Totally weird issue with 308 evap system: It starts with vacuum drawn from all of left (front) bank (5-8) and half of right (rear) (1-2) which Tee's together to vent charcoal canister fumes. Anyway, I was replacing hoses and I discovered heavy carbon soot clogging the front rail. I cleaned it with WD-40, and spinning a piece of bicycle cable. I could only get the cable to cyl 7&8, so this is where I stopped. I noticed the engine behaved like an idle circuit problem afterwards, so I connected a gas analyzer to all of the cylinders and discovered these cylinders 7&8 were not responding to mixture at all! I cleaned the idle jets, no good, then removed and thoroughly cleaned the carb, still no good. I need to crack the idle jet off its seat to get it to respond. (Acting like an air leak). Does it make sense that the carbs are not synched correctly now that I've opened up the evap draw circuit????? I thought it had only the tiniest opening and wouldn't influence carb balance (certainly not enough to render idle mixture screw out of range). Thanks for any tips or suggestions!
IIRC, the metering hole in the manifold for each cylinder is 1mm diameter. This is a substantial air "leak" at idle, more than enough to upset the pre-leak balance. Bill
I've measured the individual airflow rate (using a ball-in-funnel type rotameter) into each small hole for the fuel evap system on a '78-'79 US 308 at 0.5 Kg/hr at 1000 RPM idle (where each barrel is drawing in about 3~4 Kg/hr via the main venturi), so going from plugged to unplugged would require mixture readjustment, but that doesn't explain: See this thread (post #6) for a suggested replumbing of the fuel evap and air injection vacuum lines that will give the same 0.5 Kg/hr "leak" to each cylinder (so the Synchrometer reading of each barrel can be used directly without compensation): http://www.ferrarichat.com/forum/technical-q/121753-1978-308-evaporative-emissions-valve.html (PS Your finding of "plugged" is a good reason to put a small inline fuel filter (to act as an air filter) in the hose going from the charcoal canister to those small manifold tubes -- prevents the engine, and the tubes, from ingesting any carbon bits that might get sucked out of the charcoal canister.)
All, When I replaced my canister I took the old one apart, l actually had to destroy it to get it open . The carbon was trapped under a very thin micro perferated screen not unlike the one on the oil sump pick-up. I though that the carbon could not make it through there based on my observation. Not sure if there are different models but I decided I did not need the filter which I had on there. I am not giving advice advice and still could be wrong, just sharing my experience. I did use Steve's diagram to set up my evap hoses. I only have one canister and not vacuum sphere or its related lines, mainly just the two rails going back to the canister. I would assume a pump blowing backwards through the filter could breach the screen?? Rob (looking for his old filter to re-install
More details: My carbs got fouled with charcoal long ago, so I adopted the practice of putting fuel filters in line with breathers to keep charcoal away from carbs. Also, I think my vacuum manifold ports got clogged from carbon deposits on the intake side. The same sooty crud you see on the carb choke circuits. After cleaning the 7&8 carb breathers, the mixture stopped responding. I would still expect the carb design was setup to handle that flow loss??? I hope I don't have to plug it up to get the motor to respond properly.
Have you compared the relative air flow/vacuum between all 8-cylinders? If 7-8 purge circuits were plugged and then cleaned, manifold vacuum would go down in cylinders 7 and 8. Relatively low manifold vacuum could cause this carb to stop metering air/fuel. I would increase airflow for this carb and then try to adjust the mixture. Webers require a minimum air flow to operate properly. Bill
I plugged the manifold suction lines and it made the tiniest difference in the carb response as to be considered negligable. I was advised to start removing lead plugs to check passages. I guess 4-1/2 years of storage is long enough to mess up a carb this badly. Pulling the lead plugs out, and re-installing isn't the part I'm worried about ----it's doing this only to discover the internal passages are wide open. Then it becomes, "Now What"?!
When i got my 77 all of the emission equip. was removed. These lines on the carbs going to the vacuum canister were all capped and plugged. There was no vacuum canister. I didn't seem to be a problem in tuning / sync. the carbs.
2dinos, I would be reluctant to pull the lead plugs on the carb(s), especially if they aren't leaking. I still think you should increase the air flow through the 7-8 carb, by adjusting the balance fitting between the 5-6 and 7-8 carb, about 1-full turn. I know this is not an easy adjustment, but you can reverse it. See if the carb will now respond. Ensure the 7-8 spark plugs are clean. If in doubt, use new ones. Have extra new ones on hand and replace if in doubt. It doesn't take much to foul a plug and a fouled plug make that cylinder almost impossible to tune. Do you have a way of measuring or comparing air flow between the carbs? Check all the progression holes on the 7-8 carb. This is almost impossible to do on the car. You must do it on the bench. If the purge holes in the intake manifold were blocked with carbon, this was likely because of a miss fire, usually because the mixture was too lean. One or more of the progression holes can similarly become coked up, ask me how I know. The best way I have found to clean them is to wet them with carb cleaner and carefully probe the holes with a straight pin, held with a hemostat. Carbon is relatively hard, but can be fractured and cleared with the conical tip of a flat pin, without damage to the surrounding metal. Steve, It's likely the PO took the time to tune the carbs after the purge lines were removed and the holes plugged. When I removed the hard lines from my intake manifolds, I replaced them with threaded hose barbs so I could compare manifold vacuum for balance purposes. I now have the option of reattaching the charcoal canister soft lines or running capped. Currently, I am running capped. When I hook up to the charcoal canister (to all 8-cyiinders), both the idle speed and idle mixture are affected. The two configurations are noticeably different and should be adjusted accordingly for best running. Bill
