Carb adjustment

rdgerber2 · Posted: September 28, 2008, 1:20 pm Post subject: Carb adjustment

I have a 75 stock xs650 and recently rebuilt the carbs. It ran fine for 2 months or so after the rebuild and just recently I pulled the plugs and they are both black. When I put the carbs back on I followed the instructions in my repair manual for air/fuel mixture and turned the screw all the way in then backed it out 3/4 turn on both carbs.

I put new plugs in and took it for a spin last night and when im at low rpms the left cylinder doesnt fire until I roll on the throttle and it picks up a bit of speed. It will then kick in and take off.

I also put 2 new coils, new plug wire, and new plug caps.

What is the proper way to set the mixture on the carbs ?
Would it be worth it to go buy the vacuum gauges and set using them ?

Retiredgentleman · Posted: September 29, 2008, 1:41 am Post subject: Re: Carb adjustment

Go to 650garage.com and read the section called "650garage Carb guide"
Use the dead cylinder method to set air/fuel mixture screws. You don't need vacuum gauges. I use 22 feet of plastic tubing ( 3/16 ID, 5/16 OD) screwed to a wooden board, with some red liquid in it, as a manometer to balance (sync) the 2 carbs, cost about $7.00.

steveypsi · Posted: September 29, 2008, 11:03 am Post subject: Re: Carb adjustment

Retiredgentleman
man that is one pretty bike, can you post a full photo

Retiredgentleman · Posted: September 29, 2008, 11:47 am Post subject: Re: Carb adjustment

steveypsi; Hey steve, that's very kind to say that, you'll give me a swelled head Very Happy

...............but if you insist I do have a pic.

steveypsi · Posted: September 29, 2008, 10:31 pm Post subject: Re: Carb adjustment

thanks, I save those that are really nice to inspire me to make my 3 half as nice, or at least one of the 3.
I tried to find it in the galleries but couldn't, to bad the site doesn't have a icon on each persons post linking to their photo gallery
Thanks again
I also change my wall paper with these gook looking 650's, on the full size screen they look better

metalredneck · Full Member Joined: Aug 04, 2008 Posts: 308

I doubt idle would solve black plugs. Main jets, maybe.

5twins · Full Member Joined: Aug 05, 2006 Posts: 1616

Vacuum gauges or a manometer like RG's are more for setting carb sync than mix screws. But you may need to do that as evidenced by one cylinder kicking in later than the other. On your 2 throttle cable '75 model, the simple "no special tools" method for syncing is just to make sure both throttle cables have the same amount of freeplay. That will insure that both throttle butterfly plates begin to open at the same time when you twist the throttle open.

The main jets would have to be really, really big compared to stock to blacken the plugs. These CV carbs work on demand. The slide only opens as much as the motor can use. This can make tuning difficult because the bike will seem to run pretty good with a larger main than it really needs. The slide won't be lifting fully so you won't be getting the full flow through that larger main. It will run and flow like a smaller size to a certain extent.

If your mix screws are set too rich, they can blacken your plugs if you let the bike sit and idle for long periods. Prolonged use of the choke (longer than necessary) on start-up will also blacken them. Even normal use of the choke does it but that usually burns right off again in a few miles of normal riding.

I set my mix screws by ear. I start at the recommended setting for that particular carb set and then work the screw in/out 1/4 turn at a time until the best spot is found. Go too far in either direction and the motor will start to falter and stumble. Find those 2 spots and best setting is somewhere in between. I do some test runs and note the amount of popping on decel. Setting the screw to the rich side but still in the area between the lean and rich falter points can help alleviate this. If it won't, a pilot jet change may be needed.

It may just be time to check your points gap and timing. Both slowly change and come out of adjustment as the points rubbing blocks wear.

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Yamaha xs650