timing curve question

[JerryIrons]

I have a mercruiser 170 engine that I have been doing a lot of work on. I never even saw a point or condensor before I got this boat, so this has been quite an education! According to the merc manual, the distributor advance curve says that the total advance spark, minus initial timing should be between 25 and 29 degrees.

So, if my initial base timing is 8 degrees (BTDC) at 650 rpm, then at 3000 rpm, I should be reading a timing measurement with my advance timing light of somewhere between 33 and 37 degrees?

Does it really matter what the base timing is, or end timing for that matter, as long as whatever you measure at 2100 rpm, (according to my merc manual it flatlines about 2100 ) and higher rpm, minus your initial base timing, falls between 25 and 29 degrees? I do understand that a higher base timing can cause a harder to start problem. I also understand that too much advance can cause bad things like detonation and blowing holes in pistons. This is the part I am concerned about and how you have too much advance.

I'm guessing if you have an engine that doesn't want to run correctly with a base timing of 8 degrees BTDC, then there is something out of whack that should be fixed? (ie loose timing chain, etc) Right now my engine runs great with a base timing of 14. I was able to fix a hesitation problem by advancing the base timing. This is on the muffs, but I will measure everything and adjust in the water.

-Jerry

 

[Bondo]

Re: timing curve question

Ayuh,... Donno Jerry, but I'll send this back to the top for ya,...

 

[EddiePetty]

Re: timing curve question

Jerry,

Years ago Mercruiser recommended that the base timing be REDUCED from 8 degrees BTDC to 4 degrees BTDC.
To my way of thinking, with your reported 14degrees initial timing plus full mechanical advance you are somewhere between 39 and 43 BTDC at full advance...way too much!!! With today's fuels and our engine configuration, anything past the early thirties is flirting with danger.
I suspect that your vibration damper is slipping on the crankshaft hub and indicating false readings.
I would verify TDC with a cylinder probe (neutralizing TDC piston dwell) and compare the timing marks. Another test, to indicate excessive timing chain wear, would to be would be to align the timing marks in the normal direction of rotation, when hand bar the engine in the opposite direction until resistance is felt. Note the degree variance in the timing marks.

 

[stonyloam]

Re: timing curve question

I suspect that your vibration damper is slipping on the crankshaft hub and indicating false readings.

The rotor on a 470 is keyed to the crankshaft and can not slip. Your initial timing should be 4 degrees BTDC, gap 0.022, dwell 28-34.

 

[JerryIrons]

Re: timing curve question

Thanks everyone for your comments, I really appreciate it. I have fixed my own vehicles etc all my life, but this is my first boat that I got a little over a year ago and has been a fun challenge. It's a 1986. The 2nd time out with the boat it died on me in the water and that really ticked me off, I want to be the one helping other people, not the one needing rescued! Plus I will be doing the majority of my boating on lake erie, a lake you do not want to have boat trouble on if weather turns bad.

Anyway, before I touched the ignition I did pull out my ancient timing light and noticed that the base timing was a little higher than 10 degrees, I didn't know how high because my light did not do advance. Fast forward to today, and I have all new points, condensor, spark plugs, wires, dist cap, rotor. I gapped my points to .022, and then set my dwell and it's right in spec at 30. (took a few tries) I also have a delco dist which lists dwell 28-34, for the prestolite it's higher. I also have a new timing light which measures advance. Yes you are all right, the spec calls for 4 degrees BTDC, when I initially timed the engine I put it at 4 degrees, but it ran kind of crappy, so I bumped it up to 8 degrees, which helped. If I used the listen to the engine by ear and adjust the timing, the engine really wanted to have a higher base timing. If I moved it high enough the engine started to ping I think. In boats however adjusting timing like this (by ear) seems to be a nono from some comments that I have read. Eddie my next steps were to validate that my timing marks are correct by measuring TDC of piston #1. For now I will try this by removing all spark plugs, and putting my compression tester on cylinder 1 and identify compression stroke, and mark engine somehow. Then I will use a screwdriver or wooden dowel to measure where beginning and ending TDC is and go halfway between, I think this will at least tell me if my timing marks are wrong. Also I will ensure that when I am at #1 TDC, my dist rotor is pointed at cylinder #1. My firing order and spark plugs wires have already been verified to 1342. By the way, I did check compression and I'm getting 160 I think on cylinders 1,2,3 and 150 in 4. I wrote it down but it isn't near me right now. And I have changed out both fuel filters, and played around with mixture and idle screws on carb to see how that affects things.

I'm having fun with figuring out this engine, and can go to the manuals and go back to the engine and am not afraid to start adjusting and fixing things. What I do not have is experience with these boats and engines, and any comments that people have helping me out are very much appreciated.

Thanks,
-Jerry

 

[JerryIrons]

Re: timing curve question

Last night in the driveway, I used a wooden dowel to check my timing mark, and as far as I can tell TDC of piston #1 is pretty much 0 degrees on the timing scale. It's a little hard to be exact, but my wooden dowel stopped moving up at about 5 degrees BTDC, and then started moving down about 5 degrees on other side of 0. I did this several times and have to say I think the timing marks are right on, or within a couple of degrees. Also, I removed the distributor cap and verified that when I'm at TDC of piston 1, the rotor inside the distributor cap is pointed at spark plug wire #1, so this is all in order. I also tried seeing how much slack was in my timing chain by reversing direction of the engine rotation until I felt resistance, and a guess would be about 7 degrees. But this was confusing because I checked it at various points throughout the rotation as well, sometimes it had resistance right away, sometimes it had slack. But from TDC it was about 7 degrees. Probably I need to actually check the chain itself for slack. Anyway, this morning I went on the water to play some more. I reset the timing to 4 degrees as per spec, and engine runs crappy. Lots of hesitation when I accelerate, and it backfires as well. I know the accelerator pump is putting 2 streams of gas into the carb as I have verified that. I moved the timing up to 8 degrees and it runs better but still struggles. I move the base timing up to 14 degrees, and everything runs great. No hesitation, no backfiring, no odd sounds like pinging etc. Then I started checking timing at 1500 rpm, 2000 rpm, and once over 3000 rpms. Timing advanced like it should, at 2500 rpms I think it was at 40! 40-14 = 26 right in spec. But 40! I'm worried I may blow a hole in a piston. By the way, I tooled around for a bit like this, fished for some bass here and there nothing too strenous. Every single time the engine fires right up when I go to switch spots like it was brand new. Shouldn't the engine be hard to start if the timing is too far advanced? If I have a loose chain, the timing would be retarded I believe and I would have to advance it just to get to a "normal" state so to speak. I will check on that timing chain, not sure what is involved to get a look at that just yet. I know to change it I have to remove alternator rotor and stator according to the manual.

 

[EddiePetty]

Re: timing curve question

....... If I have a loose chain, the timing would be retarded I believe and I would have to advance it just to get to a "normal" state so to speak.

If the chain is excessively slack, by the time the crankshaft revolves enough to remove the slack, the timing marks will show advanced. Another (remote) possibility is excessive thrust clearance on the distributor shaft or worn drive gears.