Easy HP increase for CH18 engine?

[Blue Chips 173]

The only difference that I've been able to find (so far) between the CH18 used in a 2001 518xi and the CH20 used in a 2001 520xi is that the CH18S has a throttle stop screw on the carburetor that limits how far the throttle plate can open. The throttle stop screw is shown in a parts diagram for the CH18, but not in a parts diagram for the CH20.

 

Here's a photo of the throttle stop screw on my New Holland GT18 (which I believe has the same engine as a 518xi):

 

 

With the throttle stop screw in place, the throttle can't open all the way, perhaps only about 2/3 to 3/4 open, as shown below.

 

 

As an experiment, I have removed the throttle stop screw to allow the throttle to open all the way. I was able to remove it without pulling the carb by using a right-angle screwdriver:

 

 

After removing the throttle stop screw, I readjusted the governor. However, I haven't finished reassembling the tractor yet, so I haven't given it a real-world test yet.

 

Edited 21 hours ago by Blue Chips

[ebinmaine 74,401]

[ri702bill 9,974]

9 hours ago, Blue Chips said:

 

After removing the throttle stop screw, I readjusted the governor.

Adjust accordingly to prevent over-revving the engine - best not to find out why it was put there that way!!

Edited 11 hours ago by ri702bill

[ebinmaine 74,401]

1 hour ago, ri702bill said:

Adjust accordingly to prevent over-revving the engine - best not to find out why it was put there that way!!

 

The throttle shaft stop screw isn't a high rpm limit. It's only a flow limiter. 

 

On an unloaded engine you can still over rev with the screw in place given correct conditions. 

 

The reason for the governor adjustment following removal of the screw is to realign the end movement of the governor with the - now more freely moving - throttle shaft. 

[JoeM 8,794]

1 hour ago, ebinmaine said:

The throttle shaft stop screw isn't a high rpm limit. It's only a flow limiter. 

 

The high RPM is adjusted with the linkage connected to the governor on the CH. 

 

I never had an 18HP but have had a 20 and 22HP. When I tested them cutting some grass I could see no difference. 

Blue Chips is on it! 

 

 

[Blue Chips 173]

3 hours ago, ri702bill said:

Adjust accordingly to prevent over-revving the engine - best not to find out why it was put there that way!!

 

I adjusted the governor per the instructions in the Service Manual, following the procedure for carburetor-equipped engines. Someone had messed with the governor before, as it had a different (wrong) screw where the governor arm clamps to the governor shaft, and they may have been overtightened it, since I had to file the clamp just slightly to make sure there was still a thin open gap at the end of the clamp when tightened against the shaft.

 

I still need to try it under typical loading conditions to make sure the sensitivity is adjusted correctly to prevent surging, and then re-check the RPM if I made any further adjustments.

 

1 hour ago, JoeM said:

The high RPM is adjusted with the linkage connected to the governor on the CH. 

 

I never had an 18HP but have had a 20 and 22HP. When I tested them cutting some grass I could see no difference. 

Blue Chips is on it! 

 

I'm not expecting to see much, if any, noticeable improvement in power. I think you'd have to have a timed (and reproducible) high-load condition that you could use as a baseline for comparison. I think I'll be happy if it runs as well or better than before. Then maybe I'll tell people I have an "unofficial" GT20. 

 

Edited 9 hours ago by Blue Chips

[Handy Don 14,576]

14 hours ago, ebinmaine said:

[JoeM 8,794]

3 hours ago, Blue Chips said:

(and reproducible) high-load condition that you could use as a baseline for comparison.

Well kind of

I have a steep hill behind my place and wide open mowing up it separates the men from the boys. I think it is about 9 degree.

I have seen a noticeable difference on the machines using the smart spark, they are strong and loose the least rpm. 

[Blue Chips 173]

1 hour ago, JoeM said:

I have a steep hill behind my place and wide open mowing up it separates the men from the boys. I think it is about 9 degree.

I have seen a noticeable difference on the machines using the smart spark, they are strong and loose the least rpm. 

 

A hill like that might be a good test, carried out with and without the throttle stop screw, and in both high and low range. I don't know if I'll get around to attempting a scientific or semi-scientific test, at least not until I have a few other projects out of the way.

 

Varying the spark advance should improve the power curves. My 522xi has the spark advance module, but not my GT18(20).