1970 Merc 7.5hp Only Starts with Full Throttle

[LMBoater]

Motor: I have a 1970 Merc 75 (7.5hp) 2-stroke. Thunderbolt ignition.

Background: The motor has been in storage for approx. 2 years but was starting fine prior to storage. It has NOT idled well for quite a while. Just put in new spark plugs, have great spark. Fuel is also fresh, no issues with tank or line/bulb. Historically, this motor has been well maintained and winterized.

Issue: The motor will only start at 3/4 or higher throttle, which requires starting in FORWARD (in NEUTRAL, you can only adjust up about 1/4 throttle), and once started, stalls as soon as you reduce throttle below about 1/4. (I am aware you can remove a bolt to start in NEUTRAL at higher throttle, but that won't solve the underlying problem here).

I'm looking for help on where to start. I assume most will advise removing and cleaning the carb first, which I can do. I'm not familiar with this ignition and am hoping that's not in the mix at this point.

 

[Tassie 1]

G' day,
welcome,
clean the carb.
lol

I had that motor many years ago,
A trifle slow on a 12ft tinny with 2 kids and myself,

 

[tpenfield]

:welcome: to iBoats . . .

I just went through this on my 1969 Mercury 7.5 HP

https://forums.iboats.com/forum/engi...t-dis-assembly

I had to start the engine in gear to get it to stay running.. Reverse is best, assuming you are running the engine in a barrel. It had all the symptoms of a gummed up carburetor . . .

What I ended up doing was to mix 2/3 of the 2-cycle fuel and 1/3 Seafoam and mix up about a gallon of it. then run the gallon through the engine (in gear). It took a couple of hours, but it cleaned out the carburetor pretty well.

Then I took out the idle mixture screw/needle valve (on the front of the carb, just above the choke plate - black plastic thingy) and used a straw to pour some acetone into the idle mixture barrel. Also cleaned the needle itself and put it back in. Set it to about 1-2 turns out and then the engine would start and run at an idle setting. From there I was able to make a final adjustment to the idle mixture and idle setting.

 

[LMBoater]

Ted,

Great to hear from someone with the same motor...I might call on your experience again down the road. I read your earlier post as well, thanks.

Had it out on the water today...you’re right, starts at about 3/4 throttle in FORWARD, as well as at about 1/2 throttle in REVERSE. Nothing in NEUTRAL.

I ran some Sea Foam through it today and had it running for about 30 minutes, already seems to be running a little better but, like you said, I need to dedicate a couple of hours to this. I plan to put more Sea Foam through at a much higher concentration in the next couple of days.