the proper and right way in my opinion. With your older carb set, you don't need to fool with that "dead cylinder" crap. Your carbs should respond to mix screw adjustments and you should be able to hear the difference without pulling a plug. Granted, you may need to turn the idle speed down a bit, but that's all. Your carb set is one of the best of the 38s, some say THE best. Personally I prefer the '78-'79 set, but that's another story. Anyway, that set should respond well to mix screw adjustments without pulling a plug cap. If it won't, you may have other problems (timing, sync, blocked carb passages).

 

[hotrdd]

I have the Boyer ignition so I will contact them about the timing but right now I have it set dead on the T. Each mix screw is set at 1 1/2 turns out and the Carbs. Are synced. The Carbs have been cleaned twice and I've confirmed they are 100% clean and I'm running fuel filters. What else should I look at?

 

[hotrdd]

So tonight I re-timed the bike according to Boyer's instructions and things are running a little nicer. Once the bike was warm I also did a compression test, the proper way, and came up with 125 for both cylinders. Checked the chain lash and valves and then threw the manometer on the bike and got it dialed in. Then I tried to dial in the fuel mix screws but even with a very low idle I wasn't able to hear a difference while dialing in or out the fuel screws. I wasn't using the dead cylinder method this time. The left carb showed a slight change but very, very minor but the left carb didn't really change at all.

What's next?

 

[5twins]

I would start the screws at the spec, 1.5 turns out, then work each one 1/4 turn at a time in/out until I heard a change. This will establish your rich and lean roll-off points and show you the range of adjustment where the bike runs best. It may take as much as a turn or turn and a half in either direction before you see any change. If you get none no matter how far you turn the screws then you have something else amiss. Either some of your pilot circuit passages are still plugged or maybe your float level is set too rich (high).

Another way to set the mix screws is the ride and adjust method. Set them to spec and your idle to 1200. Go for a ride and take a small screwdriver with you. Ride a couple miles to warm the bike up and pull over. Pick a direction (in or out) and give the screws a quarter turn. Ride a couple more miles and pull over. Note idle speed and give the screws another quarter turn in the direction you've chosen. Keep doing this until your idle speed starts to fall off. When it does, you've found one limit or roll off point for the screws. Set them back to spec and run the same testing turning the screws in the opposite direction. Best setting is somewhere in between those 2 roll off points. I run mine in the richer side of the range because it helps with popping on decel with my aftermarket (louder) mufflers. Speaking of popping, as you work the screws in (leaner), besides the idle speed dropping, watch for the popping on decel. It will appear or increase as you get too lean. Also do some throttle blips at idle. The RPMs will hang and be slow to drop as you get too lean.

 

[hotrdd]

I honestly don't know how you store all your information. I'll give that a shot but it might be easier for me to write a computer program to just download all the XS650 knowledge out of your head. I'm sure your running out of space.

 

[5twins]

I take lots of notes, lol. I have a loose leaf notebook set up for every brand bike I own. They are chock full of info I've downloaded or discovered on my own. They also contain maintenance logs I drew up. I couldn't possibly remember what I did to which bike when unless I kept track, lol.

I have blank versions of the maintenance log in a PDF file but I'm not sure if or how I could post them here. If you'd like them, drop me a P.M. and I'll e-mail them to you.

 

[hotrdd]

5twins how does that method work for the two Carbs? How can you tell if one needs to go for rich and the other lean?

 

[5twins]

If everything is right with your carbs, the mix screw settings should be the same on each one. If you have one carb that's running richer or leaner than the other one to begin with, you really can't use the "ride and adjust" method until you get the carbs squared away and matched. The carbs are a matched set and should run the same. If they don't, there's something wrong with one of them or the ignition on that cylinder. If there is something wrong then you really can't tune them no matter what method you try. You're not ready for the "tuning" step yet, you still need to complete the "fixing" step.