0.061 big mig, leaning symptom

This certainly sounds like a tank problem. How do yo know the tank is good? Have you air tested it Have you taken the tank back off and checked to see if the pickup is cracked or soldered properly to the inside? Who makes the tank? Brodak or Perfect? Another problem is the venturi gasket. If not sealed properly, it can offer results such as your experiencing. Running a engine on a test stand isn't going to solve your issue. Fly it and document when it's occurring. Does it occur the same time every flight? changing a plug is never a bad idea. It's quick and reversible if needed. I don't feel that is the issue here but it's s simple experiment.

I agree with Ken that the symptoms may be related to impaired fuel supply when half the tank is emptied and later.

Either the venturi seal is leaking, leading to reduced engine suction, or, the bottom of the tank sits too low relative to the venturi level, and even if the venturi seal is tight, the venturi depression is not enough to lift fuel from too deep in the tank. Long and/or kinked fuel line may add to the problem.

I'm stumped ive gone through the fuel tank several times, all looks good to me, it is oriented correctly, lines were inspected and good.  The lines are neat and clean, no kinks.  It's a round clunk that I run on muffler pressure. I've gotten years of reliability out of this arrangement.

I will get one 90% flight first thing in the morning when it's cool.  Then every flight has the same behavior, no matter the tune. Some times its 3/4 full when it sags and stalls.  Fuel is fresh and the same I've always run sig 1/2a fuel.   Maybe I will try and put some foam between the tank and plane.  

Venturi is fully sealed, has new orings, and the bolt that holds it on is sealed with permitex.  An air leak was my first thought.
No bubbles coming from around the head its sealed good. I did change the prop from a MA to a Cox one, maybe the cox one vibrates way more.  Felt typical when running. Can definetly see the fuel foam when running and your holding it.  I don't know.  If it's not an engine issue should run ok on a test stand.    Or maybe try a wedge tank. Not sure why it would stop working after so many years.

Layback,
How much slop is there between the crank and case? I have a Golden Bee that does this too. I can observe dark grey oil residue leaking out of the crankcase snout during operations. I determined the case is excessively worn by swapping all other parts to a good runner crankcase. Another tell was just how much I could rock a prop tip fore and aft in line with the crankshaft. I finally gave up running the "GB."

I would also check around the needle valve, after some hours of service the vibration may have made its fit loose and allow air to seep in the carb along the threads..some silicone fuel tube added for improved sealing may help like with the COX needle valves..

Thanks for the feedback everyone. I'm in courage to give it another crack. Great suggestions glad to hear similar thoughts.

Interesting, it is a rc carb that is wired open, never thought about that area leaking great suggestion. I'll check that.

I'll double check the plane bearing wallow wear too. I think it doesn't have play yet, but need to check that too. Thanks.

I would put a spritz of Original Armor All formula in your fuel and try it again. This will stop the foaming.

I have nothing to add on the Norvel as others have covered the issue nicely-but on the McCoy 35 in your initial post, I have two suggestions/comments;

(1) McCoys (I'm assuming the plain bearing redhead or bluehead series here..) didn't have much in the way of compression to start with-and didn't wear well. Very much a case of 'built down to a price' and not 'built up to a standard'

(2) one or rather two things you might try: try using fuel with a high castor content-say 28% (one of Duke Fox's  'go to' standbys for worn motors) and secondly, try adding a couple of drops of castor THROUGH the plug hole before starting from cold. that's an old trick from the 50s used by testers on glow engines set up a bit loose ex factory. Effectively it gives you a better piston seal when you need it...for starting. Its amazing how well worn out motors will run once you get them started...even diesels-but you need that piston seal to GET started in the first place....

ChrisM
'ffkiwi'

Zero compression with a Mccoy isn't uncommon. They're a very inexpensive throw away engine. My K&B greenhead I just cleaned up is almost identical in construction. The Mccoy uses brass end pads on the wrist pins which wear grooves in the liner. Assuming this isn't the issue, the base gasket is fiber composition as well as the head gasket. Both of the gaskets are more than likely compromised now. One big problem which the Mccoy's are plaqued with is the piston wearing out. The piston is made of compressed powder which is heat formed. They can offer a very short lifespan. It's kind of ironic because the Johnson engines are high quality engines and the same manufacturer made the pistons for them.

The Mccoy piston is domed so some have tapped the crown down with a hammer which expands the sides. They lap the piston back into the cylinder which they claim offers new life to the engine. I can insure you that it will work but the engine will never run the same as it did. It usually cracks the piston or sets up micro fractures where the baffle meets the side of the piston. In addition, it's extremely short lived due to only the very top half of the piston sealing.

Ken Cook wrote:            Zero compression with a Mccoy isn't uncommon. They're a very inexpensive throw away engine. My K&B greenhead I just cleaned up is almost identical in construction. The Mccoy uses brass end pads on the wrist pins which wear grooves in the liner. Assuming this isn't the issue, the base gasket is fiber composition as well as the head gasket. Both of the gaskets are  more than likely compromised now. One big problem which the Mccoy's are plaqued with is the piston wearing out. The piston is made of compressed powder which is heat formed. They can offer a very short lifespan. It's kind of ironic because the Johnson engines are high quality engines and the same manufacturer made the pistons for them.

          The Mccoy piston is domed so some have tapped the crown down with a hammer which expands the sides. They lap the piston back into the cylinder which they claim offers new life to the engine. I can insure you that it will work but the engine will never run the same as it did. It usually cracks the piston or sets up micro fractures where the baffle meets the side of the piston. In addition, it's extremely short lived due to only the very top half of the piston sealing.

icbimproducts (the RC Gasket Company) on Ebay offers a VERY wide range of laser cut aftermarket gaskets for a huge number of different engines-including the McCoy 35....so that deals with any gasket issues you might have. Incidentally I'm somewhat intrigued as to how BRASS endpads can wear grooves in a steel liner.... I can easily see this happening if the pads were missing-but brass is softer than steel...

ChrisM

Chris, you hit the nail on the head, the pad is similar to a brass rivet. Reason for the excessive wear  is due to  many times the wrist pin hole isn't square to the piston and the rod is jumping back and forth. The nylon end pad on the crank pin  assists holding the rod from jumping for a while. The wrist pin bosses egg out and the end pad wears out. Many have a love for these engines. I will say I learned with them and from a nostalgic point of view I do like them. However, I have a cabinet with so many of them which are clapped out junk. I've spent countless hours fiddling with trying to make them work. My buddy Al essentially takes the Mccoy series 21 Dykes ringed piston and installs them in the gated Redhead Lightning bolt cylinders. No machining is required as it's a direct drop in. This piston utilizes a larger wrist pin with Delrin end pads. The baffle of the Series 21 piston can sometimes strike the stock Redhead head. This may require some filing of the baffle because of it's height. After all monies spent and the time required, your better off buying a known engine that works.

I would also suggest to those interested in the gaskets that are offered on EBAY to purchase them. The owner of the company no longer has the website he used to offer. He purchased the company from the former owner and a few months back this new owner was trying to sell the company. I contacted him last year for a specific gasket that he didn't make. He told me to send the measurements and he would charge me X amount to do the work. After some time I contacted him again and he explained to me that he would not offer that service as he wanted to sell the company. I've recently noticed that he has surfaced again on EBAY.

Hey gents, thanks for the suggestions on my old red head mccoy 35.  It's the pre lighting bolt variety.  I have a few others I've collected over the years. Might set this one aside for now or use as parts once I confirm it's not a gasket leak (didnt know they were paper style). I've been running it on 25% oil content (70% castor 30% syn), I like that the higher oil content could maybe extend the slow death. Ill try that trick next flyday if i get in the same issue again. Been blending rc car fuel that I have lying around with too much oil content for modern abc car engines.  Unless you can run wot all day long.

I like the light weight of the mccoy engine and how it fits most vintage kits mounting like the flight streak and ringmaster, with no modifications.  Also like the sound of them.  But yes, they do seem like budget quality. Got 2 with new compression I've been saving. Time to put them too work.

A Fox .35 is the same weight and will outlast the Mccoy 5 times over. While the Fox can have quality issues, it lasts and lasts. Some claim the Mccoy offers more power, I say show me. I own some pretty impressive Fox.35's which most don't believe it until they see it. The OS MAX-S .35 is also similar in weight and it offers terrific quality and power.