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Author Topic: CB750 K5 rebuilt, fights to stay alive.  (Read 1621 times)

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Offline C.Doyle

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CB750 K5 rebuilt, fights to stay alive.
« on: November 24, 2009, 02:31:41 PM »
I picked up my K5 cb750 about a year ago with 12k original miles, and started the rebuild with cafe' intentions. Here's where I'm at now. Straight 4-1 rapped, airpods, dyna-s, dyna 5ohm coils, new wires, ngk plugs and rebuilt carbs. also picked up a bunch of parts from steve over at cb750cafe.com. But I ran into a huge problem thats been keeping me off the road... I cant seem to get the needle/jets right at all! Valves were adjusted by the local shop because my feelers broke... but its driving me insane!!! >:( right now I'm at 1 notch rich on needle position, and 130 mains. It seems my carbs are making a coughing sound as well, not so much after I changed jets from 120 to 130. But now hesitates to move cant really get it over 35mph at all. I've tried just about every thing I can think of and didn't want to post until I had too. heres the old pig

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Offline w1sa

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #1 on: November 24, 2009, 02:54:25 PM »
Is the timing advancing to full advance per spec ?
Are all cylinders firing?  Equal heat at header joint?
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Offline Old Scrambler

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #2 on: November 24, 2009, 02:56:10 PM »
Great Tank.........That color is desireable!  Go back to the 120s and an airbox with a K&N filter.  Set the needles at the stock notch.  Adjust the air and idle for balance.  Read the numerous info in these pages about carb tuning. Then tell us how the bike runs.
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Dennis in Wisconsin
'64 Triumph Cub & '74 Honda CB750 Bonneville Salt Flats AMA Record Holder (6)
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Offline C.Doyle

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #3 on: November 24, 2009, 03:26:32 PM »
Timing is set so at 2000 its between the two notch's. The blue tank you see here had tons of rust and pin hole leeks so I picked up a ss tank. I loved hat blue color tho. I figured out that the cough sound is also fallowed by fuel tossed out from the pods. I don't have an airbox :( I removed it and threw it in with a dump load some 6months back thinking I wouldn't need it... stupid me... I've read the carb faqs post and with every thing I'v got seems to be the ringer. I'm starting to think the cough sound is the valves at .003,.002 and not .003, .004 like hondaman says. I could be way wrong Thanks for the replys!
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Offline Simpson

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #4 on: November 24, 2009, 03:40:17 PM »
Quote from: Old Scrambler on November 24, 2009, 02:56:10 PM
Great Tank.........That color is desireable!  Go back to the 120s and an airbox with a K&N filter.  Set the needles at the stock notch.  Adjust the air and idle for balance.  Read the numerous info in these pages about carb tuning. Then tell us how the bike runs.

My gut tells me that he is running way too rich also. I'd revisit the air screws for sure.
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Offline C.Doyle

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #5 on: November 24, 2009, 04:18:11 PM »
I put the 130's in about 3hours ago and road around, It was very choppy to get up and go. I switched down to 125's and it started wouldn't idle and died then wouldn't start... this is what I've been dealing with for the last 2months. Before the 130's it had 120's, witch I could ride around town and climb small hills. 115's wouldn't idle right and made the engine hot by sitting. air screws haven't made very much difference at all with any of the jet sizes.  ???
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Offline Simpson

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #6 on: November 24, 2009, 04:30:22 PM »
Quote from: idaninja101 on November 24, 2009, 04:18:11 PM
I put the 130's in about 3hours ago and road around, It was very choppy to get up and go. I switched down to 125's and it started wouldn't idle and died then wouldn't start... this is what I've been dealing with for the last 2months. Before the 130's it had 120's, witch I could ride around town and climb small hills. 115's wouldn't idle right and made the engine hot by sitting. air screws haven't made very much difference at all with any of the jet sizes.  ???

Keep in mind the needles and main jets only start really contributing to the A/f ratio after "about" 1/4 throttle turn. I said "about" for those about to question the 1/4. Choose your mains based on performance at higher throttle, Therefore if you have idling issues, take a look at your slow jets. Maybe they are clogged, maybe you need different size based on modifications. Sounds like your problem is in the slow jets and air screw adjustment.
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1970 CB750 K0
1975 CL/CB 360 Mix

Offline ekpent

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #7 on: November 24, 2009, 04:40:18 PM »
What color are your sparks plugs. Are they wet dark and sooty or do they look lean. Are you running a fuel filter by chance. Not enough good fuel flow will give your bike 0 power and running issues as well as plugged-dirty slow jets.
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Offline mlinder

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #8 on: November 24, 2009, 04:51:18 PM »
Come by the shop, we'll take a look. Contact me via PM. You aren't far.
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Offline C.Doyle

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #9 on: November 24, 2009, 05:35:08 PM »
Umm- I've got a inline filter all right... the forest grove honda shop had this nifty gas tank with a fuel filter they used when testing bikes. So I poled  the gas tank off the old lawn eater/cleaned it out out for testing. The SS tank has minor rust inside and didn't want to put it back on right after I rebuild the carbs about a week ago so I bungee corded the little tank on. THAT WOULD BE FANTASTIC Mlinder!! It'll probably have to be next week as thanksgiving and all, but I'll get ahold of you soon. I'm missing my 18mm for the sparkplugs.
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Offline mlinder

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #10 on: November 24, 2009, 05:37:29 PM »
You bet. There's a few of us down there. We can help.
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Offline C.Doyle

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Re: CB750 K5 rebuilt, fights to stay alive.
« Reply #11 on: January 08, 2010, 08:24:25 PM »
So the Cafe's not fighting to stay alive anymore! With the help of a charged battery, my Parts'n more rectifier/regulator must not be wired right... :( I had my good friend John take this short clip of the girl running slightly cold. I've read on the sight that the stock K5 750 runs at 6500 RPM's but in this clip it hits around 7200 rpms with no inner engine mods just pipe and filter/jets. Didn't know for sure but will changing out the cam for a hot cam/836 kit boost the rpms?

http://www.facebook.com/video/video.php?v=1243130851164 ;D ;D ;D ;D ;D
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