1978 Mercury 70hp 700 series thunderbolt

[browell05]

I'm having proplems getting my 70 HP 1978 mercuryon a 92 model 15' stratos to plane off. The motor sounds as if its running on all senlindors but doesn't seem to have full power on the takeoff. Any suggestions would be greatly appreciated.

 

[bktheking]

Re: 1978 Mercury 70hp 700 series thunderbolt

Have you verified it's got spark on all cylinders, sounds are deceiving!

 

[browell05]

Re: 1978 Mercury 70hp 700 series thunderbolt

Yes it has spark on all 3 cylnders

 

[bktheking]

Re: 1978 Mercury 70hp 700 series thunderbolt

Ok so- check compression, has the hull taken on water, are the carbs clean, what condition are the spark plugs- any water washed.

 

[browell05]

Re: 1978 Mercury 70hp 700 series thunderbolt

Plugs are new and the hull has taken on no water. I dropped the boat off at my local marine shop yesterday to check compression. I'll post the updated info as soon as they call with info on compression.

 

[ajgraz]

Re: 1978 Mercury 70hp 700 series thunderbolt

Welcome to the 70's-80's 3 cylinder Merc club. We like pain.

As mentioned, clean carbs are crucial.

Ask the shop to test coils, trigger and stator, and to look over all the wiring under the cowling. These motors are notorious for corroded wiring.

Switchbox could be bad too, but can't be bench tested as far as I am aware.

 

[PHE]

Re: 1978 Mercury 70hp 700 series thunderbolt

I have a 78 70HP. In order to get it to have good throttle response out of the hole, I have to have the idle screws backed out to the richest setting that will idle properly. I believe the manual says to start at 1 1/2 turns out. At that setting mine idles great, but will not accelerate and get on plane. It simply dies out and you have to go back to trolling. I believe that I'm closer to 1 3/4 turns out. It idles rougher, but still idles at 800 RPM in neutral and will jump right up from trolling to on plane with no hestitation when the throttle is hammered. Its best to play with that on the water. Without dropping the shroud.

This assumes that the carbs are clean, the floats are set right and the link and sync has been done properly. A plugged main jet in one of the two carbs will also cause this problem. Since it has two carbs for three cylinders, one will be running great, the middle will run OK, and the third will not get any fuel. It runs OK off plane but will not accelerate to plane speed. The main jets are inside the drain plug on the carb.

An added fuel filter, even a cheap lawnmower filter between the semi-worthless filter screen and the carbs will keep junk out.

Hope this helps. Its a good motor when running right. I believe it is the lightest 70HP ever made.

 

[browell05]

Re: 1978 Mercury 70hp 700 series thunderbolt

Hey guys. I sure do appreciate your posts to this thread...My local marine shop says " the motor has good compression and is firing on all three cylinders." , but there was a few things mentioned in the last two post that i feel certain was not checked out by the techs. I feel if i checked them out on my own and they were found to be in bad shap and fixed then the motor may run as if brand new. "That is if the un-mentioned items are the reason why the engine is running foul."..I'll check these items out and reply back soon....Thanks for the help guys...! , Browelll05