I have an Excell VR2500 pressure washer powered by an B&S 6.5 HP 123K12-0134-E1 engine, which just wouldn't start after sitting for a couple of years. I did not expect the 1/2" brass screw that holds up the float bowl to actually be a jet and thought it was plugged up and put a drill to it (yep I did. Stupid is what stupid does). Now I need a new main jet. My problem is I have several numbers to pick from. One is 498978 and the other is 498981. Also on top of the head of the jet is a number stamped on, which is 77. What does that number mean and is it important?

https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/100000-19z999-series/123k00-to-123k99/123k12-0134-e1/carburetor
 
Part #117a is the standard jet, (498981), 
the other (different # than you posted) is for high altitude.
 
Un-less your above say 5,000' altitude, the standard jet is fine..
(Doesn't look like the high altitude jet is available either..
 
 

Thanks. What does the 77 mean on the head of the jet? Is it significant?

I'd guess the jet size.
 
Does your original have a # stamped on it? 
I don't re-call seeing #'s stamped on the jets and I clean MANY carbs, especially Briggs.

SRTsFZ6
I'd guess the jet size.
 
Does your original have a # stamped on it? 
I don't re-call seeing #'s stamped on the jets and I clean MANY carbs, especially Briggs.

Yes, that's why I attached the picture. Click on it and the number will clearly show. Yes, I am pretty sure it denotes the jet size but I am curious about what it actually means

The main jet issue has been fixed, I got a brand new carburetor for less than just the jet. I tested it for about 15 minutes and it ran great. So the next day I got ready to do some serious pressure washing and it wouldn't start at all. So weird. I finally traced it down and noticed that I didn't have a spark on my plug. I tested it with a different plug and I had a spark just fine. Went back to the old plug - no spark. Now my nightmare begins. I pull up the parts manual for this particular B&S engine and the plug it says I need for this engine doesn't look at all like the old one. The mandated one (#697451 EMS or #796112 resistor) has a tread depth of about 10 mm whereas the old plug is about twice as deep, about 20 mm (see picture). Now remember this plug has been in this engine for a good 10 years without any problems but I immediately went to measure to piston clearance at TDC and the tip of the plug when I realized that I am poking around in some chamber, because as I move the piston up and down the wire I stuck through the spark plug hole is not moving up and down with the piston. I looked in there with a flash light as good as I could and I can clearly see some kind of a chamber where the plug is and off to the left I can see the piston. I go back to the parts manual to check out all the possible heads and according to my engine code 050727 FA, head #594989 is the right head, but nowhere do i see any kind of a chamber. So now I am totally confused and have no clue what to make of all of this. I double checked engine model number and code and they are correct. Help!
 
 

I'm seeing a shorter plug per the parts diagram:
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/100000-19z999-series/123k00-to-123k99/123k12-0134-e1/camshaft-crankshaft-cylinder-engine-sump-kit-gasket-lubrication-piston-group-valves
 
 
Most importantly is how much threads the head actually has.  
 
It should be easily seen if it's approx 15MM or 30MM..
 
**The flat head engines almost always, uses the shallow plug. **
 
 
 
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https://www.sparkplug-crossreference.com/convert/briggsstratton/697451 
 
Spark plugs are usually much less $ from an automotive parts store.
 
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In the meantime I informed myself a little better about the B&S small engines and I have some answers to my own questions. Without removing the cylinder head I now know what it looks like in there. What I thought was a chamber was actually the top of a valve and what fooled me was the fact that I can move the piston and the valves don't necessarily move at that time. Yes, the parts manual is clear about what spark plug to use and it is a short plug not longer than about 10 mm and the one that was in the engine was twice as long. Scary, but apparently it worked all these years. Normally with a plug that is about 10 mm longer than what it should be either the piston or the valves will hit it, but apparently there is enough clearance. But regardless, i will put the proper plug in it because the manual says so. Maybe I will treat it to a E3.10 plug instead of the Champion RJ19LM

Yep, some engines the longer plug may not hit.
 
It was definitly the wrong one... I'd put the Champion in myself.
 
Gap at .030"..      Just my 2 cents.
 
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