Author Topic: New one for me (Read 2238 times)

Dwayne · « **on:** August 12, 2011, 04:18:35 PM »

I have a merco 40 that won't run higher than 7600 rpm when i turn the needle in it will leanout and quit when I turn the needle out it goes rich just like normal. I'm using a plastic 4 0z. uniflow I've gone through it several times, I've changed nva all the tubing checked for leaks everything I can think of, this one has me stumped I've never seen anything like this before any help would be greatly appreciated.
Thank you

Steve Helmick · « **Reply #1 on:** August 12, 2011, 06:12:39 PM »

It sounds to me like a clogged spraybar, actually. Similar to what happened to my Magnum .36 at NW Regionals. It would get faster, slower, faster, slower, as I turned the needle in, without ever reaching peak. I richened the NV up until it was very rich and then suddenly went even richer as the crud passed on through. It's been running fine ever since. It might also have a brass burr inside the spraybar. That will make them grab little junk as it passes through and run unpredictably. That or maybe a leaky head gasket or loose/blown glowplug seal.

Steve

Bill Hummel · « **Reply #2 on:** August 12, 2011, 06:52:22 PM »

Hi, Dwayne, recently had a friend's LA 25 act similar...tried everything! Sat and thought, and looked at the darn thing, and finally
noticed that the venturi was tiny! Replaced it with a stock venturi, problem solved instantly. Since then, I routinely check venturi
sizes, even if they look "stock".

Jim Kraft · « **Reply #3 on:** August 12, 2011, 07:12:14 PM »

Or, it could be that the sleeve is in backwards.

Dwayne · « **Reply #4 on:** August 12, 2011, 07:30:25 PM »

Quote from: Jim Kraft on August 12, 2011, 07:12:14 PM

Or, it could be that the sleeve is in backwards.

That's what I thought as well, nope tried that too...

I'll be looking at the venturi next.

Dwayne · « **Reply #5 on:** August 13, 2011, 04:51:49 AM »

Quote from: Bill Hummel on August 12, 2011, 06:52:22 PM

Hi, Dwayne, recently had a friend's LA 25 act similar...tried everything! Sat and thought, and looked at the darn thing, and finally
noticed that the venturi was tiny! Replaced it with a stock venturi, problem solved instantly. Since then, I routinely check venturi
sizes, even if they look "stock".

Thanks Bill that's what I'm thinking as well I'll change it and see what happens.

Bootlegger · « **Reply #6 on:** August 13, 2011, 08:24:22 AM »

If the spraybar is clogged you can use welding torch cleaners an run them thru the spraybar like a pipe cleaner.
Some time when the spray hole is drilled a burr is left inside the spraybar.

rustler · « **Reply #7 on:** August 13, 2011, 01:41:27 PM »

Quote from: Dwayne Donnelly on August 12, 2011, 04:18:35 PM

I have a merco 40....

I may have an interest here - is it an old type side exhaust Merco or a Rustler-Merco?

Dwayne · « **Reply #8 on:** August 13, 2011, 08:31:10 PM »

Quote from: rustler on August 13, 2011, 01:41:27 PM

I may have an interest here - is it an old type side exhaust Merco or a Rustler-Merco?

It's a 80's Premier, abc baffled side exhaust.

Balsa Butcher · « **Reply #9 on:** August 14, 2011, 06:58:25 AM »

Hope you have better luck with yours than I had with mine, I never could get it to run right. Later found out that it (and others from that era) have major machining/quality control issues. Finally gave up. Has it ever run well?

rustler · « **Reply #10 on:** August 14, 2011, 02:53:53 PM »

Quote from: Dwayne Donnelly on August 13, 2011, 08:31:10 PM

It's a 80's Premier, abc baffled side exhaust.

Phew!!

As I guessed, not one of mine. I do have one, which I know has run, and in due course I have an Ozzie ceramic p/c to install which I hope will make it MAGIC.

rustler · « **Reply #11 on:** August 14, 2011, 02:56:53 PM »

Dwayne - in due course I will run mine and maybe compare figures with yours. If mine's o.k. and the ceramic p/c works out, you are welcome to my old p/c.

Dwayne · « **Reply #12 on:** August 14, 2011, 05:51:11 PM »

Quote from: rustler on August 14, 2011, 02:56:53 PM

Dwayne - in due course I will run mine and maybe compare figures with yours. If mine's o.k. and the ceramic p/c works out, you are welcome to my old p/c.

Wow that would be awesome! Thank you!

Now for an update, I made this venturi from a piece of brass pipe that a thought had a 9/32 ID turns out it's 1/4" I just drilled it out to 7mm (about .2756) so I think it should run much better I'll let you all know tomorow. $:-\$

Chris Wilson · « **Reply #13 on:** August 14, 2011, 08:54:31 PM »

Quote from: rustler on August 14, 2011, 02:56:53 PM

Dwayne - in due course I will run mine and maybe compare figures with yours. If mine's o.k. and the ceramic p/c works out, you are welcome to my old p/c.

Hi Ian,
Could it be a lack of taper on the liner?

I have heard of that here in Oz.

Dwayne · « **Reply #14 on:** August 15, 2011, 07:18:57 AM »

I ran today and it peaked at 8400 on the .275 venturi this is more than big enough for a 40 so I guess there's something really wrong it I'm going to try a .281 and see what happens but I don't think it's going to make much difference.

Balsa Butcher · « **Reply #15 on:** August 15, 2011, 08:33:21 AM »

A .281 isn't a bad idea. But if it runs at 8400 and swings a 10x6 or 11x5 prop consistently without overheating, that may be just how it runs. That's about how mine ran but would never run out a tank without going lean or quitting. You may not get the revs an OS or Brodak 40 will give you. I don't mind being wrong here but... FWIW: No relation to the Rustler-Merco 40's.

john e. holliday · « **Reply #16 on:** August 15, 2011, 09:08:45 AM »

If there is no burr in the spraybar, I would checkthe taper on the needle valve itself. Have taken a poor mans machining method of tapering needle valves to a longer sharper point with a fine metal file.

rustler · « **Reply #17 on:** August 15, 2011, 03:01:23 PM »

Quote from: Chris Wilson on August 14, 2011, 08:54:31 PM

Hi Ian, Could it be a lack of taper on the liner? I have heard of that here in Oz.

Chris - you mean Dwayne's problem? If so, I'm not sure. Those Merco 40's were made when I had nothing to do with the Co. I understand they could be variable. I know the one I have has run, but in an R/C model, and a starter was probably used. I'll try to give it a run at our next club engine day.

Dwayne · « **Reply #18 on:** August 15, 2011, 04:07:06 PM »

Rustler please check your messages.
Thanks

Dwayne · « **Reply #19 on:** August 15, 2011, 06:23:45 PM »

Problem solved I think,after a pm to Rustler I relized it was something I did too the case that changed the intake timing, oh well live and learn...

Thanks for all the replies.

Chris Wilson · « **Reply #20 on:** August 15, 2011, 06:46:19 PM »

Quote from: rustler on August 15, 2011, 03:01:23 PM

Chris - you mean Dwayne's problem? If so, I'm not sure. Those Merco 40's were made when I had nothing to do with the Co. I understand they could be variable. I know the one I have has run, but in an R/C model, and a starter was probably used. I'll try to give it a run at our next club engine day.

Hi again Ian,
Yes, I mean Dwayne's issue.

I have seen a side exhaust Merco with what feels like zero taper in the liner but I highly doubt that it is one of yours!

Anyway, what fixed the problem?

Dwayne · « **Reply #21 on:** August 15, 2011, 07:42:56 PM »

Anyway, what fixed the problem?
[/quote]

If you're refering to my problem it was solved but can never be fixed, I'm hopeful that with a .281 venturi it will run at a high enough rpm I can prop it to at least fly with it but it will never run at peak rpm again.