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SL350 K2 carb questions

Stutz

Stutz

Veteran Member
Joined
Dec 31, 2023
Total Posts
126
Total likes
88
Location
San Diego
I'm actually on a search for more info on the Kehin carbs for the K2 SL350, but didn't want to keep asking questions that may have been answered elsewhere. I found this post:

Mixture adjustments

Many riders and wrenchers have asked about mixture adjustments. Many have misunderstood the purpose and function of the mixture adjustments. Screws located on the engine side of the carb (between the engine and the throttle plate) are generally fuel screws. Turning them in (clockwise) makes...
www.vintagehondatwins.com www.vintagehondatwins.com

But although it somewhat addresses the adjustment of the air screw (I got far enough to figure out that I have an air screw, not fuel) I want more for these specific carbs. There is a video showing rebuild of a CB175/200, I won't mention who...that is a very similar carb. The CB350 and SL350K0 have lots of available info, just not the K2. In the past, I've always gotten away with extremely basic adjustment on other bikes, and learned a lot more about riding than fixing. But want to make this perfect, and with as few stupid questions as possible.

BTW, I've got a 17tooth front sprocket on the way from Sprocket Specialists, they finished making it, just need to ship it. That should help the freeway riding somewhat.
 
Stutz said:
I tried to find info on these carbs, I swear...but something has been troubling me. My choke will kill the engine if I close it completely, and I have been having trouble finding a good position to start the bike in. Sometimes it takes a lot of kicks, but if I turn the idle up far enough for it to be easy, it's too high at warm-up. If the bike is warmed up, it's always a 1-kick deal. I don't know if this has anything to do with it, there's an off-idle hesitation.
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Obviously I don't know exactly what cold starting procedure you're using but if you're not aware, most Hondas prefer the throttle to be left at idle or just slightly above during start-up. Opening the throttle wider, whether the choke is on or not, will tend to hinder the engine drawing in fuel as well and it will often take many more kicks. And you should never touch the idle speed or mixture adjustments when cold, the carbs perform differently when warmed up with engine heat and you'll have to go through it all again. In your climate you might only need 1 kick on choke, then at least partially turned off for the next kick. Then let the engine warm up fully (taking it for a short ride works well), then once warmed up put a box fan on it to he;lp keep it from overheating and then do your carb idle speed adjustment followed by mixture adjustments, and bringing the idle back down to 1200 or so as the adjustments improve the idle and speed.
 
Thanks, I found from experience with this bike that leaving the throttle alone works best. I got good results this morning at 56 degrees with the choke almost all the way, and no throttle whatsoever.
 
Fuel hose is leaking slightly at the petcock now, but that's pretty straightforward and it only leaks when the petcock is open. What's important was the fact that I got to ride it around most of the day and found a little mud.2CFBF5D6-CE60-49C7-9F45-690CF55536A4.jpegEB49A4B9-2680-4B70-896D-8DA98EA25EA5.jpeg
 
You want to change that fuel line to the 5.5mm Honda brand. Lose those hose clamps since they distort and cut the hose.
The Honda fuel line really doesn't need clamps but the original spring wire ones are more than sufficient.
 
I would not run fuel filters. There should be a screen in the petcock that will protect your carbs.
Filters are just extra restriction that a gravity fed fuel system doesn't need.
 
Twin Town said:
I would not run fuel filters. There should be a screen in the petcock that will protect your carbs.
Filters are just extra restriction that a gravity fed fuel system doesn't need.
Click to expand...
I had a feeling they were over-kill or Honda would have installed them in the first place.
 
I think the screen is a little to coarse for my taste. If I had a new one with a new tank, the screen is fine. It will keep acorns out of the carbs. Rusting tanks, and anything 50+ years old has some corrosion, can make some might fine particles. The emulsifiers in the carbs have some really fine holes. So my vote, for what it is worth, is add filters.

DSC02753.JPG

By the way, if you look at a finished photo of my bike, the sock filters are gone, back to stock airboxes, back to relatively stock exhaust. Bike runs better than ever.

Also I put a filter on my 1991 NH750. It's the fine stuff that clogs the carbs. I have no issues with fuel supply, even at WOT for long times. These little bikes run hard.
 
I’m thinking the tank is due for cleaning. Is vinegar a realistic solution? I have muriatic acid but don’t trust the internets with a realistic ratio.
 
I posted this already on the wrong thread…It was starting to take too many kicks to start and the choke wouldn’t help, so I pulled the carbs to take a look. The slow jet on the right carb was clogged. Going to soak it tonight and blow it out tomorrow with my compressor. I pulled the peacock apart too, because I think that was where my fuel leak was coming from and found some very crusty o-rings. I may have some fresh o-rings to match, or buy a “rebuild kit” for a fresh screen as well. I love finding problems, even if these aren’t the exact issues I get that much closer to cool runnings.
 
A torch tip cleaner is the best tool for cleaning plugged jets. People will scream "you will ruin the jet", but the jet material is hardened brass and the ridges on the tip cleaner will do no damage. Just use it to poke thought the crud. Don't use it as a file.

I have been using one for at least fifty years and have never ruined a jet.
 
LongDistanceRider said:
I view fuel filters as band-aids. It's a good temporary fix for the real problem.
Click to expand...
Bandaids often keep the wound (real problem) from getting infected by keeping dirt out. The filter on my NH750 is over 15 years old. Still clear. No issues, but if rust starts, I'll see it in the filter before the carbs are dirty. Even with plastic tanks, cars have multiple fuel filters. A filter sock on the suction line, and a filter on the outlet of some sort.

And not to be argumentative (oops, too late), but the screen on the petcock inlet and the screen on the petcock bowl are also bandaids as they are filters too, or rather "strainers". I would of prefers a better filter right in the petcock for a more integrated look.


Stutz said:
I’m thinking the tank is due for cleaning. Is vinegar a realistic solution? I have muriatic acid but don’t trust the internets with a realistic ratio.
Click to expand...
Vinegar is acetic acid. It will clean the tank. The caveat is to make sure to rinse well and coat with a petroleum product immediately. Marvel Mystery Oil, ATF, some WD 40, even fill with gasoline. The vinegar leave bare metal that will flash rust quickly. Once coated with oil or gas, it will no longer rust. I cleaned my tank this way. I used short Sheetrock screws in the tank to loosen scale. The screws are hardened and will knock scale loose. The are easier to remove than some other options.

I also considered a tank coating, like POR-15 or equivalent, but I found that the coatings are difficult to apply really well. If not done well, the coating scales off the inside and causes as many problems as it solves. Siince I keep my tank full when the bike is stores, I haven't had any internal rusting issues as my petcock filter and external filters have remained clean.
 
I've cleaned and re-assembled the carbs, all jets are clear including the small openings in the emulsifier and pilot jets. The bike runs better, but still the choke has no effect on the idle, unless it is closed completely and that will kill the engine. it starts with a few kicks when it's cold, but if it's warm it starts easily.

Overall it runs well, but cold starting isn't great without being able to add a little choke.
 
Stutz said:
...but still the choke has no effect on the idle, unless it is closed completely and that will kill the engine.
Click to expand...
I assume you're aware that the older carbs had no high idle function built into the choke mechanism, it's just choke plates that close to increase vacuum draw to pull additional fuel for cold starting. It's quite typical for full choke to kill the engine when idling.
Stutz said:
Overall it runs well, but cold starting isn't great without being able to add a little choke.
Click to expand...
Not sure I understand this since you mentioned the choke kills the engine once running, so it must be working to increase fuel draw. Honda engines like the throttle near or at idle while starting. If you're turning the throttle at all during cranking (kick or electric) it's likely making it harder to start. Since these older carbs do not have accelerator pumps, the choke is the only way to enrich the mixture for cold temps. You do have both chokes tied together with the linkage between them so both are functional when you close the left choke, yes?
 
I wasn't aware of the high idle function, so I'm glad to hear that full choke killing the engine at idle is typical. I think I've just been lucky in the past and never needed to do anything with carbs except adjust the idle speed. So far I've been having a great time, nearly everything I do makes a small improvement. Changing o-rings in the petcock stopped my fuel leak, small victories.

I don't touch the throttle when cranking, it's a kick only. Yes, the chokes for both carbs are tied together, I even checked the linkage with the carbs off the bike to see that they opened and closed the same amount at the same time. I can get it started after 3-10 kicks it idles very slowly, and actually sounds nice and even, no pops at all.

I've noticed that the carbs have both a slow air jet and a main air jet that seem to be (page 129 in the Cb-Cl250/350 manual) permanent part of the carb body, unlike the removable brass jets. I don't have access to an ultrasonic tank, so I'm just spraying carb cleaner and compressed air through the carbs. I was able to clean the slow jets and emulsifier tubes well, but thinking these air passages could be cleaned better. I was going to go in and do more cleaning.

I actually cleaned the tank with muriatic acid because I had some sitting around, it worked well.
 
I have some tiny pipe-cleaner like tools for cleaning my airbrush (I have a small one for plastic models) I used them with carb cleaner and guitar strings to clean the small passages in the slow jet.
 
Stutz said:
I don't have access to an ultrasonic tank, so I'm just spraying carb cleaner and compressed air through the carbs. I was able to clean the slow jets and emulsifier tubes well, but thinking these air passages could be cleaned better.
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Based on the above I'd say it's very possible the passages in the carbs are not yet fully clean. Those who don't use an ultrasonic cleaner usually use a chem-dip from an auto parts store, overnight in the chemical mix usually softens deposits in passages that are impossible to reach or clean properly manually, then thoroughly rinse and blow out with air.
 
I'll give that a go, thanks! I'm sure I just need to be more patient. I have a slight oil leak at the kickstart and I think the points cam seal lets out some oil as well. I'll order those seals and do some proper carb soaking in the meantime. I expect any oil on the points is going to effect them negatively.
 
I've opened up and cleaned out the carbs twice now and to see if there were significant changes, I tried 4into1 rebuild kits. Nothing life changing, same old idle niggles.

1 thing I have been happy with as a general rule, my work never ends with an engine that runs worse.

But...The issue I have is that certain nagging problems don't stay away. ONe of my most annoying was the transition off idle. At one point it was at it's best when the air screws were only 1/4 turn out. Even at 1/2-3/4 turn I get a drop in idle that can kill the engine. I consider this to be minor, because it points a flaws in the idle circuit, the slow jet, and maybe a simple air screw adjustment.

It seems to me that changing the slow jet for a different size is something never done, and I've had 2 slow jets in the same size, and cleaned out without a cure for that throttle roll off idle (Sitting at a traffic light) This only happens when the engine has 20 or so minutes riding, and once I get past that annoying point, the throttle response is great, super responsive.

I have been trying to keep my right and left carb air screws even, is there any reason not to? My thought it just that I need to make smaller adjustments because they are so sensitive. 1/16 turn.
 
Stutz said:
I have been trying to keep my right and left carb air screws even, is there any reason not to? My thought it just that I need to make smaller adjustments because they are so sensitive. 1/16 turn.
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Since they are separate carbs operating independently of each other, there's no reason to be concerned over differences in mixture screw settings unless one is grossly different than the other (which would be an indicator that one carb's passages and/or jets probably aren't fully clean). And remember that jets and other brass in kits are usually improperly sized, so it's best to clean and re-use any (hopefully) original parts.
 
Your nagging problem may also be affected by the pods you're running. Adding an 1 1/2" stand offs or switching to the long Unifilters or refurbed stock filters may do it.
 
4into1.com

UNI Foam Pod Filter Set - 52mm - Honda CB/CL350/360/450 CB500T

UNI high flow foam air filter pod set for Honda CB350, CB450 and others. This is a great replacement for the stock paper filter, sometimes they look clean but if they are 40 years old they probably aren't providing the proper airflow. These filters are 6 3/4" long.
4into1.com 4into1.com



They work good on my CB350G w/ stock 722 carbs
 
They did not work well on my CB360. See earlier pic. I ended up spending a lot of time and some welding to return my bike to stock. The OP has mikuni carbs, which if properly setup, will probably work fine. CV Carbs don't like turbulent airflow and pressure differences and temperature difference in air behind the motor can cause irregular behavior. The stock airboxes on CV carbed bikes equalize air pressure and temperature so both carbs received smooth airflow of the same consistency.

Setting up carbs, mikuni or otherwise, when you change filters and flow characteristics is an art unto itself. Needle tapers, jet changes, all need to be fiddled with. Not many people can do that. Stock gives you a known setup and makes adjustment easier.
 
I agree stock airboxes for CV's. Aftermarket set have become more affordable. The long Uni's seem to be the best of alternatives from all I've heard.
I'm going with these on my CL350K5, not sure if they would fit SL or if they actually make an SL version.

www.ebay.com

Air Filter R + L For Honda 1968-1973 CL350 Scrambler K0-K5 Element Air Cleaner | eBay

1973 CB350G Super Sport K0. 1969-1970 SL350 Motosport K0 VIN # SL350-1000001 Series only. 1968-1973 CL350 Scrambler K0-K5. 1x R AIR FILTER. 1x L AIR FILTER. 2x Air filter tube joint seal. Manufacturer Part Number17310-310-000 17210-310-000 Cleaner Cleaners.
www.ebay.com

There is always the refurb route with sheet Uni foam if stock used units are there.


Edit: Ah yes they say they do fit.

www.ebay.com

FOR HONDA AIR FILTER ELEMENT SET CL350 CB350 SL350 CL350 CB350 SL350 | eBay

For Honda 68-73 CB350 CL350 1970 SL350 Air Filter Cleaner Set. On the SL350 it only fits the 1970 model. These filters do not fit the later models of the SL350. Fits all years of the CB350 and CL350.
www.ebay.com
 
They fit the K0 Sl350, not mine. The stock air filters are unobtainable, and the exhaust is brutally overpriced. I'd go back to stock happily if it didn't cost more than the bike, registration, new tires and everything I've purchased combined. I don't want to be cheap, but gotta draw the line somewhere ;)

The UNI foam filters on 4into1 are:
  • "Adaptable between 1 1/2" - 2" (38mm-52mm) ID for universal use on many carburetors"
The inlets on my carbs have approx. 35mm diameter, so they aren't impossible, but so far everything I see has this footnote:

From the Ebay page of the more recent stock-looking air filters:
"On the SL350 it only fits the 1970 model. These filters do not fit the later models of the SL350."
 
Stutz said:
They fit the K0 Sl350, not mine. The stock air filters are unobtainable, and the exhaust is brutally overpriced. I'd go back to stock happily if it didn't cost more than the bike, registration, new tires and everything I've purchased combined. I don't want to be cheap, but gotta draw the line somewhere ;)

The UNI foam filters on 4into1 are:
  • "Adaptable between 1 1/2" - 2" (38mm-52mm) ID for universal use on many carburetors"
The inlets on my carbs have approx. 35mm diameter, so they aren't impossible, but so far everything I see has this footnote:

From the Ebay page of the more recent stock-looking air filters:
"On the SL350 it only fits the 1970 model. These filters do not fit the later models of the SL350."
Click to expand...
So they would at least be loose and a 1.5mm thick sleeve of rubber (inner tube?) may fit up ok. Still an option then.
 
The standard slide carbs on his SL350K2 are 26mm Keihins, not CV carbs. The only year the SL350 came with CV carbs was the first year K0. So, the air filter diameter is smaller for the K1 and K2 models. Agree that Unifilter foam is a good solution for his stock filter frames.
 
ancientdad said:
The standard slide carbs on his SL350K2 are 26mm Keihins, not CV carbs. The only year the SL350 came with CV carbs was the first year K0. So, the air filter diameter is smaller for the K1 and K2 models. Agree that Unifilter foam is a good solution for his stock filter frames.
Click to expand...
IDK if Stutz said he still had old frames to re-do, but that would be the best. My 160 with dual PK types liked the original filters a lot more than the pods.
 
I don't have the old frames. If I run the bike with the air screws 1/8 turn out it rolls off idle fairly well.
I noticed that if I "gun it" there is no problem, it's only when I very slowly roll the throttle when the bike is warm, from a standing start. No hesitation at all between gears either.
 
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