2008 MCX 270 stalling and lazy at low rpm

[DaveB]

OK gang...I finally got to ride the new-to-me Nytro. Had an afternoon to play around home ditch banging (3000') and had a weekend in Revelstoke to do what it built to do. And I have to say, it's not the greatest. It starts up no problems, but idle seems up and down. It seems to randomly run between 1200 and 1600 rpm. Then, once warm, if you blip the throttle a bit or perform small movements (like loading/unloading) it will stall several times. Riding in the mountains, it is lazy at low RPMs. It has SEVERE turbo lag and really does not like running below 4000 rpm...kind of missing and loading up. If I was to guess, I would say it is very rich on the bottom. Now once you clamp it and it spools up, no issues. Although boost takes several seconds to build....it's a real deal breaker for trying to boondock in the trees. I rode with another MCX 270 with LC pistons (mine is a head shim deal) and his sled has next to zero lag. Now I understand clutching very well...so I am going to look there. I suspect a higher start rate primary spring may be in order. It has a Shockwave helix, so I also suspect a lower angle my help...but honestly I feel the issues are in the motor itself. Keeping in mind I don't "speak 4 stroke Yamaha" very well just yet, where should I look first?

 

[JP270NYTRO]

I would start with clutching...whats your setup for clutching? engagment? weights? the mcx 270 kit is rich on bottom but not with alot of lag my kit has almost no lag.

 

[Northernboy]

Check for exhaust leaks...bellows, donuts & header pipe...

 

[rxrider]

My best guess without having ridden your sled is that it's like you say too rich on fuel.

I recommend getting hold of a good AFR meter with data logging capabilities. It would take out all the guess work and make it possible to reprogram the MCX computer.

Most of the time the MCX EFI Fuel mapping are pretty close but I have seen quite a few sleds with maps completely off typically to the rich side.

 

[DaveB]

OK...went and played some more today in the garage. It is definitely fat. I'm not going to buy an A/F meter when the reason I went with the MCX was because they are reported to run perfect and don't need mapping changes. I went through the vacuum/boost lines and they all appear OK. This machine has an Alpine header. Would that change the low speed A/F ratios enough to cause an issue? I've also read that the MCX injectors can stick. Maybe this is an issue?

I can get it to act up in the garage by slowly bringing the RPMs up until juuuust below engagement and it will start to miss and burble and load up.

 

[judger101]

id still get an a/f gauge. it is really nice to see where your sled is running at all times. plus in this situation its harder to diagnose as you are just kind of guessing at this point.

 

[sorenson1610]

For sure get a Afr gauge.....cheap insurance to monitor what is actually going on.

 

[BADSLED]

You have to understand that the smallest changes (Alpine Header) can have an effect on AFR. There is no such thing as "perfect", simply too many variables. Yes, MCX turbo kits are the best available kits out there but they are not to be installed and guaranteed not to have a hick-up and require some calibration change. Invest in a AFR gauge!!

 

[Climbmax]

DAve.......sounds like a header leak. The Alpines will break just like the OEM version. Or donuts or Flex joints. You may be able to listen in the tunnel for that raspy sound of a feed leak. Take off the tank and have a look. Bet you find something.

Cheers
RS

 

[Northernboy]

Maybe check fuel pressure at idle as well since you have an external fuel pressure regulator.

Also an AFR and boost gauge is a must.

Good Luck
NB

 

[rxrider]

Idle fuel pressure at 4-5 lbs

 

[DaveB]

OK...got tank off. Pulled shield off of the header connection. No signs of leaks in there. Header is all in good shape as well.

Pulled plugs. (see pics) definitely fat. Plugs are NGK Iridium CR9EIX gapped at 30 thou. Going to pick up new plugs tomorrow since it's apart now....what should I put in? Same or different?

 

[DooAssassin]

Who did you head shim? Could you be off a tooth ? This would cause bad idle and bad turbo lag.

 

[Northernboy]

I use stock plugs gapped to 18 thousands. Most use that on boosted Nytros.

 

[mbarryracing]

I use stock plugs gapped to 18 thousands. Most use that on boosted Nytros.

Try a fresh set of spark plugs, X2 on what Northernboy said to use...
Sometimes we've seen partially smoked plugs, or plugs that sat over the season, mimick a rich condition because of an incomplete burn of the mixture... and look exactly like that.
You didn't perhaps change fuel brand?