Another comprssion ?

[philbullet]

1996 mercruiser 4.3 v6 thunder bolt 4,4 barrel carb. just rebuilt.
Motor starts good ,runs good at idel and fast idel in gear.But cant get on step,kind of like no power.
compression 125 psi on 2 back cyl. 115 to 120 on the rest,just change plugs also.After motor warms up oil preasure drops reel low at idel.
Thinking i may need a rebuild which isnt a problem for me,but or these compression reading kind of low?
thanks

 

[Scott Danforth]

yes, the compression readings are low. do a wet compression check to see if your numbers come up.

if your oil pressure is dropping, that is another issue all together. you should see about 35 psi or more at idle

 

[Volphin]

Rings are probably are not seated yet & valves won't seal at their seats well until run in. That's why the numbers are low on a fresh build. Double check your firing order and that you stabbed the distributor correctly.

 

[Volphin]

Oh, and confirm the oil pressure with a mechanical gauge. DO NOT trust the dash. You only need about 7 psi @ idle.

 

[philbullet]

Rings are probably are not seated yet & valves won't seal at their seats well until run in. That's why the numbers are low on a fresh build. Double check your firing order and that you stabbed the distributor correctly.

The motor has been sitting up.I dont know for how long but got it running and and trying to get bugs out before i put it in another boat.
I bought boat for motor and out drive,etc. too replace a 470 in my boat.Only paid 400.00 complete.
My brother is a mechanic so a rebuild isnt a isue.
Did a timing check and checked to see if timing is advancing and all was good.
We may do a leak down test today before pulling it out.
Thought about dumping some seafom in cly's. but not sure if that would help with comp. at 125 and down.
I still need to set motor in place to make new motor mounts for my boat,Doing a (done) transome replacment at moment.

 

[philbullet]

Sorry about first post,i ment carb. rebuilt.When i got boat it fired up but was dumping fuel into intake,i guess trash had float stuck.He rebuilt carb. and ran good, but in water wouldn't get up on step,2500 rpm she just bogg's down.Also i had by pass fuel tank and blow lines out,chang fuel filter so iam all good on fuel and eletric fuel pump.Running it on a seperate fuel tank because fuel looked dark in boat tank.
18' fish and ski boat,I think it call a Mariner. not sure

 

[alldodge]

Sorry about first post,i ment carb. rebuilt.When i got boat it fired up but was dumping fuel into intake,i guess trash had float stuck.He rebuilt carb. and ran good, but in water wouldn't get up on step,2500 rpm she just bogg's down.Also i had by pass fuel tank and blow lines out,chang fuel filter so iam all good on fuel and eletric fuel pump.Running it on a seperate fuel tank because fuel looked dark in boat tank.
18' fish and ski boat,I think it call a Mariner. not sure

Forget the snake oil (seafoam) treatment. From what I'm reading you know how to test and set an engine up. With that it lacks sufficient power to achieve full power. Sounds like your brother will be rebuilding the engine

 

[Bondo]

Forget the snake oil (seafoam) treatment. From what I'm reading you know how to test and set an engine up. With that it lacks sufficient power to achieve full power. Sounds like your brother will be rebuilding the engine

Ayuh,..... Marginal compression, but I'm thinkin' the carb rebuild didn't fix all the carb/ fuel delivery problems,....

 

[philbullet]

Yep rebuild.Leakdown test went reel good,stright throught the crank case, LOL 2 cly, where more than 50%. so we stopped.
I done removed motor and about to size up motor mounts in my other boat.
My brother already has some heads ready to go for a 4.3 so ill tare it down and se what she looks like on the inside.Look's like ill need some riser's also.

 

[philbullet]

We pulled motor apart and it was alreadt bored out 0.40 over and some pits in cyl. ,also had trouble getting exhuast manifolds off.Rise's didnt look good neither so looks like new riser,mantifold and block.He has another 4.3 block but may look into a short block ready to go,We have a set of head ready.