Help Dialing in Engine

[ScottyShy]

I do not have the experience on these XS650 as the other souls that post on this forum. I was checking my timing because of no power in 3rd gear and up. This was after a shop install of Mikes Big Fin 650 kit. The shop says break the bike in! But I do not believe that will take care of this no power. However, I am breaking the motor in!

Well, I hear your frustrations. i'm yet to find a good shop in Nashville that knows old bikes well.

 

[ScottyShy]

I'm a bit confused because the carb guide seems to say that my BS34 carbs have a main jet size of 132.5 and a pilot size of 42.5 ... but my yamaha service manual for xs 1978 - 83 says the main jet is 135 and the pilot jet is 27.5 (in the carb guide these are the specs for the 1979's)

I currently have a main jet size 135 and a pilot jet size 45 in my carbs. @5twins, you suggested going up a size on these jets, but i'm not sure what to do here. I ordered the foam UNI filters, the 4" long ones. If I need bigger jets i'd like to get them ordered soon so i can get on with the troubleshooting. Any ideas?

 

[5twins]

There are mistakes in the service manuals. Often they didn't keep up with the changes made to the bikes. A 132.5 main and a 42.5 pilot are stock for all the '80 and later BS34 carbs, in the U.S. anyway. That 135 main and 27.5 pilot listing is for the '78-'79 BS38 carb set, pay no mind to it.

OK, so it seems you're one size up already on both your mains and pilots. The pilots could be OK but you may need another size up on the mains (137.5). One size up on the mains in these lean BS34s doesn't seem to help much with mods. In fact, it does actually help an all stock bike. I can't tell you exactly what will work best, you'll just need to experiment and see. If you want to get a few more larger jet sizes, I would recommend the next size up on the pilots (47.5) and the next 2 sizes up on the mains (137.5, 140).

 

[ScottyShy]

There are mistakes in the service manuals. Often they didn't keep up with the changes made to the bikes. A 132.5 main and a 42.5 pilot are stock for all the '80 and later BS34 carbs, in the U.S. anyway. That 135 main and 27.5 pilot listing is for the '78-'79 BS38 carb set, pay no mind to it.

OK, so it seems you're one size up already on both your mains and pilots. The pilots could be OK but you may need another size up on the mains (137.5). One size up on the mains in these lean BS34s doesn't seem to help much with mods. In fact, it does actually help an all stock bike. I can't tell you exactly what will work best, you'll just need to experiment and see. If you want to get a few more larger jet sizes, I would recommend the next size up on the pilots (47.5) and the next 2 sizes up on the mains (137.5, 140).

Thanks for the advice, i'm going to try out a couple bigger size jets. Will these bigger jets mean i'm getting more air into my mixture?

 

[5twins]

Not these jets, these are fuel jets. You'll get more fuel in the fuel/air mixture. There is an air jet but you're not changing that one (yet anyway).

 

[ScottyShy]

So, I went up a size on my pilot and mains, and i rode all day yesterday. It seems as though i'm still getting a bit of stumbling at lower rpm's and then it takes off at higher rpm's. It's not a lot of stumbling, but i'm wondering if i can fix this with a tweak on my mix screws?

But then another problem came up.. Last night, after riding fine all day, i went to drive home and about every 4 minutes it bogged down like it was running out of gas. I had this problem when i very first bought the bike 3 years ago and the problem was the gas cap seal. I have replaced the tank since then, but i'm wondering if the same problem is back. It was a bit colder last night so I don't know if that played a role.

 

[5twins]

Yes, dialing in the mix screws could help. You should re-set them any time you change pilot jet size. The new size usually changes what setting will be best.

Setting the mix screws on the BS34s is a bit more difficult than on the earlier carb sets because they're so E.P.A. strangled. They don't seem to respond much to adjustment changes, not like the early sets where the idle speed will noticeably drop off if you adjust too far past the best setting either way. The usual routine with the BS34s is just to set the screws between 3 and 3.5 turns out. The bikes usually run OK like that. Make sure the settings are matched carb to carb. Start with both set at 3, then try 3 1/4, then 3.5 turns out. See what runs best.

You should probably also make sure the carbs are in sync. If they're not, that can cause running issues as well.

 

[ScottyShy]

Yes, dialing in the mix screws could help. You should re-set them any time you change pilot jet size. The new size usually changes what setting will be best.

Setting the mix screws on the BS34s is a bit more difficult than on the earlier carb sets because they're so E.P.A. strangled. They don't seem to respond much to adjustment changes, not like the early sets where the idle speed will noticeably drop off if you adjust too far past the best setting either way. The usual routine with the BS34s is just to set the screws between 3 and 3.5 turns out. The bikes usually run OK like that. Make sure the settings are matched carb to carb. Start with both set at 3, then try 3 1/4, then 3.5 turns out. See what runs best.

You should probably also make sure the carbs are in sync. If they're not, that can cause running issues as well.

thanks. do you have any thoughts about the bike dying last night while riding? it rode perfectly all day and then struggled last night.

 

[5twins]

It could be that gas cap thing again. Sometimes the vent hole gets plugged and needs to be cleaned out. If it starts happening again, open the gas cap if you can. If you hear a "whoosh" sound from air rushing in, the vent is plugged.

 

[ScottyShy]

It could be that gas cap thing again. Sometimes the vent hole gets plugged and needs to be cleaned out. If it starts happening again, open the gas cap if you can. If you hear a "whoosh" sound from air rushing in, the vent is plugged.

it's definitely the gas cap. i cleaned it out for now but i ordered a new one. thanks for all the help!

 

[wannabridin]

One thing I've found that is worth my time is a dyno tune session. I did it on a few bikes that I had running pretty well with my butt-dyno, but they came alive after a real pro had their way with them. It usually costs $2-300, but it's done in half a day.

I don't mind tuning, but I like having it running right ASAP after the other mechanical issues are done.

Otheriwse, listen to the guys on here, they are unbeatable!!!

 

[ScottyShy]

Ok.. my bike is actually running really good right now, better than ever before. (thanks to all the good help on this thread!) But here's something strange that wasn't happening before. When i use my timing light on the right spark plug, there appears to be 2 white lines instead of 1. In other words, the line is bouncing around a bit instead of being steady. When it's on the left side, it's a solid 1 white line. It wasn't doing this before.

Again, the bike is running well, but i'm curious as to what might be causing this.