fbpx

 

Well I didn't think that I would end up here asking for help, but here I am. My 912ULS has developed a notable miss at all revs. Carbs have been removed, cleaned with all new diaphragms / orings / gaskets, needles and jets, over and over again, with no change each time. Floats weighed and in spec. Fuel level in bowl is 1/2 inch from the top of the bowl, with the floats sitting in the fuel perfectly. New spark plugs, new carb sockets. Carbs synched. Using an IR temp gun, aiming in line with the exhaust flange nuts, all seem ok except for cylinder #1, about 30 degrees difference. No difference in mag drop between A and B, both around 100RPM. Plugs on cylinder 1 were found to be sooty, both top and bottom. All others were ok. Swapped out plugs and no difference. So here lies the question. Cylinder 1 sooty, Cylinder 3 ok....same carb...Mind you, this engine ran perfectly before overhauling the carbs. 

Moving forward, my plan is:

1. to run engine at 3000RPM up to normal operating temp, shutdown and check plug color / condition. Do the same again around 2000.

2. Squirt some ether or similar between carb and intake port to try and identify any leaks.

3. Leakdown test on all cylinders.

4. Standard compression test.

Any suggestions / advice will be gratefully received.

Rod.

  • Re: 912 ULS miss

    by » 4 months ago


    Check that the spark plug boot/connector has not been pulled partially loose from the wire or coil. This can can happen when pulling plug wire off the spark plug when removing plugs without knowing it. A quick check with an ohm meter to check the plug connector, wire and coil should read about 15K ohm in series is to measure in series the top front left to right side, the top back left to right and then do the bottom the same. If the plug connector has been pulled partially off cut the plug wire off about a 1/4" and resemble with dielectric grease.

    https://441py33rout1ptjxn2lupv31-wpengine.netdna-ssl.com/wp-content/uploads/tech_docs/tds/22058.pdf


  • Re: 912 ULS miss

    by » 4 months ago


    Hi Rod,


    -- Are you confident that the ignition system is not to blame ?

    ------ Does the engine miss on single ignition A or B  ?

    -- Did you inspect the induction system for any leaks ?

    -- How about a borescope inspection to check the valves ? You could also pop off the valve cover and look for any obvious defects.

    -- --While performing leak down test spray cylinders with soap and water to inspect for cracks and leaks.

    -- Remember to also inspect cylinder 3. The issue may be there even if symptoms show up on cyl 1.

     

    Cheers,

     

    Denis

     

     

     

     


  • Re: 912 ULS miss

    by » 4 months ago


    Hi guys, sorry late reply here.... Thanks for your replies so far. Everything is being considered at the moment and your guidance is much appreciated. Today I have been looking at the coil packs, ignition leads and boots. Resistance of the boots are all in spec between 4.4k and 6.0k ohms. Primary side all in spec between 0.1 and 0.4 ohms. However high voltage side is 5.83k to 5.95k ohms, outside the 6.1 to 6.7. Any thoughts on this?

    Thanks heaps 

    Rod 

     

     


  • Re: 912 ULS miss

    by » 4 months ago


    It is unlikely to be ignition, but that is easily determined, as Denis mentioned, by running on one mag only and seeing if the miss continues or goes away.

    Then switch to the other mag and repeat the test.

    As you have two independent ignition systems, if the problem is truly with the ignition you will be able to isolate the trouble. if the trouble is the same on both the mags the ignition is not faulty.

    It is normal for #1 cylinder to be richer at idle, and to a lesser extent #2 cylinder.

    For an accurate test run the engine at 5000+ and then check the spark plugs.


  • Re: 912 ULS miss

    by » 4 months ago


    Thanks Rob.... I did a cold compression test on Monday and all cylinders were 125 psi... I was going to do as you say but at 3000 rpm but 5k will be better yes, and also a leak down test. Damn virus is holding us up here in Oz at the moment so this might be a slow process.

    Thanks heaps 

    Rod 


You do not have permissions to reply to this topic.