Carb help wanted

[the-oz-slider]

Hi everyone,
Have got my 1980 XS1100 midnight special up and running, but its running poorly. My question is....is there a rubber plug sealing the bottom of the idle [small/secondary] jet in the bottom of the carby?
There arent plugs in any of the four carbys, but the bike has been fiddled with, so Im not sure. I did a check of the Babbits parts site and the old fiches arent clear enough for me to tell definitively.
I have a set of carbs off a 1980 XS650 and they DO have the plugs, so now Im confused.......
Help me, before I have to take my carbs off one more time................
Thanks all.........
the-oz-slider

[cywelchjr]

It depends. There are two versions of the 80 carbs, early and late, the early ones have a passage drilled between the low and high speed jet passages and if that passage is there you need the rubber plug. The late carbs do not have the passage between the low and high speed jets and those don't use the rubber plug.

[3Phase]

How's it goin' Oz?

If the engine has been starting and running without the rubber plugs it probably doesn't need them.

The plugs only go into the carburetors IF there is a crossover hole drilled in the well between the top of the pilot jet and the bottom of the emulsion tube. In the carburetors designed to use the plugs the main jet supplies the fuel for the pilot jet instead of allowing the pilot jet to draw fuel directly from the float bowl.

If the crossover hole was present and the rubber plugs weren't installed, fuel would be drawn from the float bowl through the hole and into the emulsion tube but it wouldn't be metered by the main jet, only by the slide needle. The engine would run insanely rich if it ran at all.

If you put the rubber plugs in a carburetor with no crossover hole, the pilot jet would not get any fuel.

[cywelchjr]

How's it goin' Oz?

If the engine has been starting and running without the rubber plugs it probably doesn't need them.

The plugs only go into the carburetors IF there is a crossover hole drilled in the well between the top of the pilot jet and the bottom of the emulsion tube. In the carburetors designed to use the plugs the main jet supplies the fuel for the pilot jet instead of allowing the pilot jet to draw fuel directly from the float bowl.

If the crossover hole was present and the rubber plugs weren't installed, fuel would be drawn from the float bowl through the hole and into the emulsion tube but it wouldn't be metered by the main jet, only by the slide needle. The engine would run insanely rich if it ran at all.

If you put the rubber plugs in a carburetor with no crossover hole, the pilot jet would not get any fuel.

Oh, it will run, sort of. But lots of black smoke and sputtering and fouled plugs and not very happy sounding. Things are not good. I found out on my XS400 cause when I put it back on the road the Ca gas ate the original plugs (they literally disappeared from the carbs, they were there when I put them together after cleaning them, and two days later when it started spitting and sputtering and such they were just black slime in the bottom of the bowls) . I ordered nice new ones from MikesXS which are holding up just fine, and things are good. Never had my XS1100 carbs open, so don't know if I have the hybrid carbs in it or not, but my 400 even has brass floats (and they are factory, I'm the second owner and my uncle the first owner didn't do anything other than clean them each spring). Until I got on here I thought ALL 80 Mikuni's had the rubber plug, I didn't even know there were two types, maybe if I ever have mine sit long enough for the carbs to need cleaning (longest it's sat in the 2.5 years I've had it was 2 weeks while I swapped engines, carbs were drained while I did that) I'll find out about the floats, or if I make changes that need jet changes.

[3Phase]

The gas ATE the rubber plugs out of your carburetors?!

Yeah, I figured a good engine might manage to start without the rubber plugs if the carburetors were supposed to use them but anything above idle would be a whole lot worse than "running poorly". Of course, Oz could be an Aussie or British and they both tend to understate the catastrophic.

[the-oz-slider]

hohohoho, I'll have you know that my ancestors were chosen by the finest judges in Egland........
so now that the niceties are out of the way.


Bike runs good on idle so the rubber plugs ovbiously dont need to be there, but it blubbers/kinda stalls and doesnt run properly over 4500 rpm.
Maybe a new set of plugskies may help, or not.
The bike only has 5125 km on the clocks, and I have treated it to new paint, tyres, seat cover, ceramic coated exhausts, oils, fuel, filters, pads etc
Methinks I should forget the rubbber plugs and go looking for soemthing else to explain the poor performance....it sure runs nice now low though.
t-o-s

[Lbrown311]

Are the carbies recently cleaned and clutch adjusted? Does that run an octy? If so try running on prime? Could be starving at high RPM's. I'm a noob, but I went thru all this with mine and am slowly figureing this thing out. Clutch would probably rev not die, but can't hurt to adjust them correctly. Mine was so far outa wack I could barely take off before I adjusted it. Now it feels like my Dad's '07 Stratoliner.

[3Phase]

T-o-s, welcome to the forum!

There are a lot of things that could be going on with your bike if the engine won't go over 4,500 RPM. Do you have a factory shop manual?

Is the engine stock? Is it the original engine (engine serial number matches frame number) with the correct TCI?

Is the new air filter you installed an O.E.M. filter or an aftermarket filter?

Is the air box in good shape and the inlet hose under the seat is not plugged with a critter nest?

Has someone removed the air box and installed pod filters?

Is the ceramic coated exhaust a factory exhaust or an aftermarket exhaust??

Does fuel flow freely from both nipples on the fuel tap? Put a hose on each nipple and run them to a plastic container. Turn the lever to each position: Prime, Reserve, and On to see if the tap or the filter screen in the tank is blocked.

Is the octopus (fuel control valve) working and all 9 hoses hooked up correctly?

Are the carburetors jetted correctly and do they have the correct needles in the slides for the jets that are installed? Each carburetor in a stock '80 Midnight Special should have:
42.5 pilot jet
185 pilot air jet
110 main jet
5GL16 needle
Series 300 X2 emulsion tube

The jetting might be different if you have an aftermarket air filter and/or exhaust system but at 4,500 RPM the engine is mostly running on the main jets in the carburetors.

Are the carburetor slide diaphragms in good shape with no holes? With the enrichener closed, lift each slide with your finger and block the large oblong vent hole at the mouth of the carburetor. The slide should not drop or it should drop very slowly. If the slide sticks or doesn't move smoothly then remove it and check the slide and the slide bore. Hold the diaphragm up to a strong light to look for holes or tears in the diaphragm.

Is the enrichener (choke) all the way off and not leaking?

Have you checked and adjusted the valve clearance?

Have you checked the compression so you know you're not trying to tune a shagged engine?

When the engine starts to die, pull over and open the fuel tank filler cap . If you hear air hissing into the tank the cap vent is plugged and a vacuum formed in the fuel tank. The bike will run at astonishingly moderate rates of speed if the fuel can't get to the carburetors.

Check the ignition timing to make sure the static and dynamic timing are both good. If the centrifugal advance and vacuum advance are not working the engine will run retarded at higher RPM. It will gulp fuel and it won't have any power.

Test the pickup coil wires while you're checking the ignition timing. The pickup coil plate rotates with the vacuum advance and the wires can break. Pull gently but firmly on the wires. If any of them stretch and form an 'hourglass' in the insulation the conductor is broken inside the insulation.


There is more but that should get you started!

[cywelchjr]

hohohoho, I'll have you know that my ancestors were chosen by the finest judges in Egland........
so now that the niceties are out of the way.


Bike runs good on idle so the rubber plugs ovbiously dont need to be there, but it blubbers/kinda stalls and doesnt run properly over 4500 rpm.
Maybe a new set of plugskies may help, or not.
The bike only has 5125 km on the clocks, and I have treated it to new paint, tyres, seat cover, ceramic coated exhausts, oils, fuel, filters, pads etc
Methinks I should forget the rubbber plugs and go looking for soemthing else to explain the poor performance....it sure runs nice now low though.
t-o-s

Down low is where it runs fine if it needs the rubber plugs and they are missing, it's when it transitions to the mains that it all goes to heck in a handbasket. When the slides lift, raw gas pours into the venturi up through the open hole from the bowl that's been left open cause the main jet has been bypassed. The only way to know for sure with it not running right is to look in by the idle jet and see if there is a hole going towards the main jet passage that slants up somewhat towards the top of the carb. If there is, then you need the plug, if there isn't a hole there going to the mean jet passage, you don't need the plug in there.

[cywelchjr]

The gas ATE the rubber plugs out of your carburetors?!

Yeah, I figured a good engine might manage to start without the rubber plugs if the carburetors were supposed to use them but anything above idle would be a whole lot worse than "running poorly". Of course, Oz could be an Aussie or British and they both tend to understate the catastrophic.

Yup, ate them up. I know it ate them, cause they were there, and a week later when it started running like crap they were gone. I found that most of the ORIGINAL rubber parts in the carbs disappeared as well. Had to replace pretty much every rubber part that was exposed to gasoline in liquid form cause they were either gone or almost gone. Let me tell you, Ca gas is some nasty stuff. I've had 3 lawnmowers with trashed carbs as well (older ones, eaten up parts as well).

[motoman]

There is more but that should get you started!
Wow, 3Phase..........and all in one breath!.........breath.....breath......okay, better now....

[3Phase]

Cy, I haven't had any parts disappear from any of my carburetors ... yet ... but California gas is bad and getting worse. If they'd quit monkeying around with it by adding that little bit of real gasoline to it you could probably drop an olive and one of those little festive umbrellas in the filler neck, then sit down and have a drink!

Moto', like we were talking about earlier, the bike is in Australia. Unless it's an import it's possible it's a very different bike than the Midnight Specials that were sold Stateside. He needs the V.I.N. and a manual for the bike that he has, not the one we think he might have!

T-o-s, if it's not already clear as crystal from the laundry list: while you're checking the carburetors, look for the crossover holes. If the carburetors on your Midnight Special have the crossover holes in the carburetors then put in the rubber plugs!

[OregonRider]

I had the same problem with my XJ11 not going over 3500 rpm and running like guttless crap. Synced the carbs and it nearly pulled out from under me.