Hi, I have a Lister JP3 with an irratic idle. I believe there are three areas to consider, the injectors, the goveror linkages and the fuel pump. I believe the governor is OK and not too slack. The engine starts well and no smoke when warm, good compression etc. So I believe the injectors are OK but not been replaced - could need a service. This leaves me suspecting the fuel pump.

Very simply, I cannot find a suitable idle speed. When reducung the revs I have to be careful no to go too low otherwise the engine will stop. A safe idle speed to avoid cutting out is too high. When finding a suitable idle speed the engine revs rise momentarily and them retrun to a lower idle speed, and then rise again, up and down, and so on. The idle is not constant.

Have you come across this before and if so what waa the cause?

Many thanks for any help you can offer.

RAP.

I would be looking at the governor/linkage setup first as it is unlikely to be the Injectors or pump if it is running fine at higher speed.

when it is running irratically is the rack on the pump moving along with it. Basically the govenor is trying to stop the engine while running and the speeder spring pulls against it to maintain a speed.

So start with ensuring the rack in the injector pump is free moving and not sticking. (should be able to move it for one direction to the other when disconnected for the linkages with one finger, if any more pressure is needed then it needs looking at

Next check all the linkages and ensure the throttle arm is free moving with no tight spots.

Then ensure the govenor rod that runs through the block is not sticking or has any tight spots. (some time these wear over time and if you then make adjustments that alter its setup it sticks on the ridge formed by the wear.)

Then ensure the govenor mechanism itself is not sticking. you will need to remove the oil filler crankcase door to get to it, make sure the weights are free to move and the cap in the centre is not sticky.

Finally make sure you have the correct speeder spring (correct Lister item) I have seen many JP's with just any old spring fitted here. If the incorrect strength spring is fitted e.g. too weak you will get eratic running as the govenor will push the pump closed(stop engine) and then as it has nearly stopped the govenor will release and the spring will pull the rack open again and the engine will pick up speed. also if too strong at wide throttle openings the govenor will not be able to overcome the spring resulting in overspeeding which can have catistrophic results.

Feel free to give me a call if you want to discuss. Number is on the contact us page.

Regards

Martyn

Hi Martyn,

Thanks for the list of things to check. I have done them all, but not found anything wrong. Hower, I do wonder about the first point regarding the injector rack. My checks indicate that the rack is free to move, but at the limit of its travel in both direction there is very slight resistance.

I will video the engine to see if you spot any symptoms.

Many thanks,

Richard

Hi Martin,

Success! I now know how to spell erratic - doh!

I have also solved the idle problem. My JP3 manual is for the stationry engine, and hence there is no detail about its idle speed. I now have the JP3M manual, and this states that the idle should be set at 450-500 rpm. I have rigged up a cycle computer to measue the flywheel speed and converted to rpm. The engine now achieves an idle of between 300 and 360 rpm without stalling.

Thanks for your valuable assistance, I could have spent a fortune trying other things when all it needed was some rudimentary checks, the right manual and the the correct idle adjustment.

Regards,
Richard

Hi Guys,

I am fortunate that my engine idles smoothly at around 300rpm my problem comes at higher revs when I get a lot of vibration, I am new to the engine but it basically can shake plates off the table in the boat. One other question is that the engine plate says the maximum revs for the engine is 1000rpm but all the literature I can find states maximum revs for the JP3 at 1200rpm. The engine has been rebuilt and is an industrial engine fitted in a narrowboat with a PRM 2:1 box. It is smoking a bit and using some oil but the advise I have had is to keep running it and try and stretch its legs with some river work.

Any advise would be greatly received.

Cheers

Redeye

Hi Redeye and welcome to the forum.

In respone to the first bit of your post "I am fortunate that my engine idles smoothly at around 300rpm my problem comes at higher revs when I get a lot of vibration, I am new to the engine but it basically can shake plates off the table in the boat." This is unusual, the JP3's are a very smooth running engine. We regulary test run then just sat on a pallet and they dont move a bit. if yours is vibrating then I would think something is out of adjustment or there is a bit of a problem somewhere.

As for the second bit of your post "One other question is that the engine plate says the maximum revs for the engine is 1000rpm but all the literature I can find states maximum revs for the JP3 at 1200rpm." this variation is because the genuine marine engines have a max speed of 1200rpm. yours being industrial was probably from a wartime searchlight/radar generator unit. these operated at 1000rpm max to coincide with the required speed of the generator unit. some industrial ones were 1200rpm but most we see are 1000rpm.

When you say the engine has been rebuilt, how long ago, where did the parts come from. were they genuine or copies. I ask this because if it was rebuilt with the genuine cylinder liners these are generally chrome lined. this is great for a generator where they operate at high load as it reduces wear, but in a narrowboat unless worked hard the engine does not see enough load and the chrome allow for excess oil to pass the piston rings and causes them to smoke. A good hard run on a river should sort this. secondly make sure you do not have an excessive level of oil in the cylinder heads from lubricating the rockers. if the level gets to high it will find its way down the valve stems into the engine as there are no seals fitted to JP's and also cause smoke.

Regards

Martyn

Hi Redeye,

My JP3 also idles around 300rpm but it does rise and fall as much as 50-75rpm. This is annoying when warming up to gauge what position to set the speedwheel at to avoid the engine cutting out.

I too have 'rough vibrations' at about 500-550rpm. My engine is an ex generator, maximum rpm is 1,000. Most diesels have a specific point in their rpm range where they vibrate - it can be very annoying.

How is your fuel consumption?

Regards,
RAP