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> Weber 44 idf flat spot
tornik550
post Jul 4 2013, 11:59 AM
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So I recently purchased new weber 44 idf's. They didn't work out well so I went back to an old set of weber 44 idf's that I had. I thoroughly cleaned the carbs, sprayed carb cleaner through all jets, tubes, etc. I have also balanced the linkage. Set the floats, etc.

The car starts well. Idle's well. When I drive the car I get a major flat spot between 1500 and 3000 rpm's. Below and above the car accelerates normally.

What changes should I make?

My current setup for my 2258 (dual webers, big cam)
-idle 55
-main 160
-et F11
-ac 200
-vents 36

Are there other issues that can cause this type of problem (timing,etc?)
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ThePaintedMan
post Jul 13 2013, 02:01 PM
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QUOTE(tornik550 @ Jul 12 2013, 09:46 PM) *


I thought I read somewhere that smaller vents give you more low end torque?


Potentially, and with a possible loss in high RPM power. It all depends on what the intention for the engine/car is. If it's to spend most of it's time at high RPMs, larger vents (within the appropriate range for the engine displacement and breathing ability) are the way to go. Read Tomlinson's Weber book. He has a copy of the Weber venturi selection graph that clarifies this. For a street car, smaller vents are preferable to increase velocity flowing past the carb circuits, which helps draw the appropriate amount of fuel from them.

In theory, every form of the Type IV below about 2056 (especially with Jake/Len's high flowing heads) intended for the street should be running 28 mm venturis.
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ChrisFoley
post Jul 13 2013, 02:46 PM
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QUOTE(ThePaintedMan @ Jul 13 2013, 03:01 PM) *

In theory, every form of the Type IV below about 2056 (especially with Jake/Len's high flowing heads) intended for the street should be running 28 mm venturis.

That is absolutely false.
With wideband air/fuel measurement one can quickly see the effect of incremental changes to any aspect of carb adjustment.
The 1.7L engine I've been tuning for the past week has 28mm venturis.
Based on the data I believe the engine will run slightly better with 30mm vents.

The main venturis exist only to enhance the pressure differerential between the intake throats and the float bowl in order to pull fuel into the airstream.
The largest venturis which provide sufficient pressure differential as the carbs move from progression onto the main circuit are best.

Outside that transitional range the venturis are essentially unnecessary.
Below - no fuel is metered thru the main circuit.
Above - the secondary venturis can provide sufficient pressure differential by themselves.
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ThePaintedMan
post Jul 13 2013, 05:38 PM
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QUOTE(Racer Chris @ Jul 13 2013, 04:46 PM) *


That is absolutely false.
With wideband air/fuel measurement one can quickly see the effect of incremental changes to any aspect of carb adjustment.
The 1.7L engine I've been tuning for the past week has 28mm venturis.
Based on the data I believe the engine will run slightly better with 30mm vents.



I, of course, have a LOT to learn and I will always defer to guys like Chris and Rich. A wideband really is the way to know for sure what the best setup is, absolutely.

But Chris, weren't you suspecting that the engine you have in that customer's car isn't a bone stock 1.7 tho? I thought I remembered that there was a different cam in it or something?

Maybe I should have said... 28's are a good place to start with an engine under 2056? If one has access to a wideband, 30s or maybe even 32s might be possibilities?
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Posts in this topic
tornik550   Weber 44 idf flat spot   Jul 4 2013, 11:59 AM
ThePaintedMan   So I recently purchased new weber 44 idf's. ...   Jul 4 2013, 01:36 PM
Elliot Cannon   So I recently purchased new weber 44 idf's. ...   Jul 4 2013, 01:36 PM
r_towle   f7 or turbo emulsion tubes help with flat spots, a...   Jul 4 2013, 02:02 PM
brant   28 vents. Perhaps 40 mm carbs   Jul 4 2013, 02:24 PM
Starlack   Hello This is the transition area from idle jet t...   Jul 4 2013, 02:41 PM
tornik550   The interesting thing is that my car ran great in ...   Jul 4 2013, 02:57 PM
tornik550   Thinking about this situation, I think the suggest...   Jul 4 2013, 03:12 PM
McMark   The carbs weren't really flowing fuel when the...   Jul 4 2013, 03:16 PM
tornik550   Ok, I am getting a bit pissed off at weber now. ...   Jul 4 2013, 03:45 PM
Racer Chris   So for the record- does the longer side of the se...   Jul 5 2013, 06:55 AM
sean_v8_914   good advice above smaller vents smaller main jets   Jul 4 2013, 04:05 PM
sean_v8_914   long side of taper down. spray hole down   Jul 4 2013, 04:06 PM
sean_v8_914   http://www.carburetion.com/diags/40IDFDiaginfo.asp   Jul 4 2013, 04:07 PM
sean_v8_914   a wide band O2 would be nice for selecting the tru...   Jul 4 2013, 04:09 PM
tornik550   a wide band O2 would be nice for selecting the tr...   Jul 4 2013, 04:17 PM
Mblizzard   On my weber 44s, 28 vents made all the difference....   Jul 4 2013, 04:25 PM
Woody   I run 36 vents and the car runs great. I have a se...   Jul 4 2013, 08:53 PM
McMark   If you take the vent out you'll see one side i...   Jul 4 2013, 11:54 PM
tornik550   I have worked out my flat spot. I found that the ...   Jul 12 2013, 08:15 AM
Racer Chris   I have worked out my flat spot. I found that the...   Jul 12 2013, 08:28 AM
tornik550   I have worked out my flat spot. I found that th...   Jul 12 2013, 08:31 AM
Jake Raby   The only issue that I am having on the wideband...   Jul 12 2013, 06:31 PM
tornik550   The only issue that I am having on the wideban...   Jul 12 2013, 07:46 PM
Racer Chris   If you back off the nuts on the accel pump rods an...   Jul 12 2013, 11:24 AM
ThePaintedMan   I thought I read somewhere that smaller vents gi...   Jul 13 2013, 02:01 PM
Racer Chris   In theory, every form of the Type IV below about ...   Jul 13 2013, 02:46 PM
ThePaintedMan   That is absolutely false. With wideband air/fuel...   Jul 13 2013, 05:38 PM
Racer Chris   But Chris, weren't you suspecting that the en...   Jul 14 2013, 10:01 AM
r_towle   That motor in the red/ white race car had basicall...   Jul 13 2013, 05:06 PM
tornik550   Chris- I followed your instructions on tuning my c...   Jul 14 2013, 07:46 PM
Racer Chris   The true exhaust temp is a lot higher than that. ...   Jul 14 2013, 10:29 PM


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