1976 Mercury 850, Won’t hold an idle

[1976 Supersport]

Hi Everyone, First post here and just wanted to say thank you to all the members who have unknowingly helped through their posts, as I’ve tried to breathe some life into my 76 Starcraft that I purchased last year. From impeller replacement to refilling the power trim pump, this site has been extremely helpful for someone who is not very mechanically inclined.

As I stated, I bought a 1976 Starcraft with a Merc 850 Thunderbolt last year from the 2nd owner and have had it into the shop several times to redo wiring, replace the lower unit and to replace a leaking gasket. I finally had it running right earlier this year after I adjusted the fuel/ air mixture on the carbs until I used some premixed gas that had been stored since last September (I thought it would be OK to use since I had used sea foam as a stabilizer). Since I used the old fuel, the motor will no longer hold an idle in neutral. It will run great at top speed and accelerates well. However, I always need to increase the throttle in neutral to keep it from dying on me. Every once in a while it will hold an idle in neutral after I have been running it hard for a long time. I’ve heard these motors have a history of being finicky when it comes to holding an idle but I was hoping some of you may have some recommendations to help get it running right again. If it helps, the motor also feels like it runs rough when I am in gear during a slow troll maybe 5-10 mph. After I accelerate past this point, there is no bog. Thank you in advance for any help! I checked and neither idle jets are clogged, however I have not cleaned the carbs, that is on the docket as soon as I have the time.

 

[racerone]

A compression test is the #1 thing to do before spending any coins.------And no a motor like that will idle all day long if in good condition !

 

[1976 Supersport]

Thank you! I will give that a try. Do you think the bad fuel could’ve had an impact? I’m hesitant to adjust the idle screw because a shop I took it to last year turned up the idle and I’m pretty sure that led to the lower unit to fail.

 

[racerone]

Fiddling with the carburetors will not help a motor that is suffering from internal wear.-----Do the compression test.----Perhaps bleed inserts ( If equipped ) are missing.-----Time to do some real trouble shooting.

 

[Faztbullet]

+1.....

 

[pascoea]

racerone Can you talk a little more about the bleed inserts? That's a term I haven't heard before, and my Google-fu is apparently isn't particularly honed today. (I should mention, I'm having the exact same issue as the original poster, with a similar motor)

 

[racerone]

Remove the bypass covers.-----Look for a3 wee hole on cylinders 1 , 2 and 3.---If they are there the factory installed nylon inserts to restrict " bleed " to the cylinder below.----These plastic inserts are often missing.-----I install 8-32 stainless setscrews with a 0.040" diameter hole when I have a motor apart.----Never to go missing again.----And there is other trouble shooting to be done.----What motor / serial # do you have and what is the motor doing / not doing ??

 

[pascoea]

Thanks Racer. I was able to do more digging after my 2nd cup of coffee.

I've been fighting with poor idle since I got the damn thing. Cleaned/re-built the carbs, futzed with the low speed jets, idle speed, new plugs, new stator, new pickup.

When I get some more ambition and free time I'm going to go through the other usual steps suggested every time someone posts a "my engine won't idle" brings up: Compression, float height, fuel pump, and adding checking this bleed down system. If it's still fighting me I'll start a new thread (not a fan of hijacking someone else's) with more detail on what I've done.

I appreciate your willingness to help. Keep being awesome!

 

[racerone]

Many folks jump all over the carburetors when a motor acts up.-----There is more to a 2 stroke Mercury like yours.------Look at reed valves , labarynth seal grooves in the reed blocks, bottom crankshaft seals, piston and ring wear, bleed inserts..