Nissan 90a will not idle

[rogerwa]

Late last year when I pulled the pontoon out of hte water for winter layup, the engine was having a problem that I deferred looking at until now.. It would not idle. It would peter out and die at anything below 2000 RPM. Above 2000 it would run great.

I had pulled the carbs two years ago, but decided that is where I would start. I pulled them apart and they appeared very clean and I blew air through the passages. I would be very surpised if they were clogged. In fact, I could see daylight throught the idle circuit.

One anomaly I did find is that the float did not seem to be set correctly. The measurement from the manual says that it should be set at .76" +- .059". All three of mine were set closer to .625 (5/8"). At this measurement, it would put more fuel in the bowl that the specification would call for.. Could this be a cause of my issue?

If so, how can adjust these. I have adjusted floats before and this one doesn't seem to have anything to adjust. It has a plastic float and just a flimsey spring thing to attach to the needle.

Also, I pulled the fuel pump to examine whether it could be at fault. I did not find any tears in the membranes, althogh it definitely looked used. How taught should the membranes look?

I had another question about the fuel pump. My manual saya that the spring that is used on the engine side membrane is no longer needed. Does this mean that the new kits do not require it or should I pull it out??

I am looking at fuel as it doesn't look like compression issue (I believe I checked it last year and it was normal), and electrical works at high speed so I would think it is fine at low speeds..

Any help on diagnosing this would be great. I did not reassemble yet as I was seeking additional info..

 

[pvanv]

Re: Nissan 90a will not idle

Typically, float height on these is set so that the float is precisely parallel to the carb casting when the carb is inverted.

It would be rare that the height would be off... If the float height does in fact need to be changed, I know of one tech who warms the tab (with a heat gun) and then bends it. That's a bit risky, IMHO. We would likely replace the float when doing the needle and seat. Most carb parts are still available for the 90A.

The light spring is there to help the needle track the float and not bounce around.

Fuel pump delivery seldom affects idle -- more typically, if it was delivering low volume, the high speed would suffer. However, a minor pinhole could allow extra fuel to get into the crankcase, which might very well cause the motor to load up when near idle, yet work fine at speed. Plug condition should tell you whether the plugs have been running unusually rich. Look carefully at the lower plug.

I would check the condition of all 3 plugs, to help determine whether all 3 cylinders are firing correctly. Sometimes one poor cylinder will show more symptoms near idle. Of course, verify compression and ignition, just to be sure. The proper procedures and test values are in the Factory service manual. Please be sure that you aren't relying on an aftermarket "doorstop" manual.

 

[rogerwa]

Re: Nissan 90a will not idle

I replaced the floats two years ago and the positioning seems in line with what you describe (which is out of spec with the manual).

I did pull the plugs and they all look uniform. There was not excessive fouling, just a little extra oil mixture which is pretty consistent with what the plug should look like.

I also verified the measurements of the timing components based on the factory manual. Is there any other timing related info I should be looking at??

I will check the carb passages again with compressed air and also check the fuel pump membrane for any pinholes.

 

[pvanv]

Re: Nissan 90a will not idle

Sounds like you're on the right track. Verify the carb passages are perfect, possibly replace the pump diaphragms, and verify no intake vacuum leaks at the carb bases. Also check the reeds for cracks or distortion.

One overlooked area on old 2-strokes is the crankshaft seals. Since the crankcase needs to alternately have pressure/vacuum to run, a slight seal leak can make starting and idling problematic. A pressure test of the crankcase can verify the seals.

 

[rogerwa]

Re: Nissan 90a will not idle

OK stupid question. To pressure test the crankcase, can I do a compression test and just let the guage hang there for a while? Or how would I do that test..

 

[pvanv]

Re: Nissan 90a will not idle

Nah, it's more involved than that. Compression leaking by the rings would slide right out through the carb. Bacically a block-off plate is installed in place of the carbs and then that is pressurized.

But... if you don't have any oil/gas residue near the crank seals, you are probably OK.

 

[rogerwa]

Re: Nissan 90a will not idle

OK I cleaned the carbs thoroughly this past weekend. Used a can of compressed air with the little tube as well as a can of carb cleaner spray. I found gunk in one and maybe two of the low speed jets. When I pulled them out I could not see daylight and when I blew them out, it left a dark spot on the towel. So I think I may have found my issue..

When I reaasembled and tested, I was able to get her to idle down and smooth with no coughing like it had done last year. So I am optimistic I am in the right path. The boat is still on blocks so I don't have a water test yet.

The only problem I had is when I ran it and tilted it up a bit. Nothing huge may be 5-10 degrees or so.. It started to slow down and then kill. I checked to ensure there was no restrictions in the fuel line. Any idea why that might be??

 

[pvanv]

Re: Nissan 90a will not idle

Go through the carbs again. Do a thorough and proper dip cleaning as opposed to the "spray can" treatment. Afterward, verify link and sync of the carbs, and you're probably in good shape.