YZ 490 Help/advice needed

[LWC82PE]

Can any YZ 490 experts/owners help me identify a 490 I have got to get a new piston for. I need to know if its the early pre 84 or the later model

There is no crank case number stamped as it has had a new case fitted at some stage.
The number on the barrel is  - 2HJ
It had a wiseco piston fitted and the only number I can see on it is in the inside and it reads – 481. I can not see any other numbers on the top of piston as it has melted.

A spare used genuine piston was supplied in a Yamaha box.
The number on the box is – 40T-11631-00-94
The number on the inside of the used genuine Yamaha piston in the box is 5X601 and also has 9K10. This used piston doesn’t not necessary come out of this box though.

Did the later genuine pistons use the same casting as the early pistons but have different cut outs on the pistons which would mean the numbers cast into the inside of the pistons will be the same for all years?

Is there any other way to ID what year this engine is?

Also what is the FACTORY recommended pre mix ratio for a YZ 490 as stated in the owners manual?

This motor is out of a speedway car and has been running on methanol and 40 to 1 with shell advance castor/synthetic oil. I think it is not enough oil. The whole top of the piston has melted and there are holes right through the piston. The cylinder has been heavily ported, and has a really fat custom pipe made. Any tips on how to get this to run right and not melt pistons would be great.

Also it needs new reeds as the ones fitted no longer seal on the read block and there is a gap. What is available? We don’t need fancy carbon ones. It currently has single stage fibre reeds. Would they be genuine?

This engine also has an aftermarket Smythco watercooled head fitted



Thanks for your help.

Leith

[lms6201]

hi , side port exhaust 82.83   center port 84 on , 5x6 is 82  , i,m fairly certain pistons are interchangeable , but check the ring stop pins don,t allow the ring ends to go over a port , that would be the only diff , bore and stroke remain unchanged from 82 to 90 and the wr,s  ,with similar port pattern , 40t is 84 , 2hj i think is 87 , is the box 4 or 5 speed ,  the side port has a steel reed 6 petal , the center port has fiber 8 petal , wiesco make a pro lite for all centerport   , i have jetting specs and carb type here after we work out what u have , is it using the 40 mm mic?   

[LWC82PE]

It has center exhaust port so must be the later type.
Don’t know what the box is until I pull it apart. The clutch was playing up so it will probably get new plates and springs.
It also has an 8 petal fibre reed block (single stage reeds) are they still available?
Im not sure what carb size it is but it looks really big. It’s a big round slide and looks a lot bigger than the 36mm on my suzukis.
This car was supposedly the Australian champion in its class in the 90’s and the new owner was told it was running and all good and he was told to use 40 to 1 with the methanol and the shell castor oil. Apparently with methanol you have to only use oils which are compatible with it. It seems to have run really dry and hot. The new owner used it once and then it basically melted/holed the piston. He said it went really quick and once it hits the power band it just wants to take off! It has an expansion chamber that is about 7-8 inches fat at its largest spot!
There has been a lot of external bracing and reinforcing on the out side of the barrel which I think is for extra strength because of the transfer ports being machined so big. And the watercooled Smythco head is something I’ve never seen on one of these engines before, it would have to be pretty rare. I looked on the internet and I think it was on a US forum that someone said they are an Australian made head.
So I assume and hope that it has been set up/tuned and built by someone who knew what they were doing but I just don’t think 40 to 1 on castor oil is good enough. Graham Bells 2 stroke book says about 25 to 1 with methanol.

It has a Hitachi CDI unit. Is this stock?

[Hoony]

i'm no expert but methanol and 2 strokes are a black art. i was once looking for some extra horspower for my CR250 when i was doing dirt track/long track and approached a well known tuner to set it up. he asked me if i was prepared to blow a few engines till we got it right and i backed off as i didn't have the money to do that.

i have also been told that methanol jetting is finicky with two strokes and air temp/altitude can affect it much more that petrol (this may have been the reason for the meltdown). jet sizes are huge too with methanol (so that will be why the carb is so big as well) you really need to flood engines with the stuff.

Interesting that it has a water cooled head? how was the water pump driven or was it electric ?

"Smythco" Head if it was Aussie may have possibly been machined by Graeme Smythe a Tuner of note.

[lms6201]

what application is targeted for the engine , are u going to put it back to petrol ? if u are going to put it back to aircooled head the centerport head has a diff bolt pattern to the sideport , reeds are easily available , i wonder if u can turn them over ? might get another chop out of them , yamaha recomend 20 to 1fuel oil , i use 33 to 1 with tts  with methanol there are better oils than castor but ithink more oil would be good  , a variable main jet would be handy !

[LWC82PE]

it is going to be raced in classic speedway again where the cars use old bike engines up to  about 500cc
there is no water pump as it operates on the thermosiphon principle. The head appears to be cast in once piece or 2 halves  and welded together, im not sure.
it will still be keeping the water cooled head. i think who ever made it made a big mistake putting the motor right behind the seat. i hope there is a air scoop not fitted at the monemt that will direct air to the barrel!
yeah i though about flipping the reeds, but havent dont that yet.

thanks guys

[Lozza]

Methanol is LESS affected by changes in relative air density than petrol. The holed piston is from running to much advance. I would be running a ratio of 18:1 Shell M in something like that. Scoops don't make much difference a bigger rad/more water will

[lms6201]

this would be the old formula 500 class? 

[LWC82PE]

yep its in the formula 500 class and the oil been used in it is  shell advance M castor and I read the label and it said recommended for methanol. Will be getting the owner to use a lot more oil though now
got the motor apart and found it has had a bit of crank work done. Looks like its been balanced as it has had some welding done on it and all the holes have been plugged for more crankcase compression I assume.

The ignition is an internal rotor type with 2 black plastic encased coils. I went to remove the rotor and didn’t have a puller. My books for pullers say for YZ 490 it should be a 27mm thread but the thread in the rotor is no way near that big. It is more like 20 or 22mm. 2 jaws pullers wouldn’t work either. Are my books wrong in saying it should be a 27mm thread puller?

So is it not a YZ 490 ignition fitted? I can not find any name on the stator, only a few numbers.

It needs to be a puller that is hollow and will fit over the threaded part of the crank shaft.

Also if I find out this is the stock ignition then what is the std ignition timing? My information says with methanol you should use more advance. How much advance more than STD should I use as maybe it has too much and has caused the problem as lozza said. I want to set it up with a degree wheel before I put the head back on it.

The melting and the melted through hole has occurred just above the rings on the inlet side of the piston and there is only just a slight amount of melting on the exhaust size.

[lms6201]

all 490,s had the external rotor , remaining unchanged from 83 to wr500 in 92 , so it,s been swapped over (looking for more revs?)by the sound of the two black coils , probably off a 250, i know the it 250 (even though it has ex rotor )and yz 490 have the same crank taper ,  so it would be logicial the yz 250 would probably have the same taper there is another 2 types of yamaha puller , one fits the earlier c/x 125 , the other fits the later 250 and others ,timing std 2.7mm btdc ,    i know nothing about methanol except from flying 3d rc heli,s , it,s a fuel that loves nitromethane added to it ! 

[Lozza]

Lots of Yamaha's used the Hitachi inner rotor, all the rotors are the same except for the number stamped on them and the keyway position.All the stators I have are essentialy the same.You can set them up running without the key and using base advance.It will be pushing shit up hill to make one of these stay on the same lap as an RG500 equiped car, which dominate this class.

[LWC82PE]

So its not a 490 ignition then?. Ok so if its maybe a Hitachi internal rotor ignition off a YZ 250 where the heck to we set the ignition timing to???. I could put it back to where the stator plate was when I removed it but I really want to know if its right and not too advanced or retarded. Don’t want a holed piston again

From what I gather this car is in a formula 500 class for pre 85 engines only. It is the only 2 stroke single run in the class and I think there is a twin Yamaha RD maybe? And the others are heavy 500 four stroke jap multis like CB 500 hondas I think. This YZ 490 car is really light weight. It is a class that has been revived in SA as well as the old TQ cars. The owner of the 490 also owns a AJS/matchless 500 powered car as well which is raced in an older category.

[lms6201]

u would use the same timing , 2.7 btdc (std ) it,s the pitch curve , sorry , advance/rpm curve that will be different, probably find the plate has been re drilled to achieve the propper timing , or as lozza says u can clamp the rotor up without the key, something to do with the shorter stroke (250) ,  it sounds like u know how to use a dial on the piston with the head off  , get it to tdc then wind it back , mark it , use a powered timing light and see where u r at , i ,ve had a play with different ing , on things , if u put a series of numbers around the rotor , u can see where the timing is at before changing things

[LWC82PE]

ok so set it up to the 2.7 btdc (that is mm right?) and see what that does. Yeah I got a degree wheel/dial gauges etc and all the bits to do t. I left the rotor on as the seal wasn’t leaking and I checked the RH seal and that was still very soft and new looking so I could still get the motor apart with out taking the rotor off. I got it back together this arvo and am waiting on piston and Carbon Tech reeds from Serco

[lms6201]

how did u go with the bearings into the case ? all mine were an iornclad bitch (too tight ) requiring some oven time  yep mm