77 Merc 700 low power

[mckeemm]

Hi Guys,

I have a 70 hp merc that has consistently displayed the following characteristics.

When cold starting it requires lots of choke, then once started will run for a short time and quit.

Repeated starts and tickling the choke button will get it to run but it has low power on the warm up lever for a long time. If you try to put it in gear it will stall immediately.

Then while warming up with the warm up lever at max stop its as if someone has flipped a switch and the engine will shudder and then accelerate gaining about 1500 rpm and begins to run strong.

It will run like a watch for the rest of the day, start effortlessly and run at idle, does not stall when changing gear and does not bog in the hole shot and reaches 5500 rpm max.

I have rebuilt the carbs, adjusted all the "line n sinc" stuff to the book with the exception of the timing because I don't have a light. Because the engine runs so well once past it's warm up woes I believe that the set up is pretty good.

As I write this it appears more likely to me that the fuel pump is the source of my problem. Is it possible that while sitting between weekends the pump allows the fuel to bleed back to the tank and therefore has to bleed out any air over the prolonged warm up period until the system is free of air and the pump works at full capacity?

I have tried to identify all the data so please let me know if I'm on the right track by rebuilding the fuel pump.

Thanks, Mark

 

[Laddies]

Re: 77 Merc 700 low power

Your engine is lean, without a serial number it's hard to trouble shoot as they had 3 or 4, 70HP engines with differant carbs. If it is the fuel pump you can make it run properly by pumping the bulb, it sounds more like carbs.

 

[mckeemm]

Re: 77 Merc 700 low power

Your engine is lean, without a serial number it's hard to trouble shoot as they had 3 or 4, 70HP engines with differant carbs. If it is the fuel pump you can make it run properly by pumping the bulb, it sounds more like carbs.

Carbs are rebuilt, tight and function properly once the engine warms up. Squeezing the bulb while the engine is running had no effect so fuel system seems ok.

I did a little check again last night after refreshing my memory and doing some reading here.

Engine started and ran at idle. I did a dead cylinder check by pulling the plug wires off one at a time. Sure enough #1 had no effect, the cylinder was not firing.
I swapped the trigger and plug wires from #1 and #2 coils expecting the dead cylinder to move to #2. This would tell me if I had a bad coil.

Huh!! now #3 is not firing.

I looked at what I had done and everything checked out ok except for the fact that the plug wires were run tight to each other and one wire was riding hard on the switch box mount and possibly providing ground path for the old wires to short through the insulation.

I returned the trigger wires to their original locations this time rerouting the plug wires clear of all possible grounds but still close together. Now the engine starts and runs on all three.

Looks like I will be shopping for a set of wires.

This scenario still does not explain why when cold the engine runs on two cylinders and then when warm will run on all three but it does take the fuel system out of the equation.

Mark

 

[Laddies]

Re: 77 Merc 700 low power

Running cold on 2 and picking up the other one after being run awhile is typical of a switchbox getting ready to leave

 

[mckeemm]

Re: 77 Merc 700 low power

Running cold on 2 and picking up the other one after being run awhile is typical of a switchbox getting ready to leave

I was afraid of that. The switchbox is probably the single most expensive part on this motor next to the stator. Any idea where a fella can get a deal on one?

 

[mckeemm]

Re: 77 Merc 700 low power

OK Guys,

I have a Canadian Manufacture 1977 Mercury 700 3 cylinder 70 hp s/n 7101012. The switch box p/n on the old unit still installed is 332-7778A1.

From what I can see online it appears that Sierra p/n 18-5775 is the replacement switch box for this series of 3 cyl engines replaceing p/n 332-7778A6, 332-7778A9, 332-7778A12. However my original p/n is not listed.

Does the A6, A9, A12 refer to the number of poles on the alternator? Which being in multiples of three be appropriate for a 3 cylinder engine? If so I have a 12 pole stator on my engine.

I found a new Sierra 18-5775 unit on ebay for $94.00. That seems like a good price if it will work on my engine.

Comments?

 

[mckeemm]

Re: 77 Merc 700 low power

I confirmed after checking several souces that I have identified the correct swithbox. I ordered it on Friday and will post the results after installation.

 

[mckeemm]

Re: 77 Merc 700 low power

Hi Guys ,

Seems like I'm talking to myself here. I'm also dealing with this problem on another string so it's understandable. I'll end this string by posting my conclusion to the problem in the event that it may help someone else out there.

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