Magnum 14 - smoke thru carb at shutdown

[PWL216 999]

My M14, which has always run great is now hard to start and smoke comes out the carb on shutdown. I’m thinking it’s likely an intake valve issue? I’ll do a leak down test to check. I guess it could also be a stuck exhaust valve. Any ideas or suggestions for what else it could be or the best way to go about diagnosing the issue would be welcomed. Thx.

[ineedanother 1,657]

By the sounds of it if it were me, I would probably go ahead and get a head gasket and a valve cover/breather gasket set. Pull the head, decarb, adjust valves, plane the head and put it back together. That might sound like a lot but it's really just good maintenance and will probably resolve your issue. You might be tempted to lap the valves while you're that far into it but you should have a good sense for that once you've put eyes on it. That's all a pretty cheap and relatively easy project IMO. Interested  to hear others' suggestions. 

[ri702bill 9,182]

Agreed. With the head off, roll the engine over to where each valve is at the highest off the seat. Inspect both the valve and seat for abnormalities. A chunk of loose carbon caught between the valve face & seat will lead to a burned, or split, valve in a short time. In that condition, the valve cannot shed its heat adequately to the seat.....

[wallfish 17,837]

5 hours ago, PWL216 said:

smoke comes out the carb on shutdown

Is this a large amount of smoke or just vapor puffs?

Bench testing engines with the air cleaner off, I routinely see some vapor puffs in the front of the carb at shut down.

[PWL216 999]

On 6/11/2025 at 9:54 PM, wallfish said:

Is this a large amount of smoke or just vapor puffs?

Bench testing engines with the air cleaner off, I routinely see some vapor puffs in the front of the carb at shut down.

Hi @wallfish I would say closer to vapor puffs…

[PWL216 999]

@ineedanother @wallfish @ri702bill  pulled the head and valve covers off today. There was a lot of carbon buildup and some flakes around the exhaust valve. I had done a leak down test prior to and was getting air coming thru the muffler - so I expected the exhaust valve would be dirty. I used the dremel to clean it up and I think I got at least most of it. My question is, based on the pics you see, should I lap the valves in? Also, I still need to adjust both valves since it seems I have zero clearance on each. Thx again for the prior guidance. Paul

[ineedanother 1,657]

Well, I think you found your problem being hard to start. Low RPM = low compression but might run okay at working speeds. I'm not a big fan of lapping old valves but depending on the condition of the seat and face, there could be some benefit. I would probably just clean everything up, adjust the lash, check the head for flatness and get it put back together but I'm no expert with engines so others might have more reliable advice.

[ri702bill 9,182]

Paul - that exhaust valve is of concern. Way too much carbon built up that could (did??) get loose and caught between the valve and seat and cause issues.

Since you plan to adjust the valves & you have the breather removed, why not just pull the valves to check for pitting on the faces & seats?? Lapping is not a bad idea to verify that there is even full diameter contact. (The same can be done with a Sharpie - less mess to clean up.) Chuck both valves up in a low speed drill or lathe and spin them to visually check if they are bent.... Inspect the valve stems & guides for out of spec wear. A valve that wobbles in the guide does not seat evenly all the time...

Bill

 

Edited 10 hours ago by ri702bill

[lynnmor 7,698]

Pull the valves and do a proper valve job, lapping is shortcut to do a less than ideal repair.

[PWL216 999]

1 hour ago, lynnmor said:

Pull the valves and do a proper valve job, lapping is shortcut to do a less than ideal repair.

@lynnmor I thought lapping the valves is considered a ‘proper’ valve job. No?