1989 Honda Accord LX
i got a brand new starter, spark plugs and battery. but i have a rough idle and i failed the nox part of my emissions test. can gumout help my situation ? i need a new muffler too ,not a new exhaust system. i just had t replaced in 2019 and i have 137,000 miles on it, i got a brand new engine at 100,000 miles.
There are several things that can cause both high nox emissions and rough idle. Vacuum leaks are common on vehicles that old (a lean mixture can cause high nox), or you may have a problem with your EGR valve or catalytic converter. Ignition timing being off can also be a factor.
https://www.smogtips.com/failed-high-NO-nitric-oxide.cfm
In 1989 some Hondas still had carbs. If a carb has not been overhauled recently it's probably due, especially after being exposed to ethanol-contaminated fuel for years. If you have fuel injection you may want to check your fuel injectors.
thanks for the answer, i just got a new catalytic converter april 11, 2017, i got a brandnew engine with zero miles on it at 100,000 miles in 2011,and im only at 37,000 right now. no im lx.not lxi. im not fuel injectors. my timing belt is fine too .how can you tell if you have an egr valve issue? will gumout help with my rough idle? im only asking because its an older car, my carburetor was replaced august 17,2011, when i got that new engine.also i just replaced the spark plugs battery and starter in the last 6 months, thanks for the answer so soon. patrea 
also i need a new muffler but not the exhaust pipe i had that replaced April 17,2019.
The carbs on those old Hondas are very complicated and there's a good chance it needs to be overhauled after 12 years, though you could try adusting it. (In general getting a 1980s vehicle that has a carb through an emissions test is a real challenge.)
Here's some info on checking the EGR valve:
im not a mechanic.
I understand, but if you can't work on this yourself you're going to need a mechanic familiar with these older cars, old emissions systems, and carburetors.
i know, and im broke, i may be taking a taxi until i can at least afford a new muffler in 2 weeks.
I'm sympathetic, but those things are just so tough to deal with, every adjustment throws something else out of whack. Best thing is to follow the tuneup and adjustment regimen in the factory shop manual to the letter. I used to really struggle getting my 1980s-vintage AMC Eagles past the sniffer when I had them. Those things had feedback carbs and a really crazy emissions setup with miles of vacuum plumbing and gadgets that all had finicky adjustments and had to work right for it to pass.