My Jenks' Bolts Keihin CVK36 Carb Jetting Experience
Jenks’ Bolts, a motorcycle accessories business in the UK has published the best document on Keihin carburetor tuning for the 790-865cc Hinckley twin Triumphs that I have found to date. In previous posts, I have mentioned this document and my experiences with it.
This past weekend, I experimented again with the carb tuning recommendations in the document related to my 2008 Triumph Bonneville.
The relevant basic setup on my 2008 Bonnie with the stock 865CC engine and Keihin CVK40 carbs:
- Air injection removed (BC Air Injection Removal Kit)
- Airbox removed (BC Airbox removal kit)
- Free flow air filters (K&N)
- Open free flow exhausts (BC Predators)
- Billet intake manifolds (TPUSA)
Before this weekend, my Keihin CVK 40 carb settings were:
- Main Jet = 145
- Pilot Jet = 42
- Shims = 1
- Pilot Screw = out 2.5 turns
- Needle Code = stock Bonneville Needle
- Slide Air Hole = 2.95 mm
- Miles per gallon: 35-40 in city riding, 45-50 highway
The changes I made were based on the chart on Page 10 of the document:
- Main Jet = 140
- Pilot Jet = 40
- Shims = 1
- Pilot Screw = out 2.75 turns
- Needle Code = NBTZ (Thruxton Needles)
- Slide Air Hole = 2.95 mm
- MPG (Premium): 45 highway
I would have to say that I noticed very little difference in the low RPM range. However, I am experiencing some power loss or hesitation in the upper RPM range which needs to be addressed. On page 9 of the document in the section “1. The Best Main Jet” it says: “To get the best, most even top end power (full throttle from 4000 to 5000 rpm), select main jets that produce the highest top speed and pull strongly at high revs.” Based upon this, I will probably put the 145 jets back in and see how they perform. I may also need to try an additional shim based upon the results of the 145’s.
UPDATE: August 29, 2012
I have replaced the 140 main jets with 145’s and noticed an improvement in performance. My current setup is:
- Main Jet = 145
- Pilot Jet = 40
- Shims = 1
- Pilot Screw = out 2.75 turns
- Needle Code = NBTZ (Thruxton Needles)
- Slide Air Hole = 2.95 mm
- MPG (Premium): 38-40 city, 48-50 highway
Update November 3, 2012
As colder weather has arrived in Chicago, I’ve realized that my setup was still not the best. I had idle problems, and I started losing power at about 4000 rpm. So, I made the following changes and this seems to be working well for me.
- Main Jet = 145
- Pilot Jet = 42
- Shims = 3
- Pilot Screw = out 2.75 turns
- Needle Code = NBTZ (Thruxton Needles)
- Slide Air Hole = 2.95 mm
- MPG (89 octane): 38-40 city, 48-50 highway
The Dynojet Option
I do have the Dynojet Stage 1 kit that I purchased from NewBonneville. However, when reading the instructions I realized that the Stage 1 kit is not intended for Bonnie’s with the airbox removed. The jets that come with the kit are not large enough. I contacted Dynojet about this and they told me they do not make Stage 2 and 3 kits for the Bonnie. However, Michael Cory did say that: “You can use the Stage 1 jet kit with the Keihin 140 main jet.”
I plan to try this option out in the future and will write a blog post about my experience.
Nice Bonnie. I also live in Chicago and had the same black Bonnie (2009) with Predator pipes I bought from Motoworks. Unfortunately my Bonnie was stolen in Lakeview a month ago but was able to get a used 2007 Silver/Black Bonnie with only 2K miles on it last week. Now time to start the upgrades all over again! You commute downtown? -Brad
Boys, I’ve done all that shit. Save suffering about it and go get the Dyno jet kit. You’ll be riding with a grin from ear to ear. Those boys know what they’re doing. It’s all in the needle. Believe me, it’s money well spent and instead of suffering around, you’re out enjoying the ride!
my 08 scrambler has sat a year and of couse wouldnt start . i cleaned all the green crudd out of the carb and floatbowl. it runs with some constant help from the choke, but does not idle. anybody have a clue out there?
I got a “6sigma” jet kit and installed it. No choke…….no nuthin…………starts right up and runs FINE!!
Randy, Jenks bolts sells this document…it is a small company…the more people plagiarise its products/works the more it is likely to cease trading. Thanks but your enthusiasm is not helpful……..
Iain,
Thanks for your comment. I was not aware that this was a product that your company sold as it is easily found in multiple places on the Internet doing a simple Google search. It would have been helpful if the document stated clearly that it was a product for sale and not information that was freely shared. I have not been able to find anywhere where it can be purchased, nevertheless I have removed it from my website. Good luck with your business.
Randy
These numbers are invaluable to me. I have a very similar setup to yours, 08 Bonneville with airbox, AI removed, KN pods and identical (well, Toga’s) pipes.
Last test I did was with:
140 Mains
40 pilot
2.5 turns out
NBTZ needles
No shims
Stock air hole (2.5mm?)
Rough idle that will only sit with the choke on. If I throttle up to 1/8-1/4 open, then roll off, I get a backfire into the carb big enough to blow the carb clear off if not for the British Custom’s brace.
I had a local shop back my the screw out to 2.75, but I’m not anticipating that giving enough to resolve the issue. I may also have a carb sync issue and burnt up plugs, but more research will tell.
I’m up in Madison, so we have roughly the same riding conditions. Thanks for posting all this!
Edit: I have NO air box, it’s just straight pods. (who proof reads anymore?)
I’m thinking, with my smaller 40’s that referencing your 2.75 turns is probably still too low for me. To compensate the difference, would you think 3-3.5 turns would be worthy of a try?
These darn things use D screws, what red coat over at Triumph made that design decision? 😀
Matt,
I know of no carb setup where the turns out needed to be more than 3. You should be able to find the sweet spot between 2 and 3 turns. More than that means you probably need to make an adjustment somewhere else. I always replace those D screws with thumbscrews from NewBonneville.com which make carb tuning so much easier! You might try those! Good Luck!
Randy
Those screws look great, thanks for the recommendation. For posterities sake, here is my current “working” (I need to get more miles on to be truly certain) config:
140 Mains
42 pilot
2.75 turns out
NBTZ needles
2 shims
Stock air hole (2.5mm?)
This is with the airbox removed, KN filters, Norman Hyde Toga Peashooter pipes and AI removed. The carb blow back was due to a carb sync issue. Once that was sorted, I was able to find a steady idle easily.
Thanks again, Randal. Love the site and all your hard work giving back to the community and helping folks like me!
I’m happy to help! Glad you got things sorted out!