78 115 mercury outboard won't idle, could it be an elevation problem?

[jbigtrout]

I bought the boat in Pheonix (1,100 ft)and brought to Albuquerque(5,000ft).
Do I need to adjust the carbs? Or maybe adjust the idle?
Thanks for any advice.

 

[Chris1956]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Yes, an adjustment to the carb idle mixture can help at altitude. Merc recommends main and backdraft jet changes for altitude as well. See how it runs and figure out if you want to change jets. I should have recommended jet size in my service manual.

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Thanks for the info. I really don't know much about boat motors, I am new to this. Do you know if the 115 is a decent motor?

 

[Chris1956]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

It is a very good motor design. The question is what is the compression on your cylinders, and is the motor wiring harness good? Most of the original harnesses are bad by now. The distributor and other ign components are pretty expensive, so don't berak 'em or allow a bad harness to short 'em out. The distributor rotor is non-replaceable, and fragile.

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Good info, especially about being careful with the components. I will take a closer look at the harness tonight. I don't have a compression gage. Maybe I can borrow one. Thanks!

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Okay, I talked to the previous owner. He said he had the engine rebuilt along with the carbs last year. He said it is jetted for around 1000 ft elevation. I am at 5000. I took it to the closest lake today and adjusted the idle adjustment screw. It seemed to work but I was not able to really run it because it is a no wake lake. I am going to try and take it out again soon. How hard is it to change out the jets? Should I just have a real Mechanic do it?

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

It is a very good motor design. The question is what is the compression on your cylinders, and is the motor wiring harness good? Most of the original harnesses are bad by now. The distributor and other ign components are pretty expensive, so don't berak 'em or allow a bad harness to short 'em out. The distributor rotor is non-replaceable, and fragile.

Okay I checked the compression and it was around 52 psi for all cylinders. Seems low but what do I know. The previous owner said that it was a rebuilt powerhead and rebuilt carbs with about 8 hours on the motor. Maybe it still needs to be broke in. The motor, carbs and wiring all look real clean. The local shop also said I needed to drop prop pitch at least two sizes when coming from low elevation so I am dropping from a 19 to a 15. I am going to try and run it this weekend. We'll see.

Thanks again for your help.

 

[Chris1956]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

52 PSI is not good. The motor would probably not run with that compression. If the PSI is consistant across the cylinders, it is likely the gauge is bad.

Watch the max RPM with that prop. You do not want to run them too fast (>6000RPM) for too long.

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Okay, I took her out over the weekend and still having issues although she ran a lot better. Bogging down when it hit it but once the rpms get over about 1500 she really takes off and at higher rpms has tons of power. Also still not idling properly and now it will not shift into reverse.

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Okay, I had the motor looked at and the issue is that is is sucking air into the crankcase. The mechanic said that it needs a rebuild to fix it. I cannot afford a rebuild so I am stuck with a hole in the lake. oh well. Thanks for your help Chris.

 

[Chris1956]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

That is not much of a diagnosis. Actually sounds like the mechanic has no idea what is wrong, or he is a crook.

If that motor has good high speed performance, it likely does not need a rebuild. It does likely need a carb idle mixture adjustment.

Let's assume you set the carbs for smooth running when you adjusted them. That usually means they are set too lean for acceleration. Go to a lake that allows high speed operation. Open the top carb 1/8 turn (CW), and try to accelerate. Now open mid carb 1/8 turn and retest acceleration. Now open bottom carb 1/8 turn and try to accelerate. Continue richening and testing until you get smooth acceleration and reaonable idle.

 

[Gomer50]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

Take the advise given here from Chris.These inline 6's are good motors when dialed in correct.If it was rebuilt 52 psi is probably a malfunction on your tester.Most marine shops dont even want to mess with these motors anymore (parts are hard to come by and they are not trained for these old ones anymore).The book say's to adust all three carbs to 1 1/2 turns out,but on my 90hp inline 6 to achieve great acceleration all three of my carbs are real close to 2 complete turns out.On a 115hp motor a 15 pitch prop seems a bit low (of course it depends on what kind of boat its on).If all the wiring and componets are in good shape,whatever you do dont give up.Get yourself a manual (Mercury Manual).One thing for sure I never believe a previous owner when buying a used boat they all ran great last time out,fresh rebuild,etc.etc. I not saying all are BSers.
BOAT=bust out another thousand.Good luck and stay in touch here,you'll find a huge amount knowledge and gets you on the water with a smile.

P.S. Do your adjustments on the water (on the MUFFS gets you bogus results).

 

[j_martin]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

I third that.

If you have the wrong main jets in it, (and you would by about .004" too rich) and adjust the idle for smooth idle, you can expect a perfectly normal engine to bog like heck at mid range. The problem is that of balanced adjustments.

The 52 lb compression is BS. That fact that it is dead even across all cylinders is a good sign. For all cylinders to be evenly low and that to indicate a crankcase leak is pure BS. Run away from that "mechanic" at full throttle.

You need to install the right carb jets. Be sure you use new base gaskets on them if you have to pull them off. You should go through the linkage and timing adjustment also just to be sure it's on the money before you adjust things.

Put it on the water, adjust your idle, get the right prop on it, and go like a bat out of he@@.

 

[jbigtrout]

Re: 78 115 mercury outboard won't idle, could it be an elevation problem?

The compression was retested and it is about 100 psi per cylinder. (I used a bad tester previously) I took it out to the lake (Elephant Butte) last Sunday. It is running a lot better now than it was before the mechanic worked on it. It fires right up, it is idling and shifting good now and at high speeds it is running great. He replaced the jets and I cannot imagine that he did not adjust the idle mixture. He has a test pond that he uses to run the boats he works on and he specializes on old outboards. We may take it out this weekend and I'll give it a shot. I like the mechanic and would hate to think he is dogging me but I guess it is possible.

Thanks again.