365 GT 4 acceleration hole

There's another thing I'd like to have your opinion on. Since I've bought my 365 GT4 2+2 a little over a year ago, I had quite a lot of work done on it, since the car had obviously been standing for quite some time. I had the entire suspension, steering and brakes overhauled, got new ignition wires, rotors and points, installed two dinoplex units for better spark and had recently the carburators overhauled and valves and ignition adjusted. It seems to me that all work to make the engine run as good as possible has been carried out. The compression is fine, the engine revs and idles smoothly but there's still what I would call an acceleration hole somewhere around 3500 RPM. If you accelerate with full throttle from, let's say 2000 RPM it's not as well noticeable. However, when you slowly accelerate, with maybe half throttle, the engine revs to about 3500 RPM, then kind of "gulps" for a moment, before it continues revving up high.
In a thread about a 365 GTC4, that shares the same engine, this was mentioned briefly before. I would like to know from the other 365 GT4 owners if they expirience a similar problem. Maybe even the 400 GT owners. What do you do about it?

Howard

 

I've done basically the same things with my 365 since I bought it. I noticed a hesitation at about 2500 rpm--more pronounced if you were at full throttle. My mechanic said this is typical of carbed cars that have sat and to put a full tank of it gas through it short order (like a week or two) and to see how it reacts. I did that an whatdayaknow. No hesitation. Have you been driving your car regularly? Have you run a good tank of Shell or other premium gas with a good carb cleaner through it?

 

I'm afraid that couldn't be the problem. I did two trips to Italy last year and since I've had my carburators completely overhauled I did another 1500 mile trip to Switzerland just a few weeks ago.
It's not that it isn't any fun to drive but it seems odd that especially a 12-cylinder car doesn't have a smooth power curve all the way through.
Nevertheless, in this thread that I can't find anymore, one guy claimed that this was typical for 365 GTC4's and another one, as far as I remember, recommended to just turn the mixture richer, including using a larger jet.
I don't know if this makes any sense at all. I could maybe see that they had a minor problem with the 365 GTC4 but after over 500 cars somebody at Ferrari should have found a solution for that by the time they came out with the 2+2.
Another question I have is, how often do you change your points, or how often do you adjust the gap between them?

Howard

 

just purchased a 76 vin # 19065ran compression test will have to spend $15000 on motor valves burnt on two cly. may as well do all with engine out. two months time to spiff up all runnung gear. will keep you all posted. trying to think thru just how many $$$ to spend. am at 27000 to start. should i think with hart or wallet?

 

Mitchell,

Think with your heart. I was in the same position with my 330 10 years ago when it would have been smarter to part it out. I instead poured money into it and enjoyed it. Now have a really great car and the market came my way so I'm not longer underwater!

Regards,

Art S.

 

Remember, Gasoline is totally different in formulation than it was when these cars were made. If your car is stock and has not been re-jetted in the past 10 years, that is part of your problem.To me, it is one of two things or a combo of both:

1. be sure your distributor advance throwout weights are well lubricated and not hanging up

2. To me, what you describe is a classic small lean hole between the transition and main jet circuits, which can be covered up by the accell pump on hard accell. If you are certain all of your jets are clean (a dirty jet acts as a smaller, or leaner jet), then I would suggest going up one size on all 12 of your mains. With DCOEs, this is EXTREMELY easy. What you are doing is bringing in the main circuit a bit earlier to cover any lean spot. It would be best to then check with an A/F meter, but as long as you are going just a little richer, you may can get by with a butt-o-meter check.

This drivability problem also occurs in the carb'd 308s on current gas as compared to the 70s, and most solve the behavior similarly with a slightly richer main.

Hope this makes sense and helps

 

Thanks Russ for your help. I think this makes a lot of sense - I'll give it a try and let you know how it turned out...

 

Make sure your carbs are synch'ed over the entire rpm range. Takes a long time to do it right, but you can almost get rid of the dead spot once they're done right. Or at least get it to the point where you don't notice it.

 

Thanks snj5,

today I've changed the main jets on all carburetors from 125 to 130 and the problem has disappeared almost entirely. The car is so much more fun to drive.
However, I'm tempted to try 135's to see if I can get it even smoother...

 

Very cool - That sounds great and I'm sure it's A LOT of fun.

The hole should well be gone with the 135s. A quick investment in an A/F meter or a quick Dyno run will verify where you are mixture wise, but with small changes in the rich direction, the butt-o-meter is ok. Tuning Webers is fun and really quite straightforward once you understand a few very simple concepts.

 

Good Day- I'm the guy that had the c/4 that you mentioned in your 1st post. I still have the stummble that you had. I've purchased a hotter ignition setup, but have yet to install. It looks as if you've solved the problem by going to a fatter main jet. How difficult was this?? Looks like the mains go into the bottom of the emmulsifcation tube......did you have any issues with the install?? Just looking for some advice.

Thank You- Geffen365GTC/4

 

Pending Howard's specific response, I'll simply add that DCOE jet changes as on the wonderful C4 are easy. As you say, they are "press on" to the bottom of the emulsification tube, and can be easily accessed by the small round cover on top of the carburretor. I would imagine the whole process done carefully would take less than an hour.

As I alluded before, the 308 series cars feed a cylinder of the same size (365cc) and owners have found as large as a 140 main was the best choice in their 40 DCNF. For the 38DCOE as found on the magnificent C4 and it's siblings, a main size of 130 - 135 I believe is a good empiric choice for modern fuel.

David did bring up that if the car sits, a similar problem occurs. This is due to build up of fuel varnish in the jets and passageways, causing a functional restriction as if you had a smaller jet, yielding a leaner mixture. It does right itself as fresh gas is passed through and eventually dissolves the restrictive build-up and cleans the carb.

I would still highly support the ignition upgrade which sounds like you have purchased.

Best of luck!

 

This is really easy. You open the butterfly screw on top of the carburator - the two larger screws underneath it are the main jet assembly. Take them out and pull off the main jet from the large emulsion tube. You should be able to pull them off without the help of any tools. To put on the new jet (easily obtainable, just make sure they're for a DCOE carburator) might be a little trickier, since they're a little larger than the opening in the emulsion tube to make them fit as tight a s possible - you can carefully use a small set of pliers to make them fit. Put them back in, close the lid again - done...
It took me a little over half an hour to do all 12 jets.
However, before you do anything, I would suggest for you to wait and see which jets are really better. I now have the 130's and they're a great improvement. Nevertheless I'd like to try the 135's to see if they're even better. I'm ordering them today and should have a result by the end of the week.
What I'd like to know is, what ignition system did you get for your car?
When I got mine last year I got new rotors, points and cables and installed two dinoplex units with the coresponding coils, which improved the overall performance a great deal. However, I'm still not happy. There's still the occasional backfire when I'm accelerating into the higher RPM ranges and it doesn't go there as smooth as I would hope. There's always a slight "studder". Also, when I try to maintain a certain speed with anything over 3500 RPM, it backfires. Not badly but there's always that "crackle" that I think shouldn't be there.
The way I see it, I'm looking at either rebuilding my two distributors or getting myself an ignition setup that is entirely electronic, doesn't use any points and has no mechanical parts to worry about. My guess would be that the second option might be even cheaper but is way more effective.
The question is, what system to use. A while ago I started a thread on this but didn't get that much response. A lot of people, it seems, have converted their 308's to electronic ignition but I haven't found anybody who converted his 12-cylinder engine. Superformance in England, where I got my dinoplex units and who are exceptionally friendly and knowledgeable highly recommended a unit manufacured by BSM (http://www.blackstallionmotors.com.au/bsm_ignitions.html) which is sold in the US by Carobou. The price is fairly high at 1,100 BPD's over here but they seem to be the best choice I've seen so far. All alterations can be easily rebuilt to original spec with no traces left, it's programmable, fairly easy to install and it even has a toggle switch that lets you choose between "performance" and "emissions", which is definitely an issue over here in Germany.

 

I hope that you will both keep this post going. This is very helpful to the C/4 to 400i types. Between improved jetting and ignition upgrades this could be very helpful. The c/4s, 365 and 400s were pretty quick cars and well regarded for their performance, despite astehtic detractors who seem to decrease in number as time brings a greater appreciation to pf's work in the 60s and 70s. I wonder if when you're done you could estimate an objective performance gain (0-60 or 60-60) between the two improvements, which don't seem substantial, expensive or irreversible insofar as originality is concerned.

 

Let me know how the 135's work for you........I think you may be on to something here. Are the 125's stock on the c/4's.....my car is an early model...December ' 71 production...14957. The ignition that I used came from Ferraripartsexchange.com.......Lyle Tanner. In fact, if you look at post #1430 of the Dino restoration thread in the 206/246 area you can see the exact unit w/ wiring diagram. As I stated, I haven't installed it yet. One of the main advantages is the fact that the much higher spark voltage allows a wider gap on the plugs....something that I think would work well with a bit fatter mixture....plus no more fouling. These new ignition boxes look exactly like the oem units......only with much improved internals. If you call Lyle he is very helpful. He seems to have a sweet spot in his heart for the c/4's. The only downside to this upgrade is the cost...$800.00 per kit...of course we need 2........but if it makes the car more fun / driveable why not. I have another question for you.....how does your car idle??? Mine idles at about 600 rpm and after about 30 seconds it loads up and dies. My hope is that the ignition upgrade will fix this. Good talking to you.

Geffen365gtc/4

 

While not a C4 owner, an upgrade I have heard works very well in these cars from respected shops such as Chuck Wrays and Norwoods is the commonly available MSD 6A boxes. In combination with a hotter coil, they provide a capacitive discharge multiple spark at low rpms (where you may be on the relatively rich idle/transition circuit) keeping plugs from fouling and providing better combustion. Since the trigger voltage is low on these, they also add to the lifetime of the ignition points if you have not switched to an optical or Hall effect trigger.
In my Weber carburated car, it is not unusual to have Air/Fuel mixtures in the 11 - 12 while pottering around - and it would be my bet that these 4 cam DCOE cars will do the same. The MSD still is able to burn through this and keep the plugs relatively clean.

Good luck!

 

I will keep you posted on the diffrent jets. What you bought from Lyle at ferraripartsexchange is exactly what I put into my car a year ago. He gets these units from Superformance in England, where, by the way, you can get them for 235 BPd's, so including shipping and customs you should end up in the lower $ 500's.
As I mentioned before, these units are a great improvement. The engine starts, idles and generally performs much better than before. Fuel consumption is down about 20%. I can't tell you at what exact RPM my engine idles because the rev-counter needs to be overhauled - The needle trembles at low RPM's. However, it idles perfectly at, so it seems, the right engine speed.
However, the disadvantage of this system is that it still uses the original distributors, points, rotors, etc.. These are all moving, mechanical parts, prone to malfunction or disadjust themselves in one way or another, sooner or later. I, in my particular case know that these distributors have some kind of problem somewhere because I've singled out every other possible source.
So now it's for me to decide whether I want to refurbish both distributors or if I instead go for a system that guarantees optimum performance, like a modern car, throughout all RPM ranges, practically maintenace free - both at probably the same cost. Well, you probably know my answer already. I'll probably go ahead and put in the electronic unit in september. I have a very good feeling about this. First I really trust these guys at superformance. Second, the system really does make a lot of sense. You should go and take a look at their website.

 

OK, I have now installed the 135 main jets and the car runs even better now. This was confirmed by a check of the spark plugs while the 130's were still in - they indicated a mixture that was too lean. However, I still have to check the plugs on the 135's.
Now that the car is running so well in higher RPM's, I suspect that the idle jets are a little on the small side as well. The car is a quite sluggish while accelerating out of the stand - once you get over 2500 or 3000 RPM it's a dream. It seems only natural to use a larger idle jet as well if the reason for this is really the different quality fuel we use today (btw, I'm using best quality 100 octane fuel).
However, all these changes have one negative effect: they increase fuel consumption. How much, I have not yet determined.
As I'm working my way into this rather difficult subject of finding the perfect jet setup, I'm thinking there might be also a different way to do this. Rather than going all the way up to a main jet of 135, one should be able to reach a similar result by using a different air-correction-jet. The air-correction-jet regulates the amount of air - the smaller it is, the richer the mixture gets while still using the same main jet and therefore the same maximum fuel consumption.
I think it would be very interesting to run a test on a variety of those jets and emulsion tubes and fumble with them. I bet you the results would be amazing. However, unfortunately I don't have anyone around here with those facilities.
Anyway, the results I'm getting with these small changes are amazing already. Wait until I have tried a larger idle jet and then you'll know what to use for your car. You won't believe what a difference that makes...

 

have been reading with interest. after following your post i decided to do a major powertrain rebuild. my car is rust free and complete. as old as the components are, no extra risk need be taken. your opinion and input would be welcome

 

I am glad to hear the car is running so much better. From my experience, the jumps in idle jet size need to be done on a much smaller and finer scale as you finish dialing in the car. Usually, most of everyday driving is mostly done on the idle/transition circuit and these idle jets are more typically the determinant of everyday gas mileage. I had to smile as you described your decrease in gas mileage on the larger 135s suggesting that indeed you were enjoying the car more on the main circuit, if you know what I mean...

It would be interesting to see what idles are in your car. I would imagine 50s, and I CANNOT imagine you would need any larger than a 53 or 55 with the stock camshafts.

I also agree that you may want to slightly tweak the a/c. Remember that while the a/c jet has an effect throughout the range, it is primarily in effect at the very upper rpm high flow ranges. What you have done is correct a lean spot in the lower rpm range at the transition point by bringing in the main circuit sooner and dialing in the mixture into the best power region of probably in the high 12 A/Fs. However, when you start going in the lean direction, it's a bit riskier than going slightly richer. I have been exceptionally happy with my portable LM-1 Air/Fuel meter which provides a downloadable graph so I can see what the carbs are actually doing on the road to fine tune jetting to the gnat's ass.

Webers are fun, no? They are so much more mechanically soulful than the comparative inelegance of fuel injection... but I am biased.

 

Hello Snj5,

since I will be away for a week, I have given the whole matter into the hands of my mechanic. Although the overall performance has significantly improved, I still do experience some problems in the high RPM's over 5000 - hesitant acceleration and the occasional misfire. So I'm having my mechanic go through the entire ignition setup before I continue to play with the jets. I suspect something to be wrong with the distributors, since the advance has just been set perfectly about 2000 miles or three months ago. I will know by the end of next week...
The original idle jet for my car is a 60 F8, so I went ahead and bought a set of 65 F8 which will be installed once we know that there aren't any problems ignitionwise. The immediate acceleration seems to be fine - until, as I mentioned, you get over 5000 RPM, when you feel that it doesn't accelerate as freely anymore and misfires occasionally.
On the lower end the engine only seems to idle well at higher than normal RPM's (hard to tell precisely, since the rev-counter trembles and seems to be slightly off). Also, the acceleration out of the stand seems to be very sluggish. I always use a lot of clutch which I'd rather not. Hence the idea for the bigger idle jets.
Let's wait and see what my mechanic comes up with. I'll keep you posted...

 

Just a quick note to say that after putting in the 135's , this car is transformed. 95% of the stumble is gone. Next is the ignition upgrade. YES....I'm having lots of fun!!!!

Geffen

 

Woo-hoo! Congrats! These Ferraris, they lika da gas, no?

I repeat my humble suggestion: if you have a carb car that has not been re-jetted since it was made in the 70s (or before), the new gas formulations of the last few years will make it run poorly on its original jetting. You will be happily surprised at the results as to what your car is supposed to run like after a very simple and quick re-jet, typically raising the main jet one or two sizes.
Best to all!