HELP 1979 308 GTB backfires. New plugs. Proper gaps. Retard/idle micro switch adjusted. etc etc. Actions: Carbs cleaned, adjusted and synchronized IAW Street Beast thread. Situation: 8 venturis equal vac on flowmeter- engine still backfires. Put hands (loosely) over front carbs to hand choke- engine clears/ no backfiring. Remove hands (choking) engine backfires. Hand choke- no backfiring. Back and forth. Suspect vac leak in front bank carb setup. Smog devils? Suggestions for isolating vac leak?Help?!
Thanks guys. Working way through backfiring one step at a time. Not discounting ignition/jetting issues yet. First things first. engine @ idle and 1500 RPM- backfiring stops when carbs #1 are "hand choked." (HAND choked= cover carb 1&2 with) Hard to imagine ignition/timing affected by "hand choking." Thinking hand choking richens #1. #1 #2 #3 #4 have the same # on the flowmeter. Wondering about vac lines. One thing at a time. Trying to avoid multi changes simultaneously. Don't want to change several things at the same time then end up guessing what made her backfire. Notsomuch.
I am driving to Silicon Valley on Wed afternoon from Marin through Oakland, happy to swing by and give it a listen.
Does this mean your warm idle is at 1500 RPM, or that you have the same problem at a lower idle RPM and 1500 RPM? Can you put a numerical value on the flow rate (Kg/hr/barrel), and please give the warm idle RPM and ignition timing at warm idle.
Aerosol can of Starter Fluid using one of those tiny red spagetti tubes to spray it judiciously around vacuum lines in the suspected area. Once you get close the engine will rev up from idle.
I'm with Steve M here. If you go back to the basics how does the engine run. Is the idle set at 1000rpm and the micro switch disconnected and the timing on each bank is 7*btdc how does it run. How are your points and are they adjusted correctly. Are you spitting back through the carb or are you backfiring in the exhaust. Does it just sound like a miss in the exhaust and not a backfire. What is the air flow through each barrel of the carbs. Once you get back to this point then you can adjust the idle/air needle valves to richen the mixture. Also if you have not done a complete carb tune/adjustment you may need to start with that. Also while you are at it take a look at the dizzy advance to make sure both are working correctly.
Answer: How can I isolate each advance to determine if they are working correctly? The micro switch is adjusted to release as the throttle comes off idle (1000 RPM) Thanks.
11 or 17 Kg/hr in each barrel is way, way too much (even at 1500 RPM idle) and indicates that you have some/many cylinders that really aren't contributing useful work at idle (or your Synchrometer is wacky ). If you have active R2 points (i.e., idle timing at 3 deg ATDC) , your targets should be something like: idle speed = 1000 RPM idle timing = 3 deg ATDC airflow in each barrel = 3.7~4 Kg/hr If you go to a single point, R1 only, system (like the Pertronix even though it technically doesn't have points): idle speed = 1000 RPM idle timing = 7 deg BTDC airflow in each barrel = 3.2~3.5 Kg/hr Good hunting!
Steve, Roger that. Flowmeter is new and suspect. Back when one shows 3-4 Kg/hr flow @1000 RPM. Glad this isn't a business; the hourly ($) would put me in the poor house. The main reason for the 1979 308 GTB is no computer or injection issues. Probably can wrench this issue in the end. And, it beats daytime TV.
Steve , thanks for the info. I just checked my records on my last carb tune and I am running at 4 kg/hr on all of my carbs.Idle is smooth at 900rpm and I have a pertronix set up in one dizzy set at 7*BTDC. I'm glad to see I'm in the tune range. I do recall seeing somewhere that "spitting" in the carbs is due to , to lean of a mixture. Is that correct?
