Weber 44 idf flat spot Options

[tornik550]

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?)

[ThePaintedMan]

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.

[ChrisFoley]

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.

[ThePaintedMan]

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?

[ChrisFoley]

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?

The engine does have a non-stock camshaft. However no other performance enhancements were applied during the rebuild, not even an appropriate increase in compression ratio.
Any carbed type 4 should have a non-stock camshaft if improved performance is expected.