Honda small engine help

I have a Honda HRX self-propelled mower that I'm kinda stumped on. When I bought it, it would run okay, but it would surge at idle (for lack of a better word). It would rev up, puke out a small puff of black smoke, then rev down again (but not low enough to come close to dying). Repeat on a 15-20 second cycle as long as it's running. At its worst when sitting still with the blades off. Engage the blades and the drive, and it would get a little better, but not much. Was also prone to dying if you engaged the blade clutch in grass that was any kind of tall.

Preliminary internet research says that these things are tuned super-lean like everything else is now, and they HATE ethanol gas.

So I bought a new carb (along with other tune-up stuff). (An OEM Keihin carb is $18, BTW.) Installed that yesterday. Didn't appear to have much adjustabiity, although there was at least one Phillips screw that looked to be an adjustment of some type. Replaced the air filter and the spark plug, too. Plug was absolutely BLACK with carbon. Drained the gas tank, refilled with ethanol-free gas, started it up, and it STILL. FRICKING. SURGES. Not over as wide of an RPM range, and power is definitely better, but it still gives out the little puff of sooty smoke and it's still not right.

Anyone got any ideas? I really like this mower other than this issue.

Is the spark good? You can loosen the bolts for the magneto and slide a business card between the magneto and flywheel for proper gap then tighten bolts.

I have never worked on a small Honda engine but I have had luck with this fixing some problems on other lawn mowers.

Did you replace the fuel filter? Sometimes its almost like a strainer. I ended up changing the one on my riding Honda mower eventhough the filter looked fine and it made a difference.

The carbon on the plug and the black smoke indicate a rich mixture. Since it does it with the new carb, then something else is causing it. How is the intake and filter. Something closeing the intake would act as a choke richening the mixture. Does it have a fuel pump ?

As for the ethanol thing, from experience on two old engines. Much ado about nothing.

Don't think it's your issue but spark arresters get clogged up from ethonal gas issues and causes problems with running.

Aware that it's apparently pig-rich, at least when it sputters and belches smoke.

-Intake is clear, air filter is brand new OEM.

-No fuel filter per se, just a screen in the bottom of the tank. I suppose that's worth a shot that it might be all gunked up by ethanol crap as well. The mower sat for a while before I bought it. Guess I'll try draining the tank and cleaning the screen with solvent/compressed air.

-Aside from the tank, screen, and a 5 inch piece of rubber line, there is no more fuel system. Gravity feed, no pump.

Oh, and Iceracer- OLD engines don't really give a E36 M3 about ethanol. It's the newer stuff (post-2005ish?) that is jetted super lean from the factory to lower emissions, and they tend to be a lot pickier about everything being perfect and not gunked up. Old lawnmowers will run on about anything.

wlkelley3 wrote: Don't think it's your issue but spark arresters get clogged up from ethonal gas issues and causes problems with running.

Hm. Worth looking at. It does have a spark arrester.

I was having similar problems with the briggs on my toro. i finally took the govenor controlled throttle and manually revved several times to max rpm and it cleaned it right up. sort of an italian tune up for a lawn mower.

A spark arrestor being in the exhaust wouldn't cause it to run rich, just no power.

You could be right on old mowers.

I had an emissions correct 2cyl trimmer. I had to run it for several minutes on the choke an even then it would stall. I gave it away an got an electric unit.

So what sort of enriching device for starting do you have. Could be something there

I'm gonna guess either some sort of automatic choke acting up (or intake issue in general), or ignition related.

Have you tried running it with the air cleaner off?

iceracer wrote: So what sort of enriching device for starting do you have. Could be something there

No auto choke - It has a a regular cable operated choke. When it had the old carb, it wouldn't start with the choke on. It would start right up without the choke. Now, it actually likes to have the choke on when it starts but it's not necessary to keep it on for very long at all.

Kenny_McCormic wrote: Have you tried running it with the air cleaner off?

Yep. No change.

I'd be guessing ignition then, you've changed the plug, so that's more or less ruled out. I'd be guessing a bad coil, kill switch, or something to that effect.

Just for fun, have you set the valves?

Haven't set the valves yet. I'm gonna check on a new coil. Kill switch is weird- this thing has a blade clutch, so if you let go of the handle it just stops the blades, not the motor. I don't think it ties into the ignition at all.

I was hoping to avoid buying a service manual, but it looks like I'm headed that way.