# Mtd 638 rl



## StealthWyvern

Hello,
I recently came across a unknown year MTD 638 RL riding mower with a "Formula 13.5hp Tecumseh" mower. When I got this mower it was sitting for about 2 years or so with 4 flats and spindle decks rusted in position.

I've soaked the desk with rust penetrating spray on all pulleys, put a new battery, and new carb since it was only about 5 bucks more than a rebuild kit. I also cleaned out the fuel tank the best I could and put a new fuel filter. Along with fresh gas, and fix a flat in the tires and drive it down the block and back. All 6 gears work forward and reverse.

I took the "fly wheel/engine cover" off and haven't reinstalled it and also ordered a new breather tube as the other one is brittle and not installed. I haven't reinstalled either of these.

The mower ran great for about an hour or so. The throttle had full range and was easily controlled. However after cutting the grass and dis-engaging the deck the mower surged up in rpms and stayed pretty high. I obviously throttled down as ASAP but the rpm didn't come all the way down like it would normally when fully throttled down to a idle. I had to pull the governor arm back just a hair for the rpms to settle down but then they would some times go back up a few hundred rpms (I think). 

I did toy with it and adjusted the idle screw all the way out on the carb thinking maybe that would help but made no difference in bringing the rpms back down only up if I adjusted it the incorrect direction. I ended up putting a zip tie on the governor arm to allow me to manually bring the rpms back down if they surged up causing what I think was a over reving condition. Does anyone have any idea what is going on? I'm sort of new to small engines but have the basic fuel injection automotive knowledge of engines but not much on carbs. I suspect the springs shot but no clue where to get new ones out or really how to test. I've attached a few pictures of this mower for reference. Any help would be greatly appreciated


----------



## EdF

Hi Stealth, welcome to the forum.

I found the following on the internet regarding a Tecumseh governor: 

Overspeeding Engine

With the engine at idle, move the governor lever to open the throttle and push the governor arm back toward idle. Remove the governor spring. If the engine continues to overspeed or has no push toward idle, then the damage may be in motor, static adjustment or other internal parts. In this situation, take the engine to a professional for repairs.


----------



## jhngardner367

First, NEVER run it without the cooling shroud off,for more than a couple of minutes.
It sounds as if the governor is out of adjustment.
This might help.
View attachment 28193


----------



## StealthWyvern

jhngardner367 said:


> First, NEVER run it without the cooling shroud off,for more than a couple of minutes.
> It sounds as if the governor is out of adjustment.
> This might help.
> View attachment 28193


So much for NEVER as I ran it about an 45min to an hour without issue. Either way im waiting for the breather tube to come in(Friday hopefully), ill put that on and the flywheel cover and see if that improves my over reving issue. I think the ignition switch has either went bad or the battery is completely dead. So the battery is on the charger for now.

Ill have to take a look at the linkage and see if it matches up to with what you provided. I haven't ever messed with the governor adjustments and neither has the previous owner (a friend of mine). The only scews I messed with was the idle screw on the carb itself and the idle screw on the linkage to allow the linkage to come back and possibly push the governor arm back to bring the rpms down. However that doesn't seem to make much of a difference from what I can tell. 



EdF said:


> Hi Stealth, welcome to the forum.
> 
> I found the following on the internet regarding a Tecumseh governor:
> 
> Overspeeding Engine
> 
> With the engine at idle, move the governor lever to open the throttle and push the governor arm back toward idle. Remove the governor spring. If the engine continues to overspeed or has no push toward idle, then the damage may be in motor, static adjustment or other internal parts. In this situation, take the engine to a professional for repairs.


Thanks, if the above doesn't seem to make an improvement. I'll give this a shot.


----------



## jhngardner367

The shroud directs air across the cylinder,and head.
With out it,you could warp the head.
The adjustments I posted came straight out of the Tecumseh repair manual,for that engine.
the governor spring,by the way,is usually in the 3rd,or 4th hole.


----------



## StealthWyvern

jhngardner367 said:


> The shroud directs air across the cylinder,and head.
> With out it,you could warp the head.
> The adjustments I posted came straight out of the Tecumseh repair manual,for that engine.
> the governor spring,by the way,is usually in the 3rd,or 4th hole.


How can I tell if i caused any damage? It doesn't smoke nor does it seem to be leaking oil that i noticed.

Do you have anything showing this linkage setup?


----------



## jhngardner367

Well,if it did any damage you'd have known by now,as it would have seized,from the heat.
it looks as though the linkage is correct.
Yes, this should help:
View attachment 28206


----------



## StealthWyvern

Thank you jhngardner367 for your help. I got the breather tube in, installed and the cover back installed. As I suspected my ignition switch went out so I'll be ordering a new one here shortly. I kind of figured it was to easy getting this mower up and running after it sat under a friends car port for a few years neglected. 

I did bypass the ignition switch and ran power to the start and noticed that it wasn't cranking over like it did prior to shutting it down. It cranks freely but when it attempts to fire it will A fire up in the right direction and spins a few revs and die or what it does mainly right now is spin clockwise and when it attempts to fire it spins counter clock wise and stop. its odd almost like its trying to run in reverse

I need to research this issue to see what the issue is. I think it will be eaiser to research when I find out what the technical term is on that. I'm thinking possibly clogged fuel filter\line not allowing the carb to get the fuel it needs or possibly a weak spark.

I was able to upload a very short video clip showing what I was talking about. 
[ame]https://www.youtube.com/watch?v=z1p1K4X-B1c[/ame]


----------



## jhngardner367

Look on the valve cover,and see if there are som model #'s,or, they could be on a tag on the cooling shroud.
I'm looking for something like "OVM 120,or OHM 150",etc,as these models had a compression release arm,on the cam,which if it's rusty, or broken,won't open the valve,causing this condition to happen.
In the mean time,Check the flywheel timing key,to see if it's sheared.
Even a small nick can change the timing!
Remove the nut,and washer on the fly wheel,and look at it,It should be completely straight. 
When you tighten the nut back down,torque it to 50-55 ft/lbs.


----------



## StealthWyvern

jhngardner367 said:


> Look on the valve cover,and see if there are som model #'s,or, they could be on a tag on the cooling shroud.
> I'm looking for something like "OVM 120,or OHM 150",etc,as these models had a compression release arm,on the cam,which if it's rusty, or broken,won't open the valve,causing this condition to happen.
> In the mean time,Check the flywheel timing key,to see if it's sheared.
> Even a small nick can change the timing!
> Remove the nut,and washer on the fly wheel,and look at it,It should be completely straight.
> When you tighten the nut back down,torque it to 50-55 ft/lbs.


I've ordered a new flywheel key and it should be here roughly monday simply due to it was cheap and if it is a sheered key ill at least have that part handy in the tear down process. I may not be able to even look at the thing this weekend. Below is the engine info.

Model: 0V358EA
Spec: 206940G
Engine Family: 8TPXS.3582AA


----------



## jhngardner367

Yep,this may be why they let it sit,for so long.
those MCR cams had a habit of breaking the pin,and they wouldn't start.
A new cam runs around $105.,but I have heard a couple of people that repaired them,somehow.
Either way,to check it ,you'll have to pull it apart.
Although the book says to replace the crank gear,also,.I never have .
View attachment 28217


----------



## StealthWyvern

Thanks, if the flywheel key is ok I'll tear into it a bit further. I have an idea what im looking for but not 100% certain. If its not blatantly obvious ill post some pictures to confirm things. If I have to tear into it that far I might as well look into a rebuild kit. I saw something the other day that makes me think this is a 2004 year model.

I have a feeling it might of sheered a little when I was trying to engage the deck a the first few times as they were locked up. I ended up having to take the deck off and soak everything in rust penetrant and persuade them to break free with a mallet. The mower would die when I tried to engage the deck prior to breaking everything free. 

They let the mower's sat (bought 2 of them) due to it was cheaper to replace them with other used ones than to fix the current ones they had. The people I purchased from aren't very mechanically inclined. They just fuel up, get on and go for the most part.


----------



## jhngardner367

Well, if you have any questions,during the repairs,I have the manual handy!
look under the seat for a label that will have the tractor model ,and year,on it. 
That will help.


----------



## StealthWyvern

jhngardner367 said:


> Well, if you have any questions,during the repairs,I have the manual handy!
> look under the seat for a label that will have the tractor model ,and year,on it.
> That will help.


jhngardner367, I do happen to have those pictures already on my phone. Sorry, that they are at a awkward rotation. Do you by any chance happen to know what size the flywheel nut is? I'm hoping your manual will state what size it should be. None of my sockets are the right size or if they are they are heavy duty impact sockets that wont fit in that area. So I'll have to go buy one just for this task. I see most video's on YouTube are 15/16 from what I am seeing.


----------



## jhngardner367

The book doesn't say,but I believe it's a 15/16",or 1".
They used both.Measure from one flat,to the opposite flat,of the nut to be sure.
Your machine was made in 2008/2009,for Home Depot sales.


----------



## StealthWyvern

Turns out it was 1 1/8 sized nut holding the flywheel in place. Sadly i had to buy an impact socket set for that one socket but on the bright cant hurt to have more tools! The flywheel key appears to be ok from what I can tell. What are the chances the valves are way out of adjustment causing this condition?


----------



## StealthWyvern

Wells its running now. Its appears i may have a bad ground somewhere and or a weak battery. It turns out the ingition switch is working fine and that I just didnt put the trans in park before attempting to restart. 

The idle screw on the carb and linkage are backed all the way out to allow the lowest idle possible if not even to low of a rpm. All linkages and cable appear to move freely. 

https://youtu.be/kxJGu5uvx3k


----------



## jhngardner367

Glad you found the problem.
I forgot the "Rule of K.I.S.S."
"Keep it simple,stupid"
Looking at difficult problems,instead .
Key looks fine.
Doesn't hurt to have more tools !


----------



## jhngardner367

That carb shouldn't have a high-speed adjustment screw,...only a screw for the initial idle speed,which is set at 350 rpm.
The Vector Series 7 carb high speed, is done at the control linkage plate . 
Look at the pics I sent for the linkage (side view)
Maximum rpms is 3400 ,or you risk loosing a valve,or worse.
Make sure the governor spring is the right one,and set the governor,before adjusting the rest,or it can overspeed !


----------



## StealthWyvern

John (I assume that's your name),
First I'd like to thank you again on all the help you have been. I went to adjust the governor as suggested by placing the throttle at the fast notch which should be wide open without going into the choke area of the lever. It appear the governor is slightly different than what your manual states and different from everything I've read online. The concept seems to be simple but when the lever panel all in place it appears this is almost impossible to adjust. *see below image* 

I did loosen the bolt holding the arm in but it appears to be fixed by the "tab" as shown by the blue arrow. I also did loosen the bolt (red arrow) and push it a little to the left but I feel I may of pushed it to far due to the linkage having next to movement in full or low throttle. Keep in mind that I can't reach the bolt marked in red with the lever panel in place. I also haven't even bothered to attempt to start the mower as I feel I have it way out of whack now. It all seems to pivot at that bolt in red instead of my a rod with a slow like. I'm also not certain if that lever panel is not bolted to the engine if it impacts the adjustments of the governor while its removed as shown.

Edit: Toying with it a little bit more with the breather box off and watching everything while adjusting the throttle. I had noticed that really roughly 3/8th of the throttle was really full throttle with the butterfly wide open on the carb. I did notice the butterfly start to open slowly with the throttle but then would jump to 3/4 of fully open with hardly any adjustments on the throttle lever itself. This had me double check the linkages and all are correct according to your diagrams Another thing I noticed is that when the throttle was all the way down the throttle butterfly wasn't fully closed. So I'm starting to think my issues might throttle cable adjustment issues and possibly governor now.

I was able to adjust it to the point the throttle closes fully when the throttle is lowered all the way down. I need to "test" my adjustments but the Throttle plate on the carb being fully opened with a little less than half throttle concerns me. I'm not sure how I'm going to get around that one, other than stretching the spring on the linkage so it doesn't pull the governor lever towards the carb so soon. Adjusting the spring further up on the governor seems to help a little but doesn't resolve that issue.

Edit 2: So I tinkered with the carb to try and at least get the idle control under control and even with the above adjustments, its still sounds like it is idling kind of high from what I'm used to. This is with no linkages connected other then the governor arm so everything is returning to the "default/resting" state.

I did check all linkages move smooth and freely even the throttle cable without the governor spring on. I did notice something off about the spring on the linkage is that it looks like it had teeth marks on it from a pair pliers or something. So once I figure out the part number I'll be ordering a new spring due to the unknown condition on this governor spring. When the spring is on the linkage its the only time the throttle play didn't move freely.


----------



## jhngardner367

Yeah,that looks like a much heavier spring,than is normal.
P/N for it is 35945 .
$1.47 ,at Partstree.com.
When you set the static governor setting,do it this way:
Disconnect the throttle cable from the linkage.
Place the throttle in wide open position(NOT CHOKE).Note which way the governor arm moves.
Loosen the screw (red arrow,in pic),and push the governor arm in the same direction,and keep finger pressure on the arm,and turn the clamp/shaft,in the same direction,until you feel it seat(not too hard,just seating),and tighten the screw.
When you re-install the throttle cable,hook the wire into the linkage,but leave the clamp loose enough to slide, a bit,and put the throttle in choke position, while holding the carb linkage in choke position. 
Snug the clamp and see how it operates.
It may need small adjustment , after that,but should work ok.


----------



## FredM

glad you finally told the bloke how to adjust this setup, have been itching to have my tuppence worth, out of respect, I didn't.

Cheers


----------



## StealthWyvern

Thank you, The part has been ordered. I'll start tinkering with it when that part comes it. It will more than likely be next weekend.


----------



## StealthWyvern

New spring on, governor adjusted, and throttle cable adjusted and still no difference. I'm starting to think the governor inside the motor is fubar from sitting or something. I'm at the point where im tired of messing with this thing and might pay a professional to fix the rpm/throttle issue.


----------



## jhngardner367

Ok,if it's still idling high,it could be running lean,or have the float set wrong(it happens even with new ones).
I was incorrect on the idle speed,...it should be 750-800 rpm.
The 350,after reading the passage is the kick-off point for the compression release. 
My apologies.
Do you have a tachometer,to see just how fast it's idling?
Also,if the throttle cable is not letting it return properly,try putting the spring down one notch,on the governor arm.


----------



## StealthWyvern

jhngardner367 said:


> Ok,if it's still idling high,it could be running lean,or have the float set wrong(it happens even with new ones).
> I was incorrect on the idle speed,...it should be 750-800 rpm.
> The 350,after reading the passage is the kick-off point for the compression release.
> My apologies.
> Do you have a tachometer,to see just how fast it's idling?
> Also,if the throttle cable is not letting it return properly,try putting the spring down one notch,on the governor arm.


I don't have a tachometer to confirm the idle speed but going off ear it sounds like its way to high almost like its about at half throttle. I did notice that even with the throttle linkage disconnected and with literally only the governor arm connected to the carb it still didn't idle properly nor return back down to roughly sounding the same rpm each time. So I'm starting to think that the carb or governor is an issue. I also have no way to tell if its running lean but it doesn't smell like its running right. 

One thing I noticed is that I have to crank and crank on it till it starts to catch with the choke on and then it will almost start on the next crank without an issue.


----------



## jhngardner367

It sounds like an intake leak,...gasket,maybe ?
while it's idling,spray some cab cleaner on the manifold at the block,and again at the carb.
If it changes speed,there's a leak.
Check the gasket at the manifold-to - block,and make sure the gaskets for the carb are tight.


----------



## StealthWyvern

jhngardner367 said:


> It sounds like an intake leak,...gasket,maybe ?
> while it's idling,spray some cab cleaner on the manifold at the block,and again at the carb.
> If it changes speed,there's a leak.
> Check the gasket at the manifold-to - block,and make sure the gaskets for the carb are tight.


Thanks, I'll give that a shot maybe next weekend when I get time to mess with it. I have company over this weekend so I can't tinker with it just yet.


----------



## StealthWyvern

jhngardner367 - I think most of my issues are carb related. Today I went to start it to try and test for a intake leak and discovered that I had spark and fuel to the carb but no fuel in the carb while I was cranking it. It would only run on starting fluid. So needless to say the carb has been removed and in the mail to be returned under warranty.


----------



## StealthWyvern

jhngardner367, The new carb came in and it made no difference. I had a mower repair guy come buy and take a look and the internal governor is shot and or out of adjustment. ... fun fun.


----------



## jhngardner367

Yep,kinda sounded like a governor problem.


----------



## StealthWyvern

jhngardner367 said:


> Yep,kinda sounded like a governor problem.


This maybe over simplifying it but is the below list the replacement process pretty much as follows?


Drain Oil
disconnect battery
remove harness connecting to the starter and any other place on the block
remove throttle linkage\cable
remove belt from the bottom and related brackets keeping the belt in place
remove the 4 bolts securing the motor to frame.
remove engine
flip upside down and remove pulley
remove all bolts securing the "oil pan" to the case
gently pry pan away from engine, and up and over the shaft.
inspect governor on the inside and remove any broken parts
order new parts and replace in reverse order.


----------



## jhngardner367

Yep,and you should get a Tech manual,also.
P/N =695244A,...$11.60,at Partstree.com


----------



## StealthWyvern

Blown govenor confirmed bad. Just need to order a new one after i figure out how to remove it and get torque specs.

Edit: need to figure out what the c shaped metal thing. It wasnt attached to anything inside the motor.


----------



## StealthWyvern

Well I figured out what that part was in my hand. it looks like it is apart of the cam shaft as shown in the attachment. I'm thinking that broke off and tore up the governor as its bent up pretty bad. 

What I don't understand is what the purpose of it or how it was attached in the first place or even if its really needed or not? I found a new camshaft online but just over 100 bucks but don't really want to spend that if I don't really need to.

Edit: couldnt find a site that would just sell the decompression stuff so i ordered the entire thing. I bet this is why it doesnt seem to spin over as fast as it should and the hard starting issue i was having.


----------



## StealthWyvern

New cam shaft, coil pack, governor(properly adusted), and just for kicks new carb linkages, and it starts on its own. The full throttle doesnt no longer sounds like its crazy high and low throttle sounds normal! Thank you all for your help!

Now to just run it a bit to work out ither bugs and deliver it to a friend as a surprise gift. No more push mowing for my friend who s in his 60s!

Now onto my other project mower!


----------



## FredM

StealthWyvern said:


> New cam shaft, coil pack, governor(properly adusted), and just for kicks new carb linkages, and it starts on its own. The full throttle doesnt no longer sounds like its crazy high and low throttle sounds normal! Thank you all for your help!
> 
> Now to just run it a bit to work out ither bugs and deliver it to a friend as a surprise gift. No more push mowing for my friend who s in his 60s!
> 
> Now onto my other project mower!


great to hear all is finally well, this was a good exercise for you and your next pull down shouldn't be a bother for you.

you are a true gentleman for what you are going to do with the mower


----------



## jhngardner367

The piece that came off the cam is the Automatic Compression Release,and they were noted for it.
Kinda figured it was a governor problem.
Coulda got nasty,if it ran that way for very long.
The racing guys disable the governors,but they don't care if they blow an engine,either !

Glad you got it straightened out !


----------



## StealthWyvern

FredM said:


> great to hear all is finally well, this was a good exercise for you and your next pull down shouldn't be a bother for you.
> 
> you are a true gentleman for what you are going to do with the mower


This is the 2nd mower I have done this with for a friend. The 1st was on a John Deere stx38. I enjoy doing it which is one of the reasons I do it plus seeing the excitement on peoples faces is another bonus. This little MTD mower I have $222 into it so far which was a little bit more than I wanted to put into it due to the issues I had with it, that includes buying the mower. However I know he shouldn't have any issues with it now. I'll have to leave the deck up to him to get repaired (bearings, belts, and blades), but that should be around 150ish for all the parts online.



jhngardner367 said:


> The piece that came off the cam is the Automatic Compression Release,and they were noted for it.
> Kinda figured it was a governor problem.
> Coulda got nasty,if it ran that way for very long.
> The racing guys disable the governors,but they don't care if they blow an engine,either !
> 
> Glad you got it straightened out !


Yep, full throttle now sounds about half rpms of what it was doing after everything broke. I couldn't give it to a good friend like that and have him blow it up thinking I gave him a piece of crap. 

over all I dug into the power a little more than I cared to and put more money into it than I have originally planed but it was a learning experience and they are much simpler engines than I had originally thought. It makes me want to buy the ones with broken governors and flip or give them to people that could use one. Oddly enough I don't even have a riding mower for myself. I'm still looking for one I want that's cheap to fix up.


----------

