Can someone confirm ignition timing on 78 yz250E?
seen 2.5 and 1.8mm BTDC...
thx, Joe

Neither one is correct from what I found 

Official 1978 Yamaha Service Data Sheet says 1.7mm BTDC.

Also found this with a quick search.  



So 1.7mm

 

hi,
I am interested as i have a 1979 2X4 and the yamaha technical bulletin reads "1.37" btdc, however the instructional then reads "find tdc and then retard to 1.7".

surely 1.37 cannot be right, never came across timing into the hundreths???

cheers
les.

I know my methods are as rough as a bears bum but I'd just put it back where the original screw heads have left a mark. But if you were into experimenting there a fair range of adjustment in the slots so I'd think you could safely go a screw head more advanced or retarded depending on what your trying to achieve. I might be wrong but all these new fangled bikes with magic adjustment by switch on the handlebar are possibly only doing similar to that but electronically. I know my "76 yz250c annoyed me with lacking very top end like my "75 had so after comparing things & stroking it I just advanced it which sure helped  along with no detrimental affect. Just remember--I do dodgy things lol.

Les,

The 1979 YZ250F timing spec is 1.37mm in the Yamaha documentation I have.

1978 is 1.7mm.  The discrepancy you're seeing in the text is likely "classic Yamaha" mistake where they carried over information from a previous model in the new service document. The worst I've seen is the 1977 DT400 service manual still shows the earlier 1975-76 timing instructions even though the stator is completely different in 77 and the earlier instructions don't apply. 

1980-81 initial timing for the YZ250 was retarded even more.  Yamaha was really making headway into the CDI electronics and ignition curves in that era.  It looks like timing depends on what year CDI box you have installed as it appears 79-81 had pretty much the same rotor/stator but 3 different CDI boxes with 80-81 having a 0.6mm BTDC spec.

That said, timing is a lot like jetting (and just as dangerous!)  You give it what it needs.  I have an old book from the 1970's someplace called "Motorcycle Tuning for Performance".  They recommended the tedious process of making adjustments to timing and testing for best performance.  Advancing too much can put a hole in the piston pretty quick though.  So always best to start retarded and slowly advance a little at a time and being careful. 

Mark,

many thanks for an extensive reply! my problem is, without wanting to highjack a thread, is that i have a 1979 yz250 cylinder and head on top of a 1979 dt250 bottom end, with the dt250 ignition. now, i have filed the stator slots and set the ignition at 1.3 and i have a rex's speed shop dt250 cdi. so, reading into your explanation, the dt250 ignition and speed shop cdi could well be messing with the curve, because i have a serious break up of ignition right at the point of hitting the power band.

my solution at the moment is to buy a complete yz250 race ignition from stator to plug including cdi and see if that makes a difference.

any thoughts on this are welcome.
cheers
les.

If you're using the DT ignition, I'd set the timing at 2.5mm BTDC to start.

I don't know what Rex did but stock DT ignition was fixed timing.  I'd use the 2.5mm starting point and check with a timing light to make sure it doesn't advance more than that. 

Then test and you could slowly advance to 3.2mm max advance stock setting making sure to look for signs of detonation.

P.S.  "Breaking up" might not be timing?  Make sure spark plug is gapped at 0.020" to 0.024" and not the 0.032" to 0.035" they tend to be new in the box. 

Thanks Mark,
i am gradually working thru what it is not, only really the ignition left now, as everything else is literally brand new..
cheers
les.

Think I'll sit back & watch as I'm getting confused haha. So now we're dealing with a flywheel cdi of a very late dt250 but with a yz top end. Happy days. I love projects--except when they don't go to plan.

Thanks for the info...appreciate it...