Bike starts up idles on choke, dies with choke off our when warm

[littlebill31]

The non-vacuum are the way to go. Some will say otherwise, but I have had no problems, ever, with them.

Yes, the other vacuum barb will be capped.

YES, the mix screw components get nasty and need to be replaced. The little O-ring will cause a bad air leak and make it difficult to adjust the air/fuel mixture.
Last 2 items: http://www.mikesxs.net/products-37.html#products

Here is a video of checking the diaphragm.

 

[XS650 Montanian]

I have a 79 special. I've had many of these carbs apart and was a professional Yamaha wrench when these were new. I fought these carbs and now the alcohol added fuel totally wrecked the old diaphrams in my stock carbs. I ordered a new set from 650 Central http://www.650central.com/ Michael there has them set up and is very helpful on the phone. They are round slide, not CV carbs that take a bit to get used to when riding (you can't wick the throttle below 4,000 RPM) but otherwise a great conversion and trouble free. They added extra top end and I'm very impressed with the kit. There are individual choke/enrichners on each carb so it's a bit more technical getting started but anyone can figure it out quickly. For my two cents that's the best route rather and continue to fight the OEM units.

 

[evans_search]

Hello Group,

Good News and bad news. The bike is now running...yeaaaaaah! Bad news, I don't think one of the cylinders is firing at all. I had my uncle come over here today with a temperature reading gun and compression tester. Here is what happened.

1) Cold Compression in the right cylinder was 75 lbs and on the left cylinder compression was 120 lbs.

2) After squirting some oil in the right cylinder compression went up to 110 lbs

3) We also started the bike, ran it, warmed it up and checked compression. On the right side it was back at 75 lbs and 120 on the left side.

4) Spark plug on the right cylinder is black and spark plug on the left cylinder is white and clean.

Here is a picture of the old spark plugs, I pulled them out and replaced them with Iridium plugs. However, the new Iridium Plugs look exactly the same.

We checked the temperature of the head pipe coming out of the right and left side. The temperature on the right side was 30 degrees colder than the left side. Then we shut the bike off and pulled the the plugs.

The new plugs looked exactly the same as the plugs in the picture. The right spark plug is virtually wet with gas.

Then we put the the wire boots coming from the coils over the plugs while they were out of the bike to test the spark on each plug. The size and intensity of the spark was identical so we know the cylinder is getting spark but the fuel is not combusting.

We put the plugs back in the bike, started it up and then took the air/fuel mixture screw all the way down. The bike still ran the same.

What do you think is going on here with the right cylinder?

 

[evans_search]

The hanging at high rpm is a typical sign of a lean coasting condiiton. Your petcock sounds like it's thrashed. It should pour fuel like a fountain on prime. Your tank is also not vented. It should not build vacuum as the fuel is drawn out. It should be vented to atmosphere at all times. Leave the carbs alone for a while and work on the petcock. I think the tank is vented through the cap. I need some guru verification for that. Since you are able to establish a steady idle, and get snappy throttle response, the carbs are close enough to run and ride, but your float bowls are getting starved out for fuel due to the petcock and the venting. I say both because it does it with or without the cap on the tank open, and when it's closed, it pulls a vacuum.

You were right on with pretty much everything. I finally have a chance to reply now. Been a busy weekend. I unhooked the gas cap and let the bike run, the idle went down and then when the bike got warm it went back up again. I was frustrated until I looked at my idle screws and they were just set too high. But this only worked with the cap open so The only problem I had left was the tank venting issue.

So a buddy who is a bit more savvy with bikes than myself came over and we took off the fuel cap. After Completely disassembling it, we discovered how the venting system works in the fuel cap it was completely clogged. After cleaning everything and putting it back together guess what...no more vacuum in the tank. Pet cock is still thrashed and I'm going to get a non- vacuum one. But now that those issues are resolved I can get on to my current problem I just posted about before this one.

Thanks for the tip and thank you all for your support. There is still much to be done but things are off to a nice start for a winter project.

 

[evans_search]

Your carbs are CV (Constant Velocity), not VM's is what I meant.
Check to see what the numbers are stamped on the main jet, pilot jet, and the needle jet too.

I'll have the carbs back out tomorrow and I'll be back with pictures for you. Thanks for the tip!

 

[evans_search]

The lean condition may also be a low float bowl level.
At least mine lean out(increase in rpm) as I run them to empty to the carbs.

I think you may be onto something here and the one thing I didn't do the last time I had the carbs out was test the floats to make sure they floated. All I did was adjust them. I should know here pretty soon on that as well. Thanks for the tip!