1976 Running Rich

[5twins]

Yes, the diagrams JP has posted pertain to the BS34's. A BS38 set at 24mm would be about at the bottom edge of the main body .....

But, the spec for the '76-'77 carbs is 25mm, so that would put the level slightly lower than pictured above. Here's one I found set at 26mm, so you could split the difference .....

As far as the floats go, I found they are nothing special, just the common and readily available Mikuni VM24-171. These are available from many places for around $15 or less .....

I have purchased some and compared them to originals and they are an exact match.

 

[Wilsonsk]

Here is my right carb, I was in a hurry, couldn't find my caliper or pocket scale, just wanted a look-see. I started it and ran it some, then shut it off to take the pic. The left one was about midway up the belt on the bowl, not quite to the bottom lip of the body. I set my after market floats at 24mm, which is incorrect for 76-77 (supposed to be 25mm), but with aftermarket floats that's probably within the noise of the manufacturing tolerance of the floats, this is the resulting fuel level. I made a barbed fitting out of a long 8mm-1.0 bolt, used the brass washer and a jam nut, works great. I put it in my growing carburetor parts tray for future use, I put the plugs back in the carbs. I had tubes on the bottoms of the bowls of one of my Polaris sleds years ago, great for draining carbs at the end of the season, but 1 cracked and melted down a piston, they make me nervous now.

 

[Wilsonsk]

Far too early to declare a victory, BUT, I went for a quick spin at break time. Before I buttoned things up this morning, I moved the needle clips to the center position, where they were from the factory.

So,

MJ 122.5 (old, black, crusty origninals)
PJ 27.5 (new from rebuild kits)
New Z8 needle jet from XS650 direct
Old needles, clip in CENTER position, moved down from most lean position
Mixture screws 2 turns
Fuel height, left - midway on the belt on the bowl, right, just below the belt on the bowl (sorry, no better numbers at this time). I probably should tweak that some.

There may be just the slightest burble off idle, but WAY better.
In my rip around the block, I made it up to 5,000 with no stumble, that's better.

I'll do more "research" after work and see how it all shakes out, but I will say, I should have followed instructions, 5twins told me to put everything back to stock when I put my new needle jets in, I cut a corner, thinking that I would still need to be a little lean because my old crusty mains may be wore a little big, so I started with the clip on the second to leanest slot, then when that ran bad, went to the leanest, ran worse, off-idle performance completely sucked. Put in the 27.5 PJs, didn't help. While I was messing with float heights I figured I'd move those clips back to stock positions. Seems to run pretty well now, maybe a little fat on the PJs? I am still 2 turns out on the mixture screws, I can play with those.

 

[5twins]

Yes, on these older carbs, you should be able to set the mix screws just by listening to the cylinder you're adjusting. Get much past the ideal range (either above or below) and the cylinder should slow down and run rougher. I'm thinking 2 turns out may be a touch too much, especially with that larger pilot. Many times, a larger pilot requires slightly less turns out on the mix screw (about 1/4 turn).

 

[Wilsonsk]

I futtzed with the mixture screws, they’re somewhere between 1 and 1 1/2.

I AM willing to declare victory now, I rode it a bit more, enough to know that this is the best it’s ran to date since pulling it from its shed. There may be a stumble 4,000-5,000, may be my PTSD (post traumatic stumble disorder), almost nothing if it’s anything, regardless, I’ll take it. I don’t get much past 4,000 if I’m riding normally anyway. I put a 16 tooth front sprocket on it while I had the cover off to do the clutch cable, I did that with my Honda CM400, that thing doesn’t have much torque. I don’t think I really need it with the 650, it’d be nice to get a few rpms back at 60-65 mph. Thanks again for all of the help! I have the new 122.5s coming, I may put those in, see if there’s any difference, I may just ride it.

 

[5twins]

Well, a 17T front sprocket is stock, so if you put a 16T on, you've geared it down, not up, and you'll be turning more RPMs at 60-65. Yes, these bikes do benefit from a gearing change, but only a very small one. Stock gearing was 17-34. With an 18" rear wheel, I feel the ideal gearing is 17-33, just one tooth down on the rear. This will knock a couple hundred RPM off at cruising speeds but won't adversely affect the take-off power in the lower gears. In other words, it won't kill this bike's "fun factor".

 

[Wilsonsk]

Victory is fleeting, I still think it’s running as good as it has, but it intermittently seems to starve for fuel, if I get on it, it seems to starve, if I slow down it picks back up. Rode conservatively for 3 or 4 miles back home, no problem. Everything looked pretty clean when I had the bowls off, there are screens on all of the pickup tubes, they are pretty tall, I put it on reserve when it did it the first time tonight, I only had 30 miles on that tank, it did it again right after that, slowing down and riding easy seems to be fine. Tank came pretty clean, I suppose there could be some new flaky stuff in there? When it does run, it runs great.

 

[Wilsonsk]

Well, a 17T front sprocket is stock, so if you put a 16T on, you've geared it down, not up, and you'll be turning more RPMs at 60-65. Yes, these bikes do benefit from a gearing change, but only a very small one. Stock gearing was 17-34. With an 18" rear wheel, I feel the ideal gearing is 17-33, just one tooth down on the rear. This will knock a couple hundred RPM off at cruising speeds but won't adversely affect the take-off power in the lower gears. In other words, it won't kill this bike's "fun factor".

I understand I geared it slower, I meant pick up by lower the rpms, I just figured with a semi scrambler build I’d need some gearing, I hadn’t ridden it yet. I’ll change it back. Need to figure out what’s going on with my fuel now.

 

[Wilsonsk]

I think I have some flaky stuff in my tank, which may explain why I thought it was running out on ON sooner than it was, I think screens plug up, then I switch to reserve, those are lower in the tank and stay cleaner, and tonight, they plugged up too, I looked in to tank, looks like a bunch of crap I couldn’t see before broke loose, maybe from the top of the tank?

 

[Wilsonsk]

When I first got the bike I did a CLR soak, several flushes, #8 shot in the tank and shake, getting pressure washer nozzle in as far as I can. It looked much better than it did when I start, now I think what’s in there is gooey varnish, not rust. What’s best to dissolve the varnish? Turpentine? Denatured alcohol? Lacquer thinner? Paint thinner?

 

[Wingedwheel]

Acetone should get it out. Was there possibly some type of liner in that tank?

 

[Wilsonsk]

I dumped the tank, gooey, flakey, black varnish. It is right at the line where the old gas was. The rest of the tank cleaned up really well, just didn’t get through the varnish. I’ll get some acetone and a dog chain see if I can get it cleaner. I’m a little worried about acetone, my paint’s not perfect by any stretch, but the cap doesn’t seal 100% even with a new seal. I may try some diesel first.

 

[jetmechmarty]

I dumped the tank, gooey, flakey, black varnish. It is right at the line where the old gas was. The rest of the tank cleaned up really well, just didn’t get through the varnish. I’ll get some acetone and a dog chain see if I can get it cleaner. I’m a little worried about acetone, my paint’s not perfect by any stretch, but the cap doesn’t seal 100% even with a new seal. I may try some diesel first.

Buy a rubber plug from the hardware store. Acetone will dissolve varnish and most old liner material. Diesel, won’t really get it done.

 

[Wingedwheel]

It will mess up your paint if you’re not careful. Like Marty said, get a plug but you can also tape off the top of the tank with plastic. Put it over the neck, use a big rubber band and then cut your hole. Just a little insurance.

 

[5twins]

I cut a round piece of rubber from an old inner tube or some sheet rubber to cover the hole, lay it on there, and close the cap over it. This has always worked well for me in the past.

 

[Wilsonsk]

I cut a round piece of rubber from an old inner tube or some sheet rubber to cover the hole, lay it on there, and close the cap over it. This has always worked well for me in the past.

That's what I did, worked well. I also washed the tank immediately after closing the cap and rinsed it really well. I put 10 feet of dog chain in it and really focused on the sides and top. It looks really good now, there was just a bit left right on the seams at the bottom of the tank, I suppose the chain doesn't quite get in there. I put another half gallon in just to cover that last bit, I'll let it sit for a while then drain that and wash the tank again. When I first dumped it this morning I was amazed at how much loose, flaky varnish there was that just came out on the first dump, just doing that probably would have solved my starving problem for a while. When I looked in the tank with the fuel in it, the fuel looked nice and clear, I suppose the flakes were just laying there.

 

[Wilsonsk]

Looks pretty good now, there is just a little still in the seam, not sure I can get that. I'll run with it for a while and dump it again, just work through it. The screens on the after market petcocks work pretty well but they don't have much surface area so they blind off pretty quickly with the flaky stuff.

 

[Wilsonsk]

Got everything back together, I dumped the bucket of what I cleaned out of the tank, there was a LOT in there, I didn't really look yesterday, it was all churned up. Should be in considerably better shape for a while, I'll ride it a bit and drain it again.

Back to how it runs, it runs as good as it has since I got it running, still have a little stumble at 4,000-5,000, I have new 122.5s and 120s on the way, I'll try those when I get them. Even with the stumble, it pulls harder in that 4,000-5,000 range than it ever has and sometimes it just pulls through it and doesn't stumble. It seems like it starts harder now, requires more nursing when cold, wants more choke. I'll play with the mixture screws some, turning them in another 1/4-1/3 turn seems to pick it up a little, once hot it idles nice at 1,000 rpms. The mixture screws are probably at 2 turns or a little less, the changes are tough to read on those, but you can hear something changing.

 

[5twins]

I should also mention that when you measure and set the floats, you need to measure from both sides, check each float "bulb", because they are often twisted on the mount, one sitting higher or lower than the other.

 

[Wilsonsk]

I thought of the floats being twisted after the fact, I feel a little better after checking the fuel level.

Got my new 122.5 and 120 main jets today. I’ll try them tomorrow.