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Compression test on 40 hp. 98 RLEC.

B

BigMuddy

Member
Does anyone know what the compression should be for this engine? I've done every kind of search I can think of and come up empty. No manual.... I know........ Anyway engine runs rough at idle and bucks and bogs down at any rpm. Top end is dismal. I've checked the spark and spark easily jumps a half inch gap on my homemade spark tester. What I've done so far is installed new plugs, QL78YC, gapped to .030, cleaned tank and filled with new gas, checked for and stopped air leaks in hose assembly and finally ran a can of seafoam through her. I ran a compression test before and after seafoam treatment with little change, 70 on both cylinders before treatment with no change on lower but upper going up to 75 after treatment. None of the above made her run any better. Wondering if carb rebuild would help much or if those compression readings means she's too far gone and in need of some more serious engine work. Thanks
 
have you decarbonized the engine,is that what seafoam is?70psi is pretty low.should be closer to 90,95! sounds like a rering., rebuild to me sorry!!
 
The poor compression reading could very well be due to a faulty gauge. Try another gauge. It could also be due to a slow cranking engine.... the crank over rpm has a lot to do with it.

Your explanation of the problem spells "Fouled Carburetors". Clean and rebuild them. Be sure to manually clean the high speed with a piece of single strand steel wire as solvent just doesn't do that job properly. The jets are located horizontally in the bottom center portion of the carburetor float chamber(s)... way in back of the drain screw plug.
 
John.... It's pretty unusual for two cylinders to be exactly the same. It's indicated at first that both are 70 psi, then later obtaining a reading of 75 each. Yes, that is considered low for a two or three cylinder as they normally read over 100 psi easily.

If this is actually the case, that would bring to mind that the head gasket is leaking between the cylinders with the compression of #1 cylinder blowing down #2 cylinder and vice versa... BUT... if this is the case, there are water passages in the block between the cylinders and water should be gaining entrance to the cylinders and spark plugs via the failed gasket.

Nothing has been said yet about traces of water upon the spark plug electrodes... I wonder............
 
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I actually did perform the compression test with two different gauges, first with a very old homemade one that's been sitting unused in my shop for a decade or more and then, after being suspicious of the low readings, I went to town and got a loaner from NAPA but came up with almost identical results. The gauge I got from them looks to be pretty new. But I didn't realize that cranking speed could effect the readings so hopefully that's what's responsible for the low readings. I see that the carb kits for it aren't too expensive so I'll take a chance and hope that'll get the old girl back on the water again. Thank you. What do you recommend soaking the carb bodies in? I still have some seafoam left, would that work?
 
The spark plugs were wet looking looking but I just assumed it was oil/gas. I may be wrong about that. I probably wasn't clear about the readings; after seafoam treatment one cylinder went to 75 and the other stayed at 70.
 
Joe the plugs were both wet looking but I didn't notice anything like individual water droplets or anything that made me think water. My thoughts were that the wetness was just gas/oil residue, but not sure how I would be able to tell the difference between that and water. Assuming the head gasket is indeed blown and the 70/75 lb readings were accurate, would the motor still start fairly easily and be able to run at all? In the condition it's in right now it takes about 15-20 seconds of cranking to start it cold, and usually only a couple of seconds on the starter once it's warmed up. One other thing I forgot to mention that puzzled me but may help smarter folks to diagnose the problem is that spraying Seafoam into the bottom cylinder tended to increase rpm while spraying into the top cylinder made it stumble.
 
BigMuddy said:
spraying Seafoam into the bottom cylinder tended to increase rpm while spraying into the top cylinder made it stumble.
Click to expand...
I meant to say the changes in rpm occurred when I sprayed seafoam into the respective carb throats, not into the cylinders themselves............
 
BigMuddy said:
My thoughts were that the wetness was just gas/oil residue, but not sure how I would be able to tell the difference between that and water.
Click to expand...

If water was present there would be no mistaking it. It would bead right up as it doesn't mix with gas or oil. It usually presents itself as small beads of water on the spark plug electrode.
 
Didn't see anything like that. Gonna run another compression test in the morning and try turning the motor over with the rope pull instead of the electric starter just in case I can spin it any faster and see if the readings change.
 
BigMuddy said:
Didn't see anything like that. Gonna run another compression test in the morning and try turning the motor over with the rope pull instead of the electric starter just in case I can spin it any faster and see if the readings change.
Click to expand...

Good idea... I've found that I could output a few electric starters in the past.

However, make sure you have ample "elbow" room before yanking that rope (been there a few times (YOW!) :cool:
 
Seems as though I've been getting old right along with the ol outboard, I don't seem to have the umph to out-crank the starter motor anymore....Who'd have thunk it?

But the good news is that cranking with the starter I now get 80 lbs on the lower cylinder and just shy of 90 on the upper, so a bit of improvement over yesterdays readings. When I got done testing yesterday I dumped the remaining Seafoam into the cylinders and let it soak overnight so that may be why the numbers went up or it might be because I put a fresh charge on the battery to make sure it was cranking at max rpm. Had some serious smoke this morning for awhile, although not as much as I've read that others saw after treatment. Motor ran about the same as yesterday, maybe just a tad bit smoother, although definitely not right. Using my fingers to partially cover carb throats individually I see little or no change when doing the upper one but engine rpms do go up when choking the lower one, so my uneducated guess is that my biggest problem is with the lower carb. Any thoughts on that?
Anyway, carb kits on order from this site right now and will post back once I've received and installed them with the results.
Thanks for your help.
 
BigMuddy sounds like joe reeves hit the nail on the head thread#3 Compression #s not optimal but enough to run good luck with carb rebuilds!!
 
As per my reply post #3 (copy/paste), don't overlook those jets..............

"Your explanation of the problem spells "Fouled Carburetors". Clean and rebuild them. Be sure to manually clean the high speed with a piece of single strand steel wire as solvent just doesn't do that job properly. The jets are located horizontally in the bottom center portion of the carburetor float chamber(s)... way in back of the drain screw plug."

Thousands of parts in my remaining stock. Not able to list them all. Let me know what you need and I'll look it up for you. Visit my eBay auction at:

http://shop.ebay.com/Joe_OMC32/m.html?_dmd=1&_ipg=50&_sop=12&_rdc=1
 
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BigMuddy said:
Thanks for the reminder, I'll make sure they're clean. Just to be sure I understand you correctly, they are actually in the float chamber itself and not in the carb body?
Click to expand...

Unless they've changed that location since 1992... yes, that's where they're located, way in back of the float drain screw plug... held in place via 1/4x20 threads.

Fuel must travel through them freely before the fuel has access to any other fuel passageway.
 
Thank you Joe and Racerone. I finally broke down and ordered a manual today, only 17 years late......... But I'll add that the fact that I didn't have need of a manual for 17 years stands as testament to how good that motor's been to me.
 
racerone said:
Hopefully you have replaced the impeller about 3 times over those 17 years !
Click to expand...

To be honest I've only replaced the impeller once and that wasn't because of any issues. It was still pumping great when about three years ago I had to replace the original aluminum prop due to hitting the top of one too many Missouri river wing dams with it, and during the conversation with the parts guy I mentioned that I was still running the original water pump. I'm not sure if the look he gave me meant he thought I was a liar or a moron, but either way, he convinced me to change the impeller so I did. If this is unusual impeller life, all I can tell you is that if there is any secret to it, it would have to be regular running. When I say regular running I mean an absolute minimum of one day a week, usually two, and sometimes more. Between duck hunting and cat-fishing on the Missouri River that old motor ran every week in every month of every year except February for most of those 17 years. That has decreased considerably the past few years due mainly to the fact that I quit duck hunting altogether and my cat-fishing trips have become much less frequent as well as shorter as I've gotten older. Just not tough enuff for all that anymore. Outside of oil changes to the lower unit and fresh plugs every year, the motor was basically trouble free until I stopped using it regularly so...........
 
Making progress, I think...... While waiting for the carb kits to be delivered I went ahead and tore down the carbs. Top carb looked perfect except for a bit of crud around the main jet in the center of the carb body, high speed jet was clean as a whistle. Bottom carb main jet looked similar to top carb but the high speed jet was partially clogged. Cleaned them both thoroughly, and even managed to remove high speed jets thanks to the special tool instructions I found here. Everything that comes in the rebuild kit looked to be in excellent shape and impatient person that I am, I didn't want to wait any longer for the kits so went ahead and put everything back together using original parts. including gaskets which all looked like they just came out of the box. Took her down to the river this morning for a test run and ran four miles up river without a hitch, running just a bit under WOT which has always been my go to speed when going any distance. Motor ran perfect. Turned around and got about half way back when she very suddenly bogged down, eventually died. Had some difficulty getting bulb to pump back up but after several attempts it finally got hard. The gas line and bulb were brand spanking new by the way, first time out. Motor ran good for a half minute, bogged down again, repeat, repeat and repeat for the last mile in. Pumping up the bulb got more and more difficult, to the point of my hand cramping from so much squeezing. I tried pumping it up both with and without it attached to motor fitting, no difference. Tried it with tank cap screwed down tight and with it loosened, no difference. At the ramp I continued to play with it for another half hour, pulled the fill tube out of the tank, blew though it, no clog or restriction but squeezing the bulb with the tube out of the tank I could here air going in and out with each squeeze. My best guess is that there is some kind of one way check valve just behind the tank-plug fitting, inaccessible though so can't say for sure. But decided the 17 year old tank had seen it's better days so swung by Walmart on the way home and picked up a new one. Pumps up fine in the yard and motor runs great on muffs. Hope to take her back to the river tomorrow to see if the new tank helps. or not. What I know for sure is that the bulb wouldn't pump up connected to the old tank but does pump up now. Thoughts?
 
sounds like a long summer of boating ahead!!i saw a thread posted i believe came from kim about taking carb flange and making sure it is perfectly flat on a sheet of glass if not sanding carb base across glass with 220grit wetndri sandpaper.
 
Sounds to me like you're coming down home stretch a winner!

A suggestion though..... When you have a lot to say, I'd suggest you break it up into short paragraphs. One large paragraph has a tendency to have it all run together... makes for difficult reading for some of us.
 
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Back to drawing board. Took it out for another test run this morning, new tank, new gas line and bulb, and fresh gas mixed to the proper ratio.

She started up fine, and got right up on plane and ran great, for about 20 seconds, then began surging badly and kept it up until I let off the gas. After that, for the next hour or so, when I'd try to get on it she would sometimes try to accelerate but just before getting up on plane would start surging again. But other times it wouldn't even try to accelerate, just bog right down as soon as I tried to give it some gas.

I tried pumping the bulb while it was running at least a dozen different times, and one time, but only one time, it got up on plane and ran fine for a short distance before starting to surge again. All the other times when I pumped it up while running it made no difference at all. The bulb did pump up readily every time, which is big improvement over yesterday so I guess the new tank did solve that particular problem.

Things done so far, in order, compression test (with iffy results) seafoam, spark test, , new hose and bulb, thoroughly cleaned carbs, but put back together using original parts, new tank, fresh gas. It ran like a champ for several miles yesterday before it suddenly bogged down, no surging, just bogged down. The surging this morning is a brand new symptom.

I'm stumped, symptoms keep changing with every new thing I do. I guess I'll tear the carbs down one more time, clean again but install new carb kits this time instead of using original parts since I now have the kits on hand. Also will tear down fuel pump and clean it but don't have a rebuild kit so will put it back together using the old parts unless I see a problem like a torn diaphragm or something obvious like that. Then back to the river for another test run, hopefully tomorrow if I get it all back together today.

Any ideas of what else I can try before heading back to the river again? Thanks
 
I may have asked previously but can't find a record of it............

This surging........ I assume you mean that the rpms decrease and increase without moving the throttle.

Is this a sharp, quick, sudden, instantaneous change of rpms (ignition)?.... OR

Is this a gradual, sort of smooth change of rpms (fuel)?
 
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