VERY,VERY old toro

I have a 3.5 hp self propelled toro push mower, 21 inch aluminum deck. It is 25-30 years old . All of a sudden for my latest mowing of a postage stamp size lawn, it would run at what I call 3/4 throttle speed, even with the throttle lever fully advanced. There was a little bit of surging to high and medium rpms, but this went away, leaving me with a medium rpm to do my small mowing job. The idle speed seemed normal, only the advanced throttle setting was lower than normal.The throttle cable seems well attached and the spring that disappears into the mower housing is still there and attached. After I stopped the mower I couldn't start it again until I removed the air cleaner housing and propped open the choke plate. Then it started, but again stayed at medium rpm. I thought it might be time for a new filter, so I left it overnight to check again. When I checked it cold it started and stayed at medium rpm at full throttle for a moment and then died. I removed the air cleaner housing and replaced the bolt only through the choke plate, thinking this would bypass a dirty filter. It won't start. Any help will be appreciated.

 

Check the throttle cable itself. Sounds as though where in it held by a clamp for adjustment, has slipped or part of the cable itself has failed.
Most likely it will not start now because the choke does not close since the cable does not fully close the throttle.
If you find the clamp that hold the cable, and can see it has slipped put it back where it was and try it or readjust it.
To do so,set the throttle lever to full/choke position, loosen the clamp on the cable clamp and move the cable and so it forces the lever on the carb and the choke plate fully closed. Tighten the clamp.

 

Actually, the throttle cable doesn't control the choke on this engine. You have an automatic choke controlled by the carb diaphragm. I think you probably picked up a little water, and need to remove the carb, clean it and the bowl in the tank, and replace the diaphragm.

 

Thanks to both for the reply. My model is 16320, serial 9030954.
I had already checked the throttle cable and it is tight and working correctly as I tried to indicate in my initial statement. Here's kind of what I want to know, if you can help. Is water in the fuel something that can allow the engine to run at half speed while at full throttle setting but otherwise seem ok? Can a blocked filter give this symptom? This seems odd to me and I can't find this specific complaint on the B&S troubleshoot site or in my original 30 year old instructions. In the meantime, I checked for spark and it is there. Also when I crank with the filter off I see a small explosion in the choke bowl. Doesn't this mean spark and fuel delivery are ok? I would also like to know if I can start and run the engine for test purposes only without the filter in the housing as a way to prove that a blocked filter isn't causing the problem? Thanks much, as I am committed to having this mower forever!

 

You can start and run the engine with the entire air filter assy removed, just beware and insure nothing get blown or dropped into the carb.
Briggs does not offer free manuals for your model online....certainly with the service it has given u and that u expect to get still,...is worth the time and effort to replace the diaphram as Cheese suggests and a lil PM (Preventitive Maintenance) while ur at it.
I have a toro with a 3.5 briggs not self propelled but has the auto choke but is a 190...model series so I am not sure what urs may look like.

The air filter could cause slow rpm but should also run ruff as it would be a rich mixture, smoke and fouled plugs would be symtoms I would think. The same would hold true if the choke does not open completely.

 

Thanks, Az. I did get the engine to start and run for a short time with the air cleaner assembly off, just the choke plate screw in place. I still had medium rpm at full throttle. Since there was no filter in place, does this mean the filter is not the problem? Also, do you think bad gas would give a slow rpm situation like this? Any thoughts on where I would get instructions to remove and replace the diaphragm since B&S no longer supports this model?
Thanks again to all.

 

Parts are available and the task is fairly simple and straight forward as long as you know the correct end of a screwdriver

leave the carb connected to the tank, remove the bolts holding the tank on. One main bolt with a bushing and perhaps one or two on the front connecting the speed linkage bracket.
With the tank/carb assy loose from the engine, lean the assy to allow the z-bend or hook on the end of the metal rod to the carb linkage to come out...the hose can come off of the engine or carb or both, it is only stuck a bit just twist and tug persuasively not violently
Now you should be able to seperate the carb by removing 5 or 6 screws. Be sure to clean the pick up tubes and now is a good time to clean the tank as well.

 

I'm going to tackle the disassembly this weekend. Can anyone offer an opinion on whether bad gas or bad spark or actually anything other than carb diaphragm could explain an engine running ok but just at medium rpm while at full throttle?

 

Unless u r able to determine some facts and inform us via text here, there is really no other way to positively identify the problem. We are not able to hear the engine run see or smell the exhaust, poke or prod......
If the RPM are low is the mixture correct? does it bog under load, what happens if you move this spring...this lever....push on this....
If the mixture is correct and all runs exactly as it should if it were set to mid throttle, then the only logical reason is mechanical, in that the throttle valve is not opening far enuff. If all the mechanical linkage is correct including the govenor, and it will still not rev up, then the mixture can not be correct. This is caused by a restriction or lack of regulation to either air or fuel flow.

If the choke is not working correctly, if the butterfly is closing partially it could act as the throttle valve, restricting air flow which in turn is responsible for fuel flow. IE: The more the butterfly opens, the more air it draws in and the more fuel it is able to draw. This will also cause hard starting when cold as you have mentioned. As cheese mentioned you prolly have an auto choke which is controled by the diaphram.

 

The detailed explanation is very helpful in getting me to understand how this system works. I will post the results .
Thanks

 

Well, here's the result of working on the mower over the long weekend. I replaced the foam filter and the spark plug and filled the tank with the same gas can I have been using. It started on the 2nd pull and sounded better than it had in years! The speed didn't seem to be max rpm, but pretty close. I thought it was fixed, but then I dropped from full throttle down to slow and there was no change in rpm-- it stayed at high rpm as if there was no cable connection. All cables and springs were properly attached and seemed to be working, but there was no response to adjusting the position of the throttle handle. I thought the only thing left to try was the carb adjusting screw, so I tinkered with it, and after a reasonable adjustment, tried the throttle again and it began working normally! I have started the mower 3 days in a row and it seems to work properly. I don't want to look a gift horse in the mouth, but I don't understand what has happened. Why would the carb adjustment fix a no throttle response? Either way, I think I can mow again, but your thoughts on what happened will be welcome.

 

rleescott,

Your carb most likely has two adjustment screws if not three. These are the idle mixture screw, idle adjustment screw and if yours is equiped, the main jet or high speed adjustment screw. Your correct in assuming that either mixture screw would tend not to make a big difference in the rpm's and you are correct however, I am thinking that the adjustment you made was to the idle adjusment screw. If this screw had been turned all the way in it would prevent the throttle cable or governer from fully closing the throttle flap and this would explain when readjusted the throttle lever now works as normal. The idle adjust should be adjusted when the throttle lever is at it's lowest setting.

God Bless
Dave237

 

Thanks for the reply. You make an interesting point. However, the photo of my engine from toro shows only 2 adjustments, idle mixture and idle speed. I adjusted the one the illustration calls idle mixture. When I moved the other one it caused immediate changes in idle speed and rough running. I'm pretty sure I adjusted the right one. Also, no one has ever touch any screws on the carb in 20 plus years, so there's no way I can imagine a significant adjustment having occurred by vibration, since the screw is hard to turn. By the way, my next step was to get a new carb diaphragm as was suggested, but at this point it looks like I don't need it. I sure would like to know what Cheese and Azis think.

 

rleescott,

I re-read your original post with all replies. I can think of many ways for your engine to respond this way, while disregarding all the other suggestions as not the cause, I or any others can only guess being we are not there to witness the symptoms.

Cheese, and Azis will way in when they can however, I will only guess as I'm sure they will, in this case, that gas and air will mix in only an exact proportion for complete combustion. If upon adjusting the idle mixture screw your engine now has full rpms, I suggest that trash lodged between the idle adjustment needle and screw wall allowed enough gas through the idle circuit (small holes that are beyond the throttle flap) into the intake to cause the medium/full rpm scenario.

By adjusting the screw it would have loosened the trash however; I would also guess that there is some air getting into the intake from an intermitten intake leak when your problem occurred. At some point in the future your problem; I will guess, would happen again until the leak reoccurs if such be the case.

God Bless
Dave237

 

On this engine, if you happen to overtighten the air filter screw, it can push the air filter housing down far enough to interfere with the throttle linkage and keep it from moving. The air vane can also get stuck. Chances are it was just coincidence that fiddling with the adjustment screw made the changes. Look under the filter housing and you'll probably see where the linkage is very close to it.

 

It was pretty dirty under the filter housing and i used this opportunity to clean thoroughly. I think you are all correct, and I appreciate the help.