Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Bitsa (Ralph Wright - RIP) on November 24, 2011, 11:30:19 AM
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Hi to All,
Can anyone suggest a cure to my problem?It is doing my head in, totally restored sonic cleaned, completely renewed internals, timing, balance all spot on.
From cold runs spot on until its warm.When coming to a stop with throttle shut ticks over great go to pull away and she stutters and then goes like a rocket all through the rev range until you stop again.At first I thought it could be weak mixture but from cold its fine and I think if its weak it would jerk about but it does not.Plugs colour grey to tan not white or black.Apart from this problem its as good as I can remember back in the 70s when I had a k2.Everything from the head to the rear of airbox has been renewed.The carbs are off a French model k6 and the motor is US k4.I am open to all suggestions.
many thanks
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Does it have the original needles? I understand that the aftermarket Keyster ones do not work too well in the CB750, something to do with a slightly different taper.
Info from Hondaman's book suggests replacing the size 38 pilot jets with 40's. Some K6's had these but worth a look.
Float level is also a point to check.
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Hi seamus.
Thanks for that,no they are the new keysters and I had to raise them one notch to stop them sticking slightly.Slow jets are 40s already and I did the float heights with a Honda gauge to spec 26mm if I remember right.The air screws at best are only a half turn out anymore and the tickover starts to mess about.I have also tried 105 and 110 main jets same problem.
Many thanks
Bitsa
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Have you still got the old needles. If so, give them a try.
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If you wind the airscrews out further does it sound rich and hunt about as if it's on choke?
If that is so it could be too rich on slow circuit as this would clean up when you raise the revs and engine load.
Thought most air screws are usually 1 to 1.5 turns out to start with.
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If you can get hold of 'My CB750 book' by Mark Paris appendix G,tuning, would be well worth a read.
Your jetting of the carbs (40/105) is fine but the spark advance/timing/air box vent/float hight are different from the K4 to the K6
Alan
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Hi,
Thanks to all for there help.Looked around at various sites some say keyster are great some do not.
One guy had trouble with the airscrews.Others needles and gaskets so weekend will replace with the originals and see what happens.I have always thought the Honda parts are the best.
Regards
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Hi Guys,
Just an update,now its finally warm enough to work on I fitted new Honda needles airscrews to 1 and a bit re synced carbs blasted down road and everything seems to be spot on.The hesitation has gone and she goes like the clappers.I have done over 70 miles on it now and maybe I am over reacting but the plugs still look the same, grayish and going from 110 jets to 115 seems to make no difference to the colour.I am still open to any suggestions.On a different note when building the engine was finished and first started I had oil dribbling down by the plugs and thought crap the rubbers over head bolts would be the problem but no some bozo had helicoiled 2 threads where the cam caps bolt down and had drilled into the cavity above the plugs and how I do not know the oil was running down the bolts and into the plug area.Thread sealant cured this problem.Something to look at when you are re fitting head etc.
Best Regards
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I have done over 70 miles on it now and maybe I am over reacting but the plugs still look the same, grayish and going from 110 jets to 115 seems to make no difference to the colour.I am still open to any suggestions.
If you have ridden the 70 miles at lowish revs and throttle opening the bigger main jets will make only a small difference to plug colour. Raising the needle in the slide will make more difference to mid range (Clip one groove down on the needle to richen midrange) Plug chops at different throttle openings are the way to tell if your jetting is correct, unless you pay for some rolling road time.
One mistake lots of folk make when looking at plug colour is to take out the plugs after a ride and assume thats how the carbs are performing when riding, if you've come home from a ride and let the engine idle before taking out the plugs, when looking at the plugs you're bascally looking at the idle mixture setting.
Plug chops require running the engine at different throttle opening then cutting the engine on the kill switch, snapping the throttle shut and clutching all at the same time and coasting to a stop.
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After messing for weeks with the carbs I read a hondaman post saying that some airboxs were different so I managed to get a k2 box cleaned it up new gasket, filter and took her out today and had great fun on it did 130kph OK no probs and ticked over great.Popped the outside plugs and finally they looked a bit grey\brown.Your opions guys can an airbox do this?I think the old one was a k7\F1.
Regards
Bitsa