Weber gurus, Help with flat spot
07-16-2002, 05:53 PM
#1
Rennlist Member
Thread Starter
Weber gurus, Help with flat spot
Take a look at these graphs. Air/Fuel mixture is at the bottom, hp is on the top.
<a href="http://www.printroom.com/_vti_bin/ViewImage.dll?userid=valvefloat&album_id=55025&image_id=9&courtesy=1" target="_blank">Graph1</a>
<a href="http://www.printroom.com/_vti_bin/ViewImage.dll?userid=valvefloat&album_id=55025&image_id=10&courtesy=1" target="_blank">Graph2</a>
This is a friends car: 2.0L E heads, S pistons, S cams reground to unknown specs, RSR type headers, stock dual in muffler.
I have had simular problems with my L: Stock L engine, RSR type headers with individual turbo mufflers.
My question is what can be done with carb set up to minimize the richness in the midrange? Is this not a carb problem, is it something else?
Thanks for any insight
Joe
<a href="http://www.printroom.com/_vti_bin/ViewImage.dll?userid=valvefloat&album_id=55025&image_id=9&courtesy=1" target="_blank">Graph1</a>
<a href="http://www.printroom.com/_vti_bin/ViewImage.dll?userid=valvefloat&album_id=55025&image_id=10&courtesy=1" target="_blank">Graph2</a>
This is a friends car: 2.0L E heads, S pistons, S cams reground to unknown specs, RSR type headers, stock dual in muffler.
I have had simular problems with my L: Stock L engine, RSR type headers with individual turbo mufflers.
My question is what can be done with carb set up to minimize the richness in the midrange? Is this not a carb problem, is it something else?
Thanks for any insight
Joe
07-16-2002, 08:50 PM
#2
Racer
Join Date: Jul 2001
Location: Torrance ,Ca
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VF,
That's a tough one. My first thought after looking at the charts is that it's most likely a float level or excessive pressure problem. You should have a fuel pressure gauge and regulator set between 3.5 t0 4 lbs, no more. I tried using a vaccuum assisted pressure regulator, but of course at low rpm it was rich and high rpm lean due to the low manifold vaccuum pressure.
Next I would make sure your carbs are jetted close to the factory specs, I tried using the rule of thumb in Bruce's book ( air correction 60 more than mains) but found better results with the Weber original jetting.
I would also look at the amount of fuel sprayed from the accelerator pump, I believe it should be .5 cc's per single stroke. Make sure you have the correct pump exhaust valve (inside the fuel bowl) which in the 40 IDA 3c is closed, in the 46IDA 3c is a 45 or 50. This is a check valve that allows fuel to flow in one direction when the accelerator pump is actuated.
What jets are you using for main and air correction ? It could also indicate the mains are too large, above 4000 rpm the mains and air correction are responsible for the fuel ratio. If you go with just a smaller main then you will see a lean condition above 4,000, so try a proportionately smaller air correction as well, therefore if you drop the main from 160 to 150 drop the air correction the same amount, based on your graph.
Good luck !
That's a tough one. My first thought after looking at the charts is that it's most likely a float level or excessive pressure problem. You should have a fuel pressure gauge and regulator set between 3.5 t0 4 lbs, no more. I tried using a vaccuum assisted pressure regulator, but of course at low rpm it was rich and high rpm lean due to the low manifold vaccuum pressure.
Next I would make sure your carbs are jetted close to the factory specs, I tried using the rule of thumb in Bruce's book ( air correction 60 more than mains) but found better results with the Weber original jetting.
I would also look at the amount of fuel sprayed from the accelerator pump, I believe it should be .5 cc's per single stroke. Make sure you have the correct pump exhaust valve (inside the fuel bowl) which in the 40 IDA 3c is closed, in the 46IDA 3c is a 45 or 50. This is a check valve that allows fuel to flow in one direction when the accelerator pump is actuated.
What jets are you using for main and air correction ? It could also indicate the mains are too large, above 4000 rpm the mains and air correction are responsible for the fuel ratio. If you go with just a smaller main then you will see a lean condition above 4,000, so try a proportionately smaller air correction as well, therefore if you drop the main from 160 to 150 drop the air correction the same amount, based on your graph.
Good luck !
07-16-2002, 09:28 PM
#3
Rennlist Member
Thread Starter
The main jet we ended up with was a 126, we are at about 5000' elevation. When we went with a smaller main it was getting too lean after 5000 rpm when the air-fuel curve comes back on the chart. I have not experimented with changing the air correction jet, I'll have to get some and try that.
Thank you,
Joe
Thank you,
Joe
