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Discussion Starter · #1 ·
Trying to take care of a RPM surge. Once RPMs go up, they stay up, or slowly go down. Current state is after driving it up the street the RPMs hold at 3k-4k when I stop so I have to shut it off, turn it back on again to get back to the driveway.

I've disassembled the carbs 3 times. The diaphragm rubber appears to be in good shape. The carbs pass "the straw test." Not sure what else I can do with the carbs. I HATE these 3D carbs as I've never been successful with them, even with my SL350. I've been so spoiled with the CB450 carbs.

Bike ran fine about 5 months ago. I let the fuel go bad (girlfriend didn't ride it), so I flushed everything out last night with new gas.

Surging is clearly a sign of a vacuum leak or carburetor dirty-ness in my experience. Brand new NOS points/condenser last year. Perfectly timed last year as well. The ignition system probably has about 20 miles on it only, so I know it's not the ignition. I tested the floats and they are set perfectly at 26mm before fuel starts pouring in (tested it on the bike with fuel system hooked up).

Spraying some starter fluid on carb to head boots seems to create a surge. Replaced all the clamp bands. I still think the RPMs are surging a little after spraying starter fluid, even after doing that.

Turning handle bars side to side reveals no throttle cable bind; throttle levers do not move.

What else can I check? Seems like taking a closer look at the carb rubbers may be my next step.
 

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Check the advance mechanism for smooth function and spring tension.

I clean and polish the slide bores with something like autosol, flitz or s100 finish restorer. Clean the slides also.

These carbs can be a challenge.
 

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Are the carbs still synched correctly? When it's hung up at higher rpms when you're stopped, try letting the clutch out just a bit while in gear with the brakes on and see if that drops the rpms back down.
 

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If spraying the Mounting Boots results in an RPM change they are leaking - PERIOD and you will never get them to adjust until that leak is resolved.

The boots are readily available and VERY Simple to swap out.

The Rubber in them goes from good to bad just sitting, it has very little to do with the clamps.

They may have been fine when you stopped riding the last season but when you pull it out the next year POOF they have gone bad.
 

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Nash -

Just like with your 450, a "hanging idle" is almost always due to carb "synch".
Doing it without vacuum gauges is an acquired talent, takes many tries and fails to get it down.
Keep at it, you'll get it.....
 

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Discussion Starter · #6 ·
Thanks everyone.

Yes, I can force the bike to idle lower by popping it in gear. It's just not safe or rideable that way (nor fun!).

Yep, I will plan replace the boots. I'd like to remove them an inspect them first, however.

My hand was getting burnt and the engine was getting pretty warm so I just stopped yesterday. I'll run it again this evening and spray the starting fluid to be extra sure.

Thanks Bill: I remembered your throttle guide by memory and I got it pretty well in sync. I then proceeded to equalize the back pressure with the stop screws. I'm usually good at this, but after a few hours, I'm going to bet that the boots are leaking.

Anyone have any issues with these (not OEM): HONDA CB350 CB350G CL350 SL350 CARB INTAKE BOOTS MANIFOLD INSULATOR 1968 - 1974
 

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Discussion Starter · #9 ·
Thanks for the link!

I took off the insulators and I have to say, they are in pretty good shape and I doubt that they have anything to do with the problem. I see no cracks or deterioration. I even was able to get them off without rounding the screw heads!

What I think I'd like to do is put on new gaskets and put them right back on. Are the gaskets anything special? I could trace them out of the blue material that I have, but I'm not sure if the blue paper gasket is appropriate. The thickness is correct...should I use it?

When I sprayed starter fluid earlier, it didn't really seem to have much effect. Yes, the RPMS went up, but they weren't racing...they only went up a 100 or 200. It could be that some fumes got on the air socks.

I think it's more to do with what Bill said: I need to spend some more time getting them synced. I'm using the "thumb and eye" method, and I thought that I had them pretty darn good....

-Nash
 

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Your Quote - "I tested the floats and they are set perfectly at 26mm before fuel starts pouring in (tested it on the bike with fuel system hooked up)."


How did you do this with the Carbs on the bike?

The Carbs MUST be held at a 10 Deg Angle when checking the float height.

The TIP of the Needle Valve is SPRING LOADED and MUST NOT be compressed when measuring the float height or it will be off by a full 2 mm or more.
Holding the Carbs at a 10 Deg Angle allows the Lever end of the Float to TOUCH the Tip of the needle without compressing the spring.
 

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Discussion Starter · #12 · (Edited)
I removed the bowls and held my tool (https://images-na.ssl-images-amazon.com/images/I/31nHENcMdLL.jpg) underneath the floats with fuel on. The tool was set to 26mm. As soon as I lowered the gauge from the body/floats, fuel would start coming out immediately. This method assures that when the floats are 26mm away from the body of the carb, fuel is off from entering, no guesswork; any distance greater than 26mm fuel will be allowed to come in. Is this not a good way to do it?

Sounds like it's not. It's not that it's a bad way to do it, but it may be that it's not 26mm anymore that I should be shooting for. I actually check dynamic float height, not static height, so that could easily explain my issues. Sounds like I need to check the float height by-the-book. My guess that even though I'm "actual 26mm", the real spec isn't "actual" but like you said assumes 2mm off because it assumes the spring movement will always be the same, and all floats will be the exact same. So "by the book", mine may be actually set to 28mm, not 26mm.

Can I use a blue sheet of gasket material to replace the insulator gaskets, or are the insulator gaskets special?
 

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I'm thinking that the gravity pressure in the fuel lines will throw off your measurements.
I think it will be very hard to not compress the spring loaded tip of the needle valve onto the seat.

But I've been wrong when thinking in the past (just ask my wife) !

As far as the gasket material yes any Gasket paper of the correct thickness is fine.

but the most common problem is as a motor heats up the insulator mountings become softer and any small internal cracks allow air leakage.
 

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Discussion Starter · #14 ·
Took off the bowls and did it with the "by the book" method to set the floats and achieved the same exact results. Carb floats were already set to 26mm perfect...couldn't get better. Seems like my method turned out valid.

I re-cut the boot gaskets and used a little bit of high-temp RTV to assure a seal. This isn't a bike I'm restoring yet, just want to make a solid, consistent runner.

Well, it must be something to do with the syncing.
 

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Discussion Starter · #16 ·
Yep, I'm now moving to the timing. I removed the points plate. How do I go about lubricating the advance, any tricks? I have the right stuff (Dow Corning), but not sure how much, where to apply it, etc.

Auto part Engine Automotive engine part Locking hubs Machine
Auto part Engine Automotive engine part Fuel line Transmission part
Auto part Disc brake Brake Vehicle brake Automotive wheel system
 

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Discussion Starter · #17 ·
Here's a video of me operating the advance. Seems a bit easy to operate (too lose?), but maybe that's normal.


As far as greasing, which specific parts do I add dow corning grease to? I know to apply very little on the face of the cam.

Or, do I use gun oil instead and drop some on the felt and on the pivot point on the advancer, then call it a day?
 
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