Best way to test Vacuum Leaks (popping noise) - The 1947 - Present Chevrolet & GMC Truck Message Board Network

MandarinSixSeven · 07-14-2004, 04:35 PM

Hey guys/gals:
I got my carb all tuned, muffler leaks all sealed, it has quite new spark plugs and wires, but I can't seem to get rid of this little popping noise on decel. It's not really loud, but really noticable and kind of annoying.

I'm thinking it must be a vacuum leak or something to that effect, or am I way off. What would be the best way to test? I know the timing is all good, I had that checked a while back. It has been doing this ever since I put on my 1406 carb, I was told it was a vacuum leak, but I don't know how to test...

Thanks

Grim Reaper · 07-14-2004, 04:38 PM

poping noise on decell is probably a pin hole exhaust leak. It might not even be that bad of leak that you even notiice it most times.

What happens is it sucks air in and reignights unburnt fuel in the exhaust and gives a poping noise as it does.

MandarinSixSeven · 07-14-2004, 05:31 PM

Are there any other possibilities? I sealed any little holes in the exhaust that I could. I guess my question should be, is it possible that it might be a vacuum leak and how do I check so I can rule it out at least?

shuttermutt · 07-14-2004, 05:43 PM

With the engine running, grab a can of carb cleaner and spray little shots around the intake manifold and carb base. If the idle speeds up, you've found your leak. Take your time. You don't want pools of that stuff on a hot engine.

Another thought, though a bit of a longshot, would be to check your heat riser thingamajoob (if yours is still present). It could be flapping around. I've had it happen before and tracking it down will drive you batty. BTW, I'm talking about the thing that's usually mounted on the driver's side in between the exhaust manifold and the down pipe. If you don't need it, just wire it open or remove it.

GMC AMI · 07-14-2004, 05:49 PM

Sometimes unburned fuel will cause a popping noise upon deacceleration in the exhaust. This happens a lot running open headers at the drags. Good luck!

shuttermutt · 07-14-2004, 05:51 PM

"Top of Maine", eh Mike? Wouldn't be up in The County, would you?

ChevLoRay · 07-14-2004, 06:37 PM

I had an Edelbrock 1405, that ran rich...so rich, in fact, that it would gas-foul the #2 spark plug. I don't know why it was that one in particular, but it was. I knew it was about to happen when the pipe on the right side (true duals-no crossover/balance tube) started to pop like it was backfiring.

Now that I have a model 1901 Edelbrock (Q-Jet), I have had no more gas-fouled plugs and it won't pop at all, even if I'd like to hear it.

So, it may be that your carb is jetted on the rich side, for your engine's needs, and that is the source of the fuel that is popping.

Usually, a vacuum leak will cause the engine to have a "miss", rather than a pop, in my experience. Sometimes, the easiest and cleanest way to tell if you have one, is to simply remove the air cleaner ass'y, and use your hands to smother the engine at the air horn on the carb. If you don't have a leak, it will die. If there is a leak, you can vary the amount of smothering you do, and it will smooth out. If you can completely cover the carb without it dying, you gotta hella leak.

GMC AMI · 07-14-2004, 06:49 PM

Quote:

Originally Posted by shuttermutt

"Top of Maine", eh Mike? Wouldn't be up in The County, would you?

PM you shuttermutt,Mike

MandarinSixSeven · 07-14-2004, 09:48 PM

cool, thanks for the ideas.
I will definately check the plugs, the heat riser is gone, since it was rattling...and that carb cleaner trick will be tried too

07-14-2004, 04:35 PM	#1
MandarinSixSeven Seasonal Poster Join Date: Apr 2002 Location: Kelowna BC Canada Posts: 896	Best way to test Vacuum Leaks (popping noise) Hey guys/gals: I got my carb all tuned, muffler leaks all sealed, it has quite new spark plugs and wires, but I can't seem to get rid of this little popping noise on decel. It's not really loud, but really noticable and kind of annoying. I'm thinking it must be a vacuum leak or something to that effect, or am I way off. What would be the best way to test? I know the timing is all good, I had that checked a while back. It has been doing this ever since I put on my 1406 carb, I was told it was a vacuum leak, but I don't know how to test... Thanks __________________ 67 GMC Short Step 350/4-Speed/3.08 Posi Front/Rear Sway Bars Kelowna BC, Canada Member of DTPW

07-14-2004, 05:31 PM	#3
MandarinSixSeven Seasonal Poster Join Date: Apr 2002 Location: Kelowna BC Canada Posts: 896	Are there any other possibilities? I sealed any little holes in the exhaust that I could. I guess my question should be, is it possible that it might be a vacuum leak and how do I check so I can rule it out at least? __________________ 67 GMC Short Step 350/4-Speed/3.08 Posi Front/Rear Sway Bars Kelowna BC, Canada Member of DTPW

07-14-2004, 05:43 PM	#4
shuttermutt Don't say "Oops!", Doctor Join Date: Nov 2003 Location: Houston, TX Posts: 677	With the engine running, grab a can of carb cleaner and spray little shots around the intake manifold and carb base. If the idle speeds up, you've found your leak. Take your time. You don't want pools of that stuff on a hot engine. Another thought, though a bit of a longshot, would be to check your heat riser thingamajoob (if yours is still present). It could be flapping around. I've had it happen before and tracking it down will drive you batty. BTW, I'm talking about the thing that's usually mounted on the driver's side in between the exhaust manifold and the down pipe. If you don't need it, just wire it open or remove it. __________________ 'Mutt 1968 CST LWB: R.I.P. 1967 Chevelle: TPI 454 beast 1967 C10 SWB: Claimed when Bumpster didn't put it on his list

07-14-2004, 05:49 PM	#5
GMC AMI Registered User Join Date: Apr 2004 Location: The Other Maine! Posts: 2,236	Popping noise? Sometimes unburned fuel will cause a popping noise upon deacceleration in the exhaust. This happens a lot running open headers at the drags. Good luck!

07-14-2004, 05:51 PM	#6
shuttermutt Don't say "Oops!", Doctor Join Date: Nov 2003 Location: Houston, TX Posts: 677	"Top of Maine", eh Mike? Wouldn't be up in The County, would you? __________________ 'Mutt 1968 CST LWB: R.I.P. 1967 Chevelle: TPI 454 beast 1967 C10 SWB: Claimed when Bumpster didn't put it on his list

07-14-2004, 04:38 PM	#2
Grim Reaper Registered User Join Date: Jul 2001 Location: Atlanta GA Posts: 1,704	poping noise on decell is probably a pin hole exhaust leak. It might not even be that bad of leak that you even notiice it most times. What happens is it sucks air in and reignights unburnt fuel in the exhaust and gives a poping noise as it does. __________________ Grim-Reaper 70 Pontiac LeMans Sport Convertible, worlds longest resto in progress Looking for 71-72 2wd Blazer or Jimmy Project

07-14-2004, 06:37 PM	#7
ChevLoRay Old Skool Club Join Date: Jun 2001 Location: Benton, AR "The Heart of Arkansas" Posts: 10,880	I had an Edelbrock 1405, that ran rich...so rich, in fact, that it would gas-foul the #2 spark plug. I don't know why it was that one in particular, but it was. I knew it was about to happen when the pipe on the right side (true duals-no crossover/balance tube) started to pop like it was backfiring. Now that I have a model 1901 Edelbrock (Q-Jet), I have had no more gas-fouled plugs and it won't pop at all, even if I'd like to hear it. So, it may be that your carb is jetted on the rich side, for your engine's needs, and that is the source of the fuel that is popping. Usually, a vacuum leak will cause the engine to have a "miss", rather than a pop, in my experience. Sometimes, the easiest and cleanest way to tell if you have one, is to simply remove the air cleaner ass'y, and use your hands to smother the engine at the air horn on the carb. If you don't have a leak, it will die. If there is a leak, you can vary the amount of smothering you do, and it will smooth out. If you can completely cover the carb without it dying, you gotta hella leak. __________________ Member Nr. 2770 '96 GMC Sportside; 4.3/SLT - Daily driven....constantly needs washed. '69 C-10 SWB; 350/TH400 - in limbo The older I get, the better I was.

07-14-2004, 09:48 PM	#9
MandarinSixSeven Seasonal Poster Join Date: Apr 2002 Location: Kelowna BC Canada Posts: 896	cool, thanks for the ideas. I will definately check the plugs, the heat riser is gone, since it was rattling...and that carb cleaner trick will be tried too __________________ 67 GMC Short Step 350/4-Speed/3.08 Posi Front/Rear Sway Bars Kelowna BC, Canada Member of DTPW



Register or Log In To remove these advertisements.

Register or Log In To remove these advertisements.