I have a '58 Skylark with a GO 300. Had new cylinders in December, the engine has an issue with running poorly, mostly cold. I live in cold country (WI). Here is the situation: Engine starts normally, warm it up for about 3 minutes, taxi to run up area. Set RPM to 2000 and start mag check. Engine runs normally on RH mag, on LH Mag only, it will start loosing power and eventually stall if you don't go to both mags. At idle will run fine on the LH mag. Had both mags checked by the A&P, all checked good. Carb was gone through, no problems there. Spark plugs on LH mag (bottom plugs) look clean and lean. RH upper plugs look identical. Has anyone seen this concern before, and how was it resolved? Thanks in advance for your help.. Don

Have you tried swapping the left and right mags to see if the problem follows the mag or not?

[quote name='Don' date='Feb 7 2010, 01:33 AM' post='4411']
I have a '58 Skylark with a GO 300. Had new cylinders in December, the engine has an issue with running poorly, mostly cold. I live in cold country (WI). Here is the situation: Engine starts normally, warm it up for about 3 minutes, taxi to run up area. Set RPM to 2000 and start mag check. Engine runs normally on RH mag, on LH Mag only, it will start loosing power and eventually stall if you don't go to both mags. At idle will run fine on the LH mag. Had both mags checked by the A&P, all checked good. Carb was gone through, no problems there. Spark plugs on LH mag (bottom plugs) look clean and lean. RH upper plugs look identical. Has anyone seen this concern before, and how was it resolved? Thanks in advance for your help.. Don
[/quote]


Have you checked the plug wires.
Is there a short in the primary leads?
Is the ignition switch faulty?

I had a similar problem but only on the first runup of the day. After doing about everything you did, I put the plugs in a pressure tester. Found that I had 2 plugs breaking down and not firing properly under pressure. I installed a new set of plugs and haven't had the problem since.

I had a similar problem and read where Unison was having problems with their plugs breaking down under load so I switched to Champion plugs and haven't had a problem since. And no I don't own stock in Champion

Now that you mention it Herb, I was using the Unison/Autolite plugs too.

A friend of mine with a stinson just went throught the same thing. Changed most everything and it turned out to be the almost new plugs. By now you can guess the brand, first 2 guesses don't count.
Ken

We had the same problem with our Ercoupe Rick and switched to Champion plugs and it cured it also. Don't think all Unison/Autolite plugs were bad but they had a big batch.

Thanks for the response.. Since I last posted the concern, I turned the idle mixture screw out a turn and it seems to have resolved the issue, for now. I am also aware that I may be masking the actual probem, ie: the spark plugs.. will change them anyway and see what happens.. thanks again for all of your input. Will let you know what happens. Don