Tuning the Atomic BB

biknut

biknut

Well-Known Member
Now that I got most of the bugs worked out, I've turned my attention to tuning. So far I've retorqued the head bolts. Retorqued the exhaust. The intake had a slight leak so I resurfaced and cleaned up the manifold and torqued it on. 80 inch lbs for the intake and exhaust and 125 inch lbs for the head.

After 1 gallon of 16:1 break in mix I changed the spark plug and now running 50:1 Red Line synthetic. Also leaned it a little by moving the needle clip from the middle, to the next to the top position. Just started on the second gallon of 50:1. The spark plug is looking dark so I removed the float bowl and found that the jet holder was unscrewed from the carb body. I didn't make any changes, just put it back together correctly.

After riding it, tightening the jet holder didn't seem to make much difference except it's harder starting now. When I hit 28 mph it's like I hit the wall, and my speed even seems like it backs off slightly. The way it feels is a little like a governor.

I'm going to check the plug in a minute.



 
Ok this looks like a pretty dramatic improvement on the plug after screwing the jet holder back in.

This is what it was looking like before I changed it 2 days ago.



This is what it looks like now. This plug ran about 45 miles before I fixed the jet holder and about 5 miles since. Big improvment me thinks.



Maybe I should put the needle back in the middle position? What you think?
 
It feels like when I hit about 28 mph it starts starving for fuel.

Maybe I should check my float height?
 
Can anyone tell by looking at these pics if I need to adjust my float height? If I do, the next thing I'm going to ask is how do you adjust it?







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I was going ask 'bout the needle 'cause it was moved befor the jet discovery,the float doesn't look as if it goes much further down in the bowl than the brass fingers reach now,
 
corgi1 said:
I was going ask 'bout the needle 'cause it was moved befor the jet discovery,the float doesn't look as if it goes much further down in the bowl than the brass fingers reach now,
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I was just browsing the search answers, and came to the conclusion the float is probably about right. It measures about 2mm above the gasket now. It it was much lower the primer would be hitting it. I think I good to go.

the needle is now in the middle position where it started out, and seems good. I think I ride it a while like this. It's running pretty good. Thanks for looking.
 
I guess I got to take a carb apart and look,I thought the primer was primative on these and was susposed to push the float down so the fuel bowl would over fill a little and the carb could draw the over filled fuel up to the thorat easer,and maybe if the float stuck 'cause if carb angle it could shake it loose
 
I rejetted today

My stock Chinese jet is not marked, so I had to use some drill bits to figure out what size it is. I'm calling it a .70mm. I replaced it with a .68mm.

I got to ride a little bit tonight and it seemed ok, but I need to do a longer ride before I can check the plug again.
 
Rode around today and got used to how it's running. Ended up dropping the needle one slot. Back to the clip in the next to the top position.

It's running real good now. It doesn't accelerate up to 28 and then drop back 1 mph now. It just runs up as fast as it will go. That depends on the grade, wind direction, and how long the street is. I saw a little over 31 mph several times today, easier than with the 70 jet, but it takes at least 2 blocks to do it. Leaning the needle seemed to help the off idle, but I noticed it a little more before the motor warmed up all the way. I'll probably try the needle in the leanest position.

Another thing working well is the idle. Ever since I sealed intake leak, the idle is dead spot on all the time now. It probably has the best most consistant idle of any bike I own.

I think the main is pretty close because now it won't take full throttle until it's warned up a little. With the 70 jet it didn't seem to matter.

I'm pretty sure the main stopper in the system now is the exhaust. I know it's pretty restrictive, but it's quite. My bike has more gear noise than exhaust noise.
 
I dropped my needle to the leanest position today. I think my jetting efforts are paying off because riding my neighborhood race course today I set a new record 33 mph flat, on the slightly downhill section, and tail wind lol. Takes 2 blocks to do it, and have to really push the stop sign at the end. Actually it gets up to 28 in the first 1/2 block and then takes a bolck and half to go the other 5 mph.

The jury's still out on the needle, but I couldn't tell it hurt anything so I'll leave it for now.
 
I opened the spark plug gap to .030". Didn't notice much difference so I decided to leave it that way.

At this time I'm running a 28 jet, leanest needle position, .030" plug gap on a NGK PB6HS, and 40T sprocket.

It's running really well but a little cold natured. Have to use the choke to get it started, and it has to warm up a little before it takes full throttle.

So far I've been using super gas. Next I'm going to try regular and see if there's any difference.
 
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I just tried some regular gas for the first time. I didn't notice any difference from super. Top speed was still about the same. 27-30 mph.
 
biknut said:
I just tried some regular gas for the first time. I didn't notice any difference from super. Top speed was still about the same. 27-30 mph.
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There's a cold front coming in tonight so this afternoon I decided I better get some riding in while the gettins good.

I went on a 13 mile ride, and with a north wind at my back I was hitting 34 mph for a couple of miles. This was the second or third time I've got up to that speed but this was the first time I did it on a level road surface. Before it took a tail wind and downhill to do it. I'm thinking now that if anything it may be running a little better on regular gas. At any rate nothing makes me think I need to go back to super.

My motor didn't really sound all that stressed at 34 mph with my 40T.
 
biknut said:
There's a cold front coming in tonight so this afternoon I decided I better get some riding in while the gettins good.

I went on a 13 mile ride, and with a north wind at my back I was hitting 34 mph for a couple of miles. This was the second or third time I've got up to that speed but this was the first time I did it on a level road surface. Before it took a tail wind and downhill to do it. I'm thinking now that if anything it may be running a little better on regular gas. At any rate nothing makes me think I need to go back to super.

My motor didn't really sound all that stressed at 34 mph with my 40T.
Click to expand...

I noticed today the barometric pressure was high. That was probably helping.

I got to tell you, I wasn't real comfortable going that fast over rough pavement with crazy ass Dallas drivers following me. I've been noticing car drivers seem to get all jiggy when they come up behind me. It's like they're afraid to pass me.

I couldn't wait for my forks to break in half, and feel my butt skidin across the asphalt in front of a couple of SUVs.
 
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