About as fast at half throttle than at full throttle

[Jerry Wyss]

Both these engines are converted to O/B powerheads and they have exhibited the same scenario at times. I put this in the general forum to possibly attract a wider range of opinions.

I have two theories, both engines may have too small of carb openings and they may have too small stinger ID's.

I have heard of this condition on here before but didn't catch any solutions. If there has been I may have missed them.

What say all you?

 

[Andy Brown]

Yes, I've seen too small a stinger cause that, but not always.

As far as carb bore, I've more often seen that from too big of a bore.

It could also be something in the engine that is causing the fuel to not burn well.

Sometimes the half throttle setting will help to atomize fuel as the fuel rips off the sharp edge of the partially opened carb barrel, therefore promoting a more complete burn. I have seen that situation many times and could be caused by improper transfer porting/timing and or head configuration, squishband, ect.

 

[Mike Larson]

J dub..... certainly youve the motor aspect down, but relatively, if youre using the same test platform, for the same pair of engines, i'd look to get more prop, (cup) on the platform. My theory being, that, when the motor hits on the pipe youre getting slip at rpm X......

Basically the half throttle rpm is the top efficiency of the given propellor, and more top end is wasted......

typed as such, because..... my shortstroke OB reacts EXACTLY like this scenario you describe, with the launch, and hitting the pipe, it's all there, but i can achieve no speed gain ( half, is as good, full is just testing the limits of rpm on the thing)....

Boat's great! but i know the given potential is being limited, and, I'll plan on searching the prop a little at a time as I test it further..... right now it heat races OK, and while it's not a SAW contender by far, I know where I'm going to seek.... try it....

humble and possibilities..... test well... mike

 

[Jerry Wyss]

J dub..... certainly youve the motor aspect down, but relatively, if youre using the same test platform, for the same pair of engines, i'd look to get more prop, (cup) on the platform. My theory being, that, when the motor hits on the pipe youre getting slip at rpm X......

Basically the half throttle rpm is the top efficiency of the given propellor, and more top end is wasted......

typed as such, because..... my shortstroke OB reacts EXACTLY like this scenario you describe, with the launch, and hitting the pipe, it's all there, but i can achieve no speed gain ( half, is as good, full is just testing the limits of rpm on the thing)....

Boat's great! but i know the given potential is being limited, and, I'll plan on searching the prop a little at a time as I test it further..... right now it heat races OK, and while it's not a SAW contender by far, I know where I'm going to seek.... try it....

humble and possibilities..... test well... mike

Hey LB, Ya stayin' dry over there? Got a bunch of rain over here after a **** nice summer. Rain is good for all the fiddys swimmin' up the rivers ya know

Well, two engines have done this, my O.S.30VG-P-X on the TT lower and this Top21-7M on the XM lower I've bought recently.

The VG is a very nice running engine and about the limit you would want to put on a Lynx boat on smooth water. I couldn't say its the prop on that one since I switched from a jug of 60% to 65% in the middle of race heats. Didn't open the needle for the next heat I ran it. The thing came un-corked in a Big way and spun that prop just a little too good to keep it on the water. Couldn't tell if it exhibited the "faster with half throttle" deal cause you couldn't hold it down that long.

With the VG it could've been what Andy was sayin' about "complete burn". You had to keep the VG on the rich side to control it on the Lynx. You lean up and turn the Dawgs loose with that engine and you had your hands full of Wild Thing.

This new 7M is a real mistery, it does the same thing listed in this topic. I do have the 20J carb on it just like the O.S.30-VG and have used two pipes, Irwin "stamped" and a SPP NR21TD carbon unit. The engine hasn't shown much in the area of RPM, torque all over the place with a 41mm X442 but go to a prop the next step up its a weakling. You can throw more "cool with fuel" Fuel, at it than any other engine. This is an engine I have tested more than any other and it just never tells you much,, other than it don't like anything!

This engine's acting out could also be what Andy's sayin' in "complete burn" too. It's the first domed piston I've ran with a healthy sized dish in it. I have tried another Nova angled squish button with less bubble volume without success also. It looks like my RB Classico9 is goin' on there after all. Fortunately the RB has a flat piston and has a similar configured sleeve to the 7M.

Anybody else having probs with the "domed" piston arrangement?

 

[Tim_Duggan]

Anybody else having probs with the "domed" piston arrangement?

No. My domed piston motors show no signs of this phenomenon.

What does it do with a prop with more pitch and less diameter? What does it do with the pipe shortened further?

 

[Michael Costanzo]

Both these engines are converted to O/B powerheads and they have exhibited the same scenario at times. I put this in the general forum to possibly attract a wider range of opinions.

I have two theories, both engines may have too small of carb openings and they may have too small stinger ID's.

I have heard of this condition on here before but didn't catch any solutions. If there has been I may have missed them.

What say all you?

hi jerry, i had the same deal with a cmb 90 and it turned out to be that the 3rd channel needle vavle was the problem.i had opeded itup with a drill pin set and went alittle to far.bought another one for the big motors and i was good to go. good luck,mike.

 

[Mike Larson]

Interesting, and good topic..... somebody hit on fuel, and, that might be it, surely.... the 30,might be asking for fuel at song it cant get.... I normally switch the brass fuel tubes out to 5/32 on anything bigger than 20, up to you... hopper system?

Unexplained phenomenon with any mod OB, on my end, that small/reduced dia's are where it's been, albeit higher pitch for the speed...... I have, a few props, and watch intently as the sport guys pull the moon with them, and they just wont work mod.

Cite motor infancy, but, whatever hot rod cuts are in there, it takes the torque away instantly.....top end, if you can get it there, sure is nice, though.......

Mess with that pipe a little as stated, too.... i've had good times with the carbon OB, but seen the stamped varieties give better consistency easily....for the most part, i know what youre capable of, when it flips backwards hitting the pipe, at launch, ya got something...... test em up, man, i like the challenge of finding it.... Mike

 

[Jerry Wyss]

Anybody else having probs with the "domed" piston arrangement?

No. My domed piston motors show no signs of this phenomenon.

What does it do with a prop with more pitch and less diameter? What does it do with the pipe shortened further?

Yo Greg! Nice to hear from you. The same boat, HTB 290.

The tank installation was for a Mod TT on 30%. The TT lower unit created a steering prob on the 290 just like TimD had. So the tank is one of Grim's 7+ ounce aluminum units sitting in an elevated cradle I built to hold it higher,, and Yes, the Top motor has a hard time finishing a 6 lap race with that amount of fuel.

LB?, I feel like an idiot with this motor. :blink:

Hey Tim,,are your domed piston motors Nova Plus models? or what model?

I have been thru many changes of props and have ran the pipes from 9" down to 8-3/8".

The 41mm X442-(slight B/C) appeared to be the one that it would turn easy and better than the 40x53. The tips of the 442 were tweaked and and I got a little more out of it but still the engine would not wind up like most engines do. The motor height has been about 2 to 3mm below the propshaft bullet bulge is level with the sponson bottoms.

A X640 went thru the process as well and I have shortened the pipe to where it wouldn't get up at all with the above engine heights. The engine acts more like a Mac O/B that wants to run in "tractor" mode all the time,, finding any Zing in it has been non-existant. I have ran the stock piston/button arrangement and changed the H/S from .004" to .006" on 35% nitro. Thinking that the .004" might be holding the Zing back I opened the H/S progressively to .006". With the latter the engine seemed weaker.

I have also used a lesser button volume using a P-5 button with a cut down angled squish face. It must be a commandment from God, "YOU CAN RUN THUNDER TIGER AND OS BUT NOT NOVAROSSI" :lol:

 

[Tim_Duggan]

Hey Tim,,are your domed piston motors Nova Plus models? or what model?

Older R1 series Novarossi, and Top 35+M. Both have domed/dished pistons.

 

[Jerry Wyss]

Hey Tim,,are your domed piston motors Nova Plus models? or what model?

Older R1 series Novarossi, and Top 35+M. Both have domed/dished pistons.

Good, just one less thing to be fussin' about.

 

[Mike Larson]

Dub. dont know, for, I've seen massive results from any nova i've been privileged to own.... While not of the water restriction crowd, generally, and i've seen in my travels anything from trick inlets on the motor, to collets and zipties slowing the flow.... maybe a point of interest...... (like you needed more variables)... Air cooled and a cut down head, tested to be massively finicky and not worth the headache.

The 442 and 640, are the common work around blades for me too, but, they are seriously, laughingly reduced to small proportions....

I've been turned to 1440's by a friend, that has always set the bar in tunnel tuning here..... Search there! Night and day.

Unexplained as to why the OB's are so critical on pipe length, the first problem I search, (tight seal, double springs, and knocked around after kiting).... if it aint hitting normally......

Fuel it, throw it, row it..... sweating over the tunnels and scratching my head..... good testing Mike

 

[Jerry Wyss]

Dub. dont know, for, I've seen massive results from any nova i've been privileged to own.... While not of the water restriction crowd, generally, and i've seen in my travels anything from trick inlets on the motor, to collets and zipties slowing the flow.... maybe a point of interest...... (like you needed more variables)... Air cooled and a cut down head, tested to be massively finicky and not worth the headache.

The 442 and 640, are the common work around blades for me too, but, they are seriously, laughingly reduced to small proportions....

I've been turned to 1440's by a friend, that has always set the bar in tunnel tuning here..... Search there! Night and day.

Unexplained as to why the OB's are so critical on pipe length, the first problem I search, (tight seal, double springs, and knocked around after kiting).... if it aint hitting normally......

Fuel it, throw it, row it..... sweating over the tunnels and scratching my head..... good testing Mike

My biggest problem is that I don't have many props for a direct drive 21 o/b. I have a bunch for the XM but they are really on the smallish size. The only other prop I have in that catagory is a 40x53 that I bought on here and it was cut down

The engine has layed down and tried to humm a couple times out of about 40 runs but something held it back and at that point it sounded a little weak. Most of the time it acts like it has big torque. You run the 442 a little deep the engine will consistantly

blast itself out of the water but the speed doesn't continue to climb. I had tried a full size Prather 220 with a similar story but it was like being in 1st gear all the time.

I think alot of it is the stock comustion chamber volume. The known volume of the

"bubble only" is .18cc and I haven't measured the dish in the piston. I think there's alot more in that dish than I estimated,, likely may have a combined volume over .20cc.

I thought I could make it work by running lower nitro and squeeze the snot outa the headspace and get that large charge to tick off. I got it to tick off alright but I think its a little late in the burn for a cruise missle run. The max I could take off the angled squish face is .008", without a head shim that would be .004" H/S. I got no idea how much that would reduce the volume,, and like I said before I had trimmed a P-5 button to .16cc volume with a perfect angled squish cut and Nadda there too, hmmm

 

[Mike Larson]

Jerry... you get the motor at your best theory.... and PM me your address.... gimme some time.... in all seriousness, and why, the props I have that work, and spool, look like little 535's....... barring the 1440 series....

found and realized, also, that the italian conversions like the bigger belly pipes.... the C/F with the little belly rocks, but it makes for a finite measurement....

Something's wacked, for I know, other than clearance, a ton of guys just run the factory button upon conversion, and do great.....

abliity to step on the volume is yours to try, easily.... but it should be performing regardless....

The 215 is your friend.... I have taken the 20's waaaay down for testing, and may as well take a 15 up.......

do your thing, and search it out, I'll be in touch..... Mike

 

[Jerry Wyss]

Jerry... you get the motor at your best theory.... and PM me your address.... gimme some time.... in all seriousness, and why, the props I have that work, and spool, look like little 535's....... barring the 1440 series....

found and realized, also, that the italian conversions like the bigger belly pipes.... the C/F with the little belly rocks, but it makes for a finite measurement....

Something's wacked, for I know, other than clearance, a ton of guys just run the factory button upon conversion, and do great.....

abliity to step on the volume is yours to try, easily.... but it should be performing regardless....

The 215 is your friend.... I have taken the 20's waaaay down for testing, and may as well take a 15 up.......

do your thing, and search it out, I'll be in touch..... Mike

Well I Thought the Nova products had superior torque to other motors. This one certainly

exhibits that trait but looking at all the holes in the sleeve it looks like high rpm operation was a thought. I just haven't had alot of opportunities to run the thing lately other than 2 fun runs and then a race day. But it has had several trips to my test pond back in the summer, so its been run alot.

I know my club competition likes to see that boat/motor show up. I was close to getting beat (25 pts, 950 to 925) by a TT o/b that I modded 5 years ago on a TS3! It was good stuff though, the young man that was driving that boat is the son of a long time club member,, He is getting Very Good lately.

 

[Glenn Quarles]

Guys, all of the motors now except my T21M have the domed/dish piston. Jerry I don't know what yours has. I have nothing but great things on the 7 port motors. Yes head temp is very important...on my Nova OB, I never ran ANY water to the motor (I had the K&B water pickup direct to spray a bit on the header)...screamed like crazy.

Glenn

 

[Jerry Wyss]

Guys, all of the motors now except my T21M have the domed/dish piston. Jerry I don't know what yours has. I have nothing but great things on the 7 port motors. Yes head temp is very important...on my Nova OB, I never ran ANY water to the motor (I had the K&B water pickup direct to spray a bit on the header)...screamed like crazy.

Glenn

Thanks Glenn, its a Top21-7M that I bought from Aaron in Oz after I canceled the one I ordered thru you. Unlike many of the previous batch that was sold in Oz mine had the domed piston w/dish. They Had the same #28021 button I did but theirs had a flat piston.

That flat piston with the #28021 button rendered the button useless in that combination,, thats what I was told.

On the other hand I thought the combo in mine might be a better set-up but has yet to be realized concerning the "scream" factor. Normally I don't have much trouble making o/b powerheads run well, its actually one of my best talents, leaving out the Mac o/b :lol:

Most likely its to do with the prop, carb and pipe combo. The 20J may have found its limit, I have only ran two pipes and my prop variety is not that large for the reverse crank direct drive units. I would like to go the different pipe, carb and prop direction but investing that much money and time is not on the menu,, mostly beans and franks at this time.

 

[shoboat]

Jerry,

The Prather 220 reduced is a KILLER PROPELLER.

It sounds like you need some new blades.

Thanks,

Mark Sholund

 

[Mike Larson]

Aha!.... throat bore dia on the OS?..... I've either had the speedmaster, or a cmb inboard.... search the bigger bore if'n ya can.... If'n ya cant i'll send you one...... lmk... (Rodney's been using it, you may as well, too.... ).... LOL.

Give me time on a prop, or two, brother, more than so. If ya cant wait... maybe Mark'll fix you up... and he guesses good.

Keep at it man. you know..... yeeeeeeeeeeeeooowwwww/ping!. mike

 

[RodneyPierce]

Mike, I wont need that carb and needle this weekend. Robin came to the rescue with a rush shipment of the needle I needed. Since I had heard nothing from Terry, still...

And Jerry, Ive got a 220 here you can have it. Ill send it to yah.

Or, If mark is kind enough to tell us what to reduce it too, Mike has a nice prop pitch gauge, ive got the prop, im sure we can make one and send it your way. Free of charge of course. I have no use for this prop. Ill PM mark.

 

[Tim_Duggan]

OzBryan on here had his Top21-7M O/B conversion going fairly well yesterday with an OS 20J carb and stamped Irwin pipe on it. It was on a HTB290 also. It's just starting to make the right noises! Looked like a good heat racing set-up. Maybe he could give you some feedback on prop / stinger diameter etc.?