jetting a 2.5 245 merc

**bfly** · 04-24-2003, 06:30 PM

I've always jetted carbs by reading the plugs but I've seen a couple of posts about jetting by looking at the piston. I'm new to this method!!! My plugs after a high speed run are on the dark side. I think it can be leaned down. When I look at the piston it is carboned over on the entire piston except about the first half inch in from the intake port. That part looks like it has been washed clean. Does this sound like a motor running too rich

**WaterBoy** · 04-24-2003, 08:53 PM

What jets you runnin know, also it depends on what carbs are on it also, My old motor had WMH's without accelerator pumps.

Waterboy

**jmc** · 04-24-2003, 11:41 PM

I bought a 93 2.5 carb perf. back when. It came with 76 mains from the factory, those were the break in jets. I swapped them out for 72's and the thing came to life.

**bfly** · 04-25-2003, 12:27 PM

I've got 76 jets in there now but I dont have the carbs that came factory on this motor. I have whm's without accelerator pumps.

**WaterBoy** · 04-25-2003, 01:32 PM

I say try 72's and check the plugs, I ran 70's in mine, but I was pushing it though. When I sold it (94 245) it had 72's. If you try the 72's check the plugs, your further north if that makes a difference or not.

Waterboy

**bfly** · 04-25-2003, 06:16 PM

Thanks for the info. I think I'll try 74's first instead of jumping 4 #'s at once. I'd rather be safe than sorry.

**patchesII** · 04-25-2003, 07:59 PM

keep us up on your testing because i have the same setup as you that i haven't ran yet. so i'm a little in the dark on a starting point. kinda sucks to keep buying jets to figure out where your gonna be

**Superdave** · 04-26-2003, 08:29 AM

to jet and have confidence is with pyrometers
Dave

**jmc** · 04-26-2003, 08:49 AM

We dont need any stinking pyro man.......

Pyro's are only as good as the position of the probe or probes. Even at that most pyro gauges are actually a little conservative. My last pyro which was a double probe gaffrig was more trouble than it was worth. Too hard to read @ WOT.

**pinbuoy** · 04-26-2003, 08:53 AM

I have a 2.5/245 motor without accelaerator pumps. It has 72's in #2 and #5, and 70's in the others.

pinbuoy

**ProComp** · 04-26-2003, 10:33 AM

How come the carbs with accelerator pumps use bigger jets? Mine our 78's. Any downfalls to the carbs with accelerator pumps?

Thanks,
ProComp

**Superdave** · 04-27-2003, 06:02 PM

I agree, installation is everything, but it is the ONLY way to know the EGT temp. Or you could stick your finger in the exhaust a WFO. I have WH-28's and running 0.098's in 2 & 5, 0.096's in the rest. Pyro's read 1150°. Yes, I can lean it down a little, but plugs and oil are cheaper than pistons last time I checked LOL. And we ALL have our opinions. Notice I had no jet size recommendation. If a guy puts the wrong jets in and smokes his motor because some gearhead tells him to, he will probably be a little pissed! You could also read others opinions....go here

http://forums.screamandfly.com/forum...threadid=11844
Dave

**jmc** · 04-27-2003, 07:32 PM

I agree Dave; however, your actual temperature is probably under 1100 or right at it if it reads 1150, that is...... IF your probe is in the absolute hottest position. Most pyros show around 50 deg. hotter than actual.

**Superdave** · 04-28-2003, 12:02 AM

what's the MAXIMUM EGT reading you would want, WFO and not melt pistons? I've heard several from 1200° to 1300°, but I believe MOST aluminum melts around 1400°. Not referencing a chart at this time.
Dave

**jmc** · 04-28-2003, 01:37 PM

Well Dave as you can probably tell I dont trust pyro's. Its scary to check the plugs the old fashioned way and its not real healthy on you rods as well. Cutting power @ WOT is very unsettling for me . I agree plugs are alot cheaper than pistons and that little extra bit you gain is usually not worth the risk. I would say a 1200 deg. actual temp. would be ideal. I have heard 1300 is the max and I have heard 1300 in time spells melt down. I must say I dont miss my old carb motor but, I just hope this EFI lives as long as it did. (7 years) I never fouled one plug in my old carb motor but I fouled several sets with my new engine during the break in period...talking about a pain in the rear.