74 inline 6 carb problems

[Isaacm1986]

I have a 74 115hp merc. I just got. The motor has been sitting for about a year. Got it home and she fired right up, but the throttle responce is very slow and when it is under load it has no power.

The top two cylinders also run very hot.

Any thoughts for this?

 

[mr 88]

Re: 74 inline 6 carb problems

The water impeller is probably shot as it sounds like no water is being fed up top. First do a compression test to see what you have and that you did not toast it,then report back if good and water pump is taken care of for your carb issues.

 

[Bob_VT]

Re: 74 inline 6 carb problems

Remember those top two plugs are fed by the top carb. Slow response and 2 hot cylinders indicate a carb problem.

Service the carbs as a set and then make sure you do this procedure http://forums.iboats.com/showthread.php?t=168855

 

[Isaacm1986]

Re: 74 inline 6 carb problems

Remember those top two plugs are fed by the top carb. Slow response and 2 hot cylinders indicate a carb problem.

Service the carbs as a set and then make sure you do this procedure http://forums.iboats.com/showthread.php?t=168855

Thanks Bob, I was thinking a lean running carb as well. That walkthrough is very helpful

Can anyone tell me what the normal operating temp is for these motors? I have a repair manual on order, but it wont be here for a week or so.

thanks

 

[wired247]

Re: 74 inline 6 carb problems

They have no thermostat so it really depends on what your water temp is.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

They have no thermostat so it really depends on what your water temp is.

OK. So even if the motor is running at 140 that's probably too hot?

 

[wired247]

Re: 74 inline 6 carb problems

140? I can barely get mine lukewarm at WOT

 

[Isaacm1986]

Re: 74 inline 6 carb problems

140? I can barely get mine lukewarm at WOT

OK then. Looks like I'll be ordering a water pump as well.

 

[mr 88]

Re: 74 inline 6 carb problems

bingo,, after compression test.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

bingo,, after compression test.

I have done a compression test. All cylinders are 115-120psi. So I thought that was pretty good.

Couple more questions. can i get a new ignition switch anywhere for these older merc control box?

Any tricks i need to know about pulling down the lower unit to replace the water pump?

thanks guys.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

Got the lower unit off and the water impellar was toast. glad i decided to go ahead and change that.

My biggest question is, I installed new floats in the carbs and now the engine does not idle at all? I have to hold the choke to keep it running? any suggestions?

thanks

 

[Isaacm1986]

Re: 74 inline 6 carb problems

Got the lower unit off and the water impellar was toast. glad i decided to go ahead and change that.

My biggest question is, I installed new floats in the carbs and now the engine does not idle at all? I have to hold the choke to keep it running? any suggestions?

thanks

Anyone have any thoughts?

 

[mr 88]

Re: 74 inline 6 carb problems

My first thought is to adjust the idle screws.Screw them in until they GENTLY seat.Then turn each one out 1 1/2 turns and use that as a your base.Do a search here on carb adjustment. Has the fuel pump diaphram been changed? You may want to invest in a manual as they go step by step with pictures for everything on these old gals.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

My first thought is to adjust the idle screws.Screw them in until they GENTLY seat.Then turn each one out 1 1/2 turns and use that as a your base.Do a search here on carb adjustment. Has the fuel pump diaphram been changed? You may want to invest in a manual as they go step by step with pictures for everything on these old gals.

Thanks I'll try that tonight. I have a manual on order. Will be here Monday. Until then all I have is I boats.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

Got the new water pump in, and adjusted the carb air/mix screws and she is running good as new, most of the time.

I now seem to have a intermittent spark issue. she will run fine for a while, then just cut out. ill crank it over and it will just keep cranking until i let off the key as soon as i let off the key it will start. seems to me maybe the ignition switch is going bad? any other possibilitys?

Also after running it on the hose for about 10 min, i turned her off and this started coming out of the prop with the exhaust? anyone know what this is?

 

[mr 88]

Re: 74 inline 6 carb problems

The goo is exhaust residue,which should be oily as well,that's why they are slowly banning two strokes.Have you rebuilt the fuel pumps ? Sometimes the primer bulb one way check valve goes bad and fuel flow gets reversed sucking the carbs semi dry.This happens in the idle mode more than when you are underway at speed. Maybe try squeezing bulb a bit when testing/running and see what happens.When starting some of them like to be over primed,that is after you think you have a solid bulb you give it a few extra pumps, they start a lot quicker that way.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

Well after much trouble shooting, rebuilding the carbs, and several failed test runs I have decided to that I am going to rebuild the fuel pump. I ordered that kit and will rebuild this coming weekend.

I do have a question though. Can someone point me to the correct ignition switch for the older mercury control boxes with the warm up lever on the right hand side?

thanks

 

[Chris1956]

Re: 74 inline 6 carb problems

If you want the ign switch to fit the MerControl, you will need to order it from Merc, as I have not seen an aftermarket version. See a local dealer.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

If you want the ign switch to fit the MerControl, you will need to order it from Merc, as I have not seen an aftermarket version. See a local dealer.

Well that is a bummer, but makes sense, I have not been able to find one.

 

[Isaacm1986]

Re: 74 inline 6 carb problems

I replaced the ignition switch, and still the same problem. I think i have it figured out though.

somehow i think water is leaking into the cylinders. I pulled all the plugs yesterday, and they were all clean as could be. Looks like a small amount of water is washing the plugs out. I think water is getting into the cylinders causing it not to run after a few min, and then you have to wait until the plugs dry out before it will start again.

Any ideas how water could be getting into the cylinders? Blown head? bad water jacket?

any thoughts would be appreciated.