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Message |
Mike Procopio (Pupz308)
New member
Username: Pupz308
Post Number: 38
Registered: 10-2002
| | Posted on Monday, January 06, 2003 - 8:57 pm: | |
I replied to John personally, but wanted to post this for reference in the archives.
John,
Mike Procopio here. Saw your post in the FerrariChat archives regarding fuel starvatio in corners and during certain other light-throttle scenarios.
I had the exact same problem in my 78 308 GTS, and just recently resolved it. There was a very extensive thread on the FerrariList (email list-serv) regarding this very problem. I was amazed at how you described the problem, for it was mine exactly--you did a very good job at explain this (it's pretty tough to really convey what's going on).
In the end, it had a lot to do with the carb jets. It's quite complex, and withing really re-researching it much more, I can't really explain the mechanical reasons why. But from memory, too lean on the idle jets will cause the leaning out condition during mid-RPM, light throttle. In general, during this light-throttle scenario your're running off of the idle jets. Occasionally a lean condition can manifest--that slight popping and sluggishness. Add in more throttle, and the main jets come in to play, fixing (masking, really) the problem (problem being the lean condition). This behavior is apparently intrinsic to the Weber carbs in our 308s. I'd suggest going a bit richer in your idle jets--definitely not below stock (55), and possibly to 60 or 65.
The other critical part, explicitly addressing the fuel starvation in corners, was actually the accelerator pump jets. We went to 45 on these. Solved the problem of fuel starvation during turns. Floats, as you mention, were spot on.
Just got the car back last month after having done this service. It was pure bliss--I had put up for a good while with the starvation. I took it up in to the mountains and tossed it ALL around--no problems at all, not one moment ever of hesitation. Blissful.
I wanted to mention this to you to see how you fared with your problem--have you solved it? In all my extensive collaboration with many helpful people, accelerator pump jets were never mentioned--not once. It turned out to be the solution.
Let me know how things are going on this issue. Oh yea, I'm at 5000ft in Albuquerque, NM. The starvation issues were not present, to my knowledge, in Florida (sea level) where I bought the car. Perhaps you're in a high part of SF? What's the elevation?
Hoping this is of some help.
Take care,
--Mike
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John_Miles (John_miles)
New member
Username: John_miles
Post Number: 12
Registered: 7-2001
| | Posted on Sunday, July 21, 2002 - 2:14 pm: | |
The fuel pump has already been given a clean bill of health by my mechanic. Also, low volume would rule out the car's ability to accelerate strongly and keep going at WOT, wouldn't it?
My understanding (which could be wrong) is that the accelerator pumps do nothing during steady-state throttle operation, while cornering or otherwise. They just provide a transient squirt of gasoline to overcome the temporary leanness that occurs when you open the throttle in a hurry. So I don't think they're responsible for the long-term improvement in power that occurs when you ease the throttles open farther while cornering.
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David Feinberg (Fastradio2)
Junior Member
Username: Fastradio2
Post Number: 108
Registered: 4-2002
| | Posted on Sunday, July 21, 2002 - 1:42 am: | |
John,
You might want to check the fuel pump volume, as this problem (low volume) can send you down the path of thinking the float level is incorrect. As to why it appears "better" when you give it full throttle...just a thought, the pickup for the accel pump I believe is at the bottom of the float chamber, so when you mash the throttle...you're giving here an addition pump shot...hence enrichening the leaness due to the main jets being momentarily starved for fuel...
David |
John_Miles (John_miles)
New member
Username: John_miles
Post Number: 11
Registered: 7-2001
| | Posted on Sunday, July 21, 2002 - 1:15 am: | |
You know that feeling you get in a carbureted 308 when you're taking a corner fast and the car bogs down from fuel starvation?
You know how you can almost always get the engine sounding and running strong again by putting your foot down a little farther into the gas?
Why is that, _exactly_?
The reason I'm asking is that my car is behaving that way under most driving conditions, not just in curves. Idle and overrun are OK, and so is hard acceleration. Under light acceleration, though -- the kind you need to maintain a steady cruising speed on a hilly road -- the engine takes on a flat, rough sound and power fails to build. When this happens, I can put my foot down a little more, reaching perhaps 1/4 of overall throttle travel, and it's like throwing a switch. The engine note smooths out and the power comes back on strong, just as expected.
This behavior started on the return leg of my road trip to the SF bay area from Seattle a couple of weeks ago, although my gut feel is that it's been lurking just beneath the surface for as long as I've owned the car. I-5 through northern California is rather hilly to say the least, and while the car performed like a champ otherwise, the bogging symptom really got on my nerves after awhile.
When I got home, I verified the carburetor synchronization (which was perfect under both fast and slow idle conditions); removed and blew compressed air through the idle and main jets; and pulled the tops off the carbs and checked the floats. The float levels looked fine at 48 mm (+/- a mm or two allowing for parallax) with the tangs resting lightly against the needle valves, with an additional 10.5 mm of travel fully extended... all perfectly by-the-book. I also replaced the accelerator pump diaphragms just for good measure; the diaphragms I removed were in near-new condition.
At last, after I put everything back together, I fired her up and went for a drive. Lo and behold, the symptom had gone completely away, except for the same tendency to bog a little more readily than usual in corners that this particular car has always had. For several days, the car ran as well as or better than it ever had, and I was a happy camper. Evidently, the act of taking everything apart and putting it back together was enough to "fix" the problem.
Today, for the first time since tinkering with the carbs, I really, *really* bogged the car out on a long, sweeping entrance ramp at about 2X the posted speed. Had to put my foot almost the way to the floor to keep the power up... nothing unusual for a carbureted 308. But as I came out of the ramp, I could tell immediately that the problem was back. Basically, the car behaved the same on the straightaway as if it were under lateral acceleration... sluggish and flat on initial throttle tip-in, with very occasional popping from the intakes, followed by strong, healthy acceleration upon further application of the loud pedal.
So I'm thinking this can't be a coincidence. It sure sounds like a float-related problem, because it seems to be very close to what happens when the floats are inhibited from their normal travel when pulling high lateral Gs. The act of taking the carb tops off and manipulating the floats manually got rid of the problem, until I deliberately provoked it again by sending the car into G-induced fuel starvation.
Beyond the obvious "Has anyone else ever seen this behavior, and if so, how'd you fix it for good?", what I'd like to understand is this: in a normal carbureted 308, why can you mask the symptoms of fuel starvation in a corner by putting your foot down even farther? That's always been the standard advice to F-car drivers complaining about starved Webers on the high banks, but I've never heard an explanation of the underlying theory.
Any clues? |
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