This is my first time on this site. Thanks for your help. I have the same AR that I bought slightly used in 1985. I remember I paid $400 that included 250rounds, 2-20 round mags and 3-30 rounders. For the first couple of years I shot it a lot but overall it really hasn't had a great deal of rounds through it. It has always been kept clean. My friend bought a new AR, so we went to the range. My rifle started shooting bursts of 3 to four rounds. Although it was cool I can not tolerate this illegal behavior from my rifle. The rifle is stone stock. I plan to replace the fire control group and all of the springs. I have learned that there may be different pin sizes for older ARs. Any ideas what could be causing this malfunction and recommendations for a fix. Thanks
It sounds like you've already got a good idea what's wrong. Like you said it's annoying that Colt uses different pin sizes .170" instead of .154" but at least they are common parts that you can buy.
Definitely sounds like the the hammer/trigger sear or disconnector are at fault. You can dry fire to test the disconnector. Probably best to remove the upper and then cock the hammer, pull the trigger while you catch the hammer with your thumb so it doesn't damage the bolt stop and receiver. Keep the trigger pulled back and recock with your thumb (keeping it there to catch the hammer if it falls). The disconnector should now be holding back the hammer. Now you finally release the trigger and see if the disconnector releases the hammer and the trigger sear catches it.
Probably the timing between the disconnector and trigger is off or the parts are worn.
Something else to check, is make sure that A) the pin didn't move out of place,
B)look at the inside of the lower at the trigger, hammer, etc Look for Shiny, un-even pieces.
I had it happen on my ar after 200 rounds, and the pin wasn't centered, so it walked out and suddenly it was going 2 rounds at a time
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