Slightly obsessive Weber 32/36 tuning

[rcx132]

Finally getting round to tuning the Weber carb on my Suzuki SJ413. The original Aisin carbs are too complex to maintain and parts unavailable, so I bought a supposedly pre-jetted 32/36 direct from Weber. Well I'm glad I checked the jetting as it was miles off, not sure how Weber could ever sell that as ready to go.

I used a 14Point7 Wideband controller as it's cheaper than the big brands and has decent features like 0-5v data logger output and narrowband simulator. The readings jump around way too much to read while driving so I made a simple data logger from an Arduino (code here). I attached a handheld push button that starts and stops logging when the engine is in the test condition for whatever jet I'm working on, it then shows the average.

I disabled the secondary barrel and blanked off the accelerator pump to make it easier to test one jet at a time.

I've dialled in the idle circuit, main jet and air jet in the primary barrel, and main jet on the secondary barrel. But the secondary air jet (WOT at high RPM) is mega rich (AFR 11) no matter how much I change the jet size. I've read that I need to try a different emulsion tube, so will get that ordered.

 

[8ob]

That's a big carb for a small engine, I think you will struggle to find the sweet spot. Carbs were once described to me as an organised fuel leak, other than at tick over or wot they are virtually impossible get 100%.

Bob

 

[minimutly]

Emulsion tubes and air jets - do much more than given credit for, and ultimately tweak that leak to acceptable limits.
I like your logger...

 

[dave h 2]

I put a 28/36 Weber on a 2L Opel, it tweaked in fine a 1300 may be a step to small without a turbo. The AFR was set with a Gunson see through plug
Can you still change the venturis?

 

[rcx132]

That's a big carb for a small engine, I think you will struggle to find the sweet spot. Carbs were once described to me as an organised fuel leak, other than at tick over or wot they are virtually impossible get 100%.

Bob

I put a 28/36 Weber on a 2L Opel, it tweaked in fine a 1300 may be a step to small without a turbo. The AFR was set with a Gunson see through plug
Can you still change the venturis?

Ok, I hope I havent wasted my money. It's the carb Weber sell specifically for this car. Previously they used to supply a 32/32.

I've ordered the new emulsion tube, it's supposedly leaner at the top end. I'll also compare the carb size to the original Aisin.

 

[8ob]

Ok, I hope I havent wasted my money. It's the carb Weber sell specifically for this car. Previously they used to supply a 32/32.

I've ordered the new emulsion tube, it's supposedly leaner at the top end. I'll also compare the carb size to the original Aisin.

You can achieve stoichiometric each end of the spectrum but I doubt very much you will get there across the whole throttle range. It would take a lot of dyno time.

Edit. Many moons ago I had a twin cam escort, big cams,ported, high compression pistons and all the associated sillyness. General consensus was it need 45dcoe carbs, fitted them and it ran like a pig, even the lotus garage couldn’t get them sorted. Took the car to a weber guru in Hartley Witney, long story short it ended up with the 40's back on there with 33mm chokes and it went like stink. Big carbs can make the same power as fuel injection at WOT and only there, for drivability smaller can be better.

Bob

 

[Morris]

Absolutely @8ob people tend to look at peak power and forget that if it's a road car low speed torque and mid range are what's needed.

 

[rcx132]

I believe what you're saying. I just feel gutted because I trusted Weber, bought a carb specific for my car with adaptor and they told me it was jetted right. A bit more Googling has now disclosed that everyone has found the jetting to be wrong.

The primary barrel is now well tuned and performing well right through the range. The second barrel is causing me issues. It kicks in at 3/4 throttle, the main jet works well in medium gears but goes off in very low or very high gear and completely looses it at high RPM.

I have a 32/32 Weber I picked up used so I'll try that as well.

I checked the original carb and the ports are way smaller, I didn't measure but they look more like 20 and 25.

 

[dave h 2]

OK, if this is one from Classic carbs do the jets supplied match those on their web site?
If they do it's back to basics.
The carb comes with 26 / 27 venturis fitted
The fuel pressure needs to be 3.5 psi, if the pump delivers 5 psi you have to have the fuel return pipe fitted with the tee close to the carb and not blocked otherwise it will run rich.
Is the float set right? Lowering the float height will lean the whole carb.

 

[minimutly]

I believe what you're saying. I just feel gutted because I trusted Weber, bought a carb specific for my car with adaptor and they told me it was jetted right. A bit more Googling has now disclosed that everyone has found the jetting to be wrong.

The primary barrel is now well tuned and performing well right through the range. The second barrel is causing me issues. It kicks in at 3/4 throttle, the main jet works well in medium gears but goes off in very low or very high gear and completely looses it at high RPM.

I have a 32/32 Weber I picked up used so I'll try that as well.

I checked the original carb and the ports are way smaller, I didn't measure but they look more like 20 and 25.

Carb sizes are throat diameters (where the butterfly sits), the choke sizes will be much smaller, and if they are too small the engine will fall flat on its face at high rpm...
28/36 will be miles better than a 32 32 dgas, dgav or whatever.

 

[minimutly]

You can achieve stoichiometric each end of the spectrum but I doubt very much you will get there across the whole throttle range. It would take a lot of dyno time.

Edit. Many moons ago I had a twin cam escort, big cams,ported, high compression pistons and all the associated sillyness. General consensus was it need 45dcoe carbs, fitted them and it ran like a pig, even the lotus garage couldn’t get them sorted. Took the car to a weber guru in Hartley Witney, long story short it ended up with the 40's back on there with 33mm chokes and it went like stink. Big carbs can make the same power as fuel injection at WOT and only there, for drivability smaller can be better.

Bob

40 dcoe's - good for 130 bhp in my experience, any more you need 45s. Webers were never that good for a road car, think it was Vauxhall who tried to make them work on the Chevete hsr, failed completely, put some Dellortos on, dialled right in.

 

[8ob]

40 dcoe's - good for 130 bhp in my experience, any more you need 45s. Webers were never that good for a road car, think it was Vauxhall who tried to make them work on the Chevete hsr, failed completely, put some Dellortos on, dialled right in.

One of the biggest problems with over carbing is the lack of velocity in the charge to keep fuel suspended/atomised. A mate of mine built up a mildly tuned 302 ford for his Cobra, all the advice given was to go with a 650 cfm four barrel double pumper, only ever ran properly when the link bar was removed on one set of butterflies, in effect turning it into a two barrel carb.

Bob

 

[Morris]

40 dcoe's - good for 130 bhp in my experience, any more you need 45s. Webers were never that good for a road car, think it was Vauxhall who tried to make them work on the Chevete hsr, failed completely, put some Dellortos on, dialled right in.

I ran twin 40dcoe's on a 1630cc crossflow years ago, ran great. I had it set up on Aldon's rolling road, approx 135 at the flywheel.

 

[Morris]

One of the biggest problems with over carbing is the lack of velocity in the charge to keep fuel suspended/atomised. A mate of mine built up a mildly tuned 302 ford for his Cobra, all the advice given was to go with a 650 cfm four barrel double pumper, only ever ran properly when the link bar was removed on one set of butterflies, in effect turning it into a two barrel carb.

Bob

The very definition of overkill.

 

[Turbo]

40 dcoe's - good for 130 bhp in my experience, any more you need 45s. Webers were never that good for a road car, think it was Vauxhall who tried to make them work on the Chevete hsr, failed completely, put some Dellortos on, dialled right in.

Were dellortos not just a copy of a weber?

 

[Ross365]

Going back ~50 years, I seem to recall the popular choice of Weber for 1300's of all kinds was the 28/36, esp on Escorts. Did you search the web for reports of others fitting Webers to that vehicle? Working with carbs is a black art, benefiting from the involvement of skilled practitioners!

If you look at the Weber literature, I think you'll find that for any specific model, the throat size (i.e the diameter of the outlet at the base of the carb, so 32/36mm in your case) is fixed but there may be a range of different sized venturis on offer. The venturi size is usually indicated by a number stamped on the body of the carb like 25/250 (it's not a dimension, just a "type number" and the venturis could be something like 25/26mm, appreciably smaller than the throat ) and it would be good to check that you have the correct venturi size for you car.

 

[ian f]

i have a 32/36 dgv on my cortina, always had a flat spot around 2000 rpm (at a guess, no rev counter) best thing i did was make a "air horn" as described in david vizards pinto book. smoothed it right out and been perfect for over 200k miles since i fitted it (on it's second engine but i think original carb unless i swapped it at some point in the 90's) i don't use full throttle nowadays but still feels strong

 

[500e]

DellOrto have better atomisation than Weber.
A better choice for Roots group cars

 

[Jack_K]

Is the ignition perfect?

Improper combustion will cause the O2 sensor to give misleading results.

 

[Melvyn Best]

Is the ignition perfect?

Improper combustion will cause the O2 sensor to give misleading results.

As will tight valve clearances