Yanmar 1GM - low compression.

[Wandering Star]

Right, I’ve just bought myself a Sadler 25 with a dicky engine. She’s on the hard inHolyhead harbour.

My understanding is the engine has low compression but does turn over.

What is the most likely cause of low compression? is this a common fault of this engine? If a repair is simply the cylinder head gasket, is this a DIY repair?

Can anyone recommend an engineer in the Holyhead area?

Any other suggestions? I‘d like to move quickly on the repair so I can sail her round to Lymington this season and any advice would be welcomed. Don’t tell me to steer clear, it’s a done deal, I just want to fix the engine!!

Thanks.

 

[RichardS]

When you say "turn over" do you mean turn over on the starter motor or actually start and run?

Unless it is something simple like a sticking valve, incorrect tappet clearances or stuck decompressor (if fitted), you will need to remove the head as it might be a head gasket, rings, bore or valve problem. However, I believe that this engine does have an issue with the casting cracking/ failing around the exhaust valve seat so it might well need some significant head welding or even a replacement head if you can find one. I think that is becoming rather difficult these days.

Richard

 

[Bilgediver]

You could be lucky and it is just the valve seats needing the correct treatment. New seats are easily fitted by specialists but go for the ones who DO NOT have the word Marine in their Letterhead. Motor Cycles is very good The seats need to be machined and lapped to a differential angle for best results. Grinding a wide seat is not good.

Try a simple test by squirting a small amount of engine oil directly into the air intake while turning the engine over decompressed the try turning it over compressed and see if there is a difference If there is it is unlikely it is a head gasket so either rings and or valve seats.

 

[Tranona]

Three main causes of low compression. First (and least obvious) bent con rod usually caused by water getting into the cylinder. Second and maybe related corrosion of the head, sometimes bad enough to destroy the precombustion chamber. Third poorly seated valves, also sometimes caused by water getting in.

Water ingress is from two ways, either syphoned back in because no anti syphon valve fitted - most likely to cause bent con rod, or being sucked back in following failure through corrosion of the inner tube in the exhaust elbow. This can spray water back into the combustion chamber causing corrosion inside.

Worth getting the official Yanmar manual as among other things it has instructions for checking whether the rod is bent without removing the head. However removing the head is fairly easy and you can still check the rod by measuring the piston height - dimension in the manual. Obviously you can then check the condition of the head and exhaust elbow, but be aware that a badly corroded head might well be a write off and a new one lots of beer tokens. If the exhaust elbow needs replacement - check by turning upside down, filling the gap between the inner and outer tube with water with the water injection spigot blocked - replace with an after market stainless one which are made of better material than the Yanmar one.

 

[Topcat47]

Check the tappets first. If the gap has closed up, the valves won't seat properly. You really ought to at least whip the head off and lap the valves into their seats before doing anything else. It is possible to have a damaged head refurbished, a mate had his done by an engineer in Scotland. The guy welded up after water penetration between the waterways and the exhaust port. The engine's still good after four years. It was much cheaper than a new head. Valve seats and the valves themselves can be re-cut. With the head off it's easy to check if the con-rod is bent. I've had the "bent con-rod" diagnosis from two separate marine engineers on my engine on two different occasions. Each time it was a false alarm. Apparently is is quite common though. I do nearly all my own "spannering" and have been maintaining my own motorbikes for over half a century, but I don't have a lathe or mill so some things still need to be outsourced.

 

[Slowboat35]

In order of easiness the sequence to approach this is;
1) Engine in boat; remove head, check valve seats. Regardless of their condition get them re-lapped (easy to diy - see youtube tutorials). If valve seats are very bad suspect those. You will need a new cylinder head gasket and valve stem seals. parts4engines sell these. Avoid paying for Yanmar branded parts wherever you can as they £££Cost!

2) while head is off check conrod for straightness (requires straight edge and feeler gauges). Tranona's suggestion that this can be done with head in place is correct, but in my experience not at all easy - and as you'll want to have the head off to check the valves anyway why go to the trouble?

3) While head is off
a)check bore for wear. Easily detected by feel. If the 'lip' at the top of the bore is substantial you'll suspect a
rebore may be necessary. That is an engine-out job.
b) Any appreciable scoring of the bore detectable by feel ditto. This will also require a rebore, new oversize piston and rings.
c) Check bore for glazing. Rings do not seat well on a glazed bore. If the bore is mirror-shiny and lacking in fine machining cross-hatching all over it'll need glaze-busting on the workbench.

Again Tranona advises well re workshop manual which is both excellent and probably essential for an amateur mechanic. You may manage to borrow one or scrounge a scan of the section you require.

4) Engine out with a 1GM is perfecly doable diy, thereis a thread about it running here at present.

As soon as the engine comes out bills are likely to escalate big time. As an older insatallation costs can snowball rapidly (engine mounts will likely be needed at well over a £100 apiece...)

Good idea to get someone knowlegeable and experienced to take a look first.

 

[Wandering Star]

This is all very useful stuf - in typical style, I bought the boat sight unseen for a very cheap price. The chap selling her seems very honest and I bought her if full knowledge the engine was dicky. I can’t say whether it’s running or not as I’ll see yet boat for the first time on Monday! She’s a lateish model and her interior & hull look first class, the faulty engine was replacement of the original done in 2002 I believe and the photo shows it’s osmetically quite clean and has probably been maintained properly.

Stuart Marine have inspected the engine apparently, I don’t know what they discovered but they quoted £300 for a days labour (plus parts which would seem to indicate a simple valve lapping exercise - or something else not too serious and I’ll be happy to pass the job to them If that’s what it is. but anyway, armed with the advice on here, I can at least inspect the engine more sensibly on Monday.
The price I paid leaves a wide margin for fixing or even replacing the engine if required.

Thanks.

 

[RichardS]

This is all very useful stuf - in typical style, I bought the boat sight unseen for a very cheap price. The chap selling her seems very honest and I bought her if full knowledge the engine was dicky. I can’t say whether it’s running or not as I’ll see yet boat for the first time on Monday! She’s a lateish model and her interior & hull look first class, the faulty engine was replacement of the original done in 2002 I believe and the photo shows it’s osmetically quite clean and has probably been maintained properly.

Stuart Marine have inspected the engine apparently, I don’t know what they discovered but they quoted £300 for a days labour (plus parts which would seem to indicate a simple valve lapping exercise - or something else not too serious and I’ll be happy to pass the job to them If that’s what it is. but anyway, armed with the advice on here, I can at least inspect the engine more sensibly on Monday.
The price I paid leaves a wide margin for fixing or even replacing the engine if required.

Thanks.

That's a great position to be in. Congratulations on your purchase.

Richard

 

[Slowboat35]

£300 for something they probably have no more idea about than you do? Have a care, once the engine is apart and in the hands of a workshop any snowballing is all but unstoppable. From what you've gone on to say about price paid my approach would be a simple new for old replacement for complete peace of mind and rapid timescale - and refurb the old engine at leisure as a winter project. It'll sell readily and for a good price and probably leave you not too far away financially from where you'd be with the rebuild costs alone, but still with a 20yr old engine in the boat. An engine change like that - like for like, could be done in two or three days. A rebuild will likely take weeks.
(Used ones are routinely advertised for £2,000 on ebay and elsewhere!)