'02 WR426 Starting Issues - Low Compression?

Hi everyone, I’m having issues with starting my bike, and hoping the collective wisdom of everyone in the WR forum can enlighten me.

My bike is an ’02 WR426 with YZ cam timing, stock exhaust, not sure on jetting.

ISSUE: Very hard to start when bike is cold (not referring to cold weather). I am quite familiar with the WR starting procedure, but it’s not working for me. Here’s how it usually goes down:

1) Turn on fuel

2) Give throttle a couple of twists to get some gas into carb

3) Pull out choke

4) Kick slowly until I find what I (think?) is TDC

5) Pull in decompression lever, kick a couple inches past TDC

6) Release decompression lever, bring kick back up to top

7) Giver a full, hard kick

8) Seeing as the bike didn’t start, go back and do steps 4-7 about 10-20 times

9) Push in choke, pull out hot start, pull in decomp, and clear chamber with 15 kicks

10) Keeping hot start pulled out, do steps 4-7 about another 10 times

11) Lather, rinse, repeat…

The bike usually starts after about 5-10 minutes, and in the hot-start phase. Here’s what I have done in my attempt to improve things:

1) Pulled apart the carb and thoroughly cleaned with carb cleaner/compressed air

2) Checked my fuel screw while I had the carb apart. It was actually at 3 1/8 turns out, which is waaay too far (rich) from what I understand. I went with what I believe to be the stock setting, which is 1 5/8 turns. So far has made no discernable difference in starting…

3) Brand new plug with correct gapping

4) Fresh gas

IMPORTANT: Once I do get the bike started and warmed up, it starts like a champ with barely a half kick. Soo, here’s what I think might be the issue: I have difficulty finding TDC! Should I find TDC somewhere on every kick, or is it approximately every 2 or 4 kicks or something?

If I do have low compression, is it possible that I am kicking the bike too often (thinking I have TDC even when I don’t), which floods the bike, necessitating using the hot start button?

Am I on the right track troubleshooting this? I’m going to try and check compression tonight - any tips on doing this? If the compression is low, would this affect the bike noticeably when it’s running? I’m kinda a noob rider, so perhaps I’m not riding the bike hard enough to notice the issue?

I appreciate any insight on this – Thanks! If some of my conclusions or thought patterns are blatantly ignorant - I apologize - I'm just learning!


Hey Superwilly, I have a few things that you should check. It may be time to change the jet needle and the needle jet ( they are not the same thing) after a few years they wear against each other and will contribute to a flooded condition. Assuming you have checked the valves and found no tight ones then get in the habit of doing this. The issue with the low compression is a pet peeve of mine. Whenever these bikes are shut off, you should always run the kicker up to TDC and leave it there till the next time you start the engine. When the engine is shut off and either the intake or exhaust valves are in the open position they, or just one of them, may have a tendency to hang open when you first try to start the bike. I do this now with out even thinking about it and by doing this ( even when stopped for a powerbar ) the valves are fully closed and transfering heat into the cylinder head. Once the engine is warm it will usually start no problem. There should be a TDC on compression every second revolution of the engine. WR Dave.

You could have low compression, worn piston/rings or tight valve.

Easy to tell.

Put your foot on the kick starter, with a continuous movement push it slowly until till you feel resistance and you can't push it anymore. If you have good compression the kick starter will come to a stop and you will not be able to push it anymore with your leg. If you wait the air pressure will bleed off/pass by the piston rings and the kick starter will continue to move. This was your TDC -> top dead center. With good compression when you come to TDC the air is compressed enough that the kick level will support your body weight (well it does mine- 180Lb.) momentarily.

If you can make one continuous movement with a kick level than you have low (cold) compression.

As the engine warms up, metal expands, cylinder wall coated with oil, gives a better seal = better compression.

check the valve clearance, do a compression test.

Sounds like your fuel mixture could be a problem here too, esp. with your air screw being that far out.

How long have you had the bike? Has it always been like this or deteriorated to this state?

Once you have it running and warmed up (10 minute ride), how does it sound when you get off the throttle and decellerate in gear?

Does your bike drop to normal idle quickly when you give it throttle under no load or does it "hang" at higher rpm for a few seconds?

have you tried to adjust your air screw once warmed up, to see if your pilot jet is sized right?

To do so, set RPM at lowest smooth setting without stalling.

turn air screw in until rpm changes

Turn air screw out until rpm changes

Set it midway and record the number of turns out from lightly seated - this should be between 3/4 and 2 turns, if it's below 3/4 you need to go one smaller on the pilot, if it's beyond 2 out go one bigger on the pilot.

Set your rpm back up to a smooth 1300rpm idle.

Matt4x4, I've had the bike for a year, and no, it wasn't always like this. As far as I can tell, the bike drops to normal idle quickly. I will need to look into adjusting the air screw once warmed up.

Zsebi-2, the kick definitely can't support my weight, but I'm also 80 lbs (big guy) heavier than you. It is not hard for my to continually do slow, full kicks without the decomp lever pulled. So could tight valve clearances cause low compression?

Yeah tight valve clearances can cause low compression, also check the cable routing for the decomp. cable to make sure it is not binding and holding the exhaust valve slightly open. WR Dave.

Hey dude, I'm no mechanic but i own a 01 426 and if i used the same starting procedure as you i'd have the same problems. The bike is very fussy, if you want to know if you've got a compression problem try pushing the kickstarter thru without deco (not actually kicking it hard) if you can go down twice, ie complete cycle, you've prob. got a compression issue, but it should still start. more to the point tho, this is my method of starting it, first go every go.

1. choke on

2. DO NOT TOUCH THE THROTTLE. AT ALL!!! it's not a 2stroke

3. push the kick starter thru slowly until you can't push further (note compression issue earlier tho.) This is TDC.

4. NOTE THIS PART. With LIGHT pressure on the kicker at TDC pull in deco lever SLIGHTLY. ONLY enough to feel the slightest movement or release in compression with the kicker, and then let go of the deco lever and bring kicker back up (this is to give you the full travel on the kick). This step is critical!!! To much travel after TDC and you won't get it to fire, and it is likely to cycle thru and kick back and snap your leg or something hienous.

5. Give one solid, complete kick thru all the way down. DO NOT touch the throttle. let it idle on choke, do not touch the throttle, until it picks up revs, hold for a bit, then drop choke and you should be right. now play with throttle.

There is a knack to step 4, you will think i'm a nut until you learn it, i did off the guy i bought the bike off, but he was 100% correct. If i didn't get it right i was in the same position as you, i once spent 35 minutes kicking it over!! If you mess up step 4, you're in a world of pain. Never give it throttle, never travel to much with deco lever, millimetres, not inches. if you have the BK mod, don't touch the hot start unless it is really hot.

Get it right and the electric starter boys will be jealous, 1st go every go and quicker than electricity any day.

I noticed on my -01 426 that if my idle was to high there were big problem getting it started:excuseme:

A good spark plug is also an important part for easy start.

And like they said before, do NOT touch the throttle.

If my bike have been standing several weeks I usually spray some starter gas inside the airfilter for easier starts.

Works every time

Once you figure the compression/valves out you need to can this procedure and install an 03 ehaust cam. Auto-decomp is the ticket for that bike.

i was told to never use throttle when starting mine. I think this lets excess gas into the engine because of the accelerator pump. When you do this, you could be flooding the engine which is why it takes so long for the bike to start.

Scott has hit it dead on if that is what you are doing.

ever look to see just how much fuel the AP squirts in just one crank? It's quite a bit.

If my bike sat two weeks or a winter, I give it a single small blip on the throttle before doing the kick procedure without ANY throttle and it fires within 3 kicks.

If you crank the throttle twice you've dumped a lot of gas into the intake that it'll just flow in it's liquid state right into the bore when your intake valves open - likely too much gas.

If my bike sat less than two weeks, one kick is all it takes and NO gas or throttle blipping necessary.

Thanks guys, I'm leaving the throttle alone now and it's starting in the first 1-3 kicks. You're right, I was flooding it by twisting the throttle a couple of times. I was told to do that by the previous owner, and it was working for me last year, but something has changed I guess. Seems to be working great now though.

If the temp is below 60 degrees and the bike has been sitting during the week, I always give it a twist of throttle(squirt of gas) then choke and kick, it usually starts on first or second kick every time. If its below 40, then two twist on the throttle; and may even have to repeat it because after if fires up and may dies because it is to lean at that temp. This technique worked for me since 1998 when I had my first Wr400.

Once you figure the compression/valves out you need to can this procedure and install an 03 ehaust cam. Auto-decomp is the ticket for that bike.

Can you fit auto-decomp to a 2001 WR 426 F?

yes you can, but if you don't have problems starting your bike, why bother?

yes you can, but if you don't have problems starting your bike, why bother?

Cause I was having problems starting it :thumbsup: Until today, when I got the plug out, dried it, put it back and the baby fired up first kick. Anyway, good to know that i can fit the auto-decomp.



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