1970s 75 Eldorado . exhaust puffing/missing?

[Steve Lomas]

The engine runs smoothly for a few minutes at idle then starts intermittent puffing noises from the exhaust which get worse as it warms up, coinciding with a slight shake.

It seems to run perfectly smooth with some gas or accelerating. I also found it smooths out if I half close the choke plate.

The sparks look OK, maybe a tad lean so I increased the mixture a half turn with no difference.

Not sure if it's a carb issue but wanted to get some suggestions here before I tear into that..

Thanks,
Steve

[V63]

Sounds like the catalyst converter is plugged! Yes they can do that, ask me how I know!

[Steve Lomas]

I was wondering about that, the gas mileage is lousy also..

[bcroe]

Cat converter is a real possibility, all cars here
only use the later monolithic style which flows
better.  Do make sure the timing chain set is OK,
the OEMs all need to be changed out by now. I have
not seen a carb which has an external adjustment
for mixture.  But a Wide Band OX sensor will tell
the story.  Bruce Roe

[Steve Lomas]

Thanks Bruce.

I tried some Seafoam I had on the shelf, I've never had any luck with it before but it actually worked wonders here-
much smoother idle, I don't know for how long..

I should check for slack on the timing also though.

[Chopper1942]

If you are concerned abut the CAT, hook a vacuum/presure gauge to a manifold vacuum port. Take a reading at idle. Then raise the rpm to 2000 rpm. See if the vacuum drops. Now do a snap throttle test, this is with the engine at normal operating temperature. Snap the throttle wide open and if the gauge reads more than 2 psi, you have a restricted exhaust.  It could be the CAT, inside of an exhauwt pipe collasped, or restricted muffler. To check the CAT, just remove the exhaust pipe from the CAT and repeat the snap test. If the backpresure is still high, it's a CAT problem. If OK, reattach the exhaust pipe to the CAT and rrmove a different section of the exhaust until you find the restriction.