YZ250H

[whitey 43]

Hey,
Has anyone got a standard YZ250H cylinder head floating around that they would be happy to unload?
P.M or call Whitey on 0426150927
Thanks in advance.

[Zakk]

are these still stamped 3R4 or changed to 4V3?

[whitey 43]

Apparantly either one...Both the same...

[Ando]

They are not the same, they have slightly different porting.

[Viper666]

3R4 250G 4V3 250H

Ando's right slightly different porting (Cylinder) but either will fit.

But isn't Whitey asking for a head? Again slightly different conbustion/squish but either will fit.

[whitey 43]

We have an ugly detonation issue & its a process of elimination...The cylinder head is R.S anyway. The cylinder is a 3R4.
Im not sure if the ignition is the correct one because they all look the same so im working on part numbers at the moment.
We have almost eliminated the deto by fully retarding the ignition, but this can create more dramas too.
The bike is running really well now & we could probably leave it alone, but im a picky bastard & no motor should be running full retarded.
Any input is appreciated. I have mainly worked on the newer stuff & dont know many of the tricks to get the old girls runnin better.

[Zakk]

if the cylinder is stamped 3R4 then it's a 250G. unfortunately back in the day 'tuners' used to machine down the heads, does yours still have the 1mm step between the mating surface and the combustion chamber?

a couple of questions..
does the bike have VM 38mm Mikuni?
what needle jet and jet needle are you using?
what pilot jet and main jet?

[whitey 43]

We had to machine the head...it has already been done & was actually crooked on the cylinder across the piston by .5mm!
With the machine work done, we have 1.2mm squish which is heaps for one of these old girls. Running on 100 octane leaded fuel, Q2needle (from memory) 380 main 70 pilot.
The cylinder head looks like it has been through a engine failure, with lots of scarring, plus detonation damage to boot.
I am majorly concerned at the ignition as well, but want to replace the head to eliminate it from my list...

[Zakk]

your trouble isn't with the squish clearance, its the lack of volume in the head. these heads have a low volume CC to begin with and by machining it you've reduced it even more. you can open up these heads to increase the CC volume, but you're right in looking for another head.
if you're using AVGAS, plus the Q2 needle jet and it's got the original 6F16 needle, add in the 70 pilot & 380 main then that's rich enough to keep a YZ490J happy...

[Ando]

Hey,
Has anyone got a standard YZ250H cylinder head floating around that they would be happy to unload?
P.M or call Whitey on 0426150927
Thanks in advance.

My apologies, I miss read it. Head is the same 3R4 part # & is identical. Remember different prefix = different part.

[Zakk]

Hey,
Has anyone got a standard YZ250H cylinder head floating around that they would be happy to unload?
P.M or call Whitey on 0426150927
Thanks in advance.

My apologies, I miss read it. Head is the same 3R4 part # & is identical. Remember different prefix = different part.

IT250 H & J's use the 3R4 head too, i had an idea the YZ250H used the same 3R4 head as the YZ250G but wasn't sure.

[whitey 43]

I will check the needle tonight...We have beautiful colour on the plug & its going hard.  The rider is quite fit & we done tons of laps on sunday. Oh yeah, it is a original carb, but bored out to 39mm.
Im supprised that the chamber volume is too small...I actually thought it was quite open.
Thats why I am looking for advice, so your input is appreciated.
You got a head floatin round?

[Zakk]

my IT250J's 3R4 head had been machined 40thou when i got the bike and there had been some obvious detonation happening too. a mate has a couple of 3R4 heads, i'll ask if he has a standard one he wants to sell.

[whitey 43]

Thanks heaps for that.

[Viper666]

Remember different prefix = different part

Not with Yamahas

Several parts which are the same may have a different prefect especially if the part has superceded.

The same is when earlier parts are used on later models.

The conrod kit for a YZ125 from 76-80 may have the prefix 1G8, 2k6, 1W1, 2N5, 3R2, 1W2, etc.

Check this out;

http://www.boats.net/parts/detail/yamaha/Y-3R2-11651-01-00.html

At some stage this part has had different prefixes but it's the same part.