Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Fordy on April 13, 2015, 04:53:09 PM
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Hi, Can anyone help, I'm at my wits end. My 750K6 runs lovely when cold, but when it gets warmed up it will not run smooth at low revs, it still ticks over nice, open the throttle to accelerate and its smooth as silk again, settle down to a constant speed for speed limits and it runs like a bag of spanners again.
I have replaced all the jets, float needles and seats, slide needles and seats, checked carb balance which are all at a constant 10psi @ 1400 revs. replaced plugs and caps just in case timing is bang on.
But still no different.
Help!!!!!!!
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Sounds like its running rich to me have you done the float heights,clean filter etc.Has it standard pipes,whats the needles set at etc
Cheers
Bitsa
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Hi
Yes done float heights at 26mm. Fitted a new filter and the needles are set in 3rd ( middle).
Thought I might try replacing condensers, points are good.
Worth a try?
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Agree with Bitsa, it does sound like rich mixture, what do the plugs look like?
It could be condensor(s) breaking down when hot but then I would expect it to run rough all the time when hot regardless of throttle opening. Wouldn't do any harm to replace them though particularly if they are old. If the spark is on the weak side it can sell you a dummy and make a minor carb issue much worse.
Leads me to think carburation and at the lower end of the scale what do you have the slow running screws set at?
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Slow jets are set about 1 1/8 out. When I got it the old guy (68) had been running a yoshi straight thru pipe on it but put a brand new motad on to sell it, I thought the poor running was because it had been jetted up, thats why I put new jets in, (bought before I checked, idiot !!!) Yes the old plugs were sooted up.
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Fordy,
This thread any use...?
You'll have to scroll down before it gets onto needles etc
http://forums.sohc4.net/index.php/topic,114960.0.html?PHPSESSID=s38h0m3sbn7e2uqcpa1cofnhp1
Simon
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Thanks for that Simon, interesting reading, I also couldn't detect any wear on the needles or the seats but replaced them anyway. Hasn't made any difference at all.
Don't know which way to go now cos I put the new ignition plate bits in and it's just as bad. :o
Don't get this problem with my old-old stuff cos only got 2 carbs max. Haha.
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I had a ZXR400 a couple of years ago, non standard filter and exhaust fitted, it ran ok for about 2 miles and then would bog down and not want to rev until eventually it would almost cut out, let it cool to stone cold and it would run fine until warm, sooty plugs etc, I tried lots of things before deciding to go away from the standard float height settings, if I remember correctly they should have been 13mm, I took them to 15mm and it ran better, set them to 16 or17mm and it ran perfectly, plugs a really nice colour and no flat spots, could be worth a bit of trial and error on your float heights?.
(Here's the ZXR wish I still had it, 14.5k redline, what a little screamer it was! :))
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Fordy....stabbing at straws admittedly, but have you got the correct (heat range) plugs in (D8-ES). If too cold they may be fouling up / playing havoc with your spark?
Your exhaust still the Motad? If so, perhaps Ian McDonald of Motad might be able to help? His contact # within this thread...
http://www.sohc.co.uk/index.php/topic,7443.msg45133.html#msg45133
Simon