Detonation Problem?

[PaulKim]

First sorry for being so long winded. I recently purchased a 84 Merc 75HP. It worked great the first couple of times I took it out, but it was idling very fast. I adjusted the throttle with the pickup screw. It was nice and smooth. Next time I took it out, it wouldn't start, I turned up the idle and it started. Used it all day. Next time out, used an older metal gas tank which had a little rust in it. It ran great for the first few miles then the primer bulb went soft, I checked the vent and it was open. I took the gas line and plug it into my plastic tank and worked fine again. I decided to decarb using some seafoam. I took it out once again and it would go good at idle, but once I kicked the throttle up, it would just die. Starts with no problems, but would die intermittently when I would throttle it up. If I'm lucky, I can slowly throttle it up and it will plane out. But still not the same speed I was getting when I first ran the boat. Also while on the water I monkeyed around with the Spark Stop Screw. I have changed the fuel line, bulb, took off the new inline fuel filter (thought maybe it was not allow enough fuel to go through it), cleaned out the fuel filter screen that is built into the motor. I also checked the plugs,and it was fouled with some aluminum on it on clynder 1. Didn't think nothing of it, decided to use some deep creep on the carbs. Took it out to test, and still the same problems. I have pulled the plugs once again and aluminum on clynders 2 and 3. <br /><br />What are the possible causes of detonation?<br /><br />Prior to all these troubles my cold engine compression was at 135, 109, 125, the 4th clynder I could not get to measure it<br /><br />Any help would be appreciated!!!!

 

[Reel Precision]

Re: Detonation Problem?

Sounds to me like you have adjusted the timing way out of whack. You were probably turning the timing advance stop screw instead the idle adjustment. The aluminum on the plug is not good at all.

 

[Clams Canino]

Re: Detonation Problem?

Check the compession again and then go through the timing and sync procedure I posted last week.<br /><br />Also you may wanna clean up the fuel system once and for all.<br /><br />-W

 

[PaulKim]

Re: Detonation Problem?

Reel and Clams, thanks I'll read up on timing and recheck compression this weekend.

 

[Hooty]

Re: Detonation Problem?

Personally, I'd have to pull the head and see just how bad the pistons were burned.<br /><br />c/6<br />Hooty

 

[Clams Canino]

Re: Detonation Problem?

Hooty..... A 1984 - 75hp does not have a head. <br />Unless you wanna pull it with a cutting torch. <br /><br />-W

 

[PaulKim]

Re: Detonation Problem?

Couldn't wait till this weekend to check the timing or compression. There are some salmons with my name on them <br /><br />Cold Compression: 136, 100, 110, 115<br />Added couple table spoons of oil in #2<br />Compression now reads 139, 150, 121, 119 (Bad Ring in #2 ?)<br /><br />First of all I know nothing on the how's of timing, but did read Clams post on 9/24 on how to do it. (Still very confused)<br /><br />The label on motor say's 4 degrees ATDC Running. Max Spark 30 degrees BTDC at cranking speed of 400 RPM which is 27 degrees BTDC at 5500 RPM. <br /><br />I have no idea what that is supposed to mean. What I did is turned the fly wheel and looked at the piston head until it got to TDC, and read the hash marks on the wheel. It read 4 degrees (right of the zero). Had my brother crank it, and it read it with the gun and it read 5 degrees (to the left of the zero. Manually pushed the throttle to max, and read the reading while cranked and it read 29, adjusted the spark stop so it now reads 26. <br /><br />Clams, should I adjust it down to 21 as was listed in your post from 9/24? The marks only go down to 27. <br /><br />Just to let you know in a moment of weakness, I stopped by the Boat store and asked them what it would cost to re-time, they stated that it would be around $100. I was on the fence as far as if I was going to do it myself or not, but figured that I could buy a gun and a few cases of beer, and get the satisfaction of learning something from you pros. Worst case I'll at least have the beer and one miserable headache.<br /><br />Motor is a 75 HP Merc Serial # 6503474, i believe that it's an 84 based on the German Website. <br /><br />Also, when I got the motor it had the BUHW-2 plugs in it. As I look in the Clymer book the only motor with a BUHW is 3 Clynder. I think maybe I have the wrong plugs, based on the pics in the book under timing the one that looks the most like my motor is the Mercury 75, 80 (prior to 1984), 800 and 850 with Distributor-less ignition. So when I looked up the specs the spark plug for this appears to be either a AC-V40FFK or Champion L78V. <br /><br />Thank you again for all your help.<br /><br />Paul

 

[Clams Canino]

Re: Detonation Problem?

I'd take it down to 21... or 23 if I felt totally safe abou the gas, prop etc etc. L76V or L78V is close enough for govt. work...'<br /><br />-W

 

[Clams Canino]

Re: Detonation Problem?

I'd take it down to 21... or 23 if I felt totally safe abou the gas, prop etc etc. L76V or L78V is close enough for govt. work...'<br /><br />-W

 

[PaulKim]

Re: Detonation Problem?

Clams Thanks for the advice. Something I was thinking about was the reading on wheel. I kept an eye on the Piston and once it reached top, the little pointer pointing at the fly wheel showed 4 degrees. Should I adjust the pointer so it shows 0? Then crank the motor with the Gun and check the degrees at idle position on the throttle?<br /><br />Also, should I follow this same process, pushing the throttle all the way then checking the degrees?<br /><br />I also called the boat store to check on the spark plug that should be in my motor, they stated that it should be a BUHW-2, which is what I have now. So I'm assuming that it is okay. <br /><br />Thanks a million