Crankcase detonation???

Walker · Oct 28, 2003

1994 120 johnson looper. I've been having trouble with motor for about a year and its been getting worse. Now it will hardly idle at all. It started the trouble by sneezing when cold but running fine after warming up. Now it never stops sneezing (and its a hard sneeze with big puff of blue smoke). I've been thru the carbs several times and they are spotless. Idle mixture screws won't make it stop sneezing. Today I torn it down to the intake manifold and I found one reed block covered with carbon. I'm getting fire to all cylinders and compression is 110-115 on all 4. Am I getting crankcase detonation on this cylinder and if so, whats causing it.

rodbolt · Oct 28, 2003

Re: Crankcase detonation???

hello Do a cylinder leakdown test. what is happening is hot gas is blowing past a piston, usually due to stuck rings or excessive piston to cyl wall clearence. have you ever decarbed the motor? now may be a good test of ringfree or powertune or seafoam or what haveyou. a compression test is not very accurate for what this calls for. do a decarb maybe even two decarbs then do a leak down. I have seen looper rings so stuck the only way to budge them was to use a punch and a hammer and break them off. good luck and keep posting.

seahorse5 · Oct 28, 2003

Re: Crankcase detonation???

Remove that reed box from the manifold and see if the gasket got torn away. That was a common problem. Popping back thru the carb is a "lean" problem due to an air leak into the crankcase area or restricted fuel flow thru the carb passageways. Check that there is no seepage thru the crankcase halves and that all the recirculation valves and lines are clear and hooked up. Check the cylinder head temp on each bank to be sure it is above 125 degrees, or the motor is running too cold.

Walker · Oct 29, 2003

Re: Crankcase detonation???

I recently decarbed it. I think I'll pull the powerhead and tear it down. I've been fighting this thing too long.

Dhadley · Oct 29, 2003

Re: Crankcase detonation???

I would start with Seahorse's suggestion and pull the intake first. I bet its a reed gasket or it could even be the side plate on the carb. there were some "tricks" to help those seal depending on which side plate you have. I would still bet on a reed gasket. Or a burned reed tip if you have Boyesens. Good luck!

Walker · Oct 29, 2003

Re: Crankcase detonation???

Here's a couple of more symptoms. One dry, black carboned plug, top cylinder port side.Its really sucking down the fuel, smokes like crazy and there is a lot of fuel and oil coming out the exhaust into in my run barrel. I'll try new reed gaskets and carb kits before I tear it down. It has stock reeds and I looked at them yesterday when I had the intake off. I premix my gas, no VRO.

Walker · Oct 29, 2003

Re: Crankcase detonation???

Well, I tore it down and everything is okay. I'll put it back together with new seals and gaskets and see what happens.

backdraft · Oct 30, 2003

Re: Crankcase detonation???

Walker, just as question from me, Did this motor have a vro and it was taken off. I have found 2 motors that had this unit removed and rubber caps that covered the ports had gone soft due to fuel additives allowing a airleak into the crankcase. This caused a sneezing condition and because one of the plugs was under the starter , it took a while to find. It looked allright but was sucking inwards allowing a lean condition. nevjb

Walker · Oct 30, 2003

Re: Crankcase detonation???

Yes, the VRO was removed in 98 and replaced with standard OMC fuelpump. I checked all the relief valves and they are all working as designed (one way)

93bronco · Oct 30, 2003

Re: Crankcase detonation???

check all your recirc fittings & valve. along with the other suggestion about the carbs.

seahorse5 · Oct 30, 2003

Re: Crankcase detonation???

If the crankcase side of one of the reed boxes had soot on it, concentrate on that particular cylinder's carb and inspect for air leakes. If a carb throttle is open more than the others, that cylinder will run lean and "pop" also. Make sure the "synchronization" of the linkage is spot on.

Walker · Oct 31, 2003

Re: Crankcase detonation???

This motor has throttle bodies behind the carbs. One bank could get out of sync, but individual carbs cannot.

Walker · Nov 21, 2003

Re: Crankcase detonation???

This thing still won't run right. I'm getting nice oily looking plugs on the starboard side but black chalky plugs on the port. I've tested all check valves and even swapped the carbs side to side but still get the same results. I'm stumped.

R.Johnson · Nov 21, 2003

Re: Crankcase detonation???

Sounds like you have looked every where. Did you say what cylinder is acting up, or did I over look it? When you had the carb's off, did you look at the core plugs in those throttle bodies. As was suggested earlier, take a close look at that crankcase nipple where they disconnected the VRO. If any doubt, remove the nipple, and replace with a 1/8 in. pipe plug. When installing these these types of carbs to the throttle bodies, I use a light film of grease, such as needle bearing grease to hold that o-ring type seal in place.Let us know what you find.

R.Johnson · Nov 21, 2003

Re: Crankcase detonation???

The OMC manual also shows a in-line check valve in the recirc. system. This valve is in the line, not screwed in the block.

wilde1j · Nov 21, 2003

Re: Crankcase detonation???

Sneezing is almost always due to a lean condition ... from your description, one bank is running lean. There's most probably an air into the ccase on that bank, either through the induction system, halve seal, etc. I'm not too sure how to test for it, other than try to check out all the possibilites. Check all hoses very carefully for cracks (maybe flexing the while running may help if that's the cause ... you would be able to hear the leak if it's a cracked hose, even though two strokes are loud. Don't give up (and I wouldn't tear down the block until all possibilities are eliminated. Good luck!

Walker · Nov 21, 2003

Re: Crankcase detonation???

I've been over everything you guys mentioned, more than once. What really gets me is I swapped carbs side to side and nothing changed. Still carboning up the plugs on the port side.

Hooty · Nov 21, 2003

Re: Crankcase detonation???

I don' know if this 'll work or not. See what ya think. Use a can of starting fluid, (ether) and spray around the port side of the engine (manifold/crankcase seam, etc.) and listen for an increase in rpm. The thinking is maybe, if there's a leak, the ether will get sucked in and increase the engine speed and locate the leak for ya. Keep us posted. c/6 Hooty

8up · Nov 21, 2003

Re: Crankcase detonation???

I was told on this site that on my 85 140hp looper the port carbs feed the stbd cylinders and stbd feed port. Does anyone know about this one. Just something to consider if so.

Walker · Nov 21, 2003

Re: Crankcase detonation???

Yes, the carbs feed to opposite side cylinders. Hooty, I resealed the case last week and have tested the seam with carb cleaner. I sure there is no leak thru the seam or the upper or lower seals. I'll pull the heads tomorrow and see what the cylinders look like.

Crankcase detonation???

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