1991 Mercury 150 Blackmax Poppet Valve

[bmatlock01]

• I bought a boat a little less than a year ago with a Blackmax 150. The oil injection has been removed so I premix at 50:1, I do a lot of idling and it has had a problem with fouling the plugs ever since I got it. I have cleaned carbs, and check all recirc lines, and rebuilt the fuel pump. After doing some research online I ran across a thread saying if someone had removed the thermostats the engine would never warm up to proper operating temperature and foul the plugs. I pulled the housings when I got home and sure enough, the thermostats are missing. To get to my question, if the thermostats were removed, could the poppet valve have been removed also, or would that cause it to overhead while idling? Is there a sign that the poppet valve is not in place other than removing the housing? Thanks in advance.​

 

[wired247]

If the poppet were removed you would have close to zero water pressure At anything less than WOT and you'd notice it pretty quick.

 

[bmatlock01]

If the poppet were removed you would have close to zero water pressure At anything less than WOT and you'd notice it pretty quick.

Thanks for the reply. I ordered some thermostats and should have them by this weekend. Do you also agree that the missing thermostats could be causing it to foul the plugs? I love this engine when it is running good, but if I don't change the plugs after every 2 or 3 tanks of gas it is very sluggish to get on plane and only turns about 4000 rpms max.

 

[wired247]

Theres a thermostatic air valve on the starboard cylinder head that causes the mixture to richen up when the cylinders are cold. If they never warm up its running rich all the time ( or at least when its not under much load ) so yes, it could cause the plugs to foul. I really don't know why people pull the thermostats on fishing motors. I think sometimes they just get rotten and fall apart and guys pull them out and never replace them. Sure doesnt help the motor much.

 

[bmatlock01]

I got the stats and put them in, all I can say is I am very hopeful that this was the problem. After I started the engine it took about 2 minutes for it to get up to temperature and the thermostats opened, around 140 degrees, about the same time that the tell tale started peeing, the engines rpms went up from 1000 to about 1100, the idle smoothed out, and it sounded better than it has since I got it. I won't know for sure if that fixed it until I run a couple tanks of gas through it but it looks very promising!

 

[Texasmark]

I got the stats and put them in, all I can say is I am very hopeful that this was the problem. After I started the engine it took about 2 minutes for it to get up to temperature and the thermostats opened, around 140 degrees, about the same time that the tell tale started peeing, the engines rpms went up from 1000 to about 1100, the idle smoothed out, and it sounded better than it has since I got it. I won't know for sure if that fixed it until I run a couple tanks of gas through it but it looks very promising!

If plumbed like my son's 150 of that vintage, the tell tale is plumbed off the stat output meaning the stat has to open to get any pee. Also the outlet of the crankcase was a hose barb at the top rear of the block meaning that the block had to fill with water before you would have flow out the hose and down below to the pop off. Water would flow out the open holes where the stats belonged and would never completely fill the block nor use the pop off. Additionally the upper right cylinder would be partly out of cooling water. Whether this matters to that cylinder or not I have no idea but would expect it surely couldn't do it any good.

Mark