Hello to all forum members. I thought I'd share a few pictures of the leak down test equipment I have assembled and prepared to do a leak down test on my 1975 DT250B engine.  I'm doing this test because of the few horror stories I have read on this forum regarding air leaks causing engine runaway and piston holing experiences due to unanticipated lean air fuel mixture conditions.  You'll see from the pics I've plugged the carb rubber manifold inlet and exhaust pipe exit with plumbers 'freeze plugs' purchased from the USA.  All the rest of the test equipment I purchased individually from a company called Tameson in Holland (i.e. -1 to 0.6bar pressure gauge, Tee piece, plastic pipe, barb fittings and air control ball valve).  I want to make sure I test all the boundary that can potentially allow air to leak into the engine so you'll see in one of the pics I have put a cable tie around the neck of the middle pair of reeds to ensure they remain open to allow the air pressure to equalize across the reed box which means the rubber carb joint gets pressurised (tested) during the process.  If you don't do this I figure the reed valves would backseat under pressure and hence you can't test the first rubber carb joint.  To pressurise the system to 6psi(g) I am going to use my brake bleed kit hand pump which has a reversible feature allowing it to be used as a pressure pump or vacuum scavenger.  Providing I get a steady healthy pressurisation reading (6psi(g) held for more than 10mins) it is my intention to reverse the pump and try pulling a steady vacuum for 10min hence the reason for buying a gauge that covers a vacuum condition and using a brake bleed pump that can pull a vacuum. Anyway that's it for the moment and I'll let you know how it all goes over the next few days. Bye for now Andy.
   

Great job.  Pressure AND vacuum testing is important. 

You are overthinking the reeds a little though...  they would not interfere with the test at all if you left them alone.  They don't seal perfectly.

Mark T, thanks for the advice regarding the reed valves not needing a cable tie since they don't backseat completely under pressure.  Before commencing the test I thought about where to position the engine piston which is probably obvious to the experts but since it is my first leak down test it needed addressing. Clearly TDC is hopeless as it doesn't expose the whole system boundary.  Looking at my piston skirt cutaways, transfer ports and inlet port I decided BDC would expose everything to test pressure including the crankcase 'void' itself. Perhaps one of you good folk could confirm that to be the right position?  As you can see from the first pic my brake bleed pump (in pressure mode) was simply connected up to the test pipework via a ball valve.  Pic 2 shows I've raised the pressure to 6psi(g), and then shut off the ball valve to ensure leakage back down the pump line doesn't occur.  Disaster as the gauge immediately started to fall. Starting at the most likely leak sites (and the easiest to prove/eliminate) I painted detergent/water mix on all the test equipment fittings, barb connections, pipe tubing etc and spark plug adapter fittings.  As pic 3 shows "there she blows"..!!  I found multiple leaks between the rubber carb body and the metal reed valve cage.  It didn't surprise me it leaked here because last year I bought a used carb inlet rubber (off ebay) to replace my original badly cracked one. It took quite a lot of gentle persuasion to separate the original carb rubber and my reed block, so I reckon Yamaha originally applied some sort of sealant themselves (Yamabond?) to this joint, whereas I was testing it 'dry' metal to rubber.  Yesterday I bought some 'Pro Seal' (USA product) silicone gasket sealer and as pic 4 shows have carefully smeared the faces with sealant and you can just see it oozing from the edges. Anyway the combined inlet unit is back bolted lightly together as per their instructions and it's curing for 24Hrs so that's it for now.  Andy.
  

I hope I'm not the bearer of bad news but that intake boot looks like it has micro cracks all over it.  It might be shot.  

No it's a valid query you raise. Good news is those cracks are only superficial i.e. surface cracks. Many carb inlet rubbers look bad but are still serviceable. In fact the leak down test itself is a good test of their integrity. Best regards Andy.

Again, way over thinking it on piston position.  The pressure will leak past the rings faster than you can pump even if piston at TDC.  That said, BDC is the "best". 

Not sure about using most silicone sealers anywhere near fuel.  They always fail on me.  Yamabond semi-hardening (same stuff for sealing cases) works great on the manifold. 

I'd also stay at 5 psi or less. 

Thanks to those who chipped in with advice and I shall take it all on board.  Test position for piston I’m told is largely irrelevant so I’ll just leave it at BDC from now on.  Also new pressure test start condition is 5psi(g) again going on better advice.24Hr period of silicone curing is over (I checked the silicone is compatible with petrol/gasoline) and have refitted the carb/reed valve block back on to the bike.  On pumping the pressure up to 5psi(g) pic 1 shows the result..!!   Pressure fell from 5 to 1.5psi(g) over 10mins and although picture doesn’t show it very clearly it is NOT now leaking from previous sealant leak site.  The leak and bubbles are actually coming from the paper gasket side i.e. between the reed valve block and the cylinder inlet metal flange.  I had only torqued the 4 retaining bolts to 6Nm because I didn’t want to gouge into the rubber face of the bolt holes with the spring washers.  Folks in the U.K. will know decent carb boots are like hens teeth so I didn’t want to mess it up.  You guys across the pond have the luxury of buying great looking reproductions but at $135 + shipping for us over here that’s often simply too much wonga.  Anyway I fitted 4 flat washers and torqued the bolts to 8Nm repeating the test.  This time pressure fell from 5 to 2.5psi(g) over 10mins albeit with a reduced level of frothing from the gasket area.  These are 10mm head bolts (6mm thread) so I torqued them to the Yamaha manual spec limit of 10Nm and repeated the test yet again.  Same result 5 to 2.5psi(g) over 10mins so torqueing further is not the answer.  I need a new paper/card gasket I think and have looked on line at various sites and already I think I’ve found my problem.  My reproduction gasket looks awfully flimsy and thin when compared to the thicker OEM Yamaha NOS ones.  Once again I learn it doesn’t pay to be a tight@**e..!!    I did also have a good paint around head and cylinder base gaskets with detergent but they are good.  Also read up my manual and as pic 2 states: the carb rubber body and the aluminium reed case are HEAT welded together.  No wonder then my replacement carb rubber originally leaked.  Sounds like in truth second hand carb rubbers and reed boxes should and often are sold as a single unit. You live and learn. So that’s it for now, awaiting my NOS gasket. Andy.

  

I have a question for anyone more knowledgeable than I am. Does the term "welded" in this case refer to 'over time' it/they becomes "welded," or is it something you have to do at the onset?

I have a question for anyone more knowledgeable than I am. Does the term "welded" in this case refer to 'over time' it/they becomes "welded," or is it something you have to do at the onset?

With time.  No measures required at install.

Thanks. That's what I thought, but wasn't sure if some sort of adhesive was implied...