High Idle knock

Sat Aug 10, 2013 10:59 am

When I was mowing yesterday I noticed a pretty significant knock at high rpm... I couldn't hear it when the mower was running so i'm not sure when it started, but the engine was hot enough to be spitting some coolant when i noticed it. From reading on here it sounds like that's probably indicative of bad crankshaft bearings. I plan on rebuilding the engine, but I can't afford to do it right now. Is it ok to put in heavier weight oil (right now it's got 30 wt) and plug my ears until I get around to it? Or is it going to explode when i'm plowing and spill oil all over my vegetable garden?

EDIT** I forgot to mention the oil pressure was pretty low when it was hot too. like right on the line between red and white (more like gray and white on this old thing :wink: )

Re: High Idle knock

Sat Aug 10, 2013 12:18 pm

Check your timing.....It will cause a knock at fast throttle and make it run hot if its advanced to far....I guess you can figure out how I know this... :mrgreen: Dave

Re: High Idle knock

Sat Aug 10, 2013 12:52 pm

Should I just follow the directions in the operator's manual for static timing the ignition? It's not exactly clear, but I'll give it a shot.

Re: High Idle knock

Sat Aug 10, 2013 2:31 pm

I haven't checked the timing yet but I was poking around and I did notice that she's leaking a little oil at the crankshaft bearing at the front of the engine... :(

Re: High Idle knock

Sat Aug 10, 2013 4:26 pm

I was just throwing out a suggestion in hopes it wasn't something major....Got a timing light ..... only takes a second...And check your vacuum if you have a vacuum gauge....Hooks to the plug above the carb on the manifold...I'm sure some of the other members will check in shortly with more trouble shooting ideas....Dave

Re: High Idle knock

Sat Aug 10, 2013 4:58 pm

http://www.earlycuda.org/tech/vacuum2.htm For your vacuum gauge if you have one....Dave

Re: High Idle knock

Sat Aug 10, 2013 5:30 pm

What you have described sounds exactly how my 1957 CUB sounded when I first brought it home and ran it wide open. If I were you I would drain the oil and drop the pan and look that crankshaft over very carefully. I did have a scored crank and I elected to have the engine rebuilt; a VERY costly road to go.
You might be $$$$ money ahead to look for a good used engine if yours needs to be overhauled. Just to do a rebuild yourself with a machine shop doing the block will cost you somewhere in the $1500 range.
Better to check now then destroy the engine by running it.


Re: High Idle knock

Sat Aug 10, 2013 5:32 pm

Eoghan. Reread all your previous posts.

Have you flushed the engine block and lower radiator housing/bolster? Flushed out or cleaned the radiator fins? Fan belt loose? Completed a 100% tune up and tractor lubrication?

Adding a heavier than 30 weight oil will not reduce bearing knock.

Static timing the engine is normally quite sufficient. But with a timing light you can check the advance.

The vacuum gauge and compression tests will tell you a lot about the engine's condition.

Re: High Idle knock

Sat Aug 10, 2013 5:44 pm

If its knocking it needs to be parked till you can pull the pan and inspect the rod and main bearings. Worn mains can cause low oil pressure. Tried to post earlier but wifi not working:(

Re: High Idle knock

Sat Aug 10, 2013 8:29 pm

Timing will not cause a knock loud enough for you to hear it over the mower, chances are you have a bearing knock, try to pin point it , you can do this with the aid of a broom handle or the like. Put one end to your ear and the other to the engine. You will be able to tell where it is the loudest. Most likely you will end up pulling the pan.

Re: High Idle knock

Sat Aug 10, 2013 8:43 pm

What is the history on this Cub? Service history on this Cub? How long have you owned it?

I gather from reading posts that it's a recent purchase.

Re: High Idle knock

Sat Aug 10, 2013 9:03 pm

Hey guys, Just to be clear The tractor is new to me about 2 weeks ago. All I've done so far is cleaned the carb out (it was full of rust goop), cleaned the fuel strainer, adjusted the governor, changed the oil, and fitted the mower.

The knock is not loud enough to hear over the mower. it sounds like a woodpecker that's 100 yards away. It only happens when the engine is hot, and I'm not sure if the oil pressure was lower than it ought to be.. I also haven't changed the oil filter (gotta order one), so it could be a little plugged up.

Other than the knocking she runs quite well. a little smoke at startup, but enough power to mow thick grass, and no smoke when running at temp.

I planned to first go through the lubrication list in the operators manual and do a basic tuneup (plugs/wires/points etc), but i was hoping to be able to do that stuff on rainy days when i can't be plowing or mowing (i've got fields to get ready for next year!)

sounds like i might be disappointed!
Last edited by Eoghan on Sat Aug 10, 2013 9:14 pm, edited 1 time in total.

Re: High Idle knock

Sat Aug 10, 2013 9:07 pm

Oh, and I don't have a vacuum tester, but they're pretty cheap, so i might get one to have around. a timing light too. maybe i can borrow one.. i bet i can trade some tomatoes to my neighbor!

Re: High Idle knock

Sat Aug 10, 2013 9:47 pm

Ignition parts and oil filter, at your favorite auto parts store. If not on hand, one day service.

Some auto parts stores loan test equipment and tools. Be sure and ask. You can use a multimeter to static time an engine.

My guess is rod bearing(s) making the knock. But there are other things in an engine or auxillary equipment that can make a similar noise.

Re: High Idle knock

Sat Aug 10, 2013 10:36 pm

Thanks Eugene!

to check timing, should i put the multi-meter on the 1st plug wire? The manual says something about watching the ammeter with the #1 piston at tdc and pulling the power knob?

anyways I was thinking the first thing I should do is get it up to speed/temp so it makes the noise, then pull spark plugs one by one to see if I can narrow it down that way. if it doesn't cut out with any of the spark plugs then it's not a rod bearing, right?

sorry, I'm new to this stuff if you couldn't tell!