YZ400 bogging big time, real hard to start

[yzf_rider400]

Well I've got myself a bit of a situation here. My 400 recently started bogging badly at any throttle position and is extremely hard to start (I usually have to jump it).

I've completely cleaned the air filter, the carburetor and put in a fresh plug. The problem persists. What do you think I should check next, all the electrical; or is possible that my valves being too far out of spec would do this?

Oh, and I'm running all stock jets and the needle at position/groove #3 (from the top - 1 groove leaner).

Any suggestions are totally appreciated!

[yzf_rider400]

I should mention my accelerator pump was very dirty the rubber diaphragm was perma-coated in crud. as well as the accelerator pump rod. I tried cleaning them up before reassembly. You think this is the problem?

I still want some other alternatives though...

[yzf_rider400]

or... is it my throttle position sensor?

Anyone.....? ?

[Satch0922]

Disassembly the carb and clean it thoroughly. Then you can eliminate that part of it. My guess is that your carb is causing the problem.

[yzf_rider400]

I DID thoroughly disassemble and clean the carb. The only dirty area was the accelerator pump (the rest of my carb is now shiny clean). Do you think a cruddy a.p. diaphragm would cause just a huge overflow of gas and create a massive bog?

[rush66]

may as well check the valves mine were out of spec and I was experiencing some hard starting and fix them and it cured the problem

[yzf_rider400]

I broke the carb down again and fully blasted the a.p. holes clear with carb cleaner and reinstalled everything. Some dirt came out a couple of holes. Hopefully I've fixed it. I think I may have had the a.p. so clogged up it was not squirting at all. Would this cause the bog/stutter?

I'll let you guys know if I fix it or don't. Then I'll have to try something else. ?

[Fly-nBri]

How does your a.p. diaphram look? Mine was cracked and caked with goo like yours. Just replace it....it's cheap and make sure the squirt duration is good. Might need to check out the Taffy mod. I also put a little dab of silicone on top of the rubber accordian seal after it's all set where you want it to keep the dirt out.

[yzf_rider400]

the diaphragm was really dirty but it doesn't appear to have any holes or cracks in it. I'm going to throw the carb back on today; I was thinking about doing the taffy mod but I'm going to see if I can't get her back to square one first. The bead of silicone is a good idea.

I'll let you know if she roars back to life!

[yzf_rider400]

Took her out to today. Ran fine for about 3 minutes and then began stuttering and stalling. The problem persists; I have a clean air filter, no obstruction in the airbox or boot, super clean carburetor (gone through it 3 times now), new spark plug (working).

It's real hard to start the bike and it requires a little throttle to keep the thing alive. Any throttle from around 1/4 open or more will cause the bike to choke and usually stall, accompanied by a lot of popping from the exhaust.

I'm running all stock jets and they are clean. I tried the needle a groove up and down from stock with the same result. I was running the bike at roughly 2,000 ft. elevation (has always worked fine before - doubtful it has to do with my jets).

One last thing, the bike can become VERY hard to kick over sometimes. Do you think my valve clearances could be so off that it would cause these symptoms?

I'm really at a loss here guys, and I'm starting to think about chucking my bike off a cliff. ?

[yzf_rider400]

I should also mention that my accelerator pump squirt is probably around 1.5 seconds so it's doubtful it's accelerator pump related.

[PumpkinHumper]

Took her out to today. Ran fine for about 3 minutes and then began stuttering and stalling...........It's real hard to start the bike and it requires a little throttle to keep the thing alive. Any throttle from around 1/4 open or more will cause the bike to choke and usually stall, accompanied by a lot of popping from the exhaust........ the bike can become VERY hard to kick over sometimes.

Check your cam timing.

It sounds that you have pretty much eliminated the carb as a possible issue. Which makes me think cam timing.

I had my 99 YZ400 jump timing. What ended up hapening is that the intake Cam actually siezed in the head (due to stupidity on my part...overtightened the cam caps)causing the sprocket on the intake cam to spin on the cam and threw my timing off about 1 tooth. The result was a very hard to start bike. Had to give it a little gas to get it to start. Wouldn't rev out. Stumbled bad. Is any of this sounding familiar?

Now you may not have spun a sprocket but jumping a tooth cause of a worn chain or bad tensioner is not out of the question. When is the last time if ever the chain has been replaced? Last year there seemed to be a rash of valve/timingchain problems on here from the 99 yz400. I am thinking that they finially started to reach there failer point and is why so many folk were seeing that.

Pull the cam cover and line all the marks and see were you are at. Now if the cam sprocket has spun this wont show up with the timing marks. They will be right but the cam lobe will be in the wrong position. This is hard to spot unless you know what to look for. Here is a few picts of mine with wrong and correct timing. Look at the lobe positions. When timed corectly you should almost be able to set a straight edge across the top of the lobes.

good cam timing

bad cam timing

Good luck. Let me know if I can be of any help.

[yzf_rider400]

The result was a very hard to start bike. Had to give it a little gas to get it to start. Wouldn't rev out. Stumbled bad. Is any of this sounding familiar?

Answer: Yes, yes and yes.

yzman400 you've given me some hope! I was thinking about this being a possibility and my last hope (well, electrical and black box would be my last attempt) was to check the valves and cams.

Thanks for helping me out. Even if this isn't the problem you've taught me a few things. ?

[yzf_rider400]

yzman400, do you think I should replace the chain? Will I be able to tighten the chain with the tensioner if it has come loose? I would guess I should just fork out the cash for a new chain and cure future worry.

Is the part a factory item or are there higher quality aftermarket ones available?

This is all assuming of course that my problem is cam timing (it's got to be). Thanks again for the help.

[Fastest1]

Though it could be cam timing dont rule out checking the valves. This is the most likely culprit. It is a simple procedure that shouldnt take you more than 2-3 hours your first time. The results will be worth it. The stock timing chain is fine if you go to replace it and get a new tensioner while you are at it. Valves first.

[yzf_rider400]

If I'm opening her up I'm definitely doing the valves and while I'm at it, it'll be apparent (following the advice above) whether or not my cams are off.

The bike stumbles REAL bad and dies, wouldn't the valves have too be WAY out of spec to cause this bad of a problem?

I'll probably do the job this coming weekend or the next and I'll let you all know what, if anything, I find. ?

[PumpkinHumper]

yzman400, do you think I should replace the chain? Will I be able to tighten the chain with the tensioner if it has come loose? I would guess I should just fork out the cash for a new chain and cure future worry.

Is the part a factory item or are there higher quality aftermarket ones available?

This is all assuming of course that my problem is cam timing (it's got to be). Thanks again for the help.

Have you ever had the motor apart before. Has the rings ever been replaced? Re-shimed the valves. If no. Now may be a good time to.

a. Replace the rings.

b. Replace the timing chain.

c. Shim the valves.

Just buy all the parts from Yamaha.

None of this is all that much money. Valve shims are like $5each (you will need maybe 5 of them). Timing chain is like $35. Rings and gaskets are maybe $75. You will need a flywheel puller for the flywheel to get at the timing chain. They are maybe $15. I would also highly recomend getting a 1/4" torque wrench for the cam caps. Dont even start this project without the use of a torque wrench. YOU MUST NOT OVERTORQUE THE CAM CAP BOLTS . If you overtighten the caps you can cause the cams to sieze in the journals, ie money money money.... ? If you cant find a 1/4" a 3/8 torque wrench will do in a pinch. But stay away from 1/2" torque wrench. Way too big for the job.

The cam chain tensioner is a spring loaded automatic tensioner. It automaticly adjusts to chain slack. Too a point. If the chain is too far gone then the tensioner wont help ya. There is specs in the book I think about chain length. I'd just replace it.

The corect way to replace the cam chain is to remove the head so you can get the cam chain guides outta the way. You can replace it without removing the head but it is a half way to do it and requires a little prying to get the chain past the front chain guide. Not recomended. Either way the flywheel must come off.

[PumpkinHumper]

The bike stumbles REAL bad and dies, wouldn't the valves have too be WAY out of spec to cause this bad of a problem?

From what I have noticed when the valves get outta spec they get tight. What this will cause is hard cold starting. And an eratic idle ie it will idle real high then slowly come down to a normal idle. Those were the 2 big problems I had with mine that reshiming and re-ringing fixed for me.

But from what you describe you have more going on than just tight valves. Although if then have never been shimed ya never know.

[yzf_rider400]

I bought the bike used back in 2001 and it probably has never been opened up. It's high time to have a look.

good advice yzman400, I'm going to follow it. I'll get the rings the chain, the flywheel puller and a 1/4" torque wrench. Do you think a new tensioner will most likely be necessary?

Where can I pick up a flywheel puller; will I have to get a dealer to order it for me?

Once again, thanks for being extra helpful. ?

[yzf_rider400]

Also, does someone make a gasket kit for this bike? I rather buy all the gaskets I'll ever need if it's going to cost me around the same as buying one or two OEM ones.

yzman400, will I only need the head cover gasket for this job?