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H27 1979 Yanmar SB8 cooling system issues

Nov 5, 2019
Hunter 27 Jones Creek
Hi new to the forum and also Sea Plum, a 1979 Hunter 27 Cherubini. I searched the forums and found some great information that ultimately contributed to me getting the little YANMAR SB8 roaring back to life in front of the previous owner and which resulted in me completing my purchase yesterday. She was loved by her previous owner however sat on the hard for 3 years after he retired and moved away and was no longer able to use her as he thought he once would.

Hoping someone can point me in the right direction if my below assumptions or plan is not correct. I have searched multiple forums, ran internet searches and read the YANMAR service manual. If anyone has any recommendation or advisory against my methodology any and all encouragement and criticism is welcomed. I look forward to be a contributing member of the board. Thank you in advance.

I only ran her for a 2 minutes and shut her down due to no cooling water being supplied through the engine. I will complete the regular maintenance, ie ,Oil change/filter/filter screen clean, alternator belt, thermostat, fuel filter, racor filter, charge or replace batteries and drain fuel tank in the near future. Main issues I see with the particular YANMAR SB8 are related to cooling and exhaust issues besides already mentioned maintenance above Below are the 3 main issues.
#1The water isn't pumping through block.
#2Calcification in block
#3Rusted and compromised exhaust elbow ie hole

I inspected and completed or planning on the following on her however all 3 of my issues cause me concern about water backflowing into exhaust ports. Is the issue of backflow only at the mixing exhaust tube? If I disconnect the hose from the cooling water outlet to the water mixing elbow do I bypass the chance of water entering ports? After reviewing the schematics of the piping diagram in the service manual this looks like the only interface between H2O and ports, can anyone confirm before I flood my engine? , welcome any input.

#1 Yesterday, I inspected impeller it seemed flexible and intact. I cleaned off old filter material and installed new gasket. I detached intake line from sea cock and cleaned inline sea strainer filter and re-filled strainer with water and thought I primed hose line with enough fresh water. I also ensured all hose clamps were tight and placed hose in 5 gallon bucket of water. Still no uptake of water, took off impeller cover off again and turned her over to be positive it was spinning and impeller didn't detach from hub.

Sounds like air leak or worn impeller to me, any other ideas? If water pump was bad would impeller still spin? I disconnected hose immediately after cooling water pump should water that I primed with just drip or flow past impeller maybe impeller is worn even though still intact and flexible. What product should be applied to new paper filters, is it a specific type of grease? I couldn't pull out old impeller out I tried but was getting to dark. I saw there are expensive pullers but also saw long nose pliers with a 90 degree bend should work. Going with pliers before buying an actual puller. There is no set screw correct?

My Next step this coming Friday was to install new impeller, gasket and replace water supply hoses because they are severely stiff. I had the five gallon bucket originally on the galley floor when I started engine, should I set the 5 gallon bucket on the sink so I can start a siphon to confirm its primed. Cold weather is coming, I noticed a drain cock near bottom of block if I can't get winterizing fluid in there can I just drain the block for the winter and possibly blow air through cooling system to expel any water if I disconnect cooling water tube from the mixing elbow.

#2There is severe calcification and a corroded engine anode. I read about Muriatic acid wash I plan to do a 15 to 1 acid rinse instead of buying a commercially available product.

My plan was to use PPE, proper acid handling precautions, remove thermostat and disconnect rubber hose at cooling water outlet from water mixing elbow and connect it to a bucket or a longer hose that exits boat. I was going to utilize a drill pump to get solution into block, let sit 15mins and flush and capture wash in 5 gallon bucket from the mixing tube hose add baking soda to acidic water to neutralize and dispose. Possibly repeat procedure if need be. Replace anode and thermostat after wash. Does this sound like the right plan, specifically with disconnecting hose from water mixing elbow?

#3 Replace exhaust elbow due to hole and visible smoke in cabin. My local Yanmar dealer told me the part is no longer available I found stainless steel elbow online and ordered that is aftermarket for the SB8. Seems simple enough as long as elbow unscrews from block. I was going to hit with PB rust blaster until part comes in? Any recommendations or pitfalls to avoid when replacing elbow that others experienced. I plan was going to use a big pipe wrench, to heat or not to heat. Should I replace exhaust hose, I am sure its original doesn't appear to be leaking however usual dry-rot cracks visible from a 40 year old exhaust pipe? When I put the new water mixing elbow in should I use any anti-seize compound or pipe dope ectera on threads.

I hope to complete this project on Friday if parts come in and detail what I did with pictures and verbiage so I can post and provide to other members.

Thanks for any and all assistance.
Nov 6, 2006
Hunter 34 Mandeville Louisiana
Welcome aboard, Chris!
Your approach sounds fine.. Is there a raw water strainer before the pump? sometimes a very small air leak will cause the pump to not work.. Measure the width of the impeller and make sure it is exactly the same dimension as the depth of the case.. the tips have to be in contact with the ID of the case, but the sides of the impeller also have to seal against the bottom of the case and the pump cavity cover.. The gasket should be a very thin paper and not cardboard or cork.. There is no set screw holding the impeller on the shaft. A light coat of silicone grease on the impeller and in the case will help it get primed without toasting the new impeller. Sometimes the cover plate can get some wear in it that allows an internal leak.. You can polish the grooves away or if the cover is symmetrical, reverse the cover to put a new surface against the impeller..
The exhaust should go from the elbow to a "lift muffler" then to the hull outlet. If the engine is cranked without starting for a fwile, water can fill the muffler and back up into the engine.. The water outlet hose to the elbow should rise to higher than the elbow (above the outside waterline) with a "vented loop" at the top which prevents water from siphoning and filling the muffler and back flooding the engine ..
I am not familiar with that exact engine installation, but in general these are probably useful observations.. Picture is of a vented loop (check valve in the top to let air in but not water out) On the back wall, the little stainless semicircle is the vented loop.. Outside waterline is about at the lower line on the coolant catch tank.
Engine Front.JPG
May 27, 2004
Hunter 30_74-83 Ponce Inlet FL
The YSB 8 and the YSB 12 are one cylinder, flat (pancake) engines and usually sit in the Hunters below the muffler and the exit thru hull. The thru hull is above the waterline, except when motoring. Hunter did not put a vented loop on any H 27 or H 30 of that vintage that I have seen.
Might be a good idea though.
And Kloudie, that is a very clean installation in your pic.
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Nov 5, 2019
Hunter 27 Jones Creek
Thank you for the quick response hope to report back on Friday with some pics and detail the process. The service manual is really useful. Looks like the cooling water goes straight from thermostat to mixing elbow. If I disconnect the hose from the cooling water outlet to the exhaust mixing elbow I should not have any issue on water entering the cylinder.


Mar 23, 2017
Hunter 30 Snug Harbor, Lake Champlain
I have the YSB 12. I do not have to prime the water pump. When I got mine, it had been sitting for something like 16 years. So sitting for 3 years doesn't sound long to me... ;)

Make sure there is no restriction in the intake hose. My motor does not have a filter on the incoming water to the pump. Only the intake grid on the through hull on the outside of the hull. When I winterize, or run my motor on the hard, I just take the line to the pump off the through hull and drop it into a shallow pan filled with the fluid (antifreeze or water) that I'll be using. The pump pushes liquid through the motor with no problem. I think I'd be tempted to take the intake hose off the thru hull and drop it in a pan of water, disconnect the hose on the output side of the pump and see if it's pushing water through. That would isolate if it's pump side or exhaust side. Not sure how easy this is to do as I've never tried it..

Apr 22, 2011
Hunter 27 Pecan Grove, Oriental, NC
I'll second dlj's suggestion to isolate the pump from the engine and see if there is a good stream of water coming from it. If water shoots out, then you can chase down where the blockage might be.
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Nov 5, 2019
Hunter 27 Jones Creek
Thank you everyone I was able to push through some antifreeze for the time being. However when removing the mixing elbow which was already compromised broke while turning with pipe wrench
Nov 5, 2019
Hunter 27 Jones Creek
I have tried multiple sprays of liquid wrench and now some heat still will not budge should I remove the square squats silencer manifold that the mixing tube and the thermostat housing is screwed into and work on a bench or maybe see if I can crack the rest of the mixing tube.