1983 Bukh 20 hp. Indirect injection

[rotrax]

FWIW

Yanmar say it is because fuel accumulating in the cylinders could make restarting difficult !

With all due respect to Yanmar, if the decompressors hold the exhaust valves open, excess fuel will be ejected past the exhaust valves.

Old fashioned British motorbikes-as far back as 1902-were fitted with decompressors acting on the exhaust valve and they had no clutch. They had direct vee belt drive untill chains took over. If the engine was running-the bike was going forward. The decompressor was required for push or pedal starting.

To slow down-the brakes were pretty useless-the decompressor was used. The overun and gas being forced from the cylinder was quite an effective brake.

In later years-20's through 60's expert trials riders would use the decompresor when decending steep hills. It gave better control.

Fuel build up was never a problem.

Operating a decompresor at high speed-as in an emergancy situation-could cause pushrod damage if one became dislocated. I have repaired a motorcycle where this happened, a BSA B33.

 

[Gitane]

I always carry a spare engine

I tend to tow a spare yacht, just to be on the safe side

 

[VicS]

Just repeating what Yanmar say in the workshop manual and contradicting what I said earlier in the thread

Actual wording being "Doing so may leave sufficient fuel in the cylinders to damage the engine when started"

 

[VicS]

I tend to tow a spare yacht, just to be on the safe side

No No Two yachts = twice the likelyhood of things going wrong.

 

[Champagne Murphy]

I always carry a spare engine

There y’go, perfect logic.

 

[Champagne Murphy]

I tend to tow a spare yacht, just to be on the safe side

Even more perfect logic.

 

[VicMallows]

I am late to the discussion, but having owned my BUKH20 for 34years since new I will make 3 observations

DO add a heavy duty press switch in series with the Stop solenoid as already suggested. The solenoid is expensive and in my case the wiring also melted with much smoke. My keyswitch was never spring-loaded in the 'OFF' position ; the (notoriously unreliable) alarm buzzer was supposed to alert you to turn the key back to normal.

The second time I replaced the exhaust elbow I used a stainless steel one from a local fabricator. That was 20years ago and it is still in excellent condition. The fabricator obviously knew what he was doing.

When my stop-solenoid failed, I stopped the engine with the de-compressor for the rest of the season. No ill effects.

 

[robertj]

I am late to the discussion, but having owned my BUKH20 for 34years since new I will make 3 observations

DO add a heavy duty press switch in series with the Stop solenoid as already suggested. The solenoid is expensive and in my case the wiring also melted with much smoke. My keyswitch was never spring-loaded in the 'OFF' position ; the (notoriously unreliable) alarm buzzer was supposed to alert you to turn the key back to normal.

The second time I replaced the exhaust elbow I used a stainless steel one from a local fabricator. That was 20years ago and it is still in excellent condition. The fabricator obviously knew what he was doing.

When my stop-solenoid failed, I stopped the engine with the de-compressor for the rest of the season. No ill effects.

Vic

Did the stainless steel elbow alter or increase the engine noise at all?

Rob

 

[robertj]

I wonder how’s much horse power is lost by the hug flywheel on the Bukh, anyone know?
I replaced my DV20 3 years ago as it was a bit underpowered for my boat. It never missed a beat, always started eventually, clean no smoke and fairly economical for an old style engine. When I sold it, it had over 5000hrs on the clock with no work other than exhaust elbows and that luciferian stop solenoid, one alternator and a misdiagnosed starter motor.
I remember at the time I had a dilemma of what will I replace it with.

 

[VicMallows]

Vic

Did the stainless steel elbow alter or increase the engine noise at all?

Rob

Not in any way I could discern ..... but my installation is hardly quiet at the best of times!

 

[robertj]

the only problem wwith Bukh’s are the technical advice is poor. Come back Al and Norm.
Has Norman retired with his expertise lost forever or is he still available for advice?

 

[robertj]

Not in any way I could discern ..... but my installation is hardly quiet at the best of times!

Thanks vic.

I have one new spare which will keep me going for a while I suspect.

 

[RichardS]

I always carry a spare engine

Snap! That's exactly what I do as well.

Richard

 

[LittleSister]

I wonder how’s much horse power is lost by the hug flywheel on the Bukh, anyone know?

As I understand it, no power [energy?] is lost by the flywheel: it is just stored temporarily.

So it takes power to get it moving (as you will be well aware if you've tried hand starting!), but the momentum of the flywheel is what keeps the engine going through the compression stroke. This absorbs some of the power and slows the flywheel imperceptibly. The next firing stroke imparts some more power to it, speeding it back up again. And so on, until you turn the fuel off, when the momentum of the flywheel keeps the engine moving for a few moments.

The larger the flywheel, the harder to get up to speed and to accelerate; the smaller the flywheel the more vibration (all other things being equal) until the point where there isn't enough momentum to compress the charge, at which point the engine won't run.

A smaller flywheel will need to be heavier, or geared to spin faster, to have the same effect.

 

[robertj]

Thanks for that

 

[Heckler]

Generally not recommended to stop an engine with the decompressors. Dont know why, not designed to be used that way I guess

Because the valve train goes "all loose" Holds a valve open so that the pushrods or whatever have loads of clearance to deal with.

 

[Cockaigne]

Because the valve train goes "all loose" Holds a valve open so that the pushrods or whatever have loads of clearance to deal with.

In that case, there would also be "loads of clearance " when turning the engine over with the decompressor operated prior to starting the engine. Can you say why that does not cause any problem?

 

[Rafiki]

Dylan,
Engine designed as a marine engine, easy to work on. What is basically the same engine is still made, therefore spares can be bought easily albeit at a price. I actually changed mine of similiar vintage for a second hand BUKH 24, this is basically the same engine but with a re-designed cylinder head. This means that the bottom end of the engine is the same as the Bukh 20 and therefore a direct slot-in replacement - just more powerful and starts more easily.

 

[vyv_cox]

Because the valve train goes "all loose" Holds a valve open so that the pushrods or whatever have loads of clearance to deal with.

When I suggested that in post #39 I was told I was talking rubbish.

 

[dylanwinter]

I spent some time drifting up and down the Deben thinking about the Buhk engines and the comments made here.

The next boat should be for four or five years and I do not want to spend too many hours staring down that engine hatch mithering about spare parts....

The boat could be brought up to spec but not without another £10K being spent on her plus a year in the yard... as I am not the most avid fettler on the planet I shall wait for another to turn up.

thanks for your advice chaps - most useful

D