Proper jets for Weber 45s on a Cosworth 2.0l YB Red top

[Jacques]

Trying to smooth out the carbs. Getting some popping at various times but consistently after coming off the throttle.  Is there a site that specifies which jets and tubes work best with this engine?  Unfortunately I don't know if I have a standard cam.  Ideas?  James.

[inchoate]

It's probably worth giving the carbs a good cleaning before you start messing with jets etc.  I don't know what gas is like where you are but ethanol is very hard on carbs.

[Jacques]

Have been cleaning jets, they seem okay.  Any input on proper sizing for this engine?

[Croc]

I have too many webers as it is - annoying things.  I think I have 9 lurking in my fleet currently.

 

First thing to do is install a fuel cut off valve in a readily accessible location.  When you come back from a drive, flip the valve, and let the engine run until dry.  This way the fuel does not leave residue or clog up the webers over time - a real issue with the current fuel blends you find now.  Yes it takes more time to start but you will have many years of not needing to tune webers every 6 months once you have the correct tune after the process you are currently going through.  

 

Full disclosure - I have never had a weber on a YB Cosworth.  My YB is throttle bodied with injection.  I only know them from BMWs, Lamborghinis and Holdens.  

 

For the YB Cosworth I have heard everything from 40, 45, 48 and 50 DCOE.  45 and 48 are the most common.  So you are consistent with others there. 

 

Going from the Weber factory manual dating from the early 90s I have in front of me (should not have changed much?), your baseline when you buy a 45 DCOE is normally the following - generic for all engines:

Main venturi 36

Aux venturi 4.5

Main jet 145

Em Tube F16

Air Corr Jet 155

Idle jet 55F8

Pump jet 45

Pump exh 40

Needle valve 2.00

Choke  Manual - I would have thought you should have 34mm or maybe 36mm tube?

 

A baseline was intended to provide a workable tune out of the box.  May not have been the best tune but it should work reasonably well to get you to the next stage of tuning.  There is no standard set up for a YB Cosworth - they never ran Webers in production and so the individual installations are all specialized to that car.  

 

What have you got in there now?  Have you disassembled the carbs down fully and inventoried your existing set up?  Fully cleaned once disassembled?   Are they the same in each carb?  If not then that is the next step.  

 

This is the type of topic where I am not going to be able to help you remotely.  I strongly recommend you find a local weber specialist to help you tune what you have.  One wrong jet or component is the difference between a good and bad tune.  You really do need to have a lot of specialist knowledge to do this properly.  I have a specialist work on my many webers as there is so much to this art that I do not know.   Fortunately there are many specialists out there - look for old Alfa mechanics or a classic Ferrari/Lamborghini specialist.  

 

If you do wish to do this yourself then I would buy the Haynes Manual on Weber Carb Tuning.   

Weber/Zenith Stromberg/SU Carburetor Haynes TECHBOOK (Haynes Repair Manuals): Haynes: 0003834510240: Books (amazon.com)

 

 

 

 

[DavidL]

Popping on the over-run could also indicate avacuum leak in the exhaust.... air gets drawn into the exhaust and any unburnt fuel ignites, creating the pop.  My Caterham does this as well, and I'malso trying to solve it.

 

[Jacques]

DavidL,  My headers are not a particularly attractive set up.  I haven't spent any time considering them as a contributing factor but it's possible.  I figure banging out the exhaust indicates a rich mixture.  The distributor timing is suspect as my dizzy was originally a Bosch vacuum-equipped model.  The vacuum has been removed and welded over/blocked off.  My thinking is I'm not getting enough advance so I'm looking at a Burtonpower unit or an Accuspark product; the idea being to resolve the timing concern then devote time to the Webers' tuning.  

 

I'm interested to hear if anyone has experience with Accuspark vs the Burton Power (FT295K).  Accuspark is much less expensive which raises questions.  

[MV8]

One product is a points converter and the other an ignition amplifier. Neither of those products controls timing advance. Take a picture of the distributor with the cap off. Does it have a plug on the body to connect the harness or does it have a short harness built into it? One has a mechanical advance and the other is computer controlled. If the rotor can be rotated a few degrees by hand then springs back, then it is a mechanical advance system. The distributor can be taken apart to clean and lubricate the weights or change the springs to alter the timing curve. Total advance is also "adjustable". Dual carb typically remove the vacuum advance and screw on a block off plate to keep the distributor clean.

[Jacques]

Distributor has a small plug outlet on it; a 2-lead pigtail attaches to it running back to ECU.  Distributor has a mechanical advance.  Question - because the vacuum advance has been disabled, does that not reduce the amount of advance this distributor can provide? 

 

Another question - the engine has the bracket on right side for the turbo, so I assume it's a YB, now non-aspirated running Weber 45s.  If it was normally a turbo, would it have different cams, ECU, aluminum block, etc?  I'm researching this using serial number.  If anyone knows best websites for this, please advise. 

[MV8]

Yes, the total part throttle timing could be more but you can't use it except for mpg if the initial and mech adv are set up correctly. I expect it never had vac adv with dual carbs and the distributor was built for that. The vac adv usually advances (typically limit to 6deg or so) under light loads only and pull timing back under WOT to prevent detonation. Mech adv should be all in by 3000 rpm.  

[MV8]

For the popping, if it is a result of a very low restriction/loud exhaust with little to no muffling, you could use a more restrictive/quieter muffler or possible repack, fit anti reversion cones at the head flange or use an exhaust tip insert.

 

If you think the timing isn't what it should be, check it with a light. One that also displays rpm would be best. They start at around $75.

[Jacques]

MV8 - I wonder about the exhaust.  It looks pretty home-made - just 4 to a big 1.  Between those and the Webers, I wear earplugs.  I don't know how well this unit matches the engine.

[MV8]

Thanks for the pic. While the design doesn't help the output, it looks like it fits well and will last a long time. I expect the muffler outside the frame could be greatly improved. You can post or probably send me a private pic through the forum. I'd like to see more of the car. A few months ago I created a list of suitable round mufflers that could be covered or coated for aesthetics. Your hearing is much more important. I expect the wind noise is still the loudest thing at speed.

Long tubes would be much better for power and scavenging but will likely be louder with thinner wall tube, radiating more heat into the compartment than what you have now.