Fine tuningBB carbs

3878 Views 9 Replies 0 Participants Last post by David Feinberg, Oct 9, 2003

Discussion Starter · Oct 6, 2003

In the 20 years of ownership w

In the 20 years of ownership with my 365BB, I've never been particularly pleased with two distinctive running characteristics of the car.

Specifically, driveability when cold...and,
Transition hesitation...

Although I have many years of tuning experience with Webers and high-performance engines, I always accepted the "folklore" that that's just the way the early BBs run..and that the problem is typical with the Webers. I cannot discount, even for a brief moment, that the tuning phase related to the classic Weber transition can be a real bear to resolve...I had presumed, until very recently that Ferrari had made their best attempts to jet these cars properly.

However, after much re-reading of several old Weber technical manuals...and comparing their suggestions for tuning with what Ferrari specifies for the proper idle mixture setting, I came to the realization that the idle mixture/slow running jet is just too small.

Virtally all technical references specify that if the car is jetted properly, the slow running screw will be out from 1.5-3.0 turns maximum, presuming that there are no throttle shaft leaks. On my car, the best idle quality is achieved at 4 + turns out. With the mixture set that way, the car stinks at idle...and stumbles on part-throttle tip in. Looking carefully at the pitch on the idle speed screw and the depth of the "mixture hole" in the carb body, you're pretty much out of adjustment with the screw out at 4 turns.

So...I ordered a set of idle jets one size larger than stock (.50mm->.55mm). After installation, I re-adjusted the idle mixtures (now at 3+/- turns. The idle speed had jumped to 2200 RPM!

Driving impressions:
-Cold driveabilty dramatically improved
-Unburned HC smell virtually gone...
-Stumble....What stumble? 99% gone
-This is the best the car has EVER driven
-Almost docile, around town (really)

All said and done, the best $60 I ever spent on this car!

Regards,
David

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Discussion Starter · #2 · Oct 6, 2003

David,

Have you tried the

David,

Have you tried the Color-Tune yet? I'm interested in hearing your impressions.

Regards, JRV

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Discussion Starter · #3 · Oct 6, 2003

JRV,

Honestly, I haven'

JRV,

Honestly, I haven't...
I'm trying to get as many miles as possible on the fresh engine, before Winter really sets in. My goal is to get 1,000 miles on her, re-torque the heads, adjust the valves, etc...and then go through a complete set-up with the Color-Tune.

I don't doubt that it's more accurate than the method I'm currently using, just I've done it the same way for so, so many years now...hence, it's reasonably easy to make performance comparisons with jetting changes.

The engine has an entirely different "personality" now...(No exageration here, I've driven her more in the last month than I have in the last 5 years.)

Regards,
David

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Discussion Starter · #4 · Oct 6, 2003

Hi.........

Just a quick co

Hi.........

Just a quick comment about the "progression lag" or "stumble" of the Boxers with carburetors. My mechanic, Patrick Ottis of Berkeley, CA, spent two days with my BB512 and tuned the "stumble" out of the engine completely....method unknown....after years of my tolerance for the "defect". My engine idles smoothly at 900 RPM and will "lift off" of any steady RPM under load with speed and complete smoothness.

Did the increase in compression ratio derived from your engine rebuild necessitate major carburetion changes...or exacerbate the "stumble" at all ??

Frank.......23005
www.masiarz.net/bb_resource

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Discussion Starter · #5 · Oct 6, 2003

David, that's interesting.

David, that's interesting. Most of my idle screws are turned out about 2 1/2 to 3 turns. Nothing much farther than say maybe 3 1/4. I wonder if this is somthing that is a little more critical on the 365BB than the 512BB.

I have very little stumble if any and presume I am running stock jets. I know Simon Campbell just tuned up his 365BB. Maybe he could comment on how many turns out his idle screws are setting at and the degree of stumble.

Drew

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Discussion Starter · #6 · Oct 6, 2003

David,

I've often wonde

David,

I've often wondered about the effects of using today's oxygenated fuels in carb cars. One would think that if an engine were jetted for a moderately lean condition, to meet emissions requirements, a switch to oxygenated gas (gasohol, MTBF, etc.) would make matters worse. Logically, an increase in jet size should make things better. I think you also may be experiencing, to a lesser degree, the beneficial effects of the engine breaking in.

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Discussion Starter · #7 · Oct 6, 2003

Hi Bill,

I thought I'd

Hi Bill,

I thought I'd comment on the oxygenates question you posed. I'm in the petroleum refining business and we have used ethanol and MTBE in our mogas blends and run them in octane engines. I thought you might find my comments interesting. Again, remember I'm in the petroleum industry so just keep my perspective in mind.

1) ETOH and MTBE are great octane sources. They contribute about 110 and 120 R+M/2 to the blend depending upon where you are on the blend curve if memory serves me right. They are priced accordingly.

2) What matters is octane and how much energy you can cram into the cylinder. In this case say BTU's per cubic inch.

3) Octane, the abilty to burn, before temp and pressure forces detonition, is the same in oxygenated and non-oxygenated fuels because we adjust it at the refinery that way; blend in lower octane hydrocarbon components when we use an oxygenate to arrive at say 87/89/93 octane.

4) However, the BTU content per unit volume of an oxygenate vs typical gasoline is less which means you don't have as much energy in your cylinder. Because of this fuel economy suffers slightly when you run fuels containing oxygenates.

5) However whether by the effect of the oxygenate itself or by dilution it does burn cleaner.

6) I'm guessing here, but I think the reason the dragsters and funny cars that run/ran on alcohol/methanol is that they could run at a very high compression ratio and take advantage of the fuel in the cylinder before it detonited.

7) The bigger issue for me is the lack of lubricity in gasolines currently on the market especially with oxygenate in them. We also have to remove most of the sulfur in gasoline (a lubricant) by Jan 1 2005.

My best guess to answer your question is that you proabaly can't see it preformance wise but it may take just a little more toll out of your engine because of the lubricity issue. But this is just a guess.

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Discussion Starter · #8 · Oct 8, 2003

Drew,

That's very inter

Drew,

That's very interresting information you provided.

I guess my question was just a little different. How do oxygenated fuels affect the air/fuel ratio of carb cars? If the fuels are truely oxygen rich one would think that they would tend to lean out the mixture for a given flow, assuming things like viscosity and specific gravity were otherwise not affected. FI cars with an oxygen sensor feedback loop presumably would automatically compensate. A simple carb setup would not. If this were the case, a general rule would be to richen the jetting slightly on carb cars to compensate for the reformulated fuels and restore the intended factory air/fuel mixtures. Do you have any information on this hypothesis?

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Discussion Starter · #9 · Oct 8, 2003

This is just a guess.

Somet

This is just a guess.

Something that had say 10 volume percent ethanol or MTBE would have about a 3 percent oxygen content if I did my chemistry right. I just don't know if you would ever see it in a carburated car.

I don't know a lot about what goes on in the cylinder head but if there is now some oxygen in the fuel theoretically I think you would want slightly less air and more fuel since you already have some available oxygen in the fuel. So you would want to run slightly richer with an oxygenated fuel than one that did not contain oxygen. So I agree with you.

However I think the point is moot so long as the fuel you set the carbs on is consistent. If you tune your car on fuel containing an oxygenate whether by Colortune or another method it should be set. I agree with you that if you now run a non-oxygenated fuel it could be a little off in a carburated car.

I believe CT and NY require oxygenates year round so as long as it got set right in the first place you should be in good shape.

Just a side note, as of Jan 1, 04 NY and CT have banned MTBE, but not NJ. So you will probably have ETOH in you gasoline to provide the oxygen content.

Just my 2 cents.

Drew

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Discussion Starter · #10 · Oct 9, 2003

Bill and Drew...

A little f

Bill and Drew...

A little follow-up since my last post regarding the jetting change.

I've put another 200km on the car (900knm total now on the new engine), with the new idle jets. Clearly, the engine is still tight and breaking in, as today (again) I had to re-adjust the idle speed down again by 400 rpm. Cold drivability is near perfect...and the part-throttle stumble in near non-existant. Idle quality is incredibly smooth and stable!

Hopefully, the weather will hold for a while...and I'll get a chance to do some extended highway driving with her before the snow falls.

She truly is enjoyable to drive...finally!

I can't tell you guys how much I appreciate your input and support!

Regards,
David

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