7.5hp firing on one cylinder. mercury brownband

[seany150]

after changing the sparks plugs for the correct type and cleaning the carb and inspecting it, (everything looks ok, i think) i took the merc out for a test run, its still running crap, extremely slow although on four occassions it did burst into life for a second then low revs again. tried priming the bulb as it was running, no difference. i took the plugs out when we got back and the top cylinders plug was burnt as normal and the bottom one was brand new still, no burns whatsoever?

had it running with the bottom plug removed and still in ht lead and its sparking really well and compression from bottom cylinder is good, ie cant put my finger in hole without it forcing it out. also i removed the cover plates on the side of the block and both pistons and rings appear to be in excellent condition.

how does the fuel enter the cylinder head after its left the carb? i know it goes via a tube into a square headed valve near the gear shift lever? could this block somehow?

makes no sense as the other week i could run it on either cylinder or both but just slow, now only on the top one and slow.

any help would be greatly appreciated!!

cheers

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

quick update, just ordered a new gasket for the water jacket on the cylinder head and the 2 gaskets to go on the exhaust side of the engine casing as the spark plug looked a little wet.

will strip, clean and rebuild the carb again once the parts have arrived and keep posted on the progress

any suggestions in the meantime would be of help

 

[mark_gober]

Re: 7.5hp firing on one cylinder. mercury brownband

Have you checked your ignition system? With it only firing on one cylinder, I'd suspect either the coils or the switch pack. The coils, you can just switch the wires on them. Remove the plugs and ground them on the engine. If both coils will produce fire, obviously they aren't the problem. If the problem moves to the other coil, I'd suspect a bad switch pack.

P.S. I had this exact problem on mine and that is what the problem was.

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

they both spark really good, nice strong bright blue spark, so dont think its the coils. it was firing on both cylinders last week but just extremely slow out on the water as if it was gaining enough revs. seemed to go the same speed with both plugs in as it did with either one removed so i know it was firing before.

Because it was going so slow i removed the water jacket casing off the front of the cylinder head and the exhaust casing on the side to look and in doing so tore the gaskets to peices, but still put it back together after doing so just so i could continue running it, could this be where water would get into the cylinder and causing it not to fire? i've ordered new gaskets so waiting to try this out

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

think i may have cracked it! just replaced the water jacket gaskets on exhaust side and cylinder head but still same problem, not firing on bottom cylinder.

So now i've just changed the wires and ht lead from one coil and switched them to the other coil and hey guess what, its now firing on the bottom one and not the top! what doesnt make sense is, with plugs both removed i have bright blue spark on both?

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

yet another update, i mentioned in another post that it would run on either cylinder or both but just slow, i've now discovered that it wasnt and even tho the ht lead was disconnected, the outer rubber boot was resting against the plug and must've been jumping spark, didnt think this was possible as the outer leads are rubber

 

[mark_gober]

Re: 7.5hp firing on one cylinder. mercury brownband

Seany,

Now you've got me confused. In one sentence you say you have spark and the next you say you dont. This is exactly how I'd trouble shoot your problem

1. Remove both spark plugs from the block
2. Using a pair of insulated pliers or just creative wire placement, ensure the spark plug is resting against the engine block for grounding purposes.
3. Pull the rope. If all is working as it should...you should see spark on cyl. 1 and then spark on cyl. 2. They should alternate.
4. If you only have one, try switching your spark plugs just to make sure that you have two good plugs. (probably not the case, but its quick and easy to check)
5. If you do only have one of the plugs firing, look at the coils. There are two wires on each coil. One wire on each coil goes straight to ground, so you don't have to move those wires. Switch the other wires. That should make the coils fire out of sequence.
6. Now pull the rope again. If things stay the same, you have a bad coil. If the problem moves...look at your switch pack.

In your writeup you say that one wasn't firing. How did you determine that? On mine, I had the motor started and then removed the lower wire from the plug. Nothing changed. That told me that the bottom cylinder wasn't working.

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

sorry for the confusion lol! it appeared i had sparks on both, the spark was jumping. i've just had the engine running on one cylinder with the ht lead removed and the lead just resting against the plug. the spark was jumping the lead enough to keep the engine running? i didnt think electricity could jump a rubber ht lead?

I just watched this vid on youtube on how to test a coil http://www.youtube.com/watch?v=KT8rk5QWgS0

so i literally 5 mins ago borrowed an ohm meter from my dad and it shows i have a dead coil. i put the leads accross the primary circuit on the dead one and the needle jumps to zero, then i tested the secondary circuit and got no reading whatsoever. on the working coil i got a reading of 8.35 on the secondary circuit. and a reading of zero on the primary which i guess means its working?

 

[seany150]

Re: 7.5hp firing on one cylinder. mercury brownband

update on the engine probs. after help in the forums i managed to figure out the ht coil was dead so ordered one from the states, managed to get it for ?108 in total. dealers here in the uk charge ?140 +vat so i saved quite a bit doing it this way.

I put the coil in sat morning and took engine for test sunday, it started first pull very easily, throttled up straight away and ran like a dream all day, ticking over nicely and not stalling once, so problems all sorted!

Thanks guys!!