1974 merc 20hp spark but only running on 1 cylinder

[doryfishin]

hi , i have good spark on both cylinders , good compression i have cleaned carb out , replaced hi and low speed coils yet still i only get half throttle . i pulled ht leads only to find no1 cylinder not firing yet spark is good when plug is out . it will start on no2 cylinder without no1 cylinder plug in i dont know what else to try i dont have any way of checking timing as it needs to be checked at wot and i run the risk of blowing it up in my test tank as all the water flies out the tank . any other ideas on thi will be much appreciated thanks in advance .

 

[latin monkey]

Re: 1974 merc 20hp spark but only running on 1 cylinder

could be bad reed valve

 

[jbjennings]

Re: 1974 merc 20hp spark but only running on 1 cylinder

What size gap will the spark jump on a spark tester for the #1 cylinder? If not at least a 1/4 inch gap, it's not enough.
Have you switched spark plugs to see if the problem follows a plug? ANy milkiness or beads of water on the plug? Does the plug appear to be steam cleaned?
Sorry for all the questions, just trying to help,
JBJ

 

[snobear33]

Re: 1974 merc 20hp spark but only running on 1 cylinder

I have the same problem. Was told by a marine mech. in Fla. if you have good spark , compression and fuel then the next thing to suspect is a blown bottom crank seak seal on the older mercs. They were and are well known for that issue. What happens at low RPM's the seal leaks enough to loose internal preasure. But at high RPM's the motor is turning fast enough to hold compression and block preasure and fire either fully or only part of the time. My motor drops a cylinder when at trolling speed . Wide open it runs like a bear. Let me know what you find out. Still not ready here to rip my motor all the way down yet....

 

[doryfishin]

Re: 1974 merc 20hp spark but only running on 1 cylinder

thanks for the replies , i'll check out swapping ht coils tonight and spark gapping distance . i had in my mind that could possibly be reeds hoping not though . at full throttle it is same as half throttle flat as . i did read on hear that swapping thr trigger wires could sort a timing issue but only if it was'nt firing would this do damage if i try it ? will post my findings later . rob

 

[doryfishin]

Re: 1974 merc 20hp spark but only running on 1 cylinder

hi , turns out that under load the ht coil was breaking down and now sounds and runs like it should . thanks for the replies rob .