91 Tracker Exhaust Fumes and bogs

Have a 91 Tracker, took it in for a full service and ever since it has not been the same car, it bogs down, has no power, if I don’t let it run and idle for 10 mins it just stalls. Have taken it to two different mechanics who are completely stumped and can’t find whats wrong with it! When it idles the exhaust smells very strong. Had the ecm replaced a few years ago, so don’t think its that, it starts first time, every time!! Also there seems to be a lot of carbon black stuff coming from exhaust, could I have some kind of blockage!! Any advice would be appreciated, love this car and am not ready to get rid of it yet.

Have a 91 Tracker, took it in for a full service and ever since it has not been the same car, it bogs down, has no power, if I don’t let it run and idle for 10 mins it just stalls. Have taken it to two different mechanics who are completely stumped and can’t find whats wrong with it! When it idles the exhaust smells very strong. Had the ecm replaced a few years ago, so don’t think its that, it starts first time, every time!! Also there seems to be a lot of carbon black stuff coming from exhaust, could I have some kind of blockage!! Any advice would be appreciated, love this car and am not ready to get rid of it yet.
Do you know if the place you took it to ran some sort of engine cleaner in there? When you say full service what all did they do?

I think it was just a step up from a oil change, rotate the tires, lube etc etc. They did say my valve's were leaking a little and they did put some fuel cleaner in but that was about 3 months ago and it still smells terrible. It just runs really rough which is odd because previously this little thing ran just fine. Any help would be appreciated. Thanks