SL350 idle problems

w8xw8x

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Hi guys, Udo from Germany speaking.
My SL350 didn't idle nicely at all. ONE culprit was a faulty HT lead. Replaced coils, now revs ok down to around 1000-800 rpm, but has to be kept alive by hand.My take is there is sth wrong with float level, since I had to install new floaters. I know about the 26 millimeters from the manual, but would like to check level "online", so to say. Bought a screw that fits the drain plug thread in bowl, drilled a hole in it and strapped a translucent tube over it, so I can make the gas level visible outside. (I know so. has come up with that idea already; nice meeting of the minds). What I don't know is how do the 26 millimeters translate into my method? Any ideas are greatly appreciated.
Udo
 
There's now real way to translate the 26mm float height to a clear sight glass tube. The level in the sight tube should be slightly lower than the mating surface of the body to bowl, how much is the question.
If I'm in doubt about matching float bowl volumes I simply drain one carb into a measuring cup, note that volume and dump it. Repeat on other carb expecting it to be within 5cc's of the first.
 
Hi Udo! I think what you are I describing is what I did, in post #20 here: https://www.vintagehondatwins.com/f...-passage-plugged-among-other-things-81-CM400T

Where you can see the level thru a clear tube. The way my bowl empty screw works is you just loosen it a little and it allows the gas to come out the overflow tube nipple. This I attached a transparent tube to. I had to seal the threads on the screw or it would leak out there as well.

I sure would be nice if these bowls had a little glass site window built into them!

Concerning your problem of bad low idle, look over that thread and the other carb cleaning ones. The problem you describe was my same problem(bad idle, slow start). Mine was due to plugged up idle jets. These are very tiny so clog easily. If adjusting idle screw has no change this can be an indicator that these small idle jets are clogged.
 
One other thing Udo, maybe a faster idle is OK? I run my idle a little fast now, like 1200 to 1500 rpm. For a while I was overly obsessed with it having some problems at a slower idle. Some of these not good, like the red oil pressure light coming on after it is hot. But my bike is getting old and I think it is happier idling a little fast. Not sure what others would think about this, or what the FSM would prefer. I tend to ignore rules, learning at a young age that they don't apply to me ;)
 
Thanks so far!

Ok, guys, thanks so far. Tried my method on one carb and the situation went better. Just eyed the level to approximate it to just below sealing level.Will try on other side as well. What I don't understand now is this: There is a separate idle circuit, with two parameters metering the mixture: pilot jet and air screw. But then again there is also the screw that adjusts the cylindrical throttle, which is part of the main circuit So is proper idle a sort of collaboration of the two?
Udo
 
Yes, the screw that adjusts the slide (cylindrical part) is actually what adjusts the idle speed. The air mixture screw helps that cylinder run the best mixture for that speed. Sometimes after adjusting the mixture, the idle speed has to be adjusted again if that cylinder runs better (and a faster idle speed as a result). So it's a game of adjustments and more adjustments until a balance of good idle performance AND proper idle speed is achieved.
 
Yes, understood, but ...

... the way you describe it, that alone should/could suffice. How does the pilot jet factor in? I'm asking, because in the 70ies I used to have several 50cc Zündapp (one even watercooled! look it up, sharp item) that did not have separate idle circuits. Never had any porblem.
Since idle problems are so common with these twins: Has anybody tried to mount other carbs? In case so: which ones?
Greets Udo
 
w8xw8x said:
How does the pilot jet factor in?
Click to expand...

The pilot jet provides the correct amount of fuel to mix with the amount of air allowed through the carburetor at idle, and adjusting the air mixture screw gives the best engine performance at idle with the fuel from the pilot jet. Idle problems are only common on these twins when carburetors are not fully clean, or have the wrong replacement parts installed (jets, emulsion tubes, float needles). When the carbs are completely clean and the correct parts are installed, and the moving parts of the carb are not worn out (slide bore, throttle shafts on some carbs) then they run and idle as they should. Mikuni carbs are an option for you but they will require rejetting and other fiddling to make it run correctly as well.
 
w8xw8x said:
Ok, guys, thanks so far. Tried my method on one carb and the situation went better. Just eyed the level to approximate it to just below sealing level.Will try on other side as well. What I don't understand now is this: There is a separate idle circuit, with two parameters metering the mixture: pilot jet and air screw. But then again there is also the screw that adjusts the cylindrical throttle, which is part of the main circuit So is proper idle a sort of collaboration of the two?
Udo
Click to expand...
I would say that's about right. So you have those main throttle plates, the idle adjustment you can set while riding to get faster/slower idle will adjust the stop point of both so they are open more or less. It's important to have the two carbs cracked open the same amount, that's what that sync business is about. They are adjusted relative to each other. You do a rough sync with feeler gauge, or just looking at them, or blow in them if you are a caveman like me. You can find specs or notes on suggested feeler gauge crack opening at close. Now, if you were not a caveman like me, you would take it to a professional or buy the right gear and sync it with pressure gauges while running. The idle jet, at least on mine, is this pressed in small pinhole of a brass jet. Mine was a bugger to get out, and it was icky and plugged with gunk. It leads some where but eventually it goes by those air screws and eventually out a very tiny hole more on the engine side of those throttle plates. I ended up taking a small tube to verify that yes you can blow a small amount of air or carb cleaner and see it pass thru those tiny holes. The air screws provides some small adjustment to the flow or mix for the more refined members of our society. I just turned my out 1.5 turns out as suggested and and was not overly impressed with the difference they made.
 
kbongos said:
So you have those main throttle plates...
Click to expand...

If his SL350 is a K1 (and he didn't say) it has slide-type carbs, and from his 'cylindrical throttle' description I suspect it does, unlike the CV carbs on all the other 350s including the SL350K0 which have throttle plates
 
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