Hello Friends. This is a cross-post from Technical Q&A. I need advice please. I have read the whole thread on tuning the DCNFs (well, I skimmed some of it) and although I have learned a lot - especially about the best balancing technique at idle - I have another issue with my Euro-spec, single distributor, '79 308GTB and I ask your help. Here are the observations. I would appreciate your guesses as to the problem(s). + First of all, the car is bone stock. No modifications to the carbs or ignition at all. + No vacuum or fuel leaks found. + Car sat with old gas in it for a year, I added 3/4 tank of fresh 93 octane fuel. No additives in the fuel except for the 15% alcohol used here in Houston. + Just finished a 'minor service', including belts. + The car starts and idles beautifully at about 1100 rpm and does not 'hunt'. + With no choke, the idle mixture 'smells' just a tiny bit rich. + When cold, it performs okay. I stay under about 3000 rpm until I see some oil heat. + When properly warmed up, the idle is still perfect, performance is as described below. + When accelerating mildly aggressively, away from standstill, there are no issues. + Upon gently opening the throttle from cruise, say at 2500-3000 rpm, there is a lag or stumble. + Upon accelerating gently above 4500 rpm there is an intermittent and minor miss; just enough to hear and feel + When given WOT, or close to it, the car runs fantastic without a single issue. + Upon overrun, especially after a little harder exercise, the engine sometimes 'spits', 'coughs' or produces what sounds like a carburettor backfire. Happens only once on each overrun from higher rpm and happens intermittently. Given all this info, please share any thoughts you have as to remedies to these issues. Thank you.
Get two bottles of fuel injector cleaner, add it to your tank of gas and run the gas (mostly) out, then add new gas and if you still have the problem, get back to us. Doug
Excellent. They are sitting on the garage shelf just awaiting time to get back to the project. Techron, or similar name, as I recall. Thanks.
The FI cleaner works well to de-varnish your carb passages and jets, and old gas runs really poorly, even if diluted with new gas. A couple tanks of gas with two bottles each of FI cleaner made my car, which had sat for 15 years, run quite well, after the first couple dozen miles of coughing and spitting, with all brand new gas, as I changed all the fuel hoses. I was thinking I was going to have to rebuild the carbs at first, but didn't have to. Doug
Except for the old gas, it sounds like you're leaning out between high idle (that is 2500 to 4500), until at 4500 the main jets take over. My take on the whole thing is as follows (and someone please correct me if I am wrong): The idle jets mix fuel with a bit of air and thus create some emusion. The air/fuel ratio in this emulsion is determined by the size of the idle jets The emulsion is fed to the engine through the idle adjustment screws and, upon opening the throttle, through the progression holes. The idle adjustment screws are - not surprisingly - adjustable. So the baseline in emulsion delivery is determined by that. The progression holes are NOT adjustable. And the fuel-richness of the emulsion is adjustable by the idle jets. So what happens if your idle jets are too small? Your emulsion is a bit lean, you compensate for that at idle speeds by opening the idle adjustment screws a bit more. And she idles just fine. However, once you open the throttle, you need more fuel, and your progression holes come into play. BUT, with a too-lean emulsion, these just don't deliver enough fuel to the engine. Result: as you climb in rpm (power), you start to lean out. So - too small idle jets mean possibly perfectly okay idle and leaning out as you climb in rpm, to the point where your obviously excellent working main jets kick in. Sounds familiar? I've got the same thing now. I'm changing to one step up in idle jets and will go back on the adjustment screws. This will mean the same kind of fuel delivery at idle, but more effective progression holes, and thus a slightly richer mixture at mid rpm. Makes sense? Hans
I agree with hans except 4300 seems high for the main jets, so double check timing at 5k rpm Sounds like idle circuit is rich. I had sam problem and was able to tune out the mid range stumble by backing out the idle mix screws, it will worsen the idle rich problem, but adds extra fuel in mid range before the main jets kick in This is how I was able to isolate the problem stated in hans' approach above Most will disagree thay the idle screws so anything other than control fuel at idle...but in my experience you can richen up the 1-4k rpm range and tune out the flat spots withe the idle mix screws. Look at the schematic, the idle screw hole clearly contributes fuel at off-idle
I agree with hans except 4300 seems high for the main jets, so double check timing at 5k rpm Sounds like idle circuit is rich. I had sam problem and was able to tune out the mid range stumble by backing out the idle mix screws, it will worsen the idle rich problem, but adds extra fuel in mid range before the main jets kick in This is how I was able to isolate the problem stated in hans' approach above Most will disagree thay the idle screws so anything other than control fuel at idle...but in my experience you can richen up the 1-4k rpm range and tune out the flat spots withe the idle mix screws. Look at the schematic, the idle screw hole clearly contributes fuel at off-idle
Great discussion, and written in terms that I understand. Thank you. Webers are fabulous devices. My 308 is a '79 and uses twin crank-triggered ignition modules rather than points and mechanical advance. That whole system is (assumed to be) in great condition. The plugs and plug wires are new and everything else has been carefully cleaned. And... the problem pre-dates the ignition rebuild. Therefore, I look back to the charge. Again, thanks!
I wouldn't do this for just anybody........LOL! From Post #5 in the Boxer Section "Help on carbs" thread...Newman chimes in: "fatten the mains via emulsion tubes..." "on my 308GTB I went from F36 to F24 to correct for today's fuels..." So, there ya go!!!! I just had all my carbs done, JRV had restored them to exact Factory 1977 jetting, I'll ask what Ferda thinks of this suggestion.....he couldn't finish tuning them without replacing my ANSA, that had blown to bits over time......
