Backfiring

Daryl86 · Two Stroke Sniffer Joined: 06 May 2010 Karma :

After passing my cbt at the grand old age of 23 yesterday, sods law the bike decided it didn't want to work properly today.

Had some issues with the carb, finally got the bike running sweet. I then took it out for a run and it rides perfectly fine, gets up to speed ok etc. The only thing that it did was after getting up to about 60 and then backing off it backfired.

All I wanted to know was is this normal (old engine design, no ecu controlling fueling etc) or could it be an issue with it running too rich??

Thanks to anyway who offers some advice.

Oh and its a zongshen engine. Which as far as I can gather is pretty much a replica of the cg125 engine.

Daryl.

Ichy · World Chat Champion Joined: 15 Jul 2005 Karma :

Its Chinese, be grateful it runs Laughing

Backfiring on the overrun isn't a big problem and can be just a bit of fuel 'trapped' when you shut off the throttle being burnt off.

Might be related to an air leak in the exhaust but, personally, unless it gets worse and everything else is ok then just ignore it for now.

Daryl86 · Two Stroke Sniffer Joined: 06 May 2010 Karma :

Haha yeh that is true! I did some searching online about zongshen and didn't hear too many horror stories.

It had a new exhaust fitted before I had the bike so hopefully that isn't the problem.

I will just see how it runs then.

Cheers.

doggone · World Chat Champion Joined: 20 May 2004 Karma :

Might be letting air into exhaust near engine.
Probably won't do any harm but in theory the back pressure from explosions could damage exhaust valves, so doing it a lot is not good.

Occasionally, it's more of a feature - which can be kinda fun if you suss out how to make it happen on demand Wink

drzsta · World Chat Champion Joined: 20 Apr 2009 Karma :

are you sure your not running too rich on the jetting? im almost sure that'll be the problem
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Daryl86 · Two Stroke Sniffer Joined: 06 May 2010 Karma :

Ive adjusted the fueling and it seems to have got better now.

I have another problem with the bike though! When doing 60 ish in top gear if you back off the throttle slowly it seems like the bike jerks slightly. This is affecting my confidence in some corners because it unsettles the bike.

Its not like a sudden jerk though, it seems like the bike speeds up a little and slows of its own accord. Its slightly hard for me to put into words Thumbs Down

It only does it when im in 5th gear though. Any other gears, rolling back off the throttle seems fine.

Any more help is muchly appreciated Thumbs Up

stirlinggaz · World Chat Champion Joined: 22 Jul 2007 Karma :

check your chain?

Daryl86 · Two Stroke Sniffer Joined: 06 May 2010 Karma :

Funny you should mention that. I had the day off work today and spent some time tinkering with the bike. I have adjusted the chain anyway, because it seemed very slack. I didn't connect the two problems together as I'm new to working with motorbikes.

These 125's are easy as hell to work on though which is good! I haven't had a chance to ride the bike today after adjusting the chain tension but I will be out on it tomorrow and see how it rides.

Cheers Thumbs Up

stinkwheel · Bovine Proctologist Joined: 12 Jul 2004 Karma :

A lot of those chinese bikes also have a PAV valve on them that bleeds air into the exhaust on the compression stroke in an attempt to lessen exhaust emissions.

Bleeding air into the exhaust is a recipe for backfiring if the timing of it isn't exactly right
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I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.