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I very recently acquired a '91 Max II, with a B&S 16HP Vanguard V-Twin (303447-0326-01, code 91xxxxxx). This is my first ATV. I bought it mainly to gather and haul rocks and other materials around the 40 acres for erosion control and other projects.
It ran, and it moved very nicely, though it didn't idle and though it stalled when it got hot until it cooled down for half an hour or so.
I detected fuel in the oil, and the plugs showed that it was running rich.
I replaced the oil, oil filter, spark plugs, air filter, fuel filter and old fuel, and when I took it for a test drive the aforementioned stalling at temperature repeated.
When I removed the air filter assembly to look at the condition of the carburetor, I found a vacuum hose connected to the external float bowl vent and a screw that plugged the other end of the vacuum hose, that I presume was plugged for storage and forgotten by or unknown to the previous owner. That explained most or all of the symptoms, though the idle problem probably also is related to the old fuel and neglected maintenance of the carburetor and perhaps improper tuning of the carburetor.
The carburetor bowl had a lot of varnish from the old fuel. I proceeded to pull the carburetor and intake manifold, in order to clean and overhaul the carburetor, and I'm currently waiting for the overhaul kit.
The local B&S dealer was a royal pain in the neck. I told him what I found, what I wanted and what I planned to do, and he responded with misinformation and inappropriate and irresponsible opinions.
He told me that the bowl vent was for a vacuum line to a port on a valve cover, which is complete nonsense.
I told him that I knew it was a vent to atmosphere, and he still insisted that it was a vacuum line.
I played dumb. I knew that the fuel pump connects to the valve cover, because I had to remove it to pull the shroud, to pull the intake manifold and carburetor. The repair manual calls it a vent and doesn't show it connected to a valve cover or anything else. The position of the vent and the form of the upper body reveal its function. The internal vent from the bowl to the air horn makes it impossible for it to function as a vacuum master or slave, and there is absolutely no reason to connect it to the valve cover. The VW Beetle had a vacuum line from the carburetor to the distributor, but that was a different design with a specific function. Subsequently, I have not found anything online that reveals anything of the sort for that engine or any other Vanguard V-Twin or that it functions as anything other than an external vent to atmosphere, which can't be blocked if you want the engine to run correctly, because when the carburetor gets warm enough to evaporate enough fuel it'll run richer until it floods the engine to the point that the air-to-fuel mixture won't combust and to the point that it won't run well enough to respond to throttle until either enough fuel evaporates or leaks via gravity down through the tiny gaps in the piston rings and into the oil in the crankcase.
I'm not a mechanic. However, my father was an old-school VW mechanic, so I spent a few hours in the shop and know a spark plug from an oil plug.
I told him that I wanted a valve gasket set (head gaskets, valve seals, valve cover gaskets, etc.), in order to clean the heads and pistons, check the valve seals, adjust the valves and replace all of the gaskets on the top end.
Though he agreed that I should adjust the valves, he told me that I didn't need to be concerned about the carbon deposits or the rest.
Well, it was obviously running rich, and I've looked into the intakes, which revealed to me that the valves and valve stems are covered in carbon to an extent that I haven't seen in any of many videos on the subject. The repair manual lists cleaning the carbon deposits as proper maintenance, and there is no way for me to know how long overdue it is. Apparently I do need to pull the heads and at least inspect it, and I should replace the head gaskets after pulling the heads, for good measure and because it's in the manual as proper procedure.
I also wanted the parts, because it's 30 miles (including eight miles of dirt road) to the store, and it takes too long for the dealer to get the parts when I need them-two weeks to get a carburetor overhaul kit and some valve cover gaskets.
I'm at 6,000 feet above sea level, so I told him that I wanted a high altitude main jet.
He said that I didn't need it.
Without revealing the extent of my knowledge about it, I told him that I was only following the instruction in the manual. Hey, I knew what I wanted, and I wanted to order parts without a debate. I am not going to waste time with that kind of personality.
He told me that the recommendation in the manual is just a way for them to cover their butts.
Can you believe this clown? I'm almost 50 years old, and I still don't know how to get along with that kind of punk.
He was highly recommended by a friend. I can't believe this dealer is that clueless. I can only deduce that he decided that he didn't like me and wanted to jerk me around.
Well, I ordered what I wanted online, and when this guy calls to tell me that what he decided to LET me order has arrived he can go scratch.
Now that I'm done with my introductory rant, I'll get to my question, which is about sizing jets for this specific engine and carburetor.
The operator's manual, parts manual and repair manual for this engine say nothing about high altitude jets. The B&S website and the manuals for the 305000 and other Vanguard V-twin engines have minimal information about using the engines at high altitude-instructing us to see an authorized dealer about a high altitude kit. The wording regarding the altitude at which a high altitude kit should or should not be used varies from model to model-"between 2500 and 5000 feet above sea level", "over 5000 feet", "high altitude kit should not be used under 2500 feet", etc.
The carburetor is a Nikki 6100. It appears to be identical to the 809011, and the dealer identified it as such, but the stamp on the lower body is 807570. I haven't found anything online that matches that number, so I can only guess that the 807570 is an internal Nikki part number or a part number for the lower body or that it is a superseded part number.
The standard jet is the 690862. One vendor identifies it as a #92.
In the parts manual, the high altitude main jet is listed as 842311, which the same vendor identifies as a #87.
Not finding anything more than single case anecdotes about sizing jets for specific altitudes for this equipment and other Vanguard engines, I had to wonder about the sizing algorithm, so I did some research. I found a generic equation:
CF = 1.0778-.00111*T + (2.38554*10^-10)*A^2 -(1.0777*10^-5)*A;
New Jet = CF * Old Jet,
where_
CF = correction factor,
T = temperature in degrees Fahrenheit,
A = altitude in feet,
jet size corresponds to jet diameter (in hundredths of a millimeter in the examples).
Source: https://www.rollingwrenchdenver.com/...nd-tuning-help
Of course, the vendor doesn't identify what method is used to number the jets. However, I plugged in the numbers.
At 6,000 feet and 68°F, CF = 0.946, which gives 0.946 * 92 = 87.055. #92 for 690862 and #87 for 842311. BINGO! Winner! Winner! Chicken dinner!
Can anyone verify this methodology or offer a better rule of thumb, based on experience?
Is the 842311 a good fit at 6,000 feet?
I very recently acquired a '91 Max II, with a B&S 16HP Vanguard V-Twin (303447-0326-01, code 91xxxxxx). This is my first ATV. I bought it mainly to gather and haul rocks and other materials around the 40 acres for erosion control and other projects.
It ran, and it moved very nicely, though it didn't idle and though it stalled when it got hot until it cooled down for half an hour or so.
I detected fuel in the oil, and the plugs showed that it was running rich.
I replaced the oil, oil filter, spark plugs, air filter, fuel filter and old fuel, and when I took it for a test drive the aforementioned stalling at temperature repeated.
When I removed the air filter assembly to look at the condition of the carburetor, I found a vacuum hose connected to the external float bowl vent and a screw that plugged the other end of the vacuum hose, that I presume was plugged for storage and forgotten by or unknown to the previous owner. That explained most or all of the symptoms, though the idle problem probably also is related to the old fuel and neglected maintenance of the carburetor and perhaps improper tuning of the carburetor.
The carburetor bowl had a lot of varnish from the old fuel. I proceeded to pull the carburetor and intake manifold, in order to clean and overhaul the carburetor, and I'm currently waiting for the overhaul kit.
The local B&S dealer was a royal pain in the neck. I told him what I found, what I wanted and what I planned to do, and he responded with misinformation and inappropriate and irresponsible opinions.
He told me that the bowl vent was for a vacuum line to a port on a valve cover, which is complete nonsense.
I told him that I knew it was a vent to atmosphere, and he still insisted that it was a vacuum line.
I played dumb. I knew that the fuel pump connects to the valve cover, because I had to remove it to pull the shroud, to pull the intake manifold and carburetor. The repair manual calls it a vent and doesn't show it connected to a valve cover or anything else. The position of the vent and the form of the upper body reveal its function. The internal vent from the bowl to the air horn makes it impossible for it to function as a vacuum master or slave, and there is absolutely no reason to connect it to the valve cover. The VW Beetle had a vacuum line from the carburetor to the distributor, but that was a different design with a specific function. Subsequently, I have not found anything online that reveals anything of the sort for that engine or any other Vanguard V-Twin or that it functions as anything other than an external vent to atmosphere, which can't be blocked if you want the engine to run correctly, because when the carburetor gets warm enough to evaporate enough fuel it'll run richer until it floods the engine to the point that the air-to-fuel mixture won't combust and to the point that it won't run well enough to respond to throttle until either enough fuel evaporates or leaks via gravity down through the tiny gaps in the piston rings and into the oil in the crankcase.
I'm not a mechanic. However, my father was an old-school VW mechanic, so I spent a few hours in the shop and know a spark plug from an oil plug.
I told him that I wanted a valve gasket set (head gaskets, valve seals, valve cover gaskets, etc.), in order to clean the heads and pistons, check the valve seals, adjust the valves and replace all of the gaskets on the top end.
Though he agreed that I should adjust the valves, he told me that I didn't need to be concerned about the carbon deposits or the rest.
Well, it was obviously running rich, and I've looked into the intakes, which revealed to me that the valves and valve stems are covered in carbon to an extent that I haven't seen in any of many videos on the subject. The repair manual lists cleaning the carbon deposits as proper maintenance, and there is no way for me to know how long overdue it is. Apparently I do need to pull the heads and at least inspect it, and I should replace the head gaskets after pulling the heads, for good measure and because it's in the manual as proper procedure.
I also wanted the parts, because it's 30 miles (including eight miles of dirt road) to the store, and it takes too long for the dealer to get the parts when I need them-two weeks to get a carburetor overhaul kit and some valve cover gaskets.
I'm at 6,000 feet above sea level, so I told him that I wanted a high altitude main jet.
He said that I didn't need it.
Without revealing the extent of my knowledge about it, I told him that I was only following the instruction in the manual. Hey, I knew what I wanted, and I wanted to order parts without a debate. I am not going to waste time with that kind of personality.
He told me that the recommendation in the manual is just a way for them to cover their butts.
Can you believe this clown? I'm almost 50 years old, and I still don't know how to get along with that kind of punk.
He was highly recommended by a friend. I can't believe this dealer is that clueless. I can only deduce that he decided that he didn't like me and wanted to jerk me around.
Well, I ordered what I wanted online, and when this guy calls to tell me that what he decided to LET me order has arrived he can go scratch.
Now that I'm done with my introductory rant, I'll get to my question, which is about sizing jets for this specific engine and carburetor.
The operator's manual, parts manual and repair manual for this engine say nothing about high altitude jets. The B&S website and the manuals for the 305000 and other Vanguard V-twin engines have minimal information about using the engines at high altitude-instructing us to see an authorized dealer about a high altitude kit. The wording regarding the altitude at which a high altitude kit should or should not be used varies from model to model-"between 2500 and 5000 feet above sea level", "over 5000 feet", "high altitude kit should not be used under 2500 feet", etc.
The carburetor is a Nikki 6100. It appears to be identical to the 809011, and the dealer identified it as such, but the stamp on the lower body is 807570. I haven't found anything online that matches that number, so I can only guess that the 807570 is an internal Nikki part number or a part number for the lower body or that it is a superseded part number.
The standard jet is the 690862. One vendor identifies it as a #92.
In the parts manual, the high altitude main jet is listed as 842311, which the same vendor identifies as a #87.
Not finding anything more than single case anecdotes about sizing jets for specific altitudes for this equipment and other Vanguard engines, I had to wonder about the sizing algorithm, so I did some research. I found a generic equation:
CF = 1.0778-.00111*T + (2.38554*10^-10)*A^2 -(1.0777*10^-5)*A;
New Jet = CF * Old Jet,
where_
CF = correction factor,
T = temperature in degrees Fahrenheit,
A = altitude in feet,
jet size corresponds to jet diameter (in hundredths of a millimeter in the examples).
Source: https://www.rollingwrenchdenver.com/...nd-tuning-help
Of course, the vendor doesn't identify what method is used to number the jets. However, I plugged in the numbers.
At 6,000 feet and 68°F, CF = 0.946, which gives 0.946 * 92 = 87.055. #92 for 690862 and #87 for 842311. BINGO! Winner! Winner! Chicken dinner!
Can anyone verify this methodology or offer a better rule of thumb, based on experience?
Is the 842311 a good fit at 6,000 feet?
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