How do I get to the jets on my PMO carbs?

butzi73 · 08-13-2010, 09:04 AM

Hey guys,

I need to get to the jets on my PMOs to clear/clean them because I suspect I have a clogging issue. I know it's not that complicated, I've just never had anybody show me where there are, or how to get to them. I've searched around but haven't found any instructions on how to do this.

Pelican has the Weber Diagram here: Pelican Parts - Porsche 911 Parts Listings & Diagrams
I don't know if it's the same as my PMOs. What it shows as the jets in this photo (26,27,29,30) don't really look like jets to me. Why am I confused by this?

So, I don't know if this is hard to describe with words. My carb mechanic is down in Monterey this weekend (wish I was there too). Do I take the top off to get to them? Do I open up that side panel like in the above diagram?

Here is why I suspect a clogging issue: Car has a fluttery sound/feel to engine, like a cylinder isn't working. Happens on idle, acceleration and cruising, at all RPMs, but especially noticeable around 2500. This has happened to me twice before. My carb guy, John Holleran, said it was particles getting through, or from the K&Ns. Both times before it went away after a day or two, presumably when the clog cleared itself. This time I've gone all week and it's still doing it so I have to address it. It feels exactly the same as the previous times.

40mm PMOs on a 2.7. I recently (2 years ago) had the cabs tuned/updated by Holleran and PMO.

Any help is very much appreciated.

~Cary

Tom C. · 01:05 PM

If the motor was running fine before . . . and you have the proper fuel filters in place and the correct fuel pressure setting (3-4psi) and the fuel level in the float bowls are right in the middle of the inspection window . . . and the fuel system has not been 'opened' up recently . . . I don't know that I would suspect the PMO's as the problem.

But if some dirt did get into the PMO's then you really need to take them apart and clean each carb out.

Just be sure you have a clean work area, a few cans of carb cleaner, and some spare o-rings and gaskets from Richard Parr.

Then be very organized and take your time to ensure you do a good job. The only problem I ran into was gettting the fuel level in the float bowls set . . . it just took a few times to get it right. But if yours are already set properly then you should be fine.

One more thought, if your fuel system is 'dirty' you may want to check your fuel tank. It maybe be a little rusty . . .

Hope this helps.

Eagledriver · 08-13-2010, 07:53 PM

You most likely have a clogged idle jets. You can fix this very easily. There are 6 idle jets. They are contained by some screw fittings along the top of the main body of the carbs. Just undo the brass colored screw (it should have an o-ring around the base of it). Once you get the screw out it will have an idle jet plugged into it. The idle jet has a very small hole in the end of it. This is where your obstruction will be. Often it's a small spec of gasket material from the carb gaskets. Just pull each jet out of its screw and blow through it from the big end. This will clear the little hole. Then re install. If you find the one that is blocked you'll know you've fixed it. Often the blockage is so small that you won't really notice which one it was. Once they are all clear test drive the car to see if that fixed it.

-Andy

JP911 · JP911's Garage

If it's not the idle jets, pull off the fuel filters and check for dirt.

butzi73 · 05:18 PM

OK, thanks guys. I'm going to have a chance to check the idle jets tomorrow.

Are these them?

RoninLB · 08-14-2010, 05:31 PM

butzi73 · 08-14-2010, 05:39 PM

AH! I guess I was wrong. I'm glad I posted here!

butzi73 · 08-15-2010, 08:06 AM

OK, well I now know how to find and check my idle jets. Easy! Thanks for the help on that. I couldn't figure out how to separate the jets from the holders. I pulled and twisted on them a bit, but I obviously don't understand how they are connected. I blew threw the side holes and into the tip hole. I never noticed particles in there, but I remember Andy's recommendation to blow through the big end, which I can't get to because I don't know how to separate them. Do I just have to use a lot of pulling force to get them apart? Special tool?

Unfortunately after blowing through them as best as I could the car still ran crappy. Fluttery idle and revs, like a cylinder isn't firing. Either I didn't get the particle out or it's something else besides the idle jets. I wish it was just that because it's pretty easy to get to those. Any suggestions?

~Cary

JP911 · JP911's Garage

Hmmm...if you think you might have one problem cylinder you need to isolate it. Best way to do this is with a syncrometer (the cylinder pulling the least vacuum is the troubled one). You could do it another, more crude way. Pull off the air cleaners, start the engine, and then individually cap off each trumpet one-by-one. As you do this, the engine should stumble a bit more than usual UNTIL you get to the problem cylinder. When you cap this one the engine will continue to run the same way. If you find one cylinder that's problematic, you can then focus on getting it fixed. Other than clogged jets, you could have a needle and seat valve that's stuck closed. If this is the case a cylinder or cylinders could be starved for fuel (#3 and #6 have their own valves, while #1 and 2, and #4 and 5 share valves). To clean the needles and seats you must remove the carb body from the car, pull off the air cleaner plate and carb top, and then carefully pull the valve and clean it.

But, before you do all of this, I would still check and clean the fuel filters and ensure that you have adequate fuel pressure (3.5-4 psi). I speak from experience. A partially clogged filter will wreak havoc on the carbs.

butzi73 · 08-15-2010, 10:39 AM

It's good now!!

Man, sometimes you sit there and let your mind go down the "worst case scenario" road and stress out, and of course it ends up being something simple: Clogged fuel filter. And Tom had pointed me towards that early on, but I didn't peruse it because it's a fairly new filter, and I just had blinders on that it was the carbs somehow.

Well, I happen to have the PMO fuel pressure regulator/gauge, and it showed 3.0 at idle. JP911 reminded me that pressure should be at 3.5-4.0, and again to check the filter. I took the fuel filter off and poured some gas through it in the reverse direction. There definitely was silt in there. I got a lot out, but maybe not all of it. Started it up but car still ran bad (maybe a hair better). I stewed for a while imagining all manner of expensive repairs, but decided to take it for a drive. Sure enough, 1/2 miles down the road the engine sputters and coughs then vroooom, smooth as can be! Yeah, I was so happy.

Pressure was 4.0 at idle when I got back. I must have a rusty tank. It continues to amaze me what a great resource this board it.

THANK YOU GUYS!

~Cary