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post #1 of (permalink) Old 06-24-2003, 01:00 PM
kd7kmp
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metering rods

Where can I get metering rods for a 1977 Q-jet carb? I'm running lean when the secondaries open and need to richen the fuel mixture. Thanks.

Kevin
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post #2 of (permalink) Old 06-24-2003, 05:15 PM
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Re: metering rods

A decent speed shop will have what's called a strip kit, has all kinds of jets, springs and rods.

But - if it's going lean, the easy cure is increase spring tension on the flapper - look under the top edge for a tiny little screw.
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post #3 of (permalink) Old 06-24-2003, 11:01 PM
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Re: metering rods

Tell me what metering rods and hanger you have.

Then tell me what you've got for an engine.

Your best bet is a junkyard but I can give you a ballpark guess as to what metering rods and hanger you should run.
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post #4 of (permalink) Old 06-25-2003, 01:13 AM
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Re: metering rods

Carbs Unlimited sell Q-Jet parts. Jets, hangers, metering rods, etc. Try changing just the hanger. I went to the leaner one and it wasn't pretty.

Lots of possible things for you, just do one thing at a time and test it.

Good Luck,

Jim
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post #5 of (permalink) Old 06-26-2003, 03:14 PM
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Re: metering rods

I looked at the rods and couldn't find any numbers/letters. I'll have to look again. The motor is a rebuilt 350 in a 1977 Chevy G30 van. The motor has flat-top pistons and an RV cam. The carb is the stock Q-jet as is the rest of the engine. It's backed up with a TH350 and a 3.27 rear end. The problem I'm having is when the van is under a moderate load it cuts out. I checked the exhaust and it's going lean when this happens. I know and can do anything with fuel injection but I don't know carbs very well. I can rebuild them for a stock application, but that's it. Thanks for the help.

Kevin
post #6 of (permalink) Old 06-26-2003, 03:43 PM
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Re: metering rods

It'll be stamped on the body in about the middle. You can use a crocus cloth or a scotchbrite pad if you're very careful to clean them up.

If the ends that go in the gas have crud on them I recommend getting it off. Cotton shirts (the button up kind or the bed sheet kind) and denim works pretty good for cleaning them up. You don't want to use anything scratchy like sandpaper, steel wool, or a scotchbrite pad on the metering end. If you do it'll take off the coating or hardening or whatever they do to them to keep them from collecting junk and corroding.

Did you find out what hanger you've got? It'll be stamped in the top on the end away from the metering rods. Probably on the flat spot that is stamped in. Sometimes the black haired bandits that stamp them missed and put them on the side instead.
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post #7 of (permalink) Old 06-26-2003, 04:14 PM
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Re: metering rods

Several thoughts.

1 - you said it goes lean when the secondaries first start to open -- the flapper's there to prevent that. Tighten it so it doesn't open so fast.

2 - What have you done to make the engine need a different air/fuel ratio than stock? The fallacy is a different cam or higher compression needs more fuel, untrue. The cam may take bigger "bites" of the AF mixture, but the A/F ratio remains the same, just more of it. The stock carb in stock configuration with stock metering rods and jets should work just fine - if the carb's working. Stochiometeric is still stochiometric. If you are adding more oxygen from nitrous, then it's a different story.

3 - how did you check the exhaust? With CO? Remember, if the bog is caused by a weak ignition, CO drops too. Check the ignition.

4 - to actually see if it's a lack of fuel problem, use the propane trick - it will tell you for sure in seconds.
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post #8 of (permalink) Old 06-26-2003, 09:43 PM
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Re: metering rods

</font><blockquote><font class="small">In reply to:</font><hr />

1 - you said it goes lean when the secondaries first start to open -- the flapper's there to prevent that. Tighten it so it doesn't open so fast.


[/ QUOTE ]

Wrong. You can tighten it but it won't solve the problem. It'll just make it more difficult to get the air valve to open. The air valve should open 3/4 to 1 second from the time the throttle valve is opened.

The "power tip" controls how lean it is when it first begins to open.

</font><blockquote><font class="small">In reply to:</font><hr />

2 - What have you done to make the engine need a different air/fuel ratio than stock? The fallacy is a different cam or higher compression needs more fuel, untrue. The cam may take bigger "bites" of the AF mixture, but the A/F ratio remains the same, just more of it. The stock carb in stock configuration with stock metering rods and jets should work just fine - if the carb's working. Stochiometeric is still stochiometric. If you are adding more oxygen from nitrous, then it's a different story.


[/ QUOTE ]

You're half right. In theory, if you bore a motor out .060" and do nothing more you should not have to rejet. However, you must tune for every engine. A 305 doesn't use the same needles and jets as a 350. A carburetor out of a van doesn't use the same jets as that out of a Caprice.

Changing cams makes HUMONGOUS changes in jetting. At low RPM a cam with a lot of duration (and overlap) requires very different jetting than a cam with little duration. The main reason is that with a lot of overlap you can actually end up metering air twice. A carburetor works off vacuum and velocity so any change in it requires a change in jetting. Any change in volumetric efficiency requires a change in jetting.
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post #9 of (permalink) Old 06-27-2003, 01:00 PM
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Re: metering rods

Okay, here's the number I got off the rod hanger: 33797. It was on the goose neck portion. There were no numbers anywhere else. For the metering rods there were no numbers anywhere. Only two letters: DP. I did measure the diameter of the rods. The portion just above the taper is .1330" and the tip of the rod after the taper is .0640". If it matters the diameter of the rod body is .1505". Again, there was no number on the rods.

The ignition is fine. I have scoped it during all driving conditions. The plugs, wires, cap and rotor are all less than 6 months old. Thanks for the help guys.

Kevin
post #10 of (permalink) Old 06-27-2003, 01:14 PM
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Re: metering rods

The flapper's there to prevent too much air getting in until the secondary vacuum's high enough to suck the fuel out of the secondary jets (the vacuum below the flapper that does the pulling. And also there's the delay in the fuel getting up there - it's much heavier than air, so it doesn't get started as fast. It's the same reason an accelerator pump is needed.
If it opens too early, the engine goes lean for an instant. It can be adjusted easier or tighter, depending on how you turn the screw - or bend the linkage.

The complaint wasn't a bog off the line, nor was it after the secondaries opened, it was during the transition. Either the secodaries accelerator pump isn't working (as on a Holley Double) or the flappers not doing it's job.

Try it without the flapper - bog city!

Yes, slight changes are needed due to velocity changes, and the "gulping effect" of more "gulp" by the cam. But stochiometric is still stochiometric.
Increasing VE changes the total amount of the mixture needed each "gulp" but does not change the balance - ratio between air and fuel.

Yes, different engines often use different jets - not appreciably different though. Notice often the larger engines use SMALLER jets than the smaller engines - or larger metering rods, or a combination thereof. Partly due to the fact the venturi velocity is higher because of the greater total volume, making better atomization.

The reason that a big cam usually gets larger jets is because the idle and tranfer circuits are really affected by the huge gulps. Because of the high velocity during a gulp, the carb switches from idle, to intermediate, then to main run, then when the gulp stops, the main run spills over - but the engine can't use it because there's no Oxygen to go with it, the carb drops back down to the idle circuit just about the same time the next gulp hits. The next guylp lots of air gets in without any fuel due to the heaviness of the fuel. Over and over again. Poor carb just doesn't have a chance.
There are ways to minimize it depending on the carb. But the usual method is go big jets and a high idle - it's easier.

Once throttle's open and the velocity gets more even, the mixture still needs to be just slightly richer than stochiometric.





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