Technical Backfire/Bog on Acceleration When Cold

[fiatseattle]

See below the jetting I had in my Spider…European head & 4-2-1 exhaust. It ran without flat spots/backfires and pulled like a train.

Primary

Main…140

Air Corrector…170 increased to 180

Emulsion tube…F50

Secondary

Main…140

Air Corrector…160 increased to 170

Emulsion tube…F50.

Hope this helps.

I just confirmed my emulsion tubes are F50's not F11's. Remembering this is hard when you don't have your notes on hand.....

 

[s130]

I've looked at what Weber info I have. Massive details on jets, emulsion tubes etc. but nothing aligned back to any particular carburettor or vehicle.

Emulsion tubes are very varied with varying numbers of holes, diameters and height locations. Before you can use this data one needs to understand the basics of what they are and how they operate. Years and years ago I did this for my Fiat Uno to find the optimum tube.

Not relevant to your setup but I attach my more detailed F42 and F43 jet profiles and the test run acceleration data.

You can see that generally the 90 Main Jet with the F42 emulsion jet and a 185 bleed jet gave about a 1 second performance increase.

What I would like to add / suggest that this is ALL fine tuning done by trial and error but one has to have a stable working base line to work from.

 

[gengis]

I don’t think your far away…I have found that the Fiat twin cam is reasonably sensitive to exhaust extraction. I assume you have standard cast 4 – 2 manifold? I have found the Lancia Beta cast manifold has larger ports and flows better and revs out better and fits the north to south configuration of the Spider, check that you have the right exhaust manifold…it does make a difference. Bear in mind that spitting out the exhaust is indicative of a rich mixture and spitting out of the inlet tract is on the week side. I would also try a static advance of 12 deg: Maybe increase the size of your air corrector jet.

 

[fiatseattle]

I don’t think your far away…I have found that the Fiat twin cam is reasonably sensitive to exhaust extraction. I assume you have standard cast 4 – 2 manifold? I have found the Lancia Beta cast manifold has larger ports and flows better and revs out better and fits the north to south configuration of the Spider, check that you have the right exhaust manifold…it does make a difference. Bear in mind that spitting out the exhaust is indicative of a rich mixture and spitting out of the inlet tract is on the week side. I would also try a static advance of 12 deg: Maybe increase the size of your air corrector jet.

I put the Exhaust kit from Bayless on it 2 summers ago. New Manifold, down pipes and center pipe. Has a resonator and ANSA twin tip muffler. I'm thinning the jets now. I'm running 135/150 MJ, 175/180 AC, 60/65 IJ, F50 ET, 55PJ - running way too rich.