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 ?

 

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.

 

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

 

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...

 

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...

 

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...

 

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.

 

Wow! That is a lot of difference! no wonder that I keep reading posts where one guy will say, "Hey, a 140 worked great for me!" and another guy will post, "I needed a 150."

There is no substitute for taking your bike to the dyno. Same as when I have built several turbo Miata's. You have to get a base line and then get the other data and work up from there.

I hope to be ordering some stuff next week.

Hey Everyone,

Please post links to websites where I can order, jets, needles and shims.

Thanks...

 

Mike at BellaCorse is a good source for the proper jets for CVK carbs

http://www.bellacorse.com/bce011-14.htm

15 bucks for a kit (42 pilots/145 mains & 4 shims) and you can get pairs of other sized mains (say 142's & 148's) for 6 buck per pair.... That's $27 for the pilots, shims and a good range of main jets. Add a set of 140's and 150's if you want... though the 142 - 148 set should cover you.

Tracking down needles is tougher... and Keihin has a butt ton of needles for their various carb models and you really need to know exactly which ones to order for the CVK's that Triumph uses. If you have a decent Triumph dealer near you, have them order you a set of "865" or NBZT needles... should be around 12 or 14 dollars for the pair, if I remember correctly...

The carb screw kit is a worthwhile upgrade... cheap too...

http://www.bellacorse.com/bce021.htm

 

This might be of help.

Found this in my saved stuff. It is for a 32, looks pretty similar to a 36. Pics might help you to locate the jets etc.
http://myural.com/keihin_32cvk_disassembly.htm

 

I just wanted to follow up on this thread in case anyone else is searching for info like I have been lately:

I got into the carbs and it turns out that the previous owner or previous tuner had installed 152 main jets. It has the stock needles and 3 shims under each needle. I forgot to take a look at the pilot jets.

Anyway, before I did anything I got the bike warmed up and took it up a long, mild incline on a nearby highway, took it up to full throttle and then slightly backed off to about 7/8ths throttle. There was no improvement at 7/8ths so I'm assuming from that, that the mains are sized about right.

Next I pulled it back into my shop and removed 2 of the three shims and took it back out for a spin. There was marked improvement in power from about 1/4 to 3/4 throttle and in the mid-range.

Next I pulled it back into the shop and took out the last shim, but just about that time my buddy showed up with some moonshine that he made and I don't remember much after that... and I haven't had a chance to test it again.

My thoughts are that I will be even better. Here's to hoping. :ride

Next up is to check the pilot jet. I'm 3 turns out, so my suspicion is that it is the stock 40.

 

I'm now running 150 mains w/o shims, thrux needles (I understand stock '08 T100s came with Thrux needles) and 45 pilot jets. I was running 42 pilots like a lot of others after opening up the air-box and putting open flow pipes on, but it was getting an almost predictable hiccup at stops like so many others have said they were getting. So thru recommendation of another friend of mine, I upped the pilots to 45s and put the air mixture screw at 1.5 turns out. The bike is running a lot stronger at idle, the idle no longer fluctuates and I can give the bike a quick turn of the throttle with the RPM being much quicker to recover and most of all…no longer hiccupping. I haven't had it dyno'd, but I can tell just by the seat of my pants, it's a much better improvement.

Let us know how things end up.