Carb adjustment breakdown

[Gzus308]

So I've been fiddling with the carbs trying to get a nice fluid tune but with all the adjustments I'm starting to mess things up more then fix. So pretty much just wanted to clarify my understanding:

Mixture screws: clockwise turns them into the carbs reducing air in the mix
Counter clockwise turns them out from the carbs adding air in the mix
3 turns out is preferred

Main jet: for full throttle applications
Pilot jet: for idle

Slide needle: shimming or bringing the c clip down adds more gas to the mix

Is my understanding correct or am I wrong somewhere?

Thanks!

 

[bcware]

Pilot - idle to a few thousand rpms
Needle clip position - mid range rpms
Main jet - high rpms

Idle mixture screws on my SOHC deliver fuel, not air.
Around 2.5 turns out from fully closed is typical to start.
Turning them in, clockwise, tightening, or closing them, reduces fuel at idle.
Turning them out, counter-clockwise, loosening, or opening them, increases fuel at idle.

Shimming the needle or lowering the e-clip (effectively raising the needle) increases fuel in the mid range. Adjustment of the needle won't have any appreciable effect on idle or high rpms; it will simply affect the mid range and also the transition off idle and on main jet.

The needle blocks the fuel coming from the main jet. The higher the needle is shimmed or raised (or the more the e-clip is lowered) the sooner the needle rises and the mixture becomes richer at lower rpms. Also, by shimming the needle you would reach max fueling sooner because the needle needs to travel a shorter distance before it is no longer blocking the main jet at all.

This can have wonky effects at the extreme (trying to compensate for a main jet that is too small); your fuel ratio will be richest at something like 7,000 rpms, but as rpms rise further you'll actually be leaning out because no more fuel can be provided and yet the increased engine speed causes more air to be sucked in. I found that there was little need to adjust the needle height myself and often one is better served by bumping the main jet instead.

Also, you should be tuning for high rpms at WOT first. Then tune the mid range. Then tune for idle. Never tune for "choke." I just say that because some people actually try

What intake and exhaust are you running?

 

[Gzus308]

1980 xs400sg, pod filters, stock exhaust, 137.5 mains and 45 pilots, 2.5 turns out, 2700 feet above sea level. Issue I have is at around 55mph I get a bog down in the throttle and no torque for long uphills (no tach so no idea of rpms). Just got new mixture screws which I believe is the problem (before, the left cylinder was way rich while the right cylinder was lean and the mixture screw felt funny). Plugs are a little lean so just trying to enrich the mixture to try to break that power bubble. Has a lot more power than last season but still has the bog down at that 55mph mark.

 

[bcware]

45 pilot is spot on; pretty much everyone runs that with all modified setups (including me).

What's your needle clip position?

What's your float height? Do you have brass or plastic floats?

137.5 on the main is stock for my 1978. With pods and a 2-1 exhaust I am running 150's. My needle clip position is stock (4th notch down counting from the top). What was your original main size? This varies by year.

You really need a tach to tune this thing properly even if it is only temporary. You're blind without it. My '78 runs to 9,250 rpms and there is nothing stopping it from exceeding redline.

The mixture screws may have been causing problems at idle, but the pilot jet's contribution off-idle is negligible. It won't have any appreciable effect at 55 mph. At speed the pilot really just helps the rpms fall more rapidly when you go off-throttle.

What gear are you in at 55 mph? 5th? If I want power at that speed I would not be higher than 4th. You're not in the power band yet if you're in top gear at 55mph. These things rev higher than you'd think and the power band is very linear. Mine actually has plenty of guts up to 80ish and I think it might be able to come close to 100 mph.

Can you take a picture of the plugs. Reading them can be pretty tricky. Some fuels burn brown. Some burn gray and may appear leaner than they really are.

Are you using points ignition?

 

[Gzus308]

My slide needle doesn't have notches but is unshimmed. Brass floats and I forget where they're at (27mm maybe?) found the hight recommended here last season. 137.5 mains were 2 sizes up from when I bought the bike with stock filters. And as per the gear it's in 6th and the motor always sounds like it's being pushed. I thought it maybe was just maxing out until I read the thread where people seem to average 75mph. No points but a pickup. I was going to get fresh plugs today and get a better look at the plugs after a ride and I'll take a picture.

 

[bcware]

2 sizes up on the main sounds ok to me.

The float height should be 26mm if you're measuring without a gasket and 24mm if you're measuring OVER the gasket.

Your shimmed needle may be the issue.

The slightly lean looking plugs may be getting cleaned off by a slightly lean idle mixture. If you're at idle more than you're at high rpms the idle mixture will contribute more to the appearance of the plug. The place you spend the most time is going to contribute to how the plugs look the most.

Bogging or hitting a "wall" as rpms climb is usually a sign of being too rich. Don't put too much stock in how the plugs look. The only way to truly trust what you're seeing is if you do a proper single run with fresh plugs and literally cut the engine off once you reach redline and coast to a stop. Pull the plugs and chop them. Just looking at them regularly will only tell you if you're grossly out of tune.

If the rpms are hanging or they are reluctant to fall back to idle you're probably too lean. If the rpms shoot up high inappropriately or you get popping (not backfiring) you're also probably too lean.

If the rpms are refusing the climb higher or you're hitting a "wall" you're probably too rich. The wall you're hitting is probably the point at which the needle is rising maximally and you're getting full main jet contribution.

My plan would be to turn the idle screws out another quarter turn at most and to remove those shims from the needle.

Oh, and no matter the tune it's somewhat normal to be relatively low on power if you're in top gear at 55 mph. I agree that the motorcycle should be able to hit 75 mph without any issues.

 

[Scorpio1963]

One thing since you have messed with carb adjustments once you get them tuned as good as you can make sure you sync them this should prevent any other carb problems in the future.

 

[Gzus308]

Sorry, my needles are not shimmed. But I'll give it another go today and play with gearing. With no tach and going off sound may just be my issue. Thanks for all the info and I'll let you know how it goes!!

And I do resync after every adjustment

 

[bcware]

Sorry. I misread that. I see what I want to see sometimes -_-