New Memeber with a 1978 Merc 900 - Starting/Running Probs.

[JohnnyBfromPeoria]

Thank you for this forum, I've tried to search for some answers but nothing I've found addresses quite what I'm looking for. Forgive me if it's covered somewhere I didn't see.

I've only had my boat, a 1978 15.5' Crestliner open bow and the motor, a 1978 Mercury 900 90 hp, s/n 4849085 for a month or so. It ran on muffs when I bought it, and it ran for a while when I took it to a nearby AZ lake. It was hard to start (operator error, perhaps) but got me away from the ramp and on plane, but eventually, maybe after a half hour, while at about 4K rpm, it bogged down and died. It was very hard to restart and seemed to require using the choke a lot. After it finally started, it ran perfectly again for about another 15 minutes and then died and I was unable to restart it at all. A PWC towed us back to the ramp, which is not very sexy.

At home, I discovered a shorted wire going to the square box in front of the carbs. I fixed the wire and added shrink wrap for extra insulation. I then checked for spark on cylinder #1 and found it to be acceptable. The motor will start, back on muffs, but not easily, and it won't stay running.

I replaced the in-line fuel filter. I had replaced the spark plugs with the proper Champion plugs before the lake trip and after I sprayed the engine with Deep Creep (Seafoam) in the carb throats. The bottom carb will leak fuel when I prime the bulb, so I at least have a stuck/sunk float or similar issue there.

Questions:

1) Should I start with a fuel pump overhaul kit? I have one.

2) How the heck can I get the carbs off? The mounting nuts are practically buried in the castings and there's no real straight shot at them with tools I have, from my cursory examination (and I repair appliances for a living; repair Mitsubishi SUV's for "fun"). I at least want to clean the carbs out, at most install rebuild bits and pieces. I have the kits.

3) Should I block the "economizer" tubes that cross over the carbs? I've read on here some do that.

4) Did I read correctly, that the late-70's Tower of Power 6-cylinder engines run on four cylinders until a certain point in the RPM scale when they light off the 5 & 6? Would that explain why only two of the three carburetors have choke plates?

Sorry for all the questions. I'm making a lot of assumptions here: I have not run a compression check. I have not checked spark on cylinders 2-6. I have not touched or made any adjustments to the carbs, timing or advance. My fuel appears clear with no visible water and is mixed @ 50:1.

I appreciate your reading this and I hope I can help someone on here out one day.

John B.

 

[jimmbo]

My first merc was a 78 90hp, also on a 15 1/2 ft Crestliner Cruisader. 20 yrs after I traded it, it is still my favorite.

A 78 would not have an inline fuel filter, it had a screen on the fuel pump inlet. An inline filter, if up stream of the fuel pump, can cause fuel delivery problems
Try tapping the float bowl of the leaky carb with a plastic screwdriver handle. My 115 used to suffer from floats what would somehow wedge themselves in the bowl

That square box on the front cowl support is the switchbox.

1) It won't hurt, the diaphragm may have cracked or a pinhole, but it might not be the problem. There are lots of other failure points in the fuel system.

2) you will need to remove the front cowl support, the starter, the choke linkages, the fuel lines. It will be easier if you slide all 3 carbs off together as they are connected to each other on the throttle shaft. After reinstalation, a Link and Sync, will need to be done

3) It is a good idea to block the 'back drag' hoses, as today oxygenated gasolines are chemically too lean for these engines

4) the sixes always ran on 6 cylinders, unless there was a problem. You are getting confused with the later 2 + 2 4 cylinder l loopers


5) Do a compression test, check the spark to see if it will jump a 3/8" gap

 

[racerone]

Replace the water pump impeller too.-----Remove the starter.----Then remove all 3 carburetors as a set.----Also check the bleed restrictors found inside the bypass covers.-----Usually 2 or 3 of the 5 are missing.

 

[JohnnyBfromPeoria]

Thank you jimmbo and racerone. I put the fuel pump rebuild kit on. It looked like the diaphragm was indeed torn. Unfortunately, I managed to drop one of the five bolts for the top of the pump down into the case where I can't get to it, grrr. It still won't start.

I rigged up a one gallon gas can with 50:1 and added a can of SeaFoam. If I can get this things to run, I want to try running some through to see if it clears anything up before pulling the carbs. I'd also pull the plugs and spray the cylinders with DeepCreep/SeaFoam spray.

Plugs 5 & 6 are really clean, as though they're firing but not getting fuel - at all. Something's really gummed up in the bottom carb.

Again, I appreciate the responses. Good thing I have months of great boating weather left this year because, well, it's Phoenix.

John B.

 

[JohnnyBfromPeoria]

Yay, I've been able to get it started and restarted. I've had it drawing off my "special sauce" fuel can with the Seafoam in it. It smokes like crazy, as does every engine I've used the product on. It seems to be running quite a bit better, dare I say "normal," at least when not under load. I'll have to haul it down to the lake and leave it on the trailer since I can't find anything big enough to put the lower unit into with water in it. Yes, I've been running it on muffs, I know, I know.

As soon as I've run through all or most all of my special fuel, I'll hook it back up to the tank and make sure it still runs and get it under load to see what happens. Hopefully it will be good enough. I don't think those carb kits and floats will mind sitting on the shelf for a while.

I wasn't really anticipating Seafoam would be able to do this. Maybe it's the repeated soakings the carbs have had with the starting, running a bit then sitting. WalMart has the best price on the stuff around here, just in case you want to mess with it. In a car, you get the BEST James Bond smokescreen out of it after letting it sit in the engine for a hour or so before restarting it...

John B.