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Lets talk DCOE 45 on a MGB

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Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
Warning lengthy post.

An old dog and SU carb guy trying to learn new tricks.

Several of you run a DCOE 45 on your MGB, as old SCCA SU guys that is where my knowledge resides, but as I deal with cusotmer who run DCOE, and may not fully understand then I see the need for me to know/own DCOE knowledge as well.

OK what I would love to here from you guys, either here, or privately if you choose is what choke size, main jets and air correctors you use. I know this will vary on engine prep level, so if you care to elaboroate, you can for example say I ahve 12.0 to 1 MGA engine, or 13.0 to 1 MGB, whatever you choose to share would be helpful, I think not only to me, but others as well.

My gracious friend from CO., Mr Headley has help start my Weber education, and I truly appreciate the time he has spent with me on this, Dave is a jewel to our sport.

OK here goes
Sample car: MGB vintage race, 12.0 to 1, Jeff's point distributor, sport coil, SP17BK APT scatter pattern race quick ramp cam. No rev limiter or any thing like that.

The car revs to give to 6500 and then quit revving, that number has varied from 6300 to 6800, as the customer has changed both the Webers carbs and distributor in this ordeal. At one point he started to think something in the new motor we built for him may be doing this. Now mind you the engine cranks fine, sounds good, have a solid 70-75 psi of oil pressure and is not having any temp issues. I told my customer to look closely at fuel and fire, he had a DCOE 45 with Cannon 801 intake, running 160 main jets with 150 air correctors, 34mm chokes, F16 emulsion tube, the main jet was upped to 170 for dyno testing and that set up the car would run to 6800 before it missed and quit pulling, a Unlite distributor that seem to be working fine. The dyno had no O2 set up so I don't like that, it is the main reason you go to the dyno, prefect this and all else will follow. OK the customer got a brand new DCOE 45 from Mike Pierce set up with 36mm chokes, it had 170 main jet, and 150 correctors and F-16 emulsion tubes.

Ok now not really thinking the engine was issue, I could think of two things internal engine that cause this, cam timing, and possibly a cam wiping out. We strip enough off the engine while in the chassis to check the cam timing, and cam lift. while the cam timing was 2 degrees off form the spec sheet, I can't tell if that is chain stretch, or Fred just missed the number, we retimed it to the proper number, now I know first hand this would not drastically change the engine, or the amount it would rev. The lift of the cam was fine, the only odd set up on this engine (well odd to me) is the customer supplied roller rocker arms, they are Harland sharp , but 1.625 ratio on the intakes and 1.5 ratio on the exhaust, while I am not crazy about that set up, I see no reason why that would create this issue, all valve clearances are more than adequate, .100" +.

OK after checking, and resetting the cam timing and checking cam for lift at the cam and valves, we put the engine all back together. No real surprise to me, based on our finding in the engines, it still only revved to 6500 rpms, so now my concentration was on the Weber carb. So I called Headley and we talked about the jet settings used on MGBs, he felt like it was too rich on top end, and that was causing the issue, that made perfect sense to me, because I have saw thing on SU equipped cars, meaning too rich on the top end to rev out. He advised me that he felt 150 air corrector may be the problem and see if we had a 180 and try that. WE did that very thing and the engine sounds even better and the engine now revs a little more rpm, but not all the way out like we want. What this told me was we had taken a step in the right direction , but more tuning was needed. At the point the customer need to head back home, a 7 hour drive.

Now the ideal situation would be to have us and the customer go to a chassis dyno together, and a chassis dyno that has a proper O2 set up,as I feel the O2 gauge/readings are a better tuning tool for the single DCOE, than EGTs, on my SU car I have both. I am 1000% certain that the carb just need to find the correct combinations of parts to make the issue go away. Now Dave said the F16 emulsion tube is pretty much the catch all part for the racing MGB, and most cars work best with 36mm chokes, from there it comes down to what the engine wants for air correctors and main jets, the customer has wide array of these parts to work with

So there lies the question, tell me about the choke , jets, air correctors you run in your car.



Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Free Tech Advice - Alloy wheels for British Sport Cars,and others
fast-MG.com Avatar
fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
Hap, I'm a little confused: Points or Unilite dizzy? Coil resistance? Ballast or no ballast?

One comment on Weber jetting: a main jet tends to affect the fuel curve over the entire range. An air corrector tends to trim the high RPM mixture. A main jet change of 10 is equal to an air corrector change of 20. In other words 2:1 for fuel effects.These are of course approximations. BTW, the Haynes SU and Weber carbs manual will give a reasonable starting point for jetting. Float level is important to how the carb responds. Poly floats have different settings than brass floats. IMO, Poly are preferred. Accel pump spray nozzles do affect the total fuel flow. Idle jets have little affect on race engines idling at 2KRPM+. That is not the case with road engines as the idle jet affects the transition from idle to drive engine speed ranges.

Also, I suspect you will get as many opinions on this subject as there are posters. Mine are based on my personal experiences with DCOEs on a variety of engines.


Member Services:
Dave Headley, dba FAB-TEK offers full service race car parts and preperation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
Speedracer Avatar
Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
He first had a Unilite, but at this time he has a Jeff points distributor (mechanical advance only) knowing what we know now, I think both distributors are/were working just fine.

Thanks for more Weber jet education Dave. We have not seen any fluctuation in fuel pressure, so I think we are probably looking at the right combo of main jets and air correctors, and not a float issue, do the new carbs come with poly floats?

OH, yes I am fully prepared to be more confused by folks suggestions smiling smiley but it would interesting to see what combo people have had success with. At this time I instructed the customer to go to a good chassis dyno with a good AFR set up, preferably one that works with a the dyno software and provides a graph, and use the AFR as his guide. It appears what you told me yesterday by changing the air corrector to 180 headed us in the right direction, but still more work to be done.



Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Free Tech Advice - Alloy wheels for British Sport Cars,and others
Avalon Motorsports Avatar
Avalon Motorsports Robby Bork
Buffalo, NY, USA   USA
1962 MG MGA MkII De-Luxe "The Bruiser"
I agree with Dave.
I will have to check notes when I get to the shop but I agree on the F7 emulsion tube and the chokes seem fine. Mains are a little too lean (they control fuel flow) and correctors are a little to rich (they control air flow). I run 170 and 190 but can't remember which is which but with a very similar engine it gives me an A/F ratio of 11.5-12 under acceleration which gives me maximum power.

Robby

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fast-MG.com Avatar
fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
I don't think float height would show on fuel pressure gage but could affect mixture. What pressure are you running? It's been awhile since I bought any new carbs but IIRC, the last one I bought had a brass float. It's needs to be checked.


Member Services:
Dave Headley, dba FAB-TEK offers full service race car parts and preperation for MGB & MGA race cars, SCCA and Vintage. Dave is a mechanical engineer and has raced MGBs since 1963.
Avalon Motorsports Avatar
Avalon Motorsports Robby Bork
Buffalo, NY, USA   USA
1962 MG MGA MkII De-Luxe "The Bruiser"
What fuel pressure did you run? Webers like about 2.5- 3.3 lbs.

Speedracer Avatar
Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
In reply to # 3508387 by Avalon Motorsports I run 170 and 190

Robby

Main jet, or air corrector?



Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com


Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Free Tech Advice - Alloy wheels for British Sport Cars,and others
Avalon Motorsports Avatar
Avalon Motorsports Robby Bork
Buffalo, NY, USA   USA
1962 MG MGA MkII De-Luxe "The Bruiser"
mains:190
air corrector: 170

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Speedracer Avatar
Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
I forgot I had another Weber here, for another vintage customer he ran it on a 14.0 to 1 MGB engine, it was previously a SCCA engine so .040 over, it has 40mm chokes, F16 emulsion tubes, 200 main jets and 220 air correctors.



Hap Waldrop
Acme Speed Shop
864-370-3000
Website: www.acmespeedshop.com
hapwaldrop@acmespeedshop.com



Edited 1 time(s). Last edit at 2017-05-10 05:24 PM by Speedracer.


Member Services:
MG/ Triumph Performance Street/Race Engines - Cylinder Head Porting - Modified SU HS Carbs - DIY Engine Rebuild Kits With Free Tech Advice - Alloy wheels for British Sport Cars,and others
hoffman900 Bob Adams
USA, USA, USA   USA
1978 Yamaha MC TT500 "Flat Tracker"
Hap,

I was pretty much taught that the emulsion tube sets the curve, an air corrector change is equal to half a main jet change.

The main and air corrector are going to be variable and contingent on the emulsion tube. A good friend of mine likes the F2 emulsion tube.

Not sure how this will translate for your engines, but something like this works very well:
F2 emulsion tube, main jet in the 170-190 range, air corrector is going to be variable, idle jet around a 50-55, and a 2.5 on the needle and seat. Fuel pressure around 3.5psi.

Your accelerator pump cam will need to be worked out and ideally he'll need to data log the A/F ratio while on track.

As you stated, your customer really needs to get on a dyno that can log the A/F ratio. A DCOE Weber will allow the engine to idle MUCH better than a SU. Most engines like to be 12.5-13.1 for best power, but that's also contingent on a lot of things and the location of the O2 sensor.



Edited 9 time(s). Last edit at 2017-05-10 08:30 PM by hoffman900.

Avalon Motorsports Avatar
Avalon Motorsports Robby Bork
Buffalo, NY, USA   USA
1962 MG MGA MkII De-Luxe "The Bruiser"
In reply to # 3508779 by hoffman900 Hap,

I was pretty much taught that the emulsion tube sets the curve, an air corrector change is equal to half a main jet change.

The main and air corrector are going to be variable and contingent on the emulsion tube. A good friend of mine likes the F2 emulsion tube.

Not sure how this will translate for your engines, but something like this works very well:
F2 emulsion tube, main jet in the 170-190 range, air corrector is going to be variable, idle jet around a 50-55, and a 2.5 on the needle and seat. Fuel pressure around 3.5psi.

Your accelerator pump cam will need to be worked out and ideally he'll need to data log the A/F ratio while on track.

As you stated, your customer really needs to get on a dyno that can log the A/F ratio. A DCOE Weber will allow the engine to idle MUCH better than a SU. Most engines like to be 12.5-13.1 for best power, but that's also contingent on a lot of things and the location of the O2 sensor.

Good information too. Heard F2 works well, think its close to F7 as they are not numbered based on flow rates but design order.

Steve64B Avatar
Steve64B Steve Opitz
Phoenix, AZ, USA   USA
1966 MG MGB
Hap,

There is a user group on yahoo called: sidedraft_central@yahoogroups.com

There's some great input there, a library of very valuable information and the guy who runs the site sells a tool for setting the float level that's very accurate and let's you check without taking the carb all apart.

mgracer38 Avatar
mgracer38 James Darby
Rugby, uk, UK   GBR
My race engine is

83.5mm bore (1950cc)
Big valve head (Fab tek actually from Tom Brooks)
Fab tek Rockers
Piper Cam
12.5:1 Compression
Running Split 50 DCOE.
42mm Chokes
220 Main
150 Air
F16 emulsion tubes

Its worth having a look over at the yahoo group mentioned above. The guy, keith Franks has done lots of work on the tubes and jets. He has a design of some special tubes for racing which are suppose to give a much flatter fuel curve. I have a set to try but not at the rollers for another 6 weeks.
Attached is the power run from last year at peter Burgess. The fuel curve is typical of what you get that I want to get rid off.


Attachments:
(218)MGB.pdf    206.3 KB

hoffman900 Bob Adams
USA, USA, USA   USA
1978 Yamaha MC TT500 "Flat Tracker"
That fuel curve is pretty ugly, but I don't notice any holes in the power curve.

One thing to remember, an exhaust and intake of the wrong tune lengths will cause reversion in exactly the area you don't want in the power curve. There will always be reversion and the more "in-tune" a system is in one area, the more it's going to be out of tune in another.

Remember, a carburetor will flow gas both ways.

The other thing is jetting and hp are not always in relation. What do I mean by that? Well, the more efficient the port and exhaust system, typically the less jetting you need per hp because the booster signal is stronger. Think about that for a bit.

Lastly, it helps to not think of circuits like the "idle circuit" as the name implies. Think more throttle opening position. The idle circuit controls the first bit of throttle opening before it transitions to the main circuit. It has a big impact on driveability and part throttle response.



Edited 1 time(s). Last edit at 2017-05-11 09:11 AM by hoffman900.

Steve64B Avatar
Steve64B Steve Opitz
Phoenix, AZ, USA   USA
1966 MG MGB
Hap,

This was written by the guy who runs the Side Draft group, it's a great read. It is primarily focused on using a DCOE on a street car because it has information on idle and the transition from each circuit to the next, I don't think you can have too much information.


Attachments:
Weber_DCOE_Tuning_White_Paper.pdf    242 KB

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