View Full Version : fuel system problem

07-30-2008, 12:14 PM
I have a 1990 pro star with Ford 351. Here is a chronology of my problem and am wondering if anyone can help me figure this out.

1. Boat was running rough with backfiring occurring. Changed plugs and boat ran good for several hours. Old plugs were fouled heavily with black colored stuff.

2. After replacing plugs, boat again exhibited rough running and some backfiring.

3. Replaced choke controller thinking it to be the problem and installed carburetor kit. Boat seemed to run well for several more hours until it began to use a lot of gas and with black smoke coming from exhaust.

4. pulled spark plugs and discovered that two of the eight plugs were a bit loose. All of the spark plugs were fouled with same black residue except the loose plugs those plugs were still basically white on the porcelen.

We think we have a rich fuel mixture that needs to be addressed.

a. We think we should adjust the choke controller to a more lean setting. Will this solve the rich fuel at every speed? Or is there another setting or procedure that will get us a lean setting?

Thanks everyone

07-30-2008, 01:49 PM
Black smoke and the fowling is from improper air/fuel mixture. To adjust the carb: warm up the boat to operating temp, then rev the engine and wait for it to idle back down, one side at a time, tighten the adjustment screw (slowly) until the engine just starts to stumble, then back off one-half turn, rev the engine again before doing the other side. That's all there is to the air fuel adjustment. Sometimes with a bad enough backfire you might need to replace the power valve. You said you installed a carb kit. One should have been included. Carbs can be a real headache. Everything must be squeaky clean and sealed properly. Spray a little starting fluid (use the straw) around the base gaskets while the engine is running. If it speeds up there is a vacuum leak. That should be addressed. The choke should be adjusted properly as well. The engine off and cool, lput the throttle about 1/2 - 3/4 open, loosen the 3 screws and turn the choke until it just closes, tighten the screws.

07-30-2008, 02:04 PM
Once the motor warms up, the choke does nothing. That's only used to enrich the mixture when it's cold anad even then, if the air temperature is high enough, it'll already be open. Your idle and mixture screws are what need adjustment. If you replaced the plugs, it started running rough and the wires are more than a few years old, they may be past their prime and are leaking. If you can run it at night with no lights, look for arcing between the coil wire and the tower, plugs and block, plug wires and other plug wires, cap terminals and plug wires, etc. If you see this, it needs to be addressed. Good, new plug wires are worth what they cost. Likewise, so are cheap ones.

07-30-2008, 05:47 PM
Thanks for the posts. I don't know carburators so don't know how to locate the adjustment screws to a change the fuel/air mix. Can you help?

07-30-2008, 07:21 PM
they are small, about the size of the eraser on a pencil....brass flathead screws on either side of the carb. A little turn can make a lotta difference. Hope this helps. I'd try to do the same or close to the same turns on each side.

07-30-2008, 07:38 PM
The fuel/air mixture screws are for idle only.

07-31-2008, 10:13 AM
I believe there is an idle screw as well as two screws for fuel/air mixture on each side of the carb.

Can anyone confirm?

07-31-2008, 11:58 AM
I believe there is an idle screw as well as two screws for fuel/air mixture on each side of the carb.

Can anyone confirm?

You are correct. According to the SKIDIM website the technique I described (adjusting the two screws on the sides) is suggested to adjust the carb. In my '87 that is how I did it.

07-31-2008, 02:10 PM
Here it is cut and pasted right from the SKIDIM website


1. Rev up the engine to clear it out.
2. Turn air screw in until you hear engine begin to bog and miss.
3. Now, turn screw out with 1/2 increments until you hear that the engine is steady and running smoothly.
4. Repeat procedure on air screw on opposite side of the metering block.
That’s it!

07-31-2008, 02:40 PM
Thanks Angry Bill.

Could the floats inside the fuel bowls be allowing to much fuel through? Do you think the floats need to be adjusted accordingly so less fuel passes through?

08-01-2008, 09:56 AM
Thanks Angry Bill.

Could the floats inside the fuel bowls be allowing to much fuel through? Do you think the floats need to be adjusted accordingly so less fuel passes through?

Usually with a float problem you will see fuel spurting out the rounded over tubes on top of the carb (directing the extra fuel back into the carb). Adjusting the floats is important.

I bought my '87 used in '00. I had black smoke coming out of the exhaust along with some performance issues. I had to rebuild the carb and replace several components. Carbs can be troublesome. Everything must be thoroughly cleaned. Valves and adjustments must be precise. There can be no vacuum leaks.

o A good carb kit from SKIDIM (4160 specific) is like $40.
o A tank of gas in your boat $60.
o A happy carb - priceless!

08-03-2008, 07:57 PM
Update; spent the weekend trying to correct my problem, too much gas, too much rich gas getting into cylinders. I put new plugs and plug wires in just to make sure that we could eliminate these issues. Also took the bowls off of the primary and secondary and checked the clearance for the floats. They were ok. So we fired the engine up then turned the adjustment screws to zero and back them off two turns. Unforntunately, we were not able to get to a tuned carburator because as we turned the adjusting screws in, nothing changed in terms of the engine beginning to lose its tune. We got to within 3/4 turn from base and the engine ran the same at about 500 rpm. What to do? Is it time to install a new carburator?

08-03-2008, 08:07 PM
Those screws go into small cork washers. The screws and the cork can be replaced.