Low compression on kick start & not starting - 10' yz450


20 replies to this topic
  • vomitbomb

Posted October 06, 2011 - 04:07 AM

#1

Hi All,

My bike had been starting fine when cold but had been very hard to start when it was hot. Additionally, it was stalling quite a bit when hot.

I had the valves adjusted and I was told they were tight so the mechanic loosened them up for me.

Now that I have the bike back I feel very little compression (I have absolutely no trouble using the kick starter with my arm). I can't even rest my foot on it without it following through to the bottom of the kick. Is this something to worry about? I noticed the kick felt light even before I had the mechanic look at it but now it feels like kicking over a little 2 stroke or something.

I've been kicking the guts out of it and it just won't start. I hear it loudly sucking in air and doing the odd backfire but that's about it.

Suggestions? Ideas?

  • grayracer513

Posted October 06, 2011 - 07:01 AM

#2

You need to do two things: Have a leak down test done ( I think I'd use a different mechanic ), and then probably look into checking the valve clearance again. You would think he'd have gone over his work after finishing and not have returned it to you that way, but there's a chance he's got one of the valve shims tilted in the spring retainer, holding the valve off it's seat.

Another possibility would be that there may be carbon on a valve stem holding a valve part way open, which should be indicated by an excess of clearance at that valve.

  • tech24

Posted October 06, 2011 - 08:42 AM

#3

Just curious how many hours on motor? Also as to the above, carbon may be a good possibility since these things seem to run on the rich side even when warmed up.

  • Schpenxel

Posted October 06, 2011 - 12:15 PM

#4

Sounds like valves aren't adjusted right to me. Hell isn't that what's always wrong with these things? lol

  • vomitbomb

Posted October 06, 2011 - 12:49 PM

#5

You need to do two things: Have a leak down test done ( I think I'd use a different mechanic ), and then probably look into checking the valve clearance again. You would think he'd have gone over his work after finishing and not have returned it to you that way, but there's a chance he's got one of the valve shims tilted in the spring retainer, holding the valve off it's seat.

Another possibility would be that there may be carbon on a valve stem holding a valve part way open, which should be indicated by an excess of clearance at that valve.


Hi Grayracer, thanks for replying.

I'll follow up on this leak down test if I can't get it going after tonight. It's something I wanted to do after I noticed the bike burning a little bit of oil anyway :smashpc:

I wasn't sure whether or not the low compression on the kick was significant considering how these things have auto-decompression valves. I'm going to try and roll start it tonight and see how that goes too.

His a good mechanic, been using him for years and always been happy. I'll ring him today and ask him a question or two and if I can't get it running I'll get him to have a look over it. His actually recently broken his femur and has a metal rod going up his leg so I don't believe he has the ability to kick start it at the moment which may explain why the issue wasn't picked up before I got it back.

Could it be something as simple as the spark plug? It had been sitting around for a couple of months without being ridden so I drained the fuel and put fresh stuff in but no difference. Just out of curiousity does a backfire mean there's spark?


tech24,

I'm not sure how many hours the engine has done as I didn't get an hour meter when I bought it (something I'll definitely get next time). But it can't be too much. Probably somewhere around the 1,000 - 1,500km mark.

  • tech24

Posted October 06, 2011 - 01:04 PM

#6

I take it you don't have a tuner then. Backfire means there is definently fuel and usually at least a weak spark so leak down test may reveal something. There is the possibility cam timing is off especially since cams were taken out.

  • vomitbomb

Posted October 06, 2011 - 04:40 PM

#7

I take it you don't have a tuner then. Backfire means there is definently fuel and usually at least a weak spark so leak down test may reveal something. There is the possibility cam timing is off especially since cams were taken out.


I did some reading and never actually knew the power tuner had the ability to give me the hours. Sales guy explicitly told me I'd need a seperate hour meter. Could it be helpful in this instance as a diagnostics tool? A couple of people have recommended getting it for that, if not anything else.

  • grayracer513

Posted October 06, 2011 - 04:53 PM

#8

The tuner can read hours and pull stored diagnostic codes from the unit as well as reset the ignition and fuel curves. The advanced unit that the techs use at Yamaha shops also has the ability to run active tests on the individual components, among other things.

BTW, the total run time hours in the ECU can be reset to zero with the tuner, too, so in a small sense, the guy at the shop was right about an hour meter.

  • vomitbomb

Posted October 06, 2011 - 08:30 PM

#9

The tuner can read hours and pull stored diagnostic codes from the unit as well as reset the ignition and fuel curves. The advanced unit that the techs use at Yamaha shops also has the ability to run active tests on the individual components, among other things.

BTW, the total run time hours in the ECU can be reset to zero with the tuner, too, so in a small sense, the guy at the shop was right about an hour meter.


Yeah that will be next on my list after the sub-frame (destroyed my current one).

Just out of interest is it normal for such low compression on the kick starter? I'd really like to compare it to another bike. I realise a new bike will be much harder to kick so I can't really go to a shop.

Just to follow up on my previous question could it be as simple as a spark plug?

  • tech24

Posted October 07, 2011 - 03:55 AM

#10

Yeah that will be next on my list after the sub-frame (destroyed my current one).

Just out of interest is it normal for such low compression on the kick starter? I'd really like to compare it to another bike. I realise a new bike will be much harder to kick so I can't really go to a shop.

Just to follow up on my previous question could it be as simple as a spark plug?


It could be but I doubt it....Im still bettin on cams being off unless you checked that already

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  • vomitbomb

Posted October 09, 2011 - 04:29 PM

#11

Had the valves re-checked and one of the valves he had loosened up at the front (can't remember which is which) had tightened up again. Once he moved it back I could feel the compression and it fired up. Still doesn't seem like it's starting as good as it used to but it never did after I drowned it :thumbsup: Also started burning oil after that so I may be looking at a rebuild anyway.

  • grayracer513

Posted October 09, 2011 - 07:22 PM

#12

Check the suspect valve again after one or two rides. If it's tight again, the hard coat is gone, and you need a valve job.

  • MX763

Posted October 11, 2011 - 09:45 AM

#13

Subbed.

My brand new 2011 Yz450 has been like this since day one. It starts right up when cold but when it's hot it backfires through the airbox and sometimes the exhaust. It takes 15-20 kicks when its hot to get going again. Mine also does the occasional "hiccup stall" for no apparent reason.

It now has about 8-10 hours on it and it isn't getting any better.

  • tech24

Posted October 11, 2011 - 09:52 AM

#14

Subbed.

My brand new 2011 Yz450 has been like this since day one. It starts right up when cold but when it's hot it backfires through the airbox and sometimes the exhaust. It takes 15-20 kicks when its hot to get going again. Mine also does the occasional "hiccup stall" for no apparent reason.

It now has about 8-10 hours on it and it isn't getting any better.


Its all in technique...try a search for more info but I believe there is actually an issue with this one

  • Schpenxel

Posted October 11, 2011 - 09:53 AM

#15

Subbed.

My brand new 2011 Yz450 has been like this since day one. It starts right up when cold but when it's hot it backfires through the airbox and sometimes the exhaust. It takes 15-20 kicks when its hot to get going again. Mine also does the occasional "hiccup stall" for no apparent reason.

It now has about 8-10 hours on it and it isn't getting any better.


checked the valves?

  • steve_97060

Posted October 11, 2011 - 06:48 PM

#16

Subbed.

My brand new 2011 Yz450 has been like this since day one. It starts right up when cold but when it's hot it backfires through the airbox and sometimes the exhaust. It takes 15-20 kicks when its hot to get going again. Mine also does the occasional "hiccup stall" for no apparent reason.

It now has about 8-10 hours on it and it isn't getting any better.


it should start getting easier to start after about 10 hours, and use your tuner to make sure you have the idle set high enough so it doesn't stall.

  • vomitbomb

Posted October 26, 2011 - 03:31 PM

#17

Check the suspect valve again after one or two rides. If it's tight again, the hard coat is gone, and you need a valve job.


I'll check the valves again after the next ride. I think I'm confident enough to at least do that much after watching the mechanic.

Took it out for a ride last weekend and it was still exactly the same. Still a pain to start and keeps stalling. Have the idle wound right up as far as it will go too.

I checked the oil before the ride and it was up the very top of the sight glass. After about 60kms it was below the bottom of the sight glass, so it's still burning oil. And yes I checked it hot as per the manual. I also noticed a small amount of whitish smoke coming out when it was revved hardish.

When the mechanic exposed the valves and cams he said the cam position was slightly off but shouldn't make a difference. Seemed very slight to me too but then again I don't know any better.

Is that anything to worry about (Cam position) ? Or is it all good like he said?

Another thing is that I've noticed it's been backfiring a fair bit on decel. Whether or not that's related I don't know but I assume if I keep listing symptoms I'll eventually get to the bottom of it :thumbsup:

  • grayracer513

Posted October 26, 2011 - 06:31 PM

#18

When the mechanic exposed the valves and cams he said the cam position was slightly off but shouldn't make a difference. Seemed very slight to me too but then again I don't know any better.

Is that anything to worry about (Cam position) ? Or is it all good like he said?

Cam position, or more correctly, cam timing, is somewhat critical. If it's skipped a tooth, it will cause a variety of performance problems. Because of the fact that the intake cam is at the front of the head instead of the rear with the engine still turning the same way, the position of the cams is apt to look odd to just about anyone, but they both have timing marks, and as long as they line up as they should, you should be fine.

  • vomitbomb

Posted October 26, 2011 - 08:27 PM

#19

Cam position, or more correctly, cam timing, is somewhat critical. If it's skipped a tooth, it will cause a variety of performance problems. Because of the fact that the intake cam is at the front of the head instead of the rear with the engine still turning the same way, the position of the cams is apt to look odd to just about anyone, but they both have timing marks, and as long as they line up as they should, you should be fine.


Does slightly off cam timing explain any of my other symptoms though? Just wondering if it's worth investigating.

  • grayracer513

Posted October 26, 2011 - 09:38 PM

#20

Normally, the slightest it can be off is 22.5 degrees, which is what one tooth equates to. That's easily checked by removing the cam cover and using the information in the manual.

It is possible to have the cam sprockets slip on the shafts, but not normally without one of the cams being momentarily seized in the head. The trouble with this is that the only way to check for it without directly comparing it to another cam is to check the actual timing with a degree wheel. But in order to do that, you'd need the timing specifications, and finding those is pretty much impossible.





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