Small leak between expansion chamber and silencer cause mis?

[billybaggins]

Trying to work out if my exhaust is to blame for my misfire or if its another problem. Its an 1988 Honda ns125 and between the chamber and silencer there is a very very small leak I cant get sealed up ( i will consider welding if its the known culprit)

Anyway the miss is right before the powerband at 8k rpm and it hesitates for a second then revs clean up to 11k. it makes a strange noise when missing like a baow awo awo if you can understand that. Anyone feel this could be the cause?

It did it with my last exhaust which i found had a tiny hole on the chamber r4ight before the end though the silencer was sealed to exhaust ok.

engine was rebuilt and carb is well set up, has had different cdi put in too and all else seems fine.

Thanks for the help

[Kickstart]

Hi

Might not help, but doubt it is the real issue. Screwy problems like this are no unusual with a performance exhaust.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Old Git Racing]

Don't think so, more likely needle position, try moving it up a notch to richen it, if it makes it worse move it 2 notches down and try again.

OGR
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2022 Tiger Sport 660 2019 Street Twin 2003 K3 gixxer 1000 1998 Srad 600 track hack

[billybaggins]

Cheers guys, have tried most things and needle position, now engine is rebuilt and cdi changed and still did it with standard exhaust with small hole at back, now ive got this micron and it still does it. The only thing similar is that this one blows slightly between silencer and chamber so thought that might have been it. The whole carb has been changed too, I just can't see what else it could be. Also seems to be worse when its well warmed up.

[Kickstart]

Hi

A 2 stroke expansion chamber exhaust works well over a particular rev range, but just outside that range it tends to work really badly. They are set up to suck out the next lot of gas from the engine, pulling through fresh mixture, and likely pushing back a bit any of the fresh mixture drawn into the exhaust. As the pressure waves doing this move at a pretty constant speed and the exhaust is a constant size the waves that do this can arrive at the wrong time, ramming a load of dirty exhaust gas back into the engine after the fresh mixture (possibly early enough that plenty of the fresh mixture doesn't get from the crankcases to the cylinder).

This is likely what is happening on your bike, and what a power valve largely helps to solve. Unfortunately the NS had the power valve removed for the UK model.

Note that pressure waves also work on the intake side, hence a cone filter can cause similar problems (with the fresh mixture getting bounced through the carb multiple times and getting multiple doses of fuel). From memory this is what the Ledar replacement nozzles were meant to help limit.

These all add up and make sorting out a performance 2 stroke to run nicely throughout the rev range to be a bit of a long job.

Or just slip the clutch and keep it in the power band

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[billybaggins]

Cheers Keith, i might just have to live with it although its very annoying and even a rebuild and 3 exhaust and 2 cdi units can't solve it. I think i give up!

[bypass2]

what grade spark plug is in it. maybe faulty plug /cap ht lead shorting out. is the battery good /old. standard air filter or kn. standard jets or up jeted.