1984 150 compression

[grampa's toy]

I have a 84 150, I bought a rebuilt 200 hp powerhead and had it installed. It has never run right. I have replaced the coils, plug wires, rectifer-voltage regulator, rebuilt the carbs, and run seafoam through it. It has no power to get on plane, once up it runs ok but it wants to foul the plugs. I was told that some places sell rebuilt engs. with missmatched parts. my compression is 85 lbs. on every cyl. I was told I could have the wrong heads mine are #'s 328 324 & 328 325 . Are these the wrong heads, can I mill them, and if so how much?

 

[DHPMARINE]

Re: 1984 150 compression

I'd look into the bore size of the carbs (inside dia of the carb bodies) Also look at the exhaust parts on the back of the motor,between the heads,and the exhaust parts below the powerhead to the lower unit.

DHP

 

[Dhadley]

Re: 1984 150 compression

The heads are Ok for now. What's the total timing set at?

 

[grampa's toy]

Re: 1984 150 compression

the timming is set at 2 deg. less than the factory spec 30 deg. I think per the mechanic he said the motor would last longer. carbs and intake are off of the 150 and are stock. the paperwork said something about changing the rear exaust cover because of the power tilt. I think the low compression is causing this thing to short out the plugs under a load the boat is heavy

 

[emdsapmgr]

Re: 1984 150 compression

No doubt your block exhaust cover had to be changed to accomodate your power trim box-not a big deal. If you truly have a 200, the block is a high-port engine. My 200 was always a dog out of the hole, but ran great on top end. If you have a 150XP/GT, the carbs should already be 1 5/16" throats. If you have a base 150, you need to change to the 1 5/16" carbs. Also, change the heads. The combination of a high port block and low compression heads adds up to your problem. You need to get your compression up. If it is a 149 cubic inch 200, look for a set of 323456/457 heads (108-112 lbs compression), or a set of 323842/873 (115-120 lbs compression). Hole shot will never be outstanding, but you can improve it significantly. You need to be running 93 octane with the timing set at 28.

 

[Dhadley]

Re: 1984 150 compression

We've never had any issues running the combo you have. The only reason to change the back plate, which is only the water passage cover, is so you'll have the mounting bosses for the power trim & tilt. That box can be mounted anywhere.

Now, when you installed the 150 carbs, were any of the jets changed? If the motor is running at 6200 or less the jets can stay the same as the carbs were produced with. Make sure they're clean. I'd suggest starting with a good carb cleaning and new kits just so we know where we are.

As for the plugs, which ones are you using? And when you say they foul out, what do you meam? Do they start causing the motor to miss?

 

[grampa's toy]

Re: 1984 150 compression

the jets were changed (drilled out) but it ran terrible so we put stock 150 jets back in. the motor won't go over 4200 rpm, it seems to lean out. carbs are 1 5/16 fresh with about 45 min time on them. when I take off I drop cyls, 2 or 3 when the boat starts to come on plane they kick back in one at a time, I start backing off on the throttle and I pick rpm once I'm on plane I can cruise at 3600 all day the plugs are ql77jc4 if I change them every hour I have a better holeshot however if I check them when I'm dropping cyls. they are black when I'm on plane they look decent

 

[grampa's toy]

Re: 1984 150 compression

also can I mill these heads to get more compression?

 

[emdsapmgr]

Re: 1984 150 compression

Maybe. Some crossflow heads (329772/773) in later years were redesigned for lower compression. Engineering would take an earlier head (456/457) and add extra stock to the head face, This would effectively space the combustion chamber away from the block-thereby effectively lowering compression. This extra stock can be machined off. Check your heads for this condition.

 

[grampa's toy]

Re: 1984 150 compression

I milled 60 thousands off the heads and I now have approx. 95 psi. cyl. pressure is this enough ? should I go for more? does anybody know what this motor had stock?

 

[emdsapmgr]

Re: 1984 150 compression

I believe 90-95 would be a "normal" stock compression, with stock heads. Are you taking the compression readings with all the plugs out of the heads and a strong battery? Crossflow heads are not the same as the looper heads. As you cut more, the relationship between the piston dome and the head changes. You need to be careful with the piston to head clearances and airflow. I would not cut them anymore. If you still want more compression, look for another set of heads.

 

[grampa's toy]

Re: 1984 150 compression

I took compression readings with all plugs out 2 batteries, I didn't want to spend 350 $ for a used set of heads off a salt water motor. I needed to hear 90 to 95 is normal, I will update as soon as I get it back on the water. Many thanks for your expertize.