Thank you John for being the guinea pig.
Will doPull the PTO off and you'll see what I mean!
I had a Picco 21 EXR that had a pin hole in the side of the case, in the middle of the side transfer port. I had all ready run the engine a couple of times before I noticed this stream of fuel squirting out the side of the case every time I revved the engine on the beach. I tossed it in the water anyway and it ran great, just like the two previous runs. I mean it was fast and did everything right. My thinking was Wow, This this is going to be really fast when I patch that hole. Fixed the hole and ran it a week later...NOTHING changed! Ran the same speed and performed just as it did the week before.Terry, The flow problem is Dynamic, not Static. Yes it will flow off the clock on the flow meter, but an accelerating engine sucking on very massive nitro and oil through small tubes will find itself with too little or too much fuel at varying points in the acceleration curve. It takes some time to get your head wrapped around it, but once you do your boats performance will improve.
Picture this! Liquid flowing through a long tube at a slow speed will require a relatively low pressure or suction. In order to cause the liquid to move through the same line at a high speed will require more pressure or suction. You might say that the engine is spinning faster so it will suck more. That is true, but here it the problem. The fuel and the air flow rate do not necessarily increase in the same proportion as the engine speed, and therefore suction, increases. It is the relationship between the length and diameter of the air passage and the length and diameter of the fuel passage that must match in order for the air/fuel ratio to remain constant over the range of rpm. However, we do not necessarily want the same A/F ratio as the engine speed increases.There is a load dynamic that comes into play here too, and understanding this relationship between the fuel passages and the air passages will allow one to "tweak" the system to get the correct A/F ratio map for a particular set up. It's almost like computerized fuel injection without the computer once one understands how to Tweak it.
Anyway, back to the Nova 46 specifically! A big part of these above problems with the Nova 46 are related to heat build up as the laps continue. As Martin said, the engine needs time to come up to temp, but in the cases I am reading here the Temp is continuously raising without stabilizing. In this case the heat needs to be released. Water cooling may not be enough, so the heat needs to get out of the pipe. This means a bigger stinger bore, but as you noted Terry, this will cause the fuel pressure to drop. This is the reason for needing the bigger spraybar, fuel lines, fittings.
But remember, what I wrote above. Just bigger is not best. The Spraybar can be too big. That will cause the engine to come off the corner strong and the go rich half way down the straight. This is the problem that I think Jeff is encountering. The easy fix for him will be to bore out the stinger rather than make a SMALLER spraybar.
Likewise, if the spraybar is too small the engine will come off the corner strong and sag lean halfway down the straight.
Ok, now that I have shared some of my speed secrets it's time I put some of my new ones into practice!... Time to go set some new Nitro records!
Thanks for the help Andy and I do understand what you're saying but the motor isn't just leaning out on the top, it's surging up and down. I shoulda taken a video, would have been more "visual".
Everything we've done to make it easier for the motor to draw fuel has helped, it's not surging to the point of shutting off anymore but it's still doing it. It's not going rich, it's leaning out, sounds like when you get a pinhole in a tank or fuel line. It does have a fairly small cooling jacket and was coming in hot so we took the cowl off, it's not over heating.
I finally pulled it apart last night and "think" I mighta found something. The seal between the PTO and case is only a very small diameter O ring and there was some galling, similar to my 45 VAC backplate:
Could it be leaking when it comes up to temp?
Sealed it all up with RTV and after Olly finishes his therapy sessions we're gonna try it again...
Kinda reminds me of when you would blow the paper gaskets on the old motors.lolPull the PTO off and you'll see what I mean!
I recently ran a K-90 that had no backplate gasket and no O-ring. Also, the carb had none either. All just bolted up dry, no RTV. And the mating surfaces were not perfect. The engine ran consistent with no issues.Terry, The flow problem is Dynamic, not Static. Yes it will flow off the clock on the flow meter, but an accelerating engine sucking on very massive nitro and oil through small tubes will find itself with too little or too much fuel at varying points in the acceleration curve. It takes some time to get your head wrapped around it, but once you do your boats performance will improve.
Picture this! Liquid flowing through a long tube at a slow speed will require a relatively low pressure or suction. In order to cause the liquid to move through the same line at a high speed will require more pressure or suction. You might say that the engine is spinning faster so it will suck more. That is true, but here it the problem. The fuel and the air flow rate do not necessarily increase in the same proportion as the engine speed, and therefore suction, increases. It is the relationship between the length and diameter of the air passage and the length and diameter of the fuel passage that must match in order for the air/fuel ratio to remain constant over the range of rpm. However, we do not necessarily want the same A/F ratio as the engine speed increases.There is a load dynamic that comes into play here too, and understanding this relationship between the fuel passages and the air passages will allow one to "tweak" the system to get the correct A/F ratio map for a particular set up. It's almost like computerized fuel injection without the computer once one understands how to Tweak it.
Anyway, back to the Nova 46 specifically! A big part of these above problems with the Nova 46 are related to heat build up as the laps continue. As Martin said, the engine needs time to come up to temp, but in the cases I am reading here the Temp is continuously raising without stabilizing. In this case the heat needs to be released. Water cooling may not be enough, so the heat needs to get out of the pipe. This means a bigger stinger bore, but as you noted Terry, this will cause the fuel pressure to drop. This is the reason for needing the bigger spraybar, fuel lines, fittings.
But remember, what I wrote above. Just bigger is not best. The Spraybar can be too big. That will cause the engine to come off the corner strong and the go rich half way down the straight. This is the problem that I think Jeff is encountering. The easy fix for him will be to bore out the stinger rather than make a SMALLER spraybar.
Likewise, if the spraybar is too small the engine will come off the corner strong and sag lean halfway down the straight.
Ok, now that I have shared some of my speed secrets it's time I put some of my new ones into practice!... Time to go set some new Nitro records!
Thanks for the help Andy and I do understand what you're saying but the motor isn't just leaning out on the top, it's surging up and down. I shoulda taken a video, would have been more "visual".
Everything we've done to make it easier for the motor to draw fuel has helped, it's not surging to the point of shutting off anymore but it's still doing it. It's not going rich, it's leaning out, sounds like when you get a pinhole in a tank or fuel line. It does have a fairly small cooling jacket and was coming in hot so we took the cowl off, it's not over heating.
I finally pulled it apart last night and "think" I mighta found something. The seal between the PTO and case is only a very small diameter O ring and there was some galling, similar to my 45 VAC backplate:
Could it be leaking when it comes up to temp?
Sealed it all up with RTV and after Olly finishes his therapy sessions we're gonna try it again...
9 mm will run better than some would think. We have seen Gary Preusse run near 100 mph with the K&B 7.5 using the K&B red carb which is about 8.5mm.
That was my 1st thought way back but I kinda got sidetracked from that after looking at the 11mm carb on my 45 VAC. We did try the stock carb with the low end needle, idled and transitioned better but still surged.I recently ran a K-90 that had no backplate gasket and no O-ring. Also, the carb had none either. All just bolted up dry, no RTV. And the mating surfaces were not perfect. The engine ran consistent with no issues.
I don't think there are any problems because of Nova's small o-ring.
I think the real "air leak" problem is that BIG 11mm hole in the carb.
Not that 11mm is too big for a .45 engine, but because it only has a .070" spraybar and it's in a sprort hydro, 11mm is too big.
That's the next thing to try I guess, I have a 10mm OS 6B and an old 10mm Picco carb to try. It may just be the problem.
He's running a long (6" or so) rectangular tank in the sponson with an internal hopper (one of Walt's). Because of the plumbing he might be pulling fuel through 12" or more of line before it exits the tank. Even though Walt used 5/32" tubing it might just be too much to ask combined with the large carb. The "surging" might not be an air leak at all but just expecting too much in the "fuel delivery" department.
Yup, that might help. Don't like going smaller than 3/8" tho as it can add excessive heat to the piston and cause other problems. The 6280 we tried had a 3/8" I/D stinger.
Seen the split inner tube before, that'll drive you nuts! One of the 1st things we did was empty a full tank looking for bubbles, none.
Hey Tom, I wish I knew what the magic fix was for this boat, but I don't. There are a lot of very smart folks tossing out some great ideas and I know the guys working on the boat are very smart. I know these motors can be a bit sensitive to setup, but there are a lot of guys with them running great straight out of the box. I understand they were running pretty good down in Fl at the Nitro Shootout with completely stock motors with just the standard setup most guys do. I also understand that some very good boaters have had the same kinds of problems as we are talking about. I think we are all gonna learn a little after this is over. And I am working so this kinda stuff may be a thing of the past with the Nova 46 motors.
