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Discussion Starter · #1 ·
I have completed the major ser

I have completed the major service of my 308GT4. Now, when accelerating at lower revs, it pulls fine with little throttle, but with more throttle it starts to spit and hesitate, before it starts to pull. This is really bad at about 2000 revs, gets better with higher revs. From 4000 there is no hesitation even at full throttle. There is also a lot of popping from the exhaust at deceleration.
Jets are cleaned, accelerator pumps are fine and working.
I had the cams reground at Webcam. Slightly longer duration and higher lift and less overlap (Webcam's 308 performance street specification). Everything else is stock.
Another thing is that the car did not run for more than a year, did about 400 miles since.

Any ideas? Do the carbs need re-jetting due to the other cams?

Best regards
Harry
 
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Discussion Starter · #2 ·
Called a Weber specialist toda

Called a Weber specialist today: his recommendation was to go up one size with the main jets from 130 to 135. Ordered them today, will have them next week.

Regards
Harry
 
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Discussion Starter · #3 ·
If your mechanical spark advan

If your mechanical spark advance isn't SPOT ON it can cause some power problems..when was the last time the distributors were lubed?

Sitting up is the worst cause of this......

You pull them and bench calibrate them with a machine, they are either right or very wrong! LOL!
 
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Discussion Starter · #4 ·
Jets will move more fuel......

Jets will move more fuel.......do you have a gas analyzer available?

If your spark is off that will tell the tale....
 
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Discussion Starter · #5 ·
I strongly suspect that the re

I strongly suspect that the reground cams have changed the metering requirements in the low end. The very lowest part of the low end are controlled by the idle metering jets while the transistion to the mains are controlled by the main venturi, the float level and the main jet.

From what it reads like, you are lean on the transition to the main metering, so we go up 1,2,3 sizes on the main jet to get the transistion right, and then look for an air corrector that gives the desired mixture up top (13.2:1 power -to- 14.5:1 economy).

If it takes more than 3 jest sizes to bring the transistion point up, we may want to set the floats a little higher so that the partial vacuum at the main venturi does not have to pull so hard to get the gas flowing.

It is unlikely that a reground cam will change the requirements for the main venturi (larger), but not impossible. And if you end up going larger here, it makes finding a main and air jet for the transitiion harder.

Good luck
 
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Discussion Starter · #6 ·
Alan,
had the distributors ap


Alan,
had the distributors apart, parts were moving. Anyhow, will check the advance on the car when I am in the garage this week ... neighbours don't like Ferraris reving up to 5000 rpm in the yard


Mitch,
thank you, this is the information I was looking for! I will start going up with the main jets to get the transition right. The Weber guy told me that he would not go up more than one size at a time. Probably have to go on a rolling road to get the top end mixture (air correctors) right.
Is the popping from the exhaust without load connected to idle jets (too small)?
 
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Discussion Starter · #7 ·
[QUOTE]neighbours don't li

neighbours don't like Ferraris reving up to 5000 rpm in the yard
I find this hard to believe! Perhaps you need different neighbors.....

Popping is an indication of leanness, and can be coming from the bottom of the transition range or from the top end of the idle circuits.

How much vacuum are the cylinders pulling at idle? If this is smaller than (say) 15 inches you may be able to deal with it with the main jet, otherwise you may be able to get the idle bleed farther out (richer) or go up one idle jet size.

An exhaust gas analyser is very useful in setting the air jet, as the engine will run fairly well over a number of air correctors (given the rest of the metering is spot on.) Alternately, a plug cut after a long acceleration run is in order. A gas analyser on a dyno is optimal, not just for the air jet, but also for the main and idle jets.
 
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Discussion Starter · #8 ·
Will go to a friend with a sho

Will go to a friend with a shop and have a gas analyzer available as well as colortunes. Did order larger jets but they sent only 6 jets. Have to wait another two days. Grrr... Have ordered jet files (reamers?) and gauges to be able to increase jet size without waiting for days.

Neighbors did get the whole basic ignition tuning and 15 min run-in of new camshafts on a sunday morning. Neighbor resonance was a negative square function of age with a clear maximum at 18 years, crossing the zero-axis at about 6 and 30. Some exceptions of course because maths never fully reflect real life.
 
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Discussion Starter · #9 ·
Installed the 135 main jets. I

Installed the 135 main jets. Idle (45) and AC (220) unchanged.
Still hesitation from about half throttle up to 4000 rpm. But my feeling is that power in the top range has increased.

Will increase main jet size step by step now. How do I see it becomes too rich?

Regards
Harry
 
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Discussion Starter · #10 ·
Did a plug cut after a constan

Did a plug cut after a constant 100 mph drive, what means 5th gear at around 5000 rpm - plugs were nearly white, still way too lean
 
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Discussion Starter · #11 ·
Keep going up on the main jet

Keep going up on the main jet until the transition stumble is well in check, then look for an air jet for top end mixture control.

Once you get close, then try adding some idle richness (the screws) to see when/if you can control the transistion stumble. When you can control this stumble with the idle mixture screws, then we will have found the correct mains.
 
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Discussion Starter · #12 ·
Thanks Mitch,
I have found a


Thanks Mitch,
I have found a shop with a dyno and Weber experience&parts close to my home. Will have a tuning session there in two weeks. That seems more efficient taking into account the time and cost for getting different jets. Will update after the session.

Regards
Harry
 
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Discussion Starter · #13 ·
Harry,

to answer your '


Harry,

to answer your 'plug check' mixture question:
White is to lean as you mentioned, Brown (medium brown) is just right & Black is way to rich.

You will also have to take into account the heat range of the plug. The original factory reccs were to cold for average, normal driving (except for the Autobaun) & one or two steps hotter on the plug heat range is SOP for street driving on the Carbed engines.
 
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Discussion Starter · #14 ·
I use NGK BPR6ES. In fact, it

I use NGK BPR6ES. In fact, it happens that the car is used on the Autobahn. There is no way to feel faster doing 130 mph
. I will stay away from higher speeds until the mixture is set correctly.

Regards
Harry
 
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Discussion Starter · #15 ·
Also keep in mind that as the

Also keep in mind that as the airflow is increased through the motor, the plugs will want to go colder. If BPR6ES are stock-ish plugs, you might try one range colder, but wait until you have the mixture closer.
 
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Discussion Starter · #16 ·
Just back from the dyno tuning

Just back from the dyno tuning session. It took a full day! The mechanic is very experienced with Weber and perfomance engines. Facts in short:
- points gap adjusted to 0,4 mm (my cheap dwell meter is rubbish!)
- ignition advance set to 34 deg at 5000 rpm, what comes down to 11 deg at idle
- float level set to 48 mm (was a bit high)
- carbs balanced precisely

First dyno session was for part load and transition. Idle jets were taken up one size to 0.50 mm.
Then the mid range rpm and acceleration were tested. It showed that the car ran too rich, in particular during acceleration! I was completly surprised. Main jets were changed back to 130 and then 125. This was an improvement but still some hesitation at full throttle. Accelerator jets were changed from 0.45 mm to 0.40 mm. Much better, hestitation nearly gone. Unfortunatly there were no smaller accel jets on stock.
Dyno showed good results, more than 240 (corrected) hp at 7500 rpm, still rising (red line was in the way!). Torque peak at 5000 rpm
When the smaller accel jets are there, the next will be to go down to 0.35 accel jets, idle one step to 0.55 and main jet and air corrector one step up (130/215 or 210).
Surprising result, but according to the mechanic probably connected to the WebCam timing spec (105.5/106.5 lobe center with the 270/260 deg cams) with less overlap than stock.
The car feels completely different now, there is no comparison in driveablitiy, smoothness and even power. This was well invested time and money! Now I am curious about the final tuneup.

Regards
Harry
 
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