Need carb help please

[Barbarosa]

I am at wits end with a 1980 SG that dry carbon fouls plugs. Carbs are clean yada yada yada …… It will go around 50 highway miles (5 mi @ 50-75 mph, stop/go back/repeat) before running problems begin to develop. The first 20 or so miles it runs like it is supposed to, accelerates up to 8k without a sputter. The idle will drop to 800 when shutting down, but immediately level out at 1k and stay there. After that first few miles I’m guessing that the plugs have handled all the rich stuff they can and start loading up with carbon. I installed the KL 18-4676 valves and 110/42.5 mikuni jets a while back and have set the float levels from 23 to 26mm, ran without air box and with open header, advanced the timing, and ran a bps5 plug and saw little change. Compression is 145 psi across and leak down is 60/58. I have done all I know to do. I am about to order smaller main jets, and after that guess fails, I will probably try some kind of jetting kit LOL! Would rather beg mercy on my soul for help than to waste more time and resource taking shots in the dark. Is there a “carb doctor” in the community? I am not opposed to shipping them somewhere if it’s known a good source that will either get mine up to snuff or take them as trade-in on another set.

[motoman]

To make a long story short, a to low a fuel level(which is your carbs case) does, and will create a rich running cond.. Re-set those floats at a spot-on 23mm, warm up, and with a cooling box fan in front of motor, correctly re-set idle mixture screws and re-sync carbs.

[Barbarosa]

Have set at 23, 24, 25, and 26mm looking for the sweet spot. Clear tube tested from top to bottom of bowl lip with even levels and still fouls. Have tried every tip and bit of advice I've gotten and still can't dial in carbs with stock innards. That's why the call for help beyond just words unless they are a magic spell lol.

[mack]

K&l

The 18-4676 are the correct number for your year but you need to check for the mikuni logo. Jets R Us is not to good on the quality control aspect and while the chart says genuine mikuni, what you get may be from china or Taiwan. That will account for the float height issues. Lean out your idle mixture settings, like you said your doing a lot of stop and go so your idling more that normal. Last thing I would check is to ensure your pilot and main jets are mikuni as well. The K&L orifices are different diameters.

[MPittma100]

Rich

Sounds way too rich with stock jetting. Make sure that you are not setting the float height with bowl gasket in place. If all cylinders are rich, something is wrong times 4. Some of the later carbs used a rubber plug inside. Not sure if yours do, but something to check.

Make sure there are no restrictions in the air intake system - filter, air box snout, etc.

Rich means too much fuel or not enough air.

[3Phase]

How many plugs are doing this?

If you're sure the jetting is correct, have you checked the slide diaphragms for holes or the wrong needle(s)?

[dbeardslee]

I find a colortune plug very helpful when it comes to getting the jetting right. If you can't get a bunsen blue flame by adjusting the mixture screw it's a pretty good indication that the jetting is off. JAT

[Barbarosa]

[QUOTE=MPittma100;490713]Sounds way too rich with stock jetting. Make sure that you are not setting the float height with bowl gasket in place. If all cylinders are rich, something is wrong times 4. Some of the later carbs used a rubber plug inside. Not sure if yours do, but something to check.

Make sure there are no restrictions in the air intake system - filter, air box snout, etc.

Rich means too much fuel or not enough air.[/QUOTE

I have set the float levels to cover the entire "lip" area of the bowl and even a mm below the bottom. If there is a position where the level is correct I couldn't hit it moving the levels in a systematic manner of 1mm per change. Even ran it without any intake restriction; no breather or muffler. No passage for plugs and it still fouls all 4 even with new dyna coils to step up the voltage.

[Barbarosa]

The 18-4676 are the correct number for your year but you need to check for the mikuni logo. Jets R Us is not to good on the quality control aspect and while the chart says genuine mikuni, what you get may be from china or Taiwan. That will account for the float height issues. Lean out your idle mixture settings, like you said your doing a lot of stop and go so your idling more that normal. Last thing I would check is to ensure your pilot and main jets are mikuni as well. The K&L orifices are different diameters.

The float levels hold on bench test so if they are leaking it's while they are on the bike. I am pretty sure they aren't though since there is no overflow to outside the carb or into the crankcase. The primary's are at around 1/4 turn and can actually be shut completely off so not much control there. The stop and go driving is running up the rpms to 7k or so through the gears and accelerating from 50-75 mph for 5 miles. Don't really stop so much as slow down and let it go to idle long enough to turn around. Pilots, main jets and jet needles are stock mikuni. I am stumped because with stock carb innards and no restriction on air intake (running without airbox/mufflers) I still can't get them to lean out.

[Barbarosa]

How many plugs are doing this?

If you're sure the jetting is correct, have you checked the slide diaphragms for holes or the wrong needle(s)?

All 4 carbon foul. With fresh plugs you wouldn't know there was a problem, it takes about 20 miles to be noticed. Diaphragms good and numbers on jet needles match stock. Looked around for dimensions/specs for the needle so I could check for wear beyond visual inspection but could only find where taper starts, not the diameter.

[3Phase]

So whatever it is fouls all four cylinders and not just one. Hmm....


Was the bike running before you worked on the carburetors?

Did this start happening after you changed the jets and float valves?

Why were the jets and float valves replaced and do you still have the old parts?


If the fuel jets are genuine Mikuni, that leaves the K&L float valves losing their minds and sticking open after they heat up during a ride.


The Start circuit, Pilot air and Main air could be off too and goes back to the question about the bike running or not running and why the parts were changed.

The rubber face on the Start circuit enrichener valves/plungers can harden and leak but it's really unlikely they would all four start leaking at the same time unless the carbs were soaked in carb cleaner
. That would eat the throttle shaft seals too.

The Pilot air jet is removable and is supposed to be a Mikuni 185.

The Main air jet is not removable but it could be drilled out to make the hole bigger or the entire jet can replaced with a new one if you can find press-in jets.

[Barbarosa]

So whatever it is fouls all four cylinders and not just one. Hmm....


Was the bike running before you worked on the carburetors?

Did this start happening after you changed the jets and float valves?

Why were the jets and float valves replaced and do you still have the old parts?


If the fuel jets are genuine Mikuni, that leaves the K&L float valves losing their minds and sticking open after they heat up during a ride.


The Start circuit, Pilot air and Main air could be off too and goes back to the question about the bike running or not running and why the parts were changed.

The rubber face on the Start circuit enrichener valves/plungers can harden and leak but it's really unlikely they would all four start leaking at the same time unless the carbs were soaked in carb cleaner
. That would eat the throttle shaft seals too.

The Pilot air jet is removable and is supposed to be a Mikuni 185.

The Main air jet is not removable but it could be drilled out to make the hole bigger or the entire jet can replaced with a new one if you can find press-in jets.

Bike sat for 2 yrs, PO said was running fine lol. Bowls were clean so I added gas and started. Ran ok till it started dropping cylinders. Cleaned carbs and replaced float valves because 1 was mismatch. Next cleaning the unmarked jets were replaced with stock mikuni. The next 5 disassembles were float level adjustments. The last time I had them off I took them down to bones again and rechecked every passage, made sure the plungers were sealing and checked the jets on the intake side again for marking and flow. I'm stumped because I can't even induce a lean mix by purposely setting the floats low or completely removing intake restrictions. Here's an odd ball thing I discovered on the last trip out .... the ignition fuse is getting hot enough to melt the electrical tape I have it rigged up with. I unplugged the ends from the glass fuse holder and securely connected them to a spade fuse. None of the other circuits are making heat like that.

[Ranger_xs1100]

smaller jets

I just recently had the opposite problem...

Have stock mikuni jets but I could not make mine run richer!
I tried raising the needles, playing around with float levels, turning mixture screws out as far as possible. The solution was bigger main and pilot jets, actually 4 sizes bigger on the main and one size bigger on the pilot. Then I was able to effectively adjust the fuel air mixture with colortune.

Soooo, my point being, maybe you just need to go with smaller jet sizes? who knows what the PO did inside those carbs! Just a thought

[3Phase]

Maybe the ignition coils and/or the TCI are overheating. Are you still running 3 Ohm coils or OEM 1.5 Ohm coils without the ballast resistor?

[Barbarosa]

Have the right dyna coils on it. Thought the new coils fixed the problem when I first installed them. Before the new coils it would only go a couple of miles before dropping cylinders. Was really bummed when it started acting up again, but at least there was improvement in how long it took to foul the plugs lol.