Carb issues at speed

xs10shl · Post by **xs10shl** » Mon Aug 11, 2008 3:04 am

Its been 6 months, and I've finally got my 330 GTS back together for Monterey. The cooling and suspension systems are functioning perfectly, but now I've got a bit of a carb problem.

The car starts and idles fine, and revs beautifully at a standstill. When driving, It even pulls fine to 6500 RPM, but as soon as I back off the throttle at speed and try to cruise at 4000 RPM, it begins to backfire. It will continue backfiring until I either speed up to 5000 RPM, or slow down to 3000 RPM, It's always been a little testy at this particular RPM, but now it's noticeably worse.

I've done all the easy stuff like clean all the idle jets and check the fuel flow, so I'm now looking at the coils, the distributors, etc. It looks like pretty much all of the carb intakes have black deposits on them, so it appears to be happening universally.

Anyone experienced this before? Any suggestions as to how to proceed?

Post by **330GT** » Mon Aug 11, 2008 10:18 am

Is it truly backfiring back through the carbs? If so, ignition timing, perhaps the advance curve isn't right. The best bet is to take the distributors to someone who has an old Sun machine so everything including the advance curve can be checked out.

But after Monterey please, I want to see your car Thursday night! Ignore TomY's comments about parking in the shopping center. Park yours right next to the breakfast room.

If it is just a backfire noise, probably exhaust leaks, perhaps at the headers/collectors junction as that is difficult to seal well if there is any misalignment.

Post by **tyang** » Mon Aug 11, 2008 10:26 am

But after Monterey please, I want to see your car Thursday night! Ignore TomY's comments about parking in the shopping center. Park yours right next to the breakfast room.

I don't think anyone will complain if Vintage Ferraris park at the Motel.

Tom Y.

Post by **tyang** » Mon Aug 11, 2008 12:28 pm

Hi Tom,

Francois suggests adjusting the mixture first to see if there is a lean issue with the carbs. A backfire through the carbs can mean it's running a little lean at the RPMs you're describing. If nothing changes, check your timing, and see if the distributors are advancing correctly and in time with each other through the rev range.

Tom

xs10shl · Post by **xs10shl** » Mon Aug 11, 2008 3:46 pm

Thanks for the information. I took the car over to Dayal at Prancing Horse, and he dialed in the carbs. He also mentioned the advance curve as being the likely culprit, but we decided to just leave things as they are for the week, and revisit the issue later, since the car is pretty much working, just not perfectly at 4000 RPM.

I may even have time left over to get the aerial antenna working, so we can have some sweet FM tunes blaring through the speaker as we ride down the coast.

365gtc/4 · Post by **365gtc/4** » Mon Aug 11, 2008 4:18 pm

xs10shl wrote: I may even have time left over to get the aerial antenna working, so we can have some sweet FM tunes blaring through the speaker as we ride down the coast.

The sweet tunes come from under the hood. Enjoy the ride down the coast.
John

Deane · Post by **Deane** » Mon Aug 11, 2008 4:29 pm

Let me know if you can hear the radio at all when the car is running. I asked my resto shop about it while the car was being rebuilt, and they all just laughed. They were right.

Rudy van Daalen Wetters · Mon Aug 11, 2008 10:01 pm

Timing issues is my thought. Dial in those distributors.
Made all the difference on my cars.

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

John Vardanian · Post by **John Vardanian** » Tue Aug 12, 2008 11:56 pm

Hi Tom,

Backfire thru carbs would typically mean a lean mixture, as Tom suggested. If this is the case, the problem can be jetting or plain fuel starvation due to lack of delivery from the pump. If your glass filter/regulator jar doesn't top off; i.e., there is a good size air head, this would mean poor fuel delivery from the pump.

If your back fire is thru the exhaust pipe(s), then timing is suspect.

john

Colin Angell · Post by **Colin Angell** » Thu Aug 14, 2008 8:52 am

Hi there, The symptoms you describe sound almost identical to a problem I had with my 365 GTC, when I first bought it. The car would run fine up to about 4000rpm and would then almost "chug" along if you tried to level off and cruise. it turned out to be poor float setting (too high) in the carburettors. This over filled the float chambers and let raw fuel dribble into the inlets - result was very lumpy running at high speed idle which would go away if you accelerated harder. Easy and cheap to check, let us know how you get on. Colin