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Briggs 18.5 RPM problem


NCREAppraiser's Avatar
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10-28-11, 04:22 AM   #1  
Briggs 18.5 RPM problem

I have a 18.5hp Briggs on a Craftsman ridgin mower that started running very fast RPM. When I move the throttle the RPM only changes about 500RM (guessing). The head gasket was blown and I was hoping the extra crankcase pressure was messing with the governor but thar wasn't the case. I can play with the governor rod and make the motor idle down but when I let go it revs back up. I have checked the throttle cable and it is properly adjusted and has adequate travel. The way I see it is I have either a problem with the carb or the governor has gone bad internally. Is there a way to test the governor? The carb rebuilt is fairly straight forward but being real estate funds are tight and I hate to spend the time and money to rebuilt the carb for no reason. Thanks in advance!

 
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10-28-11, 06:15 AM   #2  
Since you can manually idle the engine down by taking a hold of the governor/throttle link (which is attached to the throttle shaft on the carb), I assume there is no issue with the carb throttle shaft but don't dismiss this immediately as your strength would be good enough to overcome any binding/resistance at the throttle shaft. If you have not, nor has anyone to your knowledge, "adjusted", unknowingly or otherwise, the static governor adjustment then it is safe to say you have an internal governor issue. Post back with the engine ID numbers and we can help you further.

 
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10-28-11, 06:47 AM   #3  
The engine model is 31Q77-0036-E1.

There were no adjustments to the engine. It was running fine and started doing this about 30 minutes (half way) into cutting grass. I noted when I went to empty the bag that it would not idle down.

I can move the governor linkage with little effort.

Thanks for your assistance.

 
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10-28-11, 09:54 AM   #4  
Sure sounds like the governor went bad. i suppose it wouldn't hurt to reset the governor just to be sure, but when the governor stops working mid-session, it usually means it came apart.


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10-28-11, 10:18 AM   #5  
I agree with Cheese; if you were operating this machine when this happened then it's time for the engine to be removed so you can remove the sump and have a look at the governor gear. You left out a digit in your model number but I know it to be a "7" and, just so we know how old this is, what is the code number (off the engine)? A new governor gear is Briggs part number 691968 and new sump gasket is part number 697110 and a new PTO oil seal is part number 795387. FYI, when you are inspecting your external governor mechanism, check for a loose screw/nut between the governor shaft and arm. If this is not loose then open up the engine. If it is not loose, DO NOT loosen it!!! This is your static adjustment.

 
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10-28-11, 10:24 AM   #6  
Sorry the model is 31Q777-0036-E1

The code is 04 421ZD

Thanks for your help.

***Also I checked the bolt and it is tight.

 
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10-28-11, 10:36 AM   #7  
If you find the governor gear blown apart then make sure you inspect the crankcase thoroughly for pieces of the governor and be absolutely certain all has been removed before you zip this back together. Also, the sump gasket part number is a standard thickness part; when you reassemble the sump and after you torque the 10 sump bolts be sure you have sufficient crankshaft endplay - .002" - .023".

 
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10-28-11, 02:40 PM   #8  
I pulled the motor off this afternoon and pulled the sump. The governor appears to be intactand the "wings" move freely as does the center "pushrod". It wasn't to bad of a job.

I've got to order the sump gasket and seal so I guess I will order the carb kit as well and just wait for them to get here.

 
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10-28-11, 07:27 PM   #9  
Hmm... you can freely move the governor arm back to idle the engine but the governor can't do it? Then it's either out of adjustment, the static tension spring is pulling too hard on the arm, or the linkage is binding.


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11-02-11, 05:47 AM   #10  
After getting the motor put back together I finally tracked the problem down to the throttle shaft. When I pulled the shaft out the bottom of it was worn to almost nothing. I replaced the shaft, started the mower and it immediately cut off. It acted as if it was getting no fuel so I sprayed a small amount of ether in it and it started right up..then cut off. I started it again and this time poured a small amount of gas in the carb. It ran until it ran out and again stopped. I'm thinking that the oil that was pushed into the fuel pump by the blown head gasket must have damaged the fuel pump.

 
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11-02-11, 10:00 AM   #11  
You stated "pulled the shaft out the bottom", which leads me to believe you meant to say the governor shaft (which is in the block), not the throttle shaft (which is in the carburetor); is this correct? If so, you have a very rare problem - especially for such a newer engine! You being able to run the engine with gas cheating in the carb throat does not point, necessarily, to the fuel pump. The blown had gasket should not have pushed oil into the pump. I believe you have a carburetor in need of soaking and reconditioning.

 
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11-02-11, 11:53 AM   #12  
The post should read that I pulled the throttle shaft and the bottom of it was worn to almost nothing. Sorry I left the AND out. As I previously stated the governor was ok.

While running the motor by pouring gas in the carb I pulled the hose off the carb that comes from the fuel pump. There was no gas in the line. I originally stated that the whole reason I found the head gasket problem was because oil was coming from the vent on the fuel pump. I guess I could be wrong but shouldn't there have been small amounts of gas coming through the line? All this was fine up until the blown head gasket.

 
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11-06-11, 05:08 AM   #13  
As it stands right now the motor will crank and run if I pur gas in the carb. I have replaced the fuel pump and verified that I am getting gas to it. I even bypassed the filter to seif that was problem. I pulled the crankcase hose off and while it is running there is almost no vacuum pressure. Actually it is almost like small puffs of air coming out rather than being pulled in. Is it possible that when the head gasket blew that the breather got damaged?

I spoke with the only local repair shop and that guy told me that there must be a hole in the piston or block and I would need a new motor which I don't believe. I've seen the piston and there is no hole and the motor runs fine if you cheat the carb. I just can't get the fuel pump to pull gas. I have also verified that the hoses are attached correctly.

 
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11-06-11, 03:00 PM   #14  
Hole in piston or block?? Lol. Try this: Take the fuel line to the carb off, take the gas cap off and blow into the gas tank, creating pressure. Will gas come out the fuel line? If not, there is a blockage. If it does, connect the fuel line, blow in it again to get the carb to fill with gas, then crank it up. It may be that some small piece of trash got stuck in the fuel pump and priming the pump may get it working again.

There should not be vacuum on the crankcase hose when you pull it off. Small pulses it what is supposed to be there.


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11-25-11, 01:15 PM   #15  
FYI the lack of fuel from the fuel pump was actually a bad fuel line from the tank. The layers of hose has separated and the inner layer was coming apart probably due to ethanol in the gas.

Without a load on it there is no reponse from the throttle. It basically runs wide open. Under load it runs fine and the throttle works as it should.

At least I got the leaves picked up. Time to drain the fuel and park it for the winter anyway.

Thanks for the help!

 
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11-25-11, 11:46 PM   #16  
Glad you got it, that's what I was thinking, a blockage.


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11-27-11, 06:36 PM   #17  
Except that you've had it out already and hopefully put it back together properly I was going to say the throttle plate. I made the mistake of pulling the sump on one for a friend because of high speed only after just having them tell me the throttle plate was there and looked closed. In fact one screw had come out and the plate was jammed part open.

 
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