Hi there, can anyone shed some light on what could be causing my 4rt to start dropping it’s idle speed when it’s hot. It’s fine for the first hour or so then after it’s been revving when shutting off throttle the engine speed drops below idle speed and stalls. The bike otherwise is fine. I changed the air filter prior to going out on Sunday, thinking it may have been a dirty air filter.

but it did the same after it had been running for a while.  

Could it be the plug starting to fail. I’ve never done anything with the fuel system. Ie throttle body or fuel tank, but wondered if it could be one of those causing problems. 

Any advice welcomed. 

What year is the bike?  Might give some more insight to what could be the problem...

Might consider that plug change you indicated, always wort roiling out the small stuff.  

Other considerations, low on coolant so might be heating up the fuel, valve clearance, water pump same as coolant... just some things off the top of my head.

I'm thinking one possible cause might be the throttle cable is tight when cold, and opens the butterfly valve slightly. When the cable heats up (it's running fairly close to a lot of hot stuff) it could relax that tightness some, closing the butterfly valve and lowering the idle.

Check the "Throttle Grip Free Play" per page 1-6 in the 2015 Owners Manual. If it is tight, adjust it to have enough free play. After that, you'll probably need to open up the idle air adjust screw a little on the throttle body to bring the idle back up to the right amount.

Edited March 20, 20188 yr by sportsawyer

Check crankcase breather is not kinked or fouled inside. PCV (positive crankcase valve) may be sticking. Either way it could explain idle dropping as fumes in crankcase build up making it harder for piston to travel down cylinder bore at low rpms. This problem would show up more as the breather passed less and less fumes. You’ll probably say that the engine oil is fresh but the breather(s) can be troublesome. The other possible cause is a fuel supply issue where delivery slows down and starves the engine, start with tank breather and fuel filter etc.

As jonnyc21 mentioned, tight...ish valve clearances will affect engine performance, but you would notice a drop in engine pull if this was the case. Hope you get to the cause and sort it.

Edited March 20, 20188 yr by section swept

16 hours ago, gasgasshetland said:

Hi there, can anyone shed some light on what could be causing my 4rt to start dropping it’s idle speed when it’s hot. It’s fine for the first hour or so then after it’s been revving when shutting off throttle the engine speed drops below idle speed and stalls.

Hi

Does it stall when in neutral, when riding or when clutch is held in ?

Thanks for the quick reply’s. I will get it all checked out. And hopefully it’s just something simple. 

Hi Jimmyl. It was dropping it’s idle speed in both neutral and in gear. At least I think it was. Just as a note it’s a 2015 model 

19 hours ago, jonnyc21 said:

What year is the bike?  Might give some more insight to what could be the problem...

Might consider that plug change you indicated, always wort roiling out the small stuff.  

Other considerations, low on coolant so might be heating up the fuel, valve clearance, water pump same as coolant... just some things off the top of my head.

It’s a 2015 model. 

Hi Guys. Was out again today. Had changed plug,checked coolant. Slackened off the throttle cable and brought the idle speed up, but it continued to do the same and drop the idle speed on shutting off throttle. I’ve taken the tank off to check it. Looks spotless inside. Is the teabag type filter in the small sump below the pump? Lifting the gasket I ca see a green mesh. Is it within the mesh. I will attach a photo. 

Any advice welcome  I was thinking to get the throttle body off and see if it needed cleaning. It’s all new to me as I’ve been a Gasgas/ carb user until this bike being my first fuel injected so it’s all a bit of a mystery to me??

It's under the pump assembly in a space between the green stuff.  You have to pull the pump to get at it.