Interesting thread here.  Question for the group. 

1970 RT1… can one use.. say.. the 71 RT1 decompression ported cylinder head on the 70 with that timing advance feature? 

Drac

Layman's guess here , but I don't see why not.

The decompression valve is manually operated (unlike the 1974 on models), so doesn't have to be used.

Worse case scenario on using both ignition retard and decompression valve is that the engine won't start.

I await the educated experts' replies!

Photo of the head shows a standard compression head so i guess your cylinder is just a trail one as well.
Then again on cmsnl it doesn't show one on the "71 MX but does on trail models "70 to "73 so i don't know anymore.
CMSNL show a decomp with another diagram of the cylinder down the page of the trail models.
www.cmsnl.com/yamaha-rt1_model16783/
I had a piston port cylinder with a decomp valve plus the retarding flywheel, but was a bit of a mix n match motor.
Could probably have started it by hand as was low on comp even before the decomp was open.
Kick Started So easy. Should have 351 on the base of the cylinder.
photos.app.goo.gl/TCCZ9py6j4N3spQ87

Yes you can use both the decompressor and the early ignition that retards for starting.  

Works fine.  

Mark, 

Thanks for that bit.  That being said, i am assuming Timing setting would remain at 3.4 BTDC, advance lever full advanced.


i had a question about about pistons in this scenario but its been answered in SB 243.  

Again, thanks for your time…. A precious commodity..  

Drac

Something to be said about the Layman.. the percentage of correct analysis is very high..  

I tell my wife this constantly after i avoid a 7k bill for a 20.00 part.  Yes.. this is a true story. 

Cheers,
Drac

Using both provides extra protection against the dreaded kickback on starting . A good thing in my books .

Except you can't use both on the cylinder you bought if I'm viewing the right pics as is blanked off. Timing is 2.9 not 3.4 you mention, or 2.5 if you follow MarkT advice--which I do!!. Just saying.

Just to reiterate on the above comments, both the decompression valve and retard flywheel came from Yamaha from the factory. My '71 360 bought new out of the crate from the dealer had both. I still can get wacked by the kick stater if not dedicated to the start.

Yes, timing at 2.5mm works well with modern fuels.