Stumbling 308 GT4 / Need Advice

Mine does the same and its been attributed to the float level in the carbs that drops off when going up hill. I notice it most going up hill round a right hander (!)
Could all be nonsense as I haven't dared to delve into the intricacies of the Weber. Also slightly increasing idle speed helped a little.

Others will I'm sure have more focussed answers for you! Nice car by the way...

Cheers

Mark

 

What is your ignition situation? Exactly stock as per the 91/74 OM? I.e.:

R1 & R2 points working
with advance curve per graph on page 69
6 deg BTDC on R2 at idle
(18 deg BTDC on R1 at idle)

or something else? (Being a euro car in the US, I'll be shocked if it's all still stock , and this arrangement relies on the transition from R2 to R1 to add the advance at low RPM; whereas, the US distributor advance curve is quite different and is what adds the low RPM advance).

 

Try advancing the timing 1 or 2 degrees. My 75 seems to like it but I'm also running a electronic ignition. You won't do any damage by advancing a few degrees but you may have to turn down the idle a bit.

 

Yes, as I said, the advance curve of the US version is more tolerant of running an R1 only configuration (although it still requires strangling the airflow thru the carbs some to keep the warm idle at 1000 RPM). Have you even looked inside the distributor caps to see if the R2 points are present and/or is the throttle microswitch present?

 

Hi Rob,

Did you rebuild the carbs on this car? One of many possibilities: Check the accelerator pump jets. I once had similiar symptoms over years and I actually got used to it.
While doing a carburettor rebuild I noticed, that one accelerator pump valve had shot out its valve ball. It's just held by a lead plug inside the brass valve and each time you accelerate hard, the ball hammers against the lead plug. You cannot lose it as long as the carb cover is installed. In my case I found it, together with the lead plug lying on the top of the carb body after removal of the cover. But it means, that the accelerator pump on the affected carburettor is no more working.
Repaired it, put the ball back and soldered the lead plug ---> much better throttle response again.
Like said, just one of many possibilities. An exotic one, I admit.

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Best from Germany
Martin

 

On the early 2-dist 308GT4, it is located in the acc. pedal assembly. I'll apologize for this dreadful scan quality, but it's shown on TAV 12 of the 1976 Dino 308GT4 SPC -- item 37 (it is not shown in the later 1979 Dino 308GT4 SPC that uses a single distributor -- and that's the SPC that is typically available on the web, like at Ricambi):
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If that's the case (R1 only), you'll need to make the advance curve of the distributors to be more like the US version (it would be impossible to get a 1000RPM idle speed with an R1 only advance setting of 18 deg BTDC). Martin raises a good point -- easy enough to look down the throats of the carbs and check to see if all the acc pumps are working/squirting.

 

Classic symptom of accelerator pump problem.

 

+1 very much agree. Needs a touch more fuel off the line under load. Check pump diaphragm, if those are good check that the pump jets are not clogged, which is very common.

 

Exactly

 

Hi Rob,

both work. I have fiber washers on the banjo bolts on my Alfa and copper washers on the 308. But for invisible areas like on the 308 I would prefer the copper washers. Over the decades I had one fiber washer leaking on the Alfa and also one on the 308. (The inlet filter plug has a fiber washer). The leak was not bad. The washer was just wet but no dripping. But therefore I think the copper ones are more reliable in the long run.

Best from Germany
Martin

 

I was thinking clean the jets - check the floats. You have been so busy being a rockstar you let them sit.

 

Was it running well, then suddenly started acting up? Do I understand that part correctly?
I always hope for pinpoint diagnosis. I don't always obtain this lofty goal (hence the boxes of good parts that I'm collecting from mis-diagnosis .

If it's got a running problem all over the tach, most likely ignition. If there's a specific range, sounds more like one of the several fuel circuits has a problem and the idle circuit is a common place to look when low to mid speed drive-ability issues pop up. I've had this issue and chased around for a while and turned out to be a little dirt in one idle circuit. I could actually see a hard tiny chunk of carbon stuck in the precision orifice. A quick fix was to pull all the idle jets and check each little opening, cleaning as necessary. Then, my trick is to put the idle jet holders back in without the jet and start it briefly, and gently rev a few times to help clear any particulates in idle circuit. Stop engine and put idle jets back in. This cured it, but it came back several months later. I repeated the aforementioned process and problem hasn't come back. Keeping the fuel circuits perfectly clean on 40+ year old cars is part of the hobby.

How we love them so!!

 

Do you have access to a gas analyzer? Man,,, what a cool gadget! Tells you how each cylinder is performing. Connect it to each cylinder while idling. (i.m.h.o.) Set mixture near stoich on the fat side, and behold tuning perfection. Now, you come across a cylinder which shows high HC with not very high CO, this is an indicator of lean miss and you found which cylinder to address. One cylinder dragging is like loosing 2 cylinders. Another discovery on carbs. My 308 was parked for a while, and ran fantastic when put to bed. Upon reawakening, bad off idle stumble. Sounded off when starting. I noticed I couldn't open the idle mixture screw enough to wake up one sad cylinder shown on the gas analyzer. I flushed carb cleaner & WD-40 through every passage - still not right. Took days to solve and I was ready to go carb hunting Finally, I had the lead plugs removed and a passage was discovered to be jammed with soot. Thought I kept the machine immaculate, but deep inside the carb passages, different story. Put cleaned carb back on and voila! Very easy to dial in and definitely fixed. I'd be surprised if clogged passages is your issue, but a spec of carbon soot from a pop at startup making its way into the idle circuit wouldn't surprise me. Best of luck in your hunt

BTW: I re-read your initial post. The Italian tune up helps fouled plugs which presents all over the tach. I've run colder plugs for track days which fouled driving through town on the way and this helped.

 

Robert could it be that beauty needs to be driven? Those oem dizzys gum up so easily and driving seems to be a good thing for them. I rebuilt my oem dizzys and also sent them to very reputable shop for second opinion (and mucho casholo) and results were the same. Best thing I ever did was my my Dino parts electronic ignition conversion. Your car is so perfect and original I’m sure want to keep its purity. My opinion is that it’s ignition related.

 

Here's a simple thing that helped me diagnose my V6 dino Weber woes. Does the choke circuit work on your car? I ask cuz most folks (me included) n e v e r use the choke.

If you squeak in a tiny bit of choke and this helps the prob, that's more confirmation that she's a bit lean. Also, you still have dual exhaust and can use a gas analyzer to check out each bank. If you see high HC on one bank like 300+ and CO is not way up like 5+%, it's most likely hit lean-dropoff. Now you can make small idle mixture adjustments to this bank to find out which cylinder is unhappy. Basically, see which mixture screw doesn't respond well.

Disclaimer:
Very imho ~~ as many folks will do the method of turn a mixture screw one way til stumble, then go the other direction til stumble and return screw to the middle of those settings. I get this approach - makes perfect sense, but it has never worked well for me just beyond getting it to start. I helped a buddy in the mountains fix a sno-blower carburetor, and this got it going, also worked fine for a chain saw and model airplane engines, but for tuning the multi-throat Ferrari where one cylinder can push the other cylinders, or be dragged,,this drove me crazy. As the synch of flow and mixture get dialed, it just runs sweeter and drive-ability is much improved. W.O.T. doesn't really care much about many carb adjustments.

BTW: Your car is magnificent!