Thumper Club Forum

Technical => Bike Problems/Questions => Topic started by: Smithy on July 07, 2013, 02:53:34 PM

Title: Rich or weak?
Post by: Smithy on July 07, 2013, 02:53:34 PM: Hi guys

I have had my project Suzuki GN400 running today for the first time. I have put on a free-flow conical air filter and a shorty silencer with minimal baffling. The bike started 2nd kick but was spitting back through the silencer on the overrun. It revved freely but I wondered do those symptoms mean it is running rich or weak?

Thanks for any help you can give.
Title: Re: Rich or weak?
Post by: Dick Scratcher on July 07, 2013, 03:35:37 PM: Messing about with both the air cleaner and the exhaust can throw your fuel/air mixture all over the place more often than not 'leaning' it out, check the colour of your plug, if it's sooty it's a bit rich and if it's light tan bordering on white it's running lean. Before you do anything though check the pipe to head joint and the silencer to pipe joint to ascertain that they're both gas tight, any leakage at either point virtually guarantees popping and backfiring on the overrun. Best of luck.
Title: Re: Rich or weak?
Post by: Smithy on July 07, 2013, 09:14:49 PM: Thanks man that sounds good advice. I will check them both before doing anything else and do a plug chop to see what colour we have.
Title: Re: Rich or weak?
Post by: guest1406 on July 08, 2013, 05:50:45 PM: Smithy any pics of your GN?

Mackenzie
Title: Re: Rich or weak?
Post by: Oldtimer on July 09, 2013, 07:34:46 AM: For the ideal way to check your plugs is to get the engine up to running temp and and hold the throttle/engine revs constant while riding for a mile or so, say on a good fast road, stop, kill the engine, then whip the plug out and that should give you a reasonably accurate reading. Blipping the throttle will not give you a true reading.
Title: Re: Rich or weak?
Post by: Smithy on July 09, 2013, 09:30:54 AM: I have pictures, whether I can upload them to the site is another question altogether! I will have a go and if I need help I will ask a 5 year old.

The motor is running much sweeter now I have sealed the pipe to head joint and the silencer to pipe joint. Now it is running reliably I can do as you suggest oldtimer. I don't think it is miles out to be honest so it shouldn't take too much setting up.

Thanks for the help guys I appreciate it.
Title: Re: Rich or weak?
Post by: Dick Scratcher on July 09, 2013, 10:22:42 PM: Good on yer Smithy it sounds like you're winning :) :)
Title: Re: Rich or weak?
Post by: johnr on July 12, 2013, 08:45:32 AM: to be honest, the standard settings on your carb will be a pretty good compromise for low and mid range engine speeds but unless youve done a sustained high speed run then you wont know if its running ok. removing restrictions in an exhaust system will make the engine run lean, removing restrictions in the inlet side will make the engine run lean, doing both and then not correcting the jetting is asking for trouble. running lean will result in either a hot running engine, a burned out exhaust valve, or worse, a holed piston as the heat just builds and builds the longer you have the throttle on a high opening. this can occur after just 5 or 6 miles on the motorway. unless your checks have been with the engine under load on the road at high speed, id be very wary about closing the door on this just yet. set the carb up right and your gn will fly, better still, get a carb off a klr650 and fit that, i had one on my gn/xl hybrid and it would just about crack 100mph on the long downhill bits of the motorway, no mean feat for a 400cc off roader with a lazy custom bike engine in it!
Title: Re: Rich or weak?
Post by: Smithy on July 12, 2013, 10:08:15 PM: Cheers Johnr that is good advice. I will be very careful how I set the bike up as it is a keeper and so I want to get it spot on.