Weber IDF Main jet help Options

[tornik550]

Murray is correct in that changing multiple things at once isn't the correct way to troubleshoot. You have no idea what had the most profound effect.

Increase fuel pressure = rich
Increase float height = rich
Decrease AC jet size = rich

I suggest you set a new base line with the below info and make changes from there one at a time. Note what each change does from here on out and then adjust accordingly.

#1 - Change AC jets to 2.00 - Assuming you have the F11, you never said what ET you have. 2.00 is base AC jet for the F11. Above 2.00 you lean the mixture at high velocity, below you enrichen.

#2 - Float height = 10mm w/ gasket 11mm w/out. I suspect this was the main reason you were lean with the huge main jets. At 11mm with the gasket your main fuel wells were a low.

#3 - That bogging at 2000 RPM is because your pig rich. Reduce your main jet size. I'd suggest 160's and go from there.

#4 - That new sound your hearing is a result of the larger vents. I personally love the sound.

#1 I have F11 ET tubes. I also have a set of f7 that I could use. I changed the AC to 2.00. Things ran better but still rich. I did not notice any bogging.
#2 Thank you.
#3 I went down to 170 so far and things feel pretty good however I have not had a chance to check the plugs.

[McMark]

Increasing the float height will not change the mixture unless you mess up and allow the bowl to overfill. The float work JUST like the one in your toilet tank.

When setting the float closed height (10mm or 11mm) make sure you don't compress the ball spring on the needle valve. You want the needle valve seated/closed, but not compressing the ball. If you do this wrong, your bowl will be too empty.

Fuel pressure will also not affect mixture, unless you set it too high and it blows the needle valve off it's seat. But there will be NO general mixture change between 2psi and 3psi.

Be careful. There is a LOT of misinformation about carbs out there.

[jmill]

Increasing the float height will not change the mixture unless you mess up and allow the bowl to overfill. The float work JUST like the one in your toilet tank.

When setting the float closed height (10mm or 11mm) make sure you don't compress the ball spring on the needle valve. You want the needle valve seated/closed, but not compressing the ball. If you do this wrong, your bowl will be too empty.

Fuel pressure will also not affect mixture, unless you set it too high and it blows the needle valve off it's seat. But there will be NO general mixture change between 2psi and 3psi.

Be careful. There is a LOT of misinformation about carbs out there.

You aren't kidding about misinformation.

As you increase or decrease float height you change the height of the fuel column inside the main fuel wells. The emulsion tube sits in that column of fuel. Air is sucked in through the AC jet and pulled through the center of the ET. This emulsifies the mixure which then enters the carb through the center of the venturi. The higher the column the richer the emulsified mixture.

As far as fuel pressure, the float has to fight the needle against fuel pressure to close it. The higher the pressure the harder it has to fight. With higher pressure what you actually end up with is a float that sinks a bit before it can shut of the flow. That gives you a higher float level. See above why that affects your mixture.

[McMark]

The rate at which fuel is admitted to the emulsion chamber is controlled by the main jet, not the fuel bowl level. The float height has nothing to do with that. To get nitty-gritty about it, a higher fuel level in the wells (due to a higher float setting) will provide a SLIGHTLY richer mixture just as the main jets come into play. But soon after that the emulsion tube situation stabilizes and you simply have fuel flowing at the rate allowed by the main jet and air flowing at the rate allowed by the AC jet and E-Tube.

You can't seriously be arguing that the level of the fuel being pushed by atmospheric pressure from the fuel bowl, is GREATER than the vacuum influence from the chokes? (IMG:style_emoticons/default/blink.gif)

[jmill]

You can't seriously be arguing that the level of the fuel being pushed by atmospheric pressure from the fuel bowl, is GREATER than the vacuum influence from the chokes? (IMG:style_emoticons/default/blink.gif)

I'm saying float level does have an effect on the mixture. I agree that it's effect is most apparent at transition where 1.5mm is the difference between blowing off your air cleaners or not.

I'll tell Bob Tomlinson to get this corrected ASAP.

[McMark]

I guess I read that paragraph a completely different way. If the fuel level is too high, it will spill into the throat, causing a rich condition. If the fuel level is too low, the carb can run out of fuel under hard acceleration, thereby causing a lean condition. The page you posted says nothing about the fuel level inside the emulsion tube. It makes a general statement that you assume applies to the entire RPM range. Bob is correct that a high float level can cause a rich condition, and a low float level can cause a lean condition. But that's not the same as changing the mixture overall.

[john rogers]

From several of the posts here I seem to read that many think that the float raises up and at a certain level shuts off the fuel flow and acts like the float in a toilet tank. The guys at Pierce Manifolds (Weber repair masters) explained it to me this way as did several old German Porsche mechanics: The float and it's attached needle valve is bouncing like mad inside that carb body and on a race/street car with stiff springs and shocks it is worse than the carb was ever designed for. That is one of the items that PMO rectified with their design but unfortunately they do not make a 4 cylinder version. Anyways, since the needle does not actually shut tight and stay that way when you raise fuel pressure at the bowl it causes some spill over due to the increased force against the needle valve. I was not sure of this but when notable racers such as Wayne Baker and others said to bump up pressure by 1/2 to a full pound on a track such as CA Speedway or Willow Springs Big Track where you are at full throttle a very long time and this will provide some insurance against running out of pump capacity and going too lean. Sure enough, when at full throttle the O2 mixture would go rich by 1 full number, usually from 13.51 mixture to 12.5:1 mixture. On short tracks I would lower fuel pressure to about 3.5# and the car was very happy.

Of course this bouncing also affects the set float level and is one of the reasons why it changes over time. As a matter of fact the guys at Pierce Manifolds strongly suggest that float levels NOT be set until the car has been towed to the track as the vibration of the trip can affect the float settings!

[tornik550]

I am new to carbs. I am a human mechanic and not a car mechanic. I am trying to learn and I appreciate everyones help.

Last night I only made three changes. The changes were that I increased the venturi size which should have leaned the mixture. I slightly increased the fuel pressure and I raised the float level from 11mm with the gasket to 10mm with the gasket. I then had a super rich mixture.

Today, I drove around quite a bit and played with the mains. I had AC jets of 190 the entire time. I tried 180, 170 and 160 and all were rich however 160 didn't seem too bad. I didn't have a 155 with me so I tried 150 and Now the mixture is lean. I am trying to find my 155's and i think that I should be dead on.

[tornik550]

So what is the cheapest way to get a o2 sensor, a/f fuel gauge setup? I think that my wife will just have to be pissed for a little while.

[McMark]

PLX is $269, here.
Innovate Motorsports is $209, here.

A smaller choke will create vacuum in the main jet circuit at a lower RPM. The small diameter makes the air move faster. Faster air = larger pressure drop. The opposite is also true, a larger choke will slow down the air and delay the main jet activating. If your car were perfect at idle and perfect at mid RPM, but went rich in between (cruizing down the road for example) it's because the idle jets and the main jets are working at the same time. Going to a larger choke would move the main jet up the RPM band and keep them from overlapping.

[ME733]

I am new to carbs. I am a human mechanic and not a car mechanic. I am trying to learn and I appreciate everyones help.

Last night I only made three changes. The changes were that I increased the venturi size which should have leaned the mixture. I slightly increased the fuel pressure and I raised the float level from 11mm with the gasket to 10mm with the gasket. I then had a super rich mixture.

Today, I drove around quite a bit and played with the mains. I had AC jets of 190 the entire time. I tried 180, 170 and 160 and all were rich however 160 didn't seem too bad. I didn't have a 155 with me so I tried 150 and Now the mixture is lean. I am trying to find my 155's and i think that I should be dead on.

...................terriffic......your jetting is in the range I suspected it might need to be in...., continue to keep an eye on the OIL temperature and Cylinder Head Temperature. Try a little longer trip after you get the 155,s in and see how it goes....Murray.

[jeffdon]

This is some of the most insightful carb tuning talk i have seen. And I am just getting started on optimizing my set-up, so its damn timely info.

Keep it coming.

[john rogers]

I would suggest that for optimizing a carb setup to do it the following way. I learned this the hard (very hard) way when I tried messing with the race car when I went from a 2.2L four to a 2.8L four and then into a 2.2L six.
1. Start at the front, I.E. make sure the tank is clean and rust and junk free.
2. Install a fuel filter at pump suction.
3. Make sure you have the correct fuel pump, those little Facet "cube" shakers are bad for the float settings due to pulses.
4. Make sure fuel lines are large enough, 1/2 inch are great.
5. In the engine compt. do not let the fuel lines touch anything that generates heat and even covering them with fire sleeve is a good idea.
6. Add a pressure gauge just before the carbs so you will know what pressure is acting on the needle and seats.
7. Try to either add an O2 sensor/meter in your exhaust or borrow a meter so the car can be tuned to correct mixture.
8. Make sure to use the insulating spacers under the intake runners to keep heat away from the carbs.
9. Set float heights to the middle to start with and adjust as necessary if needed. Make sure to have the Weber repair books handy and refer to then often.
10. If the jet sizes seem a bit too far out of what normal should be, look for something wrong with the engine setup such as wrong venturi size, weak spark, etc.
11. When starting out, make sure ALL other items are in like new shape such as plugs, points (if used), valve adjustment, etc is not going to have a negative and masking effect on the carb tuning.
12. Do not ever try to sync the carbs by ear, use one of the professional synchrometers and make sure they are sync'd when starting.
13. Run the car ofter, especially in CA where the gas is crap and will let lots of junk settle into the bottom of the carbs.

Hope this helps some.

[ME733]

I would suggest that for optimizing a carb setup to do it the following way. I learned this the hard (very hard) way when I tried messing with the race car when I went from a 2.2L four to a 2.8L four and then into a 2.2L six.
1. Start at the front, I.E. make sure the tank is clean and rust and junk free.
2. Install a fuel filter at pump suction.
3. Make sure you have the correct fuel pump, those little Facet "cube" shakers are bad for the float settings due to pulses.
4. Make sure fuel lines are large enough, 1/2 inch are great.
5. In the engine compt. do not let the fuel lines touch anything that generates heat and even covering them with fire sleeve is a good idea.
6. Add a pressure gauge just before the carbs so you will know what pressure is acting on the needle and seats.
7. Try to either add an O2 sensor/meter in your exhaust or borrow a meter so the car can be tuned to correct mixture.
8. Make sure to use the insulating spacers under the intake runners to keep heat away from the carbs.
9. Set float heights to the middle to start with and adjust as necessary if needed. Make sure to have the Weber repair books handy and refer to then often.
10. If the jet sizes seem a bit too far out of what normal should be, look for something wrong with the engine setup such as wrong venturi size, weak spark, etc.
11. When starting out, make sure ALL other items are in like new shape such as plugs, points (if used), valve adjustment, etc is not going to have a negative and masking effect on the carb tuning.
12. Do not ever try to sync the carbs by ear, use one of the professional synchrometers and make sure they are sync'd when starting.
13. Run the car ofter, especially in CA where the gas is crap and will let lots of junk settle into the bottom of the carbs.

Hope this helps some.

....................I would make the following observation and comment on your recommendations......add( 1A.) always add and install a seperate fuel pump cut off switch....this will allow you to run the engine,and shut off fuel supply, which empties the float boals. Then you can transport the race car without fear of flooding it,causing a hydrolic-lockup,...naturally this assumes you installed the fuel lines in such a manner as to NOT have a syphon effect going on...Your # (3) facet pumps generally are not used on race cars as most racers want a higher level of quality, fuel pump. They are fine for street cars.the pulsation effect is absorbed in the fuel line and pressure regulator. the float needles are bouncing anyway and are not going to be affected by the pump...Your # (4) 1/2/inch fuel lines is just nuts., unless you have a fuel/blown /dragster. that rediculious large size is just a larger heat sink,far more difficult to install in a protected manner,the fuel pumps and regulators do not have that inlet/or outlet size threads.(unless your going to specially made ultra high volume drag racer stuff)...Your #5...covering the fuel lines with "fire sleeve"... will just trap and hold heat against the fuel lines, obscure any ABRASIONS/or cuts that might occur and is a VERY BAD idea...Your #7... an o2 meter is ok, except that for almost the same money an EXHAUST gas temperature guage is far more accurate, and useful. (and it,s what you see in race cars not an o2 guage.)......hope this helps.

[jmill]

If your car were perfect at idle and perfect at mid RPM, but went rich in between (cruizing down the road for example) it's because the idle jets and the main jets are working at the same time. Going to a larger choke would move the main jet up the RPM band and keep them from overlapping.

(IMG:style_emoticons/default/agree.gif)

If the overlap is minimal or the mains come on just a tad late you can try and correct it with an ET change.

F11 = leaner transition - holes are placed higher, diameter of the tube leaves less room for fuel in the main fuel wells.

F7 = richer transition - holes are placed lower, smaller diameter to hold more fuel in the fuel wells.