Author Topic: I think I'm running too lean causing high hydrocarbons and low CO's.  (Read 492 times)

Offline Terry Petty

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So I rebuilt an old Q-jet it's a late model non-feedback off an 85 350 4bbl 8:1 Mexican high altitude from Mexico City.  It's on a 355 with larger valves, 8.7:1 actual static compression, cam is 205 int dur @ .050", 215 ex dur @ .050", 112LS. At first it was rich, but realized the idle wells were leaking so I repaired that with Cliff's parts.  I was not happy with how it was running but had to have it emission checked.  I was surprised it passed. Emissions readings at cruise on the rollers were all low well under the limits.  The CO's were super low but the idle hydrocarbons were 399 at idle only.  (limit was 400).  I can make it idle rough if I screw in the idle screws too far, but when I back them out it don't seem to get rich enough.   

It stinks like gas out the tail pipes at idle and will die in gear at times.  I have not modified the idle circuits or the idle air bypass.  I feel I'm close but need less idle air bypass?  Can I put some plugs in the base-plate and maybe drill them out like 1/2 the size of stock? 

Power and anything off idle is impressive.

Open to any suggestions.

Offline Cliff Ruggles

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #1 on: November 08, 2018, 02:51:44 AM »
What is the carburetor part number.

If it is a 1985 model it should be computer controlled with an M/C solenoid and TPS.....Cliff

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #2 on: November 15, 2018, 11:34:33 AM »
Main number 17082285, and another number 1293.  You may not have this number on file.  It is not computer controlled and no TPS.  Was told it came out of an 85 Monte Carlo SS 350 engine from Mexico City.  Here in the US, 350 was not an option for the Monte Carlo I'm told.  No parts personnel have ever found the number, and I've never found it on the papers that come with the repair kits.  I just order for later model Q-jet and the parts fit.  I received the entire engine, the cast iron Q-jet manifold looks like it was cast for EGR, but the passages were never machined (I still have it the manifold if you want to see a picture of it). 

I attempted to duplicate the calibration of an Edelbrock 1904, installed all the jets, hanger, metering rods.  But had to run leaner jets on the primaries and I feel that is because it has an aneroid bellows and a 5th jet. 

I love this carb, it suits my engine and emissions test under load on the rollers runs really clean and very responsive, makes really good power better than I ever expected it would.  I just won't idle right, low CO's and high HC's.  Can't screw the idle screws out far enough to help but they will kill the engine if I screw them in too far.

So, what do you think do I need to restrict the idle air bypass and richen the mixture?  The bypass passages are wide open, would be easier for me to do this at the baseplate if possible.  If so, what parts do I need and what would be a good starting point to drill them out to?

Offline Cliff Ruggles

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #3 on: November 16, 2018, 02:52:34 AM »
A few pics of the carb would really help.  You can send them here:

cliffshp@embarqmail.com

I can look at the pics and determine what you have and how it should be set-up.....Cliff

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #4 on: November 19, 2018, 10:35:07 AM »
It looks just like an edelbrock 1904.  Do you have a kit/instructions on limiting idle air bypass?

Offline Cliff Ruggles

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #5 on: November 20, 2018, 04:47:16 AM »
Not exactly sure why you are focused on bypass air.  I'd do an idle "tip-in" test first, to see if richening it up helps as the engine will smooth our and idle speed may increase slightly at the same time. 

Taking away bypass air will alter the throttle angle at any given idle speed, and expose more transfer slot.  Might even uncovered a ported source and make it act like a manifold vacuum source, and too much and you may see "nozzle drip". 

I usually don't mess with bypass air unless I've got nozzle drip and need to add some, or throttle angle is high enough it's uncovering the ported source for the vacuum advance.

It's always a case by case thing, but for sure if idle fuel to the mixture screws is inadequate I correct that issue first........Cliff

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #6 on: November 25, 2018, 06:44:15 PM »
OK so how do I do an idle "tip in" test?  Do you mean choke it a little? 

If it smooths out and idles faster, what does that mean?

How do I know if I'm not getting enough idle fuel supply to the mixture screws? 

I will follow your instructions.

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #7 on: November 25, 2018, 06:52:35 PM »
So I'm looking at another post and you talk about opening up the idle tubes .002".  Is there a special drill bit for that?  Do I need to remove the tube and collar to open it up? OR can you sell me the right idle tubes?

Offline Cliff Ruggles

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #8 on: November 26, 2018, 11:14:40 AM »
A "tip in" test can be done to determine if the idle is too lean or part throttle. 

For an idle test get the engine fully warmed up, heat soaked and adjust the idle mixture screws for best idle quality.  "Tip in" the choke flap or richen things up by gently placing your hand over the choke housing, not enough to choke it out, just enough to fatten things up and see how it responds.  If it speeds up/smooths out it needs more idle fuel......Cliff

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #9 on: December 04, 2018, 12:42:17 PM »
It speeds up and smooths out.

Offline Terry Petty

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #10 on: December 13, 2018, 05:02:35 PM »
It speeds up and smooths out doing the tip-in test.  So can you please sell the the parts to increase the flow of fuel to the idle circuit? 



Offline Cliff Ruggles

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Re: I think I'm running too lean causing high hydrocarbons and low CO's.
« Reply #11 on: December 14, 2018, 03:05:41 AM »
You will need to remove the idle tubes, I'd get replacements as often this leaves them unusable.

The rest will be done with precision drill bits.

I would need to know the size of the upper and lower IAB's and DCR's, plus how much bypass air is being used......Cliff