Mercruiser Vibration Question

only_1

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I have a 1995 Larson 195LXI with a 5.0LX mercruiser. The engine has been changed out to a 355. I am getting a vibration at idle and through the rpm band. I get the vibration in gear and in neutral. It isn't so much of an engine miss. I am looking for thoughts and experiences that help me on where to look first.
It still has the little 6" harmonic balancer, not sure if that should have been swapped to an 8".
The u-joints in the outdrive shaft seemed a bit loose when I reassembled things.
My thoughts are: Harmonic Balancer too small for new engine, rear engine coupling got bent during removal or install, engine alignment shifted, bad/loose u-joints for the drive shaft, and verify that its not a miss in the engine itself (Compression test etc).
Any other suggestions on things to look at?

I do have a second question also. When I put a new fuel filter in, the pump did not have enough capability to prime the filter and start the engine. I had to take the filter back off, fill it with fuel, then reinstall it. Is this normal, or is the pump weak?

Thank you!
 

alldodge

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Do you have the vibration with the drive removed?

Assuming an electric fuel pump, and if so then they do not have much suction like a mechanical pump. So yes the filter needs to be filled prior
 

only_1

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I have not run the engine yet with the drive removed. This is something I am certainly game to try.

It has the mechanical fuel pump. No electric pump on it.
 

alldodge

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Assuming you ave an Alpha drive, remove it and connect a hose to the thermostat housing where the line comes from the drive. If its a Bravo drive, connect hose to the inlet side of the belt driven water pump

With a mechanical pump, you either have a weak pump or the anti-siphon valve may be sticking. The valve is the barbed fitting on top the fuel tank that the fuel line connects to. There is a spring a ball inside and can get hung

anti siphon valve.png
 

only_1

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Sorry, it is an Alpha outdrive. I will give this a try.
Thanks for your help.
 

only_1

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I should add, the only other weird symptom I have is that the engine will stall. The stall happens only after holding WOT for back to back starts. I can cruise around the lake under load with no issue if at low speed. After the stall it doesn't want to start back up right away. If I hold the throttle down when starting it, it will fire up, but won't hold idle after that, it just stalls. Hard to get going because have to drop it down to idle to put it into gear. Would the fuel pump or the anti-siphon cause this?
 

Scott Danforth

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I have a 1995 Larson 195LXI with a 5.0LX mercruiser. The engine has been changed out to a 355. I am getting a vibration at idle and through the rpm band. I get the vibration in gear and in neutral. It isn't so much of an engine miss. I am looking for thoughts and experiences that help me on where to look first.
It still has the little 6" harmonic balancer, not sure if that should have been swapped to an 8".
The u-joints in the outdrive shaft seemed a bit loose when I reassembled things.
My thoughts are: Harmonic Balancer too small for new engine, rear engine coupling got bent during removal or install, engine alignment shifted, bad/loose u-joints for the drive shaft, and verify that its not a miss in the engine itself (Compression test etc).
Any other suggestions on things to look at?

I do have a second question also. When I put a new fuel filter in, the pump did not have enough capability to prime the filter and start the engine. I had to take the filter back off, fill it with fuel, then reinstall it. Is this normal, or is the pump weak?

Thank you!

What year vintage is the 355? 1-piece RMS or 2-piece RMS?

when you had the motor built, how was it balanced?

the worn u-joints are not helping
 

Scott Danforth

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I should add, the only other weird symptom I have is that the engine will stall. The stall happens only after holding WOT for back to back starts. I can cruise around the lake under load with no issue if at low speed. After the stall it doesn't want to start back up right away. If I hold the throttle down when starting it, it will fire up, but won't hold idle after that, it just stalls. Hard to get going because have to drop it down to idle to put it into gear. Would the fuel pump or the anti-siphon cause this?

which carb? was the carb gone thru when you did the motor? did you adjust the choke?

are you holding WOT under load or in neutral? dont do it in neutral
 

alldodge

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Being an LX it should be a 4bbl Rochester or Weber

Running at WOT you would be using a lot of gas, and this may be a possibility, but could also be flooding. Start by determining where the vibration is, motor of drive. Once this is determined, need to measure fuel pressure (3 to 7 psi).

As Scott mentioned, if carb has not been cleaned, it would be a good idea
 

QBhoy

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What’s a 355 ? Like a modified bored 350 or similar ? Just for my own UK based curiosity, haha.

Does the vibration get worse when you steer or tilt the drive ? Be a sure sign it’s the UJ then.
 

alldodge

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Standard 350 is 4 inch bore x 3.48 stroke - a 355 is a block bored 0.030 over

Cu in = radius square x pi x stroke x cylinders

4 in bore = 2x2=4 x3.14 = 12.56 x 3.48 = 43.7088 x 8 = 349.67 cu in

Bore block 0.030 over

4.030 in bore = 2.015x2.015=4.060 x3.14 = 12.749 x 3.48 = 44.367 x 8 = 3.54.9 cu in
 

QBhoy

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Standard 350 is 4 inch bore x 3.48 stroke - a 355 is a block bored 0.030 over

Cu in = radius square x pi x stroke x cylinders

4 in bore = 2x2=4 x3.14 = 12.56 x 3.48 = 43.7088 x 8 = 349.67 cu in

Bore block 0.030 over

4.030 in bore = 2.015x2.015=4.060 x3.14 = 12.749 x 3.48 = 44.367 x 8 = 3.54.9 cu in

So assuming this would be because a 350 went wrong and had a rebuild with oversizing ?
 

tank1949

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I have a 1995 Larson 195LXI with a 5.0LX mercruiser. The engine has been changed out to a 355. I am getting a vibration at idle and through the rpm band. I get the vibration in gear and in neutral. It isn't so much of an engine miss. I am looking for thoughts and experiences that help me on where to look first.
It still has the little 6" harmonic balancer, not sure if that should have been swapped to an 8".
The u-joints in the outdrive shaft seemed a bit loose when I reassembled things.
My thoughts are: Harmonic Balancer too small for new engine, rear engine coupling got bent during removal or install, engine alignment shifted, bad/loose u-joints for the drive shaft, and verify that its not a miss in the engine itself (Compression test etc).
Any other suggestions on things to look at?

I do have a second question also. When I put a new fuel filter in, the pump did not have enough capability to prime the filter and start the engine. I had to take the filter back off, fill it with fuel, then reinstall it. Is this normal, or is the pump weak?

Thank you!

I am not familiar with a 355? U mean 350? U first should examine plugs with rubber gloves and see if u or former owner reattached wires correctly. Then, check to see if wires are not broken internally. A bad or incorrectly attached wire will cause vibrations. Pull at distributor and watch for tack to drop RPMs. Run compression test to see if 355 has good compression on all cyl. Examine all motor mounts to make sure they are tight and no rotten wood/fiberglass. TRUST ME! Coupler bolts may not have been tighten or worked loose. What about MC motor bellhousing bolts? Are they tight? I hope u or former owner used locktight on flywheel and couple bolts. Does Harmonic balancer wobble?
Lastly, pull OD and test alignment with tool.
 

Scott Danforth

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I am not familiar with a 355? U mean 350? U first should examine plugs with rubber gloves and see if u or former owner reattached wires correctly. Then, check to see if wires are not broken internally. A bad or incorrectly attached wire will cause vibrations. Pull at distributor and watch for tack to drop RPMs. Run compression test to see if 355 has good compression on all cyl. Examine all motor mounts to make sure they are tight and no rotten wood/fiberglass. TRUST ME! Coupler bolts may not have been tighten or worked loose. What about MC motor bellhousing bolts? Are they tight? I hope u or former owner used locktight on flywheel and couple bolts. Does Harmonic balancer wobble?
Lastly, pull OD and test alignment with tool.

No, the OP means 355 (rebuilt 350 bored .030 over) as in a rebuilt motor that the OP just put in

that also means that anything the former owner did no longer matters

motor mounts would not impart a vibration throughout the RPM range

while you bring up a good point that it could be a dead cylinder due to a plug wire, most likely its a mix of vintages on the balancer, or something a bit more serious, like an improper balance job or something in the driveline.
 

alldodge

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So assuming this would be because a 350 went wrong and had a rebuild with oversizing ?

Wrong, wore out, wanted another motor or other and just standard rebuild

Folks keep doin this, and its mainly because of marketing. It would make it easier just to say a 350
 

Scott Danforth

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agreed, it would be easier, however the 355 moniker came about in the late 60's when people would brag about what they did to their motors.....and in many people, it stuck
 

tank1949

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No, the OP means 355 (rebuilt 350 bored .030 over) as in a rebuilt motor that the OP just put in

that also means that anything the former owner did no longer matters

motor mounts would not impart a vibration throughout the RPM range

while you bring up a good point that it could be a dead cylinder due to a plug wire, most likely its a mix of vintages on the balancer, or something a bit more serious, like an improper balance job or something in the driveline.

I guarantee a faulty or not tightened mount/bolt would cause vibrations: trust me. I have screwed up more than once by being in a hurry and forgetting. When RPMs or certain harmonics are reached, there is more vibrations. Also, If my memory serves me correctly, flywheels have corresponding sized HBs, but I am not 100% sure on this.
 
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