POLL: Best carbs for ported 701-760?

[egbrig]

@ProSouth, a 964 with enhancer laydown pipe dual 46's 148 pump, pump gas, in a 310 lbs. surf hull is flatwater back flippable by average riders and still ride surf. So you won't have to work too hard.

 

[Vortex]

Boss i've been riding for 15 years. And I don't back down either. I get it youre tough and want to make examples of people. I for sure won't be looking at your business the same way again.

Dasa 4 the win.

 

[egbrig]

LOL

 

[Vortex]

Have a focking weekend

 

[Pro-pulsion]

Ello lads, I am no pro tuner. Not going to play up anything because as mentioned the results in person will speak for you on what you can do. My ski runs reasonably well on dual 38's at the moment and they are set up as the typical porting/flame arrestor/b-pipe setup goes. One question I have yet to find an answer for is the stamped numbers on the carbs. I currently have a brand new pair of dual 46's, unmodified by anyone like Novi. The numbers do not match fully. The 8 002 yes, the rest no. I have asked Harry Klem if this was a concern he didn't seem to think so, I have spoken to a former racer from a long time ago, he said no, I have spoken to a Mikuni rep in Australia and he said no. As long as the 8 002 is there, they are untouched from leaving Mikuni, then they are made identically and can be run safely together.

Over the last week I have been testing these carbs trying everything to tune them and decided to toss on the 38's for now and start from the beginning again. I now believe I understand when the concern is mentioned in other threads to have matching numbers on all levels. One of these carbs requires a 1/4 turn of difference to respond equally to the other...at least on the low speed circuit. I have not gone any further with them at this point realizing this. My question is since all of the info I have been told, what do those three other stamped numbers mean? Clearly they need to match, I see that now, but does anyone know what they designate? I have read "probably a batch number" or probably this or that. There must be more to it if they are not performing identically as discovered with identical settings. Any thoughts? Do keep in mind I did not buy the second carb before consulting all the mentioned resources, I bought the second one after I was told it is safe to run them together. I wanted to not be an idiot first and jump in uninformed lol.

 

[long beach local]

I have a jetting question hopefully a Tuner will chime in and share if i am in the ballpark. I run a lightly ported 701 61x/62t in my surf/freak with a single carb intake, 1 48 powerbomb , Bpipe, enhancer etc and it ran AWESOME with a 140m 127.5p 2.5ns 95g 1turn out on both screws. Changes are I recently have rebuilt the motor and had the cases ported and glued in some V3 reeds also running a 701 industries flame arrestor which appears a bit more restrictive but keeps the water out better. I went to a 150m and its not as snappy as with the 140m so I am planning on switching back ?? IMO for my application i prefer the single carb for surf it feels much snappier off the bottom and i dont notice any difference on top compared to the 38s. Gas mileage is slightly worse but really not noticeable.

 

[egbrig]

@Pro-pulsion, are the returns drilled and is there a return jet in line? Is you popoff set the same for both carbs?
Are the jets matched? You have to verify that, I don't trust until I verify. That would be the first thing.
There are a couple of better tuners than me on this board but I can hold my own.

 

[egbrig]

@long beach local here is the theory on it. I'd have to see the case porting to determine if it really is affecting things but the biggest influence is being applied by the more restrictive flame arrestor. Because of this you will need to raise the popoff a bit to compensate, which may mean you will need to increase the low jet. But do one step at a time. Go back to the 140 if you say it worked better. That would be the best place to start.

 

[long beach local]

egbrig Thanks for the reply I was thinking the flame arrestor could be erasing the gains from the additional porting and going back to the 140m. Keep tinkering with it

 

[Jr.]

@Pro-pulsion you are correct, the carbs themselves need to be identical.
Like Erik stated, start by drilling out your returns and install a restrictor jet with a #70 in it. only use auth Mikuni parts!
the idea of using a "T" to tie the carbs together with a single restrictor is to equalize the pressures within both carbs
Make sure you have matching needle /Seats, springs? once installed, do an actual pop off ck to verify they are the same?
slight bending of float arm is acceptable to get the pop-off identical.
The Mikuni SBN manual is a very good resource for people just learning to tune for the first time.
It can be downloaded for free here http://www.mikuni.com/pdf/sbn_manual.pdf

@long beach local The Powerbomb carb once tuned is a nice carb. Most people tuning one for the first time,
might find it somewhat difficult to understand?? they normally like lower pop-off as the signal off it is weak compared to some other carbs?
When you had your cases ported, you increased the case volume, and slowed airspeed thru the case. both the arrestors and V3 further tend to restrict flow??
I agree with Erik to raise your pop-off your current pop will be about 17#?? change out your N/S to a 2.3 with a 95G spr to be at about a 20# pop
try this first with just needle adjustments? You might need to go to a 130 pilot? ( raising pop-off makes the pilot jet act like its smaller) You can also experiment with drilling out the return, and tapping the return fitting for a Mikuni main jet? start with a #70 return? you might find yourself going as low as a #55?? I agree, put the 140 main back in? overall, your probably not going to need more fuel, but rather when you need the fuel delivered?


This thread went full circle, but I like where its headed now! lets stay positive! One thing I feel we all learned is the amount of Bad info being circulated??
If you only have marginal experience? That's fine, we all have to have the opportunity to learn. Just don't come off as gospel when you try and give advise?

Ski Ya, Paul

 

[Pro-pulsion]

@Pro-pulsion, are the returns drilled and is there a return jet in line? Is you popoff set the same for both carbs?
Are the jets matched? You have to verify that, I don't trust until I verify. That would be the first thing.
There are a couple of better tuners than me on this board but I can hold my own.

I did verify the jets...numerous times lol. I think I have bought about 30 jets now trying to dial these things in. Pop off as well is the same within maybe 1-2 lbs. The returns have not been drilled, these carbs are both totally untouched and purchased from the same seller just at a different time. When I bought the first one I had intentions of running a single intake but never went forward with it, I contacted the seller a good while later after speaking with all the mentioned resources to see if he had any more of this very carb to which he did but as noted, the numbers were not identical other than the 8 002.

Thank you as well Jr. for the info, I believe I may have approached you about these after the purchase when I was having this tuning issue last season.

Needless to say I will have two single brand new 46's going up for sale in the near future lol. My main interest is the other stamped numbers, it may seem weird but it bothers me when I don't know what they designate lol. Here is the a pic of one of them...those 0 91 numbers I really want to know what they mean. I also noticed that on this carb there is a center punch dot too whereas the other one does not have that.

 

[MTRHEAD]

I’ll jump in on this one since the question says “best”. Nothing has ever performed better on any of my skis. From a 701cc Yamaha limited spec (no porting) superjet to 800+cc ported twin race skis. 50mm Novi wink or d shape booster carbs. I’d do 52mm if it wouldn’t hit the fuel passage. However on my normal fun superjet I run 38’s and enjoy the power.

 

[Quinc]

once installed, do an actual pop off ck to verify they are the same?
slight bending of float arm is acceptable to get the pop-off identical.
Ski Ya, Paul

When you verify pop off; Do you pop off each carb separate until they match. Or do you hook them both up in line (like dual 38's) and play with them until both needles pop at the exact same time/pressure?

@waxhead - For reverse jetting, I am running oem Mikuni 44's (not yamaha) at 122.5P and 125H should I try 125P and 122.5H? I was originally at 125P /125H and she was just a hair rich off idle.

 

[Jr.]

#Quinc You have to pop them seperate. And maybe adjust each a little until they match?
For the 38’s Its not as critical for the pop to be identical. First the slave carb doesnt have the same pressure as the pumper, but the real reason is the carbs have such a strong signal streangth, (they run a 55# pop) that the differance at those pressure is marginal at best.

#Pro-pulsion Im sorry I dont remember your contacting me? Hope I was able to help you?
If you still have questions you need help with? Please contact me directly via email.
Thats the best and fastest way for me to respond.


Ski Ya, Paul

 

[waxhead]

When you verify pop off; Do you pop off each carb separate until they match. Or do you hook them both up in line (like dual 38's) and play with them until both needles pop at the exact same time/pressure?

@waxhead - For reverse jetting, I am running oem Mikuni 44's (not yamaha) at 122.5P and 125H should I try 125P and 122.5H? I was originally at 125P /125H and she was just a hair rich off idle.

i normally run about a 130 low and 115 high 2.3 needle and seat, 115 spring which gives about 21 psi pop off

 

[Pro-pulsion]

#Quinc You have to pop them seperate. And maybe adjust each a little until they match?
For the 38’s Its not as critical for the pop to be identical. First the slave carb doesnt have the same pressure as the pumper, but the real reason is the carbs have such a strong signal streangth, (they run a 55# pop) that the differance at those pressure is marginal at best.

#Pro-pulsion Im sorry I dont remember your contacting me? Hope I was able to help you?
If you still have questions you need help with? Please contact me directly via email.
Thats the best and fastest way for me to respond.


Ski Ya, Paul

Thanks Paul, I really appreciate your knowledge and willingness to share. I will certainly be in touch but it will need to be a little later on. I know I need to get new carbs now lol...so first things first and get the singles ready to sell, buy a matched set and start the process all over again. And yes you did help, you recommended that I do this very thing to sell these off and start over again. Your initial thoughts were that without them being a precise match they will in a manner or speaking be fighting each other which I completely understand. Without knowing where the screws need to be set and the other variables...perhaps relative to the unmatched numbers...it is impossible to know if both carbs are flowing the same fuel. As noted, the one carb needed the low speed set at 1/4 turn out more than the other to achieve a very, very clean and happy idle. It would be playing with fire to assume the high speed screws need the same difference. I do know I could never fully resolve a flat spot in the immediate throttle response, no matter what I did it always would hit a split second pause unless I let off then back onto the throttle. The funny thing is, when I pulled the intake assembly off, it looked as though it was putting too much fuel through loading the base to a small extent. I have kept the notes on it, when the new, matched 46's find their way into my ski I will revisit this info.

 

[DAG]

With the tiny Mikuni popoff gauges it difficult to see when it pops. Use a 6" diameter large face gauge like @blaster800 and you can make out that it popped at 24.25 PSIG. You will thank yourself later by going to a larger display.

 

[Quinc]

With the tiny Mikuni popoff gauges it difficult to see when it pops. Use a 6" diameter large face gauge like @blaster800 and you can make out that it popped at 24.25 PSIG. You will thank yourself later by going to a larger display.

I use something similar with a valve hooked up to my air compressor.

 

[Jr.]

Thanks Paul, I really appreciate your knowledge and willingness to share. I will certainly be in touch but it will need to be a little later on. I know I need to get new carbs now lol...so first things first and get the singles ready to sell, buy a matched set and start the process all over again. And yes you did help, you recommended that I do this very thing to sell these off and start over again. Your initial thoughts were that without them being a precise match they will in a manner or speaking be fighting each other which I completely understand. Without knowing where the screws need to be set and the other variables...perhaps relative to the unmatched numbers...it is impossible to know if both carbs are flowing the same fuel. As noted, the one carb needed the low speed set at 1/4 turn out more than the other to achieve a very, very clean and happy idle. It would be playing with fire to assume the high speed screws need the same difference. I do know I could never fully resolve a flat spot in the immediate throttle response, no matter what I did it always would hit a split second pause unless I let off then back onto the throttle. The funny thing is, when I pulled the intake assembly off, it looked as though it was putting too much fuel through loading the base to a small extent. I have kept the notes on it, when the new, matched 46's find their way into my ski I will revisit this info.

Always Happy to help!
Let me give a little refresher and also try to explain why to those that might not know why everything needs to match.
When you set up duel carbs on a two cyl motor, you are effectively trying to tune two seperate motors running on a common crank shaft. If everything is equal, then the setup specs of both carbs will be very close to equal also.
Now take the same , only using carbs that dont match? You run the risk of two entirely differant carb specs for each cyl. As you go thru your tuning, its expected that you are getting one correct, and the other becomes an unknown, thus your tuning become chasing a ghost to figure out? Why do that to yourself when its just smart to start off initally with a matched set. Poor port work will also do this to your tune?
Another classic mistake people make is to not keep notes? You have to! Trust me, you will not remember the minor changes you make! When you do your changes, do one at a time! Start with bottom end or off idle and work your way into upper rpm ranges. Bottom definately effects fuel delivery at high rpm!
Other things that do effect your carb tune, is how you load the motor? Prop changes, water screw adjustments in B pipes, ect, all play a part in carb tuning!

Ski ya, Paul

 

[Pro-pulsion]

Firm believer in that right there ^^^ I did my own porting work, yes I did use a template but I also added some extra work in there that I saw as needed. Granted I am no pro tuner or port master but I am meticulous in my work and do my best. I can say that my new updates to my porting work have certainly yielded positive gains too. My ski runs very well, noticeably better than last year and I believe it is not just because of the time I invested but because I asked a lot of questions to the right people after reading as much as I possibly could. I believe it is counterproductive to ask and do without understanding the science and logistics behind it first. I have in the past drawn up my own templates but I figured this time around I would use the set from Yamanube with some extra inside features. I am quite happy with the results, now I just need to carb the engine the way I want, carbs are an interesting creature when it comes down to the final tune. When consulting with the guys that do this work daily and or have a long standing history of doing it well, I will never dismiss what you guys say. It might take me a good while to afford some of the recommended avenues lol, but in the end I know you're not telling me these things to bring in business, it's because you know from experience what happens when you did it the beginner way lol. We all have to start somewhere right, info like what you guys bring forward just help to keep us from going down those beginner paths lol.