The problem that i am having is that cylinder #4 is missing while running and has low compression. It has good spark and gets fuel. Did a compression test and the results of that was: cylinder 4 was 50 PSI, then poured a little oil down the cylinder and retested only to see a 10 psi improvement to 60. The other 3 where at 150 PSI on a cold engine. Checked the valve clearances, intake was at .0055 and exhaust was at .0075. So they checked out correct. Other than pulling the engine apart, any ideas on what it could be. im looking at possibly: Headgasket, burn valve or piston rings. but i could be wrong. Could use some help from you pro's thanks
Valve sealing (or lack of). Even if it's a headgasket sealing problem the engine still has to come apart, so get to wrenching...
It's really a binary thing..the engine stays in the frame and can be adjusted or tinkered with to make the repair or NOT and it has to come out and get torn down for examination of the internals. Once it's apart you will find the cause/s, so knowing in advance EXACTLY what is wrong does no good, because since you have to tear into it anyway, you might as well wait to see what you see..
Get a leak down kit and leak it down with the exhaust and throttle bodies off. You should be able to hear and possibly feel if it is leaking past the rings (into the crankcase), past an intake valve (nice breeze from the intake port), or the exhaust valve (nice breeze from the exhaust port). At 50 psi cranking compression, you are substantially leaking from somewhere. Good luck, Rocky Stargel Team Stargel
My point exactly, why bother? If it was a "customer's bike" then giving a (sort of) accurate price quote in advance of tear down is manditory, but if it's your own machine, just tear it down and it will tell you what's wrong right there on the bench, no leak down hassles involved..
Im just saying to leak the thing down whether its in the frame or not, before tearing the engine down. 100psi will show things not apparant to the naked eye... Rocky Stargel Team Stargel
I'm with Rocky here. Leak down test every motor before you tear it apart and you will have some idea of the condition of each cylinder. It might direct you to make repairs on the head or cylinder in areas not apparently malfunctioning, i.e. another cylinder might need some attention as well. And you'll have more information rather than less. It takes what? Ten minutes? Ten and a half? Just my thoughts. Please feel free to criticize my questionable methods. I am wearing Nomex shorts.
can you get a "leak down test kit" from any auto parts store or do i have to go to a specialty shop? thanks again for all the info.
Leak down motors regularly.. we just tested my 07r6 superbike motor (2mm over,KWS spec) and discovered some leak BETWEEN the cylinders. The Cometic gaskets have been found to be junk and KWS is no longer using them. Now.. we get to spend a few hours sanding out the inside of a stock bore Yamaha gasket. With the plugs out, we heard "hissing" from somewhere but not the pipe or throttle bodies.. it was comming from the plug hole from the cylinder next door!! Presto, we know what to look for specifically and there was just a little discoloration between cylinders, nothing serious. The motor is crazy fast but had lost some steam (sorry, not another pun) as of late. The CRANK LOOKED GREAT.. so, for all the riders getting "cranky" over the cranks, i have a motor with a fair amount of time and the crank looks great. Carrillo rods, and resized to correct bearings. My both of my supersport motor cranks looked just as good.
Yea, yea, leak down test em if you got em (low volume leakdown testers that is)... The high pressure and high volume testers are junk and do a good job of hiding when an engine is STARTING to leakdown. In this guys case, his engine is junk and I consider it a waste of time I could spend drinking beer or making fun of squids to haul out an expensive tester and listen to air rushing out of whatever expensive busted bits he has rattling around in the box, that's all IM sayin...dead is dead, off with it's head and THEN he can see the cyls are trashed along with the toasted valve seats and faces...
hey tuner I am new to leak down tests. Why do you say that high volume ones are no good? Can you recommend a good (and affordable) kit to purchase? thanks
The cheapo versions that most tool dealers carry (or can order for you) use too much air ressure and volume which push the rings against the cyl walls and cause false HIGH readings. They are more or less the same as sticking a 125lb air hose in the spark plug hole and listening at the carb inlet, exhaust pipe tip and/or oil fill hole to figure out how all that valuable compression is getting out of the combustion chamber corral... A low pressure leakdown tester (also known as an aircraft type leakdown tester) fills the cyl in question VERY slowly at low vlolume so that weak or worn rings are not expanded into the cyl walls by the pressure used in the act of testing them. In the mid 90's I worked part time in an automotive race engine shop and when I got caught trying to test a customer engine (that was in for a check up between overhauls) with a normal off the shelf tester, I got my ass handed to me by the head guru.. AFTER he made me finish testing using my high pressure version of a leak tester, he handed me the low pressure version and had me test again. The results were MUCH worse (in terms of showing how close the unit was to needing a freshening up) than MY tool had shown.. I think mine showed 2% leakage and his showed over 20% leakage due to the "relaxed" rings not having 125lbs of pressure pushing them tighter into the cyl walls... Lesson learned.
I have never heard of this kind of low pressure leak down tester. That certainly doesn't mean they don't exist. Let me pose this question: If you don't have the rings pushed to the bottom of the land, there's a possibility you won't get a good seal. Sometimes you can rock the piston at TDC with my inferior leak down tester and get the reading to drop (less leakage), so what you say may be true. But, wouldn't normal combustion pressures force the rings to the bottom and therefore influence/improve the seal? Wouldn't that make the low pressure tester sort of artificially skewed? Besides, be realistic. The poor guy asking this tech question is never going to find such a tool. A regular tester will do him just fine.
The purpose of a leak down test is to establish the amount of wear on the rings and valves by finding small/subtle leaks that could be/are robbing it of the the ability to seal, not to find damaged parts or gaping wounds caused by catastrophic failure. Folks, if you got one hole that blows 50lbs, you don't need to know the NAME of the parts that need replacing, you need to remove the engine and set about to fixing it. Unlike a new (or fresh rebuild) engine that has a problem in one hole where a valve or a spring might constitute a fix, older engines with problems serious enough to trigger a teardown get the full ride. SO, my point is that he does not NEED to do a leakdown test because his compression test results told him all he needs to know, which is that the engine is damaged/worn out and needs to come out (and apart) for further examination. "Aircraft leakdown testers" can be purchased through Snap-On tools and no doubt other tool sellers as well, just not Harbor Freight, or Sears..et all.. Not many techs need to know what small percentage of compression is leaking past the rings and valve however so unless you build/repair aircraft recips or high dollar race engines, you will never see or touch one.
Point one out in the Snap-On catalog for me. I have two different Snap-on testers. If you have one, let me know where you got it and I will get one. Every single leak down tester I know of is indexed at 100 psi or more. You can run them at 70 or whatever, but then you have to recompute the percentages. I am interested to know who makes such a tool. I build race engines every day. If it's better, I want to use it. I am not refuting your claim, I just want to see this tool.
The standard aircraft ones I have used all work at 80 psi. 80/60 is the min airworthy diff. pressure reading. There is a calibrated restricter orifice in them that limits the flow. I looked on the Snapon website and they don't carry a cylinder leakage tester. This it the kind I've used for a long time. http://www.aircraftspruce.com/catalog/topages/presstester.php
The one my (ex)-boss handed me was a red box with the Snap-On tools logo on it, BUT that was in the mid 90's and I don't know how long he owned the tool before he showed to me. I do not work for or represent Snap-On tools, so I suggest you contact your local rep and see what he offers if anything, along the lines of a low vol leakdown tester. The idea of low press/low volume is to not force the rings into a "better" seal against the cyl walls than the mechanical condition of each/both would reveal at lower levels. Much like "gas ported" dragrace pistons force the rings into tighter contact with the cyl walls to decrease blow-by (and increase power output) excess leaktester pressure will make worn parts test good.
