1975 Merc 7.5 thunderbolt hard starting

[Kennym2022]

Hey everyone. I picked up a 7.5 merc today, and we were able to get it started at the guys house after probably 30+ pulls and a little horse juice, which i dont like using. When i got home, i put some premix in a bottle and squirted it in the cylinders, which after like 5 pulls on full choke, it started, and idled and reved nicely with no choke. I stopped and let it cool off, and i had to do the same thing to start it. But i could start it in one pull if it was warm.

I cleaned the carb and replaced the fuel line.
I checked compression which was 120 ~3 psi between.
Good spark on both cylinders. Old plugs, so ill try some new ones, but i dont think they will do anything.

Any advice?

 

[racerone]

Does choke close and stay closed when you pull the recoil ?----Does spark jump a gap of 3/8" on a test device ?-----Could also be an issue with the reed valves .----Or an issue with crankshaft oil seals.

 

[tpenfield]

My 1969 7.5 Mercury is a '5 pull' start, but it always starts. I did at one point have to fully disassemble the carburetor (Tillotson carb IIRC) and do a re-build kit on it.

Also, assuming you have done the primer bulb thing to get fuel up to the carb. I usually have to start the engine on full choke, unless it is fully warmed up.

Make sure the fuel pump (diaphragm pump) is working. Sometimes the diaphragm gets perforated . . . easy fix.

You could run some Mercury 'PowerTune' through the engine (squirt into carb at high idle). I have also run Mercury PowerTune through an auxiliary fuel line to get it through the carburetor internals.

 

[Kennym2022]

Does choke close and stay closed when you pull the recoil ?----Does spark jump a gap of 3/8" on a test device ?-----Could also be an issue with the reed valves .----Or an issue with crankshaft oil seals.

As far as i can see the choke stays closed. I checked spark with both a spark tester,which gave me a bright red light, and by holding the plug to the head, i was able to jump probably and 3/8 to 1/2 inch gap with each plug.

 

[Kennym2022]

My 1969 7.5 Mercury is a '5 pull' start, but it always starts. I did at one point have to fully disassemble the carburetor (Tillotson carb IIRC) and do a re-build kit on it.

Also, assuming you have done the primer bulb thing to get fuel up to the carb. I usually have to start the engine on full choke, unless it is fully warmed up.

Make sure the fuel pump (diaphragm pump) is working. Sometimes the diaphragm gets perforated . . . easy fix.

You could run some Mercury 'PowerTune' through the engine (squirt into carb at high idle). I have also run Mercury PowerTune through an auxiliary fuel line to get it through the carburetor internals.

I have always set it to full choke, and it just will not start. I press the primer bulb until its hard, usually 5-6 times

 

[racerone]

Your hard to start condition likely has to do with lack of crankcase compression ----Carburetor issues or both.-----Crankshaft seals are known to go hard and crispy on those.-----In my opinion it is all easy to repair.-----Up to you to open a tool box.

 

[Kennym2022]

Your hard to start condition likely has to do with lack of crankcase compression ----Carburetor issues or both.-----Crankshaft seals are known to go hard and crispy on those.-----In my opinion it is all easy to repair.-----Up to you to open a tool box.

Ive got no problem with working on it, i was just hoping to have it on the water today

 

[Kennym2022]

Your hard to start condition likely has to do with lack of crankcase compression ----Carburetor issues or both.-----Crankshaft seals are known to go hard and crispy on those.-----In my opinion it is all easy to repair.-----Up to you to open a tool box.

It also does have non resistor plugs, is that normal, and do they wear out

 

[tpenfield]

Sounds like you have a fuel problem, since you have covered the other bases. You can try some of the snake oil stuff (PowerTune, SeaFoam, etc.) to see if that improves things.

Otherwise you would be opening the toolbox as mentioned.

 

[Kennym2022]

Your hard to start condition likely has to do with lack of crankcase compression ----Carburetor issues or both.-----Crankshaft seals are known to go hard and crispy on those.-----In my opinion it is all easy to repair.-----Up to you to open a tool box.

Is there any way to test for bad crank seals

 

[tpenfield]

Hard/impossible to see the seals, but if you can get the engine running you can see if you are getting any signs of fuel running down the outside crankcase. Not sure about the lower seal . . .

Also the crankcase halves are held together by about 6 bolts on each side. They should be plenty tight, but you can always check.

I'd get the engine running, as it sounds like you can. Go boating, but bring a set of oars.

 

[racerone]

Your motor needs 5 things to start and run.----Spark at the right time.----Cylinder compression.-----Correct amount of fuel in the carburetor.-------Correct amount of fuel going into the intake air.---Crankcase compression.----I believe you are missing one of the latter 2.---- So inspect carburetor.---Inspect reed valve to a degree with carburetors removed.------If they are in good condition you dive in further.-----Remove upper bearing housing.-----If that seal is crispy the lower seal will be in worse shape.----The lower crankshaft seal can only be replaced by splitting engine block.-------Perhaps not the information you wanted.-----But it is a summary of what you need to do.

 

[Kennym2022]

Your motor needs 5 things to start and run.----Spark at the right time.----Cylinder compression.-----Correct amount of fuel in the carburetor.-------Correct amount of fuel going into the intake air.---Crankcase compression.----I believe you are missing one of the latter 2.---- So inspect carburetor.---Inspect reed valve to a degree with carburetors removed.------If they are in good condition you dive in further.-----Remove upper bearing housing.-----If that seal is crispy the lower seal will be in worse shape.----The lower crankshaft seal can only be replaced by splitting engine block.-------Perhaps not the information you wanted.-----But it is a summary of what you need to do.

I have it idling right now, from squirting fuel into tge cylinders, and i took starting fluid and sprayed up under the flywheel, and there was no difference, so i dont think thats an issue

 

[tpenfield]

I have it idling right now, from squirting fuel into the cylinders, and i took starting fluid and sprayed up under the flywheel, and there was no difference, so i don't think that's an issue

Spraying S.F up under the flywheel ? that's a new one to me

 

[racerone]

Good luck sorting it out.

 

[Kennym2022]

Spraying S.F up under the flywheel ? that's a new one to me

Thats always been what ive done with like chainsaws and stuff, because if it is a bad seal, it sucks the starting fluid in and affects yhe engine

 

[tpenfield]

That's always been what ive done with like chainsaws and stuff, because if it is a bad seal, it sucks the starting fluid in and affects The engine

OK, understood.

Are you also using starting fluid into the carburetor to get the motor to start? (you mentioned adding some fuel directly to the cylinders . . .)

I'd get the engine running, have a hefty amount of fuel system cleaner in the gasoline, go boating so the engine gets run under load, and see where things stand.

FWIW - I did a rebuild on my 1969 7.5 Merc a while back, here is a link to the thread https://forums.iboats.com/threads/mercury-7-5-hp-motor-refresh-1969-model-year.683798/

 

[Kennym2022]

OK, understood.

Are you also using starting fluid into the carburetor to get the motor to start? (you mentioned adding some fuel directly to the cylinders . . .)

I'd get the engine running, have a hefty amount of fuel system cleaner in the gasoline, go boating so the engine gets run under load, and see where things stand.

FWIW - I did a rebuild on my 1969 7.5 Merc a while back, here is a link to the thread https://forums.iboats.com/threads/mercury-7-5-hp-motor-refresh-1969-model-year.683798/

Nope, i no longer add any starting fluid to the motor itself, since i dont want tot burn it up

 

[Kennym2022]

OK, understood.

Are you also using starting fluid into the carburetor to get the motor to start? (you mentioned adding some fuel directly to the cylinders . . .)

I'd get the engine running, have a hefty amount of fuel system cleaner in the gasoline, go boating so the engine gets run under load, and see where things stand.

FWIW - I did a rebuild on my 1969 7.5 Merc a while back, here is a link to the thread https://forums.iboats.com/threads/mercury-7-5-hp-motor-refresh-1969-model-year.683798/

Welp, i took it to the lake and ran at WOT for like 15 min, its pumping water, but the hard starting issue persists, ontop of that, the motor did not seem like it was working at full potential

 

[tpenfield]

Still thinking you have a fuel problem. Did you run a heavy dose of fuel system cleaner (of your choice) in the fuel? 15 minutes is not very long, but probably OK, if you had fuel system cleaner in the gas.

Time to open the Toolbox.

Maybe a full rebuild of the carburetor - replace internal screen, float, etc.
Disassemble/inspect/fix fuel pump