CV carb pulling fuel through main jet at idle

[nascar7613]

2000 Duece, stock cv carb. The fuel is being pulled through the main jet at idle, fouling the plugs. Took the carb apart, cleaned all the passages with carb cleaner and compressed air, set the float according to the manual, replaced the o rings, all the things you're supposed to do. The kit had the wrong float valve, so I used the old one but it looked ok. I have another bike here with the same carb and no fuel passes through the main jet at idle. Any advise would be welcome!

 

[Jeff Klarich]

My guess is the float is not quite right, try re-adjusting.

 

[dolt]

First, I would be skeptical that fuel is passing through the main jet at idle. The needle jet/needle is the control over fuel passing through the main jet; if the needle is seated properly and the needle jet installed properly, fuel cannot pass through the main jet. A more likely possibility is that the enrichener is not seating properly and allowing additional fuel to flow. No harm in checking the float for re-adjustment but I would be looking at the components that actually control fuel flow through the main as well as the enrichener and choke plug.

 

[nascar7613]

First, I would be skeptical that fuel is passing through the main jet at idle. The needle jet/needle is the control over fuel passing through the main jet; if the needle is seated properly and the needle jet installed properly, fuel cannot pass through the main jet. A more likely possibility is that the enrichener is not seating properly and allowing additional fuel to flow. No harm in checking the float for re-adjustment but I would be looking at the components that actually control fuel flow through the main as well as the enrichener and choke plug.

I know the needle jet is installed properly, larger hole facing up, but I'm not sure if the needle jet or needle itself have been replaced before. The needle has a N8E on it, but the needle jet has no markings. When the bike is idling, the fuel is coming out of the top of the needle jet. The choke cable seemed to be in good shape, the end of it looked fine. You referred to the choke plug, but I don't know what that is.

 

[fin_676]

There will always be fuel being drawn through the needle jet at idle the amount of fuel that is being drawn is controlled by the needle
the needle is tapered the further up it is drawn by the slide the more fuel is drawn through the jet
there is no cut off of the needle jet thefore it always allows fuel to be drawn
if there is too much fuel being drawn then the jet may be too large or the needle may be the wrong profile or has been raised by washers under the needle seat
Early cv carbs fitted to sportsters had no accelerator pump and to compensate had a steeper profile on the needle some people thought that fitting the early sportster needle would give more fuel and faster acceleration and so may have been fitted by a previous owner

at idle and low speeds the slow jet has a greater influence on performance than the main so ensuring the slow jet is the correct size would be the first place to start

Brian

 

[nascar7613]

There will always be fuel being drawn through the needle jet at idle the amount of fuel that is being drawn is controlled by the needle
the needle is tapered the further up it is drawn by the slide the more fuel is drawn through the jet
there is no cut off of the needle jet thefore it always allows fuel to be drawn
if there is too much fuel being drawn then the jet may be too large or the needle may be the wrong profile or has been raised by washers under the needle seat
Early cv carbs fitted to sportsters had no accelerator pump and to compensate had a steeper profile on the needle some people thought that fitting the early sportster needle would give more fuel and faster acceleration and so may have been fitted by a previous owner

at idle and low speeds the slow jet has a greater influence on performance than the main so ensuring the slow jet is the correct size would be the first place to start

Brian

It has a 185 main and a 45 slow. What I did was closed the air fuel screw and turned it out 1 3/4 turns instead of the 2 1/2 that it was set at to try to compensate for the extra fuel coming through the needle jet. The bike runs much better and the plugs are not black now. I will leave it at that, all I want is for the bike to run good, which it does now.

 

[fin_676]

From the factory as best as i can work out it would have come with 180 main and a 45 slow jet
But they are a bit on the lean side from the factory so 185 main and 45 slow out to be about right
However getiing it properly tuned is the only way to know for sure
But if it feels right and sounds right and performs right you have got it close enough for normal road use

Brian

 

[dolt]

Fin is right and I should have qualified my post. The diameter of the needle does allow some fuel into the intake stream via the needle jet at idle and up to about 1/4 throttle and even if he needle has been raised, the amount of fuel passing the needle jet is marginal will never be enough to foul plugs. I still don't believe the plug fouling is caused by fuel passing through the needle jet.

As Fin points out, the slow jet is in play at idle to about 1/4 throttle and sizing that jet properly (45 may be right) would be worth checking. JMHO but the A/F screw should not be used to control fuel passing through the needle jet; that is the job of the needle and needle jet. The A/F screw should be used to set a smooth idle. Turn the A/F screw all the way in, not hard, and the motor should stumble and idle rough or even die. If that happens, the slow jet is too big. Backing the A/F screw back out until the motor is idling smooth is the next step and that usually takes more than 13/4 turns but every motor is different. Even a 1/8 turn can make a difference so make A/F adjustments slowly and at operating temp. I don't have a chart for the CV40 needles but there are four N8 needles for the CV44, N8EL, EN, EM and EK. The diameter at the stop increases from .106" to .108" which doesn't sound like a lot but does make a difference. I don't have a chart for the CV40 needles and it is possible that a previous owner has changed needles to one with a smaller diameter.

The way to tune a CV carb is to set idle first, which may involve the slow jet; set mid range and then the main. This can be done by riding the bike (seat of the pants) and plug reading or using an A/F meter if available, I do it on a dyno. Nightrider has a great CV tuning guide.

http://nightrider.com/biketech/hd_cv_mods.htm

 

[nascar7613]

Fin is right and I should have qualified my post. The diameter of the needle does allow some fuel into the intake stream via the needle jet at idle and up to about 1/4 throttle and even if he needle has been raised, the amount of fuel passing the needle jet is marginal will never be enough to foul plugs. I still don't believe the plug fouling is caused by fuel passing through the needle jet.

As Fin points out, the slow jet is in play at idle to about 1/4 throttle and sizing that jet properly (45 may be right) would be worth checking. JMHO but the A/F screw should not be used to control fuel passing through the needle jet; that is the job of the needle and needle jet. The A/F screw should be used to set a smooth idle. Turn the A/F screw all the way in, not hard, and the motor should stumble and idle rough or even die. If that happens, the slow jet is too big. Backing the A/F screw back out until the motor is idling smooth is the next step and that usually takes more than 13/4 turns but every motor is different. Even a 1/8 turn can make a difference so make A/F adjustments slowly and at operating temp. I don't have a chart for the CV40 needles but there are four N8 needles for the CV44, N8EL, EN, EM and EK. The diameter at the stop increases from .106" to .108" which doesn't sound like a lot but does make a difference. I don't have a chart for the CV40 needles and it is possible that a previous owner has changed needles to one with a smaller diameter.

The way to tune a CV carb is to set idle first, which may involve the slow jet; set mid range and then the main. This can be done by riding the bike (seat of the pants) and plug reading or using an A/F meter if available, I do it on a dyno. Nightrider has a great CV tuning guide.

http://nightrider.com/biketech/hd_cv_mods.htm

I think the needle must be wrong for that carburetor is all I can figure, I don't know for sure. The bike has been running fine and no more fouling issues. I know that turning the air fuel screw in further is not the proper way to tune the carburetor, but it made the bike run good, so what the heck. Thank you all for your comments and advice!