SGX 45 / AB 45XT / CMB 45 HR

[Saugling]

I maidened my new SGX 45 today. It is set up according to the included setup-sheet.

Engine is a new 2007 CMB 45 HR (completely stock), pipe is one of Andy's new 45 XT with extra muffling set to 25cm / 9.85" (measured from plug to end of pipes's first cone where the thread starts).

Engine fired up well, made some rounds, emptied one tank than came back in. Noticed the plug's filament was broken...refueld, richened the needle, made some rounds, cruised around, comes back on pipe without a problem when opening throttle...did this until the next tank was empty. Plug was still good this time

Now here is my problem:

Engine fires up well, idles good, plugs last...BUT...rpms only increase to about 1/2 throttle

Takes props like 1457 4.2" / 1655 / V955 without problems...but does not show an increase in rpms from a bit more than about 1/2 throttle to full thottle

What could be the cause of this behaviour? Pipe too long? More Nitro (I only used some 20% fuel today, will try 30% & 40% next time)? Do I need bigger vent holes in the cowling? Does the engine simply need more time to brake in (but it already feels relatively free after consuming about half a gallon of fuel)?

Any help welcome!

Robert

 

[FloridaScaleBoater]

Robert;

I will be watching this post with interest!

My brother-in-law has the same engine, pipe combination in a Thomas sport-40, we are experiencing the same thing.

Engine just doesn't have any "good" top end RPM.

I run the same boat W/ a CMB45RS with a Mac's exh. 45 pipe and that thing ZING'S the RPM at top end.

And we were having the same problem with plugs

Rick

 

[Don Ferrette]

I would start by keeping it a little rich & shortening the pipe. B)

 

[tnrcboatracer]

My thoughts, too. Keep it rich, and bring that pipe in. the 9.85" to the end of the divergent cone sounds way long to me. In theory a pipe is measured to the center on the convergent cone, but that is not practical, so the manufacturer usually references the weld at the center of a two cone pipe. On pipes with a center band, the rear of the band/start of the convergent cone is used (or sometimes weld beads like the Irqin muffled pipes). Looking at your pics, that would mean that you're set up 1"~1.5" long. Start shortening the pipe 0.25" at a time, and see if it doesn't pick up. Measuring the way you do, the pipe may need to be as short as 8.25", maybe shorter. But I think the correct way to measure that pipe to to the end of the band where the rear convergent cone starts. Just be sure to keep it rich, and bring the nitro up. Does your motor have a low nitro head button, or a high nitro (small cc/large cc).? Do you know how many cc's your head button is?

 

[mrgary]

I maidened my new SGX 45 today. It is set up according to the included setup-sheet.Engine is a new 2007 CMB 45 HR (completely stock), pipe is one of Andy's new 45 XT with extra muffling set to 25cm / 9.85" (measured from plug to end of pipes's first cone where the thread starts).

Engine fired up well, made some rounds, emptied one tank than came back in. Noticed the plug's filament was broken...refueld, richened the needle, made some rounds, cruised around, comes back on pipe without a problem when opening throttle...did this until the next tank was empty. Plug was still good this time

Now here is my problem:

Engine fires up well, idles good, plugs last...BUT...rpms only increase to about 1/2 throttle

Takes props like 1457 4.2" / 1655 / V955 without problems...but does not show an increase in rpms from a bit more than about 1/2 throttle to full thottle

What could be the cause of this behaviour? Pipe too long? More Nitro (I only used some 20% fuel today, will try 30% & 40% next time)? Do I need bigger vent holes in the cowling? Does the engine simply need more time to brake in (but it already feels relatively free after consuming about half a gallon of fuel)?

Any help welcome!

Robert

hi robert,,i am running a cmb valvola 21 in my rigger,,i know its not the same but,,i would try getting more air to the carb for one,,i am running cmdi billet header and there billet pipe,but the pipe i have was made to fit over the header snugly and is not way big like yours looks,,i was told that having a pipe that wont fit over that header properly will cause problems,try a different header and start the pipe at 10'' from the plug to the flat part of the pipe,,lmk,,gary

 

[Buckshot]

Your pipe is set too long.

I don't remember what I set mine to but I think it was close to 9.25".

You need to open up your cowling also to get some more air.

-Buck-

 

[Alan terry]

What is your head to deck st at? What alt. are you at? Did you try pushing the pipe in?

 

[dwb2620]

You guys are throwing out pipe lengths of 8 1/4 and 9 1/4 inches. We are talking .45 engines ,,,Right? I run my Eagle SG 45 w/ MAC.45 at 10 to 10 1/4 and it screams. That is with 50 to 60% nitro, 1655 prop, 70 mph. Dang, if I move my pipe in maybe I could get 75 to 80 mph out of it...............................

David

 

[ALLAN YUEN]

I did put 9.75" new AB 45XT but little slow and weak power so my good friend build lathe extra aluminum tappered pipe 1.25"onto 45XT pipe Best set up 10.25" and open turn 6 to 7 out for low needle( more rich) on carb I am very glad with great fast more power easy quick corner better cut hole top vent size water jacket head on cowling and easy air breath

 

[tnrcboatracer]

The number I said was based on where he was measuring from. If the pipe was measured to the rear of the band/start if the convergent cone, it would be about 9.75"~10.25". His measurement of 9.85" to the end of the divergent cone/start of the band has the pipe at around 11.25 "~11.75" when measured to the start on the convergent cone (add 1.25"~1.63" for hte band, I'm not sure of the length, as I don't have one of these pipes). That is way too long for the 45. The number you use depends on how you measure it. It's the same as with the 180* header guys quoting a pipe length of 4" vs the 8.25" for the Valvola type motors.

 

[waltr]

I'm kind of listening here too.

I recently finished a Speedmaster with a 45 Blackhead and AB 45XT pipe. It's gettin wet this weekend, but I just might have to lengthen it a bit before I go.

I don't have it anywhere near my Crapshooter length which is 8.00" with a .375 Parabolic, but the AB 45XT is nowhere near what you guys are talking about either. :huh: We'll see soon I guess.

But his issue sounds exactly like too long of a pipe #1

And the Cowl #2. Open the left side of the cowl forward of the carb a bit. My Crapshooter had the same issue and it was the cowl.It was Prestons boat and he could never run the boat with the cowl on.It was bad enough to actually die after 2 laps. Took off the cowl and it was a rocket. It turns out that the air openings were behind the carb and created a vacuum and actually took the air away from the carb. I re-did the cowl and no more issues.

And good luck with your boat.It'll be a blast when it's all sorted out.

 

[hydro]

You have to understand that a tuned pipe system in a 2 stroke is actually a Helmholtz resonator. Once the engine RPM gets to the pipe's resonant frequency the RPM is locked at that point , meaning it will not go any higher regardless of the carby's flow rate. That's why when the pipe is long the engines tuned to that resonance or RPM and stops . Of course there are other variables as well in the equation.

Basically put on a standard issue prop for a 45 rigger like an octura 1457 or one of Andy's recommended props. Set the pipe long and try her out. Keep shortening the pipe a step at a time, the RPM and speed will increase. You can also cup up the prop more too (on a stock 1457). Keep doing this juggling act with shortening the pipe and cupping the prop keeping an eye on the glow plug and adjusting the fuel mixture & head clearance as necessary. The boat will go faster and faster until it will not come on to the pipe , then just take a little cup out of the prop (1457) and it will then get on the pipe and also add lift to the transom. That will help unload the engine. That's about the best the boat will do at that point with the stock engine, which should be plenty Fast !!

 

[Saugling]

Thanks for all your input!

I will go testing again within the next couple of days.

Cowl is going to be opened and pipe shortened to see if top end gets better.

My progress is going to be posted her.

Thanks again,

Robert

 

[Ray Sametz]

I'm kind of listening here too.I recently finished a Speedmaster with a 45 Blackhead and AB 45XT pipe. It's gettin wet this weekend, but I just might have to lengthen it a bit before I go.

I don't have it anywhere near my Crapshooter length which is 8.00" with a .375 Parabolic, but the AB 45XT is nowhere near what you guys are talking about either. :huh: We'll see soon I guess.

But his issue sounds exactly like too long of a pipe #1

And the Cowl #2. Open the left side of the cowl forward of the carb a bit. My Crapshooter had the same issue and it was the cowl.It was Prestons boat and he could never run the boat with the cowl on.It was bad enough to actually die after 2 laps. Took off the cowl and it was a rocket. It turns out that the air openings were behind the carb and created a vacuum and actually took the air away from the carb. I re-did the cowl and no more issues.

And good luck with your boat.It'll be a blast when it's all sorted out.

Robert, don't mean to hijack your thread, but Walt could you post a pic of your 45 Crapshooter cowl. B) B)

 

[Saugling]

...Does your motor have a low nitro head button, or a high nitro (small cc/large cc).? Do you know how many cc's your head button is?

I really do not know, but I made this picture of my head button:

diameter of inner "bowl" is about 12mm

 

[Saugling]

I opened up my cowling

...and set the pipe to about 9" (from plug to end of divergent cone)

 

[AndyBrown]

Nice Cowl latch!

We have just begun to make Cowls out of clear Lexan. Tough stuff!

Be sure to richen the needle a little more with the shorter pipe.

P.S. Nice job on the complete boat!

 

[tnrcboatracer]

Yes, that button looks to have a fair amount of detonation on it. You should be able to riched a bit more. Bench the head buttong with 600~1000 grit wet-or-dry paper on a surface plate or stone countertop to remove the high spots. Don't worry about the pits.

 

[Buckshot]

Yes, that button looks to have a fair amount of detonation on it. You should be able to riched a bit more. Bench the head buttong with 600~1000 grit wet-or-dry paper on a surface plate or stone countertop to remove the high spots. Don't worry about the pits.

Could also be due to high head clearance.

-Buck-

 

[Terry Keeley]

I'd pick those bits of plug wire out of the squish band too, pretty sure they'll glow red during compression and not help the detonation/pre-ignition situation...