I have been trying to get my motor timed....355 roller motor using stock Mercruiser dist and ignition (TBolt IV). When I try to set the damn thing the mark jumps about 3 degrees up and down. It does this at idle and at full advance. What should I look at that might cause this... The motor has 40 hours on it. I know the gearheads here could probally spit the answer(s) out quicker than it would take them to type it out but give me some pity!

"Bad-Habit"

Loose rotor on the dist. shaft, worn teeth on the dist. or cam, loose timing chain.

I would start with TB's suggestions first. It is probably on of those problems. You are running a roller cam, do you know what the end play is on the cam. Is this a new problem or have you set timing prior to this personaly.

Are you sure it is a Thunderbolt IV?

F3LS, If you're using a roller cam then it could be "walking" on you. This will change the timing. Or it could be like what Tinkerboater has suggested as well. Also, it could be the magnetic pick-up within your distributor.

Hey Dennis---if you're THE Dennis Moore, I got your book on Marine Performance on the BBC from HP Books---enjoyed it and good ref material. BTW, in your book you went into great detail about the flat tappet solid lifter vs. the flat tappet hydraulic lifter, but you never really addressed the flat tappet solid lifter vs. a hydraulic roller lifter---not that I could find anyway.

My question is: Which one makes more torque and horsepower from about 3000rpm up to 5500-5600rpm in your opinion? I know the solid flat tappet is less expensive and I understand how the mechanics work between the solids vs hydraulics-----I just want to know more from the torque/horsepower and reliability standpoint. That is IF you have a minute to share---thanks in advance!

Listen, I don't want to steel F3LS's thunder from this thread about asking about his timing problem. Sorry

Mark/KAAMA

have you tried another timing light? also make sure your inductive pick up is not getting signals from other plug wires.

Well Just a few more notes....Yes it is a Tbolt IV (24R I think). The dist and ignition box are from the old motor 1986 5.7 The old pickup in the distributor LOOKED bad with some rust on it so I replaced it when switching the distributor over. The pickup was working but I just didn't like the look of it. I had never timed the old motor since it was bone stock and was running great just tired. I had noticed that the old Cap which was only one year old did show exsessive pitting.....A new cap and rotor was installed along with a new MSD Blaster II coil. New wires and of course plugs. I am wondering though if my builder put loc-tite on the rotor.....I have read a post that mercury says to do this. I never heard of this on a auto. I plan on checking that first. Also I have used both of my timing lights with the same results. Both at idle in gear and at full advance. If the rotor checks out good then I will need to pull the distributor correct??? And look for what? Uneven wear on the gear teeth? I used my old distributor gear. It looked great and comp cams said if it looks o.k. I could re-use it... If the gears check o.k. then I need to check the camshaft for end-play or walking correct???? O.K. Now thats wear Im lost how do I check for that
Well I will let everyone know the results when I find out.
Thanks

"Bad-Habit"

Bad-Habit,

How much up & down play do you have in the distributor shaft??
I recently had a similar problem and it turned out to be to much play in the distributor shaft, easly fixed with the proper shims. There shouldn't be much more than .010 up & down play.
You will need to remove the distributor to check this, not really a big deal. If you are not comfortable doing this I am sure we can walk you thru it step by step.

Bad Habit, also take a look at the 3 screws that hold the shutter to the rotor, I have not seen it in a while but I have had them loosen up on me. Also look for side play in the shaft at the bottom of the distributor. The bushing has a habit of wearing out.

An oil pump / pan problem can also give you flaky timing, but probably not at idle as you describe.