JP compression test

[Andy Beckl]

Anyone performed a compression test on a lister JP. How did you do it and what results did you expect / did you get?

If you have the kit could you come check mine in Hinkley?

[RLWP]

Hi Andy,

 

What's the problem with your JP?

 

Richard

[martyn 1]

To second what Richard has asked. Whats up with your JP?

 

A compression check will require a dummy injector and then that coupling up to a suitable pressure gauge.

 

There are a couple of quick and dirty ways of identifying an issue with an individual cylinder that doesn't require any kit but we would need to know whats up first.

 

Kind Regards

 

Martyn

[RLWP]

I haven't got around to making a dummy injector for a JP yet - maybe this is the excuse I need

 

 

Richard

Edited February 8, 2017 by RLWP

[Andy Beckl]

Hi Guys thanks for the info. That makes sense, the dummy injector. I've only ever done them on petrol engines before :-)

 

The back story is as follows:

 

I bought the boat last year from an owner who had clearly mollycoddled it. It's got a JP2M driving a 20" prop through an original mechanical reverser, but _no_ reduction. He went ballistic at me for turning the throttle (speed wheel) by 1/2 a turn, and didn't have a clue what revs he was using. I got lumps of soot out the chimney first time I revved it in reverse!

 

The handbook gives 450rpm for idle speed and a max of 1200rpm.

 

Turns out he was protecting the gearbox, which was slipping quite badly, especially in reverse.

So, I've fixed that, ie adjusted it properly so there's now negligible slip in either gear.

 

I reckon a fast cruise on canals is doing around 400rpm. I can pull a reasonable wave and running steady the exhaust is as clean as I'd expect a 1950s diesel to be. No visible smoke.

 

But it won't rev beyond - lets say 500rpm - in gear. The shaft turns by hand okay, the prop is clear, no inlet blockage etc. More throttle just gives gradually more black smoke with little increase in revs. I've had this on hire craft before, but usually at the upper end of the rev range. I know diesels do smoke as they're accelerating, which mine is clearly trying to do.

 

I realise the boat won't go a fat lot faster on canals, but I am a bit concerned about taking it out on rivers.

 

So I am thinking it is probably a tad overpropped. Crowther's and Axiom both say I should be running 20 x 14. I made a rough measurement and reckon the current prop is 20 x 18. The diameter is correct, the pitch a calculated estimate.

 

Before I get Crowther's to attempt to bend my prop a bit flatter, I want to be sure that the engine is giving it's proper power. I could convince myself that it (hand) starts better flipping the rear pot off decompression first, but it's a nuance and I could be deluding myself.

 

So I thought, what's a good health check? Compression test, I thought. Yes I could dismantle and decoke, just in case, but I'd rather not!

 

Fitting a speed reducer is an option, as is getting rid of the blackstone box and installing a PRM, but these are quite expensive options and degrade the vintage appeal. Anyway I quite like chugging along at below-idle RPM, just could use a bit more oomph for maneuvering!

 

I'm up for any intelligent comment - fire away!

 

Andy

 

PS Richard, you are near enough to me (currently Hinkley) for a visit. If you decide you need a JP adapter perhaps you'd like to quote for a test & brief report?

 

PPS I do have a couple of spare injectors so perhaps I'll just swap them in and see if anything changes.

[Giant]

If one of you has the dimensions handy, I am doing a bit of CNC mill/lathe work at the moment and might be able to supply a dummy fairly easily.

[Giant]

Also: Andy, the JP and Blackstone box are usually used with a 2:1 or 3:1 reduction box after it.. Even our installation, on a big Sheffield size boat with 29" prop, is using the 2:1 box. I'd be very surprised if you could fit a prop that would work well at a 1:1 output.

[MtB]

Also: Andy, the JP and Blackstone box are usually used with a 2:1 or 3:1 reduction box after it.. Even our installation, on a big Sheffield size boat with 29" prop, is using the 2:1 box. I'd be very surprised if you could fit a prop that would work well at a 1:1 output.

 

 

I don't see why not. The blade just needs to be appropriately sized.

 

I have a 21" x 19" on my 21hp Kelvin K1 with 1:1 transmission. Somewhat overpropped (deliberately) but works fine...

[Giant]

I don't see why not. The blade just needs to be appropriately sized.

 

I have a 21" x 19" on my 21hp Kelvin K1 with 1:1 transmission. Somewhat overpropped (deliberately) but works fine...

 

Sorry don't mind me, I was thinking completely the wrong way round for a moment there. Of course the bigger boat with space for big prop needs the reduction more, not less.

[RLWP]

If one of you has the dimensions handy, I am doing a bit of CNC mill/lathe work at the moment and might be able to supply a dummy fairly easily.

 

I'd have to find an injector, measure it up, design where the NRV goes - and I may just as well make one as I do that. It's mostly simple turning

 

Thanks for the offer though

 

Richard