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miketegra

YZ426 problems after valve adjustment

9 posts in this topic

I have a 2000 YZ426F Supermoto, recently bought. General service history is unknown although oil was clean, radiator fluid normal, correct level, etc. I changed the oil, rad fluid, a fork seal, water pump seals and a few other things.

It has a new FMF Q4 slip on, jetting unknown. Note: lots of popping on decel.

It has the Hotcams exhaust cam with the auto-decompression.

When I bought the bike, I could hear the valves ticking pretty loudly so I did a check and adjustment the other night.

Here is the data:

Intakes before adjustment: .15mm, .13mm, .08mm

Exhausts before adjustment: .45mm, .37mm

It's a 2000 so the specs are looser than the 01 and 02.

OEM specs .15-.20mm intake and .25-.30mm exhaust.

I checked the Hotcams website and they suggest .17-.22mm on the exhaust.

As you can see, intakes were a little tight so I adjusted them all to about .17mm.

You can also see the exhaust valves were WAY out of spec and were probably responsible for all the clacking I was hearing before the adjustment. As far as the adjustment on the exhaust, I went with the hotcams suggestion and set the exhaust valves at .20mm

Also, while I was working on it, I loosened the throttle cables just a little since there was some, but very little free play at the throttle tube. Also put in a new CR8E plug, just like what was in there, which looked pretty good actually.

Anyway, I go to fire it up and just kick and kick, choke, no choke, plenty of gas in the tank, but it just wont start. It was popping and backfiring every few kicks. Finally got it started by giving it about 1/4 throttle while kicking it over. It always started right up before, no throttle needed, just some choke when cold.

Anyway, thought maybe I somehow got the cams out of timing, took it apart but all seemed normal. Below is a pic of how it looked before and after the adjustment. Please note its not exactly at TDC in the pic, but very close. When at TDC, the punch marks on the (OEM) intake cam aligned perfectly with the head and the punch mark on the Hotcams exhaust cam was about a mm above the head surface. Just to test it out, I rotated the exhaust cam one tooth to see if the punch mark would line up exactly like the intake, but it was way off so I set it back to how it was when I first cracked it open. In other words, rotating it one tooth was too much to get the punch mark even close to lining up.

yzcams.jpg

So I went out for a test ride and aside from having to give it throttle to get it started, it seemed to run fine, and the valve clatter I heard before was gone. Got home from a short ride and it spewed some coolant and the radiators were abnormally hot. Never had it spew coolant before and it wasnt a particularly hot day and was an easy ride.

So, any ideas on what has changed this bikes starting mannerisms or what might be up with the sudden unusual overheat? I'm taking it to the shop tomorrow to put it on an exhaust sniffer to see whats up with the fueling. Im thinking its running lean.

Should I consider setting the exhaust valves to the looser OEM specs instead of what Hotcams recommends?

Any advice appreciated!

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The cam timing looks good. I have the feeling the person installing the E Cam originally never checked the valve clearance when he did it. The kind of clearance difference there was between what it had and what it should have had would have significantly shortened the duration of the exhaust event, effecting idle jetting as well. I wouldn't alter the clearances from what HC calls for. Otherwise I think you're on the right track.

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It seemed to idle just fine once I got it started...what do you think needs adjusting in the carb?

I was thinking the same thing about the valve duration, also wondering if perhaps carbon was building up around the valves due to the exhaust valves not opening as far as they should with such a large clearance. I've got to replace the base gasket so I'll get a look at the valve situation when I take the head off.

For now, I'm hoping we can track down the problem with the exhaust sniffer and solve it with some re-jetting/carb tuning.

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2 things can be adjusted for starting up/idling : fuel mixture screw and the idle adjusting screw. A friend turned the idle adjusting screw so much out, that it didn't touch the throttle plate, and the only way to start it, was to give it a very little gas (basically lifting the throttle plate). Start it up, and by turning the idle adjusting screw, check if the idle changes. Try to set it according to the manual. Also the idle fuel mixture plays a crucial rule on starting the bike.

Also check this :

http://www.thumpertalk.com/forum/showthread.php?t=327405

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I was thinking the same thing about the valve duration, also wondering if perhaps carbon was building up around the valves due to the exhaust valves not opening as far as they should with such a large clearance.

The extra clearance is only .010" or so, and will, or would, have little effect on the total lift. It could have had a pronounced effect on the timing, however, which would be how it could have affected the way the bike runs.

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Took the bike down to the shop and the a/f sniffer was not working right. We did manage to see it was pretty lean at idle, but the sniffer crapped out before we could test it at higher rpms.

Seems like it ran better with the fuel screw at 4 turns out...less lean at idle...thinking of trying a bigger idle jet. One of the mechanics rode it and said it ran better with the choke on which seems to indicate its running lean...again, bigger idle jet?

I'm going to check out the carb this weekend and see what jets are in it and where the clip is on the needle. It's still much harder to start than before the valve adjustment.

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Beyond about 2.5 turns out, your pilot is too small or clogged,

http://www.thumpertalk.com/forum/showthread.php?t=327405

Anything over 1/8 throttle is primarily influenced by the main circuit, although the pilot circuit can have an effect on things out to around 1/3 throttle.

Look in the jetting DB, then open your carb up and check it for varnish deposits and see how it's set up.

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Beyond about 2.5 turns out, your pilot is too small or clogged,

http://www.thumpertalk.com/forum/showthread.php?t=327405

Anything over 1/8 throttle is primarily influenced by the main circuit, although the pilot circuit can have an effect on things out to around 1/3 throttle.

Look in the jetting DB, then open your carb up and check it for varnish deposits and see how it's set up.

Will do, and I'll let you know how it turns out!

Thanks...

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