308 GTB carb tuning issue

Hello everyone, I am running into a bit of an issue tuning the carbs on my 308. As some background, it is has just had a very major service, new cam belts, valve adjustment, all new fuel lines, new vacuum lines, fuel filter, and a ton more. The carbs have also been cleaned with all new gaskets and seals, and the ignition was upgraded to a Pertronix unit, and timed appropriately. There are no vacuum leaks or any issues of that sort.

My issue is that I went through and adjusted the carbs as per the really nice write up on Birdman's website. After doing so, I have the eight throats balanced perfectly with the vacuum gauge, and the car idles great as well as having great throttle response if revved sitting still. Once driving it though, it bogs under throttle almost immediately and sputters. Pulling the plugs after just a few blocks of driving, they are completely black and smell of fuel. So I am assuming it is running really rich.

After seeing this, the first thing I went back to check were my float heights which are bang on 48mm and 58.mm on all four carbs. I tripled checked this with a set of digital calipers, so this should not be the issue.

This leads me to question the idle mixture screws. I adjusted them with the air box in place with a warm engine. I then turned each one in until I heard the revs come down a bit, and then back out slowly until I heard the cylinder fully chimed in. I proceeded to slowly turn until I heard the revs slightly come down. At that point I stopped and turned the screw back one quarter turn. Which is what I thought I read as the proper amount.

On my bikes I always turn the screw in until the revs go down, and then back out until the the cylinder fully chimes in, and stop there. I am wondering if that is what I should be doing on the 308 as well, or would that make it too lean? Also I have been doing the adjustment with the airbox and filter fully in place, as I thought this should give me a better A/F ratio. It is a total pain to do this way, and the Birdman instructions say to do it with the airbox off. Should I be doing this with the airbox off? What is everyone's opinion on this?

 

Thanks for the reply. By vacuum balance I meant the flow with the synchrometer. With the synchrometer I was measuring 4 Kg/hr at each throat, which is what I had thought was in the proper range. There was pretty much no discernable difference between carbs on the synchrometer. My timing at 1000 RPM is currently set at 7 degrees.

In terms of the idle mixture, I did it based upon what was on the Birdman site: "You want to open it until the engine just starts to run slower (way too rich), then close the screw about 1/4-1/2 turn." So tuning on the rich end instead of the lean end. I am used to tuning on the lean end, which is what I have done on bikes in the past, but followed this procedure instead. This was done with the air box in place. I made a long flexible screw driver so could turn the mixture screws with the airbox on, as I thought its flow restriction could change the mixture. Would you recommend doing it with the air box off, and from the lean side instead of how I did it from the rich side?

As for the car, it is a US car that has the air pump removed but has the EVAP system in place. The EVAP has all new lines, but I thought that maybe it could have been the issue, so I plugged off the vacuum ports at the carbs, and retuned everything again without the EVAP vacuum and had the exact same issue. So it being hooked up did not seem to matter very much, except for more gas fumes in my garage without it connected of course.

 

Steve thanks! I greatly appreciate it. I am glad to hear that my numbers aren't too crazy. I will readjust from the lean side. From when I did it from the rich side I noticed it was about a turn and a half to almost two turns out from where I would have put it on the lean side. It is good to know that I can do this with the air box off. Doing it with it on is an utter pain, and results in many burns trying to get the front bank set. I'll give this a try on Saturday, and clean my plugs once again, and let you know how it goes. Thanks!

 

Thanks for the info. I can’t say too well how it ran before the service. I only drove it a couple of blocks from the trailer to my garage as it had been sitting about 30 years. But it did run ok at that point. All 8 plugs are indeed sooty in my case. With the ignition I am running new NGK plugs, new stock wires and extenders, new stock caps and rotors. I installed the Pertronix in one distributor and cleaned and rebuilt the advance in that unit as it was gummed up. I have checked timing on both banks and it seems to be ok and runs great not under a load but falls on its face when moving. I’m currently using the stock coils and am using the resistors, but have the Pertronix wired to 12V before the resistors. Your post is making me think this could be a Pertronix setup issue. I originally had it wired to after the resistors and the car wouldn’t run at all under load, would completely die. I thought by giving the Pertronix a good 12V that I had fixed it but maybe not. How do you have your unit wired up? I was afraid to run the stock coils without the resistors as I heard it could cause issues for them, but maybe I need to remove them or switch to a different coil. I had kept the originals as I was trying to keep an original look.


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Thanks, that’s definitely a possibility. I have cleaned and inspected all the jets and cleaned the carb up with carb clean and compressed air but didn’t do anything super in depth like a sonic bath


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Thanks. I checked the timing at 5k and it seemed ok, but maybe I need to do it again. It’s definitely somewhat hard to do without any assistance from a helper unless anyone knows a good trick for that.


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Thanks everyone for the suggestions. I’m assuming I probably had both an ignition and mixture issue. I rewired the Pertronix unit removing the resistors and taking both the red starter and yellow run wires directly to the coils and wired the Pertronix to the coils. I kept the small capacitor wired into the circuit as I thought it couldn’t hurt. I also ran ground wired from the bodies of both distributors to the frame. So hopefully that should sort the ignition part of things.

I decided to replace everything in the carbs while I was at it. Im waiting on a couple of parts but should be able to finish that up this weekend hopefully. I’ll get the mixture set the proper way this time.

I also noticed that I had put in the wrong plugs. I am using NGK BPR7ES which is what came in the tune up kit from Superformance. Based upon the threads I read here it seems they are way too cold and that the resistor plug isn’t a good idea. So this probably didn’t help my issue and could be part of the reason the plugs get covered in soot almost immediately. I bought a set of BP5ES plugs that should arrive later this week, which I will swap in. The non resistor plugs seem like they are becoming nearly impossible to find. I had to get a NOS set from eBay.


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Thanks. I’ll definitely keep an ear out for detonation. I’m running stock coils, wires, and plug extenders. The only upgrade is the Pertronix instead of the points and the removal of the ballast resistor. Hopefully the 5s won’t be too hot. I wanted the BP6ES but couldn’t seem to find any that were not the resistor type plug.


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Thanks, I’ll measure those this weekend before I swap out plugs. The micro switch is no longer active with the Pertronix setup but I left it and the wiring in place as I’m trying to keep a completely stock appearance


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So I checked my plug extenders today and they are very low resistance, almost nothing. It also looks like my current plug is a BP8ES. So non-resistor and really cold. I think I’m going to get a set of BPR6ES plugs and see if that helps. Then I’ll fine tune the carbs and see what happens.


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I think the distributor advance is ok on my car at least. When I installed the Pertronix I took the whole thing apart, cleaned and lubed it. It was pretty frozen before this but moves very free and smooth now. When I set the timing it advanced nicely and came back to base timing following RPM. I checked base timing and timing at 5k on both banks and they came out good. I did not check any points in between, so I’m not sure how smooth the advance curve is, but I did watch the advance with the timing light when reving from 1k to 5k and the advance seemed smooth. I’m planning on swapping out my coils now to the Pertronix Flamethrower II to match the Ignitor II. I’m starting to wonder if my original 45 year old coils just aren’t cutting it and causing it to break up under load, even though the resistance on them seems ok. Every other ignition component seems good and all tests out great with the car parked, both at idle and reving up, just not under load. One quick question, if I go with the 0.6 Ohm Flamethrower II coils, should I increase my spark plug gap? I’m currently running 0.026 and was thinking it may benefit to go to 0.035. My final setup is the 0.6 Ohm coil, non-resistor extenders and I’m switching to a BPR6ES plug. Thanks!


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Ok so I have an update. I have gone through and dialed in the carbs. They seem to be about perfect now. I checked my ignition timing again and it was spot on. I also switched to a new set of BP5ES plugs as I am running resistors extenders. It sounds great sitting still and revs really nicely but still bogs under load. I went through to check ignition again and found one brand new extender that although tests to the right resistance, when I pull it off it arcs like crazy out the side. Even though it is brand new and no visible cracks. Could one bad extender make the car run so poorly under load?


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