Budman II

iTrader: (1)

I was working on a friend's 350 Mag this weekend. He was having problems with idle quality and was getting a strong fuel odor when idling around the docks. I looked at it and found that it was pouring fuel into the engine through the primary boosters at idle. This went beyond the usual "drip" or "dribble" that is described for Q-jets - it was more of a gusher. I had previously rebuilt this carb for him (was the original carb for the engine), since he was having problems with it before. I had lent him the carb off of my 454, which was jetted a bit rich for a 350 mag, just to get him through a few boat trips. Just to rule out the 350 carb, we put the 454 carb back on there again, and noticed it was doing the same thing. That evening I pulled the original 350 carb apart to make sure there wasn't junk in the fuel bowl, and it was clean. Also rechecked the float level setting, and it looked good.

At this point I am thinking that there is not enough vacuum to pull the main metering rods down into the main jets at idle. He had the engine replaced a couple of years ago due to freeze damage, and the marine mechanic put in a used 350 with unknown hours - it has never run right since then. I actually had to set the base timing for him last year because this guy never tightened down the distributor and it backed itself off. I set it to 10* initial advance for the base timing. I didn't have a timing light with me this time to check this. I guess this is a possibility, but I am also wondering if a flat cam lobe or other mechanical issues can cause a Q-jet to do this. Has anyone run into this? I guess the next step is to hook up a vacuum gauge to see what it is getting at idle, but not sure where to hook one up since there are not really any vacuum ports on a marine engine. Leakdown and/or compression test might also answer some questions. Any ideas?

CAPTAIN JACK

Well Budman, I don't really have a pin point answer for you. But I'll give you a few things I know of Rochester, I use to rebuild crap loads of them, like more than 100, but anyway. We found a lot of times if you tip the carb over the 2 leaded plugs on the bottom would leak, we would seal them with epoxy I believe, and another thing I remember is some of the black floats would get heavy, which would act like fuel level to high. But we had a float scale and I weighed each one that I did. This might not be a lot of help but it's something to think about. My 2 cent.

MILD THUNDER

iTrader: (3)

Any chance youre getting too much fuel psi , not allowing the needle to close on the seat?

Budman II

iTrader: (1)

MT, I thought about that, but everything is stock Merc stuff - regular old mechanical fuel pump. I would have to figure out a way to add a pressure gauge in line with the hard metal lines.

I just remember reading that the power piston spring works against manifold vacuum. If the vacuum signal is too weak, then the spring will lift the metering rods up out of the jets and send extra fuel to the boosters. This is why you usually have to go to a weaker spring with a cam that has a lot of overlap - the vacuum signal is too weak and the carb runs rich at idle. This is a dead stock engine, but the fact that two carburetors are acting exactly the same way leads me to believe we have something other than the carb causing this. I would expect excessive fuel pressure blowing the needle off the seat to send fuel out the vent holes and around the accelerator pump.

Can I take a vacuum reading off of the port that goes to the choke pull-off?

compedgemarine

there are only a few things that would cause it. one is if you are opening the throttle blades too far at idle which moves it out of the idle circuit and into the boosters, too little timing can cause this. another is the float absorbing fuel and getting heavy as was mentioned. too much fuel pressure as was mentioned, and the power piston not pulling down due to too strong of spring or lack of vacuum. you can test this by cutting a plastic straw to about three inches and slipping it into the tube above the power piston. it will rest on top of the piston and as the engine starts you should see it drop and when you rev the engine it should raise up. you should be able to use the choke pull off for a test of vacuum.

Budman II

iTrader: (1)

compedge, I am going to get a timing light on it and recheck the timing, since this was an issue in the past. Also going to take a vacuum reading - I just wasn't sure if the fitting for the choke pull-off was manifold vacuum or ported vacuum.

I also read that you can get a screwdriver in there and push the power piston down manually against the spring. This will tell me if that is the issue - if the engine idles normally without dumping fuel through the boosters, then I can rule out a float issue.

compedgemarine

you can move the power piston down with a screwdriver but there is only a very small part of it under that tube so it is hard to make sure you are on it. use a small screwdriver and play with it not running so you can make sure you are able to push it down. I have worked on Q-jets for years and years and other than the small fuel resevoir they are a great carb if you know how to correct them. I had one that someone else had worked on and when they took it apart they bent the arms on the power piston up so no matter what you did it would not move the rods low enough. that one was a pain to figure out.