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Weber DGV 32/36 on an A13 motor tuning questions.


pdp8

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I got the car with the Weber and had to swap to a Hitachi, well I've moved and the Hitachi got balky so I dug out the Weber.

It really makes more low/mid power which the poor little motor needs so that's a win.

Idle is good, power is fine, low to mid cruising is lovely.

The issue is that if I tip the throttle my A/F plummets from 14-15 down to about 12:1 and pretty much stays there.

I'm having a really hard time deciding if it's a power valve issue since it's a big carb for that little engine or if I'm opening the secondaries and they are well to the rich side.  There is only the slightest difference in throttle position and I sometimes see the ration change with no throttle movement so I'm suspecting power valve.

Secondarily, what's a good A/F for power without just turning fuel into smoke on a tiny N/A motor like this?

Thoughts anyone?

I'd rather buy the likely parts before cracking the carb since I don't live anywhere near a parts shop.

 

Thanks all,

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When I put the new 32/36 on our newly rebuilt 1200cc E1, it dumped unburned gas out the backside and tended to foul plugs. A call to Pierce Manifolds resulted in acquiring primary and secondary jets from them that were two sizes down from the off-the-shelf jets that were in the new carb. It's working like a champ. I'd kinda assume that  the same jet change would probably work on your 1300. I'd have to look back at my paperwork to find the exact size change but I'd suggest a call to PM next week and give them your data. Really good helpful people.

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Thanks for the input.

 

Well, based on that recommendation I've changed the secondary from a .0535" down to .0465".  I left the primary alone because it's running pretty right-on in that range.  Test drive tomorrow and we'll see.

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  • 3 weeks later...

Based on idle and lower-speed cruise I'm convinced my primary jet size is good.

I reduced the size of the secondary by about 20%(by area) and now my higher speed cruise values look good as well.

That helped me see my other problem, if I tip the throttle it still just dumps fuel.  I'm talking ratios as low as 11:1 here at the slightest grade or acceleration.

So, I need to both decrease the sensitivity of the power valve and also reduce the flow.

I know there are theoretical A/F ratios for ideal power but I also know that the type of motor matters a fair bit, what have people found with the A13 motor to be a good power-making ratio that stops short of just converting fuel into smoke?

 

thanks,

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You can make adjustments to the power valve by shortening the rod that protrudes into the float bowl. I hit them on a belt sander and knock them completely off. You can also get different pump nozzles or "squirters" to lessen the flow of fuel.

 

Do you know if the carb is a 5A?

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I've lost my notes temporarily.  I think the jets were #140 though there is a tiny chance they were #145.  I measured them and the one on the secondary was a bit smaller but I had looked up the date and they weren't individually gauging them at that time so I'm not too surprised. 

 

OK, I'll try shortening the rod a bit and see how I come out.  Is there a length to pressure ratio I should know?

I'll have to look at the carb for the sub-model but it's progressive mechanical secondary, single-pumper, electric-choke.

 

Is it really the case that the accelerator pump and power valve share a nozzle so to change I wind up changing  the other as well?

Unfortunate but I can cope.  I'll shoot for "best lean torque" in the mid 13s then.

 

What about primary/secondary cruise?  I'm at low 15s now and I thought that was pretty thin but I did some looking and I gather newer cars are running mid 16s.  I'd worry about exhaust valves but I guess you aren't really making any heat or power at those loads so it's ok?

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Trial and error is going to net you the best results. Modifying a DGV is common, but not as common as say a Holley 4150, so finding a source of info that is DGV specific for a certain engine is not likely.

 

Shorten the power valve so it's flush with the top of the valve itself and try it. If you don't like it, you can always get another valve and start over.

 

I do know that some of the other types of Weber 2bbl carbs have an adjustable accelerator pump rod. Not so with the DGV. If you wanted to think outside the box, maybe try digging one of them up.

 

My Mitsubishi forklift has a weird Weber/Solex 2bbl that is pretty close in design to a DGV, and it has the adjustable rod, but like I said, it's weird. It has provisions for two butterflies, but only one of them is machined and used, ie- there's only one throttle shaft. With a little machining, I bet this carb could be modified to fit a second throttle shaft and butterfly, but I bet there's another one out there already. It would be an interesting experiment to find out what it is and where to source them. It could open up a whole new avenue for tuning.

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  • 1 month later...

Well, it's progress...

"You can make adjustments to the power valve by shortening the rod that protrudes into the float bowl. I hit them on a belt sander and knock them completely off."  I thought about this and started shortening the rod from the power valve, I cut it to about 1/2 it's length and then stopped.  If I knocked it flat off, I would have had no power valve action which isn't what I was looking for.  I tried it with the shorter pin and it helped a very tiny amount so I worked the other end, I considered shortening the plunger out of the vacuum-housing but decided on reducing the spring pressure instead which has worked pretty well.  I cut about 3-turns off the spring and I can now cruise carefully at almost 70 or climb a *very* slight grade without going into the power valve,  I'll take another 2-turns off and that should be good.  If I were going to do it again, I'd leave the diaphragm housing attached to the top cover since it's small and thin,  I got it assembled with a slight crease the first time and then it was on the power valve worse than when I started.

OK, so the sensitivity is about fixed but it's still way too rich.  Sounds like it uses the same nozzle as the power valve?

Oh, When I started both jets were #140s which is .0535".  I altered the secondary to .0465"(#56 drill) which nets me a cruise A/F of about 15.2:1, I could go a touch leaner.

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  • 2 months later...

This should be the more-or-less final note I have for this thread:

I reduced the orifice on the primary a bit too much (like 18.5:1 too much) and then opened it up a touch (#55 drill) so my lean cruise seems to be in the low to mid 16s.
I've removed a few more coils from the power valve plunger so I can cruise at 75MPH on flat land but the moment I hit a grade it kicks in.
After the first time I learned my lesson and left the diaphragm in the top cover and just cut the spring one coil at a time with a dremel.
 I lost count but one coil at a time was really unnecessarily gradual, two at a time is plenty fine I think and I lost count but I'd say I'm about 8 coils down from factory.
It's still too rich on the power valve but enough of that is altitude and fuel variation that I'm not positive I'm going to chase it down.

In the end, it's much more tractable than the factory carb in terms of usable torque but it's still pretty imprecise.  I set the idle to a nice 14:1 and the next day it's at 12:1 or 15:1 depending on engine temp, where I bought gas, the barometric pressure, etc.  I see less variability at steady throttle but accelerating and decelerating both show wonky irregularities and if I pull a hard corner while doing a hillclimb I really see how fuel slosh is hurting me.
 I considered a computer-controlled  air-bleed solenoid but really I fear if I want better it might be time for some sort of EFI.

Oh, mileage... too soon for real data but it looks like I'm back in the mid-30s, not great but it's almost as good as the Hitachi.  I'm interested to see if I can break 40 on a road-trip like I could on the Hitachi.

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