Engine tune up re: Advanced curve

Hello ones again, I get tired of scratching my head alone so I like to involve you guys who are interested.
While I was tuning the engines my friend told me that I should find out what's the distributer's advance curve. On a general manuel it showed at 1000 rpm 8 btdc at 2000 26 btdc. I like to find out what's the degrees for my engines ( 4.3 lt. Gmc. 6 cyl. 4 bbl carbs, prestolite distributors .) If any body have this information at hand and like to share it will be very appreciated.

captain_peter said:

On a general manuel it showed at 1000 rpm 8 btdc at 2000 26 btdc.

I like to find out what's the degrees for my engines ( 4.3 lt. Gmc. 6 cyl. 4 bbl carbs, prestolite distributors .) If any body have this information at hand and like to share it will be very appreciated.

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Please look this up in YOUR own OEM manual for YOUR engine.
The above listing for 26* advance @ 2,000 rpm is a prime example of misinformation that can cause engine damage.
You do not mention which engine the above spec was for, but I'd be very surprised if a V-8 marine gasser could take 26* TA @ 2k rpm without some negative results.
It's not the 26* that is the killer..... it's the 26* @ 2k rpm that is.
I could be wrong, but I do not typically see that much advance so early!

I just changed out the prestolite distributor on my 92 Omc 5.0 liter Ford for a new Mallory magnetic discharge jobbie, and it says that it will advanse to a max of 24 degrees btdc at around 3500, with timing set at 10 deg. btdc. It also has a little tuner info on it's performance site, for installing softer springs on the advance plate, for better performance. As it is, I do not get any knocking of any kind. Just a thought for what it is worth..., what year is your motor, I'll look it up in my manual. Rick...,

The 'problem' with a marine engine is that you only have one gear.
On a planning boat, you may hit the max engine load at about 2500-3500 or just when it starts planning.
If you at this early stage reach the max timing advance, you may very quick be 'doomed'.
The factory curve is a general setting, and may be changed depending on engine load at the different RPM range.
In short, on a light boat with a low rpm planning threshold and 'big' engine you may advance your timing at an earlier rpm than on a heavy boat with a 'small' engine.
What works in one set-up may be a disaster in another if all factors are not considered.

Thanks Ricardo for your quick and informative responce, I do not have an OEM manual and I was hoping that some one that does will be kind enough to offer this info.
The figures that I wrote down was taken from a Seloc manual general info. I do see your point that 26 degree at 2000 rpm is quite high .

Thanks Rick for your input, the engine size is 4.3 LX 1989 GMC 4 BRL Carb. Prestolite dist.

Thanks Haffiman for being so specific I can see you got a lot of knowledge in this field.
I do not have an OEM manuel and the figures I wrote down was a very generalize specs. from a Seloc manual.
My boat is not very heavy, weigh about 7000 lbs with twin engines 4.3 ltr 205 horse each, Max rpms 4800 plane at about 3100 rpm approx. 20 miles an hr.to 45 miles an hr.
The reason why is so important to find out where the curve should be at a certain rpm because both engines at about 1600 to 2000 rpms run rougher than at any other rpm higher or lower. Basicly wondering if the springs on the counter weights have the right tension.

Peter, it gets very involved when explaining the entire reasoning behind an ignition curve.
TA (aka total advance) can help make horse power, and it can also lead to increased Detonation, of which is an engine killer.
The progressive curve, at which this varying spark advance is given to each cylinder, can make or break either.
The engine build even plays a roll in this!

As per your previous comment, it's not that the 26* is too much..... it's the lower RPM where it comes in at!

My advice would be to use Extreme Caution if setting up your own advance curves via spring kits that can be purchased.
Best to always see your OEM advance curve, and only slightly vary from this, if any, unless you know precisely what you are doing. IMO that is!

You can find shops that have trained techs that can set up a distributor on a distributor machine!
They can get them Dead On.... as long as YOU provide the curve that you want!

'The reason why is so important to find out where the curve should be at a certain rpm because both engines at about 1600 to 2000 rpms run rougher than at any other rpm higher or lower. Basicly wondering if the springs on the counter weights have the right tension.'

All engines have their weak spots, this is yours.
Mostly it appears when the boat reaches its max 'hull speed', and torque is needed to start getting on plane.
It has partly to do with timing, but not ignition.
To get away or reduce, you need a variable camshaft timing, a fuel injection system and electronic ignition with programable curve and knock sensor!
In short, you need the inlet valves to open slightly earlier to give time for the Quadra Jet to feed enough fuel. The airflow through the carb is not enough to fully operate the main jets and feed the engine, the engine runs slightly lean.
Bottom line, stay away from that rpm range if possible. Just pass through.
If you need that range for trolling or similar, change props or re-calibrate carb, but you will probably loose out in either idle or top speed!
The 4.3 4BBL was never my favorite. The 2BBL worked reasonably well, but if in need of more power/torque I always went for the 5.0 2BBL. The 4BBL carbs are too rough, and the only engine that handles them properly throughout the rpm range is the 5,7 in my opinion.

Thanks Haffiman for your thorough explanation, your explanation makes a lot of sence. I actually been keeping the speed around this rpm ranges. The only problem I am getting is some time going through the river trying to keep the speed limit of 10 k it becomes difficult with out the engines doing all this vibration.

Hi Ricardo, you guys sure gave me a lot of info. to chue on I really appreciate that . My next thing I have to get myself an OEM manual so at least I can see if this curve is with in range of my timing.

Peter...I only have a 1988 OMC factory manual left but it should be fairly close to 1989??? Anyway I scanned in the page with the timing curve (also has the 5.0 and 5.7) hope this helps. I was really surprised at how low the curve is for the 4.3.

boatin_bob said:

Peter...I only have a 1988 OMC factory manual left but it should be fairly close to 1989??? Anyway I scanned in the page with the timing curve (also has the 5.0 and 5.7) hope this helps. I was really surprised at how low the curve is for the 4.3.

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IMO, and IMOO here....., you'd best be very careful if you plan on throwing anything near 34* at this engine @ 3k rpm.
To do this, you'll need high octane fuel, and probably a cylinder configuration that offers the Q/E.
Otherwise, this may be a recipe for detonation.

I am very curious as to which manual this curve is from!!!!???? OEM, Seloc, Clymers????

Thanks Bob for the extra effort of diggging in your old files, now I can compare with what I am getting of the engines.
With the rpm @ 600 6deg btdc and @ 2000 rpm I am getting 20 deg. This seems to be much closer to your charts Than what I read on seloc manual.
It gives me peace of mind knowing that the timing is not that far off.

RicardoMarine said:

IMO, and IMOO here....., you'd best be very careful if you plan on throwing anything near 34* at this engine @ 3k rpm.
To do this, you'll need high octane fuel, and probably a cylinder configuration that offers the Q/E.
Otherwise, this may be a recipe for detonation.

I am very curious as to which manual this curve is from!!!!???? OEM, Seloc, Clymers????

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Rick...slow down and take a deep breath....

We are talking about the 4.3 which is on the top part of the chart....only 18* at 3500 rpm and yes this is from an OMC factory manual PN 507605, IMO they were the only manual worth buying. (if you could get them)

Bob, Sorry, guess I had the V-8 on my mind here.