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Weber 32/36 running rich at low rpm


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Hi folks.

I have an Exmo Seven with the usual 2 ltr Pinto engine. The car came with 48 IDF carbs and - as I had to find out - a racing cam shaft with which no weber carb would run decently. So I downgraded both the cam shaft and the carb to a more standard Weber DGV 32/36 (in DE I need to get any modification from standard approved by German MoT and the 48 IDFs were not in my papers). 

Since finding somone in Germany that still knows how to tune a carb is really hard, I went at it myself. After 6 months of trial & error using my Lambda dial, I have it reasonably in tune. The mixture now stays within AFR 13 - 15 at 95% of the time, at least above 2,500 rpm. Idle mixture is fine too.

When I push down the accelerator, the mixture still takes a dip of about 1-2 AFR values for a few seconds before it recovers back to AFR 14, independent to my rpm. That means the pump value is too large. But I am already at a size 30 and there is no smaller size available. So I live with that.

However, when I just cruise along in town or on country roads, I tend to shift up and have the engine run at about 2,000 rpm. And here is my problem:  when I get below 2,200 rpm the engine starts to run excessively rich (AFR 11-12). Even when holding speed, the mixture never recovers and stays at AFR 11-12 all the time.

I tried a smaller idle valve and I also played with adjusting the idle position of the two throttles (my stage 2 runs a 40 points leaner main valve than stage 1). A smaller main valve would sure fix that issue, but then the engine runs way too lean at 2,500 rpm and above. 

Does anyone have an idea what adjustment could fix just that the low rpm issue? I am open for any suggestions.

Thanks
Claus

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I don't have this carb, I have DFTH 30/34, however in order to recondition it I got hold of a Haynes manual for weber carbs, and chapter 7 covers DGV 32/36. Looking at it there is a description of what happens from cold start all the way through to hot on full throttle. In the bit about normal running it mentions air being drawn through the "air corrector jets",  perhaps this is a factor. In the section on fixing problems, there is a mention under execessive fuel consumption of an incorrect level in the float chamber. Hope this might help.

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3 hours ago, Sparepart said:

I don't have this carb, I have DFTH 30/34, however in order to recondition it I got hold of a Haynes manual for weber carbs, and chapter 7 covers DGV 32/36. Looking at it there is a description of what happens from cold start all the way through to hot on full throttle. In the bit about normal running it mentions air being drawn through the "air corrector jets",  perhaps this is a factor. In the section on fixing problems, there is a mention under execessive fuel consumption of an incorrect level in the float chamber. Hope this might help.

Many thanks for your suggestions. But the air corrector jets are in theory responsible for the mixture of the higher rpm (3,500-4000 until rpm limit). My issue is on the exact reverse end.

But the float level might actually be a valid factor. I set up the floater to what it is supposed to be (36-38mm in closed position). But when opening the lid, I reallize that there is not more than 15-20mm of fluel in the float chamber. That is less than half full. This seems quite low to my opinion. I might actually set the float level a little higher and then recalibrate the values to that level. Maybe that fixes something. 

Assume I have a level of 15mm when full. When the engine is running and consumes fuel, the level drops by 2mm. That changes the float level (which influences the mixture) by 2/15 or 13.3%. When I increase the full level by 10mm to 25mm (which is probably about the float chamber being half full), that same 2mm drop is 2/25 or 8%. That's a lot more steady. Or do I make a mistake here ?

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The maths of your percentages seems impecable. I'd have thought though that the level in the chamber is the important thing not the volume. You say that you have adjusted the float to what it is supposed to be, I wonder what technical manual you obtained the adjustment details from. Below I am including a table from the manual I have which shows two adjustments, level and stroke, perhaps you could check your carb against this. Also could a small leak in the float reduce it's boyancy.

Weber-Float-Adjustment.jpg.252e591694c8376220087a26963a39b4.jpg

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The measurement I quoted was the closed position. I set that position to 38mm out of memory. Your table above says 39mm for DGV. So I was only 1mm off. The stroke is just the limiter to prevent that the valve cylinder falls out.
We seem to agree that the level (height) of the fuel is important. I just checked the level again: after my Hardy pump reached its max pressure and stopped, I opened the carb and used the folding rule. There was 22mm of fuel in the revervoir, its total height is 45mm. That makes it almost exactly 50% full which seems odd to me. In lack of any better idea what else could fix my low rpm issue, I think about adjusting the closed position to something like 34mm. That gives me 5mm more fuel level. Maybe that changes something. If not, I can still switch back.

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Upate: I tried to change the float level, first decreasing the floater to 34mm (= 5mm more fuel than standard), than by going to 40mm (=1mm less than standard). The effect was nothing, at least not what I intended to get. 

I then got the idea to check the timing. It was off (early) by about 10 degrees. :-( So much for trusting my garage that they checked everything...  I tried to set it back to the 10 degrees before OT that I found in my book. But my timining gun broke before I could finish, so I could not verify my final adjustment. I took a very short turn around the block and found that the mixture chances with timing. From the few miles I drove, I believe it gets more variable, but my low rpm issue seems less.

I ordered a new gun and will check the timing before I drive again. But it seems  I could have found the leverage which fixes my issue.

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Good news if it's a matter of getting the timing spot on. I just happened to read a small article in PC magazine in the problems section someone was trying to cure spark plugs getting carboned up (I'm assuming poor combustion and emissions would come with this) .. anyway among the items that the expert suggested to look at was the timing as one might have guessed, however he went further and also suggested checking the timing advance through the rev range not just idling. For example you might have vacuum advance on the distributor and a leak in the vacuum pipe from the inlet manifold etc. Also another suggestion was that the fuel pump might be providing fuel at too high a pressure for the carb. When idling the float is high and able to push the inlet valve enough to stop flooding, as the revs rise the float drops but the high pressure rush as the valve opens is enough to cause some flooding, then at higher revs the engine is taking enough petrol to prevent flooding.  You might not need all this but I thought I'd mention it in case someone else looks at this thread later.

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I just got my new timing gun today and over the weekend I will need to verify that the timing is really where it should be. But assuming that I already got a decently good spot (the engine sounds healthier already), I drove a little more last sunday and found my first impression to be true. The mixture jumps alittle more up and down at speeding up or slowing down and subjectively the car runs a little richer overall. But the particularly rich mixture issue at low rpm is a good share better than it was before.

But the word "vacuum advance" rings another bell: as of now, I do not have a vacuum pipe attached to my ignition. I have an ignition working with gravity to set the advance at rising rpm and I can clearly see the ignition timing going to advance when I hit the accelerator while I use my timing gun. When I bought the car, no vacuum pipe was attached and I never bothered to attach one since.

What does the vacuum do? The net says, the engine runs leaner at low rpm or at idle and for that timing advance it advantageous. But I have the opposite issue. I have my idle set to AFR 13 and I also still run rich on low rpm. Vacuum increases when I take the foot off the pedal and the throttle closes. Isn't that contra productive in my case?  I just set the timing to later to fix my issue. Should I attach a vacuum pipe anyway?

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Vacuum advance is mostly an economy system.

At full power it does nothing as there is no vacuum in the inlet manifold with the throttle fully open.

As you close the throttle inlet vacuum increases and the fuel/air charge entering the cylinder reduces. This reduces the pressure in the cylinder at the end of the compression stroke. The flame front of the burning fuel/air moves faster with high pressure and slower with low pressure so ignition advance curves attempt to get full burn happening at or soon after TDC to extract maximum power. If full burn comes later then engine efficiency falls.

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On 7/18/2024 at 5:44 PM, IanS said:

Vacuum advance is mostly an economy system.

At full power it does nothing as there is no vacuum in the inlet manifold with the throttle fully open.

As you close the throttle inlet vacuum increases and the fuel/air charge entering the cylinder reduces. This reduces the pressure in the cylinder at the end of the compression stroke. The flame front of the burning fuel/air moves faster with high pressure and slower with low pressure so ignition advance curves attempt to get full burn happening at or soon after TDC to extract maximum power. If full burn comes later then engine efficiency falls.

Thanks for the explanation. I attached the VA to see how the engine reacts after verifying my static timing to be at 10 degrees. And yes, the engine does run better, although it does not decrease my rich mixture issue. The vacuum advance causes the timing to forward to something around 20 degrees at idle. Is that an acceptable value?

Above 12 degrees OT I can only guess the values as my crankshaft belt wheel only has 3 notches of 4 degrees each. And there is pactically no space between the front of the engine and the radiator fan which makes it impossible to add notches or get in there with a marker. In the web I found a picture with a scale at the camshaft wheel. At least, that is a lot easier to access. But since the camshaft turns at only half the speed of the crankshaft, it takes 2 scales (180 degrees apart) and a 2 degrees change on the crankshaft means only one degree on the camshaft wheel. I plan to try that option and see if it works.

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