1995 Force 40HP Surging Issues

I have a "new to me" 1995 40HP Force engine that is having some surging issues. This has ocurred ever since i have had the engine. The surging occurs most often while running at full throttle but does happen frequently at lower rpms. Idle is fine. The carb is clean and has new gaskets. The surging was present before and after the carb was cleaned/ rebuilt. I feel this is a gas issue. I have purchased a rebuild kit for the fuel pump but have not rebuilt it yet. If you squeezed the fuel bulb while running it does not eliminate the surge. Not sure if that is even a test for the fuel pump, but i did try it. I do not believe the tacometer is reading correctly but shows a rpm of 5100 to 5200 while at full throttle.

i wanted to see if the engine needed more gas so I removed the engine cover and cupped my hand around the carb while running to simulate "choking" the engine. At full throttle the engine immediately came alive. RPMs suddenly shot up to 5900 and the engine no longer surged. It ran extremely well and the speed increased by an estimated 8-10 mph. I immediately thought that I needed a bigger jet in the carb to give it more gas. I repeated this a couple more times to verify the results. Every time the engine ran perfectly. Just for curiosity, I ran the boat for a short time without my hand in front of the carb. The engine performed almost identical as when I had my hand in front of the carb. I thought I needed more gas to the engine, but it responded extremely well when just getting more air. So, I'm a little confused on this.

While cleaning the carb, I looked at the main jet. There were no markings on it so I'm not exactly sure which one is in the carb. The jets offered for this engine are: 0.60 for 0-2500', 0.58 for 2500-5000', 0.56 for 5000-7500' and 0.54 for 7500-10,000''. I thought about ordering the 0.60 jet and giving it a try. Am I correct that this Tillotson carb has no "high end" air adjustment. The only air screw is for idle, correct? Any ideas?

Thanks for your help.

Eric

Re: 1995 Force 40HP Surging Issues

If pumping the primer bulb didn't make any difference, it's most likely not a fuel supply issue. Even if the jet was the smallest one available, I doubt that it would cause a "surging" problem. Could possibly be a carburetor issue though. The idle speed passages in the carburetors do actually feed fuel during high speed operation too.

I would check for vacuum leaks. Check the gaskets around the base of the carburetor, and the carburetor adapter. Make sure all of the bolts are tight. Also check all of the small fuel recirculation hoses. Make sure they are in good shape and connected properly.

You can also use the old carburetor cleaner trick to check for vacuum leaks. With the engine running at idle speed, spray some carb cleaner in and around the intake gaskets, port cover gaskets etc. If there is a vacuum leak, it'll suck the cleaner into the motor and the engine speed will change. Try and keep any of the cleaner fumes from being sucked into the mouth of the carburetor because that will have the same affect and can give you a false indication.