plugged idle jets

[zumwoll]

I have a 74 911 with the triple throated Webers. I can't drive 500 miles without one of the idle jets plugging. Could it be that K&Ns just do not catch the fine particulate.
Even the fine screen in the carb inlet line is clean HOWEVER the K&Ns always have an accumulation of large particulate. Makes me think the K&Ns are letting small stuff through.

[Formerly Steve Wilkinson]

One thing you should do with those K&Ns is spread grease around both the top and bottom lips of the element, to seal them against the air-cleaner body.

You should also have a good Racal diesel fuel filter. Those in-line filters are worthless.

I was having the same problem, by the way ('83 SC, PMOs). Your quasi-neighbor Steve Weiner advised that it might well be caused by minute rust particles in a gas tank that old...which gave me a good excuse to suddenly spend $2,200 on a nice new fuel cell. For whatever it's worth, I haven't had the problem since.

[zumwoll]

I think that is a great idea!. After all it is just tin that the filters sit on and can easily be bent. I will try it right now---after I blow out the jets.

[Steve@Rennsport]

One more thing,....a deteriorated engine sound pad sluffs off the tiny bits that migrate into the air cleaners and thats a big on-going problem, too.

If its as old as the car, buy a new factory pad and replace that thing.

[911pcars]

Idle holes usually don't suck. They get sucked by engine and venturi vacuum. That is, the normal direction of air/fuel is from inside the carburetor into the air passage, not the other way around. Not to say sound pad fluff and such can't get through the air filter and into the carb throats. Maybe the debris is coming from the other direction, the fuel tank. Maybe the filter isn't filtering so well and/or the tank is full of debris. You could cut apart the fuel filter and have a look and/or remove the tank sending unit and take a peek with some sort of battery-powered light (no flames please).

A float bowl full of "stuff" is another good sign, but you should perform the easier stuff first.

Sherwood

[jwetering]

Originally posted by 911pcars
Idle holes usually don't suck. They get sucked by engine and venturi vacuum. That is, the normal direction of air/fuel is from inside the carburetor into the air passage, not the other way around. Not to say sound pad fluff and such can't get through the air filter and into the carb throats. Maybe the debris is coming from the other direction, the fuel tank. Maybe the filter isn't filtering so well and/or the tank is full of debris. You could cut apart the fuel filter and have a look and/or remove the tank sending unit and take a peek with some sort of battery-powered light (no flames please).

A float bowl full of "stuff" is another good sign, but you should perform the easier stuff first.

Sherwood

+1

I run webers and KN's and the four times I've had plugged idle jets it's come from inside the carbs. Twice it was bits of o-ring from the idle mix screws (now all removed, they are unnecessary), once it was a bit of JB weld from the jerry woods style baffle job I did, and once it was ??? never did figure that out. You should be able to identify the particulate you are pulling out of your idle jets. Put it under a microscope. flakes of rust = fuel tank degradation, bits of rubber = o-rings etc.

It's unlikely to be coming through the air filter (see rationale above) and as far as I can tell, you fuel filter(s) have to be pretty far gone before particulate makes it through them as well. I run three fuel filters in my lines, cheap fram ones, change out all three for $10 and do it fairly often. So far so good.

[zumwoll]

thanks for the thougts. All may well be valid except for the sound deadener. I did that months ago and it helped. This time I saw a black fleck that could have been from those idle jet Orings. I have never checked the tank cause the inlet fuel line filter ---the one in the carb---is clean as a whistle. I will do that tommorrow. Runs great now as it always does after I do this.
WHat kind of mileage do you make Jasper if you are cruising the freeway?
I replaced the tube and went from 15 to 20. I was amazed. It was runing way too rich.

[911pcars]

"I replaced the tube and went from 15 to 20. I was amazed. It was runing way too rich."

What tube did you replace? I want to do this too if I can get 5 more mpg.

I'm not sure how many others do this, but I removed the inlet filter screens (downstream of the banjo fittings) and instead use several (two) external fuel filters; a glass see-through type for the bigger particles and a throw-away plastic filter downstream; all before the inline pressure gauge so I can more accurately monitor fuel inlet pressure. 3 psi before a partially clogged filter results in..... less pressure.

Reason why I dump the screens. I prefer not touching this area too much. The banjo bolt is relatively soft brass and tends to get beat up with frequent removal/tightening or just stuck in place. In addition, relatively good (crushable) fiber washers (2/bolt) are needed to prevent fuel from leaking. I guess I'm too cheap to stock up on Weber fuel inlet gaskets. Besides, the Weber filter screens are relatively coarse. Mine appeared squashed when I finally yanked them years ago. I think inexpensive, multiple throwaways are better and easier to maintain.

Sherwood

[RoninLB]

all good info above.


also a beat top interior carb gasket may be flaking into the bowls.

most auto fuel filters are 10 microns or above. The Racor can provide various sizes down to 2 microns. My Racor spin on filters cost around $25ea. The fully enclosed Racor filter is safer than a spin on. Costs around $75 at a marine store for enclosed and $100 for spin on with a water trap and water alarm probe option.
btw.. I tested my spin on to 50# with a CIS fuel pump without leaking.

Our carbs are dirt sensitive imo. Kinda like sometimes a Rochester is prefered over a Holley in small boats because of dirt issues. 10micron filters is borderline cleaning imo.

[zumwoll]

Two of you have mentioned a spin on filter. My BIO-BENZ uses the best Knecht German filters and they only cost $15. I spoze I could go to the wrecking yard and get the filter housing from a junker, plug the return lines and attach it to the firewall and away we go. But how come no one has mentioned the filter that comes with the CIS and Carrera cars. Does it require too much pressure? I can't imagine that it is not fine enough.

[jwetering]

Originally posted by zumwoll
WHat kind of mileage do you make Jasper if you are cruising the freeway? I replaced the tube and went from 15 to 20. I was amazed. It was runing way too rich.

My trip odometer is busted so I've never checked my mileage. It's not a daily driver so I don't pay much attention. It uses fuel though..yep, it uses a lot of fuel. I have aggressive cams, so at the bottom of the idle circuit raw fuel pretty much spits out the tailpipe on account of the overlap. eek:

On the cams however I'm a smidge lean according to a recent dyno day AF measurement, not enough to re-jet or anything, but leaner than rich.

Last note on screens behind the banjos... I converted over to PMO style fuel rails recently (inexpensive and VERY nice) and the instructions said to toss the screens, so I did. I figure anything small enough to get past my fuel filters isn't going to get caught by the screens anyway.

cheers

[Zeke]

Idle jets. Sore topic for me. I lunched two very nice 914-4 motors that were good for 135 HP because of ***** in my Webers. In my case, it was actually in the carb passages; old varnish I failed to get out completely on the rebuilds because I didn't yank the lead plugs. Once the race gas hit those things, it let loose all kinds of junk. I had all new lines, tank, filters, the works.

I guess you can't be too careful or too clean when it comes to carbs, or any induction. Start at one end and go all the way cleaning and replacing is all I can say.

[zumwoll]

Too cool!
Can you post a shot of those PM rails. What is the benefit?
My carbs were just rebuilt professionally so I feel pretty good about their basic set up. Plus they look almost new. It s really hard to fix things that aren't broke tho. The cleanup has the car running great.
Mine is the typical build, JH forged pistons, GE60 cams, lightened flywheel, electronic ignition. THis is my sixth Pcar and without a doubt the fastest until I put 16s from a 930 on it (yes it has flares) and of course now the gearing is not as aggressive, but it always puts a smile on my face when I drive it.
Zeke, I feel bad about your 914 motors but I have only done one DE day and I can see that you can toast an engine real fast. I sucked crap into the air intake and did not notice that the needle was pegged until I got off the track. WHo looks at the guages at 100plus mph! : - )

[jwetering]

Sorry no pics showing the fuel rails up close. All I have is a long shot showing the general arrangement. Note the PMO pressure control unit. Also inexpensive and very effective. I made a loop connecting the two carburetors. I wish I had better pictures for you but I don't and the car is off site.

Check out the pmo website, it's fairly informative. The PMO carbs are beauties, but expensive. Their accessory products are very reasonable though, and the proprieter is both knowlegable and hepful. He recomended the jetting and veturis for my engine and nailed it first time. He doesn't take phone calls though. You have to fax him your question and then he calls you back. At first I thought that was abit rude, but then I realized he'd bo on the phone constantly giving free advice to people like me who are only just barely old enough to remember carburetors.

[911pcars]

Here's my setup. Not PMO, but a mixture of Aeroquip and Earls.

I figured the price is a wash with PMO's manifold when you include the add'l lines and fittings from the PMOs to the fuel block.



Sherwood

[Steve@Rennsport]

Nicely done, Sherwood.

The only advantage the PMO manifolds have are less places for a potential fuel leak.

[911pcars]

[b"The only advantage the PMO manifolds have are less places for a potential fuel leak."[/b]

Thanks Steve,
So true. In fact, for an easy DIY project, I'd recommend the PMOs. As you well know, the short length of AN hose must be cut and installed at the exact length to space the banjo bolts so they thread into the carbs. If you've ever had stainless steel braid wounds, you know what I mean. I must have screwed up somewhere as there were no fuel leaks.

Sherwood

[zumwoll]

Don't laugh. I have not had time to get after the inside of the car. But otherwise is there something you think I should do? What abouit those big filters you have? Brand?

[jwetering]

Originally posted by zumwoll
abouit those big filters you have? Brand?

Those are Fram filters, $3 each from our local parts store. I have a total of three in my system. One on the main supply line to the PCU, and one each on the lines going to the carbs.

I simply went to the counter and asked for fuel filters to fit the 3/8" fuel line I am using. That's what they came up with. Fm-2 or something like that. They are pretty generic (and made in Isreal!)

Prior to that I had a CIS filter on the supply line. I gave up on that when it came time to sort out the fittings. A few posts earlier someone said that the cheapo filters are 10 micron and IIRC the CIS filters are 2 micron. I personally don't think that a 10 micron particle will cause all that much grief, bit lord knows I've been wrong before.

In another thread altogether I remember reading that a pelican was plugging his idle jets and the problem would go away when he changed his fuel filters, and then come back again. Ultimately he replaced or repaired his fuel tank and the problem went away for good.

[zumwoll]

thanks Jasper.
I will try the Fram filters and go after the tank. I pulled the tank on an old Datsun Z car and sealed it with stuff I got at a radiator shop after sloshing a chain around inside the tank. Never had another problem. Heck of a lot easier pullng a Pcar tank. Z car was a PITA.