Troubleshooting '73 LS4 backfiring through carb.

[George H.]

The 454 engine has the original Emission Control System and EGR valve set up. I'm not sure if all components are operating properly. The engine starts well, idles well, runs smoothly but will backfire through the carbonator (not up to temperature) when I step hard on the throttle to increase the RPMs. When driving, runs ok (temp at 190) but will backfire when shifting from first to second or third during hard acceleration. The engine will also backfire through the carb when downshifting from forth or third gear.

I'm thinking it may be a bad EGR valve (7047066), PVC valve (CV-769C) one of the TCS switches or a check valve (5361992). Maybe the vacuum advance but not sure. The engine was professionally rebuild by Japer Engineering in Indiana in 2018 and has about 1500 miles since being rebuilt. I don't drive it very much Any comments on what may be causing the engine to backfire? How should I start trouble-shooting the problem?

gator943
George Hunt
Ocala, FL

[Mark E.]

Re: Troubleshooting '73 LS4 backfiring through carb.

My first thought is retarded ignition timing, but you say the engine runs well. You might check the timing nevertheless.

I haven't seen EGR cause backfire through the carb, but you can disconnect it and check.

A bad or improper PCV can cause a lean or rich condition, so check that.

Another possibility is a valve leaking or burnt.

[George H.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Replacing the PVC value is not an issue. Replacing the EGR value is a major issue. No GM service parts available (17051867). I'll tray removing the hose from the carburetor to the EGR value and see if that stops it. An EGR valve I believe has a vacuum diaphragm that in response to intake vacuum unblocks the opening between the exhaust gas passages and the intake passages thereby adding inert gases to the intake mixture. It supposedly reduces NO2 production. If the valve is stock open or closed, I'm thinking it may be the issue. I trust its not a valve since since the engine was completely rebuild and has a new complete valve train and the heads resurfaced. I'll also be checking the timing. Thanks for the feedback.

[Edward J.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Gorege,, To add to Marks post, check to make sure the accelerator pump is giving a shot of fuel ( you should see a full shot of fuel through the nozzles all the way through travel on accel. Pedal) and make sure the choke is fully closed cold, and fully open hot open after engine is warmed up ( depending on outside temp. About 3 to 4 minutes should be open) if choke does not fully open after warm up the secondary’s will not function causing loss of power on hard acceleration and cause backfires ,Also Check dist. Vacuum advance operation, most recommend to run the vacuum advance with full vacuum all the time for better performance.

[George H.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Thanks for the feedback Edward. I plan to do what you advised. Here is what I will do.
1. From a cold engine, I'll take the air cleaner off and check to see if the choke is open or closed. I assume it will be open. Or, does it close when the engine cools off?
2. I'll check all the vacuum lines and air hoses off the carb and manifold to make sure they are all in place.
3. Typically, when I first go to start the engine, I step on the gas pedal two times to give it some gas and that should close the chock, assuming it was open. I'll need help to make sure its getting a good shot of gas when I hit the pedal and then check to make sure the choke is closed.
4. After cranking (sometimes I have to give it more gas) it fires up and the RPMs run about 1200-1300. After 3-5 minutes, when I hit the gas, RMP's generally fall back to about 800.
5. The engine runs generally good and I can run the RPM's up to 2000/3000 with no issues. It is when I step on the gas hard it backfires through the carb. not the tail pipes.
4. I'll let the car run for about 10 minute or until the water temperature worms up to 160 or so.
5. I'll then make sure that the choke is fully open.
6. I'll also check to make sure the vacuum advance of the distributor is working.
7. If it still backfires, I'll start looking at the TCS components. To do that, I'll need a good mechanic with lots of expertise on Emission Control issues. I know 1973 was the first year for the ECS. My system is about 50 years old!

I'll give you feedback as to how it all works out. Love to find out out it is only a choke issue.

Thanks

[Mark E.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Yes, EGR can be disconnected by disconnecting and plugging its vacuum line.

Are you sure it's backfiring through the carb (air/fuel flame seen) and it's not just hesitating?

[Edward J.]

Re: Troubleshooting '73 LS4 backfiring through carb.

George, I think you car has a TVS (thermo vacuum system)system which controls your EGR Valve, I would unhook the vacuum hose,I think Mark said that. If the valve is opening when you have a cold engine this will cause the engine to hesitate and backfire under acceleration.if you also has a TCS system that controls the Dist. Vacuum advance I would also bypass that by hooking the vacuum hose to direct vacuum.

[Timothy B.]

Re: Troubleshooting '73 LS4 backfiring through carb.

George, be careful with your face over the carburetor checking the accelerator pump shot or anything else for that matter.

[George H.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Troubleshooting update. Yesterday I had a friend help me when I ran some tests on the engine.
1. Took off the air cleaner (engine cold) and found the choke fully open.
2. Checked all the vacuum hoses and found them in place.
3. Stepped on the gas pedal once and the choke closed and gas did shoot into the primaries.
4. Careful when starting up the engine. It did fire up after cranking it for 5-8 seconds. Choke remained closed and it ran smoothly.
6. After 4-5 minutes, the manifold heater up and the thermal choke stove spring pushed the choke fully open.
7. Ran the RPMs up an done down and it popped or backfired thru the carb once after letting off the gas or when stepping back on the gas..
8. Pulled off the vacuum hose from the distributer to the carb and the engine RPMs dropped off and picked up when we reinstalled the vacuum line. It appears vacuum advance is working.
9. I let it run for several minutes and then hit the gas hard. Did not backfire I believe until I let off the gas. Sitting at idle ran very smoothly.
10. When hitting the gas pedal hard, what we noticed was the secondary's did not open completely with the RPM's up and gas pedal fully down.
The flapper valve only pushed open about 1/2" and did not get fully open (straight up and down). It did however fully close. At the time, the Engine temp was about 140.
11. As the engine heated up, I hit the gas a few more times and it ran good and backfired only once after letting off the gas.
12. I'm thinking that it may be my Rochester carb secondary's (or primaries) are not operating properly. I believe the secondary's work off a vacuum diaphragm and maybe that's the problem. Or, maybe a poor air-and-fuel idle mixture. Not at all sure that is the issue.
13 I did have the Quadrajet carburetor rebuild back in 1995 and the car sat for years not being run very much until about 2014. The engine was rebuild in 2016 and carb was put back on a fresh new engine. Was running good however until recently.
14. Plan to have a good mechanic look at gas-to air mixture adjustments and check out the carburetor to make sure secondary's are operating properly.
15. I did check out the PVC valve and it appears to be Ok. Makes a rattle when you shake it.

Thanks everyone for your feedback.

[Edward J.]

Re: Troubleshooting '73 LS4 backfiring through carb.

Troubleshooting update. Yesterday I had a friend help me when I ran some tests on the engine.
1. Took off the air cleaner (engine cold) and found the choke fully open.
2. Checked all the vacuum hoses and found them in place.
3. Stepped on the gas pedal once and the choke closed and gas did shoot into the primaries.
4. Careful when starting up the engine. It did fire up after cranking it for 5-8 seconds. Choke remained closed and it ran smoothly.
6. After 4-5 minutes, the manifold heater up and the thermal choke stove spring pushed the choke fully open.
7. Ran the RPMs up an done down and it popped or backfired thru the carb once after letting off the gas or when stepping back on the gas..
8. Pulled off the vacuum hose from the distributer to the carb and the engine RPMs dropped off and picked up when we reinstalled the vacuum line. It appears vacuum advance is working.
9. I let it run for several minutes and then hit the gas hard. Did not backfire I believe until I let off the gas. Sitting at idle ran very smoothly.
10. When hitting the gas pedal hard, what we noticed was the secondary's did not open completely with the RPM's up and gas pedal fully down.
The flapper valve only pushed open about 1/2" and did not get fully open (straight up and down). It did however fully close. At the time, the Engine temp was about 140.
11. As the engine heated up, I hit the gas a few more times and it ran good and backfired only once after letting off the gas.
12. I'm thinking that it may be my Rochester carb secondary's (or primaries) are not operating properly. I believe the secondary's work off a vacuum diaphragm and maybe that's the problem. Or, maybe a poor air-and-fuel idle mixture. Not at all sure that is the issue.
13 I did have the Quadrajet carburetor rebuild back in 1995 and the car sat for years not being run very much until about 2014. The engine was rebuild in 2016 and carb was put back on a fresh new engine. Was running good however until recently.
14. Plan to have a good mechanic look at gas-to air mixture adjustments and check out the carburetor to make sure secondary's are operating properly.
15. I did check out the PVC valve and it appears to be Ok. Makes a rattle when you shake it.

Thanks everyone for your feedback.

[Mark M.]

Re: Troubleshooting '73 LS4 backfiring through carb.

George try one thing at a time and check for results. With c3 cars I block off all vacuum from source which usually means a few lines on manifold or so and this eliminates all the many parts that can have a vacuum leak ; headlight, vacuum storage, wipers (68-72) . Check if it helps. It seems the accel pump and vacuum advance were mentioned but also check secondary butterfly tension spring may be loose. The secondaries should only open under a load as vacuum dictates. Checking manifold vacuum may show an issue; 73 LS4 I'm guessing should have 16-18 hg at idle.

[Chris H.]

Re: Troubleshooting '73 LS4 backfiring through carb.

George, have you checked the timing? If your timing is off your just going to be chasing your tail.

[George H.]

Re: Troubleshooting '73 LS4 backfiring through carb.

I plan to take the '73 out for a 20-25 mile drive and see how it performs. Will see if it backfires under heavy load or when down shifting. If it still backfires, I'll then disconnect the EGG valve and plug its vacuum line. Take it out again and see if that stops the backfire.

Question, can an EGR valve be rebuilt? The one on the engine is original, correct Rochester NR part number and date code. Love to keep it that way.

Thanks again everyone for your feedback. Ill post an update on how I make out.

[Duke W.]

Re: Troubleshooting '73 LS4 backfiring through carb.

The secondaries on a Q-jet operate on the "constant vacuum", otherwise known as constant velocity principle, like a SU carb on an English car.

When you floor the throttle the secondary butterflies open fully. The "air valve" at the top of the air horn is attached to a spring that allows them to slowly open while maintaining a constant vacuum below the valve, and as the valve opens it lifts a tapered metering rod out of the jet that the rod sits in the middle of that increases fuel flow proportional to the increase in air flow.

From your description it sounds like the secondaries on your Q-jet are operating properly.

Have you gone over the trouble shooting procedures in the CSM? Also make sure the plug wires are indexed on the cap IAW the CSM.

Duke