imq707s

I've got a 1985 21" Scarab with a 1995 Jasper Marine 350 chevy motor. It also has the stock Mercrusier Thunderbolt breakerless ignition on it. Can some of you guys give me some info on this ignition system. When the motor is idleing it's getting around 8 deg. of advance, and at 2500 rpms it's getting around 20-25 deg. of advance. I've always set the timing on my 350 street car motors to have all the timming (about 38 deg. ) come in by at least 3000 rpms. I'm sure it's different for boat motors though. What should my total advance be, and where should it all be in by? What should the initital timing be set at? Is there any way to adjust the total timing without changing the initial timing at idle? Thanks for the help.

BadDog

IMQ,
Find the ignition module, it will have an identification stamp on like 24R or 20R. That is the amount of advance that will be added to the base timing. If you have a 24R module with a base of 8 then you should have 32 total advance.

The advance is probably not full in at 2500. Depending upon the part number of the module they are not usually in until around 3000 rpm. A service manual will show the advance curve. If you can safely check the timing at near WOT rpm set it that way. So if you saw say 30 at WOT and the base was 8 you may want to bump the base up to 10 for a total of 32. Some modules do not provide exactly what they are stamped at. I would not run more than 32 total. Others on this board may be able to provide you with a good number for your total but it will be less than what you are accustomed to in the auto world.

Tim T.

bowtie

Back in 85 the modules were not stamped. The base timing is set at 8 btdc at idle, 650 to 700 in gear. Merc has no spec for maximum advance but you are pretty close where you're at.

imq707s

Thanks for the help. I'm just trying to get a little more power out of the motor and I know that playing with the timing will sometimes get you some decent gains in power. Isn't 32 deg. total advance pretty low on a low compression small block chevy? If this motor was in a car I'd be running 38 deg. total timming with no problems at all. Are the thunderbolt ignitions pretty good, or would I gain a lot from taking off the thunderbolt and installing an MSD timing computer and MSD ignition box?

Crazyhorse

Assuming you have the aluminum module (big square aluminum box bolted to the exhaust manifold), you might be able to figure out the advance by part number
390-9607A3=22 degrees, in at 3500
390-7804A3=24 degrees, in at 3750
390-9571A1=20 degrees, in at 3000
I wouldn't go any more than 34 degrees total. I like to stay at 32 total just for safety's sake.

BAD-HABIT

I have a 86 Thunderbolt IV and It had a yellow warning sticker on it. It was on the side of the box facing up. I took it off and got the numbers. GET A COPY OF DENNIS MOORES BOOK about Small Block Chevy Marine Performance. It's many of ours bible! Great info.....He lists all the modules and gives the advance curves. MOST of the 5.7 260hp motors had 24 degrees advance by 3750 rpm's. The difference is in how fast the advance comes on. As far as the boxes go If your not running over 6000rpm's and or running one hell of a cam the Thunderbolt IV is fine. By the way how do you like your Jasper engine? I ordered my short block thru them. I went through the performance dept and had them custom build mine. Roller block with roller cam,4 bolt-mains,Keith Black pistons etc.......I got a great price thru my buddies shop. At his cost!! Hoping to put it in this weekend! Don't forget to GET THE BOOK!!!!

"Bad-Habit"