How to Properly administer a leak-down test?

[chance johns]

the bune reference was from my 750.

all i plugged was the left exhaust port and intake port and one sris line that is below the cylinder.

where is the other (top) sris line?

i did not block the oil line.

thanks tz. i was freaking out about it and this makes me feel a little better. so i blocked off,(partially apparently) the left cylinder and heard it leaking from the center cylinder. so, i need to block off the oil port and top sris line. and use...a bike pump to safely put 6 psi to the cylinder? or only fill my compressor to 6 psi before i use it? and have the piston in bdc on the cylinder im testing?

[pearljam724]

How old is your head gasket ? Did you check for proper torque settings on cylinder head bolts ? You can lose compression there too. How does the bike run when it is not idling ? There are dozens of factors that can effect idle. Float levels, pilot jets, intake boots, air filter, plugs, idle air adjustment screws. Did you check all of those things ? Even a mediocre battery will cause these bikes not to idle properly. A compression test would be my very last resort. It's definitely possible that you have a leaking seal. But, as mentioned. I think you would definitely have other signs to indicate that being a problem. Like oil spewing out your exhaust or crankcase breather. If that's not happening. I think you may need to be sure of those other areas first. The bike could also be slightly out of time. But, if it runs fine when not idling. I doubt that it is.

[Coyote]

The upper SRIS lines (spigots) are not accessible with the carbs in place. Seeings how the R. Bune was referring to the 750. I assume you are working on your 550. The lines are here:

[chance johns]

ive been through this bike 10times.for 3 years i cant figure this out.

ive cleaned and rebuilt the carbs(new jets,new floats and adj), compression test,timing,points,condensors, checked and corrected resistance through the coils. new plugs and boots. carb sync(bench sync as well as manometer )air/mixture adjustment, oil pump adjustment, i am using motul 2t oil. ive checked fuel flow through petcock,and each carb through the float chamber.

the bike doesnt idle constantly, i have to blip the throttle, it drops a cylinder. at idle and low rpms. its worse when it gets to temperature (it seems to take longer than normal to warm up). sometimes drops a cylinder at high rpms.

the insides of my pipes are busted out. there are 2 walls and 3 chambers in the pipe, 3 out of four are busted. i know that this can effect how it runs, but i imagine most gt pipes are like this by this time and should only effect FINE tuning of the bike. not necessarily my issue.

THE LEAK DOWN TEST:

right now i have the boot attached to the cylinder and shut off with the old signal relay hose clamped in. i have the exhaust blocked off w a plate i made. i have the lower sris blocked off w a bolt and o ring.

WHATS LEFT. the upper sris hose by the cylinder......the 2 oil ports?

[chance johns]

i did the test and found a base gasket leak.

so i replaced the top end gaskets and my base gasket is still leaking. a little all over and up through the studs/nuts .the left side of the motor is what im testing and holds no pressure, it immediately looses any pressure. the leak from the base gasket, although seems to be all the way around the base seems very very minute from the soap bubbles. im trying to do a case seal leak and i keep having to do these top end gaskets. whats going on here? yeah warpage/gouges,....but i dont think this is the case. before there was one leak in one spot now there is seepage all over. i DID use a very thin film of rtv gasket maker on the base gasket, i did not use some around the oil port. and i basically let it dry before i torqued it down. i realize this might have not been a good idea in hindsight, but i cant imagine a situation that would cause the seal to fail. i basically smeared it rtv on then wiped it off. just to have a little tack to it. what im i supposed to do? buy another top end gasket kit?i dont want to be spending this time on money if i have to do a complete bottom end rebuild

[Suzukidave]

I dont really understand what you want us to say ? You know you have leaks , you havent fixed them so there is no way to check the bottom end seals without stopping top end leaks first and i wouldnt use RTV .. its not the right stuff to use on bike engines . Get ya some Yamma bond or Suzuki bond . The surfaces should be smooth and without gauges .

[chance johns]

ok so i didnt use rtv i used some gasket maker that was oil/gas/heat resistant. even if it deteriorated,so what, then im left with straight gasket....which should be fine. ......the only reason i should be having this leak is from warpage and gouges. im sure there isnt either,.,,,,there are some scratches and gouges in the mating surface but none that cross the full edge of the matingsurface...nothing that seems like it shouldnt make a good seal.....espescially with a little sealant on there...

[chance johns]

should the piston be at TDC or BDC?

i think ken at knk said it should be at TDC
but it kind of make sense to me the piston would be at BDC so the air would properly escape through the transfer ports into the crankcase.

[Suzukidave]

It would depend on where you are putting in the compressed air , if through the spark plug hole set it at BDC so the transfers are open . If you are useing the intake to put the air in TDC so the intake is open .

[chance johns]

i dont get it. some people say piston should be tdc some say bdc. some say 5 lbs a pressure some say 100lbs. (this article in the link says 10-15lbs) i keep getting conflicting instruction. whatever im doing is not working. if i had bad seals the pressure should slowly decrease, right? i hold 0 pressure for 0 seconds. i mean it builds up and then with in 2 seconds is back to 0. i have to be missing something. and people have conflicting advice or instruction. the bike is running and rideable , just looses a cylinder at low rpms.

http://www.dansmc.com/vacuum_instructions.JPG

[tz375]

On a 4 stroke, we use 100PSI because there are no seals and we are testing valves and piston rings. With 100 PSI, if the piton is anywhere except BDC, the motor will rotate to BDC and may cause harm, so 4 strokes BDC and 100 PSI

With two strokes, we are really testing the bottom end for leaks as well as the head gasket. 10psi is probably OK but 6 is safer and is sufficient to determine if there's a leak TDC is no good to us on a two stroke either because at TDC the transfers are close and all we are testing is the head gasket and rings. At BDC, the transfers are wide open and allow the bottom end to pressurize.

So for a 2 stroke it's BDC at 6psi.

With your leak, the issue is where is it coming from and why. You mentioned base gaskets. If they are leaking I would pull the barrels and inspect the lower face and the crankcases. You can use almost any gasket goo on the base gaskets to test the seals, and if teh seals test OK you can leak the gasket goo in there until the next time you need to strip the motor.

[chance johns]

there were a couple imperfections and one gouge in the mating surface on the case. but none that span the line of the mating surface, so i assumed it was fine. it was leaking at one point with old base gaskets. with soapy water the leak showed large bubbles. (not where any imperfection were in the mating surface)and with a very rapid decrease in pressure almost instant. with the new gaskets i have extremly small bubbles almost around the whole perimeter and up through the studs. with the same decreas in pressure. first time i tested it i did not orientate the piston. the last time i did at tdc. because thats what i think i was told to do....although keeping it a bdc makes total sense, i just feel kind of lost when doing this test because every seems to say something different.

i was reading imquattro's 550 rebuild thread where he took a junker and rebuilt it for his highschool class and he said he did a leakdown test at 100psi's. my buddy who is a self proclaimed 2t wizard also suggested 100psi's i did it at 5 because thats what the forum said to do.

i just got proper upper sris caps and freeze plugs. im going to try the test again with my one cylinder with the piston at bdc, if i still get squirelly results im going to seal off the rest of the cylinders as well, and apply preesure, and remove the spark plug (or upper sris cap) and see if i hear or feel air seepage from the adjacent jug. i feel like this would be a sure sign my seal(s) are bad. does this sound sufficient? im using a radiator pressure hand pump. any more suggestions/tips/ideas? my buddy suggested that the TINY bubbles probably arent much of an airleak at all. and do no equate to the amount of air im losing, which i agree. nothing can be THAT ***** up, because like i said it was running decent, ive done over 100MPH on it and have ridden for semi long distances...it just has trouble idling and loses a cylinder at idle. which i hear is a sure sign of bad seals, but also is a sign on an airleak. i just want to be sure before i dump EVEN MORE money into this bike(something over 4k!?) and i hear one of the seals isnt even available. and ive never rebuilt a motor of this size (only 50cc single speed motors) and it seems over whelming. and also on top of all that. i have a gt750,re5,and cobra (and van and truck and 2 dirtbikes) that all need work! im trying to work it out, and learn. help. please and thank you.

oh and anybody want to trade for NICE STOCKER!? haha

[chance johns]

ok so i may be beating a dead horse. i just might MIGHT be in denial here.

i set the piston a bdc and tried the test again. i hold 0 pressure for 0 seconds. i see an extremely miniscule amount of soap bubbles seeping from the cylinder studs. what would cause that? is it something to worry?

again, the bike operated for the most part, decent. i would lose a cylinder at idle at low rpms and it would kick in at 4k rpms give or take a little. apparently this is a tell-tale of bad crank seals. but also could be an airleak. which i found , and redid the top end gaskets. but still hold 0 pressure. w

what are the chance that i had an airleak AND bad crank seals? if it was that bad i dont think i should have been able to ride this bike as hard as i have been? 100mph+?

also when i read about people testing crankcase pressure they say it should hold 5psi for 5 minutes. if it loses 1lb per minute the seals are bad. my pressure lose is so extreme i cant help but to think im doing something wrong. and it was kind of running alright before the test, so it cant be THAT BAD.....COULD IT? ken at knk says he thinks one of the seals arent obtainable? is this true?

AM I IN DENIAL?
AM I DOING THE TEST WRONG?

[tz375]

There's not a whole lot of absolutes in life, but 100psi on a two stroke is one of them. It's wrong. Dead wrong.

Stop and think about it for a moment. At TDC, the only place that pressure can go is past the rings into the bottom end (and that will stabilize) and out of the head gasket - or past the rings and into the exhaust if the exhaust port is not sealed.

So that suggests a head gasket leak. At BDC the same applies. If the seals are OK, air will travel up the transfers and leak out of the head gasket. So pop the head off and use a layer of vaseline to temporarily seal the head and try again - or use gasket goo to seal it while you test it.

I had problems with a 750 recently and it was the intake rubbers that were allowing air to escape.

[chance johns]

umm....well my compression was at 125 roughly for each cylinder. i sprayed the whole left side of the motor at all the seams (including the head seam and exhaust block off plate) with soapy water, no leaks (except around the cylinder studs above the cylinders mounting holes) . these are brand new gaskets. i do hear air seeping when i put my ear up to the intake spigot of the center cylinder. I KNOW I KNOW. that should be my obvious sign of the seal leak. but it just seems strange that it holds 0 pressure for any amount of time. when it was running half way decent?

how did i have an airleak at the base gasket AND a seal leak? AND it was kind of running alright?

my next test is going to be sealing off all the cylinders and pumping air into the left cylinder and then pulling the plug on the center and see if then i lose pressure.