No love between 2500 and 3500 rpm??

[PRoberts317]

Hey gang,

Been working on my carbs a bit: They are stock, everything is stock other than pod filters instead of airbox.
Bike wouldn't really pull above 3500 rpm, so i turned the screws in a a half a turn (was at 2.5 out, turned em in to 2 out), that helped but wasn't great yet. So I turned them in a quarter more (now at 1.75 out). Idles OK, runs pretty darn good about 3000 and up, then stops at about 4500. Between idle (about 1200) and 3000 it barely wants to run, lurches, almost dies, etc.

Thoughts gentlemen, and ladies??

 

[VCPatel]

I can't comment conclusively on the source of your problems, but I can say that putting the mix screw in will cause it to run more lean at idle speeds and low RPMs.

It seems you were originally having trouble with the bike not pulling above 3500 RPMs, but you've altered the mix screw which, if I understand correctly, will alter your lower RPM settings. This is probably causing the lurching/almost death at lower RPMs. I'd go back and stick with the 2.25 turns out-- I don't think the mix screw settings are your problem.

 

[PRoberts317]

Why do you think she wont pull up about 3000 or 3500 with the mix screw backed out??

 

[xjwmx]

Small float changes have a powerful effect on these carbs. Have you checked the float level? For your year it should be 22 mm +-1 from the gasket surface to the top of the round part of the float, 21 +-1 if the gasket is attached to that half. The +-1 should be taken seriously.

 

[5twins]

You can't completely tune a set of carbs by just playing with the mix screws, lol. They only control low speed operation and idle for the most part. Set them at about 3 turns out for now and leave them, then install one size larger mains. The larger main should give you back the power at 3500 and above. The 3 turns out should help in the idle to 3K range. And read the carb guide in the tech section. To properly tune carbs, you need to understand a little about how they work.

 

[PRoberts317]

I'm aware of the fact that the mixture screw is not the only part of the carb that effects tuning it and how it runs. I just figured that since all the jets and everything else about the bike is stock that it was not the jets that came from the factory?

 

[5twins]

You said you put pod filters on so your bike is no longer stock. You've increased the air flow in. The BS34s are notoriously lean from the factory. Adding the pods has aggravated this condition. Turning the mix screws in (leaner) made things worse still. Totally stock BS34 equipped bikes have been reported to run better with one size larger main so you have to figure yours needs at least that.

 

[PRoberts317]

Roger that.... You think the float adjustment thing that the guy above you mentioned has anything to do with it?

 

[retiredgentleman]

The float level is what controls (any type) carb operation. It is fundamental and must be correct. Sounds like you don't want to do it.

xjwmx gave you some good advice..............ignore it at your peril.

 

[xjwmx]

^I've had my bike range from pouring out black smoke, to being hard to start and dying as soon as you roll on the throttle, the whole range of rich to lean, just by moving the floats a couple of mm. At 22 mm the stock jets work good on my bike.

 

[savage454]

My bike did the same thing, it wouldn't pull past 4500 rpm's. Give it throttle all I wanted it wouldn't go over 60mph. I one up'ed the main and pilot jets and it ran great. Even better with the 30 tooth sprocket. '75 open exhaust and pods.

 

[PRoberts317]

Thanks for all the advice guys... However, it turns out the problem was a bad charging rotor that was screwing up my timing and ignition.

 

[bradshaw106]

I know you found your problem, but when dealing with carbs it is more throttle position versus rpms.

 

[grizld1]

Ya know, I'm trying to wean myself away from replying to carb questions. The #1 rule for carb work is, "Everything else first." It's also #1 on the list of Frequently Disregarded Rules.

Why everything else first? Combustion=Compression+Ignition+Fuel. You can inspect compression and spark directly, but unless you have an EGA and a brake dyno (so you can open up the throttle and hold it there without blowing up the motor), you can only evaluate mixture by engine response, and even with that gear, you'll only get meaningful information if compression and ignition have first been eliminated as sources of trouble.