Lets grow the sport

I  won’t be showing how to modify a pump lol but I thought I might explain how the pump works and talk about a few improvements you can do. Most pumps are similar but this is Kubota info.

Remember whenever doing pump changes of any kind, remove your cross over pipe and have a metal plate ready to block intake … in case it runs away …

Here we have a plunger and barrel from a 1105.
The spill port lines up with a chamber in the pump that is supply pressure 40 to 50 psi.

I have found out recently that high pressure supply pumps of 80 to 100 psi make for a poor idle and tends to be harder to control.

The barrel (upper part) is fixed and locked into place by a pin into the notch. The bottom end of the plunger fits into a holder attached to the rack. As the cam pushes up on the roller the plunger will move upward. There are four parts to a complete cycle.

  1. The top of plunger drops bellow the spill port allowing the barrel to fill with fuel
  2. The plunger moves upward compressing the fuel
  3. The fuel reaches crack pressure the delivery valve opens fuel goes to injector over powering the spring and is released.
  4. Delivery valve closes when pressure drops stopping reverse flow.​

As you can see in the pic there is a vertical cut and a helix or angled cut on the plunger. When the injection pump is in off position or stop position the vertical cut stays in line with the spill port going up and down but builds no fuel pressure because its being released back into the spill port via the vertical cut.

Now if the pump is turned on and throttle pushed  the plunger turns and the vertical cut is no longer in line with spill port but now the angled or helix cut comes into play. As the plunger goes up fuel is pressurized but its stoke is limited to the rotation of the helix cut.

As soon as the helix cut reaches the spill port the high pressure is released back into the spill port and the stroke is over. The more the throttle is pushed the more the plunger rotates and the further away the helix cut is from spill port thus a longer stroke more fuel more RPM…  more power.

Hope that makes sense to some of you lol.


What is being done when you send your pump out to be modified? Two things first they might increase the plunger and barrel size (1105 is 5.5mm stock) this would increase output and then second they do what is known as clocking the barrels. Clocking is basically removing the locking pins from the barrel and turning it in such a way that the helix lines up with spill port further into the stoke giving more fuel. Now the issue with this is that if you clock them to far the vertical cut will no longer line up with spill port and you won’t be able to shut it down. A little of pump 101.

1105 vs 950

You can see in the pic the 950 pump is bigger than the 1105. The advantage of the 950 pump there are larger plungers easily available. The 1150 pump can take up to 6 months for the 6mm plungers.

The other difference is the rack of the 950 is gear driven a far better setup than the 1105. The gears make it easy to clock the barrels and make fine adjustments. In my opinion the 950 is a much better pump. Some have thought about putting a 950 pump into 1105. I have looked this over and don’t believe it can be done without infringing on the rules

Supply Pump

Since we run our engines at pretty high rpm its very important to have a lift pump that can maintain 40 to 50 psi for a few reasons.

First off it helps cool the pump and second of all it lubricates as well. Any excess fuel is returned to the tank via the return line.

Another good reason for the high pressure is you are forcing the fuel into that spill port quicker keep in mind that a complete stroke is milliseconds.

These pumps can be had from Summit or plenty of other places costing around 80 to 100 bucks. Use high pressure hose and good clamps and remember to add a inline filter I run mine before the pump.


I do a lot of injector testing at my place of work and I will tell you 6 out of 10 are no good even from a running engine. Don’t fool around with old worn out injectors.

It cost about 125 bucks for a set of new tips or atomizers. If you don’t have access to a injector tester; have a shop do it for you and have them raise the crack pressure from 1900 psi to around  2700 psi.

This is done by changing shim behind the spring.  The crack pressure is basically when line pressure reaches the set point it over powers the spring allowing needle to raise and release the fuel.

The higher pressure will force more fuel through the injector for the given time they are open. This will also retard the timing slightly. An option for more fuel is to replace the tips for larger ones.

The Kubota V2203 4 cyl has bigger nozzles that will fit 1105 injectors. 16454-5361-0 Kubota number remember its all at your own risk.

I have seen online its possible to build a crack tester from hyd jack unit. Look it up if you think you might like to give it a go.


On Kubota the pump timing is controlled by adding or removing shims from under the pump. Most stock engines will have two to three shims of varying thickness. By removing shims you will advance the timing and adding will retard it.

Most of us want advanced timing I have run just one thin shim and at times no shims at all. Some have found that removing shims is not enough and have modified cam gear to make it adjustable or moved the cam by one tooth. Not for the faint of heart lol. 

Stock 1105 timing is around 19 degrees. Some will go as high as 26 but remember it will increase power but heat as well.

Pump Swapping

Unfortunately the 1105 has no bigger sister pumps that can be used. The best options are to send the pump out to a pump shop. If you choose to keep your stock plungers they can increase your output up to around 85 -90 cc. If you have plungers replaced ( 3 to 6 month back order ) they can get you up to 100 -130cc.  Considering a stock pump is around 25 to 30cc that is pretty good.

Those running a smaller 662 or 722 or 902 they can use the 1105 pump but need to make some changes. The rack plate needs to be removed and rack turned around so pin is at opposite end and then the rack plate from 722 or 662 needs to be used to secure it in place. These fuel pumps have two places for fuel input line one is normally plugged. just swap them.30

A Few Improvements

So if your short on cash to have your pump done there are a few things you can do that won’t cost you much. The first thing is you want to remove the stud  otherwise known as the fuel screw. Pretty simple pop off the aluminum cap then remove the locking nut and screw out the stud completely. Replace this with a very short bolt of the same thread keeping the copper washer to seal it. This will allow the rack to travel further.


Second thing you can do is remove the delivery valves and have them cut. Just below the seat is a tiny ridge have this removed at a machine shop. I like to remove a bit off the shank as well just to clean it up.

These are found by removing the fuel lines from pump then removing the three fittings or delivery valve holders.Remove a shim or two some guys run no shims.

The shims come is a few thicknesses you can tell by the amount of holes in it. 1 hole is 25mm, 2 holes 20mm, and sizes continue down.

For more rpm look up the governor section on spring replacement. 

Pump Shops

Basically there are a couple pump shops that are getting some big cc out of our pumps. Schied Deasel and Fairvalley Performance. Both are good I find Schied more affordable but many guys swear by Fairvalley. If you keep up on the big pulls last few years you may have noticed a lot more smoke these days. Basically there are a couple pump shops that are getting some big cc out of our pumps. Schied Deasel and Fairvalley Performance. Both are good I find Schied more affordable but many guys swear by Fairvalley. If you keep up on the big pulls last few years you may have noticed a lot more smoke these days.

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