Screwdriver's : DIY homemade projects

If your Tecumseh lawn tractor or snowblower engine (OHV 110, 120, 130, 135... model) is hard or impossible to start, the most probable cause could be a lack of decompression. Decompression is necessary to allow the starter (or you for manual start) to passover the firsts compressions while starting. If not explosion will tend to force the enginge turning in opposite way (due to high pressure build-up). The symptoms are sudden jamming of the rotation in a big noise when using electric starter or sudden stop of the cord if using manual start.
The first thing to do is to check the valves clearance that must be well adjusted to allow correct operation of decompression mechanism. But if no improvement is made by adjusting valves it is possible that decompression mechanism is defective or shows wear.

Basically on OHV engines the decompression mechanism is made by a mobile pin that goes through the exhaust cam in the camshaft and increase the valve clearance. While engine is started the pin is retracted using a mass moved by centrifugal force.

Through the years this pin can worn a render mechanism ineffective. Normal process would be to replace the full camshaft but unfortunately due to closure of Tecumseh motor company such part is no more available or very hard to find. Buying an used one can be interresting but it may also be worn or close to limits.
An other cause could be a rupture of the spring that push back the pin when engine is stopped

Fortunately it is possible to disassemble the mechanism and make a new pin. Note : such pin is not sold as spare part so you will have to makes it by yourself or find a machinist that can make it for you. Important notice such operation can be done by anybody that have some mecanic knowledge but some operation as  reassembly may be not so easy and could damage the engine if not done properly.

The first step is to remove the engine from the machine where it is. At this step remove the oil. Take care to record electrical connections, location of bolts and so on, some picture may be helpfull for further reinstallation.

Then wash external of the engine to ensure no debris will be introduced in it.

The access to the camshaf is made by the back of the engine, it is nor necessary to remove breech or flywheel. You have to remove all the bolts fixing back cover. If stuck a litte hit with a soft mallet can help. Remove the back carefully, try not to damage the seal and take car to oil pump that can come with.

It is not necessary to record position of camshaft and anti-vibration shaft as there are marks to put them in place.
To remove camshaft you have to release the valves springs under the breech cover.
And then remove first :
- crankshaft gear (crankshaft will remian in place)
- oil pump housing and piston
- anti-vibration shaft

Then you can pullout the camshaft.

The decompression mechanism is maintainned on camshaft by a little pin that you have to push out using a needle or a rod. You can fix camshaft in a vice but take care not to damage cams or oil pump surfaces. (note position of the spring and the centrifugal mass).



Now you can measure your decompression pin. For an OVH110 it shoud be approximatively between 24.30 and 24.35 mm (0.957 to 0.959 in) in length. This length is very important as we suppose that only 0.1 mm (0.004 in) of less can render system inoperative. Note these values are estimated as no data is available from OEM.

To make a new pin :
you will notice that original pin is stopped by a little circlip at the end. Obviouly unless if you have specific tooling such desing may be hard to reproduce, knowing that diameter of the groove must be very precise to avoid loosening of the clip. So clip will be replaced by a shoulder in the bare metal.
The other end is reduced in diameter.
Moreover such pin is made in a very hard tensile steel to avoid damage when it hits the valve pushrod. So dont try to make your pin in metal like nails (we have tryed and pin bends only after few starts).
A good solution in regard of availability, resistance and machining possibility is to use 4 mm (0.157in) music wire available form hobbies shops in different diameters. Such steel should be hard enough to support loads.

Easiest way to machine the pin is to use a lathe, but as in our example if you don't have one it is possible to make it on a vertical drill using files and saw blades. In this case you should machine by step taking care to respect original diameter. At this step machine an overlength pin, length will be adjusted later.
Music wire is quite hard to machine but not impossible if you take time and use good tools.
The key point are :
 - respect of diameter to ensure correct translation of the pin (approx 3mm but you should measure original pin).
 - flat face and 90° angle of the shoulder face in contat with the camshaft
Note the legth of the head (shouder) must be exactly the same than original pin, a minimum gap between mass lever must be present when assembled. If not the pin will not translate and prevent engine running especially when hot (pin will remain in decompression position).

Last operation consist in adjusting the lenght to approx 0.1mm over original one, and check that it translate freely in the camshaft.

Then you have to reassemble the mechanism, while inserting the locking pin. Take care not to damage the oil pump surface, such pin must not protrude or you will damage the pump.

Last operation is to reassemble the engine in reverted way of dissassembly. You must align the different marks of anti-vibration shaft with camshaft, and camshaft with crankshaft. When you will put back the rear cover, put the oil pump and its piston on the camshaft in away it will insert in the cover support. After you put cover check that oil pump piston is aligned with cover support, if a gap subsist do not force on the cover but try to turn with a screw driver the governor gear (white plastic gear fixed on the rear cover), if you dont do this you will brake the governor.

Finally adjust the valves clearance, make some free turn with hand to check that engine turn freely with no abnormal noise.Dont miss to replenish the oil before starting your engine.

It should now start as new. But if it turn freely but do not start with a little blowing noise this could mean that your pin is too long and decompression is too strong to allow start, in such case try to increase exhaust valve clearance to see if there is any improvement.