Cleaning the low speed jets on a Briggs cast metal 798917 carb - Snowblower Forum : Snow Blower Forums
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post #1 of 2 Old 12-30-2019, 12:10 AM Thread Starter
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Cleaning the low speed jets on a Briggs cast metal 798917 carb

I watched a couple Youtube videos on cleaning a Briggs cast metal carb and all the videos I watched talked about high speed jets getting clogged up but the ones I watched never mentioned the low speed jets.

In a recent thread here,

I explained the non-existent low/medium speed operation of a Briggs OHV 205cc winterized engine and asked about getting back some low/medium speed performance because after cleaning the high speed jet, the engine ran fine at half to full throttle but would want to die at the medium speed the governor is set at. The governor would kick in twice a second to keep the engine alive (rhh - rhh - rhh - rhh ad infinitum), but I want to fix it to get reasonable operation at low/medium speed. As the subject line says, this is for a Briggs 798917 cast metal carb. The factory carb is a RuiXing with OEM number 28642216 and 795485 on the top side of the mounting flange. But to Briggs, it's a 798917. It is shown in the Briggs parts manual as a 125E model carb.

So I got out my 1963 Howards Sams Small Engines book and refreshed myself on carb operation. First Principles: the low speed venturi orifices are ALWAYS downstream of the closed throttle plate. I entirely overlooked them on the first carb cleaning.

As to whether this applies to other carbs, the concept of low speed venturi orifices being downstream of the closed throttle plate applies, how they're 'fed' would differ from carb to carb.

The photo below shows the three low speed venturi orifices.

Two of the orifices are fed by ports upstream of the throttle plate, one (red) port is downstream of the choke, the other port (green) is upstream of the choke, as shown below.

(Sorry for the photo rotation, the web site mis-rotates it no matter how I re-size or rename the file. Technology is great when it works, huh?)

The blue circle shows the large atmospheric bore vent; that vents the bowl; it is very unlikely to ever clog if one keeps an air cleaner on the inlet. I can't figure out what port feeds the 3rd venturi orifice.

When the carburetor is manufactured, the passages and orifices need to be bored/drilled in the casting. The passageways are drilled from the air inlet end, then the passageway has the brass orifices inserted. The venturi orifices have to drilled from the side of the carburetor. The red circle shows the Welch plug/cover used to cover the access hole used to drill the three venturi orifices.

The passages and venturi orifices needed to be cleaned out. My tool for cleaning small orifices is a short length of 10g (?) stranded copper wire.

The insulation in the middle keeps all the strands together. When one strand gets too bent up, I clip it off. A copper strand is soft so it tends not to score the casting, and it is small enough in diameter to just fit the venturi orifice bores.

I tried to insert the copper wire strand (with a right angle bend in it) into the venturi orifice bores and found two of the three clogged. A right angle bend in the copper wire gave me access to work at cleaning the orifices out.

Eventually the wire poked a hole in the crud.

When cleaning the passages from the air inlet end, the copper wire has to go all the way through the passageway to the Welch plug/cover, as shown by from the start of the arrow at the air inlet to the point of the arrow in the vicinity of the Welch plug/cover. If the copper wire doesn't go that far, you haven't 'cleaned' the passageway. Mark the insertion depth with your thumb or finer and when you pull it out of the passageway, compare the depth on the outside of carb to see if the wire reaches the Welch plug/cover.

The passageways seem to have a restriction orifice in them downstream of the brass entry orifice because that small diameter copper strand would not slip down the passageway even though the opening at the air inlet end is relatively large. I had to work the copper strand around and around until it passed through the restriction orifice. (I'd like to cut this carb apart to check out the actual passageways, but it's the son-in-law's snowblower, so I don't have the luxury).

After cleaning the two passageways and the venturi orifices, I could see carb cleaner coming out of the venturi orfices when I shot some into each of the passageways at the inlet end. I couldn't hold the carb and the carb cleaner spray can AND a camera at the same time, so there's no photo, but with the throttle plate choke plate fully open, one can eyeball the venturi orifices when shooting the carb cleaner.

When put back together, the engine ran like it should, except that the governed speed seems way too low (but I haven't had snow to try it on, yet). There's no mention in the owner's manual for this Briggs 922EXD snowblower for what the governed operating speed should be and I can't find a service manual for the snowblower. Maybe someone here knows what the governor adjustment is for a 12xxxx Briggs OHV winter engine (circa 2016 date code).

If pressed, I might have purchased one of the carb rebuild kits because the kits include replacement Welch plug (127x). With a replacement available, the plug could be pryed out and the orifices cleaned from the outside.

No, I don't know about the dozens of other carbs out there (it's been 50 years since I messed with small engines) other than every carb has a high speed jet and at least one low speed jet, and a throttle stop speed adjustment.

I hope this helps people with good high speed performance but lousy or non-existent low/medium speed performance.
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post #2 of 2 Old 12-30-2019, 05:44 PM
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Sorry about the delay in getting this approved. I think the system flagged it due to the amount of photos in someone so new.

Make sure the windows are up before the snow plow goes by !!

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