Merc. Prop or Jet problem

[chadbenton]

When I bought my bought from my boss he had problems with if for more than ten years{performance probs.} It had a 4in. hub prop on a 4 3/4in. hub lower unit causing major prop wash. I installed the same pitch prop, but in the cheaper composite brand. Sure cured the wash out and slippage problem. But, motor will load up and die if I try to speed up slowly. Most of the time I can slam it to wot and get my holeshot and be okay, but even then sometimes it will load up. Should I look into a better and heavier aluminum prop or re-jet carbs.

 

[rodbolt]

Re: Merc. Prop or Jet problem

I would look into what is wrong with the engine. you may have a prop problem but its not causing the engine problems you describe.
are you positive its "loading" up?
do you understand what the term means ?

 

[chadbenton]

Re: Merc. Prop or Jet problem

Well, I was an auto mechanic for 6 years and now a diesel mechanic. But, by no means do I know everything. That is why I am here for help. If it is not loading up what else could it be. The engine would run fine with the other{wrong size} prop. It would spool up very quckly, but I feel that was due to the prop slippage and no load. The engine was rejetted before to try to cure the problem with the prop, which nobody was ever able to catch. That is why I am leaning towards fuel problem. Would like to know what jets i should be running at 4000ft.

 

[Laddies]

Re: Merc. Prop or Jet problem

If it ldles alright and runs at top speed alright then you need to ck the carb pickups and back drag jets in the carbs

 

[chadbenton]

Re: Merc. Prop or Jet problem

Top end seems to be fine and even idle seems fine. But, holeshot and low speed seems to be where the problem is. Should I remove the carbs, disasseble and check everything that you stated... Thanks for the help

 

[rodbolt]

Re: Merc. Prop or Jet problem

also check the fuelpump and pump diaphrams, normally the term"loading up" indicates the fuel/air ratio goes excessivly rich. this will lead to hard restarts and fouled plugs. if your carbs are back draft models make sure the passages are clear.
also make sure the ign is working and the correct gear ratio is actually in the lower unit.
outboards are no different than any other articulated rod internal combustion engine, its still suck,squeeze,bang and blow.
one of the biggest problems we had with the V mercs at low speeds was weak fuel pumps not keeping the fuel bowls full at low speeds,this problem was made worse with the square pump.
the symptoms usually were it will idle all day but will die shortly after accelerating. pump the bulb and restart and it goes.
in your merc manual is a suggestion for various carb models at various altitudes.
if this problem has been going on 10 years its time to break the engine into its subsystems and test each one. mechanical,fuel,and ign.
but there were to many carb models used on V mercs for me to quote out of memory.

 

[chadbenton]

Re: Merc. Prop or Jet problem

Great info.... The problem that existed was due to the prop, I believe it in my heart. The fist time I took the boat out it hit almost 4500rpm and was crawling. I asked him about it and he said that is what it always did. It would finally grab water and go, but still felt funny..I still can not believe that the several shops he took it to did not notice the huge gap between the prop hub and the lower unit. I wonder about the lower unit it is a 4 3/4in. and what I have seen msot xr2's had 4in. models. Could this possibly be the problem....

 

[Laddies]

Re: Merc. Prop or Jet problem

Chad, i'm sorry but can't help with jet sizes, I have misplaced my manual on the mid 80s V/6s, you should be able to go to the BAM Marine site and get standard jet sizes though, you will find the high speeds are differant cyl. to cyl. Also a engine that is mounted to deep will cause the problem that you are having

 

[chadbenton]

Re: Merc. Prop or Jet problem

I am not sure if the motor is too deep. Last year my buddy did'nt back the trailer in far enough and I tried to power the boat up the bunks and it would bog down very heavily and die... Almost like it overfuels at a high load or the timing is slow......

 

[rodbolt]

Re: Merc. Prop or Jet problem

overfuel on a 2 stroke is very rare and will leave all kinds of unburned fuel rainbows on the water.
its a motor ,dont make it more complex.
I still dont think your understanding the term loading up verses leaning out.

 

[chadbenton]

Re: Merc. Prop or Jet problem

If it is going lean, would it pop back through the intake and carbs like a 4 stroke would. That is what I am used to, are 2 strokes that different with driveability concerns...Maybe this will help. When trying for a holeshot and it stumbles and dies. It has a hardstart and blows quit a bit of smoke. Idling up will help clear it up and then I can try again....

 

[rodbolt]

Re: Merc. Prop or Jet problem

if it pops back through the carb it means a reed is damaged.
yes 2 strokes are very different than 4 strokes.
if the issue has gone on 10 years I would start from scratch and go over each subsystem and verify correct operation using the factory shop service manual and the correct test equipment.

 

[chadbenton]

Re: Merc. Prop or Jet problem

This problem came about when I changed props, he always complained of a fludder in the engine at cruise, probably due to the wrong hub prop. Today I pulled out both thermostats and the poppet valve. I can not see anything wrong with the valve or diaphram. How does this sucker work. Does the spring relax with heat and allow water flow. Thermostats looked okay, but are different styles. One opens in and the other out, possible problem? Also, I have the newer style fuel pump. Is it better to rebuild or replace it...Can you even test it in any way...Thanks

 

[chadbenton]

Re: Merc. Prop or Jet problem

I just realized that my boat has two fuel filters. The stock plastic filter on the engine itself plus it has an automotive style with a spin on filter, which I think was added on. Could the addition of the large spin on filter cause enough restriction to cause low fuel pressure at idle, thus starving the carbs..........

 

[gss036]

Re: Merc. Prop or Jet problem

Change it and find out, they only cost about $8 and if you don't know when the last change was, it must be due. It is in all probability, a fuel/water seperation type filter.

 

[chadbenton]

Re: Merc. Prop or Jet problem

Yes, it is a fuel/water seperator. I have never had a boat with an external filter on it. How necessary are they. If not, I may just by-pass it and see if it cures any problems...

 

[rodbolt]

Re: Merc. Prop or Jet problem

bypass it and you may create more issues.
water in the fuel is a constant problem with boats.
it should be against the law to rig a boat without a fuel water seperator.
myself I would actually trouble sgoot the engine and fix it. a 20 dollar vacum guage and 10 minutes of time will pinpoint fuel restrictions.

 

[chadbenton]

Re: Merc. Prop or Jet problem

I am going to troubleshoot. But, no matter what I am going to rebuild carbs and fuel pump and replace all fuel lines. They are all old a getting brittle. About the fuel filter, I am just grasping for straws, making sure that any and all potential problems are found and fixxed.. I really appreciate all the input that I have received and I am open to all suggestions in regards to fixing my boat...

 

[rodbolt]

Re: Merc. Prop or Jet problem

if your going to play with this merc you will need a few tools. first is the factory service manual not an aftermarket one.
you will need a compression tester. a spark tester,can be home made in an hour or less and can properly load the ign system for any cranking tests.
you will need a DVM with a DVA adapter or a peak reading multi-meter.
you will need a pressure/vacum gauge to find fuel restrictions,anything under about 1/2"Hg indicates an air leak and more than 6" at wide open throttle indicates a fuel restriction. a good 10 micron filter with a minimum flow rating of 60GPH,90GPH is better, will drop about 1" Hg give or take.
a decent timing light,preferably one with a tach and advance function is very nice to test certain aspects of the various mercury ign components.
after that spend a lot of time reading the service manual until your familiar with the subsystems you will need to work with.