[KingR]

I have a Honda HT3813 lawn tractor that has served me well for about 30 years. I bought it new. At the time of purchase, I also ordered its service manual. I had done all the maintenance on it by the book, including valves, timing adjustments, etc. Other than maintenance items, a timing belt, a PTO belt, a PTO pulley, starter brushes, and a few cutter bearings, the tractor has not needed anything. Everything on it is original and has not been “overhauled”.

This summer, it developed a problem that has me stumped. I am hoping that someone here has either resolved something like it or can provide me some guidance.

First, the tractor has mostly run on Shell 89 octane gas (with cleaning additives). All its life, I have used proper viscosity Mobil 1 oil in its engine and its transmission.

The tractor starts fine on the first try (with choke when cold). Idle speed is fine. It revs up properly to full throttle. PTO engages and it cuts fine. After about ½ hour on a hot day, and an hour or so on a comfortable day, intermittently, the engine RPM suddenly goes up. The tractor accelerates in speed and the cutter turns very fast. I have to quickly turn the throttle down to prevent possible engine damage. If I disengage the PTO and move the throttle up, sometimes it revs up normally; other times it revs up with very high rpm and vibrations (the problem). The PTO does not seem to make any difference.

If I turn the tractor off and let it sit for a while, it starts and runs normally for some time and then the problem shows up.

So far, I have replaced the governor including both bearings, replaced the ignition coil, adjust timing and valves, cleaned air cleaner, checked all electrical components per specs (and they meet them), and opened and cleaned the carburetor. The fuel pump is original and works properly (I have not checked the flow; although the tractor runs normally). The fuel filter was replaced two years ago. The fuel lines and the carburetor gasket are original. I do not see leak in the lines anywhere. The carburetor overflow tube was replaced (I damaged it when removing). Today, I looked at the breather tube. The carburetor end is fine. The engine end does not look tight. I could easily move it even with the clamp on. Could this be the problem?

In trying to locate the issue, I did notice that when the engine is running normally, the link from the governor arm to the throttle has considerable force. It keeps the throttle to near closed position (spring under tension). When the tractor is misbehaving, this force is not there and the throttle link is pulled to the max (spring pulls it forward towards the front of the tractor).

Although I replaced the governor, I did not find any problems with the old one. During installation, the new governor seated properly and the weights could easily slide on the pins. Hard to believe, it looks to me that when the problem occurs, the governor seems to be disengaged. Is there a large play that can cause the governor to slide axially and become free from the large toothed gear turning with the crank?

I also thought about a problem with the vacuum. How does this work and can it affect the throttle and the governor function?

If someone has experience in this area and can provide me pointers for where to look next, I would really appreciate it.

Thanks.

 

[Tom Bauer]

if the gov arm does not push back against the spring, you must take gov apart and see why.

 

[KingR]

I suspected the governor too and was the first thing that I replaced. I have attached a page from the service manual. Governor weight holder gear assembly (includes new pins, weights and sliders) was replaced. I also replaced the two bearings, one behind and one in the governor cover. The only thing I noticed as unusual was that that the governor fork attached to the governor arm shaft had put wear marks on the closed side of the governor cover. IMO, this is a poor design. Over time and with movement, the steel fork would wear out the aluminum cover. This is exactly what happened.

To fix, I put three very thin stainless washers inside the hole in the cover before installing the shaft and then screwing the fork. This pushed the governor arm out enough to be in the center of the opening. The movement is free and the fork does not touch the cover's sides. Please note that once installed, the shaft is typically at an angle. With gravity, it is generally resting against the far end.

After I assembled and ran the tractor, the problem was still there.

I saw engine oil in the governor area with everything well lubricated. The oil was replaced in the tractor to proper level and I do not see any oil leaks. Does anyone knows if the oil is circulated by the gear in the crank or forced through some orifice by the oil pump? Theory is if oil is not circulating, then the weights could get stuck and the governor will malfunction.

 

[Tom Bauer]

fork should never touch cover. doesn't the spring arm keep it from moving in to far? gov slider goes in gear with weights hooked on and then pins put in. when together slider will not come out of gear. are the weights new? it has to be something wrong with gov.

 

[KingR]

Tom, I agree. The fork should never touch the cover. The notch in the governor arm shaft is such that when the spring arm is pushed all the way, there is about 5mm clearance between the arm and the governor cover. I did not see anything else in the assembly that would prevent the fork from touching the cover; hence my washer solution.

My first suspect was the weights. I replaced the whole assembly that included new pins, weights and the slider. I checked the pins and weights twice. The weights were moving freely. I also checked the old governor assembly. To be honest, I did not see any problem such as wear on it either.

At this point, my suspicion is moving towards lack of lubrication. If oil is not being supplied properly, the centrifugal force would force the oil out and cause the governor weights to stick, creating the kind of a problem I am seeing.

I recently adjusted valves clearances. I see plenty of oil in the valve area. I do not know how the oil is channeled to the governor. If there is some kind of a blockage somewhere in the path to the governor, that may explain lack of lubrication. Perhaps, engine flush is the first thing to do. Anyone has any suggestion what is safe and effective way to do it?

Thanks!

 

[rogerb]

Mine has been doing that too. It is very puzzling. Going to take her apart this winter to have a look see. Sure would like an answer to this mess.

Does mother Honda have a suggestion?

 

[rogerb]

How about Robert from Honda? Any answers on this?

 

[[email protected]]

Way back in 1989, Honda issued a Service Bulletin for two symptoms:

If this is determined to the to source of the trouble, then find out the oil pressure status. Depending on the data collected, the engine may require some of the oil pump gears, a new cylinder barrel assembly along 7+ parts to get it right.

Hope this helps.

 

[rogerb]

Oh boy................Thanks Robert.

 

[Tom Bauer]

yeah, that was my thought, that the drive gear on the crank was not turning. i don't know how that gear is fastened to the crank, i am very surprised it is only a light press fit that would be crazy for something this important. the good news is that now that this has surfaced, it can be fixed although it will be a LOT of work.

 

[KingR]

Thank you Robert! Your service bulletin post has verified my suspicion of lack of lubrication as the cause of the problem.

I have posted two pictures from the tractor's service manual. From the bulletin it appears that the oil pump drive gear (gear 1), or the oil pump (gear 2) could be the problem.

I am speculating that with wearable parts, the oil pump could lose its effectiveness and cause reduction in lubrication creating the problem. I have ordered the gear (gear 2), both rotors, O-rings and the gasket. The parts were not very expensive. I believe that after removing the PTO, the oil pump can be repaired without removing the engine. Once its done, I will post the result.

Repairing the slipping oil pump drive gear is a serious undertaking. From the data that Honda service may have collected, did they find if slipping was more often the cause of the engine overspeed problem?

The service manual show that the oil pump drive gear is simply pressed on. I do not see any lock type mechanism. In order to repair it, most likely the engine would have to be disassembled, which I am trying to avoid.

Thank you all for your help.

 

[Tom Bauer]

the problem is that if you make the oil pump better it will put more load on the drive gear making it more likely to slip! since the drive gear is not available without the crank the only repair option is to knurl the surface of the crank so it is a tight fit or machine a keyway.

 

[[email protected]]

Thank you Robert! Your service bulletin post has verified my suspicion of lack of lubrication as the cause of the problem.

From the data that Honda service may have collected, did they find if slipping was more often the cause of the engine overspeed problem?

As Honda has been out of the riding equipment business for 15 years now, the data related to this service bulletin has been long gone, sorry. Non-safety record retention policies, etc. Even our key riding equipment engineer has been retired for 4+ years now, so all I have are some spotty notes in old shop manuals. :-(

Sorry I can't offer more details, but hopefully you have enough now to make some decisions on the next steps.

 

[KingR]

First, thanks to all and specifically Robert from Honda. I decided to take a crack at the oil pump. My advanced non-mechanical engineering expertise was telling me that Honda must have done thorough research before deciding to press the oil pump gear on the camshaft. I could not see it slipping before some other major failure elsewhere.

I ordered all the oil pump moving parts and the gaskets. I removed the radiator and the PTO and decided to repair the oil pump with the engine in place. One peculiar thing I noticed was oil streaks going back towards the rear and oil soaked dust and grass crud. I cleaned the area and opened the oil pump.

Upon opening, I discovered that the outer rotor was installed with the punch mark facing in. The inner rotor was installed with the punch mark facing out. I had bought this tractor brand new and the oil pump was never opened or repaired. I can only speculate an assembly error.

The new rotors I received did not have the punch marks. Only difference I could see was that one side was shinier than the other. I made it a point to install rotors with both shinning sides facing in since they are rotating resting on aluminum housing. Please note the page from the service manual (attached). It explicitly calls out that the punch mark should be facing out. There must be a reason, but I could not see. Oh well!

I assembled the oil pump with all new gaskets and rings. Bolts were torqued equally. With warm November and December in the Northeast, I decided to test the tractor.

About 2 acres of grass was cut with one break. The engine never raced or accelerated suddenly. After a week, I installed the bagger and cut the grass again as well as picked up leaves etc. Again, the tractor ran flawlessly. This is the first time the tractor did not misbehave since I noticed the problem.

At this point, I believe the root cause of my problem was the lack of lubrication to the governor with oil pump malfunction. If I see this problem occur again, I will definitely post.