Rochester 2-gv carburetor running lean (From lake to ocean)

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
I recently purchased a boat with a 5.7L mercruiser and Rochester 2-GV. It was ran at a lake at an altitude of 6800' and most likely was tuned for that. I am using it at sea level. I've taken it out once so far and I believe it is running lean. The first and only time we took it out, the engine overheated at 2/3 throttle. We let it cool down and ran it for another hour at lower rpms (dumb, I know). Didnt overheat again. I pulled the spark plugs and they show a pretty lean condition (it was close to white at the tips). Do I have to rejet the carb?
I really would like to be on the water asap and don't want to waste time rejetting. Is there a carb I can purchase that is more easily tuned? I've spent quite some time redoing the upholstery, wood floor,and impeller. I'm burnt out. Summers almost over and I just wanna run the damn thing.
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,056
The first thing to address the cooling system. Remove the hose from the drive pump to the thermostat housing with the flush out on the drive and see if you can stop the water flow with your hand at idle , if you can then there is a leak between the drive and the engine. If not then check the exhaust risers by connecting a garden hose to where the water exits to be sure there is no blockage. Be sure there are no restrictions in the thermostat housing and also replace the thermostat while you are at it. Whe you replaced the impeller did you replace the entire pump ?
 

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
The first thing to address the cooling system. Remove the hose from the drive pump to the thermostat housing with the flush out on the drive and see if you can stop the water flow with your hand at idle , if you can then there is a leak between the drive and the engine. If not then check the exhaust risers by connecting a garden hose to where the water exits to be sure there is no blockage. Be sure there are no restrictions in the thermostat housing and also replace the thermostat while you are at it. Whe you replaced the impeller did you replace the entire pump ?


Yes I changed the entire pump. The risers have been removed and inspected and new gaskets were installed. We probably ran the boat close to 2 hours after the overheating issue with no problems. No overheating at idle either and runs good.
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
You'll burn up the engine running it lean, so there is no other way around fixing the problem outside of getting the carb jetted properly. You could buy another carb already jetted properly, but it's not going to save time or money.

Once you get the proper jets (and a gasket set in case you tear a gasket) it should take 1/2 hour, maybe an hour at the outside. I re-jet for altitude 1-2 times a year on a 2GC, takes 15 minutes since I've done it so many times. Remove spark arrestor, remove top off carb, empty bowl, swap jets, put everything back together. One tip, make sure the screw driver you're using to remove the jets fits the slot "perfectly". I put the screw driver in the jet slot, give the handle of the screwdriver a whack to help loosen the jet, then remove jet.

I would also check the engine's timing to make sure it's set to spec. While not advised by Mercruiser, timing can be advanced at altitude to regain some lost performance, and who knows what the previous owners did to the boat...
 

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
You'll burn up the engine running it lean, so there is no other way around fixing the problem outside of getting the carb jetted properly. You could buy another carb already jetted properly, but it's not going to save time or money.

Once you get the proper jets (and a gasket set in case you tear a gasket) it should take 1/2 hour, maybe an hour at the outside. I re-jet for altitude 1-2 times a year on a 2GC, takes 15 minutes since I've done it so many times. Remove spark arrestor, remove top off carb, empty bowl, swap jets, put everything back together. One tip, make sure the screw driver you're using to remove the jets fits the slot "perfectly". I put the screw driver in the jet slot, give the handle of the screwdriver a whack to help loosen the jet, then remove jet.

I would also check the engine's timing to make sure it's set to spec. While not advised by Mercruiser, timing can be advanced at altitude to regain some lost performance, and who knows what the previous owners did to the boat...


Where can I get the proper jets and how do I figure out what size? Would you happen to know what size I should use for sea level? I will also check the timing
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
You'll need to hunt down a good carburator shop, and may have to order online. I'm in a pretty good sized city, called all around locally and couldn't find anyone who had what I needed, eventually ordered online from a shop in southern California. No idea what the shop was called, but seem to remember it had a person's name in it: Ernie, Bob, Sam?....

It would probably be best to pull the existing jets to see what they are now, so you know what your starting point is. Then fatten up the jets accordingly.

Post up your engine serial number and carb number. Some of the Mercruiser manuals have jetting information in them.
 

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
You'll need to hunt down a good carburator shop, and may have to order online. I'm in a pretty good sized city, called all around locally and couldn't find anyone who had what I needed, eventually ordered online from a shop in southern California. No idea what the shop was called, but seem to remember it had a person's name in it: Ernie, Bob, Sam?....

It would probably be best to pull the existing jets to see what they are now, so you know what your starting point is. Then fatten up the jets accordingly.

Post up your engine serial number and carb number. Some of the Mercruiser manuals have jetting information in them.


Thank you so much for the help. Engine serial number is 0F769808 MCM 5.7 thunderbolt V
The numbers on the carb reads.
7041114
357 0 ? DB
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
Looks like you need 1.65 jets - Mercury Mercruiser Marine - JET-1.65 (#3302-9058)

Google your engine serial number (I used perfprotech) and drill down in their carb exploded diagram, shows what's stock in the carb.

Remove what you have before ordering to be sure, guessing you have 1.50 jets in there now, 150 stamped on jet. Also, in case you tear the cover gasket, so you can order a carb kit. Carb kits are cheap on Ebay, $30. Jets are on Ebay too. You most likely have a "Mercarb", not a Rochester, if the carb has 1 idle mixture screw it's a Mercarb. Rochester carb's have 2 idle mixture screws.

If I was swapping jets, on a carb that I didn't know the age of the gaskets, I'd be replacing the top gasktet.
 

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
Looks like you need 1.65 jets - Mercury Mercruiser Marine - JET-1.65 (#3302-9058)

Google your engine serial number (I used perfprotech) and drill down in their carb exploded diagram, shows what's stock in the carb.

Remove what you have before ordering to be sure, guessing you have 1.50 jets in there now, 150 stamped on jet. Also, in case you tear the cover gasket, so you can order a carb kit. Carb kits are cheap on Ebay, $30. Jets are on Ebay too. You most likely have a "Mercarb", not a Rochester, if the carb has 1 idle mixture screw it's a Mercarb. Rochester carb's have 2 idle mixture screws.

If I was swapping jets, on a carb that I didn't know the age of the gaskets, I'd be replacing the top gasktet.

Is the top gasket you speak of included in the carb kit? It is a Rochester, there are two idle mixture screws. I will be ordering a carb kit and doing a rebuild as well. The carb is very old and it looks like there is a very small leak (fuel). How did you know 1.65 is what I need? Do I also need to change the "rod"? From what I read, the jet is for WOT and the rod is for cruising speeds. Thanks!
 

mtb_prodigy

Cadet
Joined
May 2, 2017
Messages
26
I just pulled the carburetor, I'll be taking it apart in the next hour to find out what jet is in there. I'm hoping to get the boat out sometime next week.
 
Top