Throttle problem
Sadly, I have irreparably damaged my M-III. Fortunately that was all that was hurt, maybe sharing my experience will prevent it from happening to others.
I had what I thought might have been a stuck 582 carb slide. I pulled the throttle back and nothing much happened. I tried to cycle the throttle. Unfortunately, that only left me with a wide open throttle and I ultimately had to shut the runaway train down with the ignition key. In my simple post event field check, I found the front carb felt 'sticky'. By that I mean when pulling up on the throttle cables at the top of the carbs, rear was free but not the front. I didn't do any further in-depth engine checks until yesterday, in part due to the plane being at the A&P's for a damage appraisal (which you can correctly surmise went rather poorly).
So yesterday I opened up the carbs and found nothing unusual looking at all. The cup was sliding fine in the front carb and the disconnected throttle cable worked smoothly as it should. Return spring pulled throttle closed fine. Basically, I was kind of stumped and at a loss as to what the heck happened. Not to the point of questioning whether some type of malfunction had actually happened, it most assuredly did, but I started wondering if I was going to have resolution. I HATE when stuff happens and you can't pinpoint the source, leaves that lingering doubt in the back of your mind about mechanical reliability.
As I discounted any problem with the carbs themselves I started working my way back from the carbs along the cables to the throttle in the cabin. Again, no binding or friction problems at all with the carb ends disconnected, but something did catch my attention. When I bought the plane, both throttle cables were secured to the engine mount between the firewall and carbs with a black zip tie. I didn't think that was unusual, as I've always mechanically secured cable sheaths multiple times to keep internal cable function in the event one clamp fails. And, apparently, that particular zip tie did not cause alarm at the annual condition inspection only a few weeks prior. What I did notice was that it was very slightly loose and I could move it up or down along the engine mount tube.
Looking at the pics I posted, you can see from the black overspray on the engine mount tubing just how far along the tube I could slide the zip tie. When I first noticed it, the zip tie was at the bottom most position. When I reattached the throttle cables at the carb end and operated the throttle, moving the zip tie up and down the tube to various positions it became clear it impacted the function of the cable ends at the carbs. As I understand it, the zip tie somehow slid/vibrated down the tubing which had the effect of shortening the working length of the cables. This had little effect on the rear carb closest to the firewall, but put just enough stress on the cable path into the front carb to create friction and preclude the carb slide from closing. With the zip tie removed, all throttle, cable and carb operations function perfectly.
An expensive lesson for me, don't you make the same mistake.
Regards,
Doug
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