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NS30A3 - Bogging/Not reaching WOT

7

79cj7stone

New member
Nissan NS30A3 short shaft. 30hp, 2 cylinder, 1 carb

Issues: Anything over a slight increase in throttle from the start position when in gear and under a load, the engine bogs. Seems to run fine and will reach higher RPM when on muffs.

What I have determined so far:
Engine sat for probably 6 plus years in a barn. Pulled carb and cleaned. All ports were surprisingly clear. Reinstalled and engine fired up. Took on test run and encountered the issues above. I found a mouse had gotten in to the wiring harness and had chewed some of the insulation off the wires but did not damage the copper. Those have been taped up for the time beings. The main 2 were the red exciter wire and the the black and yellow wire off the coil. I verified resistance between the red and black and got 250 ohms and blue and black and got 37 ohms which is within spec on a CDI electronics sheet. Ground wire was also bare, so I replaced that.

New plugs were installed. BR7HS-10 gapped to .35 and issue is still present.

I put a inline spark tester (just the light blub one) and I am getting a light to each cylinder. I am planning on getting a spark gap tester today, setting it to a 7/16th gap and testing again. When I pulled the plugs, the top plug looked to be burning decent and the bottom plug looked like i just took it out of the box. Reinstalled the plugs and pulled the wires will running. I pulled the bottom plug wire, engine stumbled some but continued to run. I pulled the top plug wire and engine immediately dies. To me this means a lack of combustion in the bottom cylinder.

Compression check using electric starter: Top 80 psi and Bottom 80 psi
Compression check using pull cord: Top 88 psi and Bottom 88 psi

I am struggling to find a spec on the required cylinder compression. Anybody have that?
So I have unofficially verified spark, verified compression even though I think this is on the low end, so now I'm on to fuel.

Couple questions:
I believe this is a wasted spark ignition. Could I only be receiving primary spark on the top cylinder and wasted spark on the bottom cylinder and no primary spark on the bottom making it look like I am getting spark but in reality I'm not. I a little weak on this ignition system.

Could a weak fuel pump be causing fuel to the top cylinder and not the bottom?
Is the reed valve the only other possible blockage for this condition? On engine fuel filter screen is clear.
Any other possible ideas on this?
 
Thanks. Reed valves looked good and functional. Passage between crankcase and cylinder was clear. Cylinder walls looked fine. When removing the head the bolts were rusted and one broke. Any idea on where to pick up ten of those?

I'll run back through the carb again since I have it off. I am waiting on gaskets and seals to come in so it might be a little bit before its back together.
 
I got the engine put back together over the week end and tested. I replaced all seals and gaskets in the engine. Also new head gasket. Compression jumped up significantly with these changes. It was at 90 psi and is now 150 psi. I tore the carb back down. There was an open hole in the low speed jet but after some additional cleaning it did open up bigger. I also put a new fuel pump due to a nipple I broke off. It is now not wanting to idle well at all. Lowest I could get the idle was 1000 rpm and it was pretty rough. When putting it in gear, if i didn't immediately give it throttle it would kill itself. I am still having the same issues with the bottom cylinder not combusting. I switch the plug wires around and the same thing is happening.

I have been thinking timing might be off but it is not making sense to be how one cylinder would be firing while the other isn't if it was a timing issue. There is a screw on the carb that sets the carb idle. The tiller affects both the butterfly valve on the carb and the timing ring. The carb screw stops the butterfly valve but the timing ring continues to move which seems to be when the engine dies on the idle.
Do you know the timing specs?


Is there another passageway that I might have missed? Internally in the engine I do not see how there could be a blocked passage as the the intake ports are big. How I am understanding the 2 stroke to work is the carb is spraying fuel while as is mixing to form the fuel/air mixture. This is sucked through the reeds into the crank case. On the exhaust stroke the the spent vapors are flowing out of the exhaust side of the cylinder wall while the new fuel/air mixture is sucked in the intake side.

Do you have a method for verify that I am getting fuel to the cylinder?
 
Plug is wet. I got a spark gap tester yesterday. Set gap to 7/16th of an inch and consistently received good spark from one wire. Bottom other wire would not jump the gap. I closed the gap to maybe a quarter of an inch and received spark but it was inconsistent. So weak spark and inconsistent spark.
 
79cj7stone said:
Plug is wet. I got a spark gap tester yesterday. Set gap to 7/16th of an inch and consistently received good spark from one wire. Bottom other wire would not jump the gap. I closed the gap to maybe a quarter of an inch and received spark but it was inconsistent. So weak spark and inconsistent spark.
Click to expand...
Check the magnets on you flywheel. Loose or broken
 
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