omc engine problems

[zr1_lt5]

i have an 85 omc renken with a 262, and it is having trouble getting past 2500 rpms...i can rev it up to about 2500 and it sounds really rough, like detonation or something. anyway, it will eventually drop down to about 2200 and will go slow as hell...i was told it was the carburetor, but i just cleaned it and it helped a little, but it still will not go past 2500...the noises this engine makes are terrible at this speed. any ideas what could be causing this?

newly replaced or rebuilt....distributer cap & rotor, spark plugs & wires, carburetor, ignition coil.

please help me...this is driving me mad!!

p.s. Thanks to Gonzo for the help with my cooling trouble...works fine now.

[Thunderhead19]

Contaminated fuel, a leak in the fuel system sucking air (definitely worth a look, sometimes a leaking pick-up tube in tank). You may have a damaged jet in the carb. You may have a leaking head gasket.

[zr1_lt5]

well i know my gas is clean...and i just replaced the jets in the carb...any help on how to diagnose from there? how do i check to see if it is the head gasket or a leak in the fuel system?

[Thunderhead19]

Head gasket leak is easy to find, use the tool for checking compression pressure. All cylinders should be the same. Change the pick-up tube regardless of weather it looks like it needs it or not. Check for drips or stains anywhere along the fuel line from the tank to the motor. If there is any indication of a leak, replace it all. And for god's sake be careful! You might want to replace it all anyway...if ,in fact, it is twenty year old fuel line. check all connections around filters, check filters (new?) for obvious defects (installed backwards?).
You also mentioned new plugs. I've bought brand new Champion spark plugs that were defective from day one. You may be firing on one or even two cylinders short. Check air filter (probably already did that). No water in the oil? (looks like milky white scum), no oil leak?, no overheating? no raw fuel in exhaust?(sometimes backfiring), no cracks in intake manifold? not leaking manifold gasket?(I forget how to check this)

If it's none of the above, then it's time for mechanic Bob to have a look at it.

[gonzo]

have you checked the firing order in the plug wires?

[zr1_lt5]

the firing order should be right .... we had some trouble with the spark earlier on....(the boat had not run in about 5 years) we couldnt get a good spark...but that is all worked out. as far as i know their are no gas leaks...i have checked that and have not seen any. the plugs are new and are champion plugs... we had some problems with the some of the plugs not firing but that was worked out (i believe) now we have a problem where the engine will not start sometimes due to the plugs getting covered in carbon. every weeks or so we have to pull the plugs and clean them. (since they are all covered in carbon i am guessing that they are all firing?) i was told this could be because the mixture is too rich. however i have also been told that this problem with the engine not getting above 2500 could be because of the mixture being too lean....is changing the jets the only way to change the mixture? (rochester 2gv) the air cleaner is clean the oil is oily (no water) and except for a small leak at the pan it is fine. we check the oil level after every run. there was a problem with overheating (which has made a comeback i believe as of yesterday) the exhaust manifold water passages were getting clogged with carbon (thanks to gonzo with diagnosing that). there is some gas in the exhaust which again makes me think that it is running rich. it was backfiring a couple weeks ago but that seems to have cured itself...the manifold has no cracks...but one confusing point...the choke has a vacuum line that leads to one of two identical, adjacent ports in the manifold. the other port is open. just sucking in air...???...the manifold gasket seems fine to me

[zr1_lt5]

one more quick note about the overheating....it has never overheated above id say 1700rpm so i dont believe that is what is causing this seeing as how it only happens at around 2500

[gonzo]

The choke uses heated air to open. It goes in throught the hole. An overflowing carburator could cause the rich mixture. That is, the float valve not closing. A too high float can do it too. Black plugs will have a weak spark and make you loose power. Also, you are using a lot of gas. Do you know what model carb it is and the jet sizes?

[cyclops]

I would get a good check of your choke vacume port and use the right one. Also check that the choke valve in the carburator throat fully opens by 80 to 100 degrees F. Those 2 conditions can cause all your bad problems. Very easy to do on a carburator rebuild.

[zr1_lt5]

i was thinking that it could be the float but i wasnt sure...the carb is either a rochester 2gv 2gc or 2g and i dont know what size jets are in it

[arees]

Hey dude,

i hope i can help, maybe you should try to check the strainer in turbo charger, or maybe you should take it the manufacturer for engine overhaul. i think you should do that, dont try to touch anything which is not looks familiar to you, maybe ask for the assistance of the manufacturer, i think it will help.