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DCOE jetting... expert recommendation please

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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
While this isn't MG specific, its in the realm of all your expertise so I'm looking for a suggestion.

So I'm tuning the Bonneville race truck with dual Weber DCOE 50's. Current setup:

idles 65F8
Emulsion F7
Main 180
Air correctors 160

It runs close to 12.25:1 AFR until I hit 4k, so I'm assuming it needs air correctors. At that point it climbs progressively leaner from 13:1 at 5k until I stopped at 7500 rpm (no load) and saw 15:1.

My guess is that jumping down to 120 air correctors and drilling them to suit is in order? I have a set of jet drills. Or would 140's do the trick?

The hard part is that this is just for dyno tuning on Thursday. Then we go to Bonneville and race at 4000', with a rapidly changing density altitude.

What would your game plan be with these carbs? Any help warmly appreciated so we can go set a land speed record in 24 days.



jeff@advanceddistributors.com

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
Jeff, what size chokes and aux vents do you have. Also, I have no experience with F7 em. tubes. Stephen at Pierce may be a good resource to help with tuning these although 50DCOEs are not real common in my experience.


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.
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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
That's where I bought them. I can't read the numbers on the chokes. They're HUGE. Its all about volume. This is an rpm motor with a unique cam, and a head I ported on seemingly forever. The exhaust is proportionately huge, computer modeled. Its a beast, but goes lean on top. And yes all manifold and head and carb surfaces are FLAT. I guess they should be out of bed in CA by now???



jeff@advanceddistributors.com

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
Tell me more about the motor.


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.
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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Datsun L20b. Bonneville. Land-speed race only. Overbored, destroked, Honda bearings, Molnar rods, JE pistons, 15.2:1. Head flows in excess of 230 at .550 lift, no notable turbulence at.700 lift and coil bind. Severe duty Manley valves. 2 1/4 primary exhaust, 48" long open header. Running single point ignition that test past the capability of my best Sun machine at 9650. Lots of other ceramic coatings that are doing wonders for heat control and efficiency.

My question is regarding jetting that goes progressively lean above 4k. Rule-of-thumb type of question.



jeff@advanceddistributors.com

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
Post #1 suggests you are running no load. If so you are still in the idle to main jet transition range, and air correctors not in play. What are your cam specs? PM if need be

Also, what size fuel inlet needles? Fuel pressure?


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.
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pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
You're gonna go richer at 4000 feet so drill a spare set and keep the jets you have just in case. By the sounds of your engine, back pressure won't exist so being under load probably isn't going to change much. It will be interesting to see if does anyway.

Good luck in 24 days!

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Fuel inlet needled .250s.
Fuel pressure experimented 3.7-6 psi with no change.
Stephen suggested trying 140 air correctors to set a baseline and go from those results. Solder and drill. I'm using misc spares I had tucked away, stock are bagged. May need a main jet step up to get where I want to go. He had no concern about landing at 120 air correctors if that's what it takes.

Cam is roughly .600 lift, 309/319 duration, 112 LSA! Cylinder pressures at crank around 200. I was shooting for best combo of tq and hp under the curve, with a high hp peak. I'm expecting to lift the head sooner or later. Nismo's special gasket (have spares) and ARP's finest hardware.

No load, but at 7500 rpm it hits 15:1. I don't suspect loaded jetting will vary much from those numbers - at least not in my experience. I'd like to get it to stay under 13:1 at 8500, because the engine will easily go there, despite the fact we'll try to severely limit any time over 7500.

The next big question is if it will have the power to pull in 5th if we try to use it near top speed, or will we hit a brick wall with gearing and wind?



jeff@advanceddistributors.com

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
I'm confused Jeff. No load throttle opening is very low to achieve high RPM. When running WOT under load, air flow is way higher, causing a much larger depression through the venturi(choke) which controls fuel draw. Also the aux venturi affects fuel flow as well. So did Stephen know what you have for chokes and aux venturiis?confused smiley


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.
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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Don't be confused Dave. Its simple stuff. WOT testing. thumbs up

Yes, Pierce sold me the carbs so they know the setup. Its what they specifically order. Stephen said its common to need small jet changes - nothing major when setting up the 50's, since they're really only good for one thing - WOT at high rpm.

Yes Dave, I was testing WOT AFR. 100% throttle open. I have to get this thing somewhat in the ballpark before hitting the dyno so I don't completely F it up under load on the first pull. At part throttle everything looks solid. At high velocity, high rpm, it looks lean. If I end up at 14:1 top end, it'll probably be about perfect - high 12's or low 13:1 on the salt. But for the dyno I'd rather be rich and make adjustments from there. Its nice that I don't need the idle circuit, so I can use the idle jets to fine tune midrange. I'll always be above 1200, so its an easy way to fine tune 2000-4000 to keep it safe.

The dyno session isn't even being used to get the truck dialed in. Rather, its for me to see what AFR changes are made with specific jet changes so I can stock up for the trip, and to break in the motor a little more before putting race oil into it. I'm just trying to keep it in one piece until we get that funny looking snow stuck all over it. I bet we only make 2-3 pulls. Its also to get the driver in the race seat and get his fear levels calmed a bit. He's only got a total of about 10 minutes seat time in this thing so far.



jeff@advanceddistributors.com

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fast-MG.com Gold Member Dave Headley
Cortez, 4 corners, Colorado, USA   USA
Thanx Jeff, I'm less confused now.smoking smiley


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.
B-racer Avatar
B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Why am I now more concerned than ever after that comment? confused smiley



jeff@advanceddistributors.com

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B-racer Jeff Schlemmer
Shakopee, Minnesota, USA   USA
1950 Willys Jeep Pickup "Ratrod"
1971 MG MGB
2014 Dodge Charger
Well, we ended tuning at Bonneville with 198 mains and 130 air correctors. The lean mixture repeatedly got richer under load, so my initial experimentation paid off and worked - better than stock jetting anyway.



jeff@advanceddistributors.com

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Speedracer Platinum Member Hap Waldrop
Greenville, SC, USA   USA
1967 MG MGB Racecar "The Biscuit"
So talk to us about what you expect for top speed once the bugs are worked out. I know reading Chris Conrad's comments, that this is harder than it seems, and it took him a few years to get up to speed, and he has a very, very smart engine builder, who has done it at the highest levels. I guess one of your biggest hurdles is the areo drag out of the P/U. Isn't the record in the 120s for your class, I think Chris's record is in the mid to high 120s for the 1000cc MG Midget. I think this is like drag racing , oval racing, it always harder than it looks. If you remember Jeff, we met a guy at the Mitty, who wanted to take a 1800cc MGB to Bonneville, and thought with a mild race engine he could break 150, he never got there, and I don't think it was even close to being possible with his plan, or maybe any other plan. No doubt this is harder than it appears, and like most racing, humbling.



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



Edited 1 time(s). Last edit at 2017-08-28 05:37 PM by Speedracer.


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pinkyponk Gold Member Adrian Page
Berwick, Nova Scotia, Canada   CAN
You got that right Hap! I had a 350cc Yamaha 2 stroke when I was a kid. Modifying it to get it up to 120mph wasn't too difficult. Getting it to top 130 was a lot more difficult. It was aerodynamics mostly... and a lot more power. If I sat up, above the fairing it was like hitting the brakes hard.

No clue if Jeff has a belly pan (or if it's allowed) but I bet that would add quite a few mph.

Adrian



Home built Eaton M62 Supercharger with 9psi boost, "stock" high ratio rocker arms, 8:1 compression, Piper 270 cam, ported head, matched manifolds, CB Performance computerized ignition.

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