YZ 426 cold starting issues

I'm new to this forum and have ridden and maintained 2 stroke dirt bikes all of my life up until I purchased a used 2001 yz 426 in very good shape a few years ago. Although I am getting a bit older and mostly ride street, (this is largely due to no closeby riding areas anymore and a broken ankle on the 426 about a year ago) I still ride whenever I get the chance. The 426 has developed a cold starting problem over the last year. It used to fire up right away on a cold start with only the choke out. It then decided it didnt like the choke but 2 twists of the throttle made it happen and it was even better if you could grab and pull the choke after it started. If the weather is colder, say under 35 degrees, I throw a towel over it and a small ceramic heater under it and 20 minutes later she fires right up, no choke. Within the last couple of weeks, Ive replaced the plug, cleaned the air filter, fresh fuel and adjusted the fuel pilot screw out as it was running to lean. While it runs smooth now once warm, the starting problem when cold is still there. You could kick this thing for an hour and not get a sputter. I have just noticed it's top end rpms are down a little from the summer, but the starting issue is a real pain. I see from reading some posts on this that it could be different things, but from some of you seasoned yamaha 4 stroke dirt bike veterans what is the most likely cause?

Thanks, Steve

Its one of two things. The carb is dirty or you have a tight valve. Since its only hard to start while cold, it sounds more like a tight valve to me.

How cold is cold. Thumpers dont like to start when its cold outside, ie 30 deg and below, without propper jetting.

Its one of two things. The carb is dirty or you have a tight valve. Since its only hard to start while cold, it sounds more like a tight valve to me.

I'll check the valve clearances first. I'll probably sleep better knowing this is right anyway. (The engine BTW may even be a little too quiet)

Thanks very much, Steve

How cold is cold. Thumpers dont like to start when its cold outside, ie 30 deg and below, without propper jetting.

cold lately has been the 30's. I'm going to check the valves 1st as I'll feel better knowing they are within specs, and the engine seems a little too quiet. I'll tell you though, a little bit of heat on this thing and it fires right up.

Thanks, Steve

I'll check the valve clearances first. I'll probably sleep better knowing this is right anyway. (The engine BTW may even be a little too quiet)

Thanks very much, Steve

Let us know what you find. :censored:

Sounds More Like Jetting, My Snowmobile If Left Out Overnight Will Start On The Second Pull Every Time But Bring It In The Garage For The Night Your Going To Be Getting A Arm Work Out Before You Ride. And I Do Have To Say If That You Are Riding When It Is 30 Out You Are Braveir Than I Am.

cold lately has been the 30's. I'm going to check the valves 1st as I'll feel better knowing they are within specs, and the engine seems a little too quiet. I'll tell you though, a little bit of heat on this thing and it fires right up.

Thanks, Steve

Its jetting id bet $ on it.

Install a larger starter jet and it should start way easier. The starter is in the bottom of the carb next to the pilot and main. Its a stubby little sucker. Go 2 sizes bigger and see what happens.

An irridium plug helps also. The stock plugs suck.

Sounds More Like Jetting, My Snowmobile If Left Out Overnight Will Start On The Second Pull Every Time But Bring It In The Garage For The Night Your Going To Be Getting A Arm Work Out Before You Ride. And I Do Have To Say If That You Are Riding When It Is 30 Out You Are Braveir Than I Am.

It's interesting you mentioned leaving it sit out of the garage. A friend of mine who's hobby is working on street bikes as much as riding them has left certain bikes out of his garage on very cold nights (20f or less) if he suspects tight valve clearance. He claims it's just enough to detect this issue if it's borderline due to the cold temperature shrinking the clearance. (If it starts easier he checks valves first) It doesnt seem to matter where I park the 426 these days, it does the same thing unless I put heater under it and a towel over it. Kind of like a blanket on a horse on a real cold day! My youngest sons worn out yz 80 on the other hand starts all the time even in desperate need of a ring job.

Steve

Its jetting id bet $ on it.

Install a larger starter jet and it should start way easier. The starter is in the bottom of the carb next to the pilot and main. Its a stubby little sucker. Go 2 sizes bigger and see what happens.

An irridium plug helps also. The stock plugs suck.

What's an irridium plug? Thanks, S.

It could be clogged pilot jet also. Would explain the lack of starting without twisting the throttle and the lessened top end ( even though the effect on top end is minimal). If it hadnt been being used on a regular basis and this problem arose, that is what I would suspect. Always turn off the petcock and let it die from fuel starvation and dirty jets will be a thing of the past. No fuel, no varnish.

What's an irridium plug? Thanks, S.

Its just a better quality plug for your bike. I got mine at the autoparts store. Just tell them the stock plug # and tell them you want the irridium equivelent. They are like $10 but last way longer without fowling out.

The CRF's are running them and they rarely fowl out a plug. I switched to one on my old 400 and it was way better for starting.

It could be clogged pilot jet also. Would explain the lack of starting without twisting the throttle and the lessened top end ( even though the effect on top end is minimal). If it hadnt been being used on a regular basis and this problem arose, that is what I would suspect. Always turn off the petcock and let it die from fuel starvation and dirty jets will be a thing of the past. No fuel, no varnish.

Thats a good point. Thanksgiving day 2005 we rode in the morning before getting together with family. I laid it over hard and fast after hitting an icy small piece of plywood under brush on a left turn. (I think it was something left over from an old duckblind that found it's way onto the trail) I was wearing Tech 6's but still managed to break my left ankle. I kept going and didnt take the boot off till much later when we got back. (It hurt bad but I really didnt think I broke it) Anyway, the bike sat for a while in the garage as my foot sat in a boot and healed. I did start it from time to time with my one good foot but who knows perhaps it did gum up in that timeframe.

Steve

It could be clogged pilot jet also. Would explain the lack of starting without twisting the throttle and the lessened top end ( even though the effect on top end is minimal). If it hadnt been being used on a regular basis and this problem arose, that is what I would suspect. Always turn off the petcock and let it die from fuel starvation and dirty jets will be a thing of the past. No fuel, no varnish.

I'd like to thank everyone very much for thier advice on this starting issue. I appreciate it very much. Last weekend I turned the pilot screw out 3/4 additional turn for a total of 2 or 2 and 1/8 to help the lean running/backfiring and it helped a lot. Just for kicks before I had to go out of town yesterday I got it out of the garage, pulled the choke, rolled it over tdc, kicked and nothing. I then twisted the throttle twice, rolled it over, kicked again and she started. I will check the valves, but It does look like its a carb issue. The temperature was about 35-40 f. I'm not convinced it's fixed as I kicked a third and 4th time before it would continue to run, but this was a real improvement. I went for a short ride, came back and closed the petcock and ran it dry. With the standard jetting, how much is top end effected by cold weather? Although it accelerates as fast as ever, it does seem to rev slower than it used to on the upper end. If you twist the throttle in neutral it picks up fast but seems to hit a wall after about 2/3 throttle. Thanks again everyone for helping me to figure this out!

Steve

If it was jetted right at 70 degrees, the main jet will be about 4% too small at 35 degrees. That would be the difference between a 165 and a 170.

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