Brother

Can anyone talk me through the tuning of the set on my 914?

I'll take pictures and make this a tech topic.

I have the books and downloaded the stuff of the redline site, but I have NO carb experience. I am fairly handy.

jimbo3

Hi Paul-

Here is a pretty good synopsis of carb tuning by Jon Lowe from the PCA site. As he says in the penultimate paragraph, it sounds more complicated than it is.

START OF ARTICLE-

To synch your carbs properly, you need a air volume measurement device, such
as a Uni-Syn, available from various Panorama advertisers. This measures how
much air each throat is passing at idle. (Edit- Try the 3' long tube described in the Weber manual you have- Jim)

To start, warm up the engine to normal operating temp., ensure that timing
and dwell are to specs and shut the engine off. Leave all of the plug wires
on; I'm not sure why your were removing them. Then disconnect the cross
linkage between the two carbs. Loosen the lock nuts on each air
bypass/volume needle and gently tturn the screws in all in to the stop, and
snug the lock nuts. Since your car is already been running, leave your idle
mixture screws alone for now. Start the engine at idle. Alternating between
the idle speed screws, adjust the idle speed to about 950 rpm. When you are
done, screwing in one screw even slightly should advance the idle a little.

Using the synch tool, find the carb throat that is showing the highest volume
of air passing through it. You will use the idle air bleed screws to make all
of the others match the one that passes the highest volume of air. You will
leave the bleed screw for the throat that shows the highest volume shut. You
will have to go back and forth between the carbs and throats with the
synchronizing tool and the bleed screws until all of the throats are pulling
the same volume of air. As you do this, you may have to readjust the idle
speed screws, as adjusting the bleed screws may cause the engine to idle
faster. When you have all of the throats pulling the same volume of air, you
can adjust the idle mixture screws. Be sure to snug the lock nuts on the air
volume screws first. Slowly turn each of the idle mixture screws in, until
the engine rpm drops slightly, and the engine starts missing on that
cylinder. Then turn each one back out slowly until the rpm just picks up
again, and open about 1/2 turn beyond that point. Do this for each cylinder.
As you get the mixture correct, you may have to adjust the idle speed screws
again. Use the synch tool to make sure both carbs remain matched.

After you get all of the carb adjustments correct, reattach the cross linkage
to the carb closest to the throttle cable first. Make sure that attaching it
does not change the idle, and that all of the slack is out of the cable.
Adjust the linkage until the connection goes on without affecting the idle.
The adjust the linkage at the attachment at the other carb so that it
attaches freely, and does not speed up the idle. Double check everything with
the synch tool, to make sure you didn't inadvertantly preload one carb or the
other with the linkage. (Edit- Try measuring each side at around 2400 RPM or so to see how they sync up as they should be in sync at this point, too. -Jim)

This all sounds more complex than it is. It is a somewhat iterative process,
but it is very important to make sure the carbs are balanced at idle.

A good description of how to do this is in the early 911 shop manuals, or in
the factory manual for the 914/6. The dual 3 barrel carbs used for the 911s
are adjusted very similarly to what you need to do for your carbed 4 cylinder.

Jon Lowe
PCA Tech Committee, 914
13 May 2002


END OF ARTICLE

-Jim

Brother

Excellent, Jim. Thank you. I'll keep everyone posted. I have one project in front of this one -- I need to get the driver side seatbelt to retract. Then I'll do this. I have a synch tool, but I'm not sure it works. It came with the car and it looks a little beat up.

Is there anyway to tell the size of the venturi without disassembling the carb?

Art Thraen said I should use a 32mm with my engine.

jimbo3

I think you have 32's in there. I had 32's and 36's- whichever is in the baggie, the other is in the carb. There was a tech article in a recent "Excellence" that reviewed the math for determining venturi and jetting. I'll see if I can dig it up.

jimbo3

Ooops. I should have said that you have 32's and 28's, I think. It's easily checked without pulling the carbs off. Remove the air horns and filter base by taking off the four nuts. Remove the two brass emulsion tube holders near the center of the carb. Remove the five screws (and washers) that hold the top of the carb and lift off the top being carful to not tear up the gasket. (The floats tend to get tangled up with the gasket) Loosen the locking nut and screw that holds each venturi in place. Lift out the venturi. Size is marked. Installation is reverse, but don't over-tighten the emulsion tube holders, just snug them up.

-Jim

Brother

Okay, I'll check. I have some in a baggie. I'll check to see what they are. I didn't really know what they were until now.

I'm assuming the venturi is a cylinder that looks like a 1.5" section of pipe?

jimbo3

Yes, that's the venturi and the size is cast into it. There are some main jets (fuel and air correction) in the bag, too. You should be pretty well set with the current jetting. Check your Weber book to ID parts under the IDF section. Tuning is outlined in one of the first couple of chapters.

Matt Romanowski

One thing to remember is make sure your linkage is perfect. Get them so that all the geometry is correct, part throttle and WOT is even flow. That is where most people have the biggest problem.

Jay Gratton

Going off of what Matt said I battled problems with my Webbers for years. In the end it came down to my linage just being out of wack. Good luck!

Brother

I checked during lunch, and I have four 28mm venturis in the baggie, Jim. So the 32mm venturis are installed.

yarin

sold mine and built a megasquirt system

I know it doesnt help you at all, but i went that route. not a carb person myself.

gluck

Brother

UPDATE!!!

I may not have any issues with the carbs. I wanted to wait until I got some new wires and plugs installed before I tuned the carbs. I installed a new set of bosch wires and W8 plugs. WOW! I must have been nearly dropping a cylinder before. This thing hauls the mail.

I suspected the wires after using my inductive pickup on my DMM. On the number 2 wire, the RPM always showed lower than the others. All of the others were within 1-2 RPM and the #2 wire was about 20-40 RPM lower. I thought it could be noise, but apparently not. The last set of wires all bet fell apart when I pulled them out.

So, I went to start the car which is usually a 4-5 attempt affair. Fired up on the first try - also the first start of the day so it was completely cold. I took it down the street to go run it up a little. All of my misfires are gone. I think I'm probably still slightly lean, but damn what a difference.