WD backfiring

So I have a WD that I just got running. I got it a couple years ago from a friend but I'm guessing this is the first time it's ran in several decades. Anyway, the carb is fresh, the head is fresh. I didn't think I could get the tank satisfactorily clean, and the sediment bowl was too messed up to use so I put a filter and pump on it at least temporarily.
The thing starts great hand cranked, idles smoothly and revs pretty decent but maybe a bit sluggish, and when it's under load, it backfires out the intake. As it warms up it gets worse and worse till I have to shut it down. The backfiring seems rhythmic and the power cuts with it resulting in jerky motion.
I have three ideas of what could be causing it but would love some input from you experts if I can get it. The intake/exhaust manifold is in pretty rough shape and I wonder if a crack in it could be opening with heat and either just creating a vacuum leak or possibly allowing exhaust gas to ignite the intake fuel/air mix. Another possibility is ignition issues. It has the magneto system and I don't know what should or should not be happening in there as far as advance under load or with speed or what. Is there something that could be going on in there to cause this? It seems as if the system works great at least as far as the impulse stuff goes. It usually fires in about one rotation of the hand crank and I haven't had a kickback or anything. Third guess of mine is carb adjustment but other than the idle speed screw and idle mix screw I don't know of any other adjustments to make. It's the zenithh carb without the main jet adjustment on the bottom.
I have been reading a bit but this is my first post so here goes nothing! Thanks in advance!

Edited by ThatXJGuy - 16 Apr 2019 at 10:19pm

a warped or cracked manifold will cause rough running and backfiring

Sounds like it is pulling air from around the intake. Easy fix.

Lean on fuel.     MACK

Other causes; bad/worn gears on mag., bad condenser, bad plugs, as you've stated, intake and exhaust, wiring (creates an open when it gets hot).
I myself doubt it is fuel related because it would get bad and stay that way, hot, warm or cold.  BUT, you could have a problem with gas hose...??  Do you have steel gas line?  or is part of it rubber/plastic?  That could be a cause and I would get rid of any rubber or plastic.  It's bad on carbs too.

Change ONE THING AT A TIME and try it out.  They put the old part back in and change the next thing.  Do the simple little things first and work your way forward.  If new plugs (them I'd leave in, don't put the old ones back in)(But I'd also get them from Steve in NJ) don't help, try new plug wires.  Get the exhaust leaks fixed.  Air cleaner clean?  It's a mesh that gets clogged with old oil and dirt/grease and gets plugged easily.
If none of that works, I'd suggest sending the Mag to one of the guys on here to have it done up good.  (somebody on here was just selling rebuilds for cheap - check the classifieds)
Let us know as you go along and we'll keep picking our brains for you....

By the way, Welcome



Edited by Ted J - 18 Apr 2019 at 10:30pm

Oh, and to be more specific about the valves and head, it's got new valves, new seats on the ones that were removeable which I'm sure were the exhaust (it's been a year or two since then). I believe that they're all adjusted right, I went through them several times and lastly while it was hot which is what I think is the right way to go. Every time I checked them it seemed like several were in need of further adjustment but it's hard to say. The last time I did them I remember thinking: "they may not be exactly perfect but I'm confident that none of them are too tight" which is the safe way to be right? Too much lash would be inefficient, not enough is damaging?

Edited by ThatXJGuy - 27 Apr 2019 at 8:59pm

Mine would get worse as engine got hotter. Took coil off and could actually see that coil was cracked on the back of it. Worked really well when the coil was replaced!!!!

To me it sounds like the valve clearance was not set right. Set each cylinder clearance with the piston on top dead center on compression stroke.  Maybe you know that? Or a valve sticking. 

I reread your post again and see you have a magneto so forget the coil theory. LOL. I'll have to read more carefully.

Intake valves too tight common mistake on old tractor motors,I've seen it often just got thru getting an Oliver 66 running smooth because of too tight adjusted valves.I like valves on old low RPM tractor motors to be adjusted a little on the loose side.