Engine Flat spot in 365GT4

Good day Daren,

The problem could be caused by a number of things and these items could be related... in other words it may not be simply "one" thing. Certainly doing a quick check of the jets would not hurt, but I would do some further diagnoses first. Is the problem fuel related or possibly ignition related? On the simple side...It could be a plugged fuel filter, old fuel, etc. Is the problem isolated to one or more cylinders? As I said, the issue could be a number of things and so a step by step diagnoses/analysis would yield much better results.

Cheers,

Sam

 

More people can add to this but my car exhibits the same thing. Not sure how bad yours is. You definitely can't just floor the gas especially from lower RPMs. I've heard different theories including that today's gasoline is quite a bit different than it was when these cars were designed and built. I have heard that some people have put smaller venturis in the carbs and that can help the situation with a small loss of power at high rpm. Another issue can be ignition timing and specifically if you have two distributors and the static timing is off or if the advance curves are different between the two. What some people do is set up the ignition so only one of the distributors controls the the spark for both. They do this by modifying the controlling distributor to have two pickups (I believe this can only be done if converting to electronic pickups and getting rid of the points) one of the pickups is hooked up to one coil and the other pickup is connected to the other coil. This company
http://www.carobu.com
has some interesting studies where the modified the center section exhaust pipes and did an X pipe between cylinders 1,2,3 and 4,5,6 on the passenger side and then did the same thing on the drivers side. The before and after dyno results showed a much improved midrange torque curve.
Hope this helps.

 

The colortune spark plugs are usefull for any enthousiast who wants to spot an ignition/mixture problem on a given cylinder. That's basically an old system based on transparent spark plugs, just have to put it instead of the regular spark plug and you can see the combustion. Very useful in order to isolate a carb that is totally off or faulty HT leads. Usually I tune my carb with this transparent plug, and if I cannot find a good tune (i.e blue flame in the combustion chamber), I know it's time to ask my mechanic...

Dumb question: are you sure of the quality of your fuel? The petrol blend here is vastly different in winter versus summer and even between brands (the amount of ethanol is not the same). My vintage cars do need a specific winter/summer tune in order to work properly. I know some countries do have Ethanol Free fuel, that's a good solution for consistent fuel mixture.

 

Unlikely to be fuel grade - the 1970's Ferrari engines are relatively low compression and don't need a high octane fuel to run OK, 95 will be fine.

Under acceleration the accelerator pumps in each of the 6 carbs push extra fuel in to the engine, if there is not enough fuel reaching the carbs there may not be enough fuel in the accelerator pumps, easy to check fuel availabalilty by disconnecting fuel line at the engine and running pumps and seeing how much flow there is in to a container ( some fire precautions needed ). But you say you have plenty of power when it does rev, so fuel quality or flow are very unlikley the cause of the issues. You might have a clogged jet, but again one clogged jet is hardly noticeable with 6 carbs and 12 venturies.

What you are looking for is a problem that affects the entire system, not a 1/12th problem like a spark plug, jet or ignition lead. I would focus on the ignition, in particular the timing and advance curve. It is not unheard of for the mechanical advance mechanism in the distributor to "stick" and not provide the required smooth advance curve as engine revs increase.

M

 

As noted earlier by RAEMIN, the lead free 95 may not be the most appropriate solution everywhere. In Europe this fuel can contain up to 5% ethanol and even up to 10% in its version 95-10 (E10 is the new name for this fuel in Europe). Ethanol is not friendly with fuel components in our old cars.

98 is not perfect on the ethanol content either, but this content is supposed to be smaller than in the 95.

 

I agree, the ethanol content is not good for some of the rubber components and that could lead to problems with carburettor float needle seats. In Australia ( and NZ) not all fuels contain ethanol, and most 95 and 98 octance contain no ethanol.

 

Good day Daren,

I am unsure if you are aware and/or have ruled this out... but "The bogging down only after a hot start" can be caused by vapor lock (https://en.wikipedia.org/wiki/Vapor_lock) in the fuel lines. I say this, as later in you comment you state "until I have driven a bit" which would substantiate the vapor lock issue.

Cheers,

Sam

 

Good day Daren,

You are most welcome! However, Fuel Vapor lock is an unpleasant side effect of a number (if not most) of carb'd engines and so you may be unable to "fix" it. Vapor lock happens when a hot engine is switched off and the heat within the engine bay rises because of no active cooling (e.g. engine stopped means coolant has also stopped moving through the radiator, no air moving around the engine, cool fuel from the gas tank is no longer feeding the carbs, etc). The result is that the fuel within the engine bay fuel lines and carb float bowls rise to a point where it becomes gaseous...and so when you restart the car the fuel does not flow properly which negatively affects the carbs's ability to properly meter fuel... this causes rough engine running, carbs spitting, etc.. generally poor operation of the car. Once the engine is running the engine temperatures start to lower and cool fuel flows to the carbs and thus after a short while the engine starts to run properly. In your car's case it would be best to determine if this is the issue, as if so you may need to simply wait a few moments after starting a hot engine before driving off.

Cheers,

Sam

 

I would check the filters in the fuel tank as well if they have not been replaced.
These are out of one of my 400i's.

Cheers,
Al

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Good day Daren,

The fuel filter is not located in the fuel pump it is a separate assembly ... see attached diagram (item 14). The filter is part number is 95180050 and comes with the gasket... these are stock and can be ordered from Eurospares, Ferrari UK, etc.

As for the pumps... I would not replace them until you have reason to do so. If so, then a substitute for the Bendix pump is the Facet pump which looks and operates quite similarly. Although I believe that the Bendix pumps can be rebuilt.

Also, you should also examine and replace the cross over fuel hose sections (see second picture - item 21)... as these become old with age and crack/leak... and could be a fire hazard. To replace these you will need to drain the fuel tanks and so this may be a job for your service garage, as it can be messy and dangerous.

Cheers,

Sam

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