Author Topic: quadrajet secondary adjustment  (Read 8974 times)

Offline bedwards

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quadrajet secondary adjustment
« on: January 18, 2020, 09:39:28 PM »
Bone stock '85 c10 305. I bought it 3 years ago with 19000 original miles. I rebuilt the carb then. It has 40000 on it now. The plugs are almost new, the cap and rotor were replaced last year. I run no ethanol in it.
The truck has run very well all this time. I had one issue with the tank selector valve that was fixed some time ago.
This week it has started bogging under hard acceleration. It cranks and idles well and runs just fine easing around. I wasn't sure what it was so I checked the cap and rotor, then the plugs, replaced the coil, checked the egr, the vacuum secondary, the fuel pump output. The timing is set at 8 btc. I think the engine is bogging after the secondaries open. It feels like its starving for gas when they kick in. Is there an easy way to check the secondary adjustment and why would this adjustment change? Is there is something else I have overlooked? I really don't want to take the carb apart again.

update, the hoses are good, the fuel filter is brand new, the fuel pump pumped a quart in 30 seconds, I put a new coil in it because the old original just barely failed the bench test. I loosened and retorqued the air horn.
The secondaries are under tension. They are cracking open revving the engine in park.
I gently used compressed air to empty the bowl and jets and filled it with carb cleaner and let it set for a while. It cranks right up and idles well. The ignition module is original but it seems to be ok.
I'm out of ideas. I hate to just throw parts at it. I bought a carb kit today. If it doesn't start doing better this week I guess I'll pull the carb.
« Last Edit: January 19, 2020, 06:32:51 PM by bedwards »

Offline JohnnyPopper

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Re: quadrajet secondary adjustment
« Reply #1 on: January 19, 2020, 07:56:23 PM »
Do you see the Secondary Metering rods rise when you are at WOT? Not while running of course...
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bedwards

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Re: quadrajet secondary adjustment
« Reply #2 on: January 19, 2020, 09:31:00 PM »
Yes, they are rising.

Offline 75gmck25

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Re: quadrajet secondary adjustment
« Reply #3 on: January 20, 2020, 11:36:46 AM »
AFAIK you should not see the secondary flaps moving when revving it up in Park.  There should be enough tension to keep them stable.  Too little tension will usually cause a bog.

The secondary flaps are held down by a small spring on one end (you can see the flat screwdriver fitting on that end).  You loosen the small allen head set screw that is up under the end of the shaft (3/32" allen, I think) and then rotate the flap mechanism with a small screwdriver on the end until the spring just touches the flap (zero tension). Then rotate it further to set the tension based on the specs, and tighten the allen screw.  I think the specs are something like 3/4 turn after zero tension, but you would have to check it for your specific carb model. 

You might be able to adjust it with the carb on the truck, but its a real PIA to see.  Also, that small set screw might end up anywhere on the engine if you loosen it too much and falls out at the wrong time.

Bruce

Offline bd

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Re: quadrajet secondary adjustment
« Reply #4 on: January 20, 2020, 11:47:34 AM »
^^^^^  I agree.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bedwards

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Re: quadrajet secondary adjustment
« Reply #5 on: January 20, 2020, 02:43:08 PM »
ok, will try adjusting it next.

Offline JohnnyPopper

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Re: quadrajet secondary adjustment
« Reply #6 on: January 20, 2020, 02:52:17 PM »
So is the bog caused by loss of vacuum in the secondary tunnels, due to incorrect tension on the spring?

If so, I have learned again!
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: quadrajet secondary adjustment
« Reply #7 on: January 20, 2020, 03:43:39 PM »
QJ secondaries operate on a variable venturi principle.  The secondaries are fully mechanical, but the air valve responds to engine load and airflow.  If the air valve opens too fast the engine bogs because it cannot process the sudden increase in air.  Throat velocity decreases, pressure differentials drop and fuel delivery leans.  The air valve is tuned to engine load parameters by tension adjustments on the air valve spring.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline VileZambonie

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Re: quadrajet secondary adjustment
« Reply #8 on: January 20, 2020, 05:51:45 PM »
This is why we have fuel injection  ;)
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10

Offline bedwards

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Re: quadrajet secondary adjustment
« Reply #9 on: January 20, 2020, 06:15:05 PM »
Well.. I didn't have good luck. Adjusting the screw 1/8 turn clockwise just moved the flat spot higher in rpm seems like so I moved it in the opposite direction which just lowered rpm of the flat spot. Now that a cold front has gone through, it seems to be running much worse. It is surging at lower rpms also. I put the screw back where it was before the adjustments. I vacuum checked the timing and it is working. I also vacuum checked the EGR and it won't hold vacuum but I don't think this is the main problem. I am ordering a new EGR and gasket tonight. I'm leaning towards something in the carb or it is stopped up. It is also harder to start cold than it used to be and like I said earlier, the plugs look like they are running really lean. If the carb were getting in the secondaries too soon wouldn't it cough black smoke?
Did I move the screw enough or should I have gone further?
thanks again for all the ideas
« Last Edit: January 20, 2020, 06:16:47 PM by bedwards »

Offline bedwards

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Re: quadrajet secondary adjustment
« Reply #10 on: January 20, 2020, 06:47:23 PM »
there are 3 egr valves listed at Rock Auto. They all look alike. Any idea which one is right?
DELPHI EG10080 OE No 17085765, 17085767
DELPHI EG10081 OE No 17085768, 17085769
DELPHI EG10079 OE No 17085764

Offline bd

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Re: quadrajet secondary adjustment
« Reply #11 on: January 20, 2020, 07:14:23 PM »
If the carb were getting in the secondaries too soon wouldn't it cough black smoke?
Did I move the screw enough or should I have gone further?

Black smoke is the result of excess fuel making the engine run rich.  Premature opening of the secondary air valve will impose a lean condition.  The air valve adjustment is tied directly to the application and is specified by the carburetor number.  What is the number stamped onto the left rear pillar of the carburetor?  The number probably begins with 170xxxxx.

What is the number stamped into the top of the existing EGR valve?

Check the distributor vacuum advance to see if it will hold vacuum.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline bedwards

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Re: quadrajet secondary adjustment
« Reply #12 on: January 20, 2020, 08:01:51 PM »
It acts like its running lean. The plugs look like its running lean.
The carb number is 17085226. I will have to go back up to the garage to get the egr number.
The distributor held vacuum. Like I said, I doubt the egr is causing the lean condition as its not even opening. I can open it with my finger and the truck will idle down. I bought a GP Sorensen kit for the carb #96-519a. If I don't find the problem this week, I will pull the carb and build it this weekend. It worries me that the truck seems to be running worse today but the last time I ran it, it was 64 degrees and today its 30 so the lean condition might be the problem. It seemed to be surging at lower speeds.

The numbers on the egr are: csm 17085768 and 103 51n ( I guess that answers one question anyway)
« Last Edit: January 20, 2020, 08:31:37 PM by bedwards »

Offline bd

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Re: quadrajet secondary adjustment
« Reply #13 on: January 20, 2020, 09:16:57 PM »
Yes, it does...

It acts like its running lean. The plugs look like its running lean.
The carb number is 17085226. I will have to go back up to the garage to get the egr number.
The distributor held vacuum. Like I said, I doubt the egr is causing the lean condition as its not even opening. I can open it with my finger and the truck will idle down. I bought a GP Sorensen kit for the carb #96-519a. If I don't find the problem this week, I will pull the carb and build it this weekend. It worries me that the truck seems to be running worse today but the last time I ran it, it was 64 degrees and today its 30 so the lean condition might be the problem. It seemed to be surging at lower speeds.

The numbers on the egr are: csm 17085768 and 103 51n ( I guess that answers one question anyway)


DELPHI EG10081 OE No 17085768, 17085769


Air Valve Spring adjustment is air valve just closed plus 7/8 turn (image).  Pay close attention to the float adjustment, as well.  Make sure the primary metering rod piston moves freely in its bore and that the secondary metering rods didn't drop off their lifting arm.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline blazer74

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Re: quadrajet secondary adjustment
« Reply #14 on: January 20, 2020, 10:07:00 PM »
Check your pull off for a vac leak.