Pilot air/fuel screws?

[Big T]

It's most likely that the two tips you can see were over tightened at some time and expanded the seat, allowing the tips to come up higher

You could swap the tips around and see what results

[KEITH KRAUSE]

You have a Kerker and I assume the packing in it is OK. Does your stock air box have the lid? Are there any "extra" holes cut into the airbox anywhere? If the lid is on and there are no mod's to the box then you shouldn't need more than a 1/2 position richer on the jet needles. That will require some jetting spacers like the DJ stage 3 kit supplies (about .022"). I'd go up two full sizes (10) for the main jets. The stock pilot jets should work fine combined with richer than factory set fuel screw adjustments, about 1/4 turn richer or less. The pipe doesn't effect the pilot circuit like it does the main/needle circuits.
It's not as easy to get accurate plug reads when checking the pilot circuit. Just different vacuum levels can cause reads to be non-uniform. And I know what you mean about the pilot fuel screws protruding at different amounts into the carb bore.
Fine tuning the fuel screws can be difficult considering they were originally set using expensive emissions equipment. Do the best you can. If the carbs are clean, float levels correct, carbs synched and all jets in decent condition, then set them 1 full turn out from lightly seated. Set the side air screws to 2 turns out. Warm up the motor completely and set the idle to NO MORE than 1,100 rpm's using the idle knob adjuster. Start at any air screw and slowly turn in either direction to achieve best idle. Then re-set the idle to the baseline 1,100 rpm's by using the idle knob adjuster. Go to the next air screw and repeat. When done, focus on testing the pilot circuit of the bike. Low speeds only and idling. Check for excessive decel' popping (some is OK and is attributed to the freer flowing pipe). The idle should return smoothly and not hesitate to return (be sure throttle cables are routed and slacked correctly). After idling several seconds, a short, 1/5 blip of the throttle should reveal no dark exhaust coming out. Pulling away should be smooth without any flat spot or bog. If all is good, then check the plugs colors. Do what the plugs/performance say. We can help if you're not sure if you have a rich or lean condition.
What I wonder is if all 4 of your jet needles are the same size? Also, are you sure the factory setting for the jet needle e-clips is position 2 from the top? A pipe only/stock intake combo shouldn't require a full position richer on the needles. The Kerker flows better but is most noticeable at higher rpm's. About 7,000 and above. The main circuit is effected more but there is some overlap effect that I think 1/2 position richer would be right for.

[Drewbis1]

Thanks Keith, that is exactly the explanation I needed. I'm new to carburetors, so I'm learning as I go, and reading up on the subjects several hours per day so I feel I have to knowledge to tune this bike.

As far as the intake goes, it is a stock airbox, no leaks. I sealed it all up and cleaned/lightly oiled the air filter. I'll check the insulation on the Kerker. I do have new exhaust gaskets and stainless bolts coming for the headers to motor, so that should seal any leaks that may be coming from the old exhaust gaskets.

When I get my Carbtune on Monday I'll drop the needle to the 2nd richest location (2nd from bottom), fuel screw 1 turn out, air screw 2 turns out, sync carbs and test. I did swap the fuel screws around to see if it was in fact screw that was not being allowed to seat in as far as the others, but same results. I think there is more of an issue with the threads in the carbs, or the tapered seat inside the carb.

As for jetting, it is impossible to buy jets locally. I had to order a set of 110 mains as a test jet, and I grabbed 17.5 pilots just to have on hand. They probably won't be here for another week.

One more thing I have to do is check the points. The bike fires, but I'd like to check the timing and possibly look into a Dyna S ignition, coils, and new plug wires.

[OldVet66]

The needle tips in the pilot circuit are just visible below the hole in the carb body when lightly seated if they haven't been damaged. If you can't see two of them when lightly seated you might need some more cleaning in those passages. If you tweak your pilot needles in slightly so your highest idle adjusts out closer to two turns with the air screws, I think you will be in a better adjustment range. You are too far in when you get a slight stutter at low speeds 1,000-3,000 RPM.

[KEITH KRAUSE]

OK. Assuming the stock jet needle e-clip position is position 2 from the top, 2 positions richer on the jet needles is way too rich for a Kerker and stock air box. Also, the pipe doesn't effect the pilot circuit enough to change to a larger pilot jet. You'll be too rich at both circuits.
110 mains might work OK if not just right.

[Chuck78]

Specs for your float height should be 23.0 +/- 1.0mm, so you are running lean with your 25.5 setting.

Yes, the 76-77 used the lower float height (25 or 26mm). 78-9 was 23mm =/- 1mm I thought - refer to the factory service manuals and supplements, info gets mixed up even in the publications sometimes. DEFINITELY DO AN ACTUAL FUEL LEVEL CHECK using a clear hose piped from the drain port on the fuel bowl and held upright in a U-shape, I think the fuel level spec is 3mm off the gasket surface when the bike is running or in the prime position. a worn needle or seat will let fuel fill higher than wanted even if the floats are set to exact spec which is a baseline, the fuel level check is absolute and will tell you if you have a bad needle and seat. replace as a set if so. you can get some tubing and just bevel the tip a little and jam it in the hole hard for this test. I think motion pro or Suzuki makes an m6 adapter to do it proper as well.

Needles 2nd from the top will be leaner than stock.

Yes if you have the 5F21 needles. Some 78-79 models came with a leaner needle jet and a skinnier (richer at more throttle openings) dual taper 5DL36 needle that I believe was in the 2nd notch, as the taper starts a lot sooner than the 5F21's. I posted a chart on these needles a while back.

Adding a 4-into-1 will lean it a bit at the upper end, what main jets do you have? Stock is 100, with your Kerker, you might need 110s.

For your pipe, increase the pilot screws a bit, maybe to 1 turn on the fuel and 2 turns on the air.

Mine had 105's with a MAC 4-1, and that was good until I took out the baffle and then later made my own baffle. Both ran too lean on a 105 @ wide open throttle. 107.5 was good, but varying ring/cylinder/valve conditions on older engines will require you to do plug chops to really tell if you're dead on. K&N open element filters (no airbox) needed a 110 main to get the best plug chop readings for me. Yours might be a size or so bigger, as I have 850 pistons.


what Steve said on the 1 turn fuel 2 turns air screws. good baseline with a 4-1. Then carb synch. run a big fan in front of the bike while idling parked for a long run time while tuning, they are air cooled engines, don't forget. No air movement, very little cooling.

[Steve]

I knew they change a bunch in the early years, but did not realize quite how much.

Thanks for the education.

.

[Chuck78]

In the Clymer it specs my carbs from 25-27mm float height. I did notice a few drops coming out of the overflow from carbs 1 and 4 this morning.

You may have worn needles and seats, do an actual physical fuel level check like I described (see fact serv manual). If the fuel petcock is in the ON position, it should hold the fuel back if the bike is turned off, PRI prime means fuel flowing even without engine vacuum applied to the vacuum operated fuel petcock. never know, definitely check this to see if you have a leaker.

a not so known problem also is that the brass overflow tubes will develop little hairline cracks in them and seep fuel. temperature changes and dissimilar metals expansion/contraction rates. you can polish them up and inspect them very very carefully, or also fill the bowl with water, seal off one end with your fintertip, and blow in the other with 15psi from the air compressor with a rubber tipped blow gun, and watch for bubbles. soldering carefully can repair them, be sure to clean all the flux out after. Or you can ebay some or get some from the classifies WTB or ebay. I went through 12 to get 6 good fuel bowls that had no signs of cracks. two racks of junk carbs as donors.

and for your needle, a 5F21 (thicker) needle with your pipe should be in the middle or shimmed with the thin washers mentioned to be in the next lower from middle with shims to make it a 3.5 position. 4th clip notch minus thin shim washer = 3.5 so we call it. 4 is too rich I would imagine with a stock airbox. I think the other guys mentioned that as well. Good advice from Steve, T, and Kieth, I;ve learned a lot from them and others on here in the past several years since I started tinkering on the GS550.

You will really love this bike, especially when you start modding it. All it really takes is better tires and stiffer springs on top of good maintenance, and they are really awesome bikes. you can go miles beyond that with mods and keep getting sportier handling and a better ride. I never want to get rid of mine. Plenty fast enough. If you bump up your main jet and do some plug chops, you will be blown away with how much faster it is than a british/american/italian/german twin... the musclecar of motorcycles, the japanese inline 4 cylinder bikes!

[Chuck78]

Jetting and plug chops explained below. Find a deserted road that is long and straight and uphill, or a highway entrance or exit ramp that you can blast for 10 seconds for the WOT chop. If you start at 20mph in 5th gear at wide open throttle, I would consider that a little out of the normal useage range of a manual slide VM carb, so I'd hold it for longer than 10 seconds just to get you into the higher rev area that WOT is normally used in. Others may disagree, but for wide open throttle chops for the mains, I'd start at 3500 rpm and blast it uphill if possible at least 7 seconds if not 10 with some shifts in there so you don't go to the end of the tach and hear a big bang! Kill the kill switch, close the throttle, and pull in the clutch all exactly at once, swap the plug(s), and take the chop plugs home and literally cut off the threaded area to see the very base of the exposed insulator portion to see a light tan ring at the bottom, and no speckles of grey on the porcelain indicating too lean and melting pistons! on a 750, a wot chop ends up with you going VERY FAST, so hold on or just pay a dyno shop to run your bike with an air fuel ratio meter in the pipes instead. I prefer plug chops as I don't know the science of throttle ranges and afr's for air cooled engines. it varies, 10:1 is great for idle, 14:1 is perfect efficiency, but an air cooled engine needs to run a little richer, so 12.5:1 @wot...something like that. plug chops are a definitive concrete answer for me without knowing the science of air fuel ratios at various engine ranges.

http://jetsrus.com/FAQs/FAQ_spark_plugs.htm

http://www.xs11.com/xs11-info/tech-tips/maintenance/carburators/68-tuning-carbs-by-reading-spark-plugsthrottle-chop.html

http://jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm

http://www.strappe.com/plugs.html

http://www.dragstuff.com/techarticle...ark-plugs.html