engine knock

17.5 hp ohv intek has nasty knock on start up. i think its a piston knock because the valves open and close good and the push rods arent bent. never been taken apart. its on a pony. never seen mud or anything. lawn mower for mowing. any idea what could have caused it or is causing it? maybe a grenaded piston or rod?

worn crank, rods or bearings

Ok. Thank you

Those crank weights tend to go on those engines so check them if you open it up

i think it is the weight. im not opening it up since it is not mine to do that too. because when i start it it knocks and shakes the tractor. might be bearings tho since it is slowly getting worse

crazykid1994 wrote:

i think it is the weight. im not opening it up since it is not mine to do that too. because when i start it it knocks and shakes the tractor. might be bearings tho since it is slowly getting worse

I had an 18hp that was doing the same thing and the top weight let go and knocked a big whole in the block so if I was you I'd get permission from who owns it and catch it before its to late

Well. Maybe. Ill mention it too him

Check the engine mount bolts. I have had engines that knocked bad, bolts were just loose.

Ok. Ill do that

If its a knock its either the engine bolts are loose or the bores have a groove in them it it does best thing to do is get a new engine is costs about $600 just to rebore the holes

MudKing01 wrote:

If its a knock its either the engine bolts are loose or the bores have a groove in them it it does best thing to do is get a new engine is costs about $600 just to rebore the holes

Or you can get a used block in good shape its alot cheaper

matt-man wrote:

Or you can get a used block in good shape its alot cheaper

Agreed. If yours turns out to be physically damaged, just source another used one that's in good physical condition. It doesn't even have to run as long as you are well-enough informed to be able to check it. Get a similar enough match and just swap whatever parts over that you need to. Think of is as buying a short block. The new one doesn't need to have a functioning starter, coil, carb, whatever- just swap them over from the bad one.

checked all the engine bolts. they are tight. its not knocking anymore. and its running again since i cleaned the carb... idk what caused the knock honestly....

Sounds to me like the weights they will come and go as far as the knock in concerned

the engine had been sitting for 9 months so we tried starting it and it knocked and would then start and the knocking stopped and it only ran on choke so i cleaned the carb today and made sure all the bolts were tight. and started it and it runs. no knock

The fact that the knock disappeared after you cleaned the carb is most likely coincidence. Is this a pressure-lubed engine, or a splash-lube?

Splash. And I know its just coincidence but now I can't figure out what was knocking

Now hold up, mechanical collisions internally arent the only thing that makes a 'knock' sound. Briggs OHV engines generally have a high compression ratio, so older 87 octane gasoline may cause it to detonate and make it sound like a rod knock.

Also if youre exhaust valve isnt seating perfectly, the combustion whoosh goes past the valve heating it up. The orange hot valve acts like a glow plug and can cause preignition. Running rich on bronze grade gas can cause detonation as well.

Check your plug. If pieces of porcelain are broken off, and/or the insulator is bleached white, youve got a detonation problem. If the plug looks almost mechanically damaged check the piston face. You may find it has a hole or peppered surface.

Preignition shows similar signs, but is caused by over advanced ignition timing. (broken flywheel key). They can shear off if the flywheel nut is loose.

I had a 4hp tecumseh prism push mower with a TVS115 flatty on it. Got it for nothing. Ran great, smooth. But knocked so bad I thought the rod would come thru the block. I also found, if I cut the motor warm, and pulled the rope slow it ripped the handle out of my hand backfiring. Took the flywheel off, key partially sheared giving a bad timing of about 46* BTDC.

Check your keys if all else fails gentlemen!