Carburetor Tuning

[ntheyer]

I am starting a new thread from the 1974 -1995 engine swap. I have ordered a carb kit and will go through the carb once I get it. My question is; When reading the shop manual it just says, 'Adjust Choke'. How do you know when it is adjusted well? I have a 1974 Rochester 2bbl carb with a vaccum choke, but do not know how much tension to dial in. Also, the idle speed adjustment screw location is unclear on my shop manual. I am assuming it is the screw that is with a coil spring on the carb linkage on the side of the carb. Is that correct?

 

[Mahoney]

Re: Carburetor Tuning

The idle screw is the one on the side of the carb that the throttle linkage attaches too. The screw you are refering to is the fast idle cam, that is a screw located on the choke side of the carb. Your rebuild kit should have instructions with it explaing how to make the proper adjustments.

 

[Don S]

Re: Carburetor Tuning

I usually set the choke so the choke plate just closes when cold. Most manuals give settings for them, but they seem to always be to tight and take forever to open.<br />For the idle mixture adjustment, (assuming a Mercarb) with the single adjustment screw, hook up a shop tach, and with engine running, turn screw in till rpm starts to drop and engine misses. Then back screw out till engine speed is at it's highest and back out an additional 1/8 turn. Do this several times and gun the engine a few times in the process to "Clear it out" from running to rich. Also make sure you adjust the idle rpm to specs each time. <br />Disconnect the throttle cable while doing all this so you don't have it holding the throttle open. You may have to readjust the cable when reinstalled.

 

[Mahoney]

Re: Carburetor Tuning

Don S gave you the comprehensive run down. I have the Rochester carb book here if you run into any problems along the way. After getting on the water and running at cruise, take the time to read the plugs too. This will give you a good idea of how your mixture is, and can find potential issues before they become problems.<br /><br />Good luck

 

[ntheyer]

Re: Carburetor Tuning

I just got off the water a few hours ago... I rebuilt the carb and adjusted everything. I replaced the ignition cap, rotor and checked the timing, which was perfect. The engine idles perfectly now and runs smoothly until I try to go full throttle. Basically the exact same symptoms as before the rebuild. I also replaced all filters and cleaned the fuel pump. I am able to get to about 2900 rpm and anything more than that and the engine starts stumbling. The more throttle I give, the worse the stumbling and basically the engine just slows down. Once I back off the throttle to about 1/2 to 3/4 it goes again. When I creep the throttle up just to the point where it starts choking, if I back off the throttle a hair, it actually gains 50 rpm or so after it drops. What's the issue here? Is this due to main jets that are too small? Is this fuel starvation? This is a Rochester 2bbl off a 2.5 liter and put on a 3.0 liter. If it is main jets, which I think it is, where can I get bigger ones, and what size should I get for a 3.0liter. When I rebuilt the carb, there were no numbers on the jets to describe the size. I am familiar with working on Mikuni carbs for snowmobiles and jetting, but this is a little different now. Thanks.

 

[rjohson]

Re: Carburetor Tuning

let me know when you find out i was experience the same problem. when i get up to 3000rpm the engine stumbles, i cleaned out the filter going to to the neck of the carb and it improved a tiny bit then i poured mystery oil down the barrell of the carb and opened throttle to 4000 rpm if cough and chocked and stumble until it finally smooth out. than i was able to do 25 miles an hour. the next day i went out on the water and it started to act up again when i tried doing 30 miles an hour. i poured more mystery oil in and opened throttle to blow it out. now im able to do 30 miles and hour at 4000 rpm. and it holds i went about 3 miles and it held well, but if i try and go faster it will stumble again. i personally thing 4000 rpm should let me go at least 50 miles an hour. i have a 84 cobia oddesy with a 3.0 4 cylinder engine and a 2 barell rochechester knock off carb. i also heard sea foam works good.

 

[flashback]

Re: Carburetor Tuning

sounds like you may have a fuel supply problem which is starving the engine. check the fuel tank vent, make sure it's not stopped up, then check the filters, then the pump.. good luck....

 

[AdamB]

Re: Carburetor Tuning

What do your spark plugs look like? It does sound like you are going lean, although I'm not sure switching to a 3.0 liter from 2.5 would really matter that much at 2900 rpms (I'm not a mechaninc but I don't think the different engines would require that great a difference in carb jets). I know you said you cleaned the fuel pump, but I think I would probably check that again, and make sure there isn't water in the fuel etc. Also, make sure the float is adjusted properly, or you could be running out of fuel in the carb...

 

[Don S]

Re: Carburetor Tuning

You don't have a jet problem, what you have is lack of fuel. Plugged vent, plugged fuel pickup screen on the tube going into the tank, partially plugged antisiphon valve, fuel line air leaks between the fuel tank and the fuel pump. Water in the fuel filter, or a bad fuel pump.

 

[Phantom17]

Re: Carburetor Tuning

I have this problem with a rochester 2bbl 1981 cobia 2.5 OMC. It started when I removed a homemade welch plug in the back of the choke, and installed a real welch plug. Now I get to 3300 max and the then the engine boggs down. Worked perfect before this, got 4800 easy and pulled right out of the water in no time. Difficult thing is it only runs like that in the water, tachs 5000 smooth as silk. Keep us posted.<br /><br />BTW, the homemade plug was a bolt and 2 nuts with a washer, gooped in with orange gasket sealer. Wanted something a little more reliable with my kids in the boat. The plug came with the boat......the kids didn't..... : )

 

[ntheyer]

Re: Carburetor Tuning

This makes sense to me. I had the carb place jet up three sizes, and I have the exact same symptoms at the same RPM. I just replaced the pump after reading your responses. I cleaned the one on the engine, and it was unbeleivably dirty. It was completely varnished up. I thought it was good enough, but maybe not. I'll let you know what happens tomorrow when I try the new pump. I know it's not the vent because it spurts fuel when I fill the tank up. We'll see. Otherwise, everything is working just great!

 

[AdamB]

Re: Carburetor Tuning

How did the new pump work for you? Also, have you changed the fuel filter (water separator)?

 

[ntheyer]

Re: Carburetor Tuning

No luck. The other pump I had did not work with the camshaft lobe. I am about to go outside and drain the fuel tank and check the fuel pickup. It's the last thing left other than replacing the pump with a new one. The pump seems fine though? It definitely pumps, I just don't know if it pumps enough for high RPM. I want to eliminate other things before I throw money around again. Fuel filters are all replaced already.

 

[ntheyer]

Re: Carburetor Tuning

I just got back from the lake (again) after pulling the fuel tank. The bottom of the tank was somewhat dirty, but nothing major like flakes or particles, more just like brown powder dirt junk. I pulled the pickup tube and cleaned the screen (it was ok I think) and shook the tak and rinsed it out. The vent line was clear but the antisiphon seemed pretty hard to blow through so I removed it and replaced it with an open elbow for now to troubleshoot it. The boat seems better on the water. Got it up to 3800 rpm this time and it felt more like 3/4 throttle. Perhaps the fuel pump is weak and the lessed restriction on the supply line by removing the antisiphon helped it. I will buy a new pump tomorrow.

 

[JRRoadster]

Re: Carburetor Tuning

My choke plate was open and it was still pumping gas into the motor. I have a 4 cylander OMG GM motor 3.0L. When I started it in the driveway it fired right up but did smoke alot. When i got it to the lake it didnt want to start at all. I think the gas was just dumpin into the motor while in transport. I adjusted the Idle screw and the main jet screw and it still has the problem of leaking fuel into the carburator

 

[Don S]

Re: Carburetor Tuning

Clean the bottom of the boat.

 

[veedy]

Re: Carburetor Tuning

It's funny that I come upon this whole conversation. i just bought a 1978 16' Silverline with a 120 mercruiser I/O. The whole boat is in mint shape and only 52 total hours on the original motor. When I bought it , it had 1/2 tank of fuel, and decided to top it up with some high test. We ran it with the muffs and it ran good. Put it on the lake, and was great for about 20 minutes, then it started to lose power in small spurts, like it was losing fuel. I stopped and pulled the top off the fuel filter on the fuel pump, and it looked ok. I then pulled the fuel screen on the carb. There was a small amount of sediment in it, which I cleaned out. The motor fired up instantly and idled fine, but when I applied any throttle it sputtered. I tried playing with the jets ( or air mixture screws whichever they are ) and got it to run a bit better. I think there may be a bit of moisture in the gas tank from the boat sitting so long, but I didn't get any pops or farts that is associated with fuel contamination. The vent line was open because when I topped the tank, fuel spurted out the vent line with ease. I'm wondering If I need to go deeper with such a low time engine, or just look for the easy stuff first. I may remove the tank and empty it, and start with the fuel lines, then work my way back to the motor. When it runs, this motor runs strong. I topped out at 38 mph indicated. I will use my GPS next time to get an accurate speed reading. Thanks to anyone who can suggest where I start with this problem.<br /><br /> picture of my new boat .. click here