Craftsman snowblower

rrobert · Jun 7, 2014

7.5 Briggs OHV. The RPM surges up and down slightly when the choke is off. Has new sparkplug.

It will hold a steady RPM if choke is on halfway or I hold a finger over carb throat.

I have cleaned the carb as well as I can tell. There are no blockages.

I tried adjusting the governor. I tried moving the throttle tang that the spring attaches to and still surges.

To adjust governor(looking straight down on engine), I held governor lever counterclockwise(makes carb butterfly open to full throttle) and spun the governor shaft counterclockwise. The adjustment was good before I started.

A new carb is about $50 so I was thinking about opening up the small jet on the carb bowl plug and see what that does.

Is there anything inside the head that would cause it not to get enough fuel into the cylinder? The compression seems good so I am assuming the valve adjustment is ok.
engine schematic

rrobert · Jun 7, 2014

Upon further inspection, the spark is faint...

The spark is scatterred on the old plug so I think it had water hit it.

The new plug is not a bright blue spark.

I have the motor halfway apart so I might as well check the shear key and coil.

The engine is about 10 years old so it might be time for a new coil.

rrobert · Jun 7, 2014

Well, one of the arms on the magnetto was ground down and spread because it was rubbing on the flywheel. I tried a 5hp magnetto on it but no improvement. I might order a new magnetto and take a chance on that fixing it.

El Toro · Jun 7, 2014

If that engine has a carburetor with a main jet adjusting screw you need to open that. That will enrich your fuel mixture. You're running a lean mixture since the choke has to be used. Hal

Steve@Advance · Jun 7, 2014

The problem is a lean fuel mixture. If you're sure there are no vacuum leaks, the problem will be in the carb.

Does the idle mix screw have an effect on idle quality? If not, the idle circuit is clogged. If the idle mix screw will adjust, set it at 1 1/2 turns, get it running and warmed up, turn it in until the idle stumbles, back it out to best idle, then turn it out another 1/4 turn.

If it still surges, and you're sure everything is clean and the float level is correct, the main jet will need to be replaced with the next size larger, or drilled out. This can be done with a set of jet drills.

Take out only to the next size up at a time, test before going any larger, only take the minimal amount to make it run without surging.

rrobert · Jun 7, 2014

All the carb adjustments are sealed from the factory. So, I will need to drill the jet out.

I am curious about the spark being faint. If I hold the butterfly still, it runs rough like a bad sparkplug wire. The sparkplug was new and the gas was fresh.

The magnetto did get banged up rubbing against the flywheel. Could also be the magnet is weak?

joe csuti · Jun 8, 2014

If coil leg(s) are banged up or you have tracks across flywheel magnets check bearings & crank,either or both may be worn allowing crank to shift on compression stroke changeing air gap for coil and depending on the amount wear valve clearance may be affected also.

jeffcat · Jun 8, 2014

Hate to say it but this is sounding more and more like new engine or new snow blower time. I was going for fuel mix as in use Hi- test and NOT regular. weak spark is sounding like the magnets are full of little metal fines from banging into the coil. If you can wiggle the end of the shaft up and down it is RIP time. On flea bay you can find NOS carbs that are full adjustable and they work SO much better.Jeffcat

rrobert · Jun 8, 2014

There is slight endplay but I do not feel much wobble in the crank bearing. I am working on a 7.5 hp horizontal engine. I tried a coil from a 5hp vertical engine with no change. The coils appear the same except for the plug wire end. Should it work? I think the lower screw might have come loose. The magnet can be removed/replaced if I can get he torx bolt out.

old coil:

joe csuti · Jun 8, 2014

If you're checking spark while cranking eng by hand with recoil starter there may be nothing wrong with either coil,most/all briggs ohv eng require a minimum of 350 rpm to produce a weak spark & with a recoil start you have to work at it to get that speed,a better test is use an inline spark tester & see if you're loseing spark with eng running,also if I remember correctly coil is the same on 5-7.5hp vert/horiz shaft eng with the exception of plug boot/lead.

The carb problem may be the nature of the beast & main jet nozzle may require replacement/rework.I have two chipper/shredders both with briggs ohv eng,one is a craftsman with a 6hp eng other is a troy bilt with a 7.5hp eng,both of them share the same problem,their not happy unless choke 1/2-3/4 open then they run great,with choke full open & hot eng they go lean,start missing & hunting for their speed plus they won't idle unless you close choke to 1/2 or less.

Steve@Advance · Jun 8, 2014

If it makes enough spark to start, when the RPMs come up, there will be plenty of spark to run. I would set the coil gap, plain paper business card between the coil and the magnet, push the flywheel toward the coil and tighten it down.

Then go after the carb. Those non adjustable carbs are notorious for being too lean. Keep in mind you're adjusting this in warm weather, when the air is cold, it's denser, going just a little rich will be better.

Craftsman snowblower

rrobert

Member

rrobert

Member

rrobert

Member

El Toro

Well-known Member

Steve@Advance

Well-known Member

rrobert

Member

joe csuti

Well-known Member

jeffcat

Well-known Member

rrobert

Member

joe csuti

Well-known Member

Steve@Advance

Well-known Member

Similar threads