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primary drive train rattle - CB750F2

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Skoti:
Analogue gauges can go off if they've been handled roughly, I use Morgan Carbtune which have always provided an accurate reading. Bought mine in the 1980's when they used mercury filled tubes.

https://www.carbtune.com/


Another thought is to make sure your rubber manifold stubs are in good nick and tightly secured, drawing air from that area would probably also give you a rough idle. 

Hope this helps


regards

Skoti

K2-K6:
"this is very similar to the poor initial running of the cb400f i did 2 years ago and it turned out to be the slow running jets being 'cleaned' and made oversize, a new set and it runs lovely!! "

Definitely part and parcel of this scale of engine,  just having one carb per cylinder takes us toward a very fine metering component set to run profficiently.

Could be that previous ownership has compromised the jet bores, making it uncertain for you to adjust.

blackeagles:
ok, thanks.  well i am going to give it a little bit more thought before i give up and call for help

i have ordered a carbtune pro so i can, at the very least, check the setting the analogue ones are giving me.

this is going to sound a bit like i'm a simpleton but given that the factory used to churn out parts for carbs by the 1000's so they are likely to be almost identical...well i notice the adjustment screw/nut on the top of carb 1 has the screw head flush with the upper nut surface whereas 3 and 4 the screw is about 1/2 turn or so showing above the nut!! i'm grasping at straws but this surely can't be right??

robvangulik:
That is exactly what you change while synchronizing, but while you wait for the new set, why don't you put them all at the same height and see how it runs?

blackeagles:

--- Quote from: robvangulik on July 18, 2020, 05:26:49 PM ---That is exactly what you change while synchronizing, but while you wait for the new set, why don't you put them all at the same height and see how it runs?

--- End quote ---

yes, i did that last thing, did it help?, well about just over 3/4 of a turn out so they all look level(ish) by eye and yes the tach went up by maybe 100rpm but it still has the very slight wavering effect.  i know its an analogue tach so that may be the effect of the cable but the engine still sounds like its wavering (maybe that is standard).

oh, slow running needle is 1 1/2 out.  1 1/4 revs drop maybe 100 rpm but hard to tell (tried the multi meter but the reading is up and down so much its unreadable), 1 3/4 out and revs increase maybe 100rpm, 2 out revs increase a little bit more but seems to be 'hunting' and beyond this they drop so 1 1/2 it is!!

oh, oh, i see that no3 has a very occasional fire back into the carb - hope it goes away when i adjust them.

soon find out!! 

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