On Thu, 24 Nov 1994, Andrew Matthew Theurer wrote:


> The carbs are Weber 40 DCOE 18 (three of 'em).  Main jets are 135 and 
> Primary venturies are 32 or 33, I can't remember.  These settings have 
> remained the same, even though a crane 282/292 .450 cam has been added 
> along with a header, 2.25" exhaust, flowmaster muffler, etc.  So I think
> the airflow through this motor has definately changed, but I haven't 
> changed the jets throughout all these additions.  Do you have 
> a recomendation for a main jet size?  These jets are drivable, but do
> bogg somewhat, but I can live with them for daily driving, but I want some
> jets that are suited more for performance.  If so, I hope to make a purchase
> of jets, dist cap & rotor, and the plugs within the next week or so.

What size engine are we talking about here? That seems to be an awfully 
large venturi size for anything under about 3-3.5 liters. On the L28 
motor I referenced we are only running 30 's.  when you reffer to a bog 
do you mean a stumble when you open the throttles, or a "dead spot at a 
given throlle opening and engine speed? the former would go with my 
venturi's are too big theory, the latter would indicate incorrect jetting.

While I'm at this I should give you the "tune your carbs with the 
distributor" talk. Webers like a LOT of initial advance, in the range of 
18-25 degrees, depenmding on motor. They also don't need nearly as much 
total advance as stock dist's have. The dist will need to be recurved to 
get this right. I have posted my curves before but here they are again.
initial 
	(static) 20
mechanical 
	1500 RPM 0
	3500 RPM 14-16
vacuum 
	dissabled

This will give a curve like so;

	4500						*


	4000						*
	


	3500						*



	3000					 *

RPM
	2500				   *


	2000			      *


	1500			  *


	1000			  *	


		0   5   10   15   20	25	30	35

				Degrees

If you want to lay out the curve you have now against this and check. If 
you are using a stock dist, you'll find you have a very different curve. 
The problem with the stock curve is that at idle and just off idle you do 
not have enough advance and at the top end you have too much. The lack of 
advance at idle causes the poor low end and mid-range power and the 
dirvability problems (stumble on accelleration, poor accelleration below 
a certain RPM. Most of the time if you do this plotting you will see a 
point at which the stock curve equals my curve and has the same (or more) 
advance. It is usually arround 2000 and this is usually where the 
accelleration starts to pick up (although this is related to cam profile 
too, and so may not be obvious if the cam doesn't start to work well till 
a higher RPM). Vacuum advance's sole reason in life is to let you run 
shitty gas without pinging, it screws up accelleration by retarding the 
timing when you open the throttles (the opposite of what you want) and 
still gives reasonable light throttle cruise and good MPG by advancing it 
at part throttle. It is not good for performance or mileage on a hard 
driven sports car. If pinging is a problem with the available gas go to 
water injection or a knock control system to fix it, not to vacuum advance.

Back to curves. This is what happens if the initial advance is way too 
retarded (I've seen this dozens of times, its not that uncommon). The 
engine will not idle fast enough due to the lack of initial advance, and 
so you must crack the throttles open farther with the idle speed screws 
on the cabrs. This positions the throttle plates over the transition 
holes instead of the idle hole (in the bore of the carb). The idle 
mixture screws contol how much fuel emulsion is discharged out the idle 
hole, ther is no way to control the amount of fuel discharged out the 
progression holes. With the engine idling on the progression holes as 
well as the idle hole, there is not enough fuel capacity left in the 
idle/progression curcuit to cover the added requirements of part throttle 
and you get a part throttle lean out. Some times the situation is so bad 
that changing the setting of the idle mixture screw will not affect the 
idle (speed or mixture). This is because the engine is idling primarily 
on the progressionholes and the idle hole fuel plays too small a part in 
the mixture to have a noticable effect. If this is happening changing 
idle jets will alter the idle and part throttle operation by riching up the 
mixture, but adjsutments to the idle mixture screw will have little or no 
effect. Whe throttle position is right, 1/2 to 1 turn of the idle mixture 
screw will have a major effect on the idle mixuture and therefore on the  
speed.

I just realized I'm writting a short book here. I think this might be of 
interest to the Z car list and the 510'ers so I'm going to post it to 
them as well. I must go now, before GRM realizes I'm writting for free 
here. :-)


Marc Sayer
Performance Engineering
1070 W. 2nd Ave. 
Eugene OR 97402
(503) 484-0904  Fax 746-0863
msayer@efn.org