87 johnson gt 150 blown up

Bought boat last year, ran good but didnt idle well and didnt seem to pee very strong. So this year replaced water pump and rebuilt carbs and cleaned. replaced fuel lines and primer and fuel filter. Ran on muffs. Started right up idled smooth and peed much better. Ran for 15 mins or so with no issues. Took to the lake and started warmed up fine took off and within 1 min noticed the engine sound start to change (I know that sound I used to race snowmobiles.) shut down throttle right away. Very hot, towed to shore. Pulled heads at home, Took out whole bank of cylinders on one side. Just wondering what could be the cause. Thanks Chad.

Re: 87 johnson gt 150 blown up

I would check the water pump to see if it is still in one piece. How was the old one when you took it out ? If fins missing this could have lodged a piece within the cooling system and cut off circulation

Re: 87 johnson gt 150 blown up

No idea how old water pump that was in there was fins were all there seemed stiff but other than that ok.

Re: 87 johnson gt 150 blown up

Was the fuel mixed properly?

Re: 87 johnson gt 150 blown up

the vro system is still hooked up on this unit so ?. I assume it was working because when I removed the heads I only had damage to on side of the motor V6. I dont suspect a fuel mixture issue because the carbs share common float bowls. So why just one side? I want to know why before rebuild. The damage looks like a lean burn. pitted pistons from detonation and scoring on cyl walls cant feel any etching.

Re: 87 johnson gt 150 blown up

Did you do a compression test before the damage?

Air leak on one side can do it.
Or restricted water flow.
What sparkplugs?

Re: 87 johnson gt 150 blown up

No not before the damage. ran great all last year except the idle issue. which the carb cleaning and rebuild seemed to take care of it. So didnt see the need to comp check. air crossed my mind. spark plugs are champ ql77jc4.
Thinking I messed up with water pump install also crossed my mind but seemed to work better then before at home on the hose.

Re: 87 johnson gt 150 blown up

Yeh the champion plugs are good, there is talk by techs about japonese sparkplugs causing overheating.

What about water passages in the block, find any junk like waterpump vanes or water deflectors out of place? sand?

otherwise exhaust gaskets and manifold gaskets are other sources of compression leaks.

I think a temp guage in the head would give me future peace of mind.

Re: 87 johnson gt 150 blown up

Only pulled of the heads, didnt dig any further yet the water passages looked ok there. Head gaskets looked ok. Guess if I end up doing a rebuild everything will be dissasembled and cleaned and all gaskets and seals replaced. So that should take care of it. Should one have motors leak tested and comp checked every year. found out through this sight about waterpump replacment intervels. Cleaning carbs I've been doing for years. Leanrned that on the hard way too. Tried to do it right and still blew it up. Thankyou.

Re: 87 johnson gt 150 blown up

If the overheat was limited to one head, it could be related to a problem inside the thermostat housing. If the stat is stuck closed, it will overheat at idle. If the nylon pressure relief valve was ever seriously overheated, it can deform and not function properly, causing cooling issues when on plane. You can have a good telltale and still have overheat issues on one/both heads. You may want to test the overheat senders in each head to see if they activate the warning horn in the control box. Also, check the rubber water diverters in the side of the block with the overheat. Make sure all three are there and that they are in proper position. If the water does not circulate properly around the cylinders, it can cause individual cylinder overheating without a warning horn going off.