backfiring /sneezing weber DCNF

martijn

martijn

X addicted
I recently rebuild my engine with a flowed head, dual weber 36 DCNF, 292 cam and vernier campulley. I'm also running an elektronic ignition (with vaccuum advance (same type as henk sells). When i'm cruising at partial trottle. the engine somes backfires or "sneezing" out of the carbutettors. It doesn't happen continously but quite often. The car pulls strong at full trottle. But the sneezing is an annoying sound and i would like to fix it. Do you guys have the same issues and how to solve it?
regards
martijn
 
Sneezing carb

That usually indicates a too lean jetting. At light cruising you are running mostly on your idle jets. Try larger idles.

Carl in Virginia
 
More info would be handy

what size chokes, idles, airs and mains are you running?
Did you check the sync of the carbs with a syncrometer?
 
Here all the jets etc:

- mainjet 120
- aircorrector 185
- venturi 29 mm
- idle 0.45

i synchronized the carbs. I only use 1 of the vacuumpoints for the advance of the ignition, not al 4 of them.

What idle do you suggest; 0.5?



pict3789.jpg
 
Martijn,

Is the angle of the picture, or are those carbs leaning backwards??

If you have a 128/strada/ritmo type manifold (instead of the correct manifold for an x19) then the carbs will be tilted back and the float level will be compromised...which usually leads to tuning issues.

128 engine leans forward at about 18 degrees, the x19 tilts forward at 11 degrees.

SteveC
 
I agree on lean jetting.
Yes, try a 50. I ran a 55 for a while, but I have gone back to 50.
 
they are a bit tilted, i believe this manifold was a compromise between x1/9 and 128. I've driven with this acrb before i put on the flowed head. At that time i had no sneezing problems. So maybe it runs a bit lean due to the flowed head. i ordered the 0.5 so we will see if it helps. I'm also going to fit a CSC exhaust with header this weekend for more torque. (now is a standaard euro exhaust).
 
Idle jets for the DCNF are available in "half" sizes too... 0.45, 0.47, 0.50 and 0.52 are available to get it "just right"

The Weber part number is 74405.size, so a 0.50 idle is 74405.050.

SteveC
 
Martijn, Steve... Leaning engine...

Does it not appear that the ENTIRE engine is leaning backwards a bit?

Possibly an incorrect dogbone?

(Good eye Steve... I didn't notice that at first... kinda thinking it was a fish-eye lens or someting...)

But taking a longer look... the spacing between the firewall and the cylinder head seems a bit larger than normal also. Maybe you (Martijn) could measure the distance using the a common point on the cylinder head and another on the firewall, and we could compare.
 
I also thought it was tilted. I have a 1500 engine in a 1300 car with 1300 dogbone. I tried a 1500 dogbone But that wouldn't fit properly. To much tension.All the mounts are new and not worn.
 
me too

Our twin webers are gurgelly below 2,500 rpm (we checked the plugs and they were fine). Auto Turismo has got em the best they can
I met a couple of racer drivers ( one Vintage the other SCCA) both think that the air filter set up is to restrictive. Since the DCNF has the raised choke tubes the gap between them and the top of the filter is actually quite narrow. And since we are running so much carb we don't get good vacuum until 3 k rpm.
So they suggested removing the air filt. And shazaamn! it ran better! So after talking to "Mad Matt" I called Pierce Manifolds on the west coast and ordered velocity stacks. Unfortunatly the top of the carb has to be milled so gunna do that this week I'll let you know how it drives and sounds! oh yeah we'll post pics too
 
Ferrari used DCNF on more than one of their engines.. The had the tops lowered.. and they have Velocity stacks with a tallish plenum with air cleaner.

As for jetting, some chassis dyno testing with a proper exhuast analyzer should help sort out the jetting. Alternatively, a wide band o2 sensor can help if the o2 analyzer is portable, be used with the car on the road and logs the required data for later analysis.

Bernice

abarth2litre said:
Our twin webers are gurgelly below 2,500 rpm (we checked the plugs and they were fine). Auto Turismo has got em the best they can
I met a couple of racer drivers ( one Vintage the other SCCA) both think that the air filter set up is to restrictive. Since the DCNF has the raised choke tubes the gap between them and the top of the filter is actually quite narrow. And since we are running so much carb we don't get good vacuum until 3 k rpm.
So they suggested removing the air filt. And shazaamn! it ran better! So after talking to "Mad Matt" I called Pierce Manifolds on the west coast and ordered velocity stacks. Unfortunatly the top of the carb has to be milled so gunna do that this week I'll let you know how it drives and sounds! oh yeah we'll post pics too
Click to expand...
 
Let me know about the milling for the stacks

My daughter has twin DCNF's and I want to install those velo stacks and wire filters that Pierce has (surprise birthday gift). They are not cheap but oh so nice! What has to be milled to make them fit?


Thanks
 
Bernice

Thanks for weighing in I am always impressed with your opinions
some questions; by plenum you mean stack height? how would one get a hold of a portable o2 sensor and finally the big question.
What are you holding in your pic?
:hmm: Ron
 
Ferrari and others used a variant of DCNF where the top of the carb is basically flat.
http://www.fca-se.org/carbs/index.htm

The wire screens on top of the V stacks should not be used alone. Excessive particles will be sucked into the engine and could easily cause problem. This was a mind set that came from early performance engine days. Use a proper air cleaner and make sure there is enough height inside the air cleaner for proper air flow with the V stacks in place.

Bernice

karhawk said:
My daughter has twin DCNF's and I want to install those velo stacks and wire filters that Pierce has (surprise birthday gift). They are not cheap but oh so nice! What has to be milled to make them fit?


Thanks
Click to expand...
 
*If the height of the plenum does not allow proper air flow, there will be air flow problems for the DCNF due to it's high inlet shrouding. Basically the plenum or air cleaner must be high enough to no cause air flow problems with the carb.

*Innovate Motorsports is one company that makes a logging, portable wide band O2 sensor meter:
http://www.innovatemotorsports.com/

*That doggie is a pomeranian poodle mix that we met in Napa at a winery. She was so cute I had to get an image with her..

Bernice

abarth2litre said:
Thanks for weighing in I am always impressed with your opinions
some questions; by plenum you mean stack height? how would one get a hold of a portable o2 sensor and finally the big question.
What are you holding in your pic?
:hmm: Ron
Click to expand...
 
More tuning problems. Today i fitted a CSC 4-1 exhaust with manifold (free flow)(instead of the temporary euro standard 1500 exhaust). The X runs significatly better at high revs, but hesitates when opening the trottle. Is this also a problem caused by the small idle jets (0.5 is ordered) or has this a different cause.
 
OK

So the plenum is the top of the air cleaner and the inlet shrouds are the existing tubes that stick up, got it they are clearly restrictive IMO. so we are sending the carb tops out for milling to put the stacks on. I may mill the inside of the filters if I want to replace the stacks.
Nice doggie when my kids went off to college we got a dog.
A Maltie Poo.
Ok waiting on machine shop to mill carb tops.
Thank You Bernice
 
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