b00 Johnson 70 Runs Great Then Bogs Down at 3400 RPM Then Runs Great Again

"We are on our knees here, the

"We are on our knees here, the motor has been plagued with problem for 3 seasons with no fix that lasts and we are bound for the Cape in 9 days.

2000 Johnson 70hp 3cyl outboard, external fuel tanks
Problem Synopsis - Runs great then next start up bogs down then might be bad for days then all of sudden starts up fine and runs great again

Details-
Out of nowhere (3yrs ago) a motor that had run like a top from the get go, one day won't start, then when it did it barely made 3000 RPMs at full throttle. We were recomended to try a new separator and some dry gas. Worked OK but had to add dry gas frequently.(motor has been checked for dry gas damage -none found).

Next season same thing, told not use dry gas. change separator motor runs better then another problem of bogging down at 3400rpm is unaffected by another new separator. Have carbs changed/rebuilt - motor runs like top for a month or so then problem returns.

Last season, carbs cleaned and checked by Marina tech preseason. Motor runs great 80% of season and then a couple problems with the bogging down at 3400rpms at full throttle that seemed to be often corrected by shutting down briefly then priming engine lightly before start, and bang the motor's fine again and will rev smoothly up to 5400rpm with good power/speed and hole shot. Ended season running fine.

This year, late start to season (motor was winterized as usual) and first two days out motor is fine all day. third day out, it starts a very slight rough idle and will not go beyond 3400rpm again at full throttle. Mechanic checks motor at dry land Marina (same mechanic all this time) says it runs fine - in fact, motor runs fine every time we've brought it to him except the first time when he felt the carbs were clogging up.

Recently, we go out and motor is locked in bog down mode though at one point it oh so slowly makes it up to 3800rpm. We fish a few hours not driving around all that much, then kick the motor over to move again (engine had off for maybe 10 minutes) and presto the very slightly rough idle is gone and the darn thing runs great - as good as ever - the whole rest of day!

Over course of three+ years we've replaced the gas tanks, oil tank, plugs, had every filter on the engine changed more than once, carbs opened up and cleaned once, rebuilt another, had compression checked x2, tried disconnecting a couple thermostats, etc. and nothing seems to really effect the performance long term but chance - one start it's fine then next one it could stink and vice versa.

When it's bogging down it takes 60-90+ seconds to wind up to 3400rpm at full throttle. It's only happened once or twice where the motor's been running long under load bogging down at 3400rpm then suddenly reved up and worked fine w, 99% of the time it gets better at a start. the priming the motor even when it's warm thing didn't do anything this year to fix problem.

Folks, we are at our wits end on this. Please if you can offer a solution let me know. I searched the forums for hours and could not find this same on again/off again problem described anywhere. Thank you very much for your time."

"Paul.... Although I retired a

"Paul.... Although I retired a few years before that engine was manufactured, your explanation of the problem spells out "Fouled Carburetors", and the mention that the engine runs fine for a time after the carburetors have been gone thru each time would appear to back up that fact.

I would strongly suspect that althought the carburetors are clean, the engine's fuel lines may be deteriorating internally and those bits and pieces of rubber etc are being fed to the carburetors. If so, replace those fuel lines and clean out the engine's fuel system.

You don't say if the engine sits for any great length of time (months) but should it, you would be surprised at just how fast those clean carburetors can gum up the fixed brass high speed jets that are located in the bottom center of the float chambers.

http://stores.ebay.com/Evinrude-Johnson-Outboard-Parts-etc?refid=store"

"The fact that it always bogs

"The fact that it always bogs down to the same speed has me wondering if it's an ignition issue. When it's acting up, put on a heavy rubber glove and pull the spark plugs off one at a time when the engine is acting up. When you find one that doesn't seem to affect the rpm, you found the cylinder that isn't firing. This could be the carb as Joe suggests, or it could be a coil, plug wire, power pack, stator, or whatever.

When you find which cylinder isn't firing, hook the spark plug wire up to a spark tester such as the one Joe Reeves has described many times on this board. Make sure the plug is firing strong. If so, keep going on the fuel system, If not, go after the ignition system.

Dan"

"mr preston, he is right i had

"mr preston, he is right i had a johnson 3 cyc that did the same thing ,I thought the same a carb or fuel problem, but it was a bad coil on one cyl it will drive you nuts it was cracked and jumped spark at times with moisture on engine at mid range, check each coil for hair line defects any were near motor block or replace them all to be sure or check when it locks at 3400 rpm by pulling wires one at a time robert hotfoot5"

"Thanks for the info so far -

"Thanks for the info so far - have relayed it to the mechanic who was still stumped, he's going to check the coils today. Anyone else with ideas please feel free to pitch in!
Paul"

when you say bog down...does i

when you say bog down...does it sound like its running on two instead of three?my experience on that motor is that you can hear it...i have had coils get damp or wet and then dry out and cause problems but it was an obvious miss..if by chance this is happening you can revarnish them and will be ok probably...i vote for gas problems tho.

"i have a 1977 evinrude 70hp t

"i have a 1977 evinrude 70hp three cylinder i have cleaned all three carbs,installed new fuel pump. starts up fine and idles fine but when you go to give it trottle it chokes down like its losing power.could you please help me with this problem? one coil does have a crack in it."