[Shop-talk] mower engine issue

[Donald H Locker]

The rod is the connection from the governor to the throttle.  It's probably oscillating because there is no load on the engine, but it might be the spark plug or poor mixture.  The spring won't provide any damping on its own, but if you hold the rod steady, you are holding the throttle steady and the engine speed should be constant.  If it is, then I would consider the plug and fueling system to be OK and look for a sticky governor or just some way to apply a load to the engine - they aren't designed to operate with no load and the governor will hunt under those conditions.

Try to see what the other end of the rod is attached to - I'd expect either a vane in the cooling fan airstream or a flyweight governor might be buried in an impossible-to-disassemble mechanical box.  Both kinds of governors will leave the throttle wide open when the engine is off, then start to close the throttle as the sensed speed approaches the set point.

The spring should probably be attached so as to close the throttle and was probably attached to the speed control - pulling the knob or twisting the lever further releases some of the spring tenson and allows the engine to run faster.  There should still be another spring (it might be buried in the flyweight governor mechanism or attached to the wind vane) to urge the throttle closed unless the speed is lower than the setpoint.

HTH,
Donald. 

----- Original Message -----
From: "scott hall" <scott.hall at comcast.net>
To: shop-talk at Autox.Team.Net
Sent: Tuesday, August 25, 2009 7:50:15 AM GMT -05:00 US/Canada Eastern
Subject: [Shop-talk] mower engine issue

the engine's a 5.5 hp honda ohv knockoff from pepboys. cheng kang, or something. 

it's an a go-kart. couldn't help it, it was damn near free. because it didn't run. 

ah, but it didn't run because the dpo left it sitting so long the gas went...I don't know what. I've never actually seen the inside of a carb look like that. the jets were plugged solid and the gas in the tank was green. I mean, I've seen lots of bad gas...but never that shade of green. so clean out the jets, new gas, it fires! I needed a win after the pool. :-) come to think of it, the gas was the same shade as the pool is far too often... 

so it starts and runs...but it runs with an 'oscillation', I guess is how I'd describe it. if you hold the throttle pedal down, it won't maintain a steady rpm. up and down at a fairly frequent pattern. there's an actuating rod attached to what I think is the throttle plate in the carb (there are two butterfly plates in the carb--the choke and the plate to which this rod is attached). this rod runs under the gas tank and is attached to the motor in some fashion (can't seem and wouldn't understand it anyway, so didn't remove the tank). this rod is moving back and forth at the same periodicity as the engine rpm oscillation. clearly, that's what's causing it. 

anybody know what that rod is, and what it's doing? some sort of constant velocity setup? 

then there's a spring installed outside/over this same rod (the rod is running through the center of the spring). one end of the spring attaches to the 'throttle plate' (?) cam right next to the rod. the other end was attached to nothing when I got it, but judging by its length should attach under the tank somewhere. anybody know what this spring is supposed to be doing? some sort of dampener? 

I can't fit my fingers in to pull the spring, but I can hold that rod still, and when I do, it runs fine. 

so...I was hoping for a guess as to what that spring might be doing, which might help when I take off the tank and try to figure out where to attach it. or a description of what that rod is and what it's doing. or, if you know exactly what to do, that'd be cool too. pep boys apparently sells a different brand of these each week and there are no identifying markings on it to google the maker. it just looks a lot like my honda mower engine turned upright. 

thanks in advance. 

scott