Jezmo

I am going to have to search for a smaller carb. I have put the #28 venturi carb back on, checked and changed everything imaginable, and it still bogs too much for the type of flying I like to do. I am convinced that the smaller carb will fix it but I don't have anything in the size needed for this engine. Thanks for all the help and suggestions.

pe reivers

What is your prop size? Heavy prop load also provides less spare power for prop acceleration. The prop should be light as well.
In your earlier posts, you mentioned a strange muffler. Can you post a picture?
This 26cc engine is very sensitive to the kind of muffler you attach to it. The influence on power output is very large, and can be as much as 1500 rpm on a prop same size. With a decent tuned pipe (MVVS) you get 3.8hp if you prop for 9500 rpm (18x6)

Jezmo

Sorry Pe, I have been out on vacation and just returned. It is an 18X6 wood (XOAR) right now and has had numerous types on it including an 18X6 Mejzlik. It turns them all very well and yes it does help to use lighter props for better spool up. I will look for a picture of the muffler on it.


OK I found the picture from when it was on my old Sig Mayhem. You can see a lot of the muffler in this pic.

pe reivers

The 26 does not like these mufflers too much. They restrict breathing. so 18x6 may well be the largest prop size that will do the job.

Jezmo

I know this is going to sound weird Pe but I've tached mine with open exhaust and with the muffler and hard as it may be to believe it turns more rpm's with the muffler than open exhaust, same prop, same day and time, same fuel. In fairness it doesn't seem to bog as much with the open exhaust so maybe the backpressure is causing more problems at lower rpm's. Just a thought but I may see if I can get one of the EVO wraparound pitts style mufflers for it and try that.

aussiesteve

I use an MVVS 26 (SP version) in a Wild Fred Laminar Yak (profile plane) on a "standard" MVVS supplied Tuned pipe (3266) running an 18/8 prop - All is standard (carb, ignition, factory pipe, etc)All runing on 40:1 Premium pump gas / Castrol 2T synthetic oil and bogging is definitley not an issue. I do have the header cut fairly short on this one though due to space related issud for the plane.

I am sure that Pe is right about your large bore carb and your exisitng muffler both being unsuitable for your aims.

Jezmo

I changed the prop and went to a 17X6 wood (from an 18X6 wood) with a huge change in throttle response. It will now idle, then take a very quick opening of the throttle (as quick as I can move the stick) without significant "bog". I can fly it this way, but one question Pe, how many rpm's is too many? Since it came with the big 34/64" (WT561) carb maybe you know how the bottom end is built by knowing what series it is. I don't know how to tell if it has the heavy duty crank, case and rod bearing without pulling it apart. (I'm not real inclined to do that especially since it is running so good LOL) I didn't put the #34 carb from the factory back on, I am using the #28 venturi carb but it runs great now with the smaller prop.

Hey Steve, yes I know it would run good on the pipe but I don't want to have the pipe hanging below the plane (it's a Funtana 125) since there isn't any room for a tunnel in the plane. I haven's seen one running the EVO wrap around pitts type muffler doing hard 3D so I'm just curious if that would be any better than the thing I'm using now. On the carb size deal, it does it even with a 28/64" venturi and that is what the newer ones come with. If I can find a #28 venturi walbro with the accelerator pump that might be the ticket but so far it looks good with the smaller prop. Oh well figuring out these kinds of issues is half the fun for me. I started working for my uncle at his lawnmower shop 46 years ago and the love of engines is still there today so heck, I'm having a blast figuring this thing out. Like I said, if you were to put it on a sport plane and hand the radio to a normal flier they would be major impressed, it's just me and the way I fly nowadays. I just keep flying closer and closer to the ground and doing crazier and crazier stuff and need more and more performance from the motor to help cover for my dumb thumbs LOL.

pe reivers

10500 I think still safe. How many rpm do you run?
at elevated rpm the crankpin bearing loads are less severe.
Can you post a quarter view picture and built year of your engine? From the picture I can see if you have the heavy duty crank.

Jezmo

I haven't tached it yet with this prop but I'll have to do that when I get back. I got a call out and will be away until Friday at the earliest so I'll tach it then and pull the cowl to get a picture. Since I bought it second hand, is there a way to determine year built? Maybe some numbers on the case or something?

pe reivers

Ask Horizon how they mark up the serials on their engines.
MVVS has a letter on one mounting lug (for year, starting "A" = 2000), and a number on the other (for serial number)

Jezmo

It's been a while since I updated this thread but have managed to figure out what the problems were that were causing my poor fuel draw and poor transition issues. In short, as mentioned at the beginning of the thread, this is a second hand engine. One of the problems was "damage by the previous owner" and the second was a bad o-ring that was only visible on complete dis-assembly. I decided to completely tear the engine down and thoroughly inspect each part for damage or defects. Upon doing so I found the o-ring between the carb adapter and case to be damaged (the damage was hidden by the cylinder and not visible without adapter removal) and the pulse line nipple was damaged in a way that even when tight would still leak around the threads. Bottom line the little MVVS runs flawless now.

Without getting too far into the details a couple of little things added up to cause a poor running engine. Sometimes the only way to find all of these "problems" is to just completely disassemble the engine and inspect each and every piece very thoroughly one at a time.

Edited to add: Thanks to all who offered suggestions and especially to Pe who always tries his best to help us.

All Day Dan

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Great update. Thanks for posting the root cause of the problem. Dan.

pe reivers

Whenever I check an MVVS engine, I go through the pulseline and nipples. The gasket under the nipple is fortified by a drop of loctite to seal all well.
The rubber O-ring disintegrating is new to me. However, here too I use loctite to prevent all leaks. Only crash damage will lead to a necessity to replace the carb mounting base, so it can be as well glued in.

Jezmo

The nipple at the carb adapter had a small gasket under it however the one screwed into the case did not. The hex portion of that nipple had been roughed up to the point it would hold the fitting cocked to one side and not let it seat fully. The threads are not very tight hence the fitting sitting slightly crooked allowing some vacuum/pressure to leak past. I put it in the lathe and resquared the flange, then reinstalled it with a retaining compound/sealer on the threads. The threads looked fine, not stripped at all, but they were loose as if the tap had been allowed to wobble when the threads were cut. It's also possible that since this is a second hand engine someone tried to "clean up" the threads and made the hole larger. Regardless, with the retaining compound/sealer it will not be a problem again. The O-ring on the carb adapter was replaced with a high temp viton based one. Shouldn't be any more trouble there either.

pe reivers

That's the problem with second hand stuff. The previous owner most likely will not admit his lack of skill, screws up, and then puts the engine for sale.

Jezmo

I got a great value for my money but I've put some time into it so maybe it's a wash. The engine makes great power for sure.