Outdoor Power Equipment (Lawn Mowers, Snow Blowers, Chain Saws and more) Discussions

IT's a Viking single stage from MTD sold by Eaton in Canada in 1974 it runs a Tecumseh 7hp snowking engine very powerfull

Rush,
The cracked boot wouldnt make a difference....but maybe what cracked the boot would also have cracked the plug?
If it was a resistor plug....it would most certainly cause backfiring.
I cant see what plug you have...but if it is something like a Champion RJ19lm....that would be the first thing too suspect.
Those resistor plugs have a (?) resistor that looks like a tube inside the ceramic insulator. It  also has a little spring underneath the resistor.
The outside of the plug and the electrode can look perfectly fine....but if the spring is broke or burnt up....they backfire all over the place.
I get rid of the resistor plugs when I can...but I dont live in Canada....where they frown on such practices.
But changing the plug is a good and inexpensive way too start.

we could use a little more info on the problem...

backfire when ,

high speed

low speed

when attempting to start

when stopping

all the time ...

not trying to be a wise crack .but many possibilities here .and we can help.

later chris

Points or condensor problems could cause backfire, as well as a grossly mis-adjusted carb. I agree, a bad boot won't cause backfiring...

Marty

No but it could affect simply idle speed

I would think a weak spark  regardless of the cause could contribute to backfiring, but probably not be the sole cause of it.  If your spark is weak, there is a chance you are not getting a full burn of air/gas in the cylinder, allowing some of the unburned to get to the muffler and POP!  Kohler engines are known to back fire/pop if you don't "idle them down" prior to shutting them off.

Speaking of spark plug and backfiring until this evening I did not know that a dying plug could be the main cause of surges and backfires. I was clearing the heavy snow we got last night when after about 2 hours of strenuous but uneventful blowing my trustful Honda started running real rough with surges, backfires and exhaust flashes. It kept doing this regardless of throttle settings, even on full choke (I know, should've seen the huge red flag). Time to clean the main jet I thought, so I let the machine cool off and had coffee then took the bowl out, spayed the jet and nozzle with carb cleaner and finished the job with the tiny wire, which came out clean. Hmmmm. Screwed the bowl back on and turned the key.

Ran great for about 10 seconds then back to backfires, surges and flashes. Dang, must be some gummy stuff inside the carb. Took the whole carb off which isn't that easy on the HS928 especially when working with gas soaked frozen fingers (new garage not heated yet and it was like 2 degrees out there), brought it indoors for close scrutiny: sparkling clean. Not what I had expected. Fuel strainer was pristine as well. I was vaguely considering dismantling other stuff by then but didn't really know what to look for. My wife asked if I had checked the spark plug. Please don't concern yourself hon, this is serious guy stuff I thought but just mumbled something about spark plugs not causing these symptoms and such (what did I know). However after making sure wifey wasn't watching I did go out and took out the plug, both the electrode and insulator were jet black. Nevertheless I didn't think much of it but cleaned it for good measure and put it back, along with the carb,  and  started the engine. It ran perfect.

WTF?

I have no idea why but it appears that the "coked" spark was the cause of all these symptoms. So now I'm asking myself -and you guys- how can one tell for sure that running rough is not caused by a carb issue? Check the plug first? Gotta get new plug tomorrow. Storm coming.           

I'd go with checking the plug first, it kind of easier to remove and put back than the carburetor.

The basic troubleshooting process for an engine is Air, Gas, Spark. You have to have all three so check them, the order is up to you and your suspicions but I go easiest/fastest first.

1. Gas tank full with good gas;
2. Throttle on;
3. Ignition key in;
4. Check spark plug;
5. Check plug is getting electricity;
6. Primer working OK;
7. Fuel lines;
8. Carburetor;
9. Compression;
   
10. Stare at it and use mental persuasion;
11. Take to shop and pay big bucks to find out what I missed.

Good luck, hope its something in the low numbers and not the highest.