missing cylinder

[Alohawhaler]

Firs,t I want to thank all the contributors to this site because it helped me so much in rebuilding my '67 whaler and tinkering with my engine. Unfortunately, my 2001 Nissan NS25A, four stroke, is not as nice as my whaler is. I bought it off a pretty shady guy, and it was obvious that it had some issues. When I first checked it out the engine purred like a kitten and seemed to run great on the back of this guys boat. The second time I checked it (the price was reduced dramatically) it did not run as well. So I compression tested it, all cylinders were within 5 psi of each other, and bought it. After cleaning out all of the carbs and putting in new kits, new spark plugs, a new coil, water pump, fuel lines and oil it worked muuuuch better. However, it still ran a little rough. I have to push the throttle forward a number of time before it starts, and if I lift up the choke it dies completely. When I did the water test it will ran on idle all day long and purrs until about half throttle, however, it does absolutely nothing when I increase the throttle, even when full. It does not die, but the engine does not seem to react at all to the increased throttle (and I know the linkage is okay). I removed the spark plugs and saw that there was some gray residue on the top two plugs, and the bottom plug looks brand new, but a little "wet." So I ran it again and when I removed the bottom plug boot from the spark plug there was no change in how the engine ran. I spark tested the coil to that cylinder and it had a good spark. I also changed the plug, but still no change. I have read similar problems which have been caused by either a faulty fuel pump or a ruptured head gasket. However, none of the previous posts mentioned anything about the engine dying when the choke is lifted up. So I am hoping that someone can help diagnose the problem based off this and the other problems. Thanks so much

 

[Sea Rider]

Re: missing cylinder

Sorry, wrong data, errase please.

 

[TOHATSU GURU]

Re: missing cylinder

If nothing happens when you pull the boot off on a plug that means that you have a dead cylinder. If you have spark to that cylinder, then that leaves fuel or mechanical. If you have compression, then you have eliminated mechanical(probably...You need to do a leak down test to be sure). That would indicate that your problem is fuel system based...If you have accurately described the problem. Revisit the carbs. After that I would urge you to get it into a dealer. Your probably going to be better off with the short term expense of the dealer finding the problem rather than the long term expense of you finding the problem

 

[pvanv]

Re: missing cylinder

Please clarify your model and symptoms. If you have an NSF25A 4-stroke, there is no conventional choke. This 3-carb motor has an auto-by-starter enrichment solenoid on the upper carb.

If so...

When you refer to the "motor dying when the choke is lifted up", I suspect that you are actually lifting the warm-up lever on the remote control (RC) box (assuming the motor is an EP or EPT type). This lever is for motors with a conventional choke (not the auto-by-starter models), and is intended to increase idle speed during warm up, to help maintain intake velocity, which helps prevent fuel dropout in the intake manifold when the motor is cold. Again, if you have an NSF25A, you do not need to raise this lever. See the owner's manual at http://www.nissanmarine.com/tech_talk/pdf_files/Nis25_30_4str.pdf

If you have a "dead cylinder", and have verified compression and spark for that cylinder, you probably have a fuel issue, as Elvin mentioned. Of course, a leak-down test would be more definitive of the compression condition. I would review the carb settings, paying particular attention to the lower carb. If the float is not engaging the needle at the correct point, you could be dumping a lot of extra fuel to that cylinder, causing that cylinder to be way too rich to run.

 

[Alohawhaler]

Re: missing cylinder

Thanks. Yes, I used incorrect terminology. It was the warm up lever on the remote control that caused the engine to cut out. Unfortunately, after Guru's post I wanted to make absolutely sure that it was not the spark, so I went to test all the coils again, and my middle coil, not the cylinder I was having trouble with, broke. So I am waiting for two new coils to come in. In the mean time I will build the fuel tester, thanks to this forum, and check out the bottom carb. Hopefully it will not take too long to get the parts in so I can test the seals and the carb. Thanks again