Ok here's the deal, I have a 52 mercury with a 87 sbc. It's been running fine for months now and all of a sudden the other day it starts to idol rough at low rpms and hiccups through 2 and 3rd gear. When it's in park it idols fine but as soon as it's put in drive it drops to about 100 and bounces between 100 and 600. I've been messing with the mixture and choke but nothing seems to help. I'm totally confused.
Carb or 87 TBI ? ***uming HEI ? or no longer 87 distributor ? Also.....87 computer controls or none whatsoever ? Any EGR stuff still on it ? More, more, more, please......
I would almost rule out fuel if it came on suddenly. Pull the top off the carb and make sure there isn't gunk blocking a jet or something. I hope you have a filter pre-carb. Since you went carb, I ***ume you went HEI, since they are so easy to put in. Couldn't tell you unless I knew. Only other thing based on the limited info is maybe a vacuum line got disconnected somewhere? You will really have to give all the specs of the engine to get a better diagnosis.
I would surely look at all the vacuum lines and possible vacuum leaks, vacuum modulator line especially. Doesn't really sound like a fueling issue to me. Will the engine wind up, aside from the hiccups?
We are ***uming that you have a non computer HEI with a vacuum advance or other non computer distributor? Ran great up till now but ****s out off Idle now. Dirt in carb low speed circuit? Plugged fuel filter? kinked or pinched fuel line just after you did some work on the line? Brand new shiny gas cap not venting? Or tank not venting. Advance mechanism in the distributor went gunnybags? Including vacuum advance.
Yep, for some reason the vacuum modulator was my first thought. If you are running an older auto trans, crawl underneath and check the rubber line where it goes into the transmission vacuum modulator.
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