Voltage drops when cranking...

[CraigH]

THE ENGINE MUST BE ABLE TO SPIN FREELY 360 DEGREES MULTIPLE REVOLUTIONS WITH THE PLUGS OUT BOTH BY HAND AND STARTER BEFORE GOING FURTHER. There is no compression when the plugs are out.

to have 12.2V while spinning the starter, and 12.8V at rest, would be within a normal voltage drop I think. Why the engine can turn freely by hand when the plugs are out but load the battery so much when the starter is used with the plugs out is beyond me.

Until the engine spins freely with the plugs out, doing other adjustments, etc., taking an engine apart is just wasted time in my eyes.

ONE TEST might be to spin the engine by hand several revolutions with the plugs out. Disconnect the starter wire and tape it out of the way so not to short or cause another issue. Turn the key to start and see if the engine still spins freely by hand. Would a shorted alternator winding bind the engine enough and pull the voltage down? What if the PTO was wired to full on when the engine was starting and the belt was binding. Need to be sure the engine will spin freely by hand in the start position. The voltage should not fall much in the start position when the starter or starter wire is removed.

Yes, the engine spins multiple revolutions very freely using the starter with the spark plug out. It's when the spark plug is in that it seems to have too much compression and won't spin a full revolution.
The PTO is not engaged and the belt is off.

 

[CraigH]

With the plug out, turn the engine over by hand while observing the bottom valve which on the single OHV engine is the intake valve. After the intake valve closes and you turn the engine and the piston comes toward TDC you should see the intake valve bump off of the seat slightly and then close as the piston hits TDC. That bump is the decompressor. No bump which I suspect in this case will me time to open up the engine.

I'll try that tomorrow morning and report back, thanks for the advice.

 

[sgkent]

Yes, the engine spins multiple revolutions very freely using the starter with the spark plug out. It's when the spark plug is in that it seems to have too much compression and won't spin a full revolution.
The PTO is not engaged and the belt is off.

if that is the case, then the suggestions that the decompression feature is not working sounds correct.

edit (added):
That said, I read back over your notes and I believe you said that with the plugs out on a new fully charged battery, the voltage falls to 10.8V. That seems about .75 to 1V low. It implies either a weak battery or excessive current draw. Then I think you said the starter only drops the voltage to 12.2V when the starter is out of the engine. While the decompression feature may be the cause of it not spinning, you may have other issues in the electrical system once you get it spinning. If the engineers designed that much voltage drop it is a poor design. Many later automotive FI systems won't work when voltages fall that low. Here in California the new small engines are going to require FI and catalytic converters. Will be interesting if cranking systems drop that low.

 

[CraigH]

Thanks for all the help with this.

I finally got round to pulling the engine out last night to have a look at, safe to say I think it's completely shafted!
What I assumed was going to be a small problem with the starter has turned into a whole different story.
When I got the engine unbolted and laid it on the bench it sounded like a tin can filled with nails, not a great sign I figured.... Once I opened it up, I discovered alot of parts rattling around, the counter weight links are both in lots of small pieces, aswell as a white plastic cog thats smashed and a few bent springs.

If I can figure out how to post a pic I will try to later.

Now I'm wondering if it's worth the hassle of rebuilding or do I find a suitable replacement engine?
Ive had a look for that engine online and can't find any for sale so would a similar Briggs and Stratton engine with the same power work?