Ok i been trying to get this refurb" weedie running, You can read about it here [link]http://www.rcuniverse.com/forum/m_4737028/tm.htm[/link]
To me this thing doesn't have much compression, with small and larger carbs just can't get it to run right..

Would more compression help, top dead center on this thing is a long way from the bottom of the spark plug threads.
unlike the other engines I have.
If I get the bottom of jug machined what do I need to look for , will it change the timing,etc. etc.


thanks.

i have a mac 32 coming to me in the mail in about 3-4 days.when i get it ill check it out some,see if i can hand start it,check piston to head clearance ect.

I have a Mac. I'm not sure if it's a 28 or a 32.

I have it running on Gas Glow fuel.

The compression is very weak but it runs pretty good.

I pulled the cylinder and removed the ring to check end gap.

What I found is that the ring is bent in about 1/4 inch from an end, so it's not sealing well.

I'm going to Email [email protected] and order a custom made ring for it.

Check yours and see if you have the same problem.

You are not going to believe the difference with a new Frank Bowman ring once it breaks in a bit!

AV8TOR

Take a thick lenght of solder and stick it through the plug hole until it reaches the outer cylinder wall. Turn the engine over to crush the solder. Measure the solder to find the current clearance. You can lower the cylinder to within 0.020".

Dave

But yes, it will change the port timing. It will decrease the exhaust and transfer port timing and increase the intake timing.

AV8TOR

is there a website for frank bowman rings??

Thanks guys I'll try the solder trick, I have seen Av8tor post that before but I didn't understand to push it to the outer wall, because @ the spark plug hole it wouldn't even crush.
I'm assuming the inside of the jug is dommed shaped..I measure it and see what I've got..

Av8tor, I haven't found a 24,26 size carb so if you have one you'd like to get rid of , just PM me.

Ok I tried the solder trick and it wouldn't even crush,, took the gasket out and it may touch ever so slightly, so now what start filing on the ridge on the botom case?

Frank can be reached at:
Frank Bowman
1211 N. Allen
Farmington, NM. 87401
505-327-0696 (6 pm to 9pm MST weekdays)
Email: [email protected]

How thick is the solder? I use 3/32" thick solder. You can have the cylinder flange machined. Allow 0.020" head clearance at least. The exhust and transfer duration will decrease as the intake duration will increase. This was mentioned above. People usually raise the exhust port by at lease the amoumt removed. The stock duration is 140 degrees, compared to a Homelite 25 at 150 degrees.

Dave

I'm about to tear into my mac 32 also, it's maiden flight was this past sunday and it needs more power... probably only in the 5krpm range with a 18x8 windsor if that... the compression seems kinda weak, so for now I guess I'll pull the jug seal off after checking it with solder, and think about getting a set of frank bowman rings.

what other modifications should I do? I'm goiong to bore out the carb some which should help, but what about port timing and stuff? I've never done any of that but am sure I could handle it, the thing I want to be sure of is that it's still easy to start...

Fredbob,
Get an 11mm carb. To take full advantage of the new carb, you will need to bore out the manifold to 5/8" or make a new one that rotates the carb. Great care must be taken to preserve the pulse channel in the manifold or use an external pulse line. The 11 to 14mm carbs have a 5/8" throttle butterfly, which adds volume and a greater plenum effect, for better response. My Mac 28's carbs had a 9.5mm venturi and a 9/16" throttle bore, which is about right for the 21cc Mac that I use them on, but small for a 32cc.

Dave

A larger carb (not a bored one) will help and so will reworking the muffler. Those two things will give you a engine that will turn in the 6500 range.
Raising the compression and changing the timing will add 200-300 rpm, the Bowman ring will 100-200 rpm IF YOUR CYLINDER IS GOOD. A CH ignition conversion will add 200 rpm. A proper breakin will add another 200-300 rpm over a summer of flying.
So you would be looking at 7200-7500 rpm when your done.
Boring the carb messes up the venturi and the fuel draw.

Dave the solder is just standard solder 3/32 sounds about right.
Mind just doesn't seem to be able to handle the volume of the 11mm carb, Its seems way low on compression, and Im pretty sure it needs a new ring. when I took it apart I can see some blow by under the ring.

Dale,
When the ring wears, the gap grows larger allowing blow by especally on a single ring motor.

Dave

The proper ring gap is very tight on these motors. Like .001 to .003". Most used engines can use a new ring.

AV8TOR

I forgot to mention I "reworked" the stock muffler... took out the tiny 1/4" outlet, cut out the baffle and installed a 5" long chunk of 1/2" tube...

I'll check the carb in my homie saw that has a stripped plug hole, maybe it has the desired carb...

Update: I bought a Bowman ring for the little 28cc mac that I could never get to run on gas glow.. well It would run nicely on the small 20" size carb but only put out 6400 rpm swinging a 16 x8 mas prop.

New ring installed:

first tank was ok max rpm were 6600 on 16x8 with the small carb 20" size, idle dependible but little rough.
Second tank max rpm 7150 rpm 16x 8 20" carb.
third tank I changed to a 18 x8 prop and could still get 7200 rpm..
Fourth tank replaced carb with a 24" size walbro 18x8 and bammmm[X(] 7600 rpm idle is perfect smooth, transition is great .
I think its ready for a plane.

MAc 28cc weedie, gutted mufler, base gasket removed, Bowman ring, 24" size carb 7600 rpm... 18x 8 mas prop

The bowman ring really raised compression I guess, before the larger carbs would just bog down on transition (if I had it lean enough for good idle) now I may even try a bigger carb next but I afarid Im pushing my luck..

I'm going to place this on a Midwest AT-6 Texan 120 size.. thrust HP shows about 17. lbs of thrust..

I can't say enough for the Frank Bowman rings its what made the gas/glow combo work for me..
Thanks.

Good job and congrats!! I have found that most used "conversion" engines need a new ring, and anymore, when I pull an engine out of my pile to convert, I just automatically order a Frank Bowman ring for it.

Have fun,
AV8TOR