Triple Weber (45 DCOE 152) High Gear BOG!

[Guest ironhead]

Hi - Before I describe the problem I’m having with my webers let me go ahead and tell you that I have searched the archives and read both weber books (one by Pat Braden and the other by Des Hammill).

 

Engine basic specs:

2.9L

N47 Maxima Head (big valves, mild port and polish)

Mild cam (.460 lift 272 duration)

Headers (2.5 exhaust)

Mallory Unilte (Crane Ignition and Coil) Timing = 14° initial 34° total

 

Current Carb Set-up:

45 DCOE 152 (Cannon Intake)

Chokes 36mm

Main 150

Air 180

Idle Jet 60 F 9

Pump 55

E-Tubes F 11

Idle mixture 2 ¼ turns

 

I’ve experimented with many jet combinations (below are the ranges)

 

Mains 135 ~ 160

Air 170 ~ 205

Idle Jet 45 F 8 ~ 60 F 8 and 45 F 9 ~ 60 F 9

Pump Jets 35 ~ 60

E – Tubes F 16 and F11

Timing 10° ~ 16° initial (32° ~ 36° total)

 

So far the best combination is the current carb set-up

 

The problem:

The engine idles good

There’s no hesitation or bog at take-off

The engine revs smooth and quick in 1st through 4th gear

However, in 5th gear it bogs at around 3000 rpm – about the time the cam comes in.

 

It improved when I replaced the F16 E-Tubes with F 11 E-Tubes. There was less bog but it still will not transition smoothly – or I should say it will not transition at all.

 

Question: I know that the F16 e-tubes and F11 e-tubes represent two different families of e-tube design, hole orientation etc., but by going from F16 to F11 did I richen the system or did I lean the system?

 

Like I said before, this only occurs in high gear.

 

I believe my problem is with the idle jet or emulsion tube. I may be leaning out before my mains come in. I think – but I’m not sure. I hope someone has a suggestion.

 

If I need to go to a richer E-Tube, what would I go to?

Or

Do you believe that the idlejets are too small and unable to supply enough fuel at this point before the mains come in?

 

Has anybody experimented with F9, F7 or F15 e-tubes?

 

When you look at the jet sizes I’m currently using, the first thing someone is going to say you’re your carbs are jetted too rich. They aren’t – there’s no black smoke, the plugs are light brown. I don’t think we should get caught up so much in the numbers.

 

I read an article in the archives that says “all webers are not created equal†and I am beginning to believe that statement. I also have noticed from previous posted comments regarding 45 DCOE 152 webers, that the jetting seems to be slightly bigger than other 45 DCOE webers (18, 19 etc).

 

I am not going to give up. I believe the solution to my problem is just around the corner.

 

Thanks for reading through my post

 

Mike

[Synlubes]

Your setup seems to be "in the norm" for the carb/engine combo.

The F11 e-tubes will give you better response in the mid - mid high range and the F-16 have worked well for me in the higher range.

My setup (similar to yours, but with 32mm chokes) works best with the F-11 e-tubes. Better response getting into the revs.

The 36mm chokes would be recommended for a "very healthy" engine setup operating at the high rpm (4 - 7+) range.

I run 32mm chokes, but think 34mm would be the best setup for a well built Z engine.

 

I run. . .

 

Chokes 32mm (want 34mm)

Main 135

Air 170

Idle Jet don’t remember

Pump 55

E-Tubes F 11

Idle mixture about 2 turns

 

I run the same basic engine specs, but with a N42 cly head.

I have found the 45`s need to be "detuned" abit UNLESS you have a big bore, big cam all out racer turning high revs.

 

I have found one of the best books to be, Weber - Guide to Parts and Tuning.

Pierce Manifolds sells them.

[JMortensen]

I don't know much about the Weber carbs, but that cam seems small for the rest of the engine. If what Doug says is true, sounds like you have chokes for a big cam with a small cam. I say get a bigger cam, but I'd probably say that even if you only had 8.5:1 compression.

[Synlubes]

Emulsion Tube – Its task is to emulsify the previously metered air issuing from the bleed jet with the fuel coming from the main jet.

Its influence is more marked at small and average throttle opening angles and during accelerations.

 

Probably doesn’t need to be said but, make sure the carbs are sync with all the throttle linkage loose.

 

Maybe look at using a smaller air corrector jet

Check the float level settings???

Fuel volume??? not pressure

Maybe some trash in the bowl???

[ezzzzzzz]

If you're not experiencing this problem in other gears in the same rpm range I would suspect a lack of fuel to support the engine load. I would look at the emulsion tube. It is the heart of a sidedraft when the mains kick in. Maybe go up a size on idle jet and retune the carbs too if you think the transistion phase is causing the bog. You didn't say if the engine pulls fine after the bog which would point away from the emulsion tube. Lastly, I think you are over on the choke for this engine. A 34mm would be the max I'd consider with that combo. I suspect your engine would start to peak out on hp here anyhow.

[Guest ironhead]

The feedback has been very beneficial.

 

I must admit, I have been very steadfast in my decision to keep 36mm main venturis (simply because they are expensive to replace). However, after reading most of the replies it seems a better option would be 34mm chokes for my engine specs. So, I am going to replace my chokes, re-check the float levels, etc., make sure everything mechanical is up to snuff.

 

This has truly been a learning experience. Admitadly, my background is not with webers or L6 motors. I'm farely new to this generation of automobile. So it's safe to say at this point, I would not feel confident in telling someone what to do to correct a problem, but I would feel confident in tell someone what not to do. (laugh!) I feel that I have made practically every mistake possible. And believe or not I have read every post dealing with webers on this web site and zcar.com

 

Anyhow thanks again for your feedback. I will post my results after I change chokes.

 

Mike

[AdrianZ]

why do theres carbs have to be such a pain in the butt?

 

[randy 77zt]

a carb works from manifold vacuem.if you open up all those throttle blades you get no manifold vacuem.then you get bog.smaller chokes make more vacuem in the carb.the webers work best at very high rpms when used for some kind of closed course track racing.if you got 1 of those innovate lm1 mixture meters it would help on tuning by data logging the mixture while you drive.but carb guys are usually hardcore anti-electronic types.my choice of carbs for a street z would be some z-therapy modded su's.or efi.some help can be had by running some low gears in the rear like 3.9 and checking the advance curve in the distributor.

[tube80z]

a carb works from manifold vacuem.if you open up all those throttle blades you get no manifold vacuem.then you get bog.smaller chokes make more vacuem in the carb.the webers work best at very high rpms when used for some kind of closed course track racing.

 

Carbs work by creating a pressure drop from airflow, not by manifold vacuum. Put a vacuum guage on a set of tripples and you will see zilch when you open them up. That's what the accel shot is for, to cover that condition until enough airflow makes up for the suddenly lean condition.

 

Webers have so many combos because they were OEM carbs for a number of autos and needed to pass emissions. You won't find the depth of parts for Mikunis.

 

The setup mentioned above sounds like it was originally setup for a pretty hot street motor or autoxer. We used tripples on the street with a 240 and it ran very well. The chokes were 32s and we had a little more cam. This was on a 2.4. When the car was no longer driven to events we went with 38s and a larger cam. It would only pull from hard from the 3500/4000 range to around 8K. It would do the same thing you mention when flooring it in lower gears.

 

Besides checking the air/fuel ratio I'd suggest getting a decent Weber book. I have a factory manual that seems to be out of print but it has the basics to get these setup and working. I seem to recall seeing some good posts on this list if you use search that had much of the same info. Contrary to many opinions once you have these working well the seem to stay that way. Most of the issues I've seen come from carbs that are in poor shape and/or really crappy linkage. Bend a throttle shaft in one of these and it will really make tuning a nightmare.

 

Cary "ah the good ol' days"

[TheNeedForZ]

Both of you guys are right. The idle and progression circuit is controlled by manifold vacuum; the main circuit is activated by airflow and venturi.

[TheNeedForZ]

Here's my three cents about the bogging problem :

 

The problem you are experiencing may be related to air resistance counteracting with wheel torque since you do no have bog at 3000rpm in lower gears. Assuming that when you reach 3000rpm in 5th gear, you are already moving quite fast therefore air resistance is huge. Combine that with the numerically low gear ratio of 5th gear....there is very little NET wheel torque left to accelerate the car anymore. To make things worse, the contour of the Z car roof/hatch is shaped like the top of an airplane wing which creates lift. Lift reduces traction. Here's part of an article that dissects aerodynamics on a Z : "The figure that comes to mind is 120 pounds of lift on the rear of the car, but I don't recall now at what speed that figure is reached, though I believe it was either 60 or 100 mph."

 

If you insist on going that fast, here's what I'd do :

 

change the length of the airhorn/intake path to add some torque at 3000rpm

 

OR

 

Build an air box with a tunnel leading to the front of the radiator, taking advantage of ram air and cold air. This should give you several percent increase in hp compared to sucking hot air in the engine bay.

 

PLUS

 

adding a wing at the read end to create some downforce or add a spoiler on the roof to "spoil" the airflow before lift can be created.

 

Then again...maybe that's nature's way of telling you to stop...Z car is famous for getting "light" at high speed. IIRC somebody said the 5th gear was created for cruising at low rpm on the highway.

[briann510]

If you can get the car on a dyno that will answer the question on the air resistance question and also show you exactly what is going on with A/F and jetting. took us a couple hours of dyno tuning to get our jetting down and set.