Carburetor Tuning Thread for B Series

[R32RennSport]

It's about time, much over due in fact, that we have a dedicated, pinned carburetor thread for all B series engines, whether they came with carburetors or maybe are being converted. I'd like to keep this first post updated with links to jetting charts, general tuning and reference guides so that it is in one concise place. Please reach out if there is something you think should be added here.

Not to be a snob but most, if not all of this information initially will relate to side draft carburetors, specifically DCO(E) Weber's as they are the most commonly installed brand besides factory SUs. I welcome other set up information as well as it pertains to the side draft Solex, Dellorto, Mikuni, and of course the original SU's. Downdraft DGA series can be included here as well.

It is my hope that any posts following this posts are confined to sharing others setups using the following template: Engine (general specifications)/Carburetor(s) installed/Carb setup/Jetting. Of course general questions specific to carburetors are welcomed as it's my hope this will be a shared experience thread. Let's keep general engine tuning, exhaust and fuel system design to a minimum or perhaps start a new general post if needed. I also don't mind posts linking to other forum discussions with relevant information, be it here on Turbobricks or elsewhere. Many Alfa, Lotus, BMW, Nissan, etc. forums have held invaluable experience and information in the past.



Setup Charts: (I favor these newer charts as some of the older printed charts don't consider todays poor fuel quality)

Misab Chart This chart covers everything from B16 to B30

KG TRimming Chart This chart is organized by carburetor application, then by B series engine application

General Tuning:

Weber DCOE Tuning Advice This guide is specific to choosing and setting up DCO(E) for a given engine

Weber DCOE Reference and Theory Guide written by a Nissan enthusiast but very relevant and technical

Weber DCOE Guide A very abbreviate quick reference guide

Guide to Weber Intakes and Air Filter Options A Turbobricks thread article

Carburetors and Parts Resources:

Pierce Manifolds

Weber Redline

SU Burlen

Pegasus Auto Racing Supplies

Printed Publications: (some of us still like books. Search on Amazon, Ebay, etc. for purchase)

Volvo Factory Green Book (SU's, etc.)
Weber Official Tuning Manual
Haynes Techbook Weber Carburetor Manual
Speedpro Series How to Build and Powertune Weber and Dellorto
SU Official Workshop Manual
Speedpro Series How to Build and Powertune SUs

 

[spock345]

Probably would be a good idea to get this thread pinned.

 

[spock345]

My Amazon is currently set up as the following

Engine:

• B20F bored 0.015" oversize
• Balanced lower end and lightened flywheel
• 10.3:1 compression with tight squish
• Cleanup in the ports courtesy of Culberro, mostly the choke point just past the exhaust valve seat.
• D cam
• Cast 4-2 manifold with 2" exhaust
• Mangoletsi DCOE intake manifold
• Timing curve set for 15 degrees BTDC at a 950 rpm idle, fully advanced at 35-36 degrees by 3000 rpm.
• No velocity stacks, 1.75" low profile air filters

Carbs:

• Italian 45 DCOE 9
• 32mm chokes
• 4.5 auxiliary venturis
• F9 emulsion tubes
• 130 main jets
• 170 air correctors
• 55F8 idle jets
• 40 pump jets
• 40 pump bleeds

AFR is relatively stable between 12 and 14:1 with 12.5-ish on WOT and it idles happily between 12 and 13:1. Plug color looks good. Noticeably grumpy for the first couple hundred yards of driving when cold. It runs out of steam by 6000 rpm as expected given the 32mm chokes.

Most guidance will say to start with 180 air correctors but in my case that led to a lean spot with the F9 tubes as the main circuit kicked in. F16 tubes also worked but were noticeably richer at cruise. A good cruising AFR in the 2500-3500 rpm range seems to be the trickiest thing to accomplish.

Also don't be a slave to the AFR gauge, it is a guide to help explain what the butt dyno tells you and isn't a replacement for a brain. At the end of the day if the plugs look good, the car feels good, fuel economy isn't in the single or low double digits, and there isn't anything frightening on the gauge (like a lean WOT mixture) then send it.

 

[R32RennSport]

Awesome, exactly what I was looking for! I'll post my specs up tomorrow, I left my notebook at home (highly recommend keeping a carb journal for making notes on various setups as we all know this a trial and error process for most of us with other monitoring equipment).

@moderators @admin can we pin this?

 

[JohnMc]

Engine:

• B20E bored to 93mm using B21A pistons
• Balanced lower end and lightened flywheel
• 10.5:1 compression with .036" squish
• R-Sport Stage 3 head with big valves (almost touch each other) and ports, double valve springs
  • this was shaved WAY down prior to me getting it, which is why I'm using B21A pistons with a small dish
• Isky VV81 cam (have an R to go in it at some point) with 1.7:1 KG roller rockers
• KG 4:2:1 header into 2 1/4" exhaust
• Peirce style short DCOE intake manifold
• Short velocity stacks, sometimes bare, sometimes with foam slip-on filters

Carbs:

• Italian 40 DCOE 9
• 36mm chokes
• F15 emulsion tubes
• 125 main jets
• 160 air correctors
• 50 F8 idle jets
• ? pump jets
• ? pump bleeds

I'll fill in the jetting later, I can't find anything saying what I have in there, and it's too cold to go pull jets on it right now to look.

I haven't had an AFR gauge on it in a very long time, used one when initially setting them up. In my case, it showed all sorts of odd behavior, largely just normal carb stuff I think. Going rich when turning hard right, lean when going left. Rich spikes going over bumps. In general, it likes to run rich, any attempts to lean it out a little (to something more reasonably close to stoich) would make it sputtery and weaker. It tends to get upper teens MPG driving around town.

The engine pulls hard and clean past 7000, even to 7500 but that's getting into stressful territory for the stock cast pistons. It's more a matter of the R-sport head and the cam, but power is best from around 3200 rpm up to 7K+.

I always hear that 40's are too small, but these work great in my experience. It's entirely possible I'm leaving some HP on the table at higher RPM's. Per 1/4 mile times and car/driver weight, I should be around 150 - 160 hp at the wheels.

 

[spock345]

Engine:

• B20E bored to 93mm using B21A pistons
• Balanced lower end and lightened flywheel
• 10.5:1 compression with .036" squish
• R-Sport Stage 3 head with big valves (almost touch each other) and ports, double valve springs
  • this was shaved WAY down prior to me getting it, which is why I'm using B21A pistons with a small dish
• Isky VV81 cam (have an R to go in it at some point) with 1.7:1 KG roller rockers
• KG 4:2:1 header into 2 1/4" exhaust
• Peirce style short DCOE intake manifold
• Short velocity stacks, sometimes bare, sometimes with foam slip-on filters

Carbs:

• Italian 40 DCOE 9
• 36mm chokes
• ? emulsion tubes
• ? main jets
• ? air correctors
• ? idle jets
• ? pump jets
• ? pump bleeds

I'll fill in the jetting later, I can't find anything saying what I have in there, and it's too cold to go pull jets on it right now to look.

I haven't had an AFR gauge on it in a very long time, used one when initially setting them up. In my case, it showed all sorts of odd behavior, largely just normal carb stuff I think. Going rich when turning hard right, lean when going left. Rich spikes going over bumps. In general, it likes to run rich, any attempts to lean it out a little (to something more reasonably close to stoich) would make it sputtery and weaker. It tends to get upper teens MPG driving around town.

The engine pulls hard and clean past 7000, even to 7500 but that's getting into stressful territory for the stock cast pistons. It's more a matter of the R-sport head and the cam, but power is best from around 3200 rpm up to 7K+.

I always here that 40's are too small, but these work great in my experience. It's entirely possible I'm leaving some HP on the table at higher RPM's. Per 1/4 mile times and car/driver weight, I should be around 150 - 160 hp at the wheels.

I find your engine contrasted to mine as an example of what sort of chokes you'd want with a B20 in different stages. I take the old jetting charts with a grain of salt given what you and many others have experienced with choosing the choke size. The often specified 36mm seems too large for a majority of the 2.0L Volvo engines in those old jetting tables. For those I found looking to the entries specifying 32mm chokes in 40 or 42 DCOEs were far more helpful as a starting point, even when using 45s. All that despite what the books say about a 2.0L making maximum power around 6000 rpm. Alfa Romeo even fitted 32mm chokes to their street focused 2.0L engines.

These engines seem to be in between 40s and 45s being optimal, so you can get away with either until you get crazy and need something bigger than 36mm chokes. If only 42s were more common or still made.

 

[JohnMc]

When my engine was more stock (stock F head on a 2.1L bottom end, D cam) I tried the 36mm chokes, and it was too much, It would bog when the throttle was opened too quickly, you had to slowly roll into the throttle. Just not moving enough air to get the main jets activated at middle RPM's. And I went back to the 32mm chokes it came with which were working well.

Only later on when I dd more to the engine did I try the 36's again, and now they work great.

I have a set of 45's sitting on a shelf, sort of intended to go onto the 16V 2.3L motor I pulled out of the wagon. Vaguest of plans to 'low-tech' the 16V and put it in the PV. I have a DCOE manifold for it as well.

42's would be a nice middle option between the 40's and 45's, the only drawback is they don't have many choke sizes available. The 45's can go smaller than the 42's I think.

 

[spock345]

When my engine was more stock (stock F head on a 2.1L bottom end, D cam) I tried the 36mm chokes, and it was too much, It would bog when the throttle was opened too quickly, you had to slowly roll into the throttle. Just not moving enough air to get the main jets activated at middle RPM's. And I went back to the 32mm chokes it came with which were working well.

Only later on when I dd more to the engine did I try the 36's again, and now they work great.

I have a set of 45's sitting on a shelf, sort of intended to go onto the 16V 2.3L motor I pulled out of the wagon. Vaguest of plans to 'low-tech' the 16V and put it in the PV. I have a DCOE manifold for it as well.

42's would be a nice middle option between the 40's and 45's, the only drawback is they don't have many choke sizes available. The 45's can go smaller than the 42's I think.

If I were to recommend a size to most folks with a relatively tame engine that SU needle profiles aren't quite cutting it for (mine wasn't happy with the standard richer needles), I'd go for 40s and start at 32mm chokes. It is a lot easier to learn how to tune by driving around when you aren't fighting a dead low end.

 

[R32RennSport]

Engine:

• B20F bored for B21 pistons .020 oversized, 2130cc
• Not balanced, used pistons (new rings) and rods, stock flywheel and clutch
• 9.25:1 compression
• Head is mildly ported with attention to exhaust short radii, 44mm Intake 38mm Exhaust, single HD springs
• KG10 camshaft
• 4-1 header
• R Sport replica DCOE intake manifolds, ITG air filter with 60mm air horns.
• Timing curve set for 12 degrees BTDC at a 1000 rpm idle, 123 Distributor using curve xxx

Carbs:

• Spanish 45 DCOE 152 (3 hole progression)
• 34mm chokes
• 4.5 auxiliary venturis
• F9 emulsion tubes
• 140 main jets
• 185 air correctors
• 55F9 idle jets

* This list is from an old notebook entry dated 08/22 and may not reflect currently installed components, will verify

This engine has always been a rough junior engine building effort on my part and as such it performs adequately, though balanced, a properly set up compression ratio and better exhaust manifold/header would likely make it a much more efficient and enjoyable. I've tried both 36mm and 32mm chokes over the years and this engine seems happiest with 34mm installed. I've also experimented with both the F9 and F8 series of idle jets to find the best match. This engine is to be retired soon and exchanged for a Swedish Relics built 2300cc stroker with 48DCO carbs. With engine removed I may revisit this engine and rebuild it to a more suitable specification.

 

[spock345]

My Amazon is currently set up as the following

Engine:

• B20F bored 0.015" oversize
• Balanced lower end and lightened flywheel
• 10.3:1 compression with tight squish
• Cleanup in the ports courtesy of Culberro, mostly the choke point just past the exhaust valve seat.
• D cam
• Cast 4-2 manifold with 2" exhaust
• Mangoletsi DCOE intake manifold
• Timing curve set for 15 degrees BTDC at a 950 rpm idle, fully advanced at 35-36 degrees by 3000 rpm.
• No velocity stacks, 1.75" low profile air filters

Carbs:

• Italian 45 DCOE 9
• 32mm chokes
• 4.5 auxiliary venturis
• F9 emulsion tubes
• 130 main jets
• 170 air correctors
• 55F8 idle jets
• 40 pump jets
• 40 pump bleeds

AFR is relatively stable between 12 and 14:1 with 12.5-ish on WOT and it idles happily between 12 and 13:1. Plug color looks good. Noticeably grumpy for the first couple hundred yards of driving when cold. It runs out of steam by 6000 rpm as expected given the 32mm chokes.

Most guidance will say to start with 180 air correctors but in my case that led to a lean spot with the F9 tubes as the main circuit kicked in. F16 tubes also worked but were noticeably richer at cruise. A good cruising AFR in the 2500-3500 rpm range seems to be the trickiest thing to accomplish.

Also don't be a slave to the AFR gauge, it is a guide to help explain what the butt dyno tells you and isn't a replacement for a brain. At the end of the day if the plugs look good, the car feels good, fuel economy isn't in the single or low double digits, and there isn't anything frightening on the gauge (like a lean WOT mixture) then send it.

A follow up to this. The jetting here would have trouble getting farther than 5250 rpm and was out of steam by a bit above 5500.

I decided to give the set of 33 mm chokes I found a go in place of the 32s. It worked quite well with the engine pulling to 6000 cleanly. It was a little leaner and stumbly off the line so I tried 55F9 idle jets. The engine likes it and the idle mixture screws are one turn out. The wideband reads a little richer with a few blips of 11.7 or 11.8 showing up transiently, but it still maintains 12.5:1-13:1 at WOT.

I also went to a slightly different intake manifold from Hiperfauto. They raise the carburetors a little more making it easier to get the filters on and off.

 

[JohnMc]

Finally got around to looking at the jets on mine, although I didn't take the lid off and look at the accel pump jets/bleeds.

Having a 125 main jet makes me wonder if I really do need a bit more carb on it. People who would likely know always say I need a bit more carb, but I always figure since it pulls well into the 7500 rpm range that it was 'enough', but maybe I'm leaving some HP on the table. I have a set of 45's on the bench, supposedly waiting to go onto the 16V motor if I ever swap it in, perhaps I should stick those on the pushrod motor and see if they make a difference.

 

[msweeney]

Finally got around to looking at the jets on mine, although I didn't take the lid off and look at the accel pump jets/bleeds.

Having a 125 main jet makes me wonder if I really do need a bit more carb on it. People who would likely know always say I need a bit more carb, but I always figure since it pulls well into the 7500 rpm range that it was 'enough', but maybe I'm leaving some HP on the table. I have a set of 45's on the bench, supposedly waiting to go onto the 16V motor if I ever swap it in, perhaps I should stick those on the pushrod motor and see if they make a difference.

I'd say go for it. At the race shop the smallest carbs we deal with are Mikuni PHH44's and they work really well on a 1.6L Datsun pushrod engine (140-160hp or so) The 2.0L engines like anything between a large-choked 44/45 at places like Sonoma, to choked down 50's at places like the old Cal Speedway.

 

[spock345]

Finally got around to looking at the jets on mine, although I didn't take the lid off and look at the accel pump jets/bleeds.

Having a 125 main jet makes me wonder if I really do need a bit more carb on it. People who would likely know always say I need a bit more carb, but I always figure since it pulls well into the 7500 rpm range that it was 'enough', but maybe I'm leaving some HP on the table. I have a set of 45's on the bench, supposedly waiting to go onto the 16V motor if I ever swap it in, perhaps I should stick those on the pushrod motor and see if they make a difference.

Yeah, the air is moving a bit fast in those carbs to need 125 mains on a hot 2.0L. I wonder how it would do with chokes larger than 36mm.

 

[JohnMc]

My comparison is with the SU's carbs it had prior. They had some properly sized needles (for the motor at the time) as recommended by Rhys, and the DCOE's were a big improvement, especially at higher RPM's. And they were cheap, lol, bought long ago on eBay for something like $300, carbs, linkage, manifold.

I'll look over the 45's I got and see what the jetting/venturi sizes on them is. It's super easy to swap carbs out.

 

[R32RennSport]

To piggy back on what was already said, I'm sure the 40s with 36 chokes work just fine, especially on the street, but I would certainly think they are operating at the far upper end of their efficiency, especially in your application. I would certainly advocate test running 45s with 34 or 36 chokes. As I like to remind users of DCOEs that a 36mm choke on a 40 versus 36mm choke on a 45 has a much different cross section and therefore produces a different vacuum effect. Do keep in mind that when swapping onto the intake manifold that some, not all, manifolds have 40mm inlet sides and if a 45 is bolted up to them will greatly disturb airflow due to the large lip between mating surfaces.