Can I run a Yamaha F90 with no lower unit installed ?

[RedwoodInfrared]

I know this looks like a really stupid question but I just bought a boat with a F90LRD with the lower unit missing.

The seller says it ran fine but had a problem with the lower unit and a shady mechanic he hired removed it and disappeared with it.

The engine has a garden hose fitting so supposedly you can connect a hose directly to flush it without using the old rabbit ears. I have no idea whether it's safe to try starting the engine with water running through that system if there's no impeller to actually circulate it through the head etc.

It would be really nice to verify I've got a serviceable engine before investing in a replacement lower unit. If I can't get that assurance I'll probably just replace the engine and part this one out on eBay.

Thanks in advance for any replies.

[fairdeal]

NO! the driveshaft is what turns the oil pump
without an LU installed,
you will have no oil pressure and seize the engine.

Some Yamaha 4 strokes have a different way to drive the pump
and its fine to run with out the LU
but not F90 (and some others)

[RedwoodInfrared]

Thanks very much for the quick reply.

Since you seem to know this motor pretty well, is there any other way you can recommend that I can assess whether I've got something worth investing in or should I just go ahead and part it out. I have another motor available (a Honda BF90) but I've never done a re-power and not looking forward to it if it's not necessary.

[99yam40]

heck, I would find someone with a f90 and ask to borrow the lower to see if your motor was a good one

[RedwoodInfrared]

heck, I would find someone with a f90 and ask to borrow the lower to see if your motor was a good one

I thought of that but I don't know that many boaters around here so not likely I'll get that lucky. Thanks again for your help.

[TownsendsFJR1300]

A leak down test will give you a very good idea on the health of the internals..


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[99yam40]

I thought of that but I don't know that many boaters around here so not likely I'll get that lucky. Thanks again for your help.

where are you are located?

time to make some boating friends
or find a marine salvage place to see it they have a used lower unit

[RedwoodInfrared]

where are you are located?

time to make some boating friends
or find a marine salvage place to see it they have a used lower unit

Eureka, CA. Very remote and a pretty small boating community even though we have a deep water port. I'm active on the local fishing forum, but no marine salvage within a couple hundred miles of here. No dealers really even. Only a couple of boat repair shops and they're backed up for weeks. I've put out some feelers but not very optimistic I'll be able to borrow or rent such a part. I'll look into the leakdown test.

[RedwoodInfrared]

A leak down test will give you a very good idea on the health of the internals..


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Took a few days to get my hands on the tool (did I mention I'm in a remote area? ) but I did the leak down test yesterday and it seems all good. Less than 10% in three cylinders, about 15% in #2 but seems to be a consensus that's pretty good, especially a dead cold motor that hasn't been turned over for quite some time. Everything else about the power plant looks good so I'm on the hunt for a lower unit. There was a typo in my original post, the model is 2005 F90TLRD.

[RedwoodInfrared]

Late follow up on this thread but just wanted to throw out a thank you for this suggestion. After the leakdown test I went ahead and purchased a newly serviced lower unit from a reputable dealer on Ebay for $1000. Installation was easy and the motor fired right up. Now that I've had it in service for just over a year this boat has provided a lot of meals for a lot of people - rock fish, ling cod, halibut, even one successful trip 40 miles offshore for albacore. Only problems to date have been a stuck tilt/trim motor, which was a relatively easy fix to clean the brushes, but now I'm dealing with an oil leak that's probably due to a bad oil pump seal and/or wear on the drive shaft, hence my return to this forum. Seems to be a common problem with the F115 and some larger models easily fixed with a Speedi Sleeve but not finding any info regarding that on my model so about to post a question under a new thread.

In any event, thanks again for pointing me to the leak down test - in my experience it's not every tech question that is so satisfactorily resolved in these forums.

A leak down test will give you a very good idea on the health of the internals..


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