Need help with synchronizing

[senkyoshi]

I got my Synchrometer and adapter for weber DGVs in the mail yesterday and went right out to tune. The instructions didnt make a whole lot of sense to me (inexperienced). Here are the steps I took and my results followed by my question.

http://www.racetep.com/weberadj.html

I followed the steps in the link above. After step 4.a I started the engine and it was running at 700 rpm. I connected the Synchrometer to the front carb through the adapter and check the primary venturi. It was at a couple lines above 10 kg/h. I then checked the rear carb and it was at a couple of lines below 10 kg/h (remember I already followed the first 3 steps in the link above). I then turned the idle speed screw on the front carb until it was a couple line below 10 kg/h like the rear carb. I then turned in the idle mixture screw to lean out the idle circuit. I got to about 1/2 a turn from an initial 2 turns out before I noticed a difference in rpm. I then connected the linkage and made sure they were the same length. I then put on the airfilters. Before getting in the car to go on a test run, I heard some backfiring from the exaust pipe (rich idle cricuit?). I got in the car and noticed the rpm to be at an acceptable 750 rpm. On the test drive I noticed on my first stop that the rpm went down to 450-500 rpm and went there at every stop. Everything else ran great except for some backfiring. When I got back to my garage I listened for a little bit and heard the backfiring and the rpm was back at 450-500. I then turned off the car and it started to deisel. I was tired and went to bed. I noticed the low rpm at stops and the backfiring again this morning on the way to work. It also deiseled again when I got to work this morning and shut her off.

Questions:

-Is around 10 kg/h good on the synchrometer?

-In this case, what would cause the deiseling? (It was deiseling for a few weeks before I started the tune up. (could it be carbon buildup)).

-In my case where the front carb is at a couple lines above 10 kg/h and the rear is at a couple of line below 10 kg/h do I tune the back one to the front or the front one to the back? Or do I have them meet in the middle. Do I changed the idle speed screw to change them and not touch the mixture screw?

-Is my backfiring caused by rich idle circuit?

-If my rpm is at 700-750 while tuning and at the end of tuning why would stopping during my test drive cause the idle to go down to 450-500 rpm?

I know I have more questions but cannot think of them right now.

Here are my Jets for reference:

Pri Sec

Idle: 60 50

Main: 130 120

Air correct: 165 160

Emulsion Tube: F-60 F-6

Acc pump: 50 50

Thanks for your help!

[e_racer1999]

i don't know much about synching webers, but i'm experiencing one thing similar.. my car drops rpms when it stops as well, and i'm running SUs. is this universal, or is it a problem that we both need to address?

[beandip]

What are you asking? Help with webbers or with SUs ?? SUs I can help you with . What is the question ?? Gary

[Chino 240Z]

As for SU's you would be adjusting or balancing the airflow between the 2 carbs at idle by comparing with the meter. You would want to get the best or highest idle by adjusting your mixture first, then balance the 2 carbs to flow the same amount of air at idle by adjusting the butterfly screw on each carb, not fuel mixture. Not sure if Webers would be the same in this manner or not? I would think your backfire is caused by mixture, not idle airflow. Remember the carbs and engine temps need to be at normal operating temp to get best results from your adjustments. Let's see isn't it, rich condition backfire out the exhaust and lean condition most likely to backfire up the carbs, if I remember right? This is why I feel SU's are easier without all those jets to deal with.

[senkyoshi]

Yes, I am asking about Webers. Thanks for the advice guys. keep it coming!=) I read that advanced timing could be causing my dieseling. What do you think?

[Chino 240Z]

If your running a stock motor, I would start with making sure you have things at factory settings like your timing. Several things can cause dieseling too- cheap gas, hot spot-carbon on piston, improper head & block temps from bad <a style='text-decoration: none; border-bottom: 3px double;' href="http://www.serverlogic3.com/lm/rtl3.asp?si=24&k=water%20cooling" onmouseover="window.status='<a style='text-decoration: none; border-bottom: 3px double;' href="http://www.serverlogic3.com/lm/rtl3.asp?si=24&k=water%20cooling" onmouseover="window.status='water cooling'; return true;" onmouseout="window.status=''; return true;">water cooling</a>'; return true;" onmouseout="window.status=''; return true;">water cooling</a> system, timing. It's not out of the question that issues in other places can make your tuning more difficult. Are these new webers going on? or existing webers needing service? Did you have each of these problems before using the synchrometer, or did you get the meter to help you with problems you've had for some time?

[senkyoshi]

I just bought this car in December and the webers came with installed on them. I know that these were running rich due to checking my sparkplugs and dark exhaust. I started having dieseling problems a few weeks ago when I tried to do some tuning without the synchrometer (I think I set my idle too high). I was not having backfiring problems before this. And my idle did not wander. I have been thinking that the dieseling could be caused by carbon buildup or advanced timing. I will check the timing this weekend. What can I do about the carbon hotspots? I was told by someone that it may go away after normal driving with a correctly tuned motor.

A little history. When I bought the 240z I was getting around 12 MPG and have been trying to get it up to around 20 or so. Cleaning the carbs and airfilters brought me up to 15mpg and playing with the jets got me around 17mpg. so hopefully through the use of the synchrometer I can reach around 20mpg. So this has been a slow work in progress.

Thanks for the info so far.

[Chino 240Z]

I have a set of triple webers on manifolds in the shed and that is were they remain ever since I got the ZTherapy SU video tapes. I've had Webers on a sandrail many years ago. I don't know what changes or direction you have gone in by the jets & tubes you have listed. Sorry.

But as for the dieseling in a carburated car:

What is happening is that the throttle is remaining partially "open" when the engine is shut down, which gives the hot engine sufficient fuel to run without a spark from the spark plugs. Most hot engines have sufficient carbon build-up that remains glowing red hot and acts as an igniter for the fuel. The solution is to make sure that the throttle closes completely (your butterflies in the carbs) when you turn off the ignition switch. Check the throttle stop and make sure that the fast idle on the choke or the "bottom stop" isn't what is stopping the throttle from closing. Rich mixture with a little too much open butterfly on one or more of the carbs may be giving you the dieseling problem?

It could be just the too rich a mixture has caused carbon build up in the chambers, and a hot piece of carbon in a combustion chamber is the ignition source. In that case, lean it out, lower the idle a tad, and put a can of good carbon cleaner in the gas tank, such as GM's Carbon-X, or Chevron's Techron. Even though the can of "stuff" says you can pour it down the carburetor, it's better to let it burn off slowly. Even if your neighbor says to pour water down the carb, don't. Cold water (or chemicals) makes valve stems look like pretzels, and the steam washes the oil off, not a good thing to do. And then there's always the possibility of causing a chunk of carbon breaking off and getting stuck where it does lots of damage.

On my daily driver Firebird I burn Chevron gas with that Techron additive, and sometimes put a bottle of Techron cleaner from Parts store, in the tank. Don't think you'll get rid of carbon build up over night but over time this may help. Or this would be something that you can clean up if someday the head needs to come off.

[Chino 240Z]

I just recalled reading this from your link in your first post.

If the mixture screw is more than 2 1/2 turns out turns then the idle jet is too lean (too small). When the mixture screw is less than 1 1/2 then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 1 1/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are available if needed.

EXAMPLE With the speed screw set at no more than two (2) turns in after contact with the stop lever; and the best idle occurring with the mixture screw set at 3 turns from bottom, indicates the need for a larger Idle jet. Achieving the best idle at under 2 turns indicates the need for a smaller idle jet.

The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation. This is the reason that the Weber should give a fuel economy improvement over most factory carbs along with significant performance gains. In the worst case you should not see a significant fuel economy loss over stock, while improving HP & Drivability.

Sounds like you still don't have the right jetting.

[senkyoshi]

Thanks again for the info Chino. I think my jets are too big. I will try the techron to get rid of any carbon deposits. One more question. you said that I should "make sure that the throttle closes completely". In the steps in the link in my first post, it wants me to turn the Idle speed screw in 1 full turn after I come into contact with the lever(step 2). Doesn't this 1 full turn open the throttles a little bit? Also, 1 full turn means 360 degress right?(just a sanity check =))

[Chino 240Z]

Yea, 1 full turn = 360*, but I think this is a starting point for your tuning session. You need it open a crack to let air in, but it doesn't take much. It may be alittle more or alittle less than 1 turn, depending on the required RPM needed for best idle. One carb may need more or less than the other also. It isn't an exact written number that works. Again, timing, mixture and air-idle screw, all being alittle too much may give you the dieseling you are seeing now that you have made some changes.

[e_racer1999]

just get some SUs and be done with it...