Other Reverse Jetting

[Billybo455]

50 and 47 buckshots and 48 novis msd ignition. Dont remember the curve.

Itll happen because people will try stuff they shouldnt be trying and blow up.

 

[kevbo]

50 and 47 buckshots and 48 novis msd ignition. Dont remember the curve.

Itll happen because people will try stuff they shouldnt be trying and blow up.

Without a doubt, the quest for more always ends there. Someone will read this thread and install a smaller main jet without realizing hes now 20 seconds away from a stuck piston. Just like posting the virtues of 200 psi and 30 degrees on pump gas, someone will try it with an enhancer and toast his engine.

 

[TheBuzzard]

every 701 or larger motor that i have looked at or jetted, had reverse jetting with 44mm or larger dual carbs. kawasaki sxr or yamaha

 

[waxhead]

If you go that small on your main that your going to seize your engine. You will be so low on power before it happens. There seems to be this rule of thinking that engines make the most power just before they lean seize. Not true at all. They start to make power as you go down in the jet size and it finds a weak area in your setup. In reality if you were to put in a jet that cold cause a lean seize on its own then the boat would not be able to get up no the plane.
I fail to see how some one setting up a carb reverse instead of conventional would have any greater chance of seizing there boat. Carb tuning is the veyr last thing you do make sure the rest of your boat is setup correctly first. Buck shots by the way had no booster and thus very little depression caused and so need a larger main jet.
30 degrees works fine on pump gas I have had up to 36 degrees in boats on pump gas just make sure you pull the timing out when the engine starts to get some dynamic efficiency

if you were not running a decent curve on the 951 there is no way reverse jetting would have worked.

 

[waxhead]

Without a doubt, the quest for more always ends there. Someone will read this thread and install a smaller main jet without realizing hes now 20 seconds away from a stuck piston. Just like posting the virtues of 200 psi and 30 degrees on pump gas, someone will try it with an enhancer and toast his engine.

Based on that we should be saying leave the boat stock and not do anything at all. But this thread is asking about reverse jetting and how to get more power doing it. If your going to tune your boat or have some tune your boat make sure they are qualified to do so. If your tuner sets your boat up so your ready to blow then I suggest taking it back when it does or finding another tuner

 

[Cannibal]

Thanks wax for taking the time to go into such detail.

 

[Billybo455]

So we will see one of your setups at world's this year? I was merely stating all the carbs had the same effect. Your method is drastically different then mine. I start with the motor then exhaust then motor then pump and then finish on the motor. But I wish all good luck with their ventures.

 

[TheBuzzard]

i just found a taper bore 46 setup without reverse jetting. ported 701 140h 105L 2.3 80 spring. it was starving bad on the bottom so went up to 115L. test beat tomorrow.

 

[dcptr]

Thanks for the detailed info wax, I've been looking for the mythology of reverse for a while.. ALL the big bore world finals boats are set up in reverse but done properly obviously. Xscream have only ever recommended reverse jetting for all their engines due to the extreme porting, accommodated with the right timing too.

My question is- should I change any of the following settings on my msd to accommodate for the extra fuel down low- these specs were given to me for normal jetting, but I've recently got the reverse jetting specs from xscream- so obviously I want to make sure the timing is right too-
Xscream 1000, 200psi true- (fuel: 15ltrs BP 98/5ltrs AVGAS 110/130, 40:1 Castor927) pfp, 48novi's.

 

[garrcass]

Is that 110 low 130 high? What was the standard jeting on it?

 

[dcptr]

Is that 110 low 130 high? What was the standard jeting on it?

*No sorry, 110/130 octane AvGAS

Jetting recommended from chuck (xscream)
130 high….5/8 turn out
135 low……1/38 turn out
2.3 needle and seat
65 gr spring
Double check valves
#60 return jet

Jetting recommended from Thrust-
120 high
140 low
2.5 needle & seat
115 gr spring

Jetting recommended from AJSP-
142.5 high
130 low
2.5 needle & seat
95 gr spring

So far I've just changed from the thrust setup to AJSP's normal jetting.. Seemed too rich and couldn't get the crisp top end nor responsive bottom..

 

[waxhead]

I would go with the thrust setting if it was me. A bit of extra timing down low will not hurt either. Bump it up to 32.

Where abouts in aussie are you

 

[SN760]

Waxhead thanks for the great info!
Have a question for you. But first a little info on what I'm running. 62T motor, Sleeved and bored to 84.5 mm, Case and cylinder porting, B Pipe with mod chamber, Xscream 46 mm carbs, Jetting - Mains 115, Pilots 122.5, Pop off 14 psi, MSD Inhancer - 22 degrees max timing.
Going to change to a programmable Ign. For a timing curve I was going to start with 32 degrees at 2000 rpm and start pulling that out at 4800 rpm and be back down to 20 degrees at 7000 rpm.
Just wondered what your thoughts were on this?
Thanks.

 

[dcptr]

Epic, flame or msd?

 

[waxhead]

First of all your welcome. Happy to help out with info

Now your ski
I would pull the timing lower than that . I like to go back to about 16-18 up top
It depends on the engine of course some will handle more timing than others. Yours is a relitivity small bore and so the heat path is shorter. it should be fine with 17 degrees up top
Make sure you start to pull it out by about 5000 rpm with it all out at 7000. Just a flat line at 17 will be ok after that.
Do you have a tacho on your ski. You can do dome real funky stuff with ignition timing after the pipes has gone past its power peak to make it pull a bit harder if you have a programable ignition

Why is your pop off so low ?? Personally if it was me I would be looking for a 125-130 low speed with 20 psi pop off. Less chances of it dribbling as you idle around.

 

[waxhead]

Epic, flame or msd?

Very good question

 

[SN760]

Epic ignition.
Will give the higher pop off a try.
In fact I do have a egt/tack gauge that I use to use to help dial in jetting. What's ya thinking? I'm game!
Thanks.

 

[waxhead]

If I was you I would go for either the flame or the total loss msd there has been mixed results on the epic. I cant talk for the new ones coming out, but the fact they are making a new one speaks for itself as well.
Would be awesome to use the egt on the engine but kinda not worth the hassle for a free ride boat unless it way top level and even then they dont get held flat out long enough to get a stable reading. I have done it on a type 9 on a race boat before and found interesting results between cylinders. Dcptr has found simillar things with a pfp. MY money is on the uneven exhaust manifold.

 

[SN760]

Use to use e.g.t.'s to tune ski's - exhaust manifold I have on my ski now is drilled and tapped for the probes.
Giving the Epic a try - friend of mine has loaned it to me. Ran MSD total loss for about 2 years, decided to go back to a charging system and sold the Total Loss ( mistake). If I can get the Epic to work it will give me a good comparison of running 22 degrees advance (msd inhancer) verses 32 degrees (epic). If there is a noticeable difference then I most likely will buy the epic or get a flame.

 

[masterblaster]

I start with the motor then exhaust then motor then pump and then finish on the motor..

So..adjust initial jetting,timing..then adjust water injection to dry pfp pipe direct from pump ..then go back adjust jetting,and timing to work with new power curve on pipe. then new prop,nozzle for new bottom power curse..then tweak jetting,timing once more for new optimal prop......? just hypothesis