Super Jet Ideal number of turns on carbs?

[Waternut]

I had 120 pilot jets in my GRS carbs (similar to blackjacks) and the engine ran ok and I could keep it running well with constant throttle input but if I got sloppy it would blubber even at 2 turns out on the low screws. So I went up to 122.5 jets in and it made a huge difference even at only 1/2 turn out. I'm currently at 1 1/2 turns out. It keeps getting slightly better with every adjustment and doesn't require any effort to run right but I can force a blubbering condition if I try hard enough.

My question is this... Should I go up another half size jet? All jetting changes I've made in the past have made an incredible difference once I passed 2 turns but I'm wondering if that's only because I was so far off. I have 125 jets sitting on my desk and wondering if they're worth putting in or if I should keep adjusting to roughly 2 turns and hope it cleans everything up.

 

[XJCrawler]

I was told never to exceed 1.5 turns and being less than 1 turn was ideal. If you have the jets why not try it

 

[kevbo]

The bottom screw is for low idle only. Pilots around 1/4 throttle and the main/top screw is for full throttle. The pop-off will affect mainly the lower settings.

 

[motoman96]

I was told never to exceed 1.5 turns and being less than 1 turn was ideal. If you have the jets why not try it

Exactly, I try to keep it around 1 turn but no more than 1.5.

 

[Waternut]

Sounds like 1.5 is the magic the number to not exceed. I took my low screw from 1 1/2 to 1 7/8 in 1/8 increments and it progressively made the off idle hit cleaner but if I cruise around at 1/8-1/4 throttle, it got more and more lean and took a few blips to clear it up. I know technically at this point I should probably mess with the pop off but since I have the next size jet and no spare N/S's, I'll give the jet a shot first.

 

[swapmeet]

How did it work out?

 

[Mark44]

Purchase a tachometer from Randy at Watcon and take the guess work out of it, your motor will thank you.

Mark44

 

[Waternut]

I'm still struggling with this but the issue is much more complicated now... I changed the jets and it made everything run better but if I cruise at low speeds for longer than 20-30 seconds, it fouls the plugs. So I went with smaller jets but raised the pop off. Again, everything runs smoothly but if I go at low speeds too long (a little longer than before though), it fouls the plugs. I haven't bothered to mess with it again but I'll try some more combinations next weekend.

For the record, I have a tachometer but tuning with that thing is like trying to hit a pencil with a pellet gun on a windy day at 100 yards. It's hopeless... Tuning by ear and by the seat of my pants has proven far more accurate than a tach ever has in my experience.

 

[CARBONX2]

I'm still struggling with this but the issue is much more complicated now... I changed the jets and it made everything run better but if I cruise at low speeds for longer than 20-30 seconds, it fouls the plugs. So I went with smaller jets but raised the pop off. Again, everything runs smoothly but if I go at low speeds too long (a little longer than before though), it fouls the plugs. I haven't bothered to mess with it again but I'll try some more combinations next weekend.

For the record, I have a tachometer but tuning with that thing is like trying to hit a pencil with a pellet gun on a windy day at 100 yards. It's hopeless... Tuning by ear and by the seat of my pants has proven far more accurate than a tach ever has in my experience.

How big are your carbs? Few engines need bigger than 44mm's for maximum power. The airspeed is too low on large carbs hurting atomisation of the fuel and making jetting difficult. A 781cc SXR engine needs the flow of a 36mm diameter circle for peak power at 6500rpm, once you factor in the butterfly shaft that equals 40mm. My 40mm carbs I modded by taking out the step in the throat(to equal the flow area at the butterfly) outperform my SE 48mm annular discharge carbs on acceleration and consistency by far. I struggled for years to jet the 48mm's perfectly, eventually went back to books and software on carb sizing and tried the SXR carbs. Modded 38mm's will give pk hp at 6000rpm on a 701cc. Modded 40mm's good for peak hp at 6750rpm on a 701cc. It's all about putting the whole engine package together.

 

[Waternut]

Well I'm currently using dual 44 GRS (annular discharge) carbs on a ported big bore 785 yamaha engine. They guy who used to run the engine ran it with dual 48 novi's but he raced it so low speed cruising probably wasn't an issue.

After re-reading my posts on this thread, I'm beginning to wonder if jetting was never the issue and maybe it's something to do with one carb. I originally was using a stock 760 intake and it worked pretty good. However, with the way that intake is setup, my carbs were swapped with the primary carb being on the rear cylinder and the slave carb without a fuel pump being on the front cylinder. When I changed to an aftermarket intake manifold, the carbs got switched around. I did switch pulse lines but I'm wondering if the signal is screwy in it's current configuration. Both carbs have been rebuilt since my original post.

 

[D-Roc]

It sounds like if your cruising at 1/4 throttle and after 30 seconds it is loading up with too much fuel, this means you can lean the pilot. Tuning 48's is not difficult, dual 44's is where I read most people having tuning issues. waternut is using some modded 44's so they should be modded to work good once he figures out his jetting. I am sure you have read this but do it again. So many people are turning the low screws , and I know it can give you an idea of if you need to richen or lean, but read the manual. I never use to run a lanyard so I had my idle stop set low and the low screws I just set a 1 turn out each. Never touched them. I tuned all the rest of the carb settings by the manual. i now run a lanyard and have set my idle as per the manual.

http://www.mikuni.com/pdf/sbn_manual.pdf

Read the Tuning section.

 

[Watcon]

It sounds to me like you are confusing "Blubbering" & "Surging" while explaining your situation. To tune correctly, you will have best results with seat of the pants & Rpm testing. Try this?
Best regards,
Randy

Basic tuning tips for all Watercraft.



Symptom…..Falls on face when accelerating

Open low speed jets until it accelerates well with no flat spot or hesitation without worrying about peak rpm in short 100 yard acceleration bursts.

Symptom… Blubbers or misfires on acceleration

Too rich on hi-speeds. Close high speeds ¼ turn at a time until peak rpm is attained max 100 yard bursts.

Tuning procedures

Hold rpm @ approx 2000-2500 rpm for 30-45 seconds before each test to stabilize fuel flow & rpm

1 Turn in lo speed jets ¼ turn at a time until flat spot appears, open 1/8 @ time until it accelerates hard & smooth.


2 Open hi speed jets 1/8 turn at a time until it blubbers or misfires on acceleration.. then close 1/8 at a time until it accelerates hard & smooth to peak rpm.

Repeat steps #1 & 2 to double check before proceeding.

At this point you should be accelerating smoothly and reaching near peak rpm in short 100 yard acceleration tests.

You should be ready for some hi-speed rpm runs, fine tune with tachometer & plug readings... If rpm peaks and then falls off you are too lean - fatten up hi-speed and try again.. see #2



General rules

Each cylinder should be fine tuned as a separate motor/pipe/carb combination not all carb screws will be adjusted the same. Fine-tune each cyl with rpm/plug burn. Always tune in short 100 yard increments to minimize burn down’s..

If you have a difficult time... Turn in the hi-speeds ½ turn from existing setting then fine-tune the low-speeds first. See #1

Always do plug checks in the water under acceleration, don’t idle in to the beach. Take your plug wrench with you.

If ski dies, surges or picks up rpm when turning hard it’s either too lean, pop-off is too high, or too rich.

If it picks up rpm in hard right hand turns it ‘s usually too rich on the high speed circuit..
A gain in rpm in a hard left hand turn usually signifies a lean high-speed setting that requires more fuel.


The high-speed jet should be used to control fuel from 3\8 to 3\4 throttle opening
Use the hi-speed adjuster to fine tune throttle response from 3\4 to wide open throttle

The tuning tips apply to most vertical mounted carburetors with the diaphragms facing inwards and may vary if aftermarket intake manifolds have been installed.

 

[Waternut]

Ok I'll be completely honest... Blubbering, hesitating, falling on face, surging, etc don't mean anything to me. Depending on how you want to interpret it, it's like saying yes, yah, sure, ok, etc. They all mean basically the same thing. Regardless of carb tuning terminology, I do know what a lean carb sounds like and I know what a rich carb sounds like. The place where it gets a little confusing is when a plug gets fouled and then it gets tough to tell what's really going on.

That said, I want to state that I used the tuning guidelines above to get where I am now. Acceleration is good, high speed tuning is good. The place where my engine takes a dump is when I stabilize at at 2k-3k rpms for 30-45 rpms. Then the rear plug fouls.