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Derbi GPR 125 4T - techincal issues 10 Derbi GPR 125 4T 4V discussion forum:
Derbi GPR 125 4T - techincal issues

Derbi GPR 125 4T - techincal issues 
Jay said 2011-03-18 22:54:22 
I have a problem with my sons 2009 DERBI GPR 125 4T which I’m hoping someone else may have experienced and can help with?

I bought the bike back in August last year. My son wasn’t 17 until January this year so the bike was stood for a while – it was fired up and ridden up and down the driveway pretty much every weekend.

When he eventually turned 17 he tried to ride the bike and that is when the problems started…….

The bike would start – go onto automatic choke for a few seconds (not long enough I don’t think) then the revs would drop and the bike would cut out. It would then not fire up at all. The starter motor would turn but the bike just wouldn’t start.

I checked the fuel tap by taking off the fuel pipe to see if any fuel was being ‘sucked’ from the tank – result: no fuel was being sucked into the carb. No fuel into the carb = bike will not run.

I removed the vacuum pipe from the carburetor and tried to suck fuel from the tank – in theory this will replicate the vacuum caused by the throttle being opened and open the fuel tap to allow fuel to flow. This didn’t work – still no fuel (and yes the tank was full of fresh fuel).

Replaced the fuel tap with a second hand one. The bike fired up but would still cut out as before. So at least now fuel was getting to the carb but the bike still wouldn’t run right. A day or so later the bike wouldn’t fire at all so again I checked fuel was actually making it to the carb……once again there was no fuel coming out of the fuel tap!! What are the chances of having 2 faulty fuel taps????

So to eliminate the fuel tap completely I bought a new one - £60.00

The bike ran ok – but actually cut out whilst being ridden and wouldn’t start for 5 minutes.

So the next step was to remove the carburetor (this is a major job in itself!! Had to remove the rear subframe!) I stripped down the carb, cleaned out all jets with an air compressor. Re-assembled and the bike worked perfectly……until the next morning. Tried starting it and the same thing happened as before – the bike would start and run on choke (for a longer period of time than previously) and would then cut out.

Bought a new spark plug – checked the coil was connected to the plug lead securely. Have always had a decent spark but cant do any harm having a new plug.

I have heard that there is a problem with the valves not seating right so removed the airbox cover and filter, blocked the hole with a rag and turned the engine over – this in theory should seat the valves.

Have once again checked that fuel is being sucked from the fuel tap and low and behold nothing again!!! Surely this cant be the fuel tap?? So I replaced the vacuum pipe that runs from the carb to the bottom of the fuel tap incase there was a split in it and still no fuel is coming out…..that was last night. Checked again this morning and fuel is now flowing but the bike still won’t start!!!

Also have a problem with the fan not working! So when the bike does actually run it overheats – though I traced this back to being a broken wire which I have now repaired. The fan will now work (when I connect it directly to the battery) but will not come on when the bike is getting hot – so either a fuse or thermostat gone now!

Could really do with a manual but cant seem to get hold of one.

All this from a 2009 bike that has 200 miles on the clock!! I bought it privately and have no warranty.

I am just about ready to ride the thing into the nearest lake……but it won’t start!!
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