Triumphrat

Irishguy · 12-05-2012, 06:52 AM

I have a few questions for the carb gurus on the board:

When I bought my 2001 Bonneville it already had the ARK and the D&D pipes on it and a rejet, supposedly done by a dealer in Atlanta. I noticed however that it seemd to be a little low on torque in the low to mid range. This has been confirmed now that Mrs. Irish has an America with similar mods and a 2002 Bonneville with similar mods. Her Bonneville seems to have much more power than mine and seemingly the only difference is that she has Predators instead of D&Ds. Yes they have the same sprockets.

Mine seems to be fine at full throttle, but only seems lacking in the low to mid-range. This would lead me to believe that I might should try either shimming my needles or buying a needle with a different taper or maybe both. Maybe check the pilot jets too and go a little bigger...

What do you guys think I should try?

denbow · 12-05-2012, 07:39 AM

First - have a look at the countershaft sprocket, could be the PO swapped the stock 17 for an 18 or 19.
You could try 42 pilots, Thruxton needles (no shim) with one turn out on the mixture screws.
What Mains have you got in there ?

Bron-Yr-Aur · 12-05-2012, 09:16 AM

I had the same problem. A shim seemed to fix the problem, but it ended up being just a bandaid. Turned out my mains were too small. She pulled hard at WOT with a flat spot in the middle, but going up 12(!) points in my mains and no shims was what it really needed. You have to make sure you have the right mains first, or you'll just be chasing ghosts.

I don't know much about tuning 790's, but I believe usually they don't require a bigger pilot. First, find out what pilots and mains you have, and count the turns in your mixture screws and check for shims. Then you'll have a good idea of where to go from there. You may very well only need to richen up the mixture and add a shim, but it's hard to tell without knowing what you have already. Don't forget, if the jetting has been messed with and you don't know what's been done, an overly rich mixture could produce is similar loss of power, especially if the word 'boggy' would apply to your problem.

fullplat1 · 12-05-2012, 09:52 AM

You need to know what you are dealing with before attempting to fix anything. Check the main jet size, the needle part number, the pilot jet size and the countershaft sprocket size, then we will be able to help.

Phil

SCbonneville · 12-05-2012, 11:06 AM

Quote:

Originally Posted by Irishguy

I have a few questions for the carb gurus on the board:

When I bought my 2001 Bonneville it already had the ARK and the D&D pipes on it and a rejet, supposedly done by a dealer in Atlanta. I noticed however that it seemd to be a little low on torque in the low to mid range. This has been confirmed now that Mrs. Irish has an America with similar mods and a 2002 Bonneville with similar mods. Her Bonneville seems to have much more power than mine and seemingly the only difference is that she has Predators instead of D&Ds. Yes they have the same sprockets.

Mine seems to be fine at full throttle, but only seems lacking in the low to mid-range. This would lead me to believe that I might should try either shimming my needles or buying a needle with a different taper or maybe both. Maybe check the pilot jets too and go a little bigger...

What do you guys think I should try?

Hey Irish guy... Let me help you out a bit on understanding carb set up for the CVK's and what shimming needles does... My set up is somewhat similar... air box carve with Staintune exhaust. I also have the Pieman reprogram done to the ignitor, that while not being strictly "mandatory", it does optimize the ignition timing curve to take full advantage of the intake and exhaust mods... I will also add that my set up has been verified via dyno testing to give me the air fuel ratio that I was after, across the board... and should put you in the ballpark, tuning wise. Also, comparing the "torque feel" between a 360 engine (Bonnie) and a 270 engine (America) is really difficult. The 270 configuration is totally different in how and where in the rpm range, it delivers its torque.

The ABSOLUTE FIRST thing you need to KNOW, regardless of anything else... is what is in your carbs now... You can guess all you want... and that's what you'll be doing... guessing... If you don't know where you're at now, you can't make the proper adjustments. The flip side of that is to assemble a small "jetting kit" before you even tear into the carbs, have everything on hand and then just install your desired package.

Also, tuning carbs is all about the inter-relationship between the pilot jets, pilot screw position, needles (shape/profile & shimming/height) and the main jet size. They all contribute to the air/fuel flow at various throttle positions and there is effective overlap between them. If you google "tuning Keihin carbs" you will come across articles that will show and explain this.

Let's start with the Pilot jets... you can use either 40 or 42's... they will overlap in their effective range... But to get a 40 to "flow" enough you need to open the pilot screw around 3 to 3 1/2 turns to "equal" a 42 at 1 to 1/2 turns out... So you are at the end of the effective tuning range on the 40's at the point that they need to be typically set with a set up such as yours... I recommend 42 pilot jets with the pilot screws ~ 2 turns out for your set up.

Needles... ahhh... the black art of needles and shims... Ok, the needles meter the fuel coming out of the main jets and affect the amount of flow and at what throttle position that flow comes into play via needle diameter, profile (shape) and height. Keihin offers over 100 various needles for the CVK... don't worry... we're going to look at just three...

The stock 790 needle basically has the profile of a nail... and when used without any shims, brings the main jets into play very slowly. This, along with the cam profiles, creates a "torque dip" between 4 & 5k rpms... Shimming (raising) the needles helps bring the mains in sooner but the straight, "fat" profile is still not optimum for addressing that "dip"... Per the Triumph dealers service manual, Triumph uses two different profiles now, the "Thruxton needle" (used on Thruxtons & Scramblers) and the "865 needle" used on everything else... I have found that the "865" or NBZT coded needle along with one GENUINE Keihin shim works best on my set up. When looking at my torque curve from dyno runs, this has just about eliminated, or has minimized, the "torque dip" typically found in the 4 to 5k rpm range.

Main jets... this is really the easiest variable to deal with... all it has to do is flow enough gas at wide open throttle to match up with your air flow and give you your desired air/fuel ratio, typically around 13 to 13.5 to 1... Depending on mufflers used (effective flow) and your general elevation (air density), within set ups such as ours, main jet sizing between 140's and 150's is typical, give or take a few points either way. 145's are spot for me...

Ok... so now we have an idea of what gets us proper fueling across the complete throttle position range... Here's what I would recommend: assemble a jetting kit consisting of 42 pilot jets, a set NBZT needles and 4 GENUINE Keihin shims (they are flat ground and not punched out, as a washer is. And they give very finite, predictable and repetitive adjustment), and a range of main jets (I would think that 140, 145, 148's would cover your needs. add 142's and 150's if you want to have all bases covered). Look at your carbs and see if they still have the (soft) phillips head screws in them or have they been converted over to the stainless hex screws... If they have not, buy a set from one of the vendors (Mike @ BellaCorse has a kit)... cheap enough and then you won't strip out the heads and it makes doing "insitu" carb adjustments a lot easier. you should be able to assemble all this for around 50 bucks or so... I would go with the 42's, screws 1 1/2 to 2 turns out, NBZT needles w/1 shim and the 145's... install that set up, see how you like it... and then find a good dyno operator and take your bike and jetting kit over and see exactly where you are... and you should have what you need in your kit to make any needed adjustments...

If you want to get maximum "enjoyment" out of your 790 set up, I HIGHLY recommend that you contact Mike (Pieman) at Triumph Twin Power to have your ignitor reprogrammed... This will pull all your engine mods together and you'll be surprised just what a 790 engine can do... If you do the reprogram, you WILL need to install Barnett "green" clutch sprigs... as I can guarantee that you will get clutch slippage at upper rpms... the stock springs are not strong enough to hold the plates together... and no, there is not much increase in lever effort... With the above mods, you should be making around 70 rwhp and 50 lb. ft. of peak torque... with a base of around 44 lb. ft. and rising quickly...

I hope this helped and wasn't too long winded...

Irishguy · 12-06-2012, 07:03 AM

Thanks for the replies everyone, especially SCBonneville.

I know that I have a 17 tooth front sprocket, because I put it on myself. I will check on the main jet and pilot jet.

I have seen the Dyno Jet kit on ebay for $69, but where should I go online to source the jets and needles that everyone is talking about so I can get started on this thing?

fullplat1 · 12-06-2012, 07:50 AM

Irishguy
just get the float bowls off and check what you have FIRST. Obtaining replacement parts is easy once you know what you need. As for the Dynojet kit, their jets and needles are DIFFERENT to the standard ones and require a bit of knowledge to use effectively.

SCbonneville, on the 790 the pilot screw can be opened to at least 5 turns and will continue to provide adjustment, in almost every case a 42 size jet is not required. These details were worked out long ago and published at large on the Delphi forum, sadly you can't read back and most of the original research has been lost. Jojie spent many hours experimenting and testing.

SCbonneville · 12-06-2012, 08:54 AM

Quote:

Originally Posted by fullplat1

Irishguy
just get the float bowls off and check what you have FIRST. Obtaining replacement parts is easy once you know what you need. As for the Dynojet kit, their jets and needles are DIFFERENT to the standard ones and require a bit of knowledge to use effectively.

SCbonneville, on the 790 the pilot screw can be opened to at least 5 turns and will continue to provide adjustment, in almost every case a 42 size jet is not required. These details were worked out long ago and published at large on the Delphi forum, sadly you can't read back and most of the original research has been lost. Jojie spent many hours experimenting and testing.

After 3 to 3 1/2 turns out on the screws you have effectively surpassed the "overlap" point with the 42's... every tuner will tell you that that is the point at which you should step up (or down, as the case may be) to the next size... Keihin even says as much... but what the heck... what do they know? Also, the further you turn the screws out, the less tension the spring have on the screws and they can vibrate loose and even fall out...

Pilot jets are cheap.. and if you're going to go to the trouble to properly set up your carbs, why not do it the right way?

I gave Irishguy advice based on my ten years experience of working with the my and my friends Bonnies in various states of modifications... backed up by a LOT of dyno A/F ratio read outs... he and anyone else, can use it or not... the choice is his...

Also, just taking off the float bowl will only give you (easy) access to the main jet... the pilots are recessed into their carriers ahead of the mains... and of course, as we all know, the needles/shims are accessed from the top of the carbs... and even they are setting down in the slides under the spring spider...

SCbonneville · 12-06-2012, 09:02 AM

Quote:

Originally Posted by Irishguy

Thanks for the replies everyone, especially SCBonneville.

I know that I have a 17 tooth front sprocket, because I put it on myself. I will check on the main jet and pilot jet.

I have seen the Dyno Jet kit on ebay for $69, but where should I go online to source the jets and needles that everyone is talking about so I can get started on this thing?

Irish, you can certainly use the Dynojet kit... it's a good one... but you can put together your own kit that will give you a broader range of adjustment for about the same money or maybe even less.

Jets are available through a number of vendors such as BellaCorse or newBonneville, etc.. I got my needles and shims through my Triumph dealer as they deal with Sudco/Keihin.

JetsRus is another source...

bluesforchallah · 12-06-2012, 03:05 PM

Also, make yourself clear when ordering your jets. The picture below is of a main jet purchased thru British Customs. They're both 150's, but you can see the bright shiny new one is a little larger than the old tarnished one.