Sounds like spark knock...

[thundercat 9]

Hi guys,

I have a couple of questions, but first I'll tell you about my problem. I replaced my harmonic balancer on my 87' merc 350/260hp with a thunderbolt IV because the original one had a little wobble, also when I originally set the timing to 8deg BTDC, I used the "mark/line" on the harmonic balancer to set my top dead center(spark plug out and piston at the top of compression stroke...I used a compression gauge in the #1 hole to make sure) and had the "mark" pointing at the zero on the timing tab.
I started the motor and pointed my timing gun at the damper, the "mark" was reading about 6deg ATDC, not BTDC so I set it to 8deg BTDC and took it out on the water. It ran good up to 2000 rpm, anything higher and the detonation got worse. Back on land, I thought the balancer might have spun in the rubber so I decided to replace it. I picked up a chev 350 cid balancer off of an 89 pick-up truck and installed it.(when I compared both dampers, lining up the crankshaft keyway, the timing marks were about 1 1/2" apart, I thought I had this problem licked) After installing the new(used) damper, I fired it up, aimed the timing light and the timing was about 14-16 degrees BTDC.

I pulled the #1 plug and found TDC again, this time I marked the damper with some white out(were it pointed at the zero on the timing tab) and set the timing to 8deg BTDC. Back out on the water we went.....Same problem, this time I loosened the distributor and while my friend increased the RPM, I turned it and the detonation went away and I was able to get up on plane and cruise around 3500 rpm. I did not want to push it any harder as I knew I had a timing problem.

I followed the merc service manual and set the timing the way they say....I'm at my wits end.... would the thunderboltIV amplifier be the culprit. It has A8 22 marked on the side of the box.



Any info would be greatly appreciated.

thanks
jeff

 

[Don S]

Re: Sounds like spark knock...

Pulling the plug and finding 0? is only necessary to install the distributor in the correct location. After that you just turn it while using the timing light.
After you get it set to 8? BTDC, lock it down and speed the engine up to and see if the advance is working. If not advancing, the module is probably faulty.

 

[thundercat 9]

Re: Sounds like spark knock...

Hey Don,

Thanks for reading my lengthy problem. The reason I pulled the plug to find TDC was that I wanted to put the timing mark on the pulley as close to TDC as possible,because the factory "mark" was obviously off. If I took the cap off to see when the rotor pointed at #1, I could still be off a little,wouldn't I still have to pull the plug to "feel" when the piston reached it's apex?

Will I be able to perform that test with muffs or should I be under load?

 

[Reel Poor]

Re: Sounds like spark knock...

Hi guys,

It has A8 22 marked on the side of the box.

thanks
jeff

That should be V8 22. You timing advance should add 22 degrees to the base setting of 8 for a total advance of 30 degrees. It should all be in by 2800 rpms. If the total is higher or lower than specs you have a problem.

BTW, it's best checked under a load but can be checked on muffs.

 

[thundercat 9]

Re: Sounds like spark knock...

Thanks for the feedback.

 

[thundercat 9]

Re: Sounds like spark knock...

Hey again,

I checked my advance with a timing gun, it does advance when I rev the motor. My next question is should I adjust my carburator idle speed first and if so, what is the procedure? It is a rochester "quadrobog" 4 barrel. I think my idle could be off to begin with which I think would mean that when I set it at 8deg BTDC, it really isn't. I think??????:redface: