Where do you look for a flat spot?

**Magua** · 10th March 2009, 17:41

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** · 11th March 2009, 09:53

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:Me** · 11th March 2009, 10:09

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** · 11th March 2009, 10:57

Originally Posted by MSTRS

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

Nope.

Originally Posted by MSTRS

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:Me** · 11th March 2009, 12:15

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

**Magua** · 14th April 2009, 17:03

Originally Posted by MSTRS

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** · 14th April 2009, 21:00

Originally Posted by Magua

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 )