S65 carb woes

I have a 1965 S65 I recently restored but can't seem to get the fuel mixture to behave - it's running extremely rich at idle. I've cleaned out the Keihin carb three times now, by soda blasting, dipping and blowing with compressed air. Replaced all internals with the exception of floats (which I tested and can confirm do float ok) Their height is set to 19.5mm as per the manual. Also tested them/needle valve & seat etc with the bowl off and they stop and start the flow of fuel as would be expected.

Clue: Turning the mixture screw in either direction does nothing. I know most often this points to a clogged idle circuit/jet but I can confirm it's all clear.

Valve clearances & timing are set to spec with a brand new air filter. Spark is bright and blue.

Unless I'm missing something, could there potentially be an issue with the actual carb body? Possibly an internal crack etc that could be hemorrhaging fuel? It starts without choke and idles erratically but doesn't die out unless I choke it. Weirdly, it seems to rev out fine and smooth, but throws a lot of smoke and then dies when it returns to idle. Plug is dripping wet with fuel after a few seconds at idle.

Follow up question: Anyone happen to know if an S90 carb would fit? The Keihin's for those models look almost identical but not sure about diameters/jetting etc. From what I've seen, they seem to be more plentiful & generally in better condition than ones for the S65.

There's not a ton of support for these models but figured I'd try my luck here here since the engine layout/specs seem to be closest to the old CT70's. Any ideas are very much appreciated.

An update to this saga - and a bump to this post - have tried 3 different stock carbs now with 3 different sets of needles and jets and all behave in exactly the same way:

• Bike starts fine but idles and runs very, very rich (no choke needed)
• Plugs not just carbon fouled - they're dripping with wet, unburned gasoline
  
• Bike moves under its own power, but loses power in higher RPMs (power just ends up flat-lining)
• All carbs and parts are stock & set to stock settings by the book
  
• Leak down test gives a perfect score with only about 2% leakage
  
• Timing is correct (confirmed with timing light)
• Cam sprocket is correctly installed (with marks aligned)
• Spark appears fat and blue when grounded to engine
• Valve clearances check out
• Brand new (stock) air filter.

Any and all suggestions are welcome, no matter how crazy. With the same issue appearing over 3 different carbs, I had given up on bad mixture being the culprit and tried to look elsewhere. But where? With a passing grade on all the above tests I'm ready to suspect the supernatural... Somehow, from somewhere, the bike guzzles gas. Even after setting the float as low as it could possibly be set without starving out completely.

Just one question - when you say "all carbs and parts are stock & set to stock settings by the book" what specific settings are you referring to?

Wild-assed suggestion...start experimenting with float height. If, as you say, it runs the same... pig-rich with 3 different carburetors, I'd try lowering fuel level inside the float bowl.

Not by chance a z50 top end? how about running with no air filter or air box, give it extra air and see if it compensates for the fuel. Whats the compression? Was this a china ebay top end?

Was the motor rebuilt during restoration? Low compression can do some funny things. What do you mean bike moves under its own power? It will drive on its own once in gear? If that is true then your idle is way to high and your pilot jet probably isn't even in the equation. Stock rpm is probably around 1200-1500 rpm.

Also what is the crab number stamped on the flange of the OEM carburetor?

I'm with hrc pull the air cleaner element and check the inlet track through the frame before the carb. Out of curiosity are you using the original type paper air cleaner element? If I recall it's a 19mm carb. The S65 was the first to use this type engine,early models had major transmission troubles and actually had a recall to replace them. 10-11K red line is out of this world. Doubtful the head has been replaced it required a piston to match, and the cylinder has a an oil boss on it,but you never know after all this time. The top end oil comes out a boss on the right hand clutch cover up through a steel oil line to the carb then back down to a round boss on the cylinder and then up thru the cylinder head stud like all other versions of this engine. One of these lines also has a rubber insulator cover, don't understand if this lines help heat the long inlet track,which I suspect or if it can possibly help cool the top end oil

Thanks very much for the feedback everyone - will do my best to answer questions below.

dirtbkr188 said:

Just one question - when you say "all carbs and parts are stock & set to stock settings by the book" what specific settings are you referring to?

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Referring here to the settings suggested by the Honda service manual as well as Clymer's (so things like checking stamped numbers of jets to make sure they match what's in the book / turning idle mixture screw to the factory setting of 1.5 turns from fully seated, jet needle clip on 3rd position / float height set to 19.5mm etc.)

racerx said:

Wild-assed suggestion...start experimenting with float height. If, as you say, it runs the same... pig-rich with 3 different carburetors, I'd try lowering fuel level inside the float bowl.

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I tried this - I had set the float to where engine wasn't getting gas at all and then backed it up to the point where just a bare minimum amount of gas was resting at the bottom of the bowl (enough to fire up the engine) Still smoked like crazy and fouled the plug. But might do some more experimenting with this until a better option comes along.

hrc200x said:

Not by chance a z50 top end? how about running with no air filter or air box, give it extra air and see if it compensates for the fuel. Whats the compression? Was this a china ebay top end?

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Interesting point about the head - The original bike was a barn find that had been abused at some point with a bunch of broken fins, so I sourced brand new NOS head and cylinder. Both came from Davidsilverspares.com. Made sure to avoid knock off parts whenever possible.

Tried also to run without air filter but it made no difference. I used a leak down tester and numbers came out perfect, indicating no leak whatsoever. This also surprised me a bit since on other engine rebuilds I've done, I've never gotten perfect numbers, even after a complete break in.

lukelaw1 said:

Was the motor rebuilt during restoration? Low compression can do some funny things. What do you mean bike moves under its own power? It will drive on its own once in gear? If that is true then your idle is way to high and your pilot jet probably isn't even in the equation. Stock rpm is probably around 1200-1500 rpm.

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Engine was rebuilt from the crank-up and leak-down test passed with flying colors. Sorry, when I meant bike moved under its own power, was just a bad way of me saying I never ended up having to push it. However, power would flat-line quickly. It never got past around 25mph as indicated on the speedo (however accurate it may be)

lukelaw1 said:

Also what is the crab number stamped on the flange of the OEM carburetor?

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I've tried 3 different carbs in total so far, 2 Keihins and a Mikuni. The only numbers I was able to find on each one are as follows:
1st Keihin: 17B
2nd Keihin (currently on bike): S65B on top of carb / JC E (or what looks like an e with the top half missing) on side of carb facing the engine
Mikuni: 17-2

Gary said:

I'm with hrc pull the air cleaner element and check the inlet track through the frame before the carb. Out of curiosity are you using the original type paper air cleaner element? If I recall it's a 19mm carb. The S65 was the first to use this type engine,early models had major transmission troubles and actually had a recall to replace them. 10-11K red line is out of this world. Doubtful the head has been replaced it required a piston to match, and the cylinder has a an oil boss on it,but you never know after all this time. The top end oil comes out a boss on the right hand clutch cover up through a steel oil line to the carb then back down to a round boss on the cylinder and then up thru the cylinder head stud like all other versions of this engine. One of these lines also has a rubber insulator cover, don't understand if this lines help heat the long inlet track,which I suspect or if it can possibly help cool the top end oil

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This has the paper filter and there's a rubber tube that runs along the inside of the frame into the carb. However, I've tried running it with both out and makes no difference.

I've replaced the head, cylinder, piston and rings during the rebuild. All parts sourced from David Silver Spares.

Did you compare the new piston to the old one? I think I remember someone on here getting a new piston and the deck height was different. I'll look for the other post.

Does your carb look like this? Don't know if you can zoom in to see jet sizes to compare. I sold this carb to another member so I can't get any numbers off it.

lukelaw1 said:

Did you compare the new piston to the old one? I think I remember someone on here getting a new piston and the deck height was different. I'll look for the other post.

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Huh that’s interesting. I had the pistons side by side on the bench but never measured or paid any attention to it.

Having gotten the same exact results with 3 separate carbs makes me doubtful its a carb issue anymore so that’s something else to look at.

Gary said:

Does your carb look like this? Don't know if you can zoom in to see jet sizes to compare. I sold this carb to another member so I can't get any numbers off it.

View attachment 66854

View attachment 66855

View attachment 66856

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Yup that’s the same carb.

Ok then. Here are some more pictures for you to compare. In order- nos head,nos piston on right compared to ct70,markings on head and box head came in

Gary said:

Ok then. Here are some more pictures for you to compare. In order- nos head,nos piston on right compared to ct70,markings on head and box head came in

View attachment 66857

View attachment 66858

View attachment 66859

View attachment 66860

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Amazing, thanks for this Gary. I'll pull things apart this weekend for a closer look and will post my findings. Based on that part number for the NOS head in the last pic, according to CMSNL it's for 'later' models that run a different serial number to mine. I'm pretty certain the one I ordered was an 'early' model part #12200035600 according to the three listed in their fiche. But will have to dig up my invoice to make certain. Stay tuned...

Can you test compression?

hrc200x said:

Can you test compression?

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I don’t have a traditional compression tester, just a leak-down tester. (Which indicated no leaks)

hrc200x is spot on, you really should buy one. That will tell the story without taking things back apart.
I wouldn't even know but how fast is a 65cc bike supposed to go? A 50cc does 30mph if it's lucky.

OLD CT said:

hrc200x is spot on, you really should buy one. That will tell the story without taking things back apart.
I wouldn't even know but how fast is a 65cc bike supposed to go? A 50cc does 30mph if it's lucky.

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For the Z50, that's dead-on. Pre-1968 models can be a whole different story. Some of the pushrod motors, especially, can spin crazy rpm...and develop enough hp to reach speeds that a stock 6v CT70 can't match. Some factory specs:

• C50 4.8hp@10,000rpm
• S50 5.2hp@10,000rpm
• Z50A 1.9hp@5,000rpm
• S65 5.5hp@9,000rpm
• CL70 6.5hp@9,500rpm
• C70 6.2hp@9000rpm
• SL70 5.0hp@8,000rpm
• C90 7.5hp@9,500rpm
• CL90 8.0hp@9,500rpm
• CT90 7.0hp@8,500rpm
• ST90 5.5hp@8,000rpm

5hp is enough to into the mid-40s, without a tailwind or downhill assist. The S65 has the advantage of less rolling and wind resistance; it's generally considered a 50mph bike...as is the CL70. I wouldn't take these numbers as gospel. They are fairly consistent amongst themselves, good enough for model-to-model comparisons and loose benchmarks.

Ok, ordered a proper compression tester (about time I got one anyway) and will post results here as soon as I get my hands on it. The continued input and suggestions are very much appreciated (also happy to be troubleshooting something other than just carbs for a change)