96 QuadRunner LT-4WD 250 - bogs and blue smoke when warm

[erthnet]

I'm going a little nuts trying to nail down what's causing this. Any help help or feedback would be so much appreciated.

Once warmed-up, I'll get blue exhaust smoke, and the plug will gets a sooty black coating. I'm assuming it's an oil problem. My gut says rebuilt the top-end, my wallet and watch both scream no.

 

Bike was a non-runner when purchased. Story goes: was a barn-find from an old man that only rode it occasionally, seller "didn't want to" put time into rebuilding the carb. Odometer is just over 1,200.

First thing I did, was drain the crankcase: thin oil with strong gas smell. After the 1st oil fill, I ran the bike ~5 minutes to warm and circulate that new oil, and then I flushed and filled with new oil a 2nd time. Didn't overfill either time (added 3qts, 2 minutes running, 15 minute rest, and then topped to middle of oil window). New oil filters too. Also replaced the petcock, and a new in-line gas filter. Diaphragm had a hole, so I rebuilt the carb. Float level is 12.5mm (within spec), pilot screw @2.5 turns (spec is 2 7/8), needle clip in 3rd groove (I'm at 1,000ft).

Bike starts easy: slightest press of the starter, and its idling nicely. Once warmed, I got blue smoke. Engine also bogs; will not climb a 10% hill once warmed, but it'll do it when cold (if I'm gentle on the throttle).

I've changed the plug a couple times during this process. Every time, it starts and idles fine...until its warmed-up. Then it's a little more difficult to start, and it smokes once running (and warmed). Bike also tends to die if shaken side-to-side. Only times it didn't die when shaken side-to-side, is when I had the air filter off. Still not sure if the dying is due to an electrical issue or not...haven't traced that one out yet.

Leak-down shows no issues, either warm or cold (even did it with the oil filler cap off...just in-case of a plugged crankcase vent):

I can't tell if the bike is burning oil or not, as the seller left me a little surprise (yea, it drips from the cover ~10x/minute while the engine is running):

I'd expect blue smoke on starting it this was a valve issue...but there's no hint of smoke when the engine is started/ran cold. I'd expect a leak-down to show pressure losses if this was bad rings...but cylinder holds pressure whether hot or cold.

Does this still sound like a top-end rebuild project? Or am I overlooking something?

[davefrombc]

Head scratcher for sure . About the only thing I can suggest  is a possibility of a restriction on the  inlet to the air box or  filter causing  it  to  draw  hard  and run  lean and possibly  draw   air  from the  crankcase vent, and with it  a  little  oil. Idling, running  cold and  at  lower  rpm the restriction  wouldn't affect it  as  much  as   it would  at higher  rpm  and  hot . 

[erthnet]

Oh, and a straight compression test shows ~148PSI...well within spec.

 

Hmm, good catch on the airflow restriction possibility. I've tended to get impatient when warming it up in-place, so I'll rev the throttle a few times, and/or run it at about half-throttle for 5-10 seconds at a time. When I was testing it while riding around, the smoking did tend to correlate with around the same time I'd be testing into the upper-gears (e.g. riding a little faster). However, my most recent test still resulted in very light white/blue exhaust after simply idling it for 10-15 minutes (didn't touch the throttle at all).

I have a brand new high-flow K&N SU-2588 air filter on there; not supposed to be restricting. Hmm...time to start looking closer at the crankcase vent.

[erthnet]

Well, today's test was promising@davefrombc...crank vent hose was dry, but I disconnected it (and plugged the air box side) anyways to see if that made a difference.

One thing's for certain, it no longer bogs under load...heck, as a test, I pulled our one-ton rated trailer up a 15-20% incline in 1st/low, and it pulled just fine (aside from sketchy traction). Smoke is almost completely gone after a couple miles of riding today too; perhaps the earlier/thicker smoke was burning out old exhaust-side crud? I'll reconnect the vent tomorrow, and test again.

Remaining smoke smells fuel-rich, and plug is still dry/sooty, even with the jet at just 2 1/2 turns (spec is 2 7/8). Only running clear gas in it (e.g. no ethanol), and I still don't have to choke it even when starting cold...perhaps weak ignition? Choke appears to be seating all the way in.

[davefrombc]

Try  going up  a notch on the  main  jet needle  to  lower it in the jet (lean  out the main) Idle  air  needle  has little  effect on the mixture when  off idle. Sounds like you're still  running rich.

[erthnet]

On 5/10/2018 at 10:05 PM, davefrombc said:

Try  going up  a notch on the  main  jet needle  to  lower it in the jet (lean  out the main) Idle  air  needle  has little  effect on the mixture when  off idle. Sounds like you're still  running rich.

That fixed it! Now in the 2nd notch from the top, and there's no visible exhaust anymore, exhaust also doesn't stink either...and the oily exhaust deposit is also gone (although I presume this was more from burning out old crud). Crankcase vent was also reconnected, and that didn't seem to make any differences, thankfully. Machine is also performing quite nicely...was tugging wood up the hillside yesterday, and at one point I had all four wheels slipping in super-low/1st...no problems with power now!

Only issue now, is that it hunts a little when idling, and it has a tendency to stall when returning to an idle, both occurs when the engine isn't fully warmed. Regulator is good, and coil measures within spec. At this point, I'm presuming a lean idle, as jet is supposed to be 2 7/8, not its current 2 1/2 turns...so onwards to fiddling with that.

Thanks greatly for the input @davefrombc

[Parham]

On 5/9/2018 at 11:34 PM, erthnet said:

Oh, and a straight compression test shows ~148PSI...well within spec.

 

Hmm, good catch on the airflow restriction possibility. I've tended to get impatient when warming it up in-place, so I'll rev the throttle a few times, and/or run it at about half-throttle for 5-10 seconds at a time. When I was testing it while riding around, the smoking did tend to correlate with around the same time I'd be testing into the upper-gears (e.g. riding a little faster). However, my most recent test still resulted in very light white/blue exhaust after simply idling it for 10-15 minutes (didn't touch the throttle at all).

I have a brand new high-flow K&N SU-2588 air filter on there; not supposed to be restricting. Hmm...time to start looking closer at the crankcase vent.

Where would I find this crankcase vent? I have a similar problem

 

[Mech]

This is an old thread Parham..  You should download a service manual and have a read..  It will show you.