Proper pilot air screws & throttle stop screw adjustments please.

Greetings....

I've been reading many posts regarding carb settings on my stock, '73 850 Interstate. Would like some clarification on a 2016 post I found below. I like his basic, simplified process without using gauges, pulling plug wires and fussing over lots of science. His last sentence wants me to adjust the throttle screws to balance out the exhaust pulses. OK, but when are the pilot air screws adjusted from his initial 1-1/2 turn cold setting?

2016 "Come on. Let's give the guy the correct answer. First, synch the carbs. Unscrew the idle screws (the one that is angled upward) so that they don't touch the slides. Stick a finger into each carb inlets and turn the throttle adjusters so that you feel both slides moving at the same time. You have to remove the air cleaner obviously. Once both slides move exactly at the same time you can set the idle. Close the air screw in to full stop, not too tight mind you, but full stop. Then, set both air screws at one and one half turns out. Then start the bike and adjust the angled screws a little bit at a time to get close to the idle you want. Once that's done, feel the exhaust pulses coming out of the exhaust and try to set those angled screws so the pulses from both exhaust pipes are about equal at the rpm you want your idle at."

Thanks for any reply and enjoy your weekend.
Dave

IMHO best way to set the idle is when the carbs are off the bike. Set the angle screws so that the slides show just a small cresent of light under the slide when viewed from the engine side of the carb. Opening should be around .015" or about the diameter of the #78 drill bit used to clean the pilot jet. Mark the position of the angle screws with a marker or paint. Once back on the bike make very small adjustments equally to both carbs to get the idle speed you want. I've checked the vacuum after doing this method and it is usually very close.

What I do:
1. Adjust the throttle cables so that the slides open evenly, fully and close at the exact same time.
2. Turn the pilot air screws clockwise until they stop; then turn each one out 1-1/2 turns.
3. Start the engine and adjust idle for decent running. Allow engine to warm up.
4. Adjust each pilot air screw for best running; then turn each one clockwise 1/4 turn. This will normally drop the idle speed, but it is a necessary adjustment.
5. Re-adjust idle speed, exhaust note balance and test ride. If a dead spot appears "coming off the pilot" when accelerating, turn each pilot air screw an additional 1/4 turn clockwise and reset idle speed. Repeat test.

998cc said:

What I do:
1. Adjust the throttle cables so that the slides open evenly, fully and close at the exact same time.
2. Turn the pilot air screws clockwise until they stop; then turn each one out 1-1/2 turns.
3. Start the engine and adjust idle for decent running. Allow engine to warm up.
4. Adjust each pilot air screw for best running; then turn each one clockwise 1/4 turn. This will normally drop the idle speed, but it is a necessary adjustment.
5. Re-adjust idle speed, exhaust note balance and test ride. If a dead spot appears "coming off the pilot" when accelerating, turn each pilot air screw an additional 1/4 turn clockwise and reset idle speed. Repeat test.

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Yes i've recently had that dreaded dead spot just as I roll on throttle from near fully closed to maybe 1/8 rolled open. Took me a while to figure it out but adjusting air screws to give a richer mix (ie turning clockwise) a little at a time solved this issue. Was not obvious until bike was under load, say riding up a long slight hill. So cannot go by what it does at standstill. Felt like a misfire without pops or bangs.
I presume what happens is the when throttle opens mix initially goes lean until fuel catches up to new vacuum demand. If your pilots are already quite lean, more lean gives a failed or lesser explosion. Adding more pilot richness, means that early transition to needle jet stays nearer ideal mixture.

Right when I get it running great I always end up with sooty plugs...

1) buy several sets of Champion N7YC (813) spark plugs. It is what the owner's manual calls for and they really are better. Available on ebay for about $6 apiece.
2) if the carbs are off the bike find a piece of wire .038 or .040 and adjust the slide down until there is a tiny amount of drag on the wire. (this will give you a slightly high idle...after it is warm then adjust it down.
3) do you very best to feel the slides moving upward and adjust cables accordingly. It is far more important to adjust the cables at bottom that to have the slides match at top. Most of your riding will always be at the low rpms so slide balance is critical. As the slide rises it has progressively less importance.
4) Your final adjustments are done after the motor has reached full operating temperature. Carefully lower each slide for the right idle. Your air screw will affect idle too.

Interstate said:

Greetings....

I've been reading many posts regarding carb settings on my stock, '73 850 Interstate. Would like some clarification on a 2016 post I found below. I like his basic, simplified process without using gauges, pulling plug wires and fussing over lots of science. His last sentence wants me to adjust the throttle screws to balance out the exhaust pulses. OK, but when are the pilot air screws adjusted from his initial 1-1/2 turn cold setting?

2016 "Come on. Let's give the guy the correct answer. First, synch the carbs. Unscrew the idle screws (the one that is angled upward) so that they don't touch the slides. Stick a finger into each carb inlets and turn the throttle adjusters so that you feel both slides moving at the same time. You have to remove the air cleaner obviously. Once both slides move exactly at the same time you can set the idle. Close the air screw in to full stop, not too tight mind you, but full stop. Then, set both air screws at one and one half turns out. Then start the bike and adjust the angled screws a little bit at a time to get close to the idle you want. Once that's done, feel the exhaust pulses coming out of the exhaust and try to set those angled screws so the pulses from both exhaust pipes are about equal at the rpm you want your idle at."

Thanks for any reply and enjoy your weekend.
Dave

Click to expand...

Tornado said:

Yes i've recently had that dreaded dead spot just as I roll on throttle from near fully closed to maybe 1/8 rolled open. Took me a while to figure it out but adjusting air screws to give a richer mix (ie turning clockwise) a little at a time solved this issue. Was not obvious until bike was under load, say riding up a long slight hill. So cannot go by what it does at standstill. Felt like a misfire without pops or bangs.
I presume what happens is the when throttle opens mix initially goes lean until fuel catches up to new vacuum demand. If your pilots are already quite lean, more lean gives a failed or lesser explosion. Adding more pilot richness, means that early transition to needle jet stays nearer ideal mixture.

Click to expand...

The other option for this problem is to adjust needle height. I had this exact problem, cured by adjusting needles. If I remember correctly adjusting up one notch, it was a few years ago.

Dave you may find the following article of interest .

Thanks for the link HRD. The page on EI's is spot on. Every analogue equiped bike I've ever had would eventually drop idle speed and stall out if left to sit idling for a bit. The one bike I ran points on would sit there forever.

It all sounds a bit too difficult. The first thing I do is make sure the inner throttle cables do not hang up in any way. Back off the throttle stop screws. Then turn the twist grip and raise the slides to the top and make sure they are there together, either by looking or by feeling with my finger down the chokes. The slide cutaways are usually identical. Then let go of the twist grip and allow the two slides to hit bottom. The two slides should hit bottom together and you should only hear one click - not a double click. If you hear a double click, adjust one of the cables, up and down until you hear only one click.
Then I adjust the idle mixture screws. They usually control the fuel rather than the air. I wind them both out two turns from fully closed, then start the bike. I then wind each screw in, in turn until the motor coughs then wind it out a fraction. You should get smooth idle with the throttle fully closed. If it is too slow, adjust the idle speed with the slides' stop screws.
All if this needs to be done with a warm motor - no choke.
I use a quick action Yamaha racing twist grip and cables. A throttle cable with a junction box can create difficulties. Inside it there is a cylindrical piece of nylon into which two shotrer cables fir and the main cable pulls them both, sometimes the end of two shorter cables do not seat properly in the nylon, so the slides in the carbs can end up unequal. If it is an old junction box, it will probably be OK.

If you are chasing adjustments, replace the little o-rings on the screws - can make a big difference - I am also considering making changing needle jets an annual event…

Derek Wilson said:

If you are chasing adjustments, replace the little o-rings on the screws - can make a big difference - I am also considering making changing needle jets an annual event…

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It might be interesting to know what fitting slower taper needles would do. The taper compensates for loss of vacuum if you open the throttle quickly. The depression over the jets affects the rate of fuel flow. Bigger ports can require faster taper needles.. But if you are changing needle jets the difference between just right and too large is extremely small. The difference in performance is often small but significant. I tune my 850 engine as I would a two-stroke. It means I must feed the throttle on, but the bike is faster that way.

The only reason I am changing needle jets is because they are a high wear item in an Amal Concentric carburetor. The pair I have in their now only have less than 6000 miles on them but drivability is suffering again…

Here's what's always worked for me; First and most important, get the floats at their proper level. If they're not, you'll tear off your wig off trying to fight something that adjustment screws cannot compensate for. Next, make sure both throttle screws are just touching the slides with slack cables. Eyeball the slides going up and down at the same time to the same height, open and closed. Once that's good, start the engine and raise both throttle screws the same amount until you get a 2500 rpm idle. Next, pull a plug wire and concentrate on one cylinder at a time. Set both air screws at 1.5 turns and fire the engine on one cylinder. Note the idle speeds as you go. lean the air screw out until it dies and set back in one turn. Do the other cylinder the same way. If the idle on one is higher than the idle on the other after air screw adjustment, crank up the throttle stop screw on the low side until they're the same. Shouldn't take much to get them both the same. Reconnect the off plug wire and set the idle back down by moving both idle screws the same amount. You should be able to get a loping 700 rpm idle, which you'll probably need to crank up a bit to keep it running, especially when it's still cold.

If your bike cold starts without choke, you have other issues to address before synching the carbs.

If you have an ei and you take a plug wire off be sure and ground it.