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Old 02-15-2008, 09:03 PM
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why old q-jets are so pesky to tune?

I'm about to set one on fire, is driving me crazy, the problem is when it opens the secondaries acts so strange and I can't tell if it goes too lean or too rich, at 100 km/h you can't really see smoke coming of the pipes.

'93 C1500 q-jet 350/700R4

the motor sonds like bubling, like a too rich would but it should smoke black if so but seems like it ain't, have leaned it, nothing, lowered and raised the fuel level, nothing, tigthtened the air valve, nothing, changed several primary jets, nothing, sec rods, nothing...

the primary side works great, until the secondaries come in and the motor feels like crap.

swapped carbs with the one from my C30 pickup and works great, so the problem really is the carb.

I've seen this problem before, seems like old q-jets reach a level where they wear ot too far or something? do internal passages get enlarged or clogged with use/time?

any thoughts?

Augusto.

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Old 02-15-2008, 10:02 PM
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Take it off and rebuild it. If the carb set for any duration of time, fuel turns to crap and plugs up the passages.
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Old 02-16-2008, 04:57 AM
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Usually, if its running rich enough to cause bucking or popping, you would have seen crazy black smoke. that leads me to think its lean.

Dirt in the secondary jets? I agree with DV.. rebuild
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Old 02-16-2008, 05:49 AM
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Another common problem is that the throttle plate shafts wear and get loose, causing vacuum leaks. There are bushing kits to fix these.
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Old 02-16-2008, 07:45 AM
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I did rebuild it already,

tuned according to the specs,

cleaned it thoroughly, removed all the crud with laquer thinner then dipped in mild acid solution, then rinsed with a pressure washer, went through all the orifices with soft copper wires, epoxied well plugs, new Warner internals.

the shafts are in pretty good shape.

engine starts withouth opening gas rigth after cranking, drives around great, plenty of torque, until the secondaries open and it falls on it's face....

the real problem here is that I have not been able to find if it gets too rich or too lean, I'm inclined towards lean though because as curtis73 said there should be lots of black smoke, but when I instaled richer sec rods same thing happened.

any ideas before I seek for a shaman?

Augusto.
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Old 02-16-2008, 07:47 AM
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Quote:
Originally Posted by Augusto
I'm about to set one on fire, is driving me crazy, the problem is when it opens the secondaries acts so strange and I can't tell if it goes too lean or too rich, at 100 km/h you can't really see smoke coming of the pipes.

'93 C1500 q-jet 350/700R4

the motor sonds like bubling, like a too rich would but it should smoke black if so but seems like it ain't, have leaned it, nothing, lowered and raised the fuel level, nothing, tigthtened the air valve, nothing, changed several primary jets, nothing, sec rods, nothing...

the primary side works great, until the secondaries come in and the motor feels like crap.

swapped carbs with the one from my C30 pickup and works great, so the problem really is the carb.

I've seen this problem before, seems like old q-jets reach a level where they wear ot too far or something? do internal passages get enlarged or clogged with use/time?

any thoughts?

Augusto.
You say it runs like crap when the secondarys open. What does it do, does it bog? Does it surge, does it feel like it's "missing out"?. What is the symptom?

When the secondarys open, there is a shot of fuel that comes out of the transfer slots and strikes the air valve, to give it a shot of fuel. If your hanger is worn, the metering rods hang too far down into the well, causing it to be too lean. Check to be sure that your hanger holes aren't worn out. Make sure your discharge tubes aren't loose.
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Old 02-16-2008, 07:57 AM
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Read tip #3 on this pdf file
http://home.comcast.net/~chadwick.ro...une_a_Qjet.pdf
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Old 02-16-2008, 08:22 AM
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Quote:
Originally Posted by NAIRB
You say it runs like crap when the secondarys open. What does it do, does it bog? Does it surge, does it feel like it's "missing out"?. What is the symptom?

When the secondarys open, there is a shot of fuel that comes out of the transfer slots and strikes the air valve, to give it a shot of fuel. If your hanger is worn, the metering rods hang too far down into the well, causing it to be too lean. Check to be sure that your hanger holes aren't worn out. Make sure your discharge tubes aren't loose.
the engine feels like if it was rich, it makes a bubling sound, misses, cuts off, and won't rev up more, but what confuses me is the fact that there is no black smoke, at leats not the amount that an overly rich condition would, just the normal amount that an accelerating engine does, it's hard to tell with the wind blowind at 100 km/h though.

it made me think it was an electrical problem (module) but when I swapped carbs it worked great.

the sec rods have the usual wear mark at the rigth heigth, so the hanger is not worn out, the discharge holes are clear, the pickup tubes are tight.

I tried different air valve settings from 1/2 turn to 1 1/2 turn,

also I tested the carb with the air valve wired to prevent from opening, this time the engine behaved great like a 2 barrel, definitelly the problem is in the secondaries.

Augusto.
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Old 02-16-2008, 08:25 AM
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Quote:
Originally Posted by impalaman63

my problem is not off idle, is close to or while triying to reach WOT.

Augusto.
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Old 02-16-2008, 12:36 PM
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Are you certain the adjustment for the secondary still works? If you move it too far the spring will snap off the tang and the secondary plates are free to flap around in the breeze meaning they will instantly open and make it bog. With a small mirror you can check the spring to see if its broke or still on the flap tang.
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