Need carb advice 45 DCOE's

[Zedyone_kenobi]

I have a set of Weber 45 DCOE 15/16 on my 2000 roadster.

Okay I know it is not Z related, but the topic of tuning webers is near and dear to our heart, and the Roadster guys seem to have the Mikuni or go home mantra rolling.

My biggest issue is with the idle jets I think.

A little background first.

My 2000 has a B cam in it (which is suited for solex/weber/mikuni), and has a bit more compression than stock, say 10:1.

The front carb is a 45 DCOE 15 and the back is a 45 DCOE 16. Apparently this combo 15/16 was popular on BMW's so I am not so worried about it. The progressoin holes look identical on them as far as I can tell.

Not sure what chokes are in the body of the carbs.

I am currently running 55F9 idle jets.

The mixture screws are only out a bit over half a turn from fully closed (seems wrong). The car stumbles slightly off idle at part throttle. But runs smoothly in 3-5th gear. Upon deceleration in gear the car pops badly from the exhaust (header into a straight pipe). Not a nice blurble, I mean BOOM, pop pop bang. If I richen up the idle mixture screws, the popping goes away entirely upon deceleration, but the off idle missing and stumbling is horrendous.

Right now I am driving it with mixture screws about a half out from fully closed.

My plan of action is to get a leaner idle jet, say a 50F9, and then try to open up the idle mixture screws more to the 1.5-2 turns I usually achieve.

Any thoughts?

[240260280z]

The off idle/cruise is the first thing to address. It is primarily the progression holes that are operating then. The two interactive parameters are fuel level and idle jet selection.

I have found that the fuel level at 25mm down will cause the mains on a 240z to come in too early and cause an overly rich progression (40DCOE with 30mm chokes).

Try removing the mains and plugging the (spill jet/pump exhaust jet/ pump bleed) with a bolt then try to get an idle jet and fuel level that works.

Also inspect the throttle plates sealing and their locations with respect to the progression holes.... you know the drill

Have fun!

btw closing the throttle plates at high rpms (over run) should draw only fuel from your idle enrichment hole. The symptoms you mention above may be caused by the choke circuit (if you have one).... sometimes the plungers do not seal properly.

[240260280z]

btw only the later DCOE's run at 2-3 turns out on the idle enrichment screw. The early ones run 1 turn or less due to a different taper in the screw.

[Zedyone_kenobi]

Thanks Blue, I could not remember which ones ran at 1 turn or less and 1 turn or more.... The thread pitch changed at some time. I know the mains are at least close. I will go and pull the jets to see what I have. I need to yank the carbs to at least see what the chokes are.

Just did some checking on the Webers. Upon removal of the covers to the progression holes, the butterfly uncovers the first hole the instant you move the throttle, which is EXACTLY how it is suppose to be. I was a bit shocked it was so dead on. Checking the float level, as I learned with my 240Z was critical, so assuming the DCOE 45 was the same as the DCOE 40, I used the Keith Franck float level checker which measures exactly 25mm. Both my carbs float levels are too high by about 3 mm. That may explain why unscrewing the idles past half way makes the car want to die from being do darn rich. First step is to get the floats in the right place.

Then I will report back, but I have to find more carb gaskets.

Oh this is what she is running.

2000 cc engine

Mains: 125

Idle: 55F8

Etube: F9

Air C: 150

The mains and the air corrector look way to close.. but I am not there yet. I would think she would need a bigger main jet. We are talking about 500cc per cylinder.

The 55 F8 is a good median choice. I cannot mess with either until the fuel level is perfect though.

Edited May 30, 2014 by Zedyone_kenobi

[240260280z]

I would set the floats for fuel level at 27mm down. I think Frank Keith's 25mm is too high.

When you look at the configuration tables in the back of the Weber books, you can see where part of the carb set-ups includes varied fuel levels (even for 2 litre 4 cylinder engine applications, fuel levels can vary) ...I am sure this is for the reason why I see problems at 25mm for 40mm's on a 240z. 1 size does not fit all.

I hope to plot A/F against float level tomorrow on a 510. I'll share the data here

Edited May 31, 2014 by Blue

[Zedyone_kenobi]

did you say DATA!!!

DROOL

[Zedyone_kenobi]

AS luck would have it, I happen to be looking through my Weber carburetors tuning tips and techniques book and it happens to have a cutaway from a DCOE 15/16 and how to check the float level on those particular carbs. Man, the usefulness of books and literature is just staggering in this hobby.

[240260280z]

What chokes are on the webers?

Edited May 31, 2014 by Blue

[Zedyone_kenobi]

The main venturi is called out in my weber book to be a 38mm for a 45 DCOE 15. I know I have a hotter cam which would need more airflow if the head can supply it adequately. I think on the roadster forum there is a flow bench test of the U20 head. Each barrel of the carbs must feed a full half liter, a whopping 500cc, where as in the L28 I have in the Z each barrel must feed only 466cc

go down to a L24 and naturally it drops to 400cc

Going to a smaller choke size may increase my sensitivity to a throttle inputs. But I will leave that alone, as the 45 DCOE 15 was made originally on the 1800 BMW sedan. So while engines differ, my hot 2.0 liter should be able to handle it within reason.

Now I still need to verify the chokes are in fact 38mm and now some 40mm monsters.

[240260280z]

Great! Good luck today...here I am checking my daughter's 98 Rav4, P0171 (Lean code) but plugs are pig rich sooted.... now to investigate

[Zedyone_kenobi]

UPDATE:

Well I managed to find time to pull off the float bowl covers from my Webers. Unfortunately, both floats were adjusted perfectly according to what my weber book says for 45 DCOE 15/16

So I went ahead and changed out the 55F8 idle jets for 50F9. I think this should make all my problems worse. Hopefully I am right. I am more interested in getting applicable results than fixing the problem. What I need is an Innovative LM1 in my roadster. That will happen in July. June is already spent.

Anyway, if I am right, I should have crazy popping from my exhaust on deceleration and then also bad off idle performance. If this is the case, then I know lean is my culprit and I can start the process of bringing her back up

I have a set of 60F9 jets that are up next. I will report back to this thread when I get my driving review of my 50F9's.

Oh also, the threads on the 45 DCOE 15/16's are very different. A 1/16th of a turn makes a difference on these things. Very interesting.

[240260280z]

I think the taper of the needle is the key difference. Can you measure the thread pitch on your 16/17?

Here is a bit of of research I did:

http://www.classiczcars.com/forums/carburetors-s30/50073-idle-enrichment-screws-flow.html