Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem
1977 454 Quadrajet Secendary issue, pop back fire rough running.
Jeff K:
Secondary's:
About 7 years ago, I bought the best car I ever owned, a 69 el Camino SS396. I'm retiring with this one... It's worn in (like me) and it has a 1977 454 truck motor with a 400 Turbo and 2.73s out back.
This motor runs real strong. This Quadrajet has never given me one lick of trouble. Over the winter it started backfiring or running rough when the secendarys are opened. With them disconnected, it runs perfectly just on the primary cercuit. The vacuum gauge needle goes to 6 or 7 when floored then comes right back up at throttle release.
It has the original HEI distributor. I disconnected the vacuum advance and run 18 initial with 30 mechanical all in by 3000. It runs best like this. I use Sams Club, Boron or Esso 87 octane. The 781 heads are suppose yield only 8.0 or 8.50 compression, I very rairly ever heard a full throttle ping. The pinging goes away after flooring it a few times. I'm thinking this must clean the chambers out and lower the compression. It's city driven.
Yesterday, I pulled the secendary hanger (J) and it was bent.. One rod (DN) - Passenger side, was almost a half inch higher then the other, I think I did this installing a lock nut on the air cleaner stud. When I open the secendarys it makes a wierd popping sound, I think its running lean. I tried to straighten the hanger out but it still is rough but better..
The carb came with the engine. The engine came from a brand new truck that was demolished with a few thousand miles. The prevous owner bought the car with several others from Texas in a package deal. It didn't have an engine or transmission. He had this 454 and th-400 sitting in his garage for decades. He kept it oiled and removed the rockers to save the valve springs.
Should I get new rods and a new hanger?
Thank you..
Cliff Ruggles:
Without the VA you are missing some timing at light engine load so it's likely going to "load up" the combustion chambers and piston tops.
IF the engine doesn't like the VA hooked up it's simply adding too much timing and you need to modify the existing VA or get one that only adds about 10-14 degrees. This will help your engine run better everyplace.
As far as problems at WOT on the secondaries I would start by installing one of my rebuild kits, and correctly repair or replace the secondary hanger. There is a new secondary plastic cam and spring in the kit. The springs are always weak from being would up nearly 50 years and the plastic cams shrink, crack, wear down and get loose on the shaft which can have a HUGE impact on full throttle performance just like you are seeing......
https://cliffshighperformance.com/product/quadrajet-rebuild-kit-sr
Kenth:
HEI distributors rarely or never receive the service that is necessary, (they do not have points that need to be checked occasionally.)
Check that the weights can move easily and are not rusted or binding.
Jeff K:
I'm thinking the popping issue comes from number five cylinder is loosing its exhaust lobe. I've been using Mobil 1 5W-30 along with a Wix or K&N oil filter for years without issue.
Then, a week after pouring a quart of Shaler Rislone oil additive into this engine, it starts popping up out of the carburetor..
Jeff K:
The first half of the last post didn't come through.
I decided to pull the valve covers and sure enough the popping issue is comming from number five cylinder, it's loosing its exhaust lobe.
I've been using Mobil 1 5W-30 along with a Wix or K&N oil filter for years without issue.
Then, a week after pouring a quart of Shaler Rislone oil additive into this engine, it starts popping up out of the carburetor..
So time for a new cam. I'll start looking at the Crower line up since I have always had good luck with there stuff..
I'm going to see if Pontiac lifters made by Johnson would fit the 454 block. I have a bunch of Johnson lifters from the seventies.
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