P.A.W. 1.49 plain bearing - can't adjust compression

[Tom Major]

Hi folks.

Not long ago I bought said P.A.W. on fleabay. I've tried to get it running few days ago and found out that compression is not really adjustable - once it's down too much, I can't decrease it at all.

I had a quick look inside (again) and found out that cylinder liner is not sitting in the crankcase as such - it came out of cylinder head. Inside I also found some strange looking part (do you call it counter piston??) that would be the cause of my issue.

I didn't want to go too hard on this so of course I didn't fix it, don't really know what the problem is. It is my first (of hopefully many) model diesel engines, so please don't be too harsh (but banter more than welcome! ).

Is there anything I could do to try and fix it?

Here is a pic of that engine


Thanks in advance for pointers.
Regards
Tom

[Martin Harris - Moderator]

This might be a silly question, but you are using proper (and fresh) "model diesel" fuel?

The contra-piston is an exceedingly close and accurate fit but when the engine fires, it should blow back - just turning it over compression won't normally force it back up.

Tuning a diesel is a bit of a black art - difficult to describe as it's done by feel more than anything - you may need to back off the compression once its running and heating up then screw it down again until you find the sweet spot - and adjust the mixture at the same time. I know there are far more accomplished diesel afficionados on here so listen to their advice if it conflicts!

A small prime at the exhaust ports normally helps to get things moving - you may need to develop the "diesel flick" to get heat into the engine by continuously flicking while combustion gets going. I haven't tried this but maybe some gentle pre-heat with a blowlamp (keep the fuel well stoppered!) might help?

 

Edited By Martin Harris on 11/12/2017 22:56:01

[Frank Skilbeck]

Tom, have you read the article in this months RCME on starting and adjusting diesels.

[Tom Major]

Yes, fuel is fresh, got a can of D2000 after I bought the engine and opened just once when my rig was set and engine was ready to fire.

The problem I have now is that compression is so strong I can't turn it over at all. Also during first attempt I didn't notice any changes in compression. The next thing that seemed logical was to turn the compression screw in even more and since then, even with the screw out I can't do anything...

Frank
I just have the mag on my lap now, only got it yesterday.

Another edit:
Article doesn't seem to solve my issue as it mentions fully working engines. Good writing by Steve though!

Edited By Tom Major on 11/12/2017 23:14:35

[Martin Harris - Moderator]

Probably best to wait for advice from someone who has encountered this problem, but in the absence of any better ideas, I would heat the cylinder to around 150 degrees C and use a copper or aluminium drift as near to the cylinder diameter as possible and carefully drift the contra-piston back up.

Edited By Martin Harris on 11/12/2017 23:19:38

[Tom Major]

Thanks a lot Martin, I thought about it too but didn't want to risk any damage before someone actually confirms it (no to mention waking up her indoors...).

[Martin Harris - Moderator]

Safest to wait until tomorrow!

[Mike Etheridge 1]

I have 4 PAW's and plenty of past experience of starting diesels. You need to put a propeller on the engine rather than a fly-wheel.and follow all the information given by the others above. Always slacken off the compression screw a few turns so it is loose. Then as is suggested squirt a few drops of diesel fuel in the exhaust port and flick the engine over and the contra- piston should shift to the end of the compression screw. If it has continue to flick the engine over and increase the compression until it fires and then runs. It will then be necessary to slacken off the compression screw slightly till the engine runs with a 'burp burp burp' and you should notice that as the engine heats up it will run without 'burps'. If the contra- piston has been forced down too far then as it suggested it will be necessary to remove the cylinder and use a drift to tap the contra- piston back up the cylinder.

Edited By Mike Etheridge 1 on 12/12/2017 00:11:11

[onetenor]

Leave the cylinder soaking in fuel In a tightly closed/ sealed container first before applying the drift treatment.

Is that a second comp screw in the  head If so why?

Edited By onetenor on 12/12/2017 00:40:06

[onetenor]

Posted by onetenor on 12/12/2017 00:36:18:

Leave the cylinder soaking in fuel In a tightly closed/ sealed container first before applying the drift treatment.

Is that a second comp screw in the head If so why

Edited By onetenor on 12/12/2017 00:40:06

Incidentally once you have got the contra piston to move tap it up and down a few times. John

[Andy G.]

Posted by onetenor on 12/12/2017 00:36:18:

Is that a second comp screw in the head If so why?

Edited By onetenor on 12/12/2017 00:40:06

Onetenor, that's a locking device to stop the compression screw backing off when the engine is running.

[Jon H]

I think the other guys have covered it for the most part but to add my 2 cents...

turn the cylinder over and fill with fuel, then do as Martin suggests and gently tap the contrapiston back up. If you are careful you could even use a small vice as a press and this would probably be the least stressful on the components.

Once you have it moving its likely to free itself up so i wouldnt recommend removing it completely.

Mike has covered the starting procedure nicely although my 1.49 needs a hefty crank of compression in the first few seconds after it starts.

I would also remove what i think its a clutch (???) on the front and fit something like an 8x4 prop as my engine seems really happy on this.

[brokenenglish]

Tom, several people have asked this question on my youtube channel. Each time, I've replied by the text below and, in every case, they came back saying "Thanks, it worked", so you may like to read and/or try the following:

First method (if it’s only moderately stuck):

-Mount the engine and install a propeller,

-Unscrew the compression screw a lot, you can even remove it,

-Put plenty of fuel in the cylinder (huge prime),

-Make sure you can turn the engine over gently (important!),

-Then give the propeller a good whack. Overcompressed combustion will often knock the contra piston back.

If this doesn’t work, then you need to unstick the contra piston any way you can...

The following has worked for me many times.

-Mount the engine and install a propeller,

-Gently turn the compression screw down, say half a turn, to increase compression. This must have « unstuck » the contra piston. When you’re sure you’ve actually moved the contra piston, then proceed as above, i.e.

-Unscrew the compression screw a lot,

-Put plenty of fuel in the cylinder,

-Make sure you can turn the engine over gently,

-Then give the propeller a good whack.

The important thing about the second method is that you start by forcing the contra piston to move, so you know it’s no longer « stuck ». You have also increased compression slightly, so there will be more force to knock the contra piston back. You won’t do any damage providing you unscrew the compression screw before turning the engine over, i.e. the contra piston will move before you bend a con-rod!

This method has always worked for me.

If it doesn’t work, then I would try serious heating (oven), followed by the above procedure.

If all that fails, you’ll have to disassemble... but I repeat, the above has always worked for me.

[Engine Doctor]

Be careful with whacking a propped engine. It usually works but I have repaired/rebuilt many engines when this treatment fails. It can result in a bent con rod ,broken gudgeon pin,sheared crank pin or broken crankshaft or a combination of these .The safest way is to remove the contra piston by tapping it out of the cylinder an gently cleaning it and the cylinder where it sits. This usually cures it . If it still sticks then it needs some gentle abrading but be careful as the last thing  you want is a loose contra piston. If not confident then get someone who is to sort it for you. The contra piston should reset to the comp screw with a good flick if engine is primed.

[Jon H]

I agree with ED on this one. The shock loads on the innards of the engine will be extreme. Better to take it to bits than risk damage.

[Tom Major]

Thank you all for amazing advice gents.

Liner is soaking now in fuel, will leave it there for few hours and then try to move contrapiston (that's another word in my dictionary ). If that doesn't help, will try with heat and vice.

I shall post an update when I get to it later today.

Cheers.

[Jon H]

If you use a vice make sure you fit soft jaws so you dont do any damage. If its a small hand vice some small bits of ply should do fine

[onetenor]

Posted by Andy G. on 12/12/2017 07:37:42:

Posted by onetenor on 12/12/2017 00:36:18:

Is that a second comp screw in the head If so why?

Edited By onetenor on 12/12/2017 00:40:06

Onetenor, that's a locking device to stop the compression screw backing off when the engine is running.

I though it could be but just LOL

[onetenor]

Hi an y luck with this ? If you have You will need a REALLY hard flick to start. An electric starter would be a help if you can beg , buy or borrow one LOL

Edited By onetenor on 12/12/2017 16:04:54

[Jon H]

I wouldnt electric start a small diesel. Thats a sure fire way to break something. They are only teeny!

[Tom Major]

Think I'll leave it soaking little bit more

John, once again thanks for fantastic advice.

[brokenenglish]

Posted by Jon Harper - Laser Engines on 12/12/2017 10:26:49:

I agree with ED on this one. The shock loads on the innards of the engine will be extreme. Better to take it to bits than risk damage.

Jon and ED, that just isn't true!

Note that I emphasised "Make sure you can turn the engine over gently".

I must have used that procedure dozens of times in the last 50 or 60 years, and I've never damaged anything!

For me, the problem with the advice that many people offer is that anyone asking for advice will be inexperienced and possibly a "beginner". IMO, to advise delicate disassembly and abrading operations is irresponsible. It's far better for an inexperienced person to avoid disassembly No damage can occur using my method providing you go about it sensibly and check that the engine turns over OK, which is clearly stated!
Like I said, I've been doing exactly that since the fifties, and I've never broken anything.

Another valid point is that, in this case, we're dealing with an engine that has already been run, so it probably runs OK, it's just that the C/P is gummed up with congealed residue... That's no great issue and, for me, it certainly doesn't justify disassembling anything!

[onetenor]

I suggested the starter be used only AFTER the thing has been freed off not using it to free it .I've had no probs using a starter as long as I ensure there is no chance of a hydraulic lock.

[Martin Harris - Moderator]

As I understand the problem, the contra-piston has already been wound down to the point where the engine won't turn through compression. Forcing it to do so would, in my opinion, risk damage to the internals due to the greater angles involved than a normal slight over compression and it appears that it had already been stripped anyway.

Careful application of force with a drift at something approaching a normal running temperature, ensuring the tool is kept square and the load spread well (hence my suggestion of a well fitting drift) should reset the contra-piston with the least likelihood of damage.

[Jon H]

Posted by brokenenglish on 12/12/2017 17:16:33:

Posted by Jon Harper - Laser Engines on 12/12/2017 10:26:49:

I must have used that procedure dozens of times in the last 50 or 60 years, and I've never damaged anything!

As you know what you are doing I'm prepared to believe this, but you make it sound as though you basically just take a swing at it without any sort of finesse. I have seen many a bent valve on our engines as customers try to force past resistance to free a gummed up engine. Given how simple little 2 strokes are, its easier to just take it to bits which is what our protagonist had already done I believe as he turned the compression all the way up and the engine is now locked against the piston. In this case, its got to come apart.

I agree that a wet n dry rehone is probably beyond a raw beginner which is why I suggested the vice. Its much more controllable than beating it with a hammer and is a far gentler way to apply the required force. I am sure that once some fuel is in there it will free up and the engine will be just fine.