78 Merc Jet won't idle

[kilroywashere1911]

78 Merc 800 won't idle, runs fine if I start it at higher throttle- I could use help troubleshooting. It has been starting and idling fine but lately it's been doing this. I'm putting new spark plugs in today and hitting it with some seafoam and carb cleaner but if anyone has anything to offer, I'm all ears.

 

[kilroywashere1911]

I was messing with it today and while it was running I sprayed carb cleaner in the top carb and it made it run better, when I sprayed in the bottom it died immediately. I'm trying to figure out if that is telling me that the top one is not getting fuel or if there is a problem with the bottom one. Since this model only has one fuel pump that would suggest a clog in the top carbs fuel line or an issue with the jet?

 

[kilroywashere1911]

There is fuel in the bowl of the top carb, the jet bowl vent screw is pretty mangled, I'm wondering if it might be clogged preventing fuel flow through the needle...

 

[Lil' Blue Rude]

Probably should pull the carbs and clean them. If I had to guess I would say one of your low speed (idle) circuits is stopped up or partially stopped up. Most carbs have 2 circuits Idle and high speed. Some have a mid range. Any who the idle circuit has the smallest opening to feed fuel when at idle because there is such a small vacuum being produced by the motor at idle once the motor starts to draw in more air the high speed circuit starts to draw fuel in through the main jet. Both circuits are drawing in fuel at higher rpms but the low speed draws in less then what the high speed does so that's probably why your motor can run at higher speeds but not idle properly. It's still not getting the proper amount of fuel but it's getting enough to run when started on fast idle.

 

[turbotodd]

with the motor idling, pull each plug wire one at a time. Idle should change drastically. If it doesn't change when you pull a wire, that cylinder is not running. Then check for spark on that cylinder. If spark then check for compression. Compression good=it's carb or fuel system related (or an air leak).

 

[srr5008]

I am experiencing a similar issue. Take a look in my thread titled "Engine only starts with throttle advanced" and see if any of the suggestions there will help. It was recently suggested to me that I should clean the idle circuits, as recommended to you by Blue Rude. Haven't done that yet - will do later tonight and let you know how it works for me.

 

[kilroywashere1911]

I broke down and took it to a shop, mostly because last year I spent an entire taking apart and putting an older 64 merc to no avail. He pulled the carbs out acid dipped and rebuilt them, says it purrs like a kitten. I'll post tomorrow with the results.