Low Compression After Overhaul

[chip51maker]

Recently I completely disassembled and bored out My 1987 150 Horsepower Evinrude Outboard Motor. I own a machine shop (not automotive) and I did all of the machine work Myself. The reason for the overhaul is because one piston was burnt as a result of overheating due to malfunctioning thermostats. <br /> The overhaul involved boring .020" oversize and reassembling with new Serria brand piston & ring sets, new rod bearings, new water pump and thermostats. The final bore size after honing gave Me .003"/.004" clearance on the pistons. I washed the cylinders with a strong soap & water mixture before final assembly. The block surface and the heads were machined to assure good headgasket sealing. The entire engine was squeaky clean before final assembly. I disabled the "VRO" and now must premix the fuel. The first tank that I ran thru the motor was mixed 25:1 and then I started running 50:1 after that.<br /> The problem is that ever since the overhaul My compression has been 30 PSI on all cylinders. The first compression check was before the initial startup and then I have checked it several times after a breakin period. I always check the compression with a screw in gage and the engine is hot and the throttle is in the WOT position.<br /> Will you please give me Your opinion about why I may have low compression and all of the factors that determine good compression. I thought that I covered all of My bases but apparently I left out something.

 

[wilde1j]

Re: Low Compression After Overhaul

Did you use a shop manual for all disassembly and assembly procedures? Use the correct sealant for the CCase halves? Torque the heads? Something is radically wrong to get 30 psi all around ... should be ~ 100+ psi.<br /><br />Also, typically, you won't get a single piston fail from a tstat failure ... usually the whole bank will go. I would look for an induction system problem (carb, reeds, etc.) for the failure source. You need to find the cause of failure or it will reoccur.<br /><br />I would suggest going through the shop manual one page a time in the assembly section for the PH and I'm sure you will find the discrepancy causing the very low compression. It almost sounds like the overbore dimension was too big. I would pop a head off and mike the bore and check against the specs for 0.020 over and maybe check with a rebuilder to see if that dimension agrees for the same pistons you used.<br /><br />Doesn't sound good.

 

[Chinewalker]

Re: Low Compression After Overhaul

Actually, I would say there's something wrong with the gauge - as your motor simply would not run at all at 30psi... Try a different compression gauge...<br />- Scott

 

[dick]

Re: Low Compression After Overhaul

Do as Chinewalker says and try a different compression gauge.For all the readings to be even at 30 psi I'm betting you got a bad gauge.

 

[walleyehed]

Re: Low Compression After Overhaul

The first problem I see here is your mention of piston/cyl fit....what did the piston manufacturer say for bore size??? it should be a true .020 over STD. piston/cyl clearance is built into the piston.<br />My next question would be did you check piston ring land clearance????????

 

[wilde1j]

Re: Low Compression After Overhaul

Chine and Richard have a good idea regarding the gauge. They tend to be very reliable, but if you haven't used it in a while, I would retest with another gauge. If you still getb ~ 30 psig, you have a serious problem.

 

[chip51maker]

Re: Low Compression After Overhaul

Thanks for the help. I will get a new gage and recheck the compression readings. Just a note about My background. I have extensive experience in 4 stroke engines both gas and diesel. When it comes to 2 stroke engines I really do not understand how or why the reed valves or crankcase sealant would affect the compression. I am guessing that if a reed valve is stuck closed it could not let air in to the crankcase for compression.(like checking compression with closed throttle) <br /> Another thought that I have had in the back of My mind is the honeing finish. I did not get as good of a cross hatch as I would have liked to have had on a 4 stroke engine. I have been told that the cross hatch is more important in a 2 cycle engine than a 4 stroke engine. Is that true??<br /> Again I certainly appreciate the help and will try all of Your suggestions.

 

[wilde1j]

Re: Low Compression After Overhaul

Perhaps, but not 80 psig or so worth on a just rebuilt block. The right sealant is really important ... I think it's generically Sealant 1000 (oe OMC Gel Seal), and there's nothing else that should be used to seal the halves. There's another sealant, which I found one time at a good motorcycle shop, which they used for the same purpose and appeared to be the same stuff, I used it on a 4 HP, so it wouldn't have been a big deal if I had to redo. A crankcase leak can be a bugger to find after the fact and it will have a very bad affect on running (way too lean, coughing, poor WOT, etc.)

 

[Walker]

Re: Low Compression After Overhaul

There are a couple of other sealers that will work on the crankcase halves. Loctite 518 and Permatex 51813

 

[clanton]

Re: Low Compression After Overhaul

The new piston coming up to the same height as the old one. What about rings and gap? Headgasket? There were low compression head gaskets made for some of these engines.

 

[jim dozier]

Re: Low Compression After Overhaul

Since the 2-stroke cylinder always has access to air at the exhaust port, throttle position, reed condition, and crankcase seal or lack thereof will not affect the measured compression. They will certainly affect the mixture in the cylinder and the running performance but not the static compression. This guy's problem has to be either the gage, the piston and ring sealing or the head gasket or some other leak in the head.

 

[grampa's toy]

Re: Low Compression After Overhaul

chipmaker after you honed the cyls. did you break the edges on the ports? you could have cracked rings.

 

[chip51maker]

Re: Low Compression After Overhaul

In reply to Grampa's Toy,, <br /> I did lightly break the edges of the ports using a Dremel Tool with a sandpaper roll. I left a slight chamfer about .010/.015" on the edges that are in the direction of piston travel. I asked 2 motorcycle mechanics about chamfering the ports and they did not seem to concerned about it. I, like you, thought that it is pretty important. I have not started disassembling this motor yet. Possibly this comming weekend I will take the intake covers off so that I can examine the piston rings. <br /> The only thing that I really felt unsure about when We were finished is the "Hone Finish"<br />It was slicker than I usually like it. I apparantly ordered the wrong grit stones to get the finish that I wanted. "Wildeone" tells Me that the "Hone Finish" should not make 80 PSI difference in My compression. I believe that and I am encouraged by that.<br />Thanks for all of the help

 

[P.V.]

Re: Low Compression After Overhaul

What you are "reading" when you do/use a compression guage is actually the "secondary compression" versus the "primary compression" which is the ability of the crankcase to draw/hold/move internal air/fuel mixture. Anyway, "compression of the cylinders should certinly be expected to be approx. ..... what??? 100 psi or above on a "new" rebuild with new pistons, rings and a good hone job! I'll tell you a story that happened to me a few (12) years ago. I was offer'd a position at an outboard remanufacturer to over-see their operation and help them out as they were having mega-problems with warrantys on their powerheads and it was costing them big-time dollars and repeat customers..... I was given "full-reign" to do "what-ever" it took, including terminating personell if necessary, to get things back on track. Remember, 10 or 12 years ago, there were not that many rebuilders on a national level and so this company had one of the first footholds on this "new" type of bussiness. Anyway, when I first walked out to the "assembly" area and met the employees, I noticed that the ball-hones they used to clean up cylinders were missing at least half of the balls and the "techs" seemd to think that two or three passes of the hone (with WD-40) was enough to do the job and in-fact their powerheads turn'd over by hand quite ez'ily once assembled! They were quite pleased with their quality of workmanship as they did not know any better ( they had never work'd for a dealership, never actually put together a working outboard after they "rebuit" the powerhead)! So, I was frustrated from day one as I could not fire the whole crew nor did some feel that they should take orders from some new guy, "WHAT DOES HE KNOW ANYWAY???" I didn't last too long at that job as the owner really didn't want to do anything that was going to cost him money and I felt he was obligated to cover his warranty liabilities.......<br />My point is this (long winded, HUH??) After a rebuild, with good piston fit, ring gaps being correct, rough cross-hatch at approx. 45 degrees, you should be able to turn each cylinder over with a ratchet ( on the flywheel nut) and feel the piston's compression "push" the piston pass TDC briskly, spark plugs in!!!! BTW, with a ball-hone, you do not need to worry about any sharp edges at the ports. The balls will do it for you! In stock OPC outboard racing, you can rebuild motors with oversize pistons but you can not use a ball hone as the factory never did champfer the ports either. Gotta be "stock"! Ball-hones do the champfering for you!

 

[Hooty]

Re: Low Compression After Overhaul

How did you determine .003"-.004" piston clearance and what is the ring gap?<br /><br />c/6<br />Hooty

 

[dick]

Re: Low Compression After Overhaul

Possibly this comming weekend I will take the intake covers off so that I can examine the piston rings. <br />

Don't take anything apart till you confirm your readings.Very odd to have a engine run great with only 30 psi per hole.<br />Let us know what you get.

 

[ledgefinder]

Re: Low Compression After Overhaul

I'd check your gauge before you do anything else - 30 psi is WAY low. Lowest I've seen (aside from a hole in the piston top) is 80 psi, and that was a severely-scored cylinder.

 

[walleyehed]

Re: Low Compression After Overhaul

Hooty, you ARE being funny, aren't you?????<br />I hope so, because if someone fits a piston by piston/cyl clearance, they're in trouble already.<br /> And, if you're not jokin' about end-gap, it's when the ring is placed at a pre-determined location in the bore and the gap (end-gap) is measured at that point. Of course insuficient clearance will cause the ring to come together and push outward causing excessive heat, and shortly after removing all oil from cyl. wall......stuck, molted, seized, scored, destroyed, A#1 blowed up job...........<br />I still think yer joshin' here.........

 

[Dhadley]

Re: Low Compression After Overhaul

Which direction is the deflector on the piston dome pointing to?

 

[Hooty]

Re: Low Compression After Overhaul

Walleye,<br /> I know how to determine piston/cylinder clearance and you know how to determine piston/cylinder clearance and I know how to measure ring end gap and you know how to measure end gap. The reason I asked is trying to determine if, when boring, a mistake was made with the final cylinder size bore. 'kay?<br /><br />c/6<br />Hooty