Hard Starting a YZ 490

No advice on starting the thing, but that is a beautiful motorcycle.

The engine starts on the pilot jet. Check to see if it is clogged. Verify the correct jet size. Maybe 38Special can share his jetting. Looking on RMATV sight, it lists a #40 pilot.  You could jump it up just see if makes starting easy. Question is, on decel, does it pop back?  That indicates a lean condition on the pilot. 

https://www.rockymountainatvmc.com/oem-parts/yamaha/1985-yamaha-yz490/c…

I have a 79 YZ 400 that is a bear to start when the engine is cold.  A spray of ether on the filter and it lights in 2-3 kicks. When the engine is warm it takes 1-2 kicks, no ether needed.

Have you checked the float height?

I am guessing that is a #48 jet. I assume you  blew carb cleaner through the pilot circuit when cleaning the carb. Just to verify that the actual carb body is clear as well. 

The first photo looks like a main jet #480.

this is a well-known age-old problem. Even 465s had it. The best permanent fix was reworking the head. IIRC decking and changing the Squish band and/or dome shape.

The big bore Yamaha is always going to be a PITA to start.  I had a good running 465 that I raced for a few years and we did dead engine starts.  My normal routine was to kick it over a few times slow, get it to TDC and give a good kick and it would fire most times.  I did have a modified head, corrected squish and carb mods (jetting/cut slide) but was otherwise stock.  

If it was really stubborn I would lean it over till I saw fuel come out the overflow.  It would trickle some fuel into the cylinder and help fire it up.  

I almost did the cylinder decompression but solid it to a local hill climber instead.  

I'm curious to see it that helps!  I have a theory that the Boyesen Dual Stage Power Reeds make it harder to start because they are so stiff.

I think the center supports are suppose to be cut out to allow for better air flow.  At least mine are cut.

There are 3 things required for the engine to run. 

Ignition, atomized air/fuel delivery, compression. 

Prove all three, then get more granular with the troubleshooting. With all the mods to the reed block and such, I would start with compression.

My YZ465 has a mildly cleaned up cylinder, stock head, modded carb slide with V Force reed set up. Avgas and 32:1 Maxima SuperM. I put it in 2nd gear and rock it through to get some fuel sucked into the crank case. Choke on, push the kicker through to get top dead center and a good kick through. So far it's been a pretty reliable starter.

Hoik that Reed Cage!

I just don't see how it could now, seal . I have to say it, that's a piece of absolute Butchery, that I could never see the benefit from. 

It's basically Open / a Plain Piston Port, now, when you consider the air / mixture demands at Start and idle. 

Time for a Moto Tassinari / V Force, Boyesen or Mossbarger, if they are still available. 

Wow, that's bad but glad you found it.  That is unquestionably your starting issue.

FWIW, I just put a PWK38 Airstriker on my 490, and starting is a ONE-KICK affair, even when stone cold.  At the same time, I threw on Boyesens and a PC pipe, as the stock reeds were slightly chipped.  It runs very cleanly and crisp.  The way the bike runs now, I see no need for head work or a decomp.    All stock other than the mods above, with race gas.

Now if I could just get that darn rear brake sorted, the bike would be fantastic.

Would a 1988 YZ250 swingarm and rear wheel fit?

@Last2Stroke, yes I have already started making a potential parts list!  The most work will be in fabbing the m/c and brake pedal mounts.  Not sure I want to bastardize the frame.  

That you are considering this, seems to indicate you may not be racing in a 'period / as Std class, if you are indeed racing.

Whilst waiting for the SMX to start, I've spent some time looking at pictures of the 88 250 that LTS put up, comparing them to 490s from 86 to 90 - they seem to be the same thing through those years.

The 250s Brake lever pivot, seems to be a bit higher up - towards the swingarm, and a tad forward, than the Brake Arm mount.

I have no really accurate idea of where the pivot point for the 490s brake lever is, but, it may share the brake arm mount as many bikes did,, way back then?  It is, of course, an 'inside, through the frame' Brake lever. The 250, like most modern bikes, has the lever outboard of the frame.

You might be able to have the 250s brake lever pivot point tube, welded into the appropriate position, just up and a tiny bit forward, as I think the 250 is. There's nothing 'special' about most brake lever pivot points. especially of that era ( the 250s) just thicker tube, generally with a brass bush, if you are lucky, with, recesses to take  O-rings or Quad rings. Easy to have made. I'd do a better one, with a sealed bearing(s), by the way. 

As for the MC mount, a pretty simple thing  to fabricate, again. As would be  a reservoir mount, if the MC you used, required a remote one. 

Done carefully, you'd Not Butcher your bike - you may indeed retain the ability to put the drum brake and torque arm, back onto the frame, with the subtle differences I think there are between the 250 and 490 brake lever / torque arm pivot points. 

Last2Stroke may have something with trying the entire 88 rear end, you'd not have to do a caliper mount to the 490s swingarm , too. 

I know I may seem blaze' about this, but, I make frames / chassis and ' things ' - what would be required, would be a doddle for me, and, many fabricators. The average bloke, perhaps / of course, no.

Yoinks, Race Day Live has just started - WTF am I up at 3am in the morning to watch this ............? 

The swingarm pivot bolts and linkages are different between the YZ490 and the 1988 YZ250, it might be better to modify the YZ490 swingarm or make a clamp to mount the rear caliper to the swing arm.

The rear brake master cylinder could be clamped to the frame...

Heh Heh, you found a mob that have my last name there Last2Stroke, but I've No relation to them.

Basically, almost anything can be done, and, such a mod, is pretty basic.

It's a lovely looking 490 that joelster  has here, as well as that gorgeous Honda he's shown us recently.

Not sure what that guy was thinking on that reed cage but that's a butcher job.  New cage and reeds will definitely help with the starting issue.  Most folks don't think of reeds as a maintenance item but I change them every season.  My 86 was getting a little more hit or miss on the 1 kick dead engine start so I put new reeds in and now it's been a hole shot machine. 

If you don't like the weak drum brake you can always do was I did and switch to the 81 YZ465 rear wheel and drum brake.  It's a lot larger and works way better.  

I also have had great success with how I dress my rear drum.  I use a porting sand paper tool to scuff  the drum before installing new pads.  If we have a bad mud race I'll rescuff the drum and brake pads. 

Thanks for the info.  I am seeing the 81 YZ465/250 using the same pads as the 84-90 490.  

You are saying the 465 drum is a larger diameter, using larger diameter pads?

They went to those terrible Z-spoke hubs in 83.  82/83 is the small pads, 1980 and older is a smaller axle.  I forgot that I avoided z-spokes when I went to the 81.  Mine is a 250 so it was the smaller pads starting in 81 till they went to disk in 88.