Troy Bilt bronco 17.5 hp mower is slow

Rivets

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Did you only replace one drive belt? With a variable speed drive system you should have two belts and they should be replaced as a set, which Bert stated earlier.
 

StarTech

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Okay it is an auto drive version. Hopeful you used OEM belts. PTO to variable 5Lx 90.8" and Variable to Transmission 5L x 40.9". A couple thing you can try to increase the speed. One is the Auto drive linkage at the twin idlers. It has an adjustable tie rod end and the other is to try either a 5/8 x 41.5" or 5/8 x 42" Variable to transmission belts. Just got make sure the tension pulley doesn't hit the frame. Also check the front double stack pulley for excessive wear and that the belts are actually the correct lengths.

One last thing to check which I have seen but not too common is that engine mounting hole are worn allowing the engine to slide back. I got one MTD here with a P90 engine that the owner let the motor to stay loose until the mower quit pulling all together. He won't spend the money on the repairs so it is now being parted out.
 
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Appreciate the advice!
Unfortunately, you’re a little too advanced for me. I don’t even know what the auto drive linkage is or where to find it.
 

StarTech

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Appreciate the advice!
Unfortunately, you’re a little too advanced for me. I don’t even know what the auto drive linkage is or where to find it.
No harm in admitting you are over your head. I wish a lot my customers would, it save me a lot of undo their attempts.

Just you are interested the circle is the where the adjustment is. Even I admit it is not the easiest adjustment to get to or to adjust.
Auto-drive setup.gif
 

bertsmobile1

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Looks like Star beat me to posting the parts break down .
To understand why your mower is slow you have to understand how it works.
And it all comes down to spring tension .
When you changed the belts did you check that the sliding element in the sheave # 78 moved fully from top to bottom ?
People who use the cruise control for hours & hours will end up wearing a little notch in the sheave which stops the sliding element moving all the way up or down.
No solution for that apart from replacement $$$$$$$$ .
In the diagram, spring # 39 is connected to pulley # 64 and over time it wears a groove where either end is anchored.
This drops the tension in the upper belt # 60 which usually stops the mower going slow, if it has been put back into the wrong hole so it is too tight then the sheave will not be able to get to the top and the mower will go too slow .
The same thing happens to either end of spring # 20 in fact I have had a couple in where the spring cut right through the speed controler # 5 and fell off.
If the controller can not put more tension in the lower belt # 61 than is in the upper belt #60 then the controller will not be able to force the sliding element to move up & down.

The sliding element moving up & down is what changes the speed just the same as a pair of stacked pulleys on a bench drill except there is an infinate number of combinations of pulley sizes.

Tightening the control rod # 55 will mean that there is more tension on the lower belt # 61 at every position of the speed control pedal # 42.


While this is a bog simple system there are a lot of places where small amounts of wear will create a bit difference to the speed attainable.
Parts to look at are the bushes # 51 ( 2 of them )
Pivot holes in # 72 & # 69 & # 70
And lastly the idler pulleys # 63 ( 2 ) & # 64 .
As a rule they rarely outlast 2 sets of belts.
They must be in good condition externally with no groves or flat spots and spine both freely & quietly.
 
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