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Merc 50 wonbt start

W

willys_wonka

Member
"i've been working on a no

"i've been working on a non-running 5.0 merc w/electronic ignition. new plugs, dist.caps and rotors. double checked that all plug wires are correct. lots of spark getting to plugs but you have to keep grinding the starter and it tries to start but won't, farting and burping all the way. had a few lovely explosions out of the carb. rochester quadrajet carb. opening the secondaries you can see the intake is wet with gas, and when you pull a plug it's wet. i thought that maybe the float valve wasn't seating properly so i replaced it. put everything back and now nothing. batteries getting low so leave them to recharge overnight. getting gas, getting spark but nothing. if this thing had a timing belt i would guess that it skipped a few teeth and the timing is way off. the guy said that one day it just wouldn't start and had always run well.
i did a rough comparison of distributor orientation with other engine and it looks okay so i doubt ignition timing is the culprit"
 
"that engine doesnt have a tim

"that engine doesnt have a timing belt, btw. I would suspect ignition timing off or firing order (plug wires). No need to crank the crap out of it. Check the ignition timing and set to TDC if you have reservations - will always start at TDC.

Use a can of carb cleaner to provide a blast of known good fuel before cranking. blast then crank.

You may have bad fuel. Doen't take much to have bad fuel with this ethanol infused junk these days."
 
"not sure what that means, but

"not sure what that means, but it's a GM 5.0 litre. easy to keep things straight becuase the dizzy cap leads are numbered as are the cylinders on the intake manifold. I'm positive i got the leads right and besides, one day it worked and the other day it didn't.
i also have it's twin to compare it to..."
 
Coil?
Ignition module?
A sud

Coil?
Ignition module?
A sudden cessation of running on a previously healthy engine sounds like a spark problem to me.

Rod
 
spark is good. measured at coi

spark is good. measured at coil and output of dizzy and output of hi tension leads. bright and will jump 1/4" gap no problem. new rotor and dizzy cap.
 
""...bright and will jump

""...bright and will jump 1/4" gap no problem. new rotor and dizzy cap." What color is the spark? Is there a good snap sound when it sparks? Perhaps you are only seeing the spark that occurs when it "...tries to start but won't, farting and burping...".

Since you also had some backfiring, I'm w/MistaHays; check the timing. In addition, there is an optic sensor on TB ign. under the rotor. If it is full of rust it can be failing intermittently."
 
"A reverse rotation engine is

"A reverse rotation engine is one that runs right hand rotation (counter clockwise as you look at the crankshaft from the front of the motor). This was very common on twin engine boats a few years back, not so much anymore.

But it it's a reverse rotation engine you have to use 12756348 as the firing order, instead of the usual 18436572."
 
"Check timing
check timing
C

"Check timing
check timing
Check Timing
you've got an ignition problem either timing is of, fire computer is failing. plug wires are out or coil trigger wire is loose
CHECK TIMING
If you don't have a timing light AutoZone or some other car parts shop will let you borrow one for free you just have to give them your license for collateral

-Kevin
Deep Blue Marine Repair LLC
San Diego, Ca"
 
"okay, so timing it is. now i

"okay, so timing it is. now i know how to set static timing with points, but how with optical? i can check that the rotor points to plug 1 at tdc, and with a point dizzy i would turn it until the points open and make sure it sparks near tdc but how do you do it with optical points? same way?

i was also wondering if the coil was getting weak. i could swap it with a known good one. i could also swap over ignition module, but my experience is that they either just blow, or they go thermal and starting packing it when hot.

i can't hear a spark snap because of the noise of the motor turning over.

one weird thing - when the starboard engine is running the timing light can pick up the firing signal in the non-running engine! it will sometimes flash even when i'm not trying to start the port engine"
 
"#@$%&!! turns out that it

"#@$%&!! turns out that it was the coil. weak, but only just weak enough to not fire plugs hot enough, but not so weak that it was obvious. miller time."
 
"Coils can be tempermental.
W

"Coils can be tempermental.
What was the first word in my first response?
"Coil?"

The good news is they don't cost much, so they are a good bet to change out when chasing an ignition problem.
Glad to hear you got her licked.

Rod"
 
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