Yup, another one of those pesky threads.

History:
Recently bought a 300 with a known starting issue (came from higher elevation, sat for 2 years with corn syrup in the carb, got a cleaning by a mechanic and new jets).
When I brought her home, I inspected the carb, it is definitely nicely clean, checked the jets which were wrong. Replaced idle jet from 25 to the proper 12.5. Replaced main jet from 120 to 130.
I also checked the valve clearance. Exhaust valves had too much lash (.35+) and intake valves were tight at -.15. Yup, the valves were cracked open at all times.
Re-shimmed all 4 valves and they now have proper clearances.
Kick-starter (the ONLY starter on the 300) was very easy to kick, now it is much harder, as it should be on a 300cc motor.

Facts:
Local density altitude below 1000'. Possibly up to 2000' in the summer.
The airbox lid is missing
Aftermarket muffler/spark arrestor
The engine has a Mikuni TM 36-68 carb
12.5 idle jet, 130 main jet, unknown needle jet and clip position, will find out
Int. valve clearance: .12 and .15 (within spec)
Exh. valve clearance: .20 and .22 (within spec)
KACR functional
Timing chain tight, timing marks on cams and crank aligned
Spark plug correct, CR8E, gap correct at .80, clean
Compression good (not actual measured numbers just yet)
Fresh fuel, clean fuel tank, clean carb bowl.

Situation:
She is very hard to start cold. With choke, without choke, using accel pupm or not, throttle closed or crack open, fully open. So many different options and none of them work initially.
After a few minutes of kicking (nice workout), she eventually burbles a little, might start and die and then finally start and run after a few more dozen kicks.
Once warmed up, she purrs like a kitten and is very easy to start (after all the work I've done). And she pulls strong out there.
But cold starts are still a bear.

Question: (finally, right?)
Could it be anything but an AFR issue?
I've heard "weak spark" from 1 person. I will be checking it one night this week but in the meantime wanted to post this up to get more opinions from the smart folks here.

 

Some members have mentioned that the KACR is not really needed on these and can actually cause the bike to start harder by not developing enough compression.
I'd try disabling it (simply remove the spring) and see if that makes any difference. I would also disable it to get a proper compression check, so after disabling would be the time for one. If disabling it fixes the problem, I'd be tempted to just remove it.

Also, see my TM36-68 jetting in my signature line below and compare to yours. They should be fairly equivalent. I took my settings from TNC (another Texan on this forum), who took them from others with more experience. Mine for a 250 with 351 big bore should be close to yours with a 300. I'm at around 3,000 ft and typically ride higher. IIRC, there was some discussion about adjusting the pilot jet and air jet (I think air went from 1.0 to the 0.9 size).

 

Thank you, sir. I've ready some of your replies. The 351 sounds like a beast.

My 300 seems to be stock so I went with stock jet settings and it seems right with a warm engine. (the 25 idle jet was dumping in way too much fuel compared to the stock 12.5 jet)
My mixture adjustment screw is somewhere below 2 turns out, could be 1.5, haven't counted.
I forgot to state above that the airbox lid is missing, I will update it now.

When I have time this week, I'll try to get compression readings for different conditions, including the KACR disabled. I wonder how different/harder the kick lever will feel.

 

Was the TM36-68 stock on your KLX300? I thought they had the same carb as the KLX250.

The mixture adjustment screw setting of 2 turns out gives me the best (highest) idle. Don't necessarily follow 2 turns out as gospel. We usually turn this screw in and out, noting when idle rpm rises, then begins to drop going both ways, and set in the middle of the 'begins to drop' points. This should be about the fastest idle, and should be between 1 and 3 turns out. Adjusting these setting should also let you determine if the pilot jet is too big or small. If you're not familiar with this, I can explain further. Although I'm not completely sure, I don't think the air box lid should have much impact on starting.

It's best to do the compression test with the engine warm, throttle fully open (turn the fuel off). If compression seems a low, squirt a little oil in the cylinder and note if this makes a difference.

 

Negative, the stock carb on the 300 was a Keihin, according to documentation. The bike has had a few small modifications, the CV carb being one of them.
I looked up here what jetting people run on stock KLX 300s. Seems like 12.5-15 idle and 127.5-130 main.

Thanks for the advice, already familiar with mixture adjustment screw and procedure. Mine happens to be running well "somewhere" around 2 turns out. Again, last time I adjusted it, I did not bother counting. I just wanted to get it idling well which I achieved but with no follow-up data. My bad.

The airbox does not have effect on idle, agreed, I just wanted to state it in the facts anyway for completeness.

I will definitely follow up here with compression results when I have them, dry and wet.
But I am already preparing mentally for head removal and OH. The valve seals are leaking and I really want to clean the valves and piston since the piston top surface seems pretty dirty through the spark plug hole. But that's a job for the future, I just want to figure out the cold start problem so that I can enjoy her some during the nice warm spring we're having.

Any thoughts on the "weak spark" theory? If not, that's fine. Just wondering.

 

What year is the KLX 300? Kawi changed the ACR timing somewhere in the 2002 model year. The newer timing holds the exhaust valve open later in the compression stroke, making it easier to kick over.

Ride on
Brewster

 

Good point. 1999. Sorry, completely forgot that it is pretty important. LOL
Now if only I could finally get off work and go home to wrench on stuff.

 

Yes, but probably not enough to be useful. New wires. Check coil resistance and/or new coil. A new spark plug even if you're running a new one. An occasional dud gets on the shelf.

It's good to see Brewster chime in. He KNOWS these bikes.

 

BBL, most of us running the 36mm pumper carb are running a 17.5 pilot jet. My '06 KLX has the factory KLX300 top end, so it should be about identical to yours, though many of us seem to run the same jetting on the 300 all the way to 351.

However, I don't see the pilot jet from 12.5-17.5 having much to do with your cold starting issue since you're on the choke circuit at that point. One thing the pumper carb alleviates almost totally is cold starting issues like some OEM carbs had. I'm wondering if your choke circuit still has some residue in it from the "syrup" you were initially confronted with. The choke circuit on this carb is robust and will pretty much always yield enough fuel for cold starting.

You mention "stock" settings on the pumper carb and the 12.5 pilot jet. I'm guessing you mean what the carb came with originally, because there probably really isn't a stock value since this carb can go on all sorts of applications. Do you know if the air jet is a 1.0 or a 0.9? But really your choke circuit should allow an easy cold start even if you're off a good deal from an optimum jetting setup for normal running.

One other issue could be fuel level in the bowl. Unfortunately the pumper doesn't have an easy way to see "real" fuel level like the CV carb does using a clear plastic tube on the outside of the carb. An extremely low fuel level/float height can cause poor fuel delivery to the choke circuit.

I'm still suspect of something blocking a circuit in your carb, especially the choke circuit, since your carb was fouled badly from old fuel. The choke assemble/plunger can be easily removed. Spraying some carb cleaner in the hole and using some compressed air may clear out any obstruction in that circuit.

Weak spark from one of the ignition components is a possibility, but usually they either work or they fail completely...but that's not a guarantee. I'll add one other thing. Are you sure there isn't an air leak at the rubber intake manifold. Obviously this carb isn't a factory fit to the OEM manifold, and I've seen some booger up the manifold by making a small cut or tear using poor technique or clamps to attach the pumper carb. Just something else to eliminate.

 

Thank you for all the good feedback, gentlemen.

As for the "stock" jetting, that was poor choice of words on my side, I meant "usual" or "standard" as per my research into what others are running in a TM36 on their 300.
So far, the warm idle is pretty darn good with the 12.5 idle jet and leaning it further with the mixture screw yields a drop in RPM so I can only assume that I have the right jet in there. Every application is different and the 12.5 seems to work well for me.

Update on the cold start: I managed to get her to start last night with less than the usual excessive effort. As I suspected, she runs too rich with the choke in (at ISA-5) which explains why she won't start with the choke. But she won't start without the choke either, still too lean for a cold start.
I managed to start her at about 1/2 choke and she idled fine until warmed up when she ran without the choke.

So TNC, you might be onto something with the choke circuit.
I am new to the CV carbs so I will have to learn about the choke on them but I am willing. It is entirely possible that, as you mentioned, the choke circuit might not have been cleaned right.

As for the air jet, I have no idea. I will pull the carb out again to clean the choke circuit and check the air jet too.

The rubber boot between the carb and the intake manifold is in great shape, nothing su****ious there.

And as far as compression goes, checking my compression gauge last night, I found out that it has a 14mm adapter for smaller plugs but not a 10mm adapter for li'l motorcycle plugs. $5 later, an adapter is coming tonight from Amazon.

What are the coil resistance specs? Primary and secondary. I am not sure whether I've seen them in the service manual but I will double check.

I will keep you posted but it seems that we are narrowing it down.
Thanks for all the help.