1976 Merc 3 cyl 70 hp wont run WOT

[Crow6603]

I bought a 16' 1970s Starcraft with this motor on it about 1 year ago. Aside from a new impeller it ran pretty good for the price I paid and the age. It did seem to be cold blooded when starting and bogged frequently when trying to go WTO though. Once it was running WTO it ran great. This year I decided to adjust the low idle screws using postings I found online to address idle and bogging and... Now I can't get any adjustments to do much and putting the throttle down will only run at half speed (If it will stay running!). All of the shops are busy till spring here in MT and I was hoping for some fall fishing. Any tips on where to start/what to look for? I'm decent when it comes to wrenching on things and willing to give a try with decent tips. Oh, and I did check compression and it seems to be too wide ranging at 100, 130 & 140. Thanks!

 

[Georgesalmon]

Sounds like its possible you dislodged some debris when you started turning screws. My suggestion would be to clean and rebuild the carbs. Kits can be had right here and they don't carry a huge price tag. Some carb cleaner and a few small tools are all you need.

 

[Georgesalmon]

double post

 

[Crow6603]

Thanks Georgesalmon. That was going to be my next step. I'll give that a try.

 

[Texasmark]

1 1/2 turns out is where to start on the low speed screw. Only close finger tight so that you don't score the tip of the screw by closing too hard. The low speed will be the one to get you going and if you hit the throttle trying to get on plane and it boggs, open the jets more a little at a time till it will take the throttle. The high speed jets are fixed and should be in the bottom of the carb bowl. If you dislodged some crud, it may have would up in the orfice of the high speed jet and that is the source of your problem. In the rebuild process, after you have chemically cleaned things, be sure you blow out everything with compressed air. I had a problem in a high speed jet that cleaning didn't catch....couldn't see it.... but when I brought in the air to finish out the job lo and behold this little piece of crud blew out from one of my high speed jets and the problem was solved.

HTH,
Mark

 

[Crow6603]

Thanks for the pointers Texasmark. I'm hoping to dive into things this week. One thing I thought of though... I've seen postings about linking and synching carbs after a carb rebuild. Anything I need to do/pay attention to in that regard when I get started?

 

[Texasmark]

I had the 90 and it's setup is probably the same as yours. There are two things to get right. One is all the butterflies opening the same amount as you operate the vertical "link" rod and the other is getting your ignition timing "in sync" with the butterflies. You really need a manual as there are (on mine) marks that you have to line up and you have to check/setup the WOT timing.

Mark

 

[Texasmark]

I had the 90 and it's setup is probably the same as yours. There are two things to get right. One is all the butterflies opening the same amount as you operate the vertical "link" rod and the other is getting your ignition timing "in sync" with the butterflies. You really need a manual as there are (on mine) marks that you have to line up and you have to check/setup the WOT timing. Suscribe to the Seloc, upper right of the page where you can get directions on how and what to do with pictures.

Mark

 

[Crow6603]

Thanks. Sounds like a manual and carb kits will be in order!

 

[racerone]

With those compression numbers I would pull the transfer cover and inspect pistons and rings.----Carburetor cleaning does not help a motor with scored cylinders !

 

[enginepower]

Racerone is right. You don't want your lowest cylinder any lower than 10% of your highest cylinder. You can build those carbs and getting running somewhat ok but the engine probably isn't going to last you very long. What you will probably mostly notice is a rough idle and lack of it's full capable power. If that thing has been sitting a while, run it a few minutes then retest. If compression numbers are still same, it may run good but not great.

 

[Texasmark]

Racerone is right. You don't want your lowest cylinder any lower than 10% of your highest cylinder. You can build those carbs and getting running somewhat ok but the engine probably isn't going to last you very long. What you will probably mostly notice is a rough idle and lack of it's full capable power. If that thing has been sitting a while, run it a few minutes then retest. If compression numbers are still same, it may run good but not great.

Geez, I overlooked the compression disparity. Shame on me.

Mark

 

[Crow6603]

Well, I got the carb kits and cleaned the carbs. There was a lot of junk when I removed the low idle screw/needle on the lower carb. I need to get everything put back together and may get to some water this weekend to run it. I was a bit worried about the compression and after seeing some of the latest posts it doesn't look good... I'll double check the compression for good measure, but for the price I paid for the boat, I might be able to live with it running good and not great until it retires.

 

[Crow6603]

Well, got the boat out to the lake and gas started coming out of the bottom carb overflow, when priming. Guess I should have checked that before heading to the lake. . Still used the kicker motor to go a short distance and hook a few fish, so not all was lost. Now to check the float, needle and seat before tweaking again. As long as I figure in a "slowly but surely" attitude combined with Murphy's Law I guess I'll be in good shape!