John Deere F935 Questions

[mlenkowski]

I have a F935 that I purchased used with 2300 hours. The machine was in fair condition and I have replaced a good amount of parts to get this machine running at top condition; fuel lines, injectors, glow plugs, filters, valve adjustment, bearings, tires, etc. The machine runs well and I have a 72" side discharge mowing deck and 60" rotary broom, both in good shape. I have had a JD dealer go through the machine as well, giving it a thumbs up on status, including compression test on the yanmar. When using this machine I notice and experience the following that lead me to get other F935 owners thoughts regarding engine performance:
1. I have to start the machine at 1/3 or 1/2 throttle or else it's a very "rough" start, i.e. I can't start it with the throttle all the way down or else it runs rough until I give it more fuel. Once started I can set the throttle at the low setting with no problems.
2. When engaging the pto with the 72" deck I notice (hearing) a drop in engine rpm, maybe a few 100 rpms. I do not get this with the broom and I understand the 72" deck is a big attachment to get moving. This might be a sign of the motor adjusting to its torque curve, but I am not sure.
3. The mower performs well in normal conditions; dry to damp cutting 2-3" of grass. When cutting 4" or more of grass or wet to very wet grass the engine bogs down. It will produce black smoke when the load on the engine gets to be much. When it bogs I have to slow up a bit to let it catch back up. I do keep my deck scrapped and blades sharp.
4. I also have a F1145 and 855 CUT, both with the direct injection 3TN75RJ engine. When these machines get loaded I notice they bog down differently, mainly they work through it better, produce more black smoke, and recover quicker. They also start better. I do understand that they are bigger motors and are direct injection unlike the 3TNA72 engine. I just get concerned that I am missing something obvious on the F935 that can be easily resolved to improve engine performance.

I have checked and adjusted slightly the throttle cable and I have checked for air leaks in the fuel lines which are new from the tank all the way to the injectors. I have also replaced air intake hoses and I maintain very clean filters. I have not ventured into the governor or exhaust yet as there have been no major visible signs of issues that I believe to be causing anything to go wrong in these areas. Maybe these items I noted above are just the limitations of the 3tna72uj power plant in the F935. In any event I am looking to get thoughts from other users of F935s and tips on some checks I can run to make sure I am getting the most from the motor and that it is operating at its peak. Thank you

 

[RoyJackson]

I've had three Deere tractors with Yanmar engines. The starting procedure is to open the throttle about halfway...try that.
Now, I never started any of those tractor like that...always started them with the throttle closed, then once the engine started I'd throttle up to 1500 RPM.

The F935 engine produces 25 HP...that isn't much for a 72" mower deck. The F1145 produces 31 HP, IIRC, with the same size deck. More HP, less bogging and quicker recovery. I don't have a broom, but I'd guess there is less inertia with the 60" broom then there is with a 72" mower deck. You could try engaging the mower in the raised position...that might help.

 

[zzvyb6]

I have an early F935 with similar black smoke symptoms that I attribute to the large hour meter: Compression might be down. However, sharp blades help this. Also, the mower deck gets choked because of the design of the housing and the cutting height settings. This problem goes away when I hook up my trailer leaf and grass sucker. I've used the Gator brand blades for more mulching effects, but prefer the high lift lighter weight blades. Less smoke issues with them. No problem with motor bogging when engaging the deck, except that I note the starting inertia of it is high with 3 large blades and the electric clutch slips. I don't engage it at full throttle.

The starting issue may be related to glow plug(s) failure. I replaced all of my glow plugs and it starts immediatedly, even now in sub freezing temperatures. I had a serious problem with fuel lines in both tanks. They both cracked about 1/2 way down and the unit ran out of fuel despite 1/2 full guage readings on both tanks. Sucking air may have resulted in the fuel pump failing, but a new one from O'Reilly auto has put the machine back in full sevice. I even run my Winpower PTO alternator off of it. I bought a 3:1 gearbox with a std farm type spline output and swap the 935's prop shaft onto it. You just never know what situation will come at you. I can also run this 50kW genny off my JD1070 but why bother since the 935 handles the deal without a burp. 'They' say you might need your tractor for use other than electricity generation, so I have a few options (including my new Honda EU6500IS toy).
Only other thing I put on it was a new seat. I hate a wet butt.

 

[mlenkowski]

Great insight and feedback. The compression is good on the machine. I am primarily looking to see if other f935 owners with the 72" deck also notice engine rpm drop when they engage the mower and if and what happens when their machines get loaded and bog down. Any and all feedback is welcomed, thanks!

 

[pmsmechanic]

My F-935 also bogs down when I engage the 72" deck. I have no other attachments so cannot comment on the engine performance with them.

I have about the same amount of hours you do. I also had to do a few repairs when I bought the machine. Mowing in a straight line was impossible as the lift pivot bushings and the mount pins (the two pins on each lift arm) were wore out. Once new bushings were installed all was good again.

I always had trouble starting after the mower sat for a while. If I bled the fuel filter all was good again. I suspected the fuel lines on the tank but before I got to repairing those, the bleed screw on the filter stripped when I was bleeding the air out of the filter one day. I debated about how to repair the screw but in the end I just went and ordered a new filter housing from John Deere. I no longer have to bleed the fuel system. I don't know exactly what was wrong but it's now fixed.

I can cut off 4" of dry grass at full speed with new blades. I don't get a perfectly clean cut but it works. The motor will bog down on tough wet grass if I go too fast. When I got the mower I wondered if it was running properly but in the five years I've had it the motor runs ok. I doesn't start good in cold weather but I suspect it's because of the indirect injection as my 955 and later my 4410 both start quite fine in cold weather. As a comparison my 4410 works harder mowing the same grass with a 72" Farm King rear discharge mower than my F-935 does with a 72" side discharge mower.

 

[mlenkowski]

Sounds as though my concerns are mitigated by these helpful and comparable responses. I am realizing the limits of the machine. Don't get me wrong, it's a great and capable piece of equipment. I have and plan to continue to keep everything well maintained. It is tough to power the 72" cut with a 881cc engine, especially that heavy yellow deck. I can live with starts at 1/3 or 1/2 throttle, which the manual provides for as well. I was just concerned with the rpm drop when engaging the deck. Thank you to all that replied. I am also grateful for any one who can offer insight on my F1145 engine question thread. Thanks!

 

[zzvyb6]

You should check the rpm at max throttle position to see if you are at the expected or designated power level. Mine was down a bit so I upped it to get more 'jump'.

 

[mlenkowski]

I will, what is the proper procedure for this? In other words how should I perform? Also are you referring to an adjustment of the throttle cable or is this more of a screw setting adjustment? Thank you in advance!