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Discussion Starter · #1 ·
Hey everyone, new here.

I'm a college student in central MA and I grew up working on engines, but I just recently got into working on motorcycles..very cool machines.

What I have on my hands is a 1971 cb450 k4.
I bought it in decent shape and have been riding it strong for about 4 months...figured it'd be time for some maintenance other than oil and a tappet adjustment.

With the help of the shop manual, my tune up went great. I started by pulling the engine, then:

-Broke and re-timed cam chain (off by a few millimeters each mark!)
-Re-set ignition timing (also quite off)
-Valve clearance set to 0.0015" w/ lobe not engaged (closest feeler gage I had)
-Changed plugs
-Cleaned carbs
-Oiled the UNI air filter (stock air boxes but it looks like the old material was burned out and replaced)
-Compression reads 165/170psi left to right, a good increase over what is was prior (can't exactly remember...)

Anyway, now the bike runs poorly.

It'll kick over and idle just fine but unless you give it throttle VERY slowly and consistently until about 3000rpm, the engine will die...
Even at that, at about 3200rpm, the climb gets very choppy and you have to punch it or else the shuddering gets much worse until the engine dies...
After about 4000rpm the motor runs like a dream, very responsive...thats where I've been doing all of my test riding, 4000rpm and up.
It will also cruise at higher speeds in higher gears at about 2000-2800rpm without much hesitation, but to consistently accelerate (in any gear) I have to take it easy on the throttle until about 3000rpm, then punch it to get over 4000rpm, then I'm set.

Please know that I've gone though so many suggested CV carb tuning methods on this forum, including Bill Lane's, and this is the BEST configuration I've gotten.
Whenever I fiddle with mid-range power in the carbs, I lose high range and the bike becomes very hard to start.

Couple thing to note:

- I re-installed the same carbs with the same parts in the same configuration. (jets/needle height/mixture screw set-point/float height)
- The carbs had a similar 'sticky spot' before I tore into the bike just MUCH more subtle, the momentary stutter at around 3200rpm barley affected riding.
- I can make the engine smoothly accelerate if I choke it a bit just before the stutter at about 3000rpm and then open it again past 4000rpm, this IS NOT an ideal way to ride...

Please let me know what you think the issue is, and if it is a matter of carb tuning if I should even bother...I'm eyeing a pair of Mikuni VM32's that would alleviate all my problems ;)

Thanks for reading.
 

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Which UNI filters? Some have boots that block airflow from directly entering the air jets in the intake, which will affect the mid-range more than open throttle.
 

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Sounds like a mechanical advancer issue. I was getting the same hesitation and backfiring around 3000rpm and messed with my carbs for days. Finally got fed up and installed a PAMCO ignition with the electronic look up table (removing my mechanical advancer) and the bike has never worked better. The cheaper solution might be to find new springs for the advancer and see how that works - I hear Mike's XS has ones that will work.

Paul
 

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Just a note, I had some VM32 carbs on the bike when I first got it, and the height of the carb gave me issues with clearance to the fuel petcock (CB500T). This caused my left carb to be tilted inward slightly meaning that it was no longer level when riding normally. That would affect the float level and suction into the main and idle circuits of the carb. I returned to the stock carbs using K&N Pod filters.
 
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