Other Reverse Jetting

Cannibal

Cannibal

Tasty Human
Location
Summit Lake, WA
In the "Affordable Power" thread, Waxhead mentioned reverse jetting, I've seen it mentioned here before as well.

I'd like to hear more on the subject. What advantage does this have?

Can anyone elaborate on how to switch over a carb from standard to revese?

If you are running 80 pilot and 120 main, do you simply run a 120 pilot and 80 main? Is there more to it? Popoff change? Other considerations? Or do you have to start from scratch?

Thanks!
 
Big Kahuna

Big Kahuna

Administrator
Location
Tuscaloosa, AL
It may not be that simple, but that is the basic premise. The Low Speed does not cut off when the High Speed Kicks in. Your putting more fuel down low, but you also have to have the right sized Needle and Seat for this combo to work, Just another way to tune a ski.


Years ago when I had my first port job done by TLR. I asked him what jetting to run on my Dual 38's. Todd said he would rather me run his single carb setup, but I did not have the money at the time. So he told me what jetting to run.

Instead of the standard jetting of 80/140 (think that is what everybody use to run - BPipe, head, etc) He had me run like a 115 Pilot. It worked and worked well. A while later I started having some Tuning problems. John at JSS told me to try the 80/120 or 140 (again, cannot remember the HS Jetsize). I told him that would not work on my ski. But, I tried it anyhow, Changed jetting on the bank of the lake, It ran like a raped ape for about 1-2 minutes and then stuck a piston, more like blew the top off of it.

Here the point. The porting I was running liked alot of fuel down low to run. It did not like a lean mixture at all. Swapping the jets will make you richer but you have to have the correct balance with Jetting, Needle and Seat and Pop-off to make it work right.
 
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JGringo

JGringo

When in doubt, Throttle-out.
Location
Lafayette, Louisiana
Interesting... I want to hear more as well.

Honestly I have never heard of this before, it seems like a great way to burn down a piston...

Subscribed,
-Gringo
 
TheBuzzard

TheBuzzard

Location
the jetski grave yard
pilot jet contributes to overall fuel delivery, even on top end. you can have a huge pilot jet and a high popoff or a small pilot and a low popoff. small pilot means you have to have a big main. big pitot means small main. the motor always requires the same ammout of fuel wide open. on wide open, you can give it a bunch from the pilot and less main or the other way around. low popoff isnt that great since there isnt very much spring pressure to keep a good seal on the needle and seat. uneven fuel delivery from vibration/ leaky seat.
 
kevbo

kevbo

Location
dfw
Cannibal said:
In the "Affordable Power" thread, Waxhead mentioned reverse jetting, I've seen it mentioned here before as well.

I'd like to hear more on the subject. What advantage does this have?

Can anyone elaborate on how to switch over a carb from standard to revese?

If you are running 80 pilot and 120 main, do you simply run a 120 pilot and 80 main? Is there more to it? Popoff change? Other considerations? Or do you have to start from scratch?

Thanks!
Click to expand...

There is more to it than blindly picking numbers. Reverse jetting is a phenomena that occurs with with SBN44,46, and some A/M carbs when installed on big engines. Freestyle throttle blippers need part throttle to be rich enough for good response, this means larger pilot. Larger engines, especially with low transfer timing have strong signal to the main at full throttle so that a smaller jet is required. Reverse jetting with smaller engines/carbs makes 1/4 throttle very rich and often require the top screw opened a lot.
 
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Cannibal

Cannibal

Tasty Human
Location
Summit Lake, WA
Good info!

So kevbo, you are saying that it's not really done on purpose, just where the jetting ends as a result of tuning a larger engine?

The reason I am curious about a difference in jetting is because of my current carbs, Blackjacked SXR 40 I bodies. Art does not "Blackjack" (annular discharge) these carbs or SJ38s any longer (he does taper bore/thin tube atomizer them) because of a load up condition on low throttle which fools people into leaning out the low to the point of engine damage to get rid of it.

I'm running the jetting he recommends and he has basically said there isn't any improvment but to modify them to the new style for a hefty price. I thought maybe this could help, but it sounds like it might actually increase the problem, but with higher popoff, maybe. Might be something I need to fool with before I commit to a different carb (which I could surely use).
 
W

waxhead

wannabe backflipper
Location
gold coast
I will explain it when I get on my laptop I'm writing on an iPad and I hate that. There is a very good reason why it works so well
 
W

waxhead

wannabe backflipper
Location
gold coast
Ok it seems that the mb in my laptop is cactus and so going to be a while. So here we go please excuse my more mistakes than normal.

As you know your carbs have a butterfly in that controls air movement. Well when you crack these open the air starts to move and for that brief second that the air speed is not enough to create a depression in the carbs there is low fuel flow. As the air speeds up the fuel starts to flow. So we can then determine that fuel is flowing slightly behind what ever the throttle plate is doing this is just one of the advantages of fuel injection. By adding more fuel down low we create a rich situation that when we crack the throttle opening and ask for the power the fuel is already there. There is no set figure as to what jet works with all carbs the same as conventional jetting. Some carbs out there that have poor signal in there booster will require a larger main jet as they don't create the same depression across the main jet. ( I will write about that later)
ok. So now we have a rich situation down low and if we keep the same main jet as conventional jetting we would have a rich situation up top as well so we reduce the top end jets size. The top end jetting is a combination of all the circuits not just the high speed jet and adjuster.
so how do we fix the issue of rich down low. How to get around this easy bump some timing into it. This will get over the slow burning rich mixture by lighting it earlier. The result massive torque when you grab the throttle. I have seen race boats with a huge low speed jets. In fact I know of a certain hx that had custom machined low speed jets as I couldn't get them large enough.


Another benefit of the extra fuel is that you get cold cases. Anyone who has driven a car on a cold night will know that you have extra torque. You get the same with cold cases.
 
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W

waxhead

wannabe backflipper
Location
gold coast
Please if you don't understand don't decide that it can't work but ask away.
now I talked about depression across the main jet.
When you speed up the air flow you get a low pressure this is what makes your planes fly. When you do it in a carb throat you decrease the air pressure in relation to the pressure in the float bowl or in a pumper carb ( metering body). So now due to higher pressure the fuel is forced out the jet into the airstream. The more efficient the booster the smaller your main jet will have to be. Be careful a restrictive flame arrestor will mimic it to a certain degree. I have seen carbs that people sell that they say you buy a certain carb for a certain pipe and that carb will not work on another pipe. A carb with an effective booster will not have this issue.
 
kevbo

kevbo

Location
dfw
The only bad part of using very large pilots is when you are forced to cover distances at low speed like crossing the rough part of a lake to find big boat wakes. The fuel burn is much higher than it needs to be. Bigger carbs increased my fuel burn by 20% on the average long ride because they had to be set richer at part throttle in order to get good response. With the top screws set 20 rpm rich of peak, it had to be fourcycling a lot at part throttle.
 
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waxhead

wannabe backflipper
Location
gold coast
Fair enough. I'm interested in fuel burn in my car not in my ski. How good do they sound with loads of fuel and lots of timing. Like a drag car and they hit like it as well

i can understand that if your going for miles that the extra fuel may be an issue. I haven't found it to be excessive but of course it will use more fuel. Got to feed the horses
 
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Billybo455

Billybo455

Location
Central FL
Umm more linear power? Bigger then you could get pilot jets on a hx? My old mans twin piped 951 hx with 50mm carbs wouldnt run it was so rich reverse jetting at 155 pilot 110 main. In fact it wouldnt run at all until it went back to conventional jetting. Dont make reverse jetting sound like the holy grail. We are going to see more blown up motor in the carnage thread. Yes, in some setups reverse jetting is the tits. Whether or not its right for your setup is another question. The only way to know is trial by error.
 
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waxhead

wannabe backflipper
Location
gold coast
How is reverse jetting going to equal more blown up engines in the carnage thread ? What 50mm carbs did it have. What ignition system did it have in it with what curve. Just because it didn't run it doesn't mean it couldn't have run. I'm not here to say what you did was wrong. I'm here to speak only of my experience with reverse jetting. Every engine I have built over the last ten years of porting and building race/ free ride skis has been reverse jetted. Before that I used to do some conventional and some reverse.
 
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