Installing new main jets when installing oversized pistons

**Davidpkl** · 11-09-2015, 11:12 PM

What does everybody do to The main jets when they bore out a cylinder? Is it best to install 2 size larger jets on bored cylinders with oversized pistons?

**flabum1017** · 11-10-2015, 12:00 AM

Generally, if you are not increasing compression, no change is necessary

**outasite** · 11-10-2015, 05:56 AM

Original jet should be fine.Just be sure to find the cause of the first failure,so there is not a second!

**dompie99** · 11-10-2015, 09:49 AM

I had a session like this in a very close past. One cylinder went out to lunch right after a complete rebuild. Pistion number one was destroyed. I rebuilt the engine but when I checked the jetting, cylinder number one (1) was two (2) size smaller (.072 instead of .074) and since I increased the compression (300X heads and .030 os.) it happened.

Now all holes are two (2) sizes larger and I respected the patern of jetting from Merc. It makes the sparkplugs ''café au lait'' a little on the dark side but the highest ROM I ran it was around 5500rpm on a sustained period.

My .02.

**Davidpkl** · 11-10-2015, 03:02 PM

Ok thanks I was just wondering if it made that much difference and was worth it to do.

**tlwjkw** · 11-10-2015, 03:07 PM

merc offers up ta 30 over for tha 2.0 but only 15 over on any 2.5.. that kinda suggest something might need checkin' when/if bigger than 15 ona 2.5.....mercs are naturally a tad fat from production anyway... jmo

**flabum1017** · 11-11-2015, 12:14 AM

Originally Posted by tlwjkw

merc offers up ta 30 over for tha 2.0 but only 15 over on any 2.5.. that kinda suggest something might need checkin' when/if bigger than 15 ona 2.5.....mercs are naturally a tad fat from production anyway... jmo

It does not matter how big you over bore it, the fuel ratio will remain the same regardless of cylinder volume. Re-jetting is only needed if you increase the compression or do other mods like porting and such.

**tlwjkw** · 11-11-2015, 06:29 AM

Originally Posted by flabum1017

It does not matter how big you over bore it,

guess we'll just hafta disagree on that......

**olboatman** · 11-12-2015, 06:37 AM

My 25hp 22 ci outboard has the same bore carb as my 25hp 31.8 ci motor. The 31.8 carb has a way larger jet than 22ci.(don't have my notebook for #s its in the shop) Just puttin this out there! Gary

**flabum1017** · 11-12-2015, 07:21 AM

Originally Posted by olboatman

My 25hp 22 ci outboard has the same bore carb as my 25hp 31.8 ci motor. The 31.8 carb has a way larger jet than 22ci.(don't have my notebook for #s its in the shop) Just puttin this out there! Gary

is the compression the same on those two motors?

That is a big difference in bore size, but when your'e talking only a 30 thou increase, it's not enough to warrant increased jet size.

I have rebuilt hundreds of engines going as much as 60 over without jet changes and never had a problem

**olboatman** · 11-12-2015, 09:46 AM

Originally Posted by flabum1017

is the compression the same on those two motors?

That is a big difference in bore size, but when your'e talking only a 30 thou increase, it's not enough to warrant increased jet size.

I have rebuilt hundreds of engines going as much as 60 over without jet changes and never had a problem

The compression is about the same BUT you are correct the displacement is a BIG difference. Gary

**rude tim** · 11-12-2015, 03:05 PM

When you increase bore size you increase the vacuum signal to the carb which will make that cylinder run richer.

**tlwjkw** · 11-13-2015, 08:32 AM

Originally Posted by rude tim

When you increase bore size you increase the vacuum signal to the carb which will make that cylinder run richer.

thats a first for me with these 'ole mercs....

this is tha internet so ya gonna get all kinds of answers.. best do what any good guy will do.. do whatever ya gonna do to tha motor then monitor it closely till ya "both" satisfied...

jan.. you have no problem with oversize and i had no problem with comp till i get ta 160 or better on "stock" fuel curves....

**rude tim** · 11-15-2015, 05:59 PM

When I say richer I do not mean it will be fat and 4 stroke. It will most likely be not even be noticed.

**Magcat 62** · 11-17-2015, 08:42 PM

[QUOTE=dompie99;2807258]I had a session like this in a very close past. One cylinder went out to lunch right after a complete rebuild. Pistion number one was destroyed. I rebuilt the engine but when I checked the jetting, cylinder number one (1) was two (2) size smaller (.072 instead of .074) and since I increased the compression (300X heads and .030 os.) it happened.

Now all holes are two (2) sizes larger and I respected the patern of jetting from Merc. It makes the sparkplugs ''café au lait'' a little on the dark side but the highest ROM I ran it was around 5500rpm on a sustained period.

My .02.[/Not all jets on v6 mercs carbs are the same from factory certain cyclinders run inherently leaner or richer clymer manual has chart for years carb1 2 3 factory sizes for cylinders 1 thru 6 QUOTE]