Thunderbolt IV Timing Jump

[jgrant911]

I recently noticed a rough/low idle on my 1995 4.3LX (Maxum 1800 SR). When I got home I did a tune-up (plugs, distributor cap and rotor) but when I checked the timing it was 4 degrees after TDC instead of 8 degrees before. Now, I am the only one who has worked on this engine and I have never adjusted the timing. When I went to adjust the timing the distributor hold-down bolt was still very tight. I did adjust the idle timing back to 8 degrees BTDC and the idle smoothed out to 700 RPM.

Here's the question. Does anyone know of any reason the timing on this engine might spontaneously adjust itself? I am worried about an aging ignition module that might be randomly retarding/advancing the timing.

Any ideas are appreciated!

Jim

 

[Fun Times]

Hi there, welcome to iboats!

Do you happen to know if you have Thunderbolt IV V? It'll make a differences for us to know on how you set the timing to know if it was adjusted properly as that is a big jump in timing.

With Thunderbolt V, you'd need to ground out a purple/white wire while warm & @ idle speed of 650 rpm.

Have your engine serial number handy?

 

[jgrant911]

Not to be a smartass but... see the thread title!

Build number is 0F605187.

Jim

 

[Fun Times]

Not to be a smartass but... see the thread title!

Build number is 0F605187.

Jim

You're not being one and yes I did see it but I'll admit as I was thinking of a response and what could be wrong I kind of forgot you mentioned it in the title and focused on the question but experience of your description had me thinking something doesn't seem to be adding up right. While in 1995 Merc was switching from TB 4 to 5, just moving the timing can be dangerous depending on what system you really have.

Now to add to the confusion, (and this could still go either way) The engine serial number is normally pretty important to know to help solve some confusions between the thunderbolt numbers, engine components, etc. etc. that happens quite often on the boating forums. One may say thunderbolt 4 while the engine serial number shows your engine to be a thunderbolt 5 but if you look at the parts catalog it will show the thunderbolt 4.:facepalm:

Running your serial number comes up as an MERCRUISER TB5, W/PS, TWOPCEX, WORO 4.3LX ALPHA (Gen II) MERCRUISER - (4.3LX H.P. (1996) Serial No: 0F605187 Model No: 443L100JS Parts Catalog No: 90-80187496

http://www.crowleymarine.com/search_results.html?string=0F605187

So either a photo of your ignition module is needed on our part to help confirm timing was done correctly or not or if you recall what your ignition module looks like using these two links.

TB 4, http://www.ebay.com/itm/NIB-Mercrui..._Accessories_Gear&hash=item4625075b68&vxp=mtr

TB 5, http://www.ebay.com/itm/OEM-MerCrui..._Accessories_Gear&hash=item5d301137ee&vxp=mtr

 

[jgrant911]

Thanks for the info Fun Times. I went and took a closer look at the ignition module. Definitely a TB4 (pic attached) and that's what it says on the intake cover. That's why I measured the timing with no jumper attached (not required for TB4). As I mentioned, in my experience ignition timing rarely changes unless you disturb something, certainly not by 12 degrees.

I will say that it's possible the idle has been gradually getting slower and rougher over the years and I just didn't really pay much attention until it reached the point it got to this year. In that case maybe it was just a matter of setting the timing properly and be done with it. I will be keeping a closer eye on it in the future. I am still curious as to what might cause that big a timing change either as a jump or a slow change.

Cheers!

Jim

 

[jgrant911]

Okay, I just went out and did a timing check and it is right in spec (-8 deg at idle, -16 at 1500, -19 at 2000 and -22 at 2500 rpm). This is after my initial adjustment of proper idle advance the other day. I'm happy that it seems to be good now and will just chalk this up to a gradual slip out of spec over the years. I admit to being lazy about checking the timing in the past just because in all my years of working on automotive ignition I have never seen timing change unless there was an obvious cause (distributor work, timing belt work etc).

My annual tune-up will now include a timing check!

Cheers!

Jim

 

[Fun Times]

After a few runs, give it another check to see if it has moved again. It really shouldn't move much but that is part of the tune up. In the past I had found many to be off a few degrees but always chalked it up to the last person that set the timing must not have been as accurate as I.:watermelon: Looking at the paint of the distributor hold down bolt helped tell the story if the timing had been played with.

If it continues, then like you mentioned, checking all the working mechanicals of the distributor for looseness should be considered as well as the timing chain & ignition module.

If it moves again, be sure to let us know. Thanks and ah good luck.