89 70hp 3cyl Bogging Still

[Grant Brown]

Title says it all, the boat bogs and won't get up to top speed. Runs at about 25 mph and gets there very slowly. RPM's taking a long time to build up to speed. After running like that for awhile, I'll shut it off and fish or whatever then fire it back up and it will run great for the rest of the day. When running good, it has a hell of a hole shot and gets up to top speed of about 34mph in a just few seconds.

I've had the carbs cleaned and rebuilt, the stator and rectifier replaced (they fried last year), the coils were checked too.

Could there be problem with the linkages?

I am having a problem with the battery holding a charge and I was wondering if that might make affect WOT spark performance.

Any ideas or input would be appreciated.

Thanks,
Grant

 

[mckeemm]

Re: 89 70hp 3cyl Bogging Still

Grant,

My 70 was acting very similar when I was able to get away from the dock with it. My engine was dropping a cylinder when cold and would pick it up once warmmed up and work fine. See my string "77 merc 700 low power" to see what I have done so far.

Mark

 

[Grant Brown]

Re: 89 70hp 3cyl Bogging Still

Is the 77 the same 3 cyl style as the 89 I have?
Mine has no real problem when idling out. It's only when I hit it that the problem becomes obvious.
One time the power kicked in when while it was running at about 24mph. Buddy was standing up and almost went over it hit so hard.

Lost here...

 

[Silvertip]

Re: 89 70hp 3cyl Bogging Still

Have you pulled the plugs and checked them? Instead of hammering the throttle, gradually bring up to full throttle. If it performs ok then, you probably need to adjust the idle mixture screws. They affect the transition from low to high speed. If slowly advancing the throttle doesn't help, then I'd be looking at a dropped cylinder (no spark/weak spark) situation. The plug should tell the story. If two of the three are the same, the different one is probably the bad one.

 

[mckeemm]

Re: 89 70hp 3cyl Bogging Still

Grant,

I played with my 70 last night and discovered that when first started the engine was only running on cylinder #3. I was pulling the plug wires off while the engine was running to determine which ones were firing. Top cylinder #1 and middle #2 were not getting spark, than after a few minutes it picked up #2. Top #1 still had no spark and did not come on line even after an extended warm up. I swapped the trigger wires and plug leads on the #1 & #2coils expecting the dead cylinder to move to #2 but when I fired it up it ran on all three with good hot spark.

I figure the coils are working fine if they are propperly triggered. There are some checks you can do with a meter on the trigger assembly under the flywheel. The mystery is in the swithch box. There doesn't seem to be much you can do with it but swap it out and see if it cures the problem. If trouble shooting the rest of the system fails to cure the problem then it must be the swith box at fault.

Others here have said that this kind of thing is a sign that the swith box is on it's way out. I'm ordering one today.

Mark

 

[Grant Brown]

Re: 89 70hp 3cyl Bogging Still

Mark,

Where are you ordering the switch box from and what's it worth? I notice you're in Canada too and it's always nice to find suppliers that aren't afraid to ship across the line.

Grant

 

[Grant Brown]

Re: 89 70hp 3cyl Bogging Still

Had the boat out again yesterday. Same bogging thing again, this time with an intermittant alarm. After I dropped the crab traps I fired it up and it ran rough for a minute while tons of oil smoke poured out of the motor. After that, it ran awesome all day. This engine is oil injected so I figure one cylinder wasn't firing then got clogged with oil. When that cylinder finally caught it blew out all the oil then ran fine.
The coils tested fine so how can I test the switchbox?

Thanks,
Grant in Vancouver

 

[mckeemm]

Re: 89 70hp 3cyl Bogging Still

Grant,

I ordered a sierra p/n 18-5775 switchbox from a vender on ebay for $94+$23shipping. That was the best price I could find on a new one. There are used ones on ebay starting at $10 but I preferred a new unit. Best price in Canada I found was $140 plus taxes and shipping. I was quoted $218 from the local marine supplier for these same boxes and he had to order it out of the states. I should have it by friday and I will post my results. I also did all the resistance checks to to eliminate the trigger, stator, rectifier and coils as potential problems which left only the switch box. The manual specifies a piece of test equipment that you do a drop voltage test accross the switch box terminals to determine if the box is good. In my case I swapped switch box to coil and plug leads between the running and dead cylinders to prove that if the coil got a good trigger signal then if fired just fine

 

[Grant Brown]

Re: 89 70hp 3cyl Bogging Still

Hey Mark,

I'm looking forward to hearing how that unit works out for you.
What's the eBay item number?

Grant

 

[mckeemm]

Re: 89 70hp 3cyl Bogging Still

Grant,

I received the switch box Tuesday afternoon in the mail, 3 days sooner than expected. I installed the unit and sure enough the engine starts strong and runs well as would be expected when actually firing on all cylinders. I hope to get out for a water test in the next few days and I will post the results.

ebay item number 27013853152.

link

http://cgi.ebay.ca/ws/eBayISAPI.dll...EAFB:IT&viewitem=&item=270138653152&rd=1&rd=1


Bob (rob212) was excellent to deal with and made the effort to ship same day. He even upgraded the shipping to priority, which meant I got the part 4 days including the weekend.

Mark

 

[Grant Brown]

Re: 89 70hp 3cyl Bogging Still

I'm all set to buy the switchbox but the trigger resistance still hasn't been checked. What are acceptable values?

My mechanic says to replace the coil on the bad cylinder first as it's only crapping out under load and that bench testing it without load might not show any problems. For $20 or so bucks I might try it.

Grant