balconio 606 #1 Posted February 9, 2020 Looking for opinions here. I have two 312-8s from the late 80's both with over 1400hrs on M12 engines. They both run great, start without hesitation in any weather and neither seems to burn oil or anything like that. But the exhaust stinks badly, smells like they're running too rich maybe. When I start them on full choke I see some gray/black smoke as expected and they want the choke to be put full off almost immediately after starting even in freezing temps (again they seem rich). Once the choke is off I don't see any gray/black smoke and they have good power with no surging stalling or backfiring. My question is, should I try to mess with the fuel mixture? Do valves start to seal poorly over time? Is this just how it is with a well worn Flathead and do you guys notice the same out of your machines? I'm thinking about running a rear exit exhaust when I build one into a mud mower and high stacking the worker horse just to get the stink away from the operator. Share this post Link to post Share on other sites
ebinmaine 70,795 #2 Posted February 9, 2020 If an M12 carb is as easy to rebuild as a K carb I'd start with that... Share this post Link to post Share on other sites
tunahead72 2,518 #3 Posted February 9, 2020 3 minutes ago, balconio said: … My question is, should I try to mess with the fuel mixture? ... I would. It's an easy thing to try, and it might solve your problem. If it doesn't, you can keep looking. Do you have manuals for your engines? Share this post Link to post Share on other sites
balconio 606 #4 Posted February 9, 2020 No I don't think I have manuals but I'm sure I can find it online and agree it's worth a shot. Sounds like you guys don't find your machines to have a queasy level of hydrocarbon emissions... The one makes my whole jacket stink after blowing off the driveway. Share this post Link to post Share on other sites
ebinmaine 70,795 #5 Posted February 9, 2020 2 minutes ago, balconio said: makes my whole jacket stink after blowing off the driveway Oh yeah.... Way high.... Share this post Link to post Share on other sites
The Tuul Crib 7,338 #6 Posted February 9, 2020 That many hours l would definitely re- adjust the valves rebuild the carburetor and give it a good complete tuneup. Share this post Link to post Share on other sites
lynnmor 7,576 #7 Posted February 9, 2020 You don’t want a Köhler running lean, that is a good way to burn valves. There is a Cub Cadet dealer in my area that smoked many an engine with his lean adjustments. Share this post Link to post Share on other sites
Digger 66 3,488 #8 Posted February 9, 2020 9 hours ago, balconio said: Do valves start to seal poorly over time? Yes ^ Especially if the valves & seats were designed for leaded fuel . Share this post Link to post Share on other sites
953 nut 57,995 #9 Posted February 9, 2020 10 hours ago, balconio said: They both run great, start without hesitation in any weather and neither seems to burn oil or anything like that. I would say a little prayer of thanks and move on. Running a little rich is a good thing for your engine's longevity. A little Seafoam in the gas couldn't hurt. 2 Share this post Link to post Share on other sites
Pollack Pete 2,273 #10 Posted February 9, 2020 I'm with 953-Nut...….if it ain't broke,don't fix it.Do they use any oil at all? Burning even a trace of oil will make the exhaust stink to high heaven. Share this post Link to post Share on other sites
Retired Wrencher 5,651 #11 Posted February 9, 2020 Did you look at the spark plug to see how it is running? A tan color or a very little on the white side. Share this post Link to post Share on other sites
pfrederi 18,337 #12 Posted February 9, 2020 Many Magnums came with Walbro Fixed jet carbs Is yours one of those? Cleaning is about it as there is no high speed adjustment. Frankly they usually act up by getting to lean (crud build up blocking the jet) Share this post Link to post Share on other sites
Lee1977 7,044 #13 Posted February 9, 2020 13 hours ago, ebinmaine said: If an M12 carb is as easy to rebuild as a K carb I'd start with that... Mine don't have both adjustmentson on the carb. You can remove the dirt and trash but it is what it is. Best to use nonethnoal gas. 1 Share this post Link to post Share on other sites
tom2p 2,394 #14 Posted February 9, 2020 (edited) 17 hours ago, balconio said: No I don't think I have manuals but I'm sure I can find it online and agree it's worth a shot. Sounds like you guys don't find your machines to have a queasy level of hydrocarbon emissions... The one makes my whole jacket stink after blowing off the driveway. replace the carb with programmed electronic fuel injection and install new exhaust system with catalytic converter and say goodbye to stink jacket or maybe some bean oil will provide better aroma ... Edited February 9, 2020 by tom2p 1 Share this post Link to post Share on other sites
