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Discussion Starter · #1 ·
I just got my bike Dyno'ed today, and without going into detail about my set up....... I would like to ask a question of either:

South Bay,
British Custom guys,
or
a knowledgeable authority of jetting for Predators

Those of us who frequent these on-line sites eventually become self proclaimed authorities of all things Bonneville. Self included.
NOT!

My jetting has resulted in a terribly rich condition, an odd lean condition, and not quite so lean condition amounting to 60.72 hp & 49.57 torque, and....... room for improvement.

The wisdom shared with me was to either contact "Dynojet" or "Factory" for a jetting kit. (and a few choice words about those of us on the Internet who think we know what the hell we're talking about......)

All that having been said,
is there some standard jetting suggested for Predators when sticking them on an otherwise stock bike? And that would be to include either:
with the factory needles of an '05 790
- or -
the substitution of standard needles with Thruxton needles

:hammer:
????????????
:???:

[ This message was edited by: Olderyoungster on 2007-03-08 19:27 ]
 

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FWIW I just installed the Preds with the Airbox removal kit on my 06 Thrux. I know it is not the exact same, but BC sent a set of 140 mains with 42 pilots, no shims, no adjustments to the D screw. I have no dyno results or any other remarks other than it feels helluva lot stronger than stock. When I get to civilization, I will find a dyno, I am curious.
 

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Discussion Starter · #3 ·
Unbelievable......

Impatient to wait for a response to my post, I went out to the garage and took the bowls off my carbs, changed out the pilots & mains, put it all back together, and fired the bike up all within a half hour!!!

I mean.... I've done this plenty times before, and the first time took me a few days, but under thirty minutes to do everything this evening?!?!?! The correct sequence, a couple precautions, some choice techniques, good light and reading glasses, that's all it takes. Amazing.

Okay, this is what prompted me to take action:

I had difficulty starting my newly assembled bike during the freezing months a few ago. So I figured what with the other changes I made, I should switch out the 40 pilots with some 42's. Didn't make any difference..... or little, depends on the power of suggestion. Turns out my ProCom igniter with 22,000 miles on it was the problem..... JoJi suggested that I stick the factory ignition module back in there and *VOILÀ* it started up with no drama.
BUT TODAY'S DYNO RUN SHOWED A REALLY REALLY RICH CONDITION AT/OFF IDLE......
so I stuck the 40's back in tonight.

For the last 2,000 miles the bike was running pretty danm good, but I didn't like the subtle delay in throttle response off cruising speed. Oh.... I might have stuck a shim on my Thruxton needles, but I still had an almost stumble that I've been living with. Well today's run shows it's pretty lean right there between 3200 & 4800 rpm.

From 5000 rpm to 7000 rpm my air/fuel was hovering at 14. So in addition to returning my pilots to the factory 40, I increased my mains from 122.5's to some 135's tonight.

So, all that said..... I'm running a NH bellmouth, K&N air filter, divider plate in place, Thruxton needles, and Predators on an '05 790cc. I wonder what "seat-o-the pants" will tell me over the next few days. It cost me forty bucks to have the local Harley dealership sniff the bike for me..... and people came from all corners of the building to look at the neat Bonneville. It was because I took my other Bonnie to them a year ago that the owner (of the Harley dealership, the local Yamaha dealership, and Honda, and another Harley dealership) jus' hadda have him a Scrambler.... and he loves it. Nice guy, he was pretty impressed with my red Bonnie this afternoon.
:-D

[ This message was edited by: Olderyoungster on 2007-03-08 20:31 ]
 

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I am no authority on anything...but it is very easy to get both rich and lean conditions with carbs...the different circuits of the carbs work at different throttle openings and overlap somewhat.

IMHO, too many times the picture received is that similar setups will mean that one should have similar jetting--NOT necessarily so. These carbs can be very different while being very alike.

My suggestion would be to get the jets you need and set it up on the dyno--I did this on mine. The result is that my mid-range is a little rich 12 A/F ratio with the stock needles. The pilot circuit and the main jet circuits are spot on at 13 A/F. I am running NH Togas, NARK, and modified stock igniter on a 790cc bonnie. I get 50+ mpg and I am not easy on the throttle. The main jets are 158, the pilots are 42--40's would have worked also, and the needles are stock.

If I were starting out with a stock bike (790cc) and just put Predators or NH Togas on it, I would begin with 120--125 main jets and see how the mid-range felt. If there were flat spots in the mid range, I would try up to two shims (0.5mm thickness) one at a time on the needles. I would begin the pilots at 3 turns out and adjust from there--now here is a BIG difference--you have a CA emissions bike and mine would have none of that stuff. :-D Until you open up the intake side of the equation, you still have a big restriction on airflow no matter how open the exhaust side. As you begin changing the intake to have less restrictions, you can go up some on the main jets. With the airbox out of the picture and K&N cone filters, most I have read about run between 150 and 158 main jets on 790cc bonnies. This is not the case for the 865cc bonnies--their jetting appears to me to vary over a wider range from what I have read on this forum. The Speedmaster and America and Scrambler are different engines (270 degree) so I don't read much about their jetting.

Remember the old rule about jetting: get the main jets sized correctly first, then get the mid-range at the proper mixture, and lastly get the idle jets and idle mixture tuned properly.

On my first suggestion of tuning on the dyno--I probably wouldn't spend the money until I got rid of the airbox and the CA emissions if I were in your situation.

My experience has been that the guys and gals on this forum have a lot of expertise and good advice. How well you use it depends Greatly on how you describe your problem, situation, etc., and how you interpret and filter the responses. An example I ran in to is the Thruxton needles for a 790cc. Many have used them with good results. Some described the reason for trying them being a "flat spot" in the mid range assumed to be from a lean condition. Some used shims very successfully to remedy this problem. I could feel no flat spot, but since age could be a factor here :-D I tryed some shims and Thrux needles...I could tell NO difference in the mid range--it ran very good whatever I did. When on the dyno with EGA, the reason was apparent because my carbs are a little rich in the midrange with the stock needles. The advice here got me in the range with the main jets and the aftermarket supplier sent main jets that were reasonably close.

Good luck...you might want to think about trying one of the EFI bonnies after they have been out a couple of years--I sure never want to go back to carbs on autos. :-D

Larry
 

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Discussion Starter · #5 ·
By the way, an interesting comment made by the Dyno tech'....

When I suggested that quite a few guys are going to pod air filters and removing the air box, he looked a bit puzzled and said, "WHY? It's obvious that your carbs are getting enough air, given your current lean conditions. I'd be willing to bet that there isn't much to be gained with pods!"

That having been said,
If my BonnieBlack 790 produced 61 hp & 48 torque with the divider plate removed and D&D's
- and -
My red café racer 790 produces 61 hp & 49 torque with the divider plate in place and Predators.......

- what does a 790 sans air box produce? -
 

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Discussion Starter · #7 ·
I had no problem getting main jets for the carbs at either the local Harley shop or Yamaha.....
- but -
the pilots are a different story. After many attempts at OTHER motorcycle shops, I ended up orderint the 42 pilots from the Triumph stealership an hour away.
 

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"-what does a 790 sans airbox produce?-"

Mine produced 67.5 hp at 7800 rpm and 54 ft lbs of torque at 3600 rpm--elevation of dyno was 714 ft.

Remember, dyno results can vary quite a bit--not all are created equal. Some have said that the dyno's found at dealer's shops can be a little more liberal with the hp figures. :-D I had mine dynoed at an independent shop--not a lot of choice around here.

I have also read some comments about having to "tweak" adjustments on carbs sans airbox. In 12000+ miles, I have done no adjustments and it has run perfectly in all Wx conditions throughout the rpm range. I checked the carb balance once at 12000 miles and it needed no adjustment. I have ridden in rain and sunshine--couldn't tell any difference. I did put some "outerwears" pre-filters over the K&N cone filters just in case I get caught in a real hard down-pour.

Larry
 

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Larry......
I would LOVE to see a copy of your Dyno run!

If your torque curve and power comes on as smoothly and in the same linear fashion as mine with the air box in place, then I MIGHT consider all that time, money, and effort may be worth another 6 horsepower.

After I switched out my 122's to 135's last night, my 30 mile ride proved to be VERY gratifying. But far more important to me than peak power, is the delivery of said power. Put me in the camp of concluding (after running both) that the divider plate in place is VASTLY superior to running with it removed.

Dyno runs used EXCLUSIVELY FOR their peak power figures are absolutley worthless! When I get THIS configuration dialed in, I'll post the power curves and air/fuel ratio runs of both my 'divider plate in' run and 'divider plate out' run. My run yesterday already shows how erratic (by comparison) the air/fuel delivery is with the plate out.

For a couple years now, I've been asking for proof via Dyno run charts that all these mod's we're doing affords the rider any advantages. Boasting of peak horsepower figures is worthless if the rest of the chart is all over the map.

And in the natural course of events...... I may just have to find out for myself what removing the air box will do. I challenge somebody ANYBODY (again) to post his Dyno chart results (power/torque/air fuel) of a run without the air box in place.
 

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I had TOR's and a K&N and it dynoed at 62hp with 49 torque. It was a 790 engine at the time. BC told me that they got really good results from the TOR's, same as their Predators. Going with the TOR's on the big bore as well, among many other upgrades.
 

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First let me say I also am no authority on anything at all, but what I do know is that throttle position is critical to tuning your bike. The best thing you could do is to place tape marker's on your throttle at 1/4-1/2-3/4 so you can see what position your throttle is in when at 3200/4800 rpm's.

Thats going to determine what adjustment you make, be it a extra shim, a larger pilot, or a larger main. I've read 13.7 is where you get really good performance out of these bikes...( I believe Bill Gately told me that )...if you were at 14 with a 122.5 I'd be willing to bet your fat with a 135.


But, what position was your throttle in?
 

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Pat,

I would gladly send you a copy; however, the printer was out of ink the day I did the runs and I never did get a printout. I never dynoed mine with the airbox in place, so I couldn't offer any comparison. We must have done twenty runs trying to find some needles leaner than the stock needles, but we were unsuccessful. The power delivery is just as linear as with the airbox, except there is more of it across the rpm band, IMHO (butt dyno comparison here).

I did see a few dyno run printouts posted about a year ago for 790's (I believe) after airbox removal.

Larry
 

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Thanks joje.... how exactly was your 790 set up then? I'm assuming you removed the air box and kept the factory carb's. But did you do anything more, say.... to the ignition at that point?

I don't know what 72 translates to with regard to torque, what is it in ft-lbs?

And given what you know now, would you consider that the simple removal of the air box (with a free flowing muffler)..... nothing more, would produce significantly more power/response/drivability through the whole power range than a set up similar to what I'm running?

I'm going on the assumption that with the proper jetting for my bellmouth, K&N, and pipes..... that rear wheel power, though slightly less, would be more linear and more "drivable." Dare I say, NOT for the drag strip.... but for everyday riding and canyon carving.
 

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72NM=53 ft-lbs

It had K&N's and some $20 megaphones with inserts, not much louder than TOR's. Nothing else was changed at that time, it was only a week old..

Well, I have found that removing the airbox is worth more than changing pipes, at least for torque. My 904cc has 74hp and 66 ft-lbs with stock pipes, last baffle removed.

To say that it doesn't need more air since it's already lean is just wrong, you can plug the airbox and still make it run lean with smaller jets.
http://www.stabbarps-auto.com/images/74hp.jpg
 

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And.. It's really simple to tell if an filter/airbox mod works or not, if you need bigger jets to get the same A/F it works. And I needed 155 on that 64hp Bonnie

[ This message was edited by: jojje1963 on 2007-03-10 16:11 ]
 
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