Experience the runaway engine even when switching ignition off. Here’s state of motor which starts and runs and performs terrific besides this (a how was the other half of the play mr Lincoln)
- new top end, base gasket with yanabond on both aides
- new genuine Yamaha crank seals properly and gently installed with crank puller vs hammer
- carb cleaned and adjusted, flange and insulator filed flat since all were warped
- timing properly set with dial indicator 
​​​​​​- million other things that don’t relate to engine

Knowing all of this, does anyone have any ideas where I might focus to find an air leak? 

epoxyed tank seam and thought I was done, but no such luck!

thanks for your help!



 

 Ps plug looks pretty good after ride. 

Never had that so only guessing at something like a plug hole that's been hellicoiled & the end of hellicoil is 'just' poking out & glowing. Hell i dunno--head gasket overhang, but they're sort of folded over so wouldn't happen. Is throttle closing off to an idle.

thanks RT
I am going to pop the head for sure to take a peek and then put it back together and try to pressure test it again.
the condition was like a runaway diesel, hard to imagine that I have an air leak large enough, but then again what the heck do i know! will report back

I would do the pressure test before you take off the head, that way you haven't changed the fault conditions.

Good point, plan revised!

You said; DT1-E

I would ask if the cylinder is piston port?
Some change to Reed valve ( big leaker ).
yama bond base gasket - huh???

your air leak is in the carburetor flange.

asco 

 

Hi asco - yes a 1971 DT1E
No reed valves, piston ported
I put a very light smear of Yamabond on both sides of the cylinder base gasket so it would seal perfectly against the incredibly cleaned mating surfaces

Def could be carb flange as the PO warped the crap out of the flange by ~1mm which I had to remove with a very large file. It def was flat after filing, but definitely worthy of another go through once i pressure test.

you were right asco!
flange that i had flattened was warped
stuffed a fat oring into groove on carb flange and it rode perfectly!
I think this bike is done
thanks a ton
martin