Mercruiser 5.7 Thunderbolt 5 Hard to Start

[Georgearia]

99 Mercruiser 5.7 very hard to start.
Recent work: Carb Rebuild, New Coil, Wires, Plugs, Cap, Rotor, Sensor, Batteries. Timing chain and gears replaced.
The motor is extremely hard to start. One started it runs rough then dies and wont start again until next day. I have tried adjusting choke, timing. Gas is fresh. It is getting fuel after several tries it does flood. Getting good spark I believe because when turning key tries to power right up but wont keep running. If I continue to keep starter going it will kinda gallop along.

Any help is greatly appretited

 

[Scott Danforth]

Any corrosion on the trigger assembly under the cap?

 

[Scott06]

What necessitated replacing the timing set ?

I have the same ignition if there’s a whiff of gas In there it starts right up.

 

[isaacs]

If it runs when the starter is engaged (presumably it's grinding away) and then falters when you release the key, this would indicate low voltage on the plus side of the coil. Turn your ignition on without starting the engine and check voltage between coil plus and ground. It should be around 9V. It could be a bad resistance wire to the coil, bad connection, bad lanyard kill switch, bad ignition switch.

 

[isaacs]

Any corrosion on the trigger assembly under the cap?

He said he replaced the sensor. Is that the same as the trigger assembly?

 

[Georgearia]

If it runs when the starter is engaged (presumably it's grinding away) and then falters when you release the key, this would indicate low voltage on the plus side of the coil. Turn your ignition on without starting the engine and check voltage between coil plus and ground. It should be around 9V. It could be a bad resistance wire to the coil, bad connection, bad lanyard kill switch, bad ignition switch.

I will recheck the voltage as u stated at the the coil + and -. And recheck the ignition key. Question, I had read on another forum to check tach ground by disconnecting tach wire which is on - side of coil. When I did that. I had no spark... Should there be spark with it disconnected?

 

[Georgearia]

What necessitated replacing the timing set ?

I have the same ignition if there’s a whiff of gas In there it starts right up.

Timing chain was very sloppy, about a half in of play...

 

[Georgearia]

Can someone point me to the location of the resitor wire and color, on the Thunderbolt 5? I do not see it in the ignition diagram.... Thanks for everyone's help

 

[Scott06]

Can someone point me to the location of the resitor wire and color, on the Thunderbolt 5? I do not see it in the ignition diagram.... Thanks for everyone's help

There’s no resistor wire on a a TBV ignition that was only used on older points systems.
your comment above on the tach wire. There are two wires going to negative side of coil. One that goes to the ignition module one that goes to the tach. If there’s only one wire they split of somewhere in the harness so you can’t remove the one from the coil without killing the ignition. Disconnect the grey wire at back of tach instead

did you set base timing ? on the TBV you have to put it in base mode to initial timing set timing

 

[Georgearia]

Got it. There is only one wire on the negative side of the coil. And I had thought that there was no resistor wire on the TB5. Thank you for verifying that.

 

[Scott06]

Got it. There is only one wire on the negative side of the coil. And I had thought that there was no resistor wire on the TB5. Thank you for verifying that.

Did you set or verify the timing by putting it in base mode? I’m wondering if the timing is not correct and that may be causing hard starting

 

[isaacs]

Got it. There is only one wire on the negative side of the coil. And I had thought that there was no resistor wire on the TB5. Thank you for verifying that.

Sorry...I missed the TB5 part in your post.

 

[Georgearia]

Ok, I check the voltage at the coil + with key on. 13volts. Rechecked all the grounds. Checked engine wiring for chaffing etc. Checked ignition switch. Power is 12v in the on position. Crank motor... Go to run position...run position has power.

Tried to start, immediately fires but will not idle(run), Dies. If I pump the throttle after it fires, it will not run. If I throttle to full as if it were flooded, it does even try to fire.

In summary, if I leave the throttle at idle, it tries to start...gallops.... And that's as far as I get. I'm befuddled....lol

 

[Georgearia]

This may be a stretch, what is the possibility of exhaust manifold being plugged? They are 5 yrs old. Always fresh water flushed, raw water cooled.

 

[Scott06]

To run you need compression,fuel, and spark. You verified 12 v at the coil in run but do you have adequate spark at the plugs? If you have spark dump a little gas down the carb if it coughs or runs a bit you know it’s fuel related, either supply or carb needs rebuilding. Somewhere InThere verify you have compression on all cylinders.if you have those verify the ignition timing is correct

 

[Georgearia]

Compression is 170 to 175 on all cylinders except for #8 Which is about 155.
The carb was fully rebuilt by a rebuildeer in Tampa. The motor is getting fuel. Odd thing is when it floods, only the spark plugs on the port side are wet and fouled.

 

[Rick Stephens]

Not to change your direction of thought here, but this sounds like the carb rebuild needs doing again. You said you're flooding. You have spark. TV is very trouble free, or it doesn't work at all. I would suggest you open the service manual for your motor, if you don't have it, use the free online one from boatinfo.no It has a very detailed carburetor rebuild and settings section. My bet is you have some crud opening a throttle pump check ball, or a float height that is wrong. Bad power valve an also dump fuel.

 

[Rick Stephens]

'99 have a Weber carburetor?

 

[Scott Danforth]

Compression is 170 to 175 on all cylinders except for #8 Which is about 155.
The carb was fully rebuilt by a rebuildeer in Tampa. The motor is getting fuel. Odd thing is when it floods, only the spark plugs on the port side are wet and fouled.

you may want to check the gauge. any reading greater than 150 is suspect or there is water or oil in the cylinder. a brand new engine from mercruiser or volvo penta or GM will have 150psi as cranking pressure

with a dual plane manifold, only doing one bank would also be suspect.

with a dual plane manifold on an SBC, one half the carb feeds cylinders 1, 4, 6, and 7, the other side feeds cylinders 2, 3, 5, and 8

only way to get one bank to be wet and fouled is to have a single plane manifold. then the lower of the two banks would flood.

you may want to check if its your exhaust leaking. that would wet and foul one bank vs the other.

 

[Georgearia]

I'm not very knowledgeable about intake manifolds. This what I have on the boat. The lower of the two sides is flooding... Cylinder side 1, 3, 5, 7.

Would the base gasket be a contributor... Its the thin open plenum gasket. Not the spread bore...4 holes...

Thanks for all your help