Tuning 40 DCOE Weber 151 on an L24

[LeonV]

Ok, so here is a question for you 151/152 guys. Having discovered that my 42DCOE 8's are rare, having been only OEM on Ferrari, Maserati, and Morgans, I trolled around the internet until I found a couple guys looking for these to do correct restorations on a Maserati 3500. One of them has made an offer of $1000 for my three. I think I can get more than that but that is another conversation.

If you were going to pick a carb set for an L28 bottom end with flat top pistons and an E31 big valve ported and polished, would you pick 151's or 152's and why?

Rob,

I may be picking up some new triples in the future as well. For the L28 I plan to build, I plan to go with the 45DCOE152.

Why? 'Cause they're bigger!

[Lazeum]

I would say bigger isn't always better but I would also agree.

With my current setup: F54, flat top, p79 ported/polished, Rebello Cam, I'm a little bit short with my 40DCOE. Some 45DCOE with small chokes would probably work better.

[LeonV]

To recap the last week or two of my quest to rid myself of the sudden-open-throttle hesitation:

1. Tried stiffer pump springs. They seemed to help my hesitation but they were so stiff that they overwhelmed the throttle return springs (new ones) and didn't let it drop back to idle.

2. I replaced the pump springs with my old ones but soldered up my bleed back jets. The setup was 40 pumps and 55 bleeds, and was now 4- pumps and zero bleed. It drove better with less hesitation, but it was still there when suddenly going WOT and sometimes when rev-matching.

3. Keith sent me some .53mm pump jets and a drill bit to upsize to .61mm if necessary, as well as some slightly stiffer springs. Holy cow, what a difference!!! The rev-matching and WOT at low rpm hesitation is gone! Once in a while I'll get a short lag in power output because of AFR going lean, but no more bucking-bronco take-offs. Not only that, but my idle is at 14:1 and cruise is 13-13.5:1 (I want this leaner). It feels so much better to drive this car now. My current accel pump setup is stock springs, 53 pumps and zero bleed.

I'm thinking I may need some leaner hypojets to get the cruise AFR where I want it. It's close but I want it leaner, at 14.5:1 or higher. I might as well try the slightly stiffer accel pump springs to see if they eliminate some mixture lag, but I won't want to use them if they mess with the throttles returning. I'd rather upsize the pump jets.

All in all, you can barely tell this thing is carburetted now (especially if you don't look at the AFR meter )!

[LeonV]

I would say bigger isn't always better but I would also agree.

With my current setup: F54, flat top, p79 ported/polished, Rebello Cam, I'm a little bit short with my 40DCOE. Some 45DCOE with small chokes would probably work better.

Bigger is better, if your engine can support it. An L28 with an aggressive cam can easily run 45s.

Basically, if (when) I decide to get some new carbs and I have the option between 40 and 45, I'm going with 45.

[conedodger]

Thanks guys. Leon, don't burn valves. I wouldn't go that lean at cruise with today's pump gas. 13.5:1 is the leanest I would go.

[LeonV]

Thanks guys. Leon, don't burn valves. I wouldn't go that lean at cruise with today's pump gas. 13.5:1 is the leanest I would go.

I should be ok. The stoichiometric point of E10 is around 14.1:1 but the AFR meter normalizes what it measures to the stoich point of pure gasoline (14.7:1) since it calculates based on lambda (measured AFR = pure gas AFR * lambda). If I can lean it out a bit more, I'll be happy, and so will my wallet!

[conedodger]

I should be ok. The stoichiometric point of E10 is around 14.1:1 but the AFR meter normalizes what it measures to the stoich point of pure gasoline (14.7:1) since it calculates based on lambda (measured AFR = pure gas AFR * lambda). If I can lean it out a bit more, I'll be happy, and so will my wallet!

Oh yes, I forget that you are paying Bay Area gas prices. We are about 25cents under Sacramento...

[LeonV]

Oh yes, I forget that you are paying Bay Area gas prices. We are about 25cents under Sacramento...

Lucky bastard...

We finally dipped under $4/gal recently.

[madkaw]

Guys,

still following this thread-even if I'm a Mikuni guy-

I have contacted mr. Franck and he responded. I have never been able to get into Sidedraft Central Yahoo group-which was mandatory if I was going to be part of this experiment. I have resorted to getting a Yahoo e-mail account to see if that helps.

keith wrote me saying that I might benefit from the tubes if my car did not pass the F.I. test-you guys might have touched on this in this thread. I'd say I am border line in that aspect. I have been reading the last few posts and it sounds like I am where you are as far as a slight hesistation during hard tip-in at lower rpms. No coughing, sputtering, spitting-just a millisecond of hesistaion, so maybe i'm at a pretty good point. At higher RPMS, above 3500, there is no hesitation.

I have tried quite a few things, but don't know if I have ever completely gotten rid of that slight studder. Other than that, these carbs have really been wonderful. I am going to put some fresh needle&seats in the floats so I know everything is good, and i am going to take to the dyno!

This weekend I even played with valve timing to see if that would change anything concerning hard tip-in at lower rpms-it didn't. I did quite a swing on the cam timing and basically put it back where it was-slightly advanced.

I have noticed watching my AFR's that WOT AFR is dictated by when hard tip-in occurs. If I stomp it at 2500rpm the AFR's look super rich for a while thru the rpms before getting to the 13 range. If I start out at a higher rpm the AFR's look much more optimum thru the remaing rpm range.

[240260280z]

Just to add another data point to this thread:

My good friend has a 510 with rebuild L16 motor and dual DCOE40 side drafts with 32mm chokes. It came with 135 main jets.... and fuel-fouls the plugs. I have not had a chance to run with the O2 sniffer.

The L16 per chamber volume is the same and an L24 so carb sizing/jetting should be similar.

To recap:

L16 DCOE40 32mm Choke 135main Pig Rich ~ 7-8 AFR (Estimated)

L24 DCOE40 30mm Choke 125main Rich ~ 10-11 AFR (Measured)

L24 DCOE40 30mm Choke 120main Rich ~ 11-12 AFR (Estimated.. will be measured Spring 2013)

L24 DCOE40 30mm Choke 115main TBA ~ 12-13 AFR (Planned for Spring 2013)

[LeonV]

Just to add another data point to this thread:

My good friend has a 510 with rebuild L16 motor and dual DCOE40 side drafts with 32mm chokes. It came with 135 main jets.... and fuel-fouls the plugs. I have not had a chance to run with the O2 sniffer.

The L16 per chamber volume is the same and an L24 so carb sizing/jetting should be similar.

To recap:

L16 DCOE40 32mm Choke 135main Pig Rich ~ 7-8 AFR (Estimated)

L24 DCOE40 30mm Choke 125main Rich ~ 10-11 AFR (Measured)

L24 DCOE40 30mm Choke 120main Rich ~ 11-12 AFR (Estimated.. will be measured Spring 2013)

L24 DCOE40 30mm Choke 115main TBA ~ 12-13 AFR (Planned for Spring 2013)

Phil,

I just ran my L24 today with 120 mains (40DCOE with 30mm choke). Saw about 12:1 on AFR meter when I was looking. It was fairly cold out though, so it will be richer in warmer temps. I may go with 115s but it runs so well, I'm not in a hurry.

I'd imagine at 7-8AFR, that 510 would be billowing black smoke out the exhaust pipe.

[240260280z]

yup... the plugs would foul after ~ 25km and black smoke. I had to give my friend new plugs to get home. He lives ~ 25km from my place and decided to take a drive over on Sunday. He barely made it home.