Alfa Romeo Forums banner

1 - 17 of 17 Posts

·
Registered
Joined
·
320 Posts
Discussion Starter #1
So, the TI is finally here. Unfortunately, the carb setting makes the car not drivable at low RPMs. To take off, got to rev it up to pass 2500 RPM before a successful take off. Once take off, it just goes and goes.

As much as I love high RPMs, it is just not practical for street use.

Currently, it has the following setting:

Venturi: 36 mm
Main jet: 155
Air corrector: 170
Idle jet: 50 F6
Emulsion Tube: F16

Any recommendation on changes to make the car more drivable? I'm willing to invest in a set of chokes with different sizes.
 

·
Registered
Joined
·
6,892 Posts
First off, the 36mm chokes are a big looser for low end power. My 1750 with 45s has 33mm chokes and has great idle and low end. Also, the cams are not too big, so that helps too.
I can let you know the other jets tomorrow if you like.
Randy
 

·
Super Moderator
Joined
·
11,342 Posts
Wouldn't 40DCOEs be all together better than 45s?
Well probably. But since sriswit hasn't told us the displacement of his engine, who knows? If it's a 1600 then yea, 45's are going to be "challenging" for street use.

My favorite article on Weber tuning, written by Dave Andrews, is available at How Do I/Tune Weber Carburettors - L o t u s S e v e n C l u b

Andrews says that the optimal ratio of venturi id to choke diameter is 1.25. That is, if your engine needs 32mm venturis then the carbs ought to be 1.25X that size, or 40mm. The Andrews article should have a chart linking engine characteristics to venturi size, but that seems to be broken on the Lotus Seven website (probably running on a Lucas server). I had saved a copy of Andrews' chart, and have posted it below. As the chart shows, you need a pretty large-displacement, high-performance engine to justify 36mm venturis. Note that the "RPM" lines are the rpm where peak power is developed - not the engine's redline.
 

Attachments

·
Registered
Joined
·
320 Posts
Discussion Starter #5
Here is the engine spec: 2.0 with high compression pistons and semi-hot cam.

I also agreed with Fangio8C that 40DCOEs might be better but this is how the car came to me yesterday.

So I have been tweaking with the carbs this morning. So far here is the result:

Try this configuration:

venturi: 36mm
main jet: 155
Air corrector: 210
E-tube: F16
idle jet: 50F6

Much better than before but still pretty rough at idle and the flat spot is pretty large.

Try another configuration:

venturi: 36mm
main jet: 130
Air corrector: 170
E-tube: F16
idle jet: 50F8

It is now drivable. Still some flat spot but not so big.
 

·
Registered
Joined
·
505 Posts
I would go smaller on the mains like 145;
Emulsion F16;
airs 155;
idles 55f8;
pumps 45;

But I don't know what cam you have in there and what the timing is, you can play with the opening of each cam.

Good luck.
 

·
Registered
Joined
·
196 Posts
I am planning to install a 8v TS into my Guilia Bertone. Eventually will put in some hot cams and move to carbs from the std setup what is the general consensus on which carbs to use and what setup then?

Cheers
Chris
 

·
Trained (ex)Professional, , 1953-2018 RIP
Joined
·
16,232 Posts
I'm with Randy.
If you want a tractable street engine, I'd consider loosing the 36mm vents.
 

·
Premium Member
Joined
·
22,365 Posts
I had 36s on my 2000 with 45s when I got it and went down to 34. I'm much happier with its driveability. I'll have to see what main jets and air correctors I have, but it's easier all around now now. 10:1 Motronics, big valves on both sides, some porting, IAP tube headers, ITG filter, IAP cams. Track driven, to about 6500.
People over-carb these frequently. 45s are really too much for most engines til you get into the stratosphere. 40s are fine in most cases.
Andrew
 

·
Registered
Joined
·
5,303 Posts
You said your problem was with the low rpm end, and transition. This is controlled by the idle jet; But you have not varied that much. Keep at it. You should be able to set the main jet by looking at the plugs on a good back road. Get those right (jet + air), then work on the idle jet. This controls the low rpm end and the transition, which is your problem.

Another common cause of this problem is too much advance. The Shankle advance curve was real popular for hot performance, but it made a steep advance change right in the middle of the transition rpm. You often end up with too much advance at low rpm, which bogs the engine. Modern fuels are have improved, and do well with less advance.

Robert
 

·
Registered
Joined
·
6,892 Posts
I think his problem is slow air speed thru those 36mm venturis at low RPM.
All of the jet changing you want will not fix that.....
 

·
Registered
Joined
·
2,270 Posts
Try same configuration as below with 135 or 140 mains and 200 air correction.

Pump jet, pump bypass/bleed as well as type of accelerator pump spring all will have effects on transition, as well as which type of 45's you have. Try 35 or 40 pump jets initially.

You can make a 2000 with hot cams and 45's very tractable and torquey with 36 venturis and they will give nice top-end - I do run 36's on my car that has a rather warmed over 2000.

However for street use 34's are probably still a better choice for most people.

Now if you want to set up for fuel economy F9 emulsion tubes may be preferable but then you have to start changing everything else too ...


Try another configuration:

venturi: 36mm
main jet: 130
Air corrector: 170
E-tube: F16
idle jet: 50F8

It is now drivable. Still some flat spot but not so big.
 

·
Registered
Joined
·
320 Posts
Discussion Starter #13
The flat spot is hard to get rid off. One problem I had with the 50F8 idle jet is that even when the mixture screws are 4 full turns away from seating, the carbs still spit from the intake. Currently, the pump jet is 35. I also have 45 pump jets that I will try to use.

I'm going to put the new idle jets in today. They are 55F8. I'm pretty happy with the top end with 145 main jet and 170 air corrector. I will also have 40 pump jets and 200 air corrector jets coming in on Monday. Will try those combination too.
 

·
Trained (ex)Professional, , 1953-2018 RIP
Joined
·
16,232 Posts
Currently, the pump jet is 35. I also have 45 pump jets that I will try to use.
Please keep in mind that changing to a larger pump jet will only decrease the time that fuel is delivered (if pump bleed and springs remain the same). The quantity of fuel delivered remains the same.
Larger pump jets will also richen the high end mixture.
 

·
Registered
Joined
·
1,752 Posts
The Dave Andrews carb software does a good job of selecting baseline settings. I still have a link on my site: Alfa 105 Technical Info

Dave moved his tech info to his own website: Daves home page

My 2L cylinder head was built to use 45s by Rich Goodrich. The engine takes a little care under 15-20 mph, but is otherwise well mannered. The aluminum flywheel, which contributes to its free-reving nature, may, in fact, be more responsible for any low-speed driveability issue than the Weber 45s. Here are my carb settings, thanks to Weber wizard Gron Perry and a dyno session:
Chokes -- 36
Idle -- 50f9
Accel pump jet -- 40
Main -- 145
Air corrector -- 180
Emulsion -- f16
Aux 4.5
 

·
Registered
Joined
·
320 Posts
Discussion Starter #16 (Edited)
After three weeks of trials and errors with various jets and chokes, the stutter does not go away. So yesterday, I advanced the ignition timing at idle significantly. It is now half-way between the static timing mark (F) and (M). With this timing, the stutter goes away.

The car has the Marelli Plex, recent cap and rotor. I don't think this timing is normal for this engine. The manual states 5 degree BTDC. However, I once remembered that the advance curve for this particular distributor is pretty conservative. I actually enjoy driving the car in this setup. At the same time, I am a bit concerned that this timing can cause damage to the engine.

What do you all think?
 

·
Registered
Joined
·
1,752 Posts
Did your engine originally have FI? If so, the timing is different than for a carbureted engine. Timing for a carbureted 2L is 34-35 degrees at maximum advance.

I'm on prescription drugs at the moment, so you should check this with one of the tuning wizards.
 
1 - 17 of 17 Posts
Top