How to jet an aftermarket cam, #91 Webcam Options

[Kansas 914]

The symptoms (with 2 different sets of carbs Webers and EMPI same jettings) were the same. It will sit and idle at 1000 RPM with no problem (after a warm up) but is boggy on the low end under load - really have to keep the revs up to pull away from a stop light. Slight throttle increase after rolling creates a studder. Stomp it (WOT) and it runs great - pulls strong. It is like the throttle is an on/off switch, all or nothing - not much in between!!

I think I am close with the jetting but still not quite right. I wonder what the timing should be on a 1.8 with carbs and the webcam? The car also has electronic ignition (Crane Fireball).

I read in depth the thread on jetting Zach's 2056 with 44 Webers. I also read a good bit on Harney's Carb Clinic (great read - printed it for reference). Using the formulas in the Harney Guide I am close based on the 28mm venturis.

Idle jets should be 1.6 X 28mm = 44.8 (mine are 50)
Main jets should be 4.1 X 28mm = 114.8 (mine are 115)

I have 1.8L with a #91 Webcam (hydraulic cam & lifters) and Dual 40MM Webers.

Here is my current Weber jetting:
Vents = 28mm
Emulsions = F11
Idles = 50
Mains = 115
Air Correction = 200

Here are the cam specs:
LIFT = .430
DURATION = 265
DURATION @ .050 Degrees = 234
DESCRIPTION = Hydraulic grind for high performance street engines. Not recommended for engines with fuel-injection.

TIMING CARD:



As always your thoughts are appreciated.

[904svo]

Which distributor are you using? Sounds like you have a timing problem.

[jmill]

Your mains seem small and your AC seems large. The F11 will also lean up the low end. I like the F7. I would run at least a 130 to 150 main and around a 175 AC jet. The F11 ET, small mains and large AC jets all will lean you up down low. When you stomp on it you toss a bunch of fuel in there from the accellerator pumps and all is good. You need to get some more fuel in there.

[Jake Raby]

The jetting sounds fine to me for a default..

Are these new carbs? Webers or HPMX Empis??

Have you re-set the float heights? Whats the fuel pressure?

We have had to modify every carburetor we have used or sold for over two years now and each make has it's own challenges.

Carbs don't like hydro cams and neither do I so we'll skip that topic all together :-) Just remember that you still need to adjust your valves and inspect the valvetrain every 8-10K even with them...

I bet 50 bucks I know what your gripe is and its not in the jetting... People start swapping jets when they should be looking for other forms of faulty operation, like accelerator pump circuit issues..

[Kansas 914]

Which distributor are you using? Sounds like you have a timing problem.

Thanks for the reply - I am using a Bosch 009.

[Kansas 914]

The jetting sounds fine to me for a default..

Are these new carbs? Webers or HPMX Empis??

Have you re-set the float heights? Whats the fuel pressure?

We have had to modify every carburetor we have used or sold for over two years now and each make has it's own challenges.

Carbs don't like hydro cams and neither do I so we'll skip that topic all together :-) Just remember that you still need to adjust your valves and inspect the valvetrain every 8-10K even with them...

I bet 50 bucks I know what your gripe is and its not in the jetting... People start swapping jets when they should be looking for other forms of faulty operation, like accelerator pump circuit issues..

Hi Jake,

I tried to sets of carbs both with nearly the same symptoms. The Webers (Italian versions with velocity stacks) were older and on the car since the motor rebuild 2 years (500 miles) ago. The EMPI's are new and are HPMX (with velocity stacks) and are on the car now. Where can I find your EMPI tuning DVD??

Because symptoms were similar with both carb sets I too think it is a timing issue.

Fuel pressure is 3lbs through a regulator. I might switch the fuel pump to a rotary I have sitting here (was going on the other car). I will also reset the float heights.

Thanks as always for your response.

[jmill]

People start swapping jets when they should be looking for other forms of faulty operation, like accelerator pump circuit issues..

I thought that too at first. What threw me for a loop was the fact that this is the second set of carbs he put on with the same issues. The one constant was the jetting. Seems strange that the accelerator pumps on both sets of carbs had issues.

IMHO with a 200 AC jet and a 115 main it would seem to me that the mixture would be on the lean side, especially at low velocity. I usually start with an AC jet about 30 larger than the mains and go from there.

Jake is obviously the expert here and has tuned alot more carbs on a type IV than I have. I bow to his experience.

[Jake Raby]

Unless you specifically order the EMPI carbs prepped by Art Thraen (I also sell them fully prepped) they require some mods.

I know what your issue is, undoubtedly as we have to address this one every set of carbs that we prep.. We are working with EMPI to rectify the issue.

We can do this prep work for you, for an hours labor... Or if you can take the top of the carbs off I can walk you through it..

The 28 vents love a 200/115 set up here in my world.. It optimizes on 85% of the combos, BUT is dependent upon fuel pressure and float settings.

The EMPI DVD that Art and I did will be available soon from EMPI, we are just the two guys showing you what we know and how to do it..

[Shade Tree]

Which distributor are you using? Sounds like you have a timing problem.

Thanks for the reply - I am using a Bosch 009.

I've read about a new american made 009 that has 28 degrees of advance...or close. However, if you don't have one of these, most 009's don't have nearly enough advance for a T4.

[type2man]

I had this same exact problem with mine. I had the same jets as yours although I have a 2.0 with a mild cam. Get 52 or 55 idles and switch up to 125-135 mains and you'll see an improvement.