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Discussion Starter #1
OK, at the risk of sounding stupid. What is the difference in a 71 k4 carb and k5 carb? I see the main jet size is different but is there anything else? Reason I ask is I have a k4 that I just slid a k5 engine into and reused the k4 carbs. new rebuild kits, sparkly clean and I went through the synch process but at around 5k RPM it seems like it hits a dead spot. if you nurse it along it keeps building and seems to run better around 7k RPM. Using uni filters and stock 130 mains. Checked the plugs and both look good. Am I missing something?

Dave
 

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Dave,

If you put another engine in, maybe the ign has a different setting.
Is the K4 and K5 engine the same? Camshaft?
What bike are you speaking of? Or is THAT a dumb question?
I have done the same thing, BMW, old type bike, newer engine in it, old mufflers, carbs, intake.
the "only" diff was bigger valves.
The whole carburetion had to be changed.

wiemer.
 

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Discussion Starter #3
Sorry about not being clear on the bike. I'm talking about a 71 cb450 K4 with the 72 K5 engine. They should be the same engine..not sure if they had some slight differences or not. the carbs seem to be the same outside the jetting. K5 had 140 mains which I tried but was running to rich so I dropped back down to 130 where it seems to give me the best performance. I grabbed up the k5 engine as it only had 8k miles on it and had been fitted with electronic ignition. That and my 24k mile k4 was totally locked up made up my mind for me. I passed on the carbs and starter opting to use mine.
thanks for the input.

Dave
 

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rustinaway said:
I grabbed up the k5 engine as it ...had been fitted with electronic ignition. .........
I'd say that's a pretty good place to start right there.........maybe there's some sort of "advancer" problem.
The only sure way to know (at our level) is to put the points back in, time it up, and see if it acts right. That might be a lot of work, can't say.


There may be a difference in the mixture screw from K4 to K5, otherwise they're the same, as far as I know.
But that wouldn't affect you at 5K rpm.
You might try some 135 mains, sort of split the difference, see if it helps.
There's nothing you can do with the needle on a 450 carb, so..........

When the bike is "stumbling" like that, see if applying some choke helps or makes it worse........
 

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Discussion Starter #5
Thanks... I'm gonna try one more thing. I tried your soda straw hint last night and I felt like I had to blow a little too hard and it really didnt move that much or easy.... so, perhaps I wasnt as sparkly clean as I first claimed. So off with them once again for another cleaning and inspection.

Thanks again

Dave
 

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It's possible that one of your carb tops is warped or bent.. If the slide doesn't rise or fall smoothly then it's a definite possibility..


GB :mrgreen:
 

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Discussion Starter #7
Tell me if anyone has seen this before. I tore apart the offending carb, removed the needle and it struck me that the needle set screw seemed larger than I remebered or felt it should be. I removed the left carb for comparison and that was one teeny set screw. When I rebuilt these one was rebuilt in Aug 09 and stuck in a box. I didnt get around to the other till Jan 10. So being just south of 50... my memory is that of a small bird. It looks like the PO maybe lost one and replaced with whatever he had. I ground it down to approx the same size as the left, cut a new slot and reinstalled. Took it out for a spin and what a freakin difference. I guress the added weight was just enough to make it out of balance at hard throttle? I had never run across this before and will look for a replacement set screw just to make sure they are the same metal etc. I'm still kicking myself for not noticing when I rebuilt.
 
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