135 Merc Thunderbolt Low idle Low RPM

[Wowgamer]

I bought a 17foot formula with a Merc 1971 135 on the back. It has taken me about a year to get it into operating condition. I replaced the lower unit because of some stripped out gears. Pulled all carbs and rebuilt. I finally got it into the water this past weekend, but it would die whenever I put it into gear. In a 55 gallon trash can it would idle at 800PRM, but it appeared to idle lower in the water 5-600RPM. The idle is smooth, but low. My idle adjustment screw on my dist is all the way in. My carbs are out 1 3/4 turn. When I did get it up and going I could get about 3000RPM at WOT. The throttle linkage appears to be all the way open at WOT. I could get about 30MPH out of it, but I am pretty sure I should be up at 5-5500RPM at WOT. Gas is new and I use stabilizer. Any suggestions would be great.

 

[Wowgamer]

Re: 135 Merc Thunderbolt Low idle Low RPM

So I was able to get a change to work on this today and tried a couple of things.

I squeezed the primer bulb while it was running and saw no effect. The primer bulb was not stiff, but not flat either.

I pulled the plug wires off while idleing at 1000rpm to see any affect on the motor. Almost all of the plugs killed the motor within seconds minus cylinder 2. It would idle regardless. I pulled the plug and there appears to be gas inside. The spark was there and when I ran it without the plug in there was a very light mist coming out. I swapped spark plugged with the cylinder below it and saw no change. I was not able to check for compression, however I will pick up a set to test that.

Upon testing the plugs I saw the gap was off. They ranged from .025 - .040. I am unsure what they are supposed to be.

I ensured that my throttle was fully opening.

I have a 13inch prop with a 19inch pitch which if I am reading the chart correctly should do the trick.

I have manual trim and tilt. Could this be an issue with the pitch I have set?

Anyone know where I could buy a nice manual for this?

 

[Chris1956]

Re: 135 Merc Thunderbolt Low idle Low RPM

Shouldn't that motor be running gapless spark plugs? Examine spark plug wire #2 for arcing to ground.

The low idle sounds like the ign timing is off. You probably need a OEM service manual. Most of the battery powered CDI ignitions with distributors had an idle timing of 4-6* BTDC and max timing of 21* BTDC. I am not completely shure about your motor's specs. You might set the motor to TDC, and remove the distributor pulley cover and see if the cast in arrow is pointing at the punch mark (or dot) on the timing decal of the flywheel.

if the boat is heavy, that 19P prop may be just too much for the motor's power, although if you get 3000RPM, that is wierd, as it shouldn't get over idle, if you are that overpropped.

Look up Clams Canino's "link and sync for merc inlines" post. It is likely in the top secret posts, should help.

 

[Wowgamer]

Re: 135 Merc Thunderbolt Low idle Low RPM

Well first I want to say thanks for the link. Excellent info. I followed all the steps and am now confident that it is timed properly. However it did not solve any of my issues.

I believe I have found the issue, but I am not sure of how to proceed. After checking and tuning with the link and sync I checked and nothing had really changed. I purchased a compression tester and found a bad cylider.

1 - 100psi
2 - 65psi
3 - 100psi
4 - 110psi
5 - 110psi
6 - 120psi

These readings were done when the engine was not recently running (cold) not sure any affect that has.

Obvious problem with #2, however what now? I am not affraid to tear it apart, but would it be a seal or piston rings or block wear? Is it worth me even trying to get it back to full power or just run it? What kind of cost for parts am I looking at? Also I have an identical 115 (Mine is 135) setting idle beside it for a parts motor. Using it for parts is also an option.

 

[Wowgamer]

Re: 135 Merc Thunderbolt Low idle Low RPM

Took the engine off today and scoped things out. One of the pistions and it's rings are toast. The block itself looks fine. Any ideo what piston I am looking for? I have a 1971 135 Mercury thunderbolt ing. I do not know the serial number. I will also need the gasket set. Anyone have a part number on a gasket set? I don't want to buy the wrong stuff. I do have a parts motor that is a 115. Could I take a piston and rings out of it? Are they compatible?

 

[Wowgamer]

Re: 135 Merc Thunderbolt Low idle Low RPM

This weekend I was able to spend some time with the motor. I removed the spent pistons from the 135 and replaced them with the 115. They appeared to be the same. I also took the block from the 115 as it looked in better shape. I retimed it, but it still dies when thrown into gear below 1000 rpm... any ideas?

 

[Chris1956]

Re: 135 Merc Thunderbolt Low idle Low RPM

If the motor stalls when shifted into gear, make the carb mixture settings a bit richer.

So it sounds like you swapped the 115HP motor onto your midsection. good luck.

 

[Wowgamer]

Re: 135 Merc Thunderbolt Low idle Low RPM

Awesome. Worked wonderfully. Yes I used parts from both motors to rebuild the 135. I also noticed a hole in the plug wire that was causing #6 to ground out. Made the necessary adjustments and I now am over 120psi on all 6 and she sounds great. I can't wait to get skiing. Thanks for the help guys.