B&S Vertical Twin

[mr-moe]

[b]I've got a B&S twin 404577-0291-E1 that I'm having decompressor problems with.
It just won't turn past the top dead-center point when trying to start.
Give it a little twist of the debris screen and away it goes.
Adjusted valves to .005" for both, and checked grounding to the starter motor, the battery is good, and the starter checked out for worn
bushing and rubbing armature. All OK.
What does this engine use as a decompressor, is it on the camshaft as the singles are?
Need some help here.
Thanks.
/b]

[KE4AVB]

As far as I can tell there is no compression release on the camshaft,

Camshaft images.
https://www.ebay.com/itm/154515213017?chn=ps&norover=1&mkevt=1&mkrid=711-117182-37290-0&mkcid=2&mkscid=101&itemid=154515213017&targetid=1263433205254&device=c&mktype=&googleloc=9013163&poi=&campaignid=14859008593&mkgroupid=130497710760&rlsatarget=pla-1263433205254&abcId=9300678&merchantid=114155107

Check for voltage drops.

[bgsengine]

Yep, what KE4 said - I would start with a voltage drop test (and battery load test) as well as checking cranking amps (DC Shunt or an accurate inline ammeter) to preclude a bad starter.. if all else checks out, I'd do a compression test (direct jump to starter and engine ground from a bigger CCA Automotive battery if necessary) and see if compression is in range (Also if it spins over fine with spark plug wire unhooked (which it would be if doing a compression test..), you might think about checking for a sheared flywheel key!)

[Arkie]

Does the engine run good and smooth after it starts???
Have you ever seen the engine crank good or is it a new to you engine? Reason I ask this is chance someone has installed a smaller frame starter. (weaker starter)
If not move the engine outside remove both spark plugs and crank the engine and see if it blows out any gas. (hydrolocking due to flooding)Be aware that spewing gas can become ignited.

Couple hints: If engine runs ok or don't start at all.

Leave spark plugs in place but remove the spark plug wires.
If it turns over ok then, check the flywheel key for slight distortion.
Next, instead of doing voltage measurements, take a set of GOOD automotive jumper cables and

jumper your neg ground wire, jumper to the motor frame. This is checking that your existing cable has a good ground.

Leave the key off on the mower, take a good automotive battery, leave the NEG ground jumper disconnected and make a good positive cable connection the the starter 7/16 NUT.
As the last connection with the NEG lead to the mower frame and expect sparks. The reason you are making the ground jumper connection LAST is to keep from arcing/sparking and ruining the copper stud threads on the starter.

I have all kinds of electrical test eq but sometimes using good set of jumper cables and another battery is more user friendly that taking voltage/current reading.

If you find your starter is ???'s check out dbelectric replacement starters.

[mr-moe]

Thanks for all the replies, will make all those checks.