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Stumbling 308 GT4 / Need Advice

Discussion in '308/328' started by robertgarven, Jan 6, 2020.

  1. robertgarven

    robertgarven F1 Veteran
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    #1 robertgarven, Jan 6, 2020
    Last edited: Jan 6, 2020
    Friends,

    My euro 74 GT4 seems to stumble when I take off from a stop. I doesn't do it going downhill, only on the flats or uphill. When the clutch engages it seems to lurch a bit. Once going it runs fine with no problems with acceleration. It also hesitates a bit off idle when you rev the engine. I am looking for suggestions. The ones I have so far are; increase the idle speed, open the idle mixture a bit on each carb.

    I have a new clutch, the plugs were out recently and none were fouled or showed any indication of lean running. Thanks for any suggestions in advance. I have allot of other thing to take care of, but want to get it running good first! Ive done the Italian tune-up of course, to no avail.

    Rob
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  3. marklintott

    marklintott Formula Junior
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    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
     
  4. Steve Magnusson

    Steve Magnusson F1 World Champ
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    #3 Steve Magnusson, Jan 7, 2020
    Last edited: Jan 7, 2020
    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).
     
  5. robertgarven

    robertgarven F1 Veteran
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    This is the rebuilt car so I haven't had the distributors off yet. They are the dog drive units and I have no experience with taking them off and I have no attachments to my Sunn distributor machine. My 75 US car has one set of points that rock. I guess I was hoping this was all set up correctly. I am not sure how to check the workings of the two sets of points, but I am sure there are a couple of sentences of explanation in the WSM!
     
  6. Stephen B.

    Stephen B. Karting
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    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.
     
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  8. Steve Magnusson

    Steve Magnusson F1 World Champ
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    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?
     
  9. robertgarven

    robertgarven F1 Veteran
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    I know this is embarrassing, but my 75 car has run so good since I did my last service, Im not sure if I remember correctly how to do this correctly. My life has been intense periods of working on the Ferrari, then getting it running well, then long periods of working and thinking about other topics! HA I did mark my flywheel well when I had it off. I noticed the oil pressure unit on this car has a wire hanging right over where you look down to check the timing, on my other car it is bent! I dont want to bend this new wire and have it break off!
     
  10. Martin308GTB

    Martin308GTB F1 Rookie

    Jan 22, 2003
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    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
     
  11. robertgarven

    robertgarven F1 Veteran
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    #9 robertgarven, Jan 7, 2020
    Last edited: Jan 7, 2020
    I never had a microswitch on my US car, and I just looked in my WSM and on the euro car and I cant find where it is located? Is it on the carbs or on the pedal assembly. What makes it engage when you touch the loud pedal???
     
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  13. robertgarven

    robertgarven F1 Veteran
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    Friends,

    Allot of money, and time was spent on this car. Here is a video of the engine on the stand after rebuild and one of the distributors, looks like only one set of points, installed, which would make me happy! HA Well I couldn't upload a video...
     

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  14. Steve Magnusson

    Steve Magnusson F1 World Champ
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    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.
     
  15. plugzit

    plugzit F1 Veteran
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    Classic symptom of accelerator pump problem.
     
  16. Ferraripilot

    Ferraripilot F1 World Champ
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    +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.
     
  17. robertgarven

    robertgarven F1 Veteran
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    Thanks,

    I will check that. This engine and carbs were rebuilt, but I wasn't involved in the process, but everything was done to a high level of detail. Does anyone here know how to post a short video clip, or if it is even possible?

    Ive never had this problem, so should remove the filter cover and have someone press on the accelerator pedal while I'm watching down throats?

    Rob
     
  18. plugzit

    plugzit F1 Veteran
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    Exactly
     
  19. robertgarven

    robertgarven F1 Veteran
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    Friends,

    After much trial and error, I just switched the accelerator pump jets from4.5 to 5mm, and replaced all the pump valves. The new ones are not calibrated and have a much larger opening. Its been a while since I worked on these. I am just tightening all the parts just snug right?

    My question the car had fiber washers on the fuel banjo bolts, but then noticed that my other car has copper ones.

    Can I use the fiber washers?

    Thanks in advance as always,

    Rob
     
  20. Martin308GTB

    Martin308GTB F1 Rookie

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    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
     
  21. sltillim

    sltillim Formula 3
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    I was thinking clean the jets - check the floats. You have been so busy being a rockstar you let them sit.
     
  22. robertgarven

    robertgarven F1 Veteran
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    Jets fine, floats to spec, just need some sorting out. :)
     
  23. 2dinos

    2dinos F1 Rookie

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    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!!
     
  24. robertgarven

    robertgarven F1 Veteran
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    Only as you start out it stumbles. Had a few clogged accelerator pumps which I cleaned, checked the timing which was off a bit and corrected, increased the idle mixture screws to about 3 full turn which made it better any more and it got worse. Of idle it screams, not as much as my other GT4 but that one has a hot rodded distributor. It seems like it is striving for gas a low rpms, so I hope this helps!
     
  25. 2dinos

    2dinos F1 Rookie

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    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.
     
  26. Nino1964

    Nino1964 Karting
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    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.
     
  27. robertgarven

    robertgarven F1 Veteran
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    No gas anaylizr and this car had an engine swap in Italy sometime before its arrival in the US and there are no sniffer tubes. I dive it at least every 2 weeks at Italian tune up speeds. Remember about 500 miles ago this was a total engine out engine rebuild and frame off restoration. Keep the suggestion coming I am listening to them all! :) I still thinks its starved for gas, but have to wait till my copper washers come in!
     
  28. 2dinos

    2dinos F1 Rookie

    Jan 13, 2007
    2,507
    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!
     

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