Old 4 cyl Merc 650

[Joe Baublis]

Hello marine engine forum experts. Thank you all for your sharing your expertise. Thanks in part to all of you, I was able to pull the lower unit on an old Merc 650, replace oil seals, water pump, even the bearing carrier seals. Thanks. The new pump squirts water well.

Problem - engine starts up very well, and smooth. But dies when I engage into gear. Is this a synchronizing problem? Fuel is high test, 1 day old, new 2 cycle oil, and seafoam, the grease fittings were lubed, new fuel filter. The engine seemed to run under load for a whle when I had accidentally left the manual choke on. But mostly it dies on load.

 

[galamb]

Did you clean and rebuild the carb(s) as part of your rehab?

It could be a sync issue - idle jet(s) may be set a lot too lean or the idle speed screw (linkage adjustment) may be way out of whack.

Do you have a tach connected? If so what's it reading at idle in neutral?

Does it idle fine in neutral but gag under load? Can you give it some throttle and keep it going? Those would be signs of "not enough gas getting through the carb(s)".

If the carb(s) were not cleaned/rebuilt they would be high on my "probable" list.

If they were, then the sync could be way out.

Alternately, you could have gasket issues (sucking too much air somewhere - carb base, transfer cover) or upper/lower main seals on the crank.

Any fuel getting blown back out of the carb throat(s)? That would tend to indicate a reed issue.

Having said all that, a motor that will run "with the choke on" tends to indicate carb or seal issues (either clogged carb(s) or blown seal letting in air) - the choke is allowing "extra gas" into the crankcase which helps keep the motor running that is "otherwise starving"...

 

[Joe Baublis]

Thank you Mr. Galamb. I agree. I did NOT rebuild the carb, but I have been gradually replacing the 40 year old fuel lines which were degrading with black crud, possibly clogging the carb jets. It's quite possible that a section of fuel hose after the filter needs replacement, and that debris is in the carb jets. I'll get on that. Thank you for your sharing your expertise.

Joe

 

[Joe Baublis]

Did you clean and rebuild the carb(s) as part of your rehab?

It could be a sync issue - idle jet(s) may be set a lot too lean or the idle speed screw (linkage adjustment) may be way out of whack.

Do you have a tach connected? If so what's it reading at idle in neutral?

Does it idle fine in neutral but gag under load? Can you give it some throttle and keep it going? Those would be signs of "not enough gas getting through the carb(s)".

If the carb(s) were not cleaned/rebuilt they would be high on my "probable" list.

If they were, then the sync could be way out.

Alternately, you could have gasket issues (sucking too much air somewhere - carb base, transfer cover) or upper/lower main seals on the crank.

Any fuel getting blown back out of the carb throat(s)? That would tend to indicate a reed issue.

Having said all that, a motor that will run "with the choke on" tends to indicate carb or seal issues (either clogged carb(s) or blown seal letting in air) - the choke is allowing "extra gas" into the crankcase which helps keep the motor running that is "otherwise starving"...

Dear Mr. Galamb,

I attached 2 photos of the engine - now with a view of the 2 carburetors. My pinky points to the upper fuel inlet (which is in the process of being replaced) and my index finger points to the old fuel hose (also being replaced) from the upper carb to the lower carb. After replacing the fuel lines, I intend to clean the carbs. That will include removal of jet pins, and spray B12 into the jet seats. I could conceivably spray carb cleaner into the carb reservoirs through the fuel line inlets. Is that advisable? How would you clean the carbs? I prefer to leave them attached because I'm a novice and I don't want to have loose parts.

Joe B