STILL need some help with tuning!

[yamanube]

Ported 61x cases, ported 64x 760 cylinder, stock head and ignition, Riva Powerbomb carb. Currently at 130p 150m, 115 spring 2.5ns at 20psi popoff. Idle is nice and power seems good right off idle but after 1/2 throttle it struggles to buid RPMs. If I get a little air in the pump it will get past the hang in the revs and makes nice power, sometimes if I stay up in the higher Rs it will rev nicely to the top but if I keep the pump loaded it doesn't like to get past it. If I do a turn sub it will struggle to build RPMs but if I give it a little hop and unload the prop it takes off like a raped date. Most of the time it's just blehh then it will take off and brap nicely. I do not have access to a tach.

Also I had started with MattE's specs in the carb database (117.5p132.5m) and worked up from there to where I am at now.

I ended up putting in a 155 pilot and 155 main just for s and giggles and it helped the problem. Still slow to respond when tooling around at slow speeds but when riding around in the midrange it is clean and revs quick to the top.

*update: I have swapped on an SBN 44 but still having issues, see post #23

 

[Philip Clemmons]

Ported 61x cases, ported 64x 760 cylinder, stock head and ignition, Riva Powerbomb carb. Currently at 130p 150m, 115 spring 2.5ns at 20psi popoff, carb does not have the restrictor drilled. Idle is nice and power seems good right off idle but after 1/2 throttle it struggles to buid RPMs. If I get a little air in the pump it will get past the hang in the revs and makes nice power, sometimes if I stay up in the higher Rs it will rev nicely to the top but if I keep the pump loaded it doesn't like to get past it. If I do a turn sub it will struggle to build RPMs but if I give it a little hop and unload the prop it takes off like a raped date. Most of the time it's just blehh then it will take off and brap nicely. I do not have access to a tach.

Also I had started with MattE's specs in the carb database (117.5p132.5m) and worked up from there to where I am at now.

I have only worked with one of these setups, so my info may be off base.....................BUT, I could never get clean, seamless carburetion with the Powerbomb on the 61X cases.................

My theory is the reduced size of the reed cage restricts airflow through the carb to the point that the Powerbomb circuits cant meter properly................AKA not enough signal.

Just my .02 from the one I messed with. Bolted on a SBN 46 and it ripped!

 

[yamanube]

I had a mildly ported 61x 701 with a bpipe ADA heAd and powerbomb and it ran beautifully I just have no idea what jetting I ran. I also ran the same PB on a 790 62t/62t motor and it ran very well. Lesson learned, write down jetting that works.

 

[yamanube]

I ended up putting in a 155 pilot and 155 main just for s and giggles and it helped the problem. Still slow to respond when tooling around at slow speeds but when riding around in the midrange it is clean and revs quick to the top.

 

[SUPERTUNE]

Ported 64X cylinder with a stock head will aggravate this problem, 760 cylinder needs some decent compression to work! Stock cdi? MSD Enhancer will help a little, a MSD TL, Flame TL or Epic will help a LOT!

 

[SUPERTUNE]

OH and I did send you a pm as well...

 

[Matt_E]

I have only worked with one of these setups, so my info may be off base.....................BUT, I could never get clean, seamless carburetion with the Powerbomb on the 61X cases.................

I have gotten one to work great with 61X cases and ported 760 cylinders. But that was a few years ago, and I think Riva has changed something since then.
Over the past year I keep reading how people have great trouble with those carbs.

I am a bit fuzzy on the rest of the motor, but I think it was this:
home-ported 61X cases with ported stock manifold
home ported 760
B-pipe with limited chamber
MSD Enchancer at first, then stock CDI
stock reeds

That thing hit really nice. I tried a 48 Full Spec, the Riva, and a 46 SBN on it. The Full Spec and the Riva were almost identical, and the 46 wasn't far behind. In fact, bang for the buck, the 46 would be my choice.
180 psi compression

 

[yamanube]

Yeah the last powerbomb I ran was a silver one and it has the internal restrictor drilled, it seemed much easier to tune for some reason. It feels pretty close now and I am pleased with my homebrew porting, if I can clean up the midrange a bit I will be happy.

 

[yamanube]

Still fighting with soggy low end, just bumping this back up to see if anybody has any ideas.

 

[Billybo455]

Have you checked your needle and seat? Maybe the pop off is off.

 

[yamanube]

I checked the popoff at 20psi with a PO guage.

 

[Billybo455]

i couldn't get my am 46's to clear up on the bottom until about 24-26 psi. have you watched the carbs while the ski is idling on a stand? is it dribbling fuel? if so does it look excessive?

 

[yamanube]

I haven't checked that but I am going to try raising the popoff and see how it effects it. I don't have a popoff guage and probably won't make one just going to play with it. I had checked the previous setup and know it was opening at 20psi. That was with a 2.5ns and 115g spring, the only other NS and spring set I have is a 1.5 and 80g spring. Going to test it out and see what happens, maybe play with the screws a bit...getting sick of tinkering with it so I will likely try swapping on a SBN44 if this doesn't help.

 

[kevbo]

Do not play with oversized carbs until you have absolutely mastersd your stock carb.

 

[orangefinger]

1.5 n/s ....

 

[yamanube]

Do not play with oversized carbs until you have absolutely mastersd your stock carb.

All I have had and used for the past 3 years is a powerbomb, both as singles on 62T and 61X motors and as duals on a 66E, I have never had as many problems tuning as I am with this particular carb. I have borrowed an SBN44 from a friend that I will try to run once I get a rebuild kit for it.

 

[yamanube]

1.5 n/s ....

??

 

[yamanube]

Bumping up my popoff helped considerably, it is still lacking a little bit of power on the bottom but it doesn't seem to bog or burble anymore and rips on the midrange.

 

[Billybo455]

what are the high and low speed adjusters set at? i'm a little curious what he means by 1.5 n/s to. mikuni states in their manual that a 1.5 n/s will flow more fuel then the actual carb can... i want to say my best luck was with a 2.3 n/s with a 115gr spring. if you ever want to make a trip my way i've got a pop off setup, but it's not really portable :/

 

[Jr Freestyle]

are you using a return restrictor jet ?