Where do you look for a flat spot?

[Magua]

I've just recently completed a rebuild of my cb400n. I had hoped a flat spot would leave with the top end work I did, but alas, it remains.

Shortly before the rebuild I overdid my valve clearances, pushing my exhaust valves into the engine, burning them. The flatspot seemed to coincide nicely with this, however after a lapp, there seems to be little difference. I know you're meant to do your clearances before a carb balance, so they somehow affect each other, could this overdoing of my clearances somehow have thrown my carbs out?

I understand flat spots usually tie in with exhaust systems or carbs. My question is, how do you isolate it?

[Magua]

I can get carb repair kits from Davidsilverspares (all the gaskets / orings, Jets, Needles and the float valve), but I don't want to rush into that if the problem lies in my exhaust system.

[MSTRS]

The exhaust valve should not affect your carbs (think about where the leaking pressure is going). Still, as you say, a flat spot may not be due to a carb issue. Since doing the valve grind, have you done a leak test to make sure they are seating right?
Still - carbs are usually the problem, and it may just be co-incidence. Or it'd been that way for a while, and you become more aware due to the valve issue.
CB400N is an old bike (with mega-miles on it?) so a rebuild of the carbs may well be in order regardless.

[Magua]

Since doing the valve grind, have you done a leak test to make sure they are seating right?

Nope.


Still - carbs are usually the problem, and it may just be co-incidence. Or it'd been that way for a while, and you become more aware due to the valve issue.
CB400N is an old bike (with mega-miles on it?) so a rebuild of the carbs may well be in order regardless.

48,000km from memory, though it is 29 years old. I'll enquire about a leak down test, thanks.

Two carb repair kits are $80, pretty cheap really, but it's $80 a student would rather spend elsewhere atm.

[MSTRS]

Fair enough. Especially when it would be a fishing exercise...

[Magua]

Fair enough. Especially when it would be a fishing exercise...

I went fishing with the carb refurb kits and it seems to have made quite a difference. However it still seems 'tempermental'. Somtimes it will rev freely, other times it'll change its mind and will bog down again.

I checked my sparkplug leads with a cheap spark plug tool (no idea how accurate it is) and it tells me that the left lead hasn't as much juice flowing through it as the right. Perhaps this will be my next target.

[roadracingoldfart]

I checked my sparkplug leads with a cheap spark plug tool (no idea how accurate it is) and it tells me that the left lead hasn't as much juice flowing through it as the right. Perhaps this will be my next target.

Check the leads with a multimetre to be sure , easy and accurate
(OHMS test )

[Magua]

Check the leads with a multimetre to be sure , easy and accurate
(OHMS test )

Hm, not so accurate actually. Both Haynes and Clymer manuals say "get an auto electrician to do it" and don't give any resistance values.

[The Pastor]

you can check that they both are reading about the same value....

trick is to know weather the tollerance is 1unit, 20 units or 200 units lol.

[roadracingoldfart]

Hm, not so accurate actually. Both Haynes and Clymer manuals say "get an auto electrician to do it" and don't give any resistance values.



And the Auto Electrician will do what ??? :whistle:

he will check they are the same and within known values. ;)
Why pay someone to do it when you can do it yourself in 5 mins ?:wacko: