308 tune-up issue

I got my first Ferrari about a month ago after a lifetime of fiats, lancias and alfas. it's a US '78 GTS with about 60k miles. it hasn't been run much for several years and had some carburetion issues - particularly upper rpm missing and skipping - but idled fine. when I got it home I checked the carb balance and it was way off. I replaced all the leaky hoses and did the birdman fusebox swap. I also took off the carbs and cleaned them in an ultrasonic tank and replaced all the parts that come in the rebuild kit. I put it back together with 60 idle jets and 130 mains and reinstalled with new base plates. I set the float levels and checked them about 10 times since. while apart I also replaced the plug wires and plugs but not the extenders . the records show a recent tune up and belt swap. I also replaced the exhaust with a superformance stainless system since the exhaust on there was junk. I worked through the birdman sync procedure and have the carbs balanced on the cable and at idle.
so now my problem is that it spits and backfires at idle. the misfire is coming only through the back two carbs. no matter how far out the air screws are. it runs smooth at 2700 where I balanced the linkage but it spits back and sometimes backfires at idle. it also revs up smoothly. the flow gage is around 6 at idle. the front carbs react as expected to the air screws but the back ones are not so responsive. those screws don't seem to do much. I took the back two carbs back off twice to check all passages are clear.
I thought that since it ran fine at idle before and since I didn't disturb the ignition timing or cam timing that it must still be carbs.
any advice on what to explore next would be so greatly appreciated!
Bill

 

Isn't that 6 (whatever the units were) really high at idle?
Shouldn't it be in the 3.x range? (That's off the top of my head early in the morning!)

What did you do about shaft bearings? It's nice to replace them with sealed.

 

I had my carbs rebuilt recently and also had a hell of a time getting them set right. Similar symptoms to what you're describing. In the end it was an ignition problem that was solved with a Pertronix kit. Now it runs like a dream.

 

forget your carburettors, look at the ignition. ( really look at them, you might have a centrifugal weight stuck or something similar.

don't question, just do it

 

I didn't replace the shaft bearings but it doesn't seem to change when I spray those with carb cleaner. I'll go to ignition checks next. that makes sense that the back distributor may have a problem.
thanks for the advice!

bill

 

Isn't there that old saying that most carbueration problems are in the ignition?

 

If you put a pertronix on the front dizzy it will sync the timing for both banks. If it is a dizzy advance problem it will go away or double.

You should also replace your plugs in the mis firing holes. These plugs foul out easily and once gone they never come back. You are probably starting and stopping your engine a lot without warning it up fully as you chase your problem. That will foul new plugs. That alone can drive you nuts chasing a problem that is not really there. Ask me how I know😄

 

Another thought. Only do one hange at a time before testing results. This will allow you to sleep at night rather than lying awake wondering what you did wrong.

 

amen to that! I didn't want to make so many changes at once but I had everything apart to do the fuel hoses so I chanced it. I'm going to closely check my new plug wires and everything mentioned. It sounds like the pertronix or electromotive ignition is the answer to lots of problems. any advice on which way to go there?

 

pertronix is so freakin simple and reliable.

 

i checked the plug wires and they were all the same so I think they're fine. I'm having a hard time seeing the marks on the flywheel so I'm not sure about timing yet.
I'm LeMons racing this weekend. I'll dig into this next week. thanks for the help!
bill

 

Those flywheel marks are almost impossible to see.
Best bet is slowly bar the engine over until you can finally clearly identify one and mark it with a color, and then use different colors (with a chart!) for different marks.

 

Hi Bill.

Proper ignition operation needs to be verified with a strobe light. Make sure the advance is advancing/retarding smoothly along with actual timing values at idle and 5krpm.

I think you balanced the carbs at too high (2700?) an rpm. Balance is most important at idle and just off idle, about 1000-2000 rpm. This is where I normally engage the clutch and I want torque at low rpm to get going. At 2700 rpm, balance is no longer an issue IMHO.

Try balancing at 1200-1500 rpm using only the cable to support the linkage. Just off the throttle stops is best, but balancing may not be too easy with an SK flow meter. I like an 8-bank manometer for easy and very sensitive balancing. You also see the results of adjustments in real time, continuously, and hands free.

Bill

 

Hi Bill.

How did you set the idle mixtures?

Bill

 

If you are doing extensive tune up work with start and stopping the engine then switch to a hotter plug so you don't foul them. I always ran BP5ES plugs in my 77 and it ran great in all types of weather. Also disconnect all linkage and set air meter for each throat at 3.5". Idle at 1000 rpm. Start needle valves at 1 1/2 to 2 turns out. This is because you touched the carbs which is the last thing to do. Ignition especially the 2 dizzy's is the 1st thing to check/verify. Good luck

 

Oh , remember that "popping " means you are to lean and the carb cars like to run a little rich

 

My '77 runs in the the 3.8 range on the flow meter at idle.

Also, 99% of carb problems are really ignition issues (learned that one tuning Quadrajets). One thing that I did to my car this year was replace my plug wires with Taylor Hemi wires. These have a nice long built-in extender. Car was backfiring off idle and refusing to rev before. Now runs very well.

My story here:
http://www.ferrarichat.com/forum/308-328/480783-real-bad-backfiring-looks-like-carb-rebuild-need-parts-sources.html

 

An exhaust gas analyzer looking at each cylinder is handy. And 6 does sound high at idle.

 

BTW, Weber recommends installing hotter range plugs during tuning sessions to avoid the fouling problem mentioned.

 

Excellent advice! Thanks so much to you all for the tips. I'll be working on this this week and will report back.
Bill

 

I have BPR6EIX plugs now. Is BP5ES hotter?
I also don't understand "setting the airflow at 1000rpm". I thought the idle adjustment was the only thing that affected airflow at idle. What adjustment do I make to reduce that value at idle?
the idle mixtures were difficult to set on the rear bank because they didn't seem to cause much change no matter where they were. the front bank responded well and I set those to about 1 turn out from where the idle dropped.
thanks again!
Bill

 

That refers to using the throttle cable to raise engine RPM to 1000.
If one bank idle adjustments doesn't seem to do much, you probably need to check the balance between the two banks. I'm sure someone has posted the link to Birdman's tutorial. It should contain information on this.

 

I understand the sync procedure and have the airflow readings balanced at idle and with the cable holding the linkage open. What I'm not understanding is how to change that balanced reading at idle to a lower airflow.
The airscrews are what seem to have no effect on idle speed or the spitting on the rear bank only.
Steve recommended setting that value lower at 1000 rpm, I'm not sure how to do that.
Sorry if I'm missing something obvious!

 

first of all I want to thank you all for the advice given so far. I really appreciate the guidance! I'm not quite home yet though.
good news - bad news.... I set the timing, which was off by a good bit on both banks, and re-did the carb sync. procedure. I replaced the wet plugs (worst ones were on rear bank) with BP5es and got it running without spitting or backfire. I was pretty excited as I put the airbox back on and set out for a ride. it was the first drive where it wasn't running terribly so it was fun..... until I realized that it had NO power. crap, I turned around and came back and pulled plug wires on the back bank, and nothing... it's running on 4.
I plugged in a spare plug and it's getting spark. then I did a leak down test on #4 and it was 5%. both seem like good signs. not sure what it can be. i'll take the airbox back off next and make sure it's fueling the back carbs. I can't imagine how it would not be getting fuel on those two but still getting fuel on the front right carb.
any ideas?