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Discussion Starter · #1 ·
My 73 CB350 is in excellent shape and I've gone through pretty much all maintenance procedures, but it gets a weak idle and wants to die when at peak running temperature. The same thing used to happen on my old CB750 when I would sit in stop-go traffic and it would get too hot, so this seems like an overheating issue.

Engine:
-Valve clearances in spec
-engine rebuilt a few thousand miles ago, compression good and even between cyls
-oil always changed frequently, good flow to head
Electrical:
-electrical system is perfect with everything clean and well-grounded, charges perfectly
-dyna coils with new iridium plugs
-Charlies place EI, timing set
Carbs:
-stock carbs synced, clean, with stock jetting. They seem to like having pilot screw 3/4 turns out. Idles at 1000-1100
-clean petcock, no fuel filters or apparent fuel flow problems
-Stock airbox setup, clean filters
-exhaust is stock headers with long, straight-through emgo mufflers

The only other thing I can imagine is stepping up the primary and second main jets to compensate for the relatively open pipes. Maybe it is a fuel flow issue and I'm getting a lean condition that makes it run hot? I don't really know what else to check... it runs perfectly until it gets to maximum temperature. It does seem to be related to temperature, and using a laser thermometer I've clocked it anywhere from 230-275, even when riding in fairly cool temperatures.

Thanks in advance.
 

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Sounds like a lean condition, so adjust or replace your jets.

First though, I assume you played with your idle screw?
 

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Discussion Starter · #3 ·
Yeah, I've opened up the carbs as they seem to be the last area I could look. Inside parts were a little varnished and worn looking so I'm rebuilding them with new emulsion tubes and thoroughly blowing the passages clean. I have also played with the idle mixture a good bit and haven't had any luck there.

When I had them apart a few years ago, I put in 110 and 70 main jets. I guess my main question now is whether that is appropriate for a 350 with a stock airbox and aftermarket exhaust. Again, my new mufflers are the straight-through emgo types so at the time I figured it would benefit from slightly richer jetting. I remember on my 79 CB750 that this was the era where EPA regulations had honda jetting bikes on the leaner side from the factory. I'm just not sure if a 1973 CB350 is before or after that period began.
 

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Why not just replace carbs with aftermarket? I have been blessed with aftermarket working well. Saving me a lot of time, money, and headache.

 

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Discussion Starter · #5 ·
I considered going to mikunis and read a lot of opinions from people who made the switch. I prefer the rideability of the stock CV keihins... no adjustment by altitude, no tuning headaches, keeping fuel economy good. They seem great for racing but in the end I trust honda's designs for basic street use
 

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As cheap as they are, you may consider keeping a pair for when your current carbs are not working like they are. My feeling is that you are going to go with what works. I have sacrifice whole summers of not riding because I am having an issue or I start some long project. Having spares when it comes to these old bikes is always a God send. It is always easier to combat an issue when you are not tearing your hair out.

if you bought those $67 carbs and slapped them on there and they worked beautifully. I am sure you may never look back. Some of these carbs even tell you how they are jetted and tuned.You can also buy a plan where you can get tour money back if they crap out.

I bought my carbs for my CA250 for $40 from china, but there is a made in Japan stamp kn them. I think they are just assembled in China.
 

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Discussion Starter · #7 ·
100% agree with the thought about spares. It's also a nice diagnostic tool to just pop on a new carb or system to see if a problem gets resolved... helps narrow down where you have to look. Luckily my carbs will be rebuild and back on the bike within the next 48 hours so I will know by then
 

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Discussion Starter · #9 ·
Thanks, much appreciated. Just so I know, when you bought those aftermarket carbs, did you have to buy new boots, jets or cables? Do these plug and play with the stock airbox tubes and the intake rubbers? Did you need to tune them any further?
 
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