87 Evinrude 70HP Dropping Surging RPM's

[revsigns]

Ok, need some help or at least some ideas as I am about out of troubleshooting know how.

Motor: 1987 Evinrude 70hp VRO (vro removed a long time ago)

Problem: my motor will run great at 5000 rpms for about 4-5 miles. Usually around that distance/time the motor will seem to bog and it will drop to 4000 rpms. Sometimes it will surge between 4 and 5000 rpms. It will stay on plane and I run all the way to the first spot.. Sometimes, however; after the first wade or drift it wont even make it on plane...sometimes it fires up and runs full rpms. If it bogs automatically at the start i will just shut it off wait a few mins and go again. Sometimes that does the trick.

Interesting find: if i repeatedly push the key in to prime the carb i can keep it running at higher rpms.

On the water check: Yesterday while on the water it started bogging when I would give it full throttle. We removed the air intake plate and accessed the carbs. My buddy unscrewed the drain plugs because it sounded like the carbs were flooded. When he removed the top carb's plug, fuel drained out considerably. When he removed the middle drain plug hardly any fuel dripped out. Could it be I'm not getting fuel to the middle carb? What would cause that?

What Ive done:

New fuel pump
Cleaned the carbs
Cleared the fuel tank intake
Replaced fuel line and connectors



Any other ideas? I would appreciate the help. Thanks

 

[fishfeatures]

Re: 87 Evinrude 70HP Dropping Surging RPM's

if it goes better when you push in the primer and it surges too then you show classic signs of a fuel issue... you say you've replaced the pump,line etc... did you replace any filters? are you sure the carbs where fully cleaned , were the jets removed and passages behind the carb core plugs done ? it real easy to miss these spots when cleaning carbs.

 

[revsigns]

Re: 87 Evinrude 70HP Dropping Surging RPM's

I replaced the in-line fuel filter a couple of months back. We used an air compressor and copper wire to flush and clean out the carbs. I am sure its possible we missed something so I may have to go back over them again. I am planning on ordering the rebuild kits to get that done as well.

 

[revsigns]

Re: 87 Evinrude 70HP Dropping Surging RPM's

Talked with my buddy who helped me clean the carbs. He said we didnt mess with the core plugs cause i didnt have the rebuild kit

 

[fishfeatures]

Re: 87 Evinrude 70HP Dropping Surging RPM's

if the carbs have been done in a while and your getting this much trouble, i'd get the rebuilt kits and give them a proper going over.

 

[revsigns]

Re: 87 Evinrude 70HP Dropping Surging RPM's

rebuild kits will be ordered today.

Question: what is the difference between the Sierra Rebuild Kit with Float and the Mallory Sierra Kit with Float?

Will let you know how it goes afterwards.

 

[revsigns]

Re: 87 Evinrude 70HP Dropping Surging RPM's

I'm guessing its just a difference in brands? Is either one better?

 

[eclark53520]

Re: 87 Evinrude 70HP Dropping Surging RPM's

Either kit will work.

Make sure you remove all the core plugs, clean thoroughly, and replace as directed. Ensure all passages are open with wire before closing it up.

You're issues definitely sound like a fuel problem, Don't run your boat until you figure it out. Running lean means running without proper lubrication.

 

[revsigns]

Re: 87 Evinrude 70HP Dropping Surging RPM's

I agree with you. Its just frustrating because the motor runs great in the driveway. Its when its under load that it acts up. Makes it tough because what i get at home isnt neccesarily what i get on the water.

Hoping the rebuild will solve my issue. Thank you for all of your helpful suggestions. I really appreciate it.