Weber 50 DCOSP Progression Tuning

[rwdfords]

Yes trumpets do have a large effect on the power curve, we would be using a much longer inlet manifold but as it is we are running the trumpets pretty close to the inner wing, the air filter is sitting right on the inner wing

Looking at rolling road testing interestingly trumpet length does change the power curve and pulse tuning waves but only at WOT, at part throttle the butterfly interferes with these waves and makes trumpet length tuning have no effect at part throttle, only at WOT does it take effect

[rwdfords]

This is quite a good read, with back to back testing, amazing how much difference trumpet length can make

http://www.emeraldm3d.com/articles/cat/ ... th-intake/

[Runit]

By removing the main jet assemblys you make it impossible for the main circuit to supply any fuel. If you run the engine it will not go above a certain rpm no matter what you do. That rpm is important. Below it the low speed jets are what 's running the engine and they are what you tune. Above it the main jet, e-tube and air jet control things and they are what you tune. A small race engine with big venturis may run to 5000 on the low speeds. A big stock engine with small venturis may only run to 1200. The engine will be very lightly loaded and you won't hurt it by running with no main assemblys. If your problem lean spot is below the RPM tune the lowspeeds, above, tune the mains.

Theres another effect that may be useful here. You may be able to limit the richness at the top of the low speed circuit. A 60f8 is roughly equivalent to a 45f9. The f8 has a 1.2mm air hole. The f9 a 1.0mm. The 60f8 will supply fuel to a higher rpm because the total hole size in the jet is more than in a 45f9. You may be able to find a lowspeed jet with smaller holes that's rich enough to cure the flat spot but not too rich at higher rpm. There are only a few stock low speed jet air hole choices though. You may have to drill.

[rwdfords]

60F8 idle jets are MILES Richer than 45F9 .. fuel gets sucked up into a 0.60mm hole in the "60" F8 from the fuel well, and a 0.45mm hole in the "45" F9, a change of just "5" in idle jet number is quite a lot and that is only a 0.05mm difference in fuel hole inner diameter, the fuel hole ID acts like an orifice to restrict fuel flow, air hole ID has a much smaller effect than a change in fuel hole size

[rwdfords]

Surely there must be some more quite experienced weber tuners on this forum willing to share some useful ideas/tuning methods in this area, I am Always open to learning more

Curious to know why this thread has been moved from "advanced engine tech" to "engine tech" is there any particular reason why it was moved? what should I Not post in AET?

[Runit]

The point was that for lowspeed jets supplying the same mixture at any given speed the one with the smaller holes will be leaner at higher rpm. An example. A race winning 4cyl Datsun circle track engine had a low speed flat spot with ??f6 lowspeed jets. The driver delighted in driving caution laps at the speed that produced the most backfiring and missing to point out the problem to the tuner. The jets had been installed in the belief that a small air hole was necessary to get the low speed jetting rich enough. The engine ran great except for the flat spot. It was cured by going to ??f8jets.

[SWR]

Surely there must be some more quite experienced weber tuners on this forum willing to share some useful ideas/tuning methods in this area, I am Always open to learning more

Curious to know why this thread has been moved from "advanced engine tech" to "engine tech" is there any particular reason why it was moved? what should I Not post in AET?

It might have been moved by some of the other Mods - I did not touch it - to get more hits, that happens now and then if things are tricky. You haven't done anything wrong, we just tried to help.

[rwdfords]

Cheers Bjorn no worries

[lada ok]

[quote="lada ok"]well ok, ......... why don't you try extra holes in the prog /circuit you can always block them off again[/q

AND ?

I bought a good selection of drills and a pin chuck ....... you drill out the holes to suit ... if it's too big you solder up the hole and start again

gas speed @ 220 ft/sec... 7000 rpm =
+/- 46.3 i/v dia
38.9 port
44.9 bowl
41.67 throat
could you down drought these heads ?
what are your cams specs ?..... if you can

[Caprimaniac]

RWD; when do you see the lean spike?

When you hold the engine at a steady rpm or when you do a idle to full throttle run, or both? Will tell you wther it's main curcuit or acc curcuit related.

I would look at the acceleration curcuit. Maybe spring- related. Tune when the squirting starts, it might like to start sooner.

[AC sports]

I had the exact same issue with 45DCOEs. I tried every combination of idle jet/ holder assemblies to the point the idle circuit was so rich the car bogged down and wouldn't go and guess what?..... Still had a lean spot at the point the throttle plate had to make a jump to the 1st progression hole. You could physically see that the plate was way behind the 1st progression hole in the case of 45 dcoe type 152. Not sure on the 50's. I ended up setting them up on a mill and drilling an extra 0.8mm progression hole right under the butterfly. Problem fixed! No popping of the intake or bucking of the car. Another solution i have not tried is to get throttle butterfly's made with a different angle so that they close further back closer to the 1st progression hole. I remember somebody mentioning the name Kinsler fuel injection as doing this. Ive personally never heard of them given my location. Hope my experience helps .
If it means anything....Im on Dellorto DHLA's on all my cars and they in my opinion are superior from a drivability point of view. Their progression circuits as you have posted pix of seems to run smoother. For a car needing to go from lower rpm and up Ive alwAys found them more friendly.
Good luck.

[Calypso]

Not Weber guru here, but I would consider increasing idle jet size together with idle air bleed size, not to mess up the AFR while extending the range. Take it FWIW.

[Erland Cox]

I am having a problem with 50 DCO carbs in that they seem incredibly weather sensitive.
At hot weather they go very rich below 5500 rpm, 0,69 lambda.
When it is cold they work fine from 4000 rpm but are very lean below when opening the throttle.
I believe the acceleration pump valve is 100 so I have bought 40 and 0 to test with.
The carbs came with F7 emulsion tubes but they were very rich around 4000 rpm.
Changing to F9:s cured that but the car was lean from low rpm.
I have tried 65F8 and 60F9 idle jets and have bought 65F9:s to test also.
I have 44mm chokes, have tried from 170 to 200 main jets and have 170 air correction jets.
2340cc engine with 232 hp at 6700 rpm.

After reading this thread I believe that I am tuning the wrong circuit, maybe using too rich emulsion tubes to fix a lean spot
in the progression between the idle and main jet system.
The colder weather makes the lower rpm leaning show more and the hot weather makes the mid rpm richness show more.

Erland

[MadBill]

Not a Weber expert, but I have read the words of several that say the choke should be ~ 72% to a maximum of 80% of the butterfly size, which would be 40 mm in this case. They say bigger is usually worse for power and makes tuning very tricky...

In his How to Make Horsepower Vol. 2, David Vizard recounts dynoing a 1600 cc engine by a top builder which had big pair of Mikuni side drafts with the largest available chokes. It made 117 HP. Downsizing the chokes to 80% immediately gave 160 HP and eventually 197.

[Erland Cox]

Going down from 44 to 42 looses 10 hp but may give better response.
There are also 2 different kind of bell mouths for DCO carbs, 48mm or 55mm I.D.
I have tried the smaller ones that come on 48 DCO carbs but I did not feel much difference.
I believe that the fault that I have made is to rich high speed system and to lean low speed to get the transition right.
And that shows up when it is colder or warmer.
I will it a try tomorrow but these carbs probably need larger idle jets than I am used to.

Erland