Weber 44 idf flat spot Options

[ChrisFoley]

I have worked out my flat spot. I found that the accelerator pumps on one carb were partially plugged.

I have ordered several sets of venturis in different sizes. Although my engine is running very well right now, would there be any reason for me to change to a different size venturi? I have weber 44 idf's with 36mm vents. I get decent low end torque and excellent high end power. I ordered 34 and 32 vents.

I would leave it alone unless you have data from a wideband O2 which suggests otherwise.

[tornik550]

I have worked out my flat spot. I found that the accelerator pumps on one carb were partially plugged.

I have ordered several sets of venturis in different sizes. Although my engine is running very well right now, would there be any reason for me to change to a different size venturi? I have weber 44 idf's with 36mm vents. I get decent low end torque and excellent high end power. I ordered 34 and 32 vents.

I would leave it alone unless you have data from a wideband O2 which suggests otherwise.

The only issue that I am having on the wideband is that I am very rich at idle, everywhere else is perfect. If I go down on idle jet size, then idle improves but screws up everything else.

[ChrisFoley]

If you back off the nuts on the accel pump rods and completely remove the main jet assemblies you can drive around to see what the mixture is on the low speed progression circuit without any other influences.
The engine probably won't run over 3000rpm, maybe even lower.
Size the idle jets to give you 16-17:1 driving around at very light throttle.
Then adjust the idle A/F with the mixture screws.
You should be able to get around 14:1 at 850-900 rpm.

Reinstall the jet stacks and size the Main jets for 12.8:1 at wide open throttle above 4K rpm.
Then increase the air corrector jet size to get rid of leanness in the midrange.
It may be necessary to increase the mains one more size, depending on how big you had to go with the A/C jets.
The last step is to screw the accel rod nuts back in to eliminate any lean hesitation when you step on the throttle.

If you can't get rid of the lean transition switch to the next smaller main venturi size.

[Jake Raby]

The only issue that I am having on the wideband is that I am very rich at idle, everywhere else is perfect. If I go down on idle jet size, then idle improves but screws up everything else.

Then adjust the mixture control circuits.

[tornik550]

The only issue that I am having on the wideband is that I am very rich at idle, everywhere else is perfect. If I go down on idle jet size, then idle improves but screws up everything else.

Then adjust the mixture control circuits.

I figured that. I just haven't got around to doing that yet.

My engine is running very well. The only reason that I asked about changing vent size was cause I have them and I can change if needed but if there is no reason, then I will just leave everything put.

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

[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.

[r_towle]

That motor in the red/ white race car had basically no ventures, and it had no main jets.

Thing was fast once you got it moving

[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.

[tornik550]

Chris-
I followed your instructions on tuning my carbs. The car is running great. CHT was max around 300- both banks. My oil temps got up to 220 with heavy driving.

I do have a AEM wideband. I am getting air fuel numbers in the ranges you mentioned in your post. Since I only have 1 wideband for all cylinders and I do not have egt gauges on each header- I used an IR thermometer at the 1st curve of the headers (triad headers- sorry Chris). I adjusted the mixtures until I got almost equal temps on each header and still was within the normal ranges on the wideband. To me- the temps seemed a bit high. After the car was idling for about 15 minutes- the IR thermo was reading about 575 at the curve. Does that seem right? I am probably looking too much into things since everything else seems fine.

Thank you for your help!

[ChrisFoley]

The true exhaust temp is a lot higher than that.
Reading the pipe temps with an IR thermometer is a good way to check for all 4 cyls working equally but won't give any data on A/F mixture.
CHT could be between 250 to 400 possibly, depending on load and rpm.
High load combined with low rpm will generate the highest cyl head temps.
Oil temp peaking at 220 under hard driving in hot weather sounds perfect to me.