Idle Jets Gumming Up

[Bill Sargent]

I am experiencing a problem with the Weber 46IDM carbs on on the 2L motor in my 59 A. The idle Jets plug very easily, which I believe is caused by the “up to 10%” ethanol gas around here. I had not driven the car for 2 weeks. That drive was 3 weeks after the previous one and it was popping some in the transition range - 3K to 4K. I had cleaned the idle jets a few weeks before that.

Before going on a drive today I checked the idle jets and found 3 out of 4 had gummed partially closed. After cleaning the jets I also swapped idle jet carriers for a set of old originals I had - same size markings but the holes in the sides were slightly larger. Night and day difference! The motor ran perfectly. Interestingly, I had to adjust the idle up after the change.

Is anyone else experiencing this kind of problem? The 46 IDM, 46 IDA and 48 IDA all use the same idle jet and carrier and are located in the same place in the carb so I assume they all might react the same.

The only other thing I can think of is fine material in the fuel, so I plan to install a fuel filter.

Any other thoughts welcome.

[Rainer Cooney]

Bill, I usually don't run pump gas in a 4-cam engine due to the lower octane and it going bad in short order when stored. Try either Cam-2 or 100 ll avgas. If this is not an option then shut off the fuel pumps before stopping the engine and run the carbs dry for storage. The jets aren't gumming up when running, it's when the car is in storage and fuel evaporates to form the residual gumming.

[Greg Bryan]

Perhaps the fuel lines are deteriorating on the inside - I’ve started to use Gates FI hose after giving up on the German braided stuff.

P.S. - I’m in California so running a minimum of 10% ethanol. The car actually seems to run ok if I keep the fan belt on it (private joke)

[Harlan Halsey]

I have a similar problem with my Honda emergency generator. Ideally, I'd like to run it 4 times a year, but if I run it dry the main jet often gums up and it won't start. Now I'm not running it dry and starting it more frequently. Old tractors used to start on gas then switch to cheaper kerosene. Maybe we should have such a dual fuel system for the Carreras: start and stop on race gas and street run on street gas!

[Ken Tuvman]

Perhaps the fuel lines are deteriorating on the inside - I’ve started to use Gates FI hose after giving up on the German braided stuff.

+1 on Gates hose vs braided hose - every time I had a fuel leak on my BMW airheads was from the braided hoses!

[Brian R Adams]

Bill, I usually don't run pump gas in a 4-cam engine due to the lower octane and it going bad in short order when stored. Try either Cam-2 or 100 ll avgas.

Bill, is ethanol-free "Blue" gas not available at Maverik stations in your area?

Edit: I see Maverik stations are only in E. Washington. The nearest Maverick to Seattle w/ ethanol-free gas is in the Tri-Cities. Pity, as that stuff works wonderfully in my 356.
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[neilbardsley]

I had an issue on my the 40 IDFs Webers on my 914 with the idle jet too. I installed a 2nd fuel filter but it didn't have so I installed some jet doctors. They stop dirt from the top of the carb being pulled into the air feed for the idle jet. Maybe they will help you. Cost about £35 and took an hour to install

[Bill Sargent]

The problem is not fines in the fuel blocking the idle jets. I have a good filter upstream of the carbs. The car runs fine after the jets are clean for as long as I want to drive it. And continues to run fine if I drive it every day or every other day. The problem occurs when the car is not driven for 1 to 2 weeks.

On Weber 46IDM carbs (also 46 and 48 IDA) the tips of the idle jets are below the level of fuel in the float bowls. As long as there is sufficient fuel in the float bowl to submerge the idle jet tip, all is good. But if not driven for a week or two, enough fuel evaporates from the float bowls to leave the idle jet tips high and dry - with an ethanol gum residue partially or wholly blocking the jet. The solution if this happens is to remove and clean the idle jets - which is a 10 minute job on these carbs.

A preventative measure, which Rainer stated earlier in this thread, is to turn the fuel pump off after a drive and let the float bowls empty and motor die. This helps a lot as the air flow dries the idle jets, but some gum still forms and periodic idle jet cleaning is required.