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Discussion Starter · #1 ·
Been trying to figure out my lean running problem on my '75 cb360T. It's not bad, I think, but I get some backfiring decelerating from anywhere above 4000 rpm, and some tendency to sputter (miss briefly) when easing speed downward with the throttle.
Timing is carefully set, valves carefully adjusted, running premium fuel, compression 155 and 157. I used the old exhaust pipe gaskets when putting the engine back together. Could that be a culprit?
I know re-jetting is a possibility, but the hassle of dismantling the carbs and finding the right jets leaves me interested in a trick a fellow told me about today: Take the tops off the carbs, pull out the diaphragm and slide, unscrew the needle keeper screw, remove the needle, and put a small washer on the needle before re-assembling, so it won't project quite so far down into its jet.
I tried this and began the re-assembly, but noticed that the washer that elevates the needle allows the keeper screw to screw down tight against the top of the needle, so it no longer has that little amount of slack which, I suppose, allows it to center itself without binding in the jet. I could file off a bit of the end of the keeper screw, equal to the increased height of the needle, but that'd be a non-reversible process.
Is it possible to find replacement jets for these carbs anywhere, if this method doesn't work, or you guys talk me out of it?
I'm all ears. Thanks!
 

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I can't comment on the fix you suggested, but I'm thinking if you are running stock pipes and air filter you really shouldn't have to rejet or move the needle any. I'd look into other causes like the exhaust gaskets you mentioned. Also, check for leaks around the manifolds.Have you checked the float heights?
 

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Where did the bike come from? If you went up in elevation you may need to jet up a small amount.
 

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Your fix doesnt sound like something that I would do but that is my opinion. These bikes run so nice when everything is correct. The problem with one that doesnt run right is almost always something pretty simple. There are just so many simple things that need to be right and that is what makes them hard to trouble shoot some times. I know it is tedious but start from the beginning and I bet you find the problem.
Don
 

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Discussion Starter · #5 ·
Hmmm...I'm reconsidering the needle modification...
Perry said if I have stock pipes and air filter I shouldn't have to re-jet. I don't have either: Bike ran better when I replaced the paper in the stock filter with a filter foam sprayed with oil. I glued it in where the paper had been. And I have Dunstall replacement mufflers, not stock. The exit hole on the stock muffler was only about 17mm diameter. The Dunstall's is about 48 mm diameter, a huge difference, which I figure, using pi r squared, is an 8 times bigger hole. I tried plugging some of the tubes packed into that 48 mm opening, as shown below, to increase the back pressure, and the performance seemed almost exactly the same. I even tried plugging all the holes, figuring that the spaces between the holes were still bigger than the the stock exhaust hole, but couldn't detect any improvement.
Regarding Dirtbag's question, I'm only about 100ft above sea level.
And Don's advice sounds wise: find the simple thing that causes the problem before making changes to the carb.
Could the mufflers be the sole problem?
Thanks, All.
 

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how do your plugs look? they are the best way to know how the engine is running, short of an mix sensor in the exhaust. i would also try tweaking the throttle balance between the carbs. the "feel the exhaust pressure and adjust till they are the same" method leaves lots of room for error, and out of syn carbs can screw with stuff. is the 360 meant to run on premium? if the fuel grade is too high, that can mess with things.

thats all i can think off... good luck. and yes, raising the needles with raise the mix slightly over the whole range of throttle.
 
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