Carb Over Fueling At Low Revs

[tas_ae71]

hey guys,

 

pulled the weber off my 4ac today as it has been running like a bag of shit (and had to fix the linkage). cleaned out the jets, blew everything out, checked and cleaned out accelerator pump. noted one of the jets in the bottom of the bowl had a bit of shit in it and was half blocked.

 

put it all back together, sealed up and bolted it back on. got it running and it was still doing the same thing that it was before.

 

when you first start it, its pooring so much fuel down it, it will not run any faster then about 700rpm. over the next 3 minutes (as it starts to warm up i guess), it gradually builds up until it clears and is drivable (still very boggy at low revs). now tonight after it had been cleaned out, its shooting more fuel down it!!

 

when you look down the barrel of the carb, fuel is coming out thick and fast. also in large droplets not mist.

 

accelerator pump is working correctly, it only sprays when the throttle in increased.

 

if you open the throttle more it doesnt pick up revs, fuel mist is then kinda spraying back up out of the throte and onto my strut brace. if i can manage to get it up and cleared, it drives fine, revs fine upto 7000

 

 

so is there something farked in it or something that i missed cleaning out???

 

thanks

Aaron

 

 

 

BTW, engine is 4AC

carb is 32/36 weber

ignition is converted to electric, new plugs, cab, button, coil etc

large cam

exhaust

[kangaroosa]

Reset the float level. It could be set so the fuel level is too high and the excess fuel is ending up in the chokes.

Also make sure your fuel pressure is around 3psi - no higher than that. If its too high, it will be forcing fuel past the needle and seat and filling up your fuel bowls.

Edited June 26, 2009 by kangaroosa

[tas_ae71]

its mechanical pump, i replaced the needle and seat a while back so it shouldnt be worn. standard 32/36 should be able to hold back quite a few PSI pressure anyways.

 

it doesnt run any type of choke set-up, had been removed when i brought the carb... or do you mean something else?

Edited June 26, 2009 by tas_ae71

[kiahn]

have you attempted to adjust the idle mixture screw??

had exact same problem with my 4k, turns out the mixture screw was just screwed in all the way and the idle was turned way up.

if its not that it could be the jets are too big. what engine did the webber come off??

Edited June 26, 2009 by kiahn

[tas_ae71]

ive been using it for a year or two, problem has been gradually getting worse. jetting isnt ideal but thats not the problem i feel

 

idle screw hasnt been changed since i tuned it last.

 

idle screw and mixture screw are 2 different things btw.

[kiahn]

sorry edited: i seperated them into mixture screw and idle screw,

anyway try a retune and it might fix it, if that doesnt work then try getting the right jets for it.

 

if all else fails go through here http://www.webercarbsdirect.com/v/vspfiles...ing%20Guide.pdf

or try getting a pdf manual on how to tune the carby properly

 

 

heres what you need from the guide:

 

Edited June 26, 2009 by kiahn

[kangaroosa]

its mechanical pump, i replaced the needle and seat a while back so it shouldnt be worn. standard 32/36 should be able to hold back quite a few PSI pressure anyways.

 

it doesnt run any type of choke set-up, had been removed when i brought the carb... or do you mean something else?

 

Chokes as in the 'Venturies'

Actually the bloke that tuned my car years ago when i had a weber, said anything over 3.5psi is not good for a weber. If you have over 3.5psi, you should really install a regulator. Webers need high flow but low pressure fuel pumps. Being a std Mechanical pump, you should be fine.

 

I would still be reseting the float level.

[kiahn]

the websites i was reading bout this is made by weber and they said it is RARELY the float that causes problem but rather the tuning of the carby or the wrong jets being installed.

ive heard that webers need constant tuning/recalibrating to keep them running perfectly, if he hasnt touched it in ages then id be checking that before ripping it apart.

[Felix]

I'd be checking the float level. If you can see fuel coming out of the auxillary venturi at idle the float level is most likely to high.

 

Also check the powervalve. The powervalve is supposed to add fuel enrichment under load. If the diaphram develops a crack/pinhole you'll get full fuel enrichment under vacuum, which isn't what you want.

Edited June 26, 2009 by Felix

[Old fella]

hey guys,

 

pulled the weber off my 4ac today as it has been running like a bag of shit (and had to fix the linkage). cleaned out the jets, blew everything out, checked and cleaned out accelerator pump. noted one of the jets in the bottom of the bowl had a bit of shit in it and was half blocked.

 

put it all back together, sealed up and bolted it back on. got it running and it was still doing the same thing that it was before.

 

when you first start it, its pooring so much fuel down it, it will not run any faster then about 700rpm. over the next 3 minutes (as it starts to warm up i guess), it gradually builds up until it clears and is drivable (still very boggy at low revs). now tonight after it had been cleaned out, its shooting more fuel down it!!

 

when you look down the barrel of the carb, fuel is coming out thick and fast. also in large droplets not mist.

 

accelerator pump is working correctly, it only sprays when the throttle in increased.

 

if you open the throttle more it doesnt pick up revs, fuel mist is then kinda spraying back up out of the throte and onto my strut brace. if i can manage to get it up and cleared, it drives fine, revs fine upto 7000

 

 

so is there something farked in it or something that i missed cleaning out???

 

thanks

Aaron

 

 

 

BTW, engine is 4AC

carb is 32/36 weber

ignition is converted to electric, new plugs, cab, button, coil etc

large cam

exhaust

 

 

G`day mate, I would personally be looking at the needle and seat before any adjustment is made. You say that the carb had a chunk of poop in one of the jets, so that means that it had to have entered past the float needle. If it is a neoprene tipped needle it may have been scratched or split. If the needle is just metal, check it for signs of wear and replaceas neccessary.

There is always a chance that there is some warpage to the top section of the carb too. When you have the top off to look at the float needle, check the top with a straight edge. (steel ruler or similar) If it is warped slightly, come back to me and I will explain how to fix it.

Good luck.

[kangaroosa]

the websites i was reading bout this is made by weber and they said it is RARELY the float that causes problem but rather the tuning of the carby or the wrong jets being installed.

ive heard that webers need constant tuning/recalibrating to keep them running perfectly, if he hasnt touched it in ages then id be checking that before ripping it apart.

 

Don't believe everything you read on the internet. For fuel to get into the Venturis at the rate that was described, the fuel level in the float has to be waaay high. I went through this exact issue not 2 weeks when tuning my rotary to run on E85 Fuel.

If your weber is setup and tuned 'properly' once, then it should be fine for an extended period of time. I mean jetting, and (balancing if you have twins), and timing etc.

He hasnt mentioned changing jets, or any recent engine mods. So that rules out wrong jetting.

Most of the time you will find that a manifold leak or fouled plugs will cause problems which most people convince themselves is all because of the weber carby.

Edited June 26, 2009 by kangaroosa

[kiahn]

ok, i was just going off what i read in all the links on the first google results page.

well the link i gave is the weber troubleshooting guide,

the reason i was pushing for a retune was that checking the float level ruins a gasket.

anyway goodluck

[tas_ae71]

kiahn, typically i believe that its more sidedrafts that need to be fine tuned every 6month or so.

 

old fella, needle and seat are full brass type, i replaced it prob 18months ago.

 

kangaroo, how exactly do you check the float level the proper way? the level in the bowl (without the floats in there) doesnt look abnormally high. turning the top housing of the carb upside down, the top of the floats isnt even as high as the over flow hole/passage.

 

what part are the venturis??

 

ive had the only man out there mucking around with it with me all morning. its still bogging down at low revs. over 2500revs it pick up again and seems to run as it should.

 

 

I'd be checking the float level. If you can see fuel coming out of the auxillary venturi at idle the float level is most likely to high.

 

Also check the powervalve. The powervalve is supposed to add fuel enrichment under load. If the diaphram develops a crack/pinhole you'll get full fuel enrichment under vacuum, which isn't what you want.

 

what is the "power valve"?? the above the floats with the stick and big spring on it??

Edited June 27, 2009 by tas_ae71

[kangaroosa]

kangaroo, how exactly do you check the float level? the level in the bowl (without the floats in there) doesnt look abnormally high. turning the top housing of the carb upside down, the top of the floats isnt even as high as the over flow hole/passage.

 

They need to be set to whatever the manufacturer states.

Heres a link to some diagrams. Floats

[Felix]

what part are the venturis??

 

Have a look in this PDF file.. Part no. 83 is the auxillary venturi's. Fuel should only be coming out of the center of it when the carb is on the main circuit, NOT at idle.

 

what is the "power valve"?? the above the floats with the stick and big spring on it??

 

Yep that is the power valve diaphram (part 14). The power valve (part 21) itself is actually in the bottom of the float chamber, and the "stick" (of the diaphram) as you called it pushes on the power valve to control fuel enrichment under load. At idle under high vacuum the diaphram should pull up closing the powervalve. If the diaphram is rooted you will have full fuel enrichment at idle.

 

With the top of the carb off you should be able to press in the power valve diaphram, and with a finger block the vacuum port for it, and it should stay in until you remove your finger.