Surging RPMs

[will vivian]

Hello all,
I have a maranized kubota z751 in a 30ft sailboat. About two years ago the engine began to surge at certain rpms. It only does it when in gear and under load. It idles fine at any rpm. When I put it in gear and throttle between 1000 and 2000 rpm the engine surges up and down about 100-300rpms. Above 2000 the engine seems to smoothen out and surging is either less noticeable or gone.

I have just replaced both primary and secondary fuel filters and all fuel lines from the tank, to filters, to IP. The tank pickup looked pretty clear but I'll service the shut off valve just to be sure. I bled the IP/filter. It looks as though there is a copper crush washer at the IP bleed screw. I didnt have a new crush washer so just reused the one I had. I doubt this is the issue but if anyone thinks it is I will get my hands on a new one. There is fresh fuel in the primary and secondary filters so fuel is not the issue. The only thing I can think of is the governor failing or somehow there is a bit of air getting in somewhere.

Does anyone have any advice or ideas for me?
I'm stumped.

s

 

[North Idaho Wolfman]

It's not air or very likely a governor issue.
It's most likely dirty/worn Injectors or injector pump seals.
Pull the injectors and have them tested or just replace them with a new set.

 

[will vivian]

It's not air or very likely a governor issue.
It's most likely dirty/worn Injectors or injector pump seals.
Pull the injectors and have them tested or just replace them with a new set.

I've heard these injector pumps rarely fail.
I put the fuel return line into a glass jar to check for air but nothing came out. No diesel, no air.
Could it be timing or valve adjustment?
The flywheel inspection hole is very difficult to access so I doubt the timing or valves have ever been done.

I'll have the injectors cleaned.

Does anyone know where I could find a IP rebuild kit for my 2 cylinder z751?

 

[North Idaho Wolfman]

I've heard these injector pumps rarely fail.
I put the fuel return line into a glass jar to check for air but nothing came out. No diesel, no air.
Could it be timing or valve adjustment?
The flywheel inspection hole is very difficult to access so I doubt the timing or valves have ever been done.

I'll have the injectors cleaned.

Does anyone know where I could find a IP rebuild kit for my 2 cylinder z751?

You could check the valve adjustment.
These motors don't get out of time unless you tear them down and rebuild them.
Have the injectors cleaned and tested, but buying new ones might be cheaper.
You are very unlikely to find a kit or the parts to rebuild the Injection pump, you also need a lot of special equipment to rebuild and teat them, you will much better off sending it to be tested and possibly rebuilt.

 

[elcojo2]

Hello all,
I have a maranized kubota z751 in a 30ft sailboat. About two years ago the engine began to surge at certain rpms. It only does it when in gear and under load. It idles fine at any rpm. When I put it in gear and throttle between 1000 and 2000 rpm the engine surges up and down about 100-300rpms. Above 2000 the engine seems to smoothen out and surging is either less noticeable or gone.

I have just replaced both primary and secondary fuel filters and all fuel lines from the tank, to filters, to IP. The tank pickup looked pretty clear but I'll service the shut off valve just to be sure. I bled the IP/filter. It looks as though there is a copper crush washer at the IP bleed screw. I didnt have a new crush washer so just reused the one I had. I doubt this is the issue but if anyone thinks it is I will get my hands on a new one. There is fresh fuel in the primary and secondary filters so fuel is not the issue. The only thing I can think of is the governor failing or somehow there is a bit of air getting in somewhere.

Does anyone have any advice or ideas for me?
I'm stumped.

s

Hi,

Have you found a solution to the problem you were having, I am having a very similar problem. Did yours hunt also when cold engine rather than hot.

Best,