Mercury Outboard 70 (3 CYL.) rough idle, low rpms trying to figure it out

Hello guys, im back again with another issue with my engine

Not good to rev outboard that high in neutral.Seloc new on this site $35. Idle should be around 800 RPMs,in gear 650-700,WOT 5000 - 5500.WOT ,if everything is set up correctly,depends on prop size,weight of boat,the weight distrubution and a variety of other factors,but these the main ones.
If the carbs are linked and synced correctly and the idle is set,then no adjustment are needed on the carbs.Cable need only be adjusted if it's not opening the butterflies all the way or closing.
If it's shaking a lot at lower RPMs,idle might need playing with,but I would have a look at the upper and lower engine mount bushings first.
I usually let my auld Merc800 sit in high idle for a few minutes to warm up before I leave the pier.

hey thanks for the reply. thats some good info there sir. thank you. well i didnt really have the rpms higher than 3000 in neutral, other than when in high idle with the throttle lever up. it did reach close to the 5000-5500 in that mode. i dunno i been playing with the carb screws all day, it seems like i get it running nice and smooth and then for some reason it just jumps up real fast...i dunno. then i turn them in a little and then i go too far and it stalls. and then im starting over. probably started over like 5 times today. frustrated. but i cant seem to get the idle down to 800, seems to always be around 1000 rpms in with throttle lever down. the boat is an old 1987 15 1/2 foot starline sierra 150. not sure of total weight. not a big boat at all really. can seat 4 people. these bushing you mention they wouldnt happen to be the bushing on the two shock absorbers would they, because all the bushings on those need replaced, but i cant find them anywhere. wonder if i could find some really big rubber washers maybe. also, another thing i noticed when messing around today was, when i put the lower gear unit and prop in a tote full of water and run it, it starts producing a sludge in the water, where as if i just used hose and muffs it dont do that. is that normal or what the hell is going on there? im new too marine engines and i dont fully understand them. very confusing to work on. anyway thanks, and i'll be looking forward to more replies.

Very familiar with those motors, and I've never owned one that would idle properly below 1,000 rpm.

You're moving the idle screws too quickly. You need to make a 1/4 turn max adjustment, then WAIT and let it settle out. (And you need the motor deep in the water, too.)

I've had success using an infrared temp gun on the metal part of the plugs to balance the three cylinders out. It takes time and PATIENCE to get all three similar. I then add /12 turn OUT (CCW) on both screws to prevent bogging.

Jeff

I bought replacement bushes from Bob Grubb in PA a good number of years ago, www.oldmercs.com .May still be around. The other bushings I had in mind were the mount bushes,19 and 25 in this parts diagram http://www.marineengine.com/parts/m...thru-5579016-usa/drive-shaft-housing-assembly.
Jeff is the man with these motors.

fastjeff said:

Very familiar with those motors, and I've never owned one that would idle properly below 1,000 rpm.

You're moving the idle screws too quickly. You need to make a 1/4 turn max adjustment, then WAIT and let it settle out. (And you need the motor deep in the water, too.)

I've had success using an infrared temp gun on the metal part of the plugs to balance the three cylinders out. It takes time and PATIENCE to get all three similar. I then add /12 turn OUT (CCW) on both screws to prevent bogging.

Jeff

Click to expand...

Thanks Jeff! Really appreciate the reply. I was turning the screws basically to a starting point of 4 half turns or 2 whole turns out. and then starting it up and started with bottom carb first then the top. i would only make either 1/4 turn adjustments or half turn adjustments, seemed to get the rpms up when i turned in a little from that starting position, i had it real close to perfect i think and then i had to touch it again and i messed it up haha. so your saying it is more than likely normal for it to be idling at about 1000 rpms? also, you state it has to be in water, so in a tote full of water wont be as good? also, about that sludge is that normal? when i put in tote?

haventaclue said:

I bought replacement bushes from Bob Grubb in PA a good number of years ago, www.oldmercs.com .May still be around. The other bushings I had in mind were the mount bushes,19 and 25 in this parts diagram http://www.marineengine.com/parts/m...thru-5579016-usa/drive-shaft-housing-assembly.
Jeff is the man with these motors.

Click to expand...

thanks for reply. i checked out that link, i didnt find anything i needed that he had. and i saw those mount bushings and damn they expensive haha. i'd have to locate on engine and see if they bad first but. thanks again

So, ya those absorber bushing are like impossible to find. part number 28198 need 4, and part number 32171 need 4. now i have found complete mount and bracket assemblies with the shock absorbers on them, with ok looking bushings on ebay or somewhere i forget, but i dont need that whole assembly. and with shipping i think i'd be looking at about $150 just so i can get a few bushings off it. i dont think so, haha.