Engine seems to start but rolls to a stop

[JustinT]

Thanks Growley, makes the mud clearer

[GrowleyMonster]

Let me try one more time, now that I have verified that the cylinders do indeed fire on ALTERNATE TDC positions, in turn.

When the pistons are both at TDC, and they will both be at TDC at the same time, one cylinder will have both valves fully closed so that it can fire when fuel is injected. The gap between rocker arm and valve rod will be at maximum, and that is when you check the valves for that cylinder. The other cylinder at the same time is also TDC but the valves will be pushed open or partly open, and there will be NO GAP TO CHECK, if they are even close to correct. If there is a gap in all for valves at once, then one of your cylinders CANNOT FIRE. That may well be the least of your problems, too.

Valves do not have a TDC position. They are fully up, i.e. closed, or fully pushed down by the rockers, i.e. fully open, or somewhere in between. Watch your rocker arms on cylinder #1 as you roll the engine over by hand. Don't mess with the compression levers. If they operate the way I think they do, they prevent your valves from closing fully. In fact you might even want to remove the whole assembly to cut through the possible confusion. When both rocker arms are as high over the valve as they can go, not one but both, the valves are held fully closed by their springs. At that point, check THOSE TWO VALVES ONLY, not the valves on #2 cylinder. Those two valves should be pushed open or partly open as they cross over between exhaust and intake strokes.

Next, roll the engine over by hand one complete revolution. Now, both pistons are again at TDC, but now #1 valves should be pressed down by the rockers, and #2 cylinder's valves should be open, and the rocker arms over the valves at their highest point. Now, check the gap on THOSE valves.

If you want, you can roll the engine one more complete turn and re-check #1's valves, and then one more complete turn and check #2 cylinder again.

Remember, TDC is when the piston of a given cylinder is at it's highest point, not necessarily either or both valves for it. You can't see the piston. Unless you marked the flywheel, your only indication of TDC is the valves of one cylinder being in the crossover condition and in the case of your engine, the valves of the other cylinder being fully closed, with their rockers as high as they go. Roll the engine back and forth and see where the rockers begin to go down toward the valves, Between the two points, right in the middle, should be TDC, close enough for our determination. Likewise, the other cylinder as I said will be at the crossover between exhaust and intake.

If after studying your engine and rockers in particular, you still can't figure it out, find an old salt locally who is willing and able to show you. I swear if you do it and do it right one time, you will never forget and you will wonder how something so simple could be so hard to understand.

If you visualize what is going on with your pistons, crankshaft, camshaft, pushrods, and rockers and valves, I know it will come to you like a brick in the face. The valves of a cylinder are adjusted only when the gap between rocker arm and valve rod is greatest. This condition can not possibly be true for both cylinders at once. Not in a million years. Not even with all 9 planets perfectly aligned and a total solar eclipse happening all at once. A full revolution of the crankshaft will make it so on the other cylinder, and the gap will no longer be there on the first cylinder's valves. The pistons go up and down together but they do not fire together.

[deblen]

Originally Posted by GrowleyMonster

Let me try one more time, now that I have verified that the cylinders do indeed fire on ALTERNATE TDC positions, in turn.

When the pistons are both at TDC, and they will both be at TDC at the same time, one cylinder will have both valves fully closed so that it can fire when fuel is injected. The gap between rocker arm and valve rod will be at maximum, and that is when you check the valves for that cylinder. The other cylinder at the same time is also TDC but the valves will be pushed open or partly open, and there will be NO GAP TO CHECK, if they are even close to correct. If there is a gap in all for valves at once, then one of your cylinders CANNOT FIRE. That may well be the least of your problems, too.

Valves do not have a TDC position. They are fully up, i.e. closed, or fully pushed down by the rockers, i.e. fully open, or somewhere in between. Watch your rocker arms on cylinder #1 as you roll the engine over by hand. Don't mess with the compression levers. If they operate the way I think they do, they prevent your valves from closing fully. In fact you might even want to remove the whole assembly to cut through the possible confusion. When both rocker arms are as high over the valve as they can go, not one but both, the valves are held fully closed by their springs. At that point, check THOSE TWO VALVES ONLY, not the valves on #2 cylinder. Those two valves should be pushed open or partly open as they cross over between exhaust and intake strokes.

Next, roll the engine over by hand one complete revolution. Now, both pistons are again at TDC, but now #1 valves should be pressed down by the rockers, and #2 cylinder's valves should be open, and the rocker arms over the valves at their highest point. Now, check the gap on THOSE valves.

If you want, you can roll the engine one more complete turn and re-check #1's valves, and then one more complete turn and check #2 cylinder again.

Remember, TDC is when the piston of a given cylinder is at it's highest point, not necessarily either or both valves for it. You can't see the piston. Unless you marked the flywheel, your only indication of TDC is the valves of one cylinder being in the crossover condition and in the case of your engine, the valves of the other cylinder being fully closed, with their rockers as high as they go. Roll the engine back and forth and see where the rockers begin to go down toward the valves, Between the two points, right in the middle, should be TDC, close enough for our determination. Likewise, the other cylinder as I said will be at the crossover between exhaust and intake.

If after studying your engine and rockers in particular, you still can't figure it out, find an old salt locally who is willing and able to show you. I swear if you do it and do it right one time, you will never forget and you will wonder how something so simple could be so hard to understand.

If you visualize what is going on with your pistons, crankshaft, camshaft, pushrods, and rockers and valves, I know it will come to you like a brick in the face. The valves of a cylinder are adjusted only when the gap between rocker arm and valve rod is greatest. This condition can not possibly be true for both cylinders at once. Not in a million years. Not even with all 9 planets perfectly aligned and a total solar eclipse happening all at once. A full revolution of the crankshaft will make it so on the other cylinder, and the gap will no longer be there on the first cylinder's valves. The pistons go up and down together but they do not fire together.

Yep!! / Len

[deblen]

Single cylinder push rod four cycle engine.
This example is only one piston/cylinder .

Your DV20 has two of these pistons/cylinders,operating exactly as this example-except when one of your 2 cylinders is in- say power stroke,with both intake and exhaust valves closed & seated & with the piston being driven down from the explosion of firing fuel,the other piston is also at the top,but it's intake valve is starting to open,it's exhaust valve is closed & air will be sucked in thru the open intake valve by the vacuum created by that downward moving piston.
Ignore references to petrol carburettor & spark plugs.
Your diesel valve cycle works identical .

https://www.youtube.com/watch?v=7_AdfS7Rllw

[Wotname]

This is turning into one of CF's better engine mysteries - where is Agatha Christie or Sherlock when you need them. I keep thinking I must have missed some early clue but everytime I re-read the thread, I come away more confused. The clues are confounding to say the least. One basic principle in fault finding is what was the previous event before the fault showed its face. The best I can find is this post

Originally Posted by JustinT

Thank you all. So much information to work through!
Last time it worked was last year when I bought it in from the swinging mooring. It did act a bit odd then. Once I tied it up (I moor it on the beach for the winter) the engine started to race and wouldn't shut down. I had to jump the solenoid to the negative. Didn't think much of that really, it's done odd things before, until trying to get it going this spring. Being on land, testing it is tricky. I remove the impeller to try starting or wait for high tide.

I'll work though all the info above and get back to you all. Thank you.

JT, can you describe this 'racing' in more detail? Did the engine suddenly rev up without any command from the throttle? Was it hot or cold? What was the approximate throttle setting when it 'raced'.

----------------------------
Moving on we have these 3 posts and when looked at together, it tells us one cylinder was not firing and presumably loading down the good cylinder.

Originally Posted by JustinT

Well, I tried the above on high tide tonight.
It really seems to be running but as soon as I let go of the key the engine slowly stops, so very slowly.
There is fuel going back to the tank.
The injectors are spraying. However, I loosen one and it makes no difference. I loosen the other and it's very sick.
Also, on first turning the key it sounds super sick but then rapidly cranks to what is sounding like a running engine. Let go of the key... putt putt putt putt putt stop

I assume I must not keep cranking&cranking and destroying the starter motor?

Originally Posted by JustinT

Success! It was the injector. Fiddled with the fittings again and tried the compression, both levers were fixed together but I made them independent and that worked. I turned the engine with the wonky one compressed and the other normal. As it turned I flicked the lever on the wonky one and the engine ran. Was no impeller or cooling so I didn't run for long. Will go have a fiddle on the super high tide tonight when I have cooling.
Thanks to all on this thread and the problem solving. Couldn't have done it without you!

Originally Posted by JustinT

Premature jubilation. I went back on high tide with cooling and it ran for 10 mins and then just stopped. Cranking again sounded sick.

I had checked the injectors and spray patten, I can only assume it just hadn't gone back together well
Idk, why didn't I buy a newer boat?

This is confirmed when we can see one cylinder has the bent pushrod and repairing that fault should have been the end of the matter.

Originally Posted by JustinT

On doing the decompression levers I find a problem:
https://youtube.com/shorts/nPZRVyC0D...pBMWLot-wCTqbr

Next we learn the cam follower on the dud exhaust valve is stuck

Originally Posted by JustinT

The pushrod appears not to be moving at all (with rocker assembly removed)

This was fixed but why did it happen initially?

More clues - the oil in the pan looked clean but the oil on the rocker gear looks very dirty? Why the difference? JT, have you ever changed the oil (& filter) and if so, when and what did the old oil look like?

The fault did seem to be intermittent (i.e ran for some minutes and then not) but now the fault seems to permanent. Permanent faults has easier to solve than intermittent ones so maybe it is good the fault is permanent...

So I go back to guessing...hopefully informed guessing.

It HAS to be a compression or a fuel problem! But is it a compression or fuel problem on one cylinder or both? What is the best way for a DIY guy to diagnose this?

One way to determine if it is isolated to one cylinder is to pull one injector completely out of the head and see if it will kind of run on the other cylinder (it should). Then try the other cylinder. If it runs on one cylinder and not the other, we can explore why! Again, it must be a compression or fuel issue on the crook cylinder. If you leave the removed injector connected to the pump, you can see if the removed injector is spraying properly while cranking.

Perhaps the original racing was caused by old dirty sticky oil gumming up the governor and also gummed up the cam follower which then rusted into place. Perhaps there is still sticky old oil around the governor and one side of injector pump causing lack of fuel to one cylinder.

Perhaps the original racing caused a broken ring or ring land on one cylinder and a loss of compression on that cylinder but still enough to load down the other cylinder so the engine won't start.

My current suggestion is try the technique posted in blue.

[Wotname]

Originally Posted by JustinT

I see, thanks, I think I'm getting it now. I'm being very dense about this I realise.

Don't beat yourself up; it is a learning curve and we all started somewhere.

[Wotname]

Regarding setting the valve lash - we are all assuming the cam followers are actually following the cam lobes and for 99.99999% of the time, this is a reasonable assumption. With this engine, we might be looking at the 0.00001% event.

I would be inclined to give the rocker a slight (hammer) tap on the side that goes to the pushrod before checking the gap. Just to make sure the pushrod and cam follower is fully down against the low side of the cam lobe.

Can't be too careful when strange stuff is occurring!

[GrowleyMonster]

It was Colonel Mustard, in the conservatory, with the candlestick!

[JustinT]

Thanks chaps, really apricate the problem solving. At least I know now it wasn't just me being dumb but the problem being tricky.
I did in-fact forget that I got it running a bit before... however, I do remember it was hard to shut down when it did go.

I've not changed the oil as at one point a lot came out due to over-filling. What shows on the dipstick looks clean.
https://youtube.com/shorts/YNe9w0Jgy...Q9m-ZPcb0Jozz1
I did change the filters. The fuel filter was clean and the oil filter had clearly been on there a number of years.
I expect I should just go ahead and change the oil.

I have sourced a local 25hp outboard which we may use just to get to Inverness and our new mooring. Working on the engine there will be so much easier, maybe a full rebuild there would be possible. There is also no tradesmen here, in Inverness there would be physical help too.

Today I'll try the gap issue again. I'll also try the pulling an injector each side. I can see then also if they are dirty again.

[skipperpete]

Did we talk about the impression of the valve head that’s hammered into the piston crown? , it’s hard to believe that the valve stem came through that event unscathed. Thanks for the retrospective Wottie, it’s always useful to look back over the chain of events from day 1.

[JustinT]

The chap who did the injectors did them very tight, I have be unable to remove them since. I did think of swapping them over to see if that made a difference. Don't know why he did them so tight though.

[JustinT]

Well, today, I think the gaps are right. Got it TDC, did the valves on cylinder 1. Rotated to TDC again. Gaps on cylinder 2. (when I had the head off I marked TDC on the flywheel with the cylinders at the top).
However, the batteries really weren't charged enough, so after realising it was sick I have spent the day charging batteries. One of them appears rather flaky. I was using three until now.

The engine sounded pretty terrible but I hoped this was down to the batteries. So two batteries charging at the boat on solar (which were charged at home for three hours too) and two back at home charging.

[GrowleyMonster]

Originally Posted by JustinT

The chap who did the injectors did them very tight, I have be unable to remove them since. I did think of swapping them over to see if that made a difference. Don't know why he did them so tight though.

Spray them hourly, lightly, with a good penetrating oil. Then soak them down good before you leave the boat for the night. There was a thread recently, rating the various penetrating oil products with actual torque required to break bolts free. I have always used Liquid Wrench but the test results favor a mix of acetone and ATF, Automatic Transmission Fluid. When you are ready to start the loosening process, alternately try to (gently) turn in the tightening direction and the loosening direction. Don't force it. Use the correct tool always, no adjustable wrench of any sort should ever come in contact. Worry it a little bit every hour. By the end of the second day you should be able to remove them. After two days of such treatment start trying more torque, or tapping your wrench with a small hammer. Be very careful to seat wrenches or sockets or allen keys properly, with correct sizes, and not round corners, etc.

[JustinT]

Thanks, I did buy some Liquid Wrench on your recommendation.

[JustinT]

Hi all. I've decided to get an outboard and a mounting bracket in order to get the yacht to our new mooring in Inverness. Once there, on a pontoon, I shall resume restoration on the Bukh. Pretty sure it needs a lot more TLC before its a useful engine again and I don't want a time limit on it and to get it done properly.

Thanks for all the help on this thread, I shall refer back to all the info once I start work on the Bukh again.

Cheers all, Justin