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Previous PostTech Posts > Problem Solving > scratching my hair out , fluffin stupid
Sir Alan of Ingol
sespit of the north,
United Kingdom

Posts: 13811

18 May 2017 12:12
OK an update

spoke to steve at motocarb , he advised not to use a lorge carb on a 125 , the CV type carb is bigger than a VM because of its design , a 26mm cv would be the same as a 22/24 vm , however , the smallest carb that steve can supply jets etc for is a 26mm , 22mm and 24mm are made and sold direct to manufacturers pre set

so another "ebay special" cv carb was ordered , a 26mm CV , (bs26) and fitted , ran like a pig , coughed spluttered and basicly just ran on full choke

stripped it down , no markings on jets , cleaned the pilot and put in a 102.5 main (as fitted to genuine carbs) and it ran!! ended up haveing to geta new altenator (stator) from ebay , £23 inc CDI pickup coil b, put a new ign coil , plug and cap and basicly rewired the CDI wiring

so ahe runs at last !! found an article on fine tuning the carb ,
(Do to the size of the engine (on the Suzuki GN125), the pilot jet size needs to be pretty close to original size. If you didn't notice any brass getting reamed out when you used the drill bit, then you're probably ok. But that would have been a really tiny drill bit. If you think you made the pilot jet bigger, but not too much, then you SHOULD be able to compensate by adjusting the idle air/fuel mixture screw as you suggested.

If your pilot jet is now letting more fuel in than stock, then you'd want turn your idle mix screw IN or clockwise just a little, to cut cut the mix back down. Turning in 1/8 turn (.25) may do it. Depends on how large you made your pilot jet. If it were me I'd do the following:

1. Use fresh gas only and drain the bottom of the carb. Make sure you have spark and the the spark plug is clean or new.
2. Get the bike started and able to idle at 1000rpm or as close to that as you can. Leave that idle mix at 2.25 turns out. Let it warm up.
4. Twist throttle a little and let it back. If the idle drops back down, good. If not, or if it stays high for a long time before dropping back down, there is probably an air leak (see previous post) or the idle speed adjustment is screwed in too far (throttle valve open too much). Back it off and try again. Once you have throttle/idle under control, continue.
5. Turn the idle mix IN 1/8 turn. Wait 10 seconds. Did the engine speed pick up? Or no change. If no change, repeat. If the idle speed starts going down, then turn the idle mix OUT 1/8 turn. Wait 10 seconds. Repeat. If it starts getting bad going the other direction, put it back to where it was best. You want the engine running as smooth as possible (you have to listen very carefully) and the engine speed should pick up a little. That is okay.
6. Each time you get the engine running smooth, adjust the idle speed adjustment out to get closer to 1000rpm. The goal is to have the butterfly valve (throttle valve) nearly closed (but you can't see it with the air filter on unfortunately) so that the engine runs off the pilot jet and idle mixture circuit entirely and NOT THE MAIN JET.

Goals to obtain:
1. Throttle Valve hardly cracked open at all, just a tiny bit.
2. Engine running smooth at idle, 1000rpm.
3. Spark plug looks clean or light tan after running idle for a while.
4. When you blip the throttle, the engine speed should pick up and drop back down quickly. Once you get the idle going well, if you open the throttle and get sputtering, backfire, or engine speed stays up too high for too long... there are more problems to sort out.)

will attend to this later , as at the moment the plug is the right colour and at least she runs

Nick , if you PM me your details , I will post this carb back , as it will be more use to you

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