Plugs

**Merc 2.5** · 03-16-2018, 04:40 PM

What's best plug to read for 2.4 200 ?

**FORBESAUTO** · 03-16-2018, 06:27 PM

Ngk BR9HS. What compression you running? I use the 9's for 135-160 compression. If less than that you can go the hotter 8's. Here is a good link on reading plugs for mixture, heat range, and timing.
http://www.strappe.com/plugs.html

**Merc 2.5** · 03-16-2018, 10:02 PM

Runnin 33cc heads on 2.4 200. Just installed and haven't got chance do compression which I expect to b round 140ish thanks for the great read gonna grab the plugs and tryin to get sum jets thanks

**FORBESAUTO** · 03-16-2018, 10:25 PM

Duh, I knew about the heads from the other thread. I was having one of those moments racervboat and I were talking about in that thread

. Definitely use the 9 plugs then.

**Merc 2.5** · 03-17-2018, 12:54 PM

thanks

**Merc 2.5** · 03-25-2018, 02:18 PM

How long should it take to color up new plugs for a read? Say 4k rpm 5k ? Got new plugs, jets ,heads , compression 135-140. Thanks

**Motv18** · 03-25-2018, 03:06 PM

That’s a good read, but outboards a bit different. Meaning you have to be in a restrained power mode where as a motorcycle enters a loafing mode close to 14.7:1 af or higher. Outboards run 10.2~:1 in full power and whatever between 10.2-14.7:1 keeps rpm constant.

A few min full throttle will be a good read. On the plug.

The more over minimum octane rateing you run due to alcohol free things, and more middle range the hotter plug is better. Alcohol can also warsh the plug (doing what it’s supposed to do clean things) and through a reading off. The other cleaning additives can as well.

Excessive octane can also occasionally benefit from advance. (Based on speculation of burn rates( its more cold cylinder) to start the burn sooner) I find it has more to do with alcohol fuels) normally don’t mess with it for a daily driver.

**Merc 2.5** · 03-25-2018, 06:08 PM

Thanks for Info! My bote way over rated so a throttle chop at wot is out of the question ! It will drown the boat (alot of water!] Lol. I try slow down fast as possible and turn out of wake and kill it with minimum run time after wot. Think i can still get a decent read or just go by pistons?

**Merc 2.5** · 03-25-2018, 06:11 PM

I do have 93 octane with little octane boost cause of ethanol gas. May not need all that with the compression I'm at, was just little paranoid first run. Thanks guys for your very valued suggestions!

**Motv18** · 03-25-2018, 06:23 PM

Run no alcohol if possible 91+doesn’t hurt 140psi will run it fine. Just sometimes can cause issues had a Nissan 50 that’s 5.2:1 compression stock about 90psi now that one didn’t like 91. Most are fine on it. The main thing I see is fouling plugs so we run hotter plugs.

Sounds like you you have a plan.

**XstreamVking** · 03-25-2018, 06:39 PM

http://www.biggerhammer.net/mercury/ Just Scroll down. Jay Smith talks about his piston reading instead of just plug reading. He knows a few things. Just some more info to add to your pile.

**Merc 2.5** · 03-25-2018, 09:07 PM

Thanks guys. I Think I'm really close on my jetting so far according to plugs , I gotta order a light to look at pistons tho , thanks for all the help.

**coffeeguy** · 03-26-2018, 07:58 AM

Jay Smith also said to run .040 gap on the BR9HS plug in earlier discussions.

**gmacrae** · 05-12-2018, 05:31 PM

my motor specifies BU8H surface gap plugs (laser 220)... is this a uncommon for mercs or do you guys run the normal type plugs anyway for some reason?