what is best replacement Carb ?

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andy2175m4

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I have a 1975 xs650 with the original 38mm mikuni carbs, K&N filters, worked fine for 46 years, until the pandemic, bike sat for over a year, carbs full of gas, gas corroded and gummed up carbs, now it won't run, wont even start. I have thoroughly cleaned the carbs, and appear to be normal now, but still won't even start, classic case of an inaccessible passage way clogged deep in carb, any ideas on where that hidden clog is ? or your thoughts on 34mm mikuni (or clones) retrofit ? I think these 38mm CV mikunis are done for, as good as they were for so many years.

Re. 34mm slide carbs, is the jetting really that big of an issue ? can you use mikuni jets in the $40 clones ? or are the chinese mikunis a waste of time give there would be no tech support or parts (jets) avail ?
 
Forget about the knockoffs unless you want more trouble than they're worth (thought you were asking about the best). Re. jetting: important but not an "issue" if you know how to tune or are willing to invest some time, money, and study. As to the title question: how much money and ability do you have? The "best" would be a custom pair or Keihin FCR flat slide pumpers; you'll need deep pockets for those, they'll run well into 4 figures. Re. Mikuni VM cable slide carbs: they're a long way from the best, but IMO they're the best for the money. "Classic case of an inaccessible passage way?" Funny, that's a case I haven't yet encountered. Takes more than one try sometimes, but carb spray followed by an air gun with serious pressure behind it will get things clear. Don't forget to inspect the ignition.

Edit: For a stock engine, the Mikuni VM32 gives you more unobstructed venturi than an OE BS34.
The smaller carb delivers better low rpm behavior and is a bit easier to tune.
 
The Kawi 500-650 carbs are popular but used, may need cleaning and WILL need to be jetted for the XS650, there's a thread...
On your BS 38's, Pilot jets open, orifice in bottom of float bowl open?
75 carbs have only one choke circuit with a cross over tube everything in place?
74-75 BS38s.JPG
 
Hi Guys, thanks for the replies, I have worked on dozens of carbs over the years and sometimes they just quit, just dont work and I found the only way to fix them, is to replace them, that's why I suspect that there may be a clogged mystery path somewhere I cant get to .

So for these Mikuni 38's they are 46 years old, worked fine, had a few tuneups over the years, but always fired up. this time, no go, so If I could buy new exact replacements that would be a slam dunk, but they are not available, So is there something else I should check ? I took wires of various sizes and chased thru all the jets and ports and orfices I could find, but no clog found yet. any other ideas ?
 
If you tried to start the bike before cleaning the carbs, you may have very well sucked gunk into the small passageways between the jets in the bowl and the main carb bore, in particular, the pilot circuit passageway. With the bowl off, on the bottom of the carb body, you'll see the pilot jet inlet. This is how the fuel/air mix from the pilot jet gets transferred into the carb's main bore .....

4yA1K20.jpg


Spray some carb cleaner in it then blow it through with compressed air. This passageway exits into the main bore through 4 tiny holes around where the butterfly plate closes. Three come directly from the passageway, the fourth from a branch of it that runs to and by the mix screw. You'll need to remove the mix screw for this cleaning process. Block it's hole on the outside of the carb with a finger as you blow the cleaner through. You want to see it coming out all four of the little holes in the main bore .....

7bk6grC.jpg


You can "backflush" it as well. Block the 4 little holes in the main bore, spray cleaner in the mix screw hole, and blow it through. It should come out the inlet hole on the bottom of the carb body. Blocking the inlet hole and the inlet for the mix screw in the main bore will only allow the cleaner to come out the 3 little holes. So, by working it from these different directions, and alternately blocking holes so the cleaner comes out others, you should be able to clear the circuit.
 
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Doing what has been suggested above , should dislodge any blockage. The BS38’s are good carburetors that almost always can be brought back to life as long as they are not physically damaged. My favorite cleaning spray is Barrymans B-12 Chemtool, cuts gunk fast! PS...wear eye protection.
451376ED-30A5-45C1-827B-67B9F5D3C4EE.png
 
If you do end up throwing the towel in on that set, I'm quite partial to original carbs and would seek out a later linked set of them. A nice set along with the parts needed to renovate them (gaskets, all new jets, throttle butterfly shaft seals) may set you back as much as $100 to $150, but that's still like half the cost of brand new ones.
 
I concur with Mr. 5twins and that's my plan with the TX. I haven't taken them apart yet but the throttle shafts on the 74 TX are rusty and frozen so I got a set from a 77 with the linked throttles off Ebay for $50 in very nice shape. They went cheaper than most because they were missing a few little parts which I found in Dad's parts stash but could likely have come off the set on the bike. I like the idea of the linked set as it seems like they would stay synchronized better than twin cables.
 
This passageway isn't a straight shot into the main bore so you're not going to be able to poke it clear, you'll need the compressed air. Get one of those tapered rubber tips for your blowgun. They let you stick the tip into passageways and holes, and seal into them so you get a good blast of air through it .....

lMoGlHJ.jpg


A selection of needle tips can also be very handy, especially for clearing out the choke jet .....

EdtANal.jpg


..... which is something I didn't mention before. It's one of the first things that plugs up if the carbs get gummed up because of it's location off to the side in the bottom of the float bowl. If plugged, the choke won't work and the bike will be very difficult to start cold, and maybe not start at all. So, this could be part of your issue too. Here's how to check if it's clear .....

IzS1tsu.jpg
 
I admit I don't Know much about carburetors . Reason for that they have always worked
What do You have in Your gasoline over there ?? -- Syrup ????
Here in this... the impression is that a bike parked little over a year ..Not only corrodes the carburetors .
Both of them are scrap .. gummed up .Cant remember any of that talk over here.
And some enthusiast here I know are very experienced ..and I think I would have heard of it more
Some people use start gas. In the spring . And some additive in the beginning rarely any long time.
But both carburetors finito after little over a Year. In the garage .Something here is not adding up
Do you have strange additives in your fuel
What does the UK and Australian Canadian .. gentlemen say about this.
I Remember a Honda CB 350 that had been sitting about 10 years outdoors. Not even sure how --long Winter and summer.
It started after some time without a tear down of carburetors.
Ran all in all fine ... After a while... Is that just luck ???
 
I think a lot depends on the climate in your location. Carbs from a hot, dry area like Arizona or Texas are more prone to be gummed up pretty good with varnish and sort of a black tar-like residue from the liquid parts of the fuel all evaporating away. The '83 I got recently, even though it had been sitting for like 12 years, still had some syrup-like liquid in the carbs, lol. I'm in the cooler, wetter N.E.
 
Jan_P said:
I admit I don't Know much about carburetors . Reason for that they have always worked
What do You have in Your gasoline over there ?? -- Syrup ????
Here in this... the impression is that a bike parked little over a year ..Not only corrodes the carburetors .
Both of them are scrap .. gummed up .Cant remember any of that talk over here.
And some enthusiast here I know are very experienced ..and I think I would have heard of it more
Some people use start gas. In the spring . And some additive in the beginning rarely any long time.
But both carburetors finito after little over a Year. In the garage .Something here is not adding up
Do you have strange additives in your fuel
What does the UK and Australian Canadian .. gentlemen say about this.
I Remember a Honda CB 350 that had been sitting about 10 years outdoors. Not even sure how --long Winter and summer.
It started after some time without a tear down of carburetors.
Ran all in all fine ... After a while... Is that just luck ???
Click to expand...

Ya I wouldn't assume it was the carbs, especially after that short time guys have gotten these carbs running after decades of sitting. I kind of doubt it's the carbs?
Does it sound like it sparks? Does it run briefly if you spray gas or fluid into intake?
 
Yes, you've got to get these carbs all clean, you can't miss spots. The previous owner of my '83 said he always had carb issues and they were never quite right. He didn't go into specifics but when I went through them, I think I discovered why. He didn't do a complete, 100% job. He didn't change out the mix screw o-rings and they were flattened out and hard as a rock. He also didn't change out the butterfly shaft seals. I see he tried but a couple of the butterfly plate screws were buggered up so I guess he gave up. After taking care of those couple things he missed, plus a complete proper cleaning, the carbs work just fine as far as I can tell.
 
OK so that takes care of the street bike, the track bike is still in pieces and has no carbs yet, I do have a pair of rusty old 34mm CV's but am leaning toward the flat slide 34mm Mikunis for the final track bike configuration, comments on that would be appreciated too,
 
andy2175m4 said:
OK so that takes care of the street bike, the track bike is still in pieces and has no carbs yet, I do have a pair of rusty old 34mm CV's but am leaning toward the flat slide 34mm Mikunis for the final track bike configuration, comments on that would be appreciated too,
Click to expand...
I have linked flatslide Mikuni 34s on my 77 standard, which still has a pretty much stock engine. Except the carbs, K&N filters, 1 1/2" single wall headers, and "Commando" mufflers.
So far I am super happy with these carbs, the only gripe being limited access to the LH side petcock. This because of the throttle linkage and cables.
I got them from this vendor in Germany:
https://www.mikuni-topham.de/ENGLISHSITE/ENGLISH/Frame_English.html
 

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