Another Carb/Timing Question, I am really not good at this stuff Options

[tornik550]

I have recently gotten my 914 up and running after a rebuild. I have been trying to work on the carbs and timing. The car runs ok but I am shooting black soot out of the exhaust and the exhaust backfires on decel. Engine idles fine. I have been reading and searching for a long time. I have read too much and I am now super confused. I keep reading that the backfiring on decel is from a lean condition and the black soot is from mixture being too rich? I am confused, can anybody help me? To confuse me more, I recently saw some white/blue smoke- I checked my compression and I am getting 150 in all cylinders.

car info
-73' 1.7 liter however I now have 96mm jugs, 78.5mm stroker crank- making engine displacement just under 2.4l.
-the previous owner put in a new camshaft however I do not know the details other than that it is from webcam
-2.0 heads
-cromoly pushrods with clearance set to 0 cold
-timing set at 27 degrees at 3000 rpm
-bosch 050 dizzy with pertronix
-msd 6al

carb info (all the jet/venturi/etc sizes were placed by the previous owner)
-dual weber 40 idf's
-carbs just rebuilt last week
-32 main venturi's
-120 main jets
-f7.5 emulsion tubes
-200 air correction jets

I am so confused.

Your help is greatly appreciated,
Steve

[r_towle]

Well, you probably have two issues.

Backfiring out the exhaust upon Decel is an Exhaust leak.
Go find and fix that.

Running to rich...need more data.

Who rebuilt the carbs?
What are the Idle jets?

Rich

[tornik550]

I will look for the exhaust leak. Any good ideas on how to find it?

My friend rebuilt the carbs. I looked everything over and I think that he did everything ok. He has rebuilt many carbs but no webers until now.

I will check on the idle jets.

[SirAndy]

I am so confused.

Like rich said, backfire through the exhaust is usually due to an exhaust leak.

A few things on your setup:

- 40 IDFs will probably be too small for a 2.4L ... I ran 44 IDFs on my 2056 ...

- 32 Venturis probably won't flow enough air, again, i ran 32s on my 2056, you might need to step up a notch.

- What exhaust/muffler setup are you running?

(IMG:style_emoticons/default/idea.gif) Andy

[r_towle]

to find the leak put a 2*4 over the exhit tailpipe while the car is running...the leak will get really loud and should be easy to spot.

What are the Idle jets?

Rich

[tornik550]

it appears that my idle jets are 50

[r_towle]

I would say first fix the exhaust leak.

I had 40mm carbs on a race car with a 2.4 and a very aggresive camshaft.
I had almost no venturis...they were paperthin.
I would fix the exhaust leak first, then call aircooled.net on monday.
they have a list of all known carb setups for motors.
They will offer good advice on what vents you need to lean out your mixture.
For now your carbs are setup nicely for a 2.0 liter motor...so if you are running rich you have small valves and low velocity.
By increasing the inner diameter of the venturis you will add more air.
Aircooled.net is very cool.
They swaped out vents and jets for me till I got it right.
I would agree with Andy..start with bigger vents...
Once you get close you may need to add larger main and idle jets to really get the most from your motor...

Rich

[904svo]

With the Weber 40's , I would suggest you change the venturis to 38MM and
change the main jets to 135. This will allow the engine to run much better.

[tornik550]

where can you get 38mm vents for a 40idf? i just checked aircooled and they only have up to 36mm.

[Joe Ricard]

1st of all NEVER trust what the internet Yahoos say.

Most of the above is pure crap.

Now to know exactly what you have there needs to be some more details of the engine.

Heads and exhaust compression ratio.
Spark plug gap and type of coil and wires.

all these things greatly affect the tune of a motor.

Now it really doesn't matter what is stamped on the jet. I suspect your engine would not even run on 120 jets. NO EFFING WAY............

So somebody had to have reamed out your jets.
Also you can probably back off on your acceleration jet stroke linkage.

[r_towle]

Make sure you listen to Joe, he knows everything.

Rich

[Joe Ricard]

See at least Rich knows who's the "Mac Daddy" of carbs.

[roadster fan]

1st of all NEVER trust what the internet Yahoos say.

Hey, doesn't posting here make you an internet Yahoo Joe? (IMG:style_emoticons/default/av-943.gif)

Seriously tho, there are alot of people here that can help you get it running right. Do all your research before you start swapping parts.

Jim

[r_towle]

Now it really doesn't matter what is stamped on the jet. I suspect your engine would not even run on 120 jets. NO EFFING WAY............

A set of 40mm carbs setup as stated will start and run the car.
the carbs are the restriction, kinda like restriction plates in Nascar..
Same concept, same idea, and they will start the car and run.
Real world factual statement made with experience.

AFAIK, SCCA has restricted the size of the dual carbs to 40mm bodies for the past twenty odd years.
Not sure if that finally changed...

The 2.4 will run with 40mm carbs, you will just need to set the carbs up correctly.

He did state that the car runs like crap, right.

Rich

[Joe Ricard]

What you still can't read?
You even re-posted in a quote.

JETS size as in main jet size of 120 will not run a 2.0L engine. let alone a 2.4L

GEEZE some people.

[r_towle]

I know you like picking a fight, but read what I wrote.
Real world experience talking.
120,s will run and start the car...yes it will run like crap.

Be careful when you make a statement like no Effin way...you have not lived long enough to try every possible thing in life.

This one is true...

I think that making blanket statements is just uncalled for and its not helping the guy. I did offer him a few suggestions and he is well on his way to solving this problem.

He did not post the thread to allow you to make statements and throw insults...he was asking for help.

I dont think that he is the first person that has bolted on carbs right out of the box and had issues...

Rich

[Joe Ricard]

That would be one heck of a starter to spin a motor fast enough to fire on main gas jets.

Help would be to stop easter egging around with carbs.

1st is to understand how all the circuits work
Idle jets control idle through the mixture screws
Idle jets also provide fuel through the progression ports up about 3400 RPM
HIS SYMPTOM IS BAD POWER
He probably has not been into the main gas jets and if so it has been terribly lean
Acceleration jets work anytime you push the throttle down to smooth out opening of the throttle plates THATS IT

Main gas will not kick in till you are above 3200 ish RPM depending on intake air velocity.

[wertygrog]

JETS size as in main jet size of 120 will not run a 2.0L engine. let alone a 2.4L

GEEZE some people.

Ummm...I've had 120 mains in my 2.0L weber 40 IDF's since I've owned the car. It is too lean and I have bigger jets on the way, but it sure as heck runs!

GEEZE some people. (IMG:style_emoticons/default/biggrin.gif)

[r_towle]

JETS size as in main jet size of 120 will not run a 2.0L engine. let alone a 2.4L

GEEZE some people.

Ummm...I've had 120 mains in my 2.0L weber 40 IDF's since I've owned the car. It is too lean and I have bigger jets on the way, but it sure as heck runs!

GEEZE some people. (IMG:style_emoticons/default/biggrin.gif)

Brent,
You still up here?

PM me about this weekend and the guy you got the motor from.

Rich

[SGB]

It has been a long time, but I thought (ouch) that I had a buncha 125s down there in a bag because they sooted up the plugs on my mildly cammed 2 liter.
I run 120 and 55 with 200 air correctors. I tried a lot before I got a clean burn, but ended up going back to 120s. Am I mistaken? I messed with accel pump return (1/2 or 0, now at 1/2) and 125s, 50s, and 180s. Thats on a 98% street car with 40ides.
Also
Joe, do you have a street 914?