1987 OMC 4.3L Ignition timing

[hayward6]

I have done a lot of work to this boat over the past year and its finally in the water! After replacing the cylinder heads I only timed the engine by ear, and I learned yesterday its way off! It has one of those pertronix electronic kits and at idle the timing was off the scale quite a bit on the BTDC side... I turned the distributor until it was running right at TDC and it would then go up to about 8 BTDC at about 2k rpms.

I'm not exactly sure if this is right for a couple reasons. The last time I took the boat out the motor had a slight tap when running WOT and I thought the timing might be the cause since I have gone over everything else. I have new heads, and valves as well as gaskets, I have 175-180 psi in every cylinder and the motor oil is as clean now as when I changed it a few weeks ago. Its running great! When I changed the timing it brought the idle way down so far I had to adjust the idle screw on the carb, and it does sound real good... All I can do is wait until I take it back out tomorrow.

Does it sound like I have it adjusted properly? I do have SELOC book, but I didn't see the actually adjustment in there...

Thank you,

 

[Maclin]

Re: 1987 OMC 4.3L Ignition timing

That sounds like not enough advance. At 2400 and above the timing should be up around 24 degrees total, maybe more as I am not sure of the exact spec for that engine, but 8 total is not enough for sure.

Make sure the mechanical advance is working well by turning the rotor with a light hand as far as it will go, there should be light spring loaded resistance, then see it it snaps back sharply. That will check the action of the weights under the plate. If that action is not smooth, or the weights do not snap back then your distributor needs some work. If the weights do not stay "in" at idle you will get a false reading on the static timing (at idle) check, and the total advance will be severely limited.

 

[hayward6]

Re: 1987 OMC 4.3L Ignition timing

That sounds like not enough advance. At 2400 and above the timing should be up around 24 degress total, maybe more as I am not sure of the exact spec for that engine, but 8 total is not enough for sure.

Make sure the mechanical advance is working well by turning the rotor with a light hand as far as it will go, there should be light spring loaded resistance, then see it it snaps back sharply. That will check the action of the weights under the plate. If that action is not smooth, or the weights do not snap back then your distributor needs some work. If the weights do not stay "in" at idle you will get a false reading on the static timing (at idle) check, and the total advance will be severely limited.

I don't know I think i'm OK with the advance. The weights and springs all look good and move freely. I read that it should be about 8 at 2k and I have that, it does go out farther when I give it more throttle but its off the scale at that point. I guess I'm wondering if timing it at 0 at an idle is proper?

 

[Maclin]

Re: 1987 OMC 4.3L Ignition timing

I would think it needs about 8 at idle which is why you had to bump the carb idle up, and 20+ at 2k and above. Where are you getting those specs?

 

[hayward6]

Re: 1987 OMC 4.3L Ignition timing

I would think it needs about 8 at idle which is why you had to bump the carb idle up, and 20+ at 2k and above. Where are you getting those specs?

I didn't see the specs in the book, but now that I think about it they're probably listed in the back with all the torque specs. The book only showed how to test it and it made mention of it being at 0 TDC. I'll check again when I get home, one this is certain it's at least close now, and before it was way off. At WOT before the plugs must have been firing way early!

Thank you,

 

[Reinell-BRXL-191]

Re: 1987 OMC 4.3L Ignition timing

the 4.3 should be 1 BTDC if using regular gas, and 6 BTDC if using Premium - this is made at idle.. Anything higher than that, may/will cause detonation at higher RPM

 

[hayward6]

Re: 1987 OMC 4.3L Ignition timing

the 4.3 should be 1 BTDC if using regular gas, and 6 BTDC if using Premium - this is made at idle with the vacuum advance disconnected. Anything higher than that, may/will cause detonation at higher RPM

Thank you. This doesn't have a vacuum advance though, it's all mechanical.

 

[Maclin]

Re: 1987 OMC 4.3L Ignition timing

Marine distributors will not have vacuum advance Reinell.

 

[Maclin]

Re: 1987 OMC 4.3L Ignition timing

After looking up the timing curve specs for that vintage of OMC 4.3l I will recant my previous posts about the advance you posted as being out of spec. I am reserving the right to be puzzled by the low advance numbers though!

Here is a chart I found for the advance curve, shows 5.7l and 4.3l, it appears the 4.3l distributor only has 12 degress of mechanical advance built in and the 4.3l seemingly can only tolerate so much total advance.....

 

[Reinell-BRXL-191]

Re: 1987 OMC 4.3L Ignition timing

Sorry, I am a auto mechanic... I don't think about marine applications some times.. But the timing I stated is dead on for the OMC model and year in question - At 24 degrees as stated in one of the posts above, on pump gas,at full operating temperature.. he will be in full on detonation..

 

[hayward6]

Re: 1987 OMC 4.3L Ignition timing

the 4.3 should be 1 BTDC if using regular gas, and 6 BTDC if using Premium - this is made at idle.. Anything higher than that, may/will cause detonation at higher RPM

Confirmed with my manual last night and you are right on! I went ahead and set it to 6 BTDC as I always run premium gasoline in my boat. It did kick the idle up a little so I adjusted it, and it sounds great. I won't be sure of anything until I get it back in the water of course and get a load on it. I should be doing that Friday. Thanks so much for the help!

 

[hayward6]

Re: 1987 OMC 4.3L Ignition timing

I just thought I would post back with the outcome. After setting the timing to 6 BTDC its running like a sewing machine! Much more powerful, faster, and even the exhaust sounds stronger. For being a 1987 engine it sure runs smooth! Thanks again for all the help.

 

[Reinell-BRXL-191]

Re: 1987 OMC 4.3L Ignition timing

Glad we could help... Happy Boating!