Triple sidedrafts - last carb running bad (video)

[frogish]

Hi everyone,
Just recently completed my conversion to triple DCOE.
Have run into some issue with the last carb, and I am hoping someone here can offer som guidance.

The carb does supply fuel to the cylinder as the spark plugs are wet when i pull them out. I do have spark, although I will be buying new wires. and rotor+distr cap.
I removed some of the jettings to see of they were clogged but have not fouind anything yet. Still haven't removed the spray nozzle.

Thanks for any kind of input!

 

[Stevehose]

Why do you have the rear carb unattached from the linkage? Opening that without the others is not going to give you much info. Attach all three to the linkage and synch them at idle and 2500 rpm. Often times a rear carb does not fire because the throttle plates are not open enough at idle, so open up the throttle on the last carb until the 5&6 cylinders fire, you can verify this by pulling a plug wire on each cylinder, if the idle goes down when pulled the cylinder is firing, if not then open the throttle plates a little more with the screw until it does. Basically, the rear carb is not getting enough air. And often the reason that #3 carb is the problem is because it isn't as open as much the other 2.

 

[frogish]

I deattached the rear carb when I noticed it wasn't firing, by pulling wires, and tried what I did in the video. Did the same thing for the front and center carb just for reference, and they were much more alert. I have previously only synced them at idle. It makes sense what you say about access to air, I will try that.

 

[frogish]

Opening up the throttle plates a little did the trick, thank you Stevehose!

 

[DWMBMW]

Steve,

Is this because the last carb is the first one being fed from the fuel line?

I have the fuel line feeding the first carb on my set up.

Doug

 

[Stevehose]

No I think it's probably linkage related, it's farthest from the pedal linkage attachment point (usually) which is between #1 & 2 carbs, so my theory is the first 2 carbs open pretty much instantaneously together whereas the 3rd is susceptible to some sort of torqueing delay or bearing slop. So when synching it's improtant to watch that carb #3 is opening with the others and getting enough air at idle to fire the plugs.

Steve,

Is this because the last carb is the first one being fed from the fuel line?

I have the fuel line feeding the first carb on my set up.

Doug

 

[Ohmess]

Hey Frogish -- where did you get those short socks?

 

[frogish]

Ohmess, these are Pipercross filters (I believe #C1050) which I have just cut to length. They are about twice as long originally.

 

[Ohmess]

Thanks!

 

[frogish]

I still have som problems with the carbs.
The car runs good when driving normal/cruising with slow or moderate acceleration. If i try to accelerate fast, the engine starts to stutter and I have to back off on the throttle and go more gently.

It's a 2.8 with 45's.
Jetting is:
Chokes 36 mm
Main jet 135
Air jet 170
E-tube F9
Needle 200
Acceleration 40
55F8

If anyone has input on this little information, it would be great

 

[m5bb]

45's are big for a 2.8 motor. Did they come with 36 chokes?
I have 45's on a 3.6 liter motor with higher compression and a cam.
I don't have my numbers here in front of me but I did have to go to a smaller choke because there just wasn't enough ventura effect to draw the gas through the jets.
Here is the best investment you can make for Weber tuning.
http://www.piercemanifolds.com/product_p/98.1517.05.htm
Great source for jets and all the stuff you need.

 

[Stevehose]

I agree with Gary, your chokes are too big for the engine, so when you open the throttles quickly you're losing the venturi effect which draws fuel in along with the air. Typically a stock 2.8 engine would call for DCOE40's and 32mm or 34mm chokes. Try replacing the 36mm chokes with 34mm, if it still hesitates then try 32mm although that's pushing the bottom limit for 45's.

 

[zinz]

The tuning is the fun part! Sort out your ignition before everything, though. Proper timing is essential with sidedrafts. (Steve has some very good #s developed on his setup)

Here is a wonderful file on tuning Webers with LOTS of good info. http://www.lainefamily.com/images/WeberTuningManual.pdf

Where is your fuel level set? This is extremely important on setting up a DCOE.

I think you are going to also have some airflow troubles with those filters being situated right at the mouth of the velocity stack. I think the accepted standard is about 1" of gap above the mouth, or something like that... more is better, regardless. You want as much undisturbed air at the mouth of the trumpet as possible. I understand the problems of fitting everything in the small space, and one must absolutely run a filter. For experimenting...perhaps remove all the filters and see if you get any improvement. But I think you will have troubles with the filter too close to the mouth of the stack.

I Googled some things on "velocity stack standoff" : http://www.enginelabs.com/news/engine-theory-quick-fuel-standoff/

Here's a decent discussion on VWs and velocity stacks and filters: http://www.shoptalkforums.com/viewtopic.php?f=15&t=122047

Tuning sidedrafts must be done methodically; change one thing at a time and keep notes. If you don't have a wide band O2 meter, check your spark plugs regularly for color/condition to check for rich/lean mixtures.

Good luck!

Ed Z

 

[frogish]

Thank you guys.
Yes the carbs came with 36 mm chokes.
I ordered a set of 34 mm chokes now.

I went with 45's because my intake (mangoletsi) is designed for 45, although I had heard that the most common choice is 40.

My float level is set at 14 mm.
Will also try to remove the filters once!

//Erik

 

[Stevehose]

The best way to check the float level is to take out the main jet stacks, fill the float bowls by running the fuel pump, then make a dip stick out of cardboard etc and insert it down the hole until it just touches the fuel. This should be 25mm from the top level of the hole.

 

[Ohmess]

Frogish - I agree that your venturis appear to be too large. This would be consistent with the stumble, because it appears your engine is not pulling enough vacuum to sustain quick acceleration at high load.

After you change the venturis, check your fuel level. I have found my carbs are very sensitive to fuel level. Following Steve's advice, I made up a crude measuring tool using a cut off zip tie and with a sharpie black line at 25 mm. I remove each jet stack, shine a flashlight into the hole and dip the zip tie in the center, then check the fuel level relative to the black line. This is somewhat crude because of the displacement of fuel caused by dipping into the well, but I am living with it for now.

 

[zinz]

shine a flashlight into the hole and dip the zip tie in the center, then check the fuel level relative to the black line.

I have found this method superior to dipping paper or cardboard into the jet well. With a bright flashlight ("torch" for those across the pond ) you can see the fuel ripple as the dipstick touches the top of the fuel. I also agree that sidedrafted engines are quite sensitive to fuel levels.

Frogish... you stated the rear plugs were wet with fuel...check all plugs and compare. A WBO2 meter is great for tuning and such, but unless you have one on each exhaust runner, it won't tell you which cylinder is misbehaving; spark plugs are best evidence of individual cylinder's running condition.

I agree with others about your choke sizes, but if fuel level is too high, it will over-fuel at low rpms, run rich, and delay throttle response. In this instance, I prefer the Solex side draft design as you can adjust the fuel level externally; greatly simplifying this step.

It's a 2.8 with 45's.
Jetting is:
Chokes 36 mm
Main jet 135
Air jet 170
E-tube F9
Needle 200
Acceleration 40
55F8

I checked my old Weber Manual that has a host of settings in the index for many different engines. The closest engine-size I could compare to was an Alfa 2600 Sprint Coupe. Here are those settings to compare to yours:

2584 cc
45DCOE 9
Choke - 36mm
Main jet - 145
Air jet - 155
E tube - F16
Needle - 200
Accelerator pump jet - 45
Starter jet - 60f5

I will close with my #1 recommendation for side draft tuning. Ignition timing... side draft engines require additional ignition advance, especially at lower rpms. Where is yours set? What distributor are you using?

Hope the info helps!

Ed Z

 

[Ohmess]

Agree with Eds comment on ignition advance. I just moved to 15 degrees at idle (kicking in at 500 rpm so as not to make starting more difficult).

 

[JFENG]

FWIW, way back 30 years ago I had a set of 40DCOEs on a 3.3liter M30. I went through a lot of parts getting them dialed in and 34mm was as big as I could go and still have good drivability on the street. 36mm chokes seem way too big for street use on a stock or stage 1 motor at 3liters or even 3.2liters. Maybe it's fine for 3.5 displacement?

I don't know what your engine specs are so my experience may not apply.

 

[frogish]

Just recieved my 34mm chokes today, will hopefully be able to install them tomorrow and try them.

My engine is original apart from the carburettors, but I plan on getting headers and different cam in the future.

I use original distributor from -69 (I'm away from the car so can't see a number on it) with new breaker point, cap, rotor and condensator + new wires and plugs. When advancing the ignition, will simply turning the distributor be enough or will it need to be recurved?

Again thank you for all your input, even though I don't answer it all!