Idle Jets Gumming Up

Discussion of 4-Cam Type 547 engines (and all the Fuhrmann racing variants) and cars that powered them.
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Bill Sargent
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Idle Jets Gumming Up

#1 Post by 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.
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Bill Sargent
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Rainer Cooney
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Re: Idle Jets Gumming Up

#2 Post by 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.

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Greg Bryan
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Re: Idle Jets Gumming Up

#3 Post by 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)
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Harlan Halsey
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Re: Idle Jets Gumming Up

#4 Post by 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!

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Ken Tuvman
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Re: Idle Jets Gumming Up

#5 Post by Ken Tuvman »

Greg Bryan wrote: Mon Jan 30, 2023 3:17 am 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!
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Brian R Adams
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Re: Idle Jets Gumming Up

#6 Post by Brian R Adams »

Rainer Cooney wrote: Sun Jan 29, 2023 8:01 pm 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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Neil Bardsley
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Re: Idle Jets Gumming Up

#7 Post by Neil Bardsley »

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

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Bill Sargent
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Re: Idle Jets Gumming Up

#8 Post by 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.
Regards,

Bill Sargent
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Alan Klingen 2
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Re: Idle Jets Gumming Up

#9 Post by Alan Klingen 2 »

Webers tend to have very small idle jets so step #1 would be to put a fuel filter in the fuel line to catch all the junk. Alan The Stable.

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Re: Idle Jets Gumming Up

#10 Post by Harlan Halsey »

I only run my Carrera on 110 octane race gas. Sunoco, now available from a Mobile gas station down the street. Sunoco offers an amazing range of racing gasoline's available in 5 gallon cans for about $100 ea. I get the nearest to the now defunct ERC 110K. The car sits for months, a GT Carrera is no fun on the street. Never a problem with the Solex 44 P 11s. I have no idea what mileage the car gets on the street, but it burns about 3 gal. in a 10 lap race (20 miles ?) at Laguna Seca.
I've never had a problem with the 40DCOEs on either of my English race cars which run only race gas but sit for many months.

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Re: Idle Jets Gumming Up

#11 Post by Alan Klingen 2 »

Webers have very tiny idle jet openings so the fuel has to be extrely clean also the air to the carbs has to be very clean also. Alan The Stable.

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Re: Idle Jets Gumming Up

#12 Post by Alan Klingen 2 »

Idle jets are sosmall that even a small drop of water can clog them . Alan The Stable.

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Re: Idle Jets Gumming Up

#13 Post by Alan Klingen 2 »

The idle jets are so small that any debis will clog them my advice to you is put a fuel filter inline with the feed line and if possible use the all metal Bosch filter. I find them superior in filtering and Quality. Also any debris even small water drops will stop the fuel flow. Also keep a clean air filter on the motor. Alan The Stable.

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Re: Idle Jets Gumming Up

#14 Post by Brian R Adams »

What are symptoms of slightly stale gasoline? I had a 5-gal can of Maverik 'Blue' ethanol-free, about 6 - 9 months old. I let my clean < 2-year-old fuel tank get down to near empty, and added the "old" gas. Could be coincidence, but now while the engine still idles and accelerates great, on coast-down I'm now getting sporadic mild backfires.
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Martin Benade
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Re: Idle Jets Gumming Up

#15 Post by Martin Benade »

Slight exhaust leaks are frequently the cause of that.
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