Quadrajet secondaries

[bedwards]

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.

 

[Snoots]

Check ALL of your vacuum lines.

 

[C10MixMaster]

Things i would check

fuel pressure/clogged fuel filter

secondary air door does it feel sloppy loose or or some tension

secondary cam that lifts the secondary rods its a plastic fiber material that could break.

also is it a bog or does it start dropping cylinders a coil can pass all the bench tests and still fail under load.

 

[Blue Ox]

Accelerator pump, although if it's really a late bog probably not.

The choke pull-off also acts as a damper to delay the air door opening on the secondaries, so make sure that holds vacuum and the linkage isn't boogered.

 

[SirRobyn0]

Bone stock '85 c10 305. I run no ethanol in it.

IMO the ethanol issues are way over played. If your tanks are clean, and rubber lines are in good shape it's not that big of a deal.

Things i would check

fuel pressure/clogged fuel filter

secondary air door does it feel sloppy loose or or some tension

secondary cam that lifts the secondary rods its a plastic fiber material that could break.

also is it a bog or does it start dropping cylinders a coil can pass all the bench tests and still fail under load.

The above is what I would check.

 

[78C10BigTen]

IMO the ethanol issues are way over played. If your tanks are clean, and rubber lines are in good shape it's not that big of a deal.



The above is what I would check.

I daily my square and dont have ethanol issues. But i believe ethanol cannot sit for long periods of time.

 

[bedwards]

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.

 

[Snoots]

Get Cliff Ruggles book on Quads.

If you're getting gas and you have spark and it stumbles, make certain that your vacuum advance is working.

 

[SirRobyn0]

I daily my square and dont have ethanol issues. But i believe ethanol cannot sit for long periods of time.

I daily mine to and run nothing but regular pump 87. Your right the ethanol will attract moisture. It is slightly harder on rubber hoses, but at 10 - 15% at the gas pump, it's not going to hurt rubber in good condition. Also it is a great cleaner, helps keep the inside of your carb clean. Conversely if you have a moderately rusty tank it'll tend to break that up then it goes up the line and causes problems.

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.

Get Cliff Ruggles book on Quads.

If you're getting gas and you have spark and it stumbles, make certain that your vacuum advance is working.

I'd change the module just because the one you have has way out lived it's normal life. Snoots makes a good point don't overlook a timing / advance issue. Also back pressure in the exhaust can seem like fuel issues in the beginning. Does the truck have duel tanks? And if so does it do it on both tanks?

 

[bedwards]

Yes, it does it on both tanks. The advance is working....but I don't know if its working right. Will check that next.

thanks for all your ideas

 

[C10MixMaster]

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.

if the secondary air valves are cracking open revving the engine in park that doesn't sound right . You are unlikely to have sufficient load or air velocity to open them in park especially with a 305.

 

[bedwards]

ok, they are cracking open less than a 1/8 in maybe. So which way do I turn the screw to tighten them? 1/4 turn Clockwise?
The truck has always run rich and left some carbon on the plugs, well... they are burned clean now as though its very lean.

 

[Snoots]

Do you have headers or the stock manifolds with the heat-riser valve?

 

[RoryH19]

Glad I saw this thread. Hopefully you can find a solution.
I'm ready to dropkick the quad from my 83 k5 and get a holley.

 

[C10MixMaster]

ok, they are cracking open less than a 1/8 in maybe. So which way do I turn the screw to tighten them? 1/4 turn Clockwise?
The truck has always run rich and left some carbon on the plugs, well... they are burned clean now as though its very lean.

loosen the lock screw and then turn 1/8 clockwise see if there is any improvement.