Off topic carb problem...

[Truckedup]

It's on my ATV, a CV Mikuni.......I'm supposed to know all about bike type carbs but this is giving me fits.... The engine runs well on the throttle but idles poorly.But when the throttle is opened, all is normal it seems..I've been through float levels, cleaning , replacing idle jets etc...No air leaks to be found...
Think about this ....the engine is idling rough...I block the carb bowl vent and the idle picks up about 1000 rpm and runs well, I lower the idle speed and the engine has a nice regular idle....With the vent still plugged, if I open the throttle, it spits out the carb...This says it's lean off idle...
My question, what effect should a plugged bowl vent have? Should the carb go lean or rich ? This will help me figure out what is really the problem.. Thanks

[pamotorman]

on a automotive carb blocking the bowl vent will make the carb go rich as you are increasing pressure above atmospheric in the float bowl

[Roundybout]

I've been playing around all summer on the kids air cooled bikes with the Mikuni carb. Unscrew the idle mixture screw and check the tip. The ones that control fuel with the sharp taper especially can get screwed up making it hard to get consistent idle. Blocking the vent and it running better is a sign it's lean. It should run the same with a partial choke as it does when blocking the vent. Those idle jets are tiny. I don't know what size you're dealing with but replacing them wouldn't be a bad idea as cheap insurance.

[Truckedup]

I've been playing around all summer on the kids air cooled bikes with the Mikuni carb. Unscrew the idle mixture screw and check the tip. The ones that control fuel with the sharp taper especially can get screwed up making it hard to get consistent idle. Blocking the vent and it running better is a sign it's lean. It should run the same with a partial choke as it does when blocking the vent. Those idle jets are tiny. I don't know what size you're dealing with but replacing them wouldn't be a bad idea as cheap insurance.

Thanks I replaced the idle jet with a new one of the correct size and probed and blew compressed air into the passage....There may be a piece of crap caught inside an idle mixture passage..

[Tuner]

on a automotive carb blocking the bowl vent will make the carb go rich as you are increasing pressure above atmospheric in the float bowl

^^ As a blanket statement, this can be considered to be exactly backwards. ^^

Blocking a bowl vent can make a carb lean or will cut off fuel flow altogether. Atmospheric pressure is what pushes fuel through the carb metering circuits. When the vent is blocked the engine vacuum will will pull enough vacuum in the bowl it will reduce or stop fuel flow.

The exception is if the carb temperature is high enough to boil the fuel so vapor pressure is higher than atmospheric pressure, in which case the vent is passing vapor to the atmosphere, instead of atmospheric pressure to the bowl. In the case of high fuel temperature and vapor pressure, a blocked vent can make it rich.

It is possible that a restricted vent can cause richness at very small fuel flow, such as standing idle, but be lean when fuel demand is increased.

When emissions raised idle speeds engine run-on "dieseling" on shut off became a problem. To prevent dieseling on engine shut down, many (mostly Japanese) carburetors have solenoids that cut off idle circuit fuel flow, some (mostly English) have solenoids that close bowl vents to stop fuel flow, most American carbs used a solenoid on the throttle to cut off air when the key is switched off.

It's on my ATV, a CV Mikuni.......I'm supposed to know all about bike type carbs but this is giving me fits.... The engine runs well on the throttle but idles poorly.But when the throttle is opened, all is normal it seems..I've been through float levels, cleaning , replacing idle jets etc...No air leaks to be found...
Think about this ....the engine is idling rough...I block the carb bowl vent and the idle picks up about 1000 rpm and runs well, I lower the idle speed and the engine has a nice regular idle....With the vent still plugged, if I open the throttle, it spits out the carb...This says it's lean off idle...
My question, what effect should a plugged bowl vent have? Should the carb go lean or rich ? This will help me figure out what is really the problem.. Thanks

The trick in answering your question is at idle it can go either way, R or L, depending on temperature and how much restriction. At large throttle opening plugging the vent will definitely make it lean.

[Truckedup]

Ok..I cleaned the carb one more time.......Well it now idles better but it's too rich...I made the mistake of doing two adjustments at the same time...I put in one size larger idle jet (same type jets as my race bikes ) and raised the float level 1mm...I'm going back to the original idle jet and try that...it's a pain in the ass, the carb need to be removed even for an idle mixture screw adjustment due to lack of room..

[pamotorman]

on a automotive carb blocking the bowl vent will make the carb go rich as you are increasing pressure above atmospheric in the float bowl

^^ As a blanket statement, this can be considered to be exactly backwards. ^^

Blocking a bowl vent can make a carb lean or will cut off fuel flow altogether. Atmospheric pressure is what pushes fuel through the carb metering circuits. When the vent is blocked the engine vacuum will will pull enough vacuum in the bowl it will reduce or stop fuel flow.

The exception is if the carb temperature is high enough to boil the fuel so vapor pressure is higher than atmospheric pressure, in which case the vent is passing vapor to the atmosphere, instead of atmospheric pressure to the bowl. In the case of high fuel temperature and vapor pressure, a blocked vent can make it rich.

It is possible that a restricted vent can cause richness at very small fuel flow, such as standing idle, but be lean when fuel demand is increased.

When emissions raised idle speeds engine run-on "dieseling" on shut off became a problem. To prevent dieseling on engine shut down, many (mostly Japanese) carburetors have solenoids that cut off idle circuit fuel flow, some (mostly English) have solenoids that close bowl vents to stop fuel flow, most American carbs used a solenoid on the throttle to cut off air when the key is switched off.

It's on my ATV, a CV Mikuni.......I'm supposed to know all about bike type carbs but this is giving me fits.... The engine runs well on the throttle but idles poorly.But when the throttle is opened, all is normal it seems..I've been through float levels, cleaning , replacing idle jets etc...No air leaks to be found...
Think about this ....the engine is idling rough...I block the carb bowl vent and the idle picks up about 1000 rpm and runs well, I lower the idle speed and the engine has a nice regular idle....With the vent still plugged, if I open the throttle, it spits out the carb...This says it's lean off idle...
My question, what effect should a plugged bowl vent have? Should the carb go lean or rich ? This will help me figure out what is really the problem.. Thanks

The trick in answering your question is at idle it can go either way, R or L, depending on temperature and how much restriction. At large throttle opening plugging the vent will definitely make it lean.

it will push fuel out of the bosters if you block off the bowl vent at idle. the air pressure generated by the fuel pump output has to be vented somewhere.

[Tuner]

Dude, please. The float and N&S hold the pump pressure like they usually do whether the vent is blocked or not. Unless the fuel is hot enough to boil vapor pressure, the bowl will draw a vacuum and fuel flow stops. Several cars, Jaguars, MGs, Hondas, et al. close the bowl vent to stop fuel flow to prevent engine run-on when the key is turned off.

[grandsport51]

Dont know about Morri Garages
But many of the jap carbs used an idle cut
Solenoid (fuel)to eliminate run on keihin, etc.

92E1A5D6-5569-401F-B990-0B169A569FE9.jpeg

[pamotorman]

Dude, please. The float and N&S hold the pump pressure like they usually do whether the vent is blocked or not. Unless the fuel is hot enough to boil vapor pressure, the bowl will draw a vacuum and fuel flow stops. Several cars, Jaguars, MGs, Hondas, et al. close the bowl vent to stop fuel flow to prevent engine run-on when the key is turned off.

at idle fuel is going out of the float bowl so the N/S is open to refill the float bowl and air is coming in along with the fuel that must be vented

[Geoff2]

pamotorman,
Tuner is correct.
How can air be entering via the n/s? It is fuel only, drawn from the tank, pumped via the pump, into the carb.

[pamotorman]

all the foam you see in the float bowl on the bo laws float bowl N/S dyno test is air in the fuel.

[GRTfast]

Sometimes the little air passages in the air side of the carb inlet (that feed air to the pilot circuit) can be clogged, and will cause exactly what you're describing. use a very tiny copper or brass wire to make sure they are clean.

[Roundybout]

A lot of these mikunis employ an external and internal vent to atmosphere. The external is a passage way from an opening in the air horn to above the float chamber. The internal is a balance tube that equalizes the pressure of the air coming in thru the filter and the pressure in the bowl. Without it the bowl pressure could be higher that the air entering through the filter if really dirty for instance. This would cause a rich condition. With either one blocked, these interconnected passageways can pass fuel.

[Tuner]

all the foam you see in the float bowl on the bo laws float bowl N/S dyno test is air in the fuel.

No, not entirely. Some is air bubbles similar to what you would generate squirting a garden hose nozzle into a bucket, but most is fuel vapor blowing bubbles in the liquid. Like water vapor bubbles in boiling water and the foam head on beer or champagne, the gas pressure that makes the bubbles comes from a portion of the liquid becoming vapor.

You just need to take it as gospel, blocked bowl vent = lean. Use your closest engine with a Holley carb on it and use tape to block the bowl vents, see if you can get it to run and if it is rich or lean.