Engine idle problem

[John Vardanian]

My carb throttles do not return to full rest. Maybe I can describe it this way, the car rolls to stop the engine idle is at 1100RPM, I nudge the gas pedal up with my foot, the idle drops to 800. Throttle linkages operate freely (non-cable set-up) and the adjustable return spring (on the cam cover) is at full tension. Any advise. Thanks.

john

[tim]

Greetings... Did this just start on an already running car or is this on a fresh rebuild? I had something similar on my 330 and one of the things that helped was a bit of oil on all the swivels in the linkeage and bearings.
Tim

[John Vardanian]

Hi Tim, this has been going on for a few months, I tensioned the return spring and it didn't fix it, but I'll revisit the linkage as you suggested. I was trying to see if anyone else has experienced this.

john

[DWR46]

John: How much wear do you have in your throttle shafts? Your problem is quite common. Usually it is worn throttle shafts leaking air. When you pull up the accelerator pedal, you reposition the butterflys and shafts, and close off the air flow enough to lower the idle rpm. Honestly, most 250's and 275's do this to some extent.

[John Vardanian]

Thanks Dyke, I thought you might have the answer. I don't know how much wear there is. All three are covered with an even coat of varnish, so I thought of making a nice winter project of it.

john

[cory]

John
Just as an FYI, if you decide to do your throttle shafts pay attention to the actual size of the oversize shafts and the worn openings. I just did mine and one carbs shaft hole was worn oblonged just a touch. After line boring and installing the oversize shafts this one shaft is sucking a bit of air. All be it fine I too will have another winter project bushing this oversize shaft to get it perfect.
Cory

[peterp]

For lubing the throttle linkage, I suggest using ATF. I'm not sure why, but ATF works a lot better and lasts a lot longer than regular oil. I learned about this from Mercedes forums, apparently some older Mercedes models are prone to linkage issues and ATF is a pretty widely agreed upon solution.

It might be worthwhile to try adding Marvel Mystery Oil to the gas. This is supposed to lubricate the throttle shafts and internals of the carburetor. I don't know to what degree it does this, but a lot of people swear by MMO.

In newer Ferraris, it's very common to have idle problems and even unintended cruise control from the floor mats. It's probably not the case here, but tossing it out as a long shot solution.

[Colin Angell]

Hi John

Just before you strip those carburettors it is worth checking that your throttle cable is moving freely. It is quite common for the cable to stick in its sheath, especially if you have neglected a bit of lubrication there. Also in a forty something year old car there is a surprising amount of wear between the throttle cable and the hollow brass adjusting bolt at the end of the cable, which can cause it to bind.

Good luck and let us know what you find

Colin

[afwrench]

Hi John also make sure you are getting the full Monty from the return springs.They do wear out over time. Best regards, Mike

[DWR46]

Mike Pierce has just released his latest DVD on Weber carbs and it is an advanced tuning course for DCOE's. I watched it yesterday and was impressed. Everybody who works Webers should get a copy. To this subject, it addresses John's problem somewhat and Mike and I spent time today on the subject. He said they were having terrible problems getting carbs to idle down with today's fuels. Even with the buterflys completely closed, in the past, the air around the plates would give adequate idle, but now the engines are idling at 1,400 rpm with the plates closed. They have had to rebore the barrels and make new oversize butterflys or sleeve the barrels to get the air flow low enough to bring the idle speed down to acceptable levels. All blamed on the stuff that passes for gasoline today.

[Steve Meltzer]

John, this problem comes right out of Steve Meltzer's "A moron's guide to Ferrari Carburetors"! What happens when the carbs are run and are disconnected from the linkage? Does the idle return to normal when you've activated the throttle ("blipped" it) and let it return to home? If it does, it's a linkage problem, somewhere, and count yourself lucky 'cause it's not likely to be in the Webers. However, if the car continue to idle high, which is my guess, it's getting air from somewhere it's not supposed to. Probably, as Dyke said, it's the throttle shafts and/or non-sealing butterflies. Three other things you could do to help narrow it down are:

1) See what each barrel is pulling at idle on your synchrometer. On my GTC you couldn't drop the "draw"/vacuum on #1/Left no matter what you adjusted. Air was leaking past that (1/L) butterfly, worse than the other two carbs. As you know, this test is done with the car warmed up, linkage, dc'd. Again, if the problem is in the linkage, these numbers will be very, very close to the same in all 6 barrels.

B) Again car warmed up, linkage off at idle. Can you change the idle by temporarily "sealing" the throttle shafts in their orifices with modeling clay? (this is a test Mike Pierce does on this flow bench) Obviously, if you can, a problem is there at the shafts.

iii) Use a flowing propane torch (unlit!), WD 40, or Brake Kleen to look for vacuum leaks. I've not been real impressed with this method in all circumstances, but it's easy to do and might contribute to your knowledge.

Pierce just did my GTC carbs and has the DCNs from my Daytona Spyder conversion in his shop now. All of those old carbs were warped and the bores not square with respect to the butterflies. He'll certainly be able to pay his expenses this month

Best of luck. steve

[John Vardanian]

Thanks Steve, the suction is equal among the six barrels. I have been thinking quite a lot about what Dyke said about the worn shaft bores and am still unable to correlate the the symptom to the problem.

Dyke, are you saying that the shaft (or shafts) sticks in its bore and resists the force of the return spring and then when I pull the pedal up with my foot the joint force of the spring and my foot together are able to reset the throttles to where they should be? In other words the drag is at the shaft bores, correct?

john

[330Jim]

Another possibility... I worked on a 212 that had the same issue. Linkage was good, throttle shafts were good, but the idle was inconsistent. After pulling the carbs off and going over them I found the return springs that wrap around the throttle shafts were set incorrectly, the tension was too low. This was difficult to find because it was easy to set them up the way they were but a little tricky to get the tension set where they should be, plus the tension did not feel unreasonable. Once these springs were installed correctly the idle was perfectly consistent (the tension on the throttle shafts felt better too: when they are right, the butterfly's will snap shut with authority). Note it took multiple tries to get the technique to do it right. If you are working on your PF coupe, take a look at the brass covers over the throttle shafts on the forward side of the carbs, inside those covers I would expect to find a hair spring. If this spring does not have enough tension, the idle will be inconsistent. Note the carbs on the 212 were professionally built and this issue was missed.

Cheers Jim

[250GT]

Another possibility... I worked on a 212 that had the same issue. Linkage was good, throttle shafts were good, but the idle was inconsistent. After pulling the carbs off and going over them I found the return springs that wrap around the throttle shafts were set incorrectly, the tension was too low. This was difficult to find because it was easy to set them up the way they were but a little tricky to get the tension set where they should be, plus the tension did not feel unreasonable. Once these springs were installed correctly the idle was perfectly consistent (the tension on the throttle shafts felt better too: when they are right, the butterfly's will snap shut with authority). Note it took multiple tries to get the technique to do it right. If you are working on your PF coupe, take a look at the brass covers over the throttle shafts on the forward side of the carbs, inside those covers I would expect to find a hair spring. If this spring does not have enough tension, the idle will be inconsistent. Note the carbs on the 212 were professionally built and this issue was missed.

Cheers Jim

Yes I also think this is a good point Jim330 mentioned.
See spring 31 and 36 on this explosion view on 40 DCZ.
Youre 36DCL-6 (PF) ans 36DCS ( Lusso) are the same inside as the pic below.

P.S I do own a 40DCL-6 set which came with overhaulbill of PierceM these are till today FAULTLESS (with modern fuel).

C.

[DWR46]

John: NO, the shafts do not stick in the bores. Usually the shaft bores wear oblong due to the linkage pressure over time. When the throttle is closed by the return springs, the shafts rest at a given position relative to the worn bores. Then when you manually apply pressure to the linkage, you move the shaft in the worn bore and many times the butterflys move enough to cut some air flow and slow the idle.