Re-jetting the Keihin CVK Carburetor

As I have shared in earlier posts, I’ve had to learn about re-jetting the stock Keihin CVK carburetors used on the 2008 Hinckley Triumph Bonneville because I modified the air intake and exhaust system. To take full advantage of these performance modifications, the stock carbs need to be able to breath more freely to allow for extra airflow.

Recently I decided to switch my carb needles back to the stock Bonneville, after installing and testing Thruxton needles for a month or so. Here are some photos I took during the process to help you understand the process better.

Step One – Preparation

I jacked up the Bonneville on my floor motorcycle jack. I removed the seat, side covers, and then the gas tank. Next I removed the carburetor brace provided by British Customs in their Airbox Removal Kit.

Carburetor Brace

British Customs Carburetor Brace

Step Two – Removal of Carbs

I disconnected the wires attached to the bottom of each carburetor. This was followed by loosening the hose clamps attaching the carbs to the intake manifolds… in my case the TPUSA billet manifolds. This allows the entire carb assembly to come off the engine and can be turned sideways on the left side of the bike. I found that a 2×2 wood placed between the transmission case and the carb assembly works well at holding it in place.

Carburetor Assembly

Carburetor Assembly Disconnected and pulled sideways, resting on wood block

Step Three – Carb Bottom Adjustments

The next step was to remove the bottom float bowls. I use a 3MM hex wrench because I have replaced all the stock philips head screws with Stainless Steel ones from New Bonneville. These are much more rugged than the stock screws and allow for multiple on-off cycles without stripping. I use an old towel below the carbs to soak up the gas that spills out of the float bowls, when these are removed.

Removing S/S screws from bottom bowls

Removing S/S float bowl screws using 3mm hex

Here are a couple views of the carb bottom with the float bowls removed and call out to the various adjustment locations.

Bottom view with float bowls removed

Bottom view with float bowls removed

Here’s another view of the carb bottom with float bowl removed.

Another view of carb bottom with float bowl removed

Another view of carb bottom with float bowl removed

Once this was done, I was able to access the main jets, pilot jets and idle mixture screw to install the sizes and settings I wanted to test. In this case I was only changing the pilot jets from size 40 to size 42.

Here's what the Keihin pilot jets look like

Here’s what the Keihin pilot jets look like

Installing the pilot jet with a small flat end screwdriver

Installing the pilot jet with a small flat end screwdriver

Then the float bowl is reattached using the same s/s screws.

Although the idle mixture screws I have can be adjusted when the carbs are fully mounted, I prefer to adjust them at this point using a needle nose pliers with some tape to protect the brass. In my case, I was following the recommended 2.5 turns out.

Adjusting the idle mix screws with a needle nose plier.

Adjusting the idle mix screws with a needle nose plier.

Step Four – Carb Top Adjustments

Now it is time to address the top of the carb, and in my case replace the Thruxton needles with stock OEM Bonneville needles.

First you remove the 4 screws holding on the diaphragm cover. It is important to know that you must hold the cover down while you loosen the last screw or the spring inside the carb may push the cover off unexpectedly, and you may lose or damage a part.

Removing top diaphragm cover with 3mm hex

Removing top diaphragm cover with 3mm hex

After the diaphragm cover is carefully removed, remove and carefully store the spring. Now it is time to remove the vacuum piston. I discovered that an easy way to do this and avoid damaging the rubber diaphragm is to insert a finger into the center hole until snug, then left the entire piston straight up and out.

Vacuum piston with diaphragm cover removed.

Vacuum piston with diaphragm cover removed.

Vacuum piston shown half way out.

Vacuum piston shown half way out.

Once the piston is out, I removed the Thruxton needle, and replaced it with the stock needle. In my case I also left 1 shim on the needle. Here are the parts laying on my work bench.

Top carb parts

Top carb parts

Once the needle was replaced, I inserted the piston back into the carb using the single finger method again. Notice in the top view of the carb (below) where I have indicated the hole where the needle will insert into the center of the main jet that was installed from the bottom of the carb. As the piston floats up and down, it raises and lowers the needle inside this main jet. This action is what regulates the gas flow into the carb.

Top view showing main jet inside where needle inserts

Top view showing main jet inside where needle inserts

After the piston is back in place and seated correctly all the way around, it is time to reinstall the cover. Make sure you remember to tighten the 4 screws down the same amount. If one is a little loose, outside air will be pulled into the carb and it will run lean, causing the engine to not run correctly.

Here’s an exploded view diagram of the Keihin CVK carb for additional reference.

My Keihin CVK Carb Settings

My Keihin CVK Carb Settings

I hope this description and my photos will help you in your Keihin CVK carb modifications and adjustments. Let me know your experiences.

32 thoughts on “Re-jetting the Keihin CVK Carburetor”

  1. Hi there Randy, I see you’ve done some more dialing in on the carburetors on your bike?Wondering how you find the results? I was also thinking of Thruxton needles but see you’ve changed them back to the std needles?
    All up now, how are you finding the jetting combination of your bike? Your a few steps ahead of me here seeming there seems to be nobody carrying carburetor parts for these Kehins down here in New Zealand. BC seem to have a limited sized jet supply and the Dynojet Kits are designed to run with the stock airbox.
    My bike still isn’t right but runs better on the open road since my modifications. Would love to get it properly sorted. If I could find someone with a Dyno that understood carburetors I’d have it sorted straight away.
    Look forward to hearing any positive results you may want to share.

    Cheers, Pat Reardon

  2. Hey thanks for the diagram. I own 2008 bonneville. It has been sitting in my garage for 2years. Now I am having a problem. I figure I need to clean a carburetor first. Can you advise me how to clean it?
    thanks,

    Saehwan

  3. I have gotten the Pilot Jet spring assembly stuck inside the carb. Any suggestions on how to get the spring and washer out?

    1. Ahhh, this happened to me once also. I believe that I just used a sharp thin ice pick type of tool to carefully straighten and tug the spring/washer out. A small needle nose plier might also be helpful. I remember that I did remove the carbs from the engine to get at it better. Hope this helps! It’s a pain, I know!

  4. I have a 99 Triumph Thunderbird with three carbs similar to yours in the demonstration. I have taken the three carbs out twice to cleaned them but the left carb backfires at less than 2,000 rpm. I know it is only the left carb because it goes directly to the left exhaust pipe while the other two go to the right exhaust. Idling is a little rough as well. Above 2,000 rpm the motor runs fine.

    Any help will be greatly appreciated.

    1. Robert,

      I don’t have experience with your year and model of Triumph or version of the Keihin carb. It may be possible that you need to do more than clean the carbs to fix this problem. You may need to check things like the pilot and main jets, float levels, carb synchronisation, and for just any part that has more wear than the others. Do you have a manual (I use a Haynes Manual) for your bike and have you followed those steps for checking and cleaning the carbs? Do you have a carb sync tool?

      Randy

    2. hello Robert, im having exactly the same problem with my 2002 thunderbird. its really got me snookered! if you rectify yours can you please let me know what it was. many thanks cameron.

      1. Cameron, I will let you know sometimes next week as I am in the process of putting the carbs back on but I only work on the bike a few times a week when I have time. Robert

        1. hello robert
          thanks for your swift response. my last eureka moment was when i realised that one of the OE jubilee clips on the offending cylinder had “bottomed out” and couldn’t be tightened any more which would lean out the mixture. so i stripped out the carbs once again and changed the clips for readily available 13mm wide clips……. still no go! truth be known its now worse so ive know ordered some 8mm wide clips which will properly fit in the grooves of the rubbers.
          n.b. im 100% positive that the carbs are rebuilt correctly and the only thing to do is set the balance…….once it runs consistently.
          cheers for now .
          cameron

    3. Are you balancing your carbs as a final step as well? You will need a quality sync tool, they cost around $85 USD.

      1. Balancing was never an issue with me becasue all three carbs are held together by a metal rod that goes the link of the cars. My problem is I consider the 99 Thunderbird as new since it only has 7,000 miles After it dawned on me that rubber and by products stiffen over time which caused me to not be able to put the three back into place. After I put new rubber intake manifolds and the rubber piecve that attaches the carb to the filter they fit right back in and I have had no problem since.

  5. Very informative many thanks,
    i have a 2007 T100 and want to change the main jets…. My tank has no fuel tap to drain it down, any clues.

    Regards

    Keith

    1. Keith,

      A stock T-100 should have a shutoff valve where the fuel line exits the tank and goes down to the carbs. You would turn the lever so it is 90 degrees to the fuel line. If you look close it should even say on it what position to close it.

      It should be visible on the left side Near the rear of the tank just above the carbs.

      If you don’t have it, then someone has altered it from stock. You will need to get an actual petcock valve either from Triumph or an aftermarket one like a Pingel.

      Let me know if this helps you locate it.

      Randy

  6. It seems we are all at one time or another having carb problems. My 2005 Thruxton was left untouched for a while. Started after charging the battery but ran only on one cylinder. after stripping the carbs, I found our dreaded enemy, corrosion! It was a real challenge because when all together again, the other cylinder failed! I must have had the carbs on and off ten times. Finally it is moderately reliable once again, but I have ordered the re-jet kit from Thunder bikes NZ, I have standard filters. My thought are that the corroded jets were really not retrievable. Great site this hope to see more. Mike

    1. Michael,

      Thanks for sharing your experience. How long was your Thruxton sitting unused? Was it stored in an unusual setting… high humidity, rain, etc.? I am trying to understand the corrosion… how that could have occurred. But, I agree, once corroded, it would be best to clean and re-jet the carbs as well as replace the needles. Those parts on a Keihin CVK need to be very clean and fresh.

      Randy

  7. Just celebrating a happy ending.

    After a lot of head scratching and soul searching I bit the bullet and stripped out the crankshaft position sensor. It turned out to be a GILL component. These are renowned for their unreliability. The coils turned out to be the same. So they were all replaced with PVL bits and now she runs like a train! SPRINT MANUFACTURING of Warminster deserve a mention as suppliers of fine components. Both reasonably priced and swiftly delivered.

    Thanks everyone ….. now where’s me helmet?!

    1. I have learned that the Hinckley Triumphs fuel system impacts the exhaust and air filters. Changes or modifications do not work well. I think this discussion started with questions about servicing the air filters and Keihin carbs for my 99 Triumph Thunderbird. I talked to the dealer about changing the air filter set up he said you could install three separate filters but the re-jetting and changes to the Keihin Carbs would be cost prohibitive. I discovered that the flexible plastic(?) intakes from the air filter to the carbs and from the carbs to the cylinders had hardened. It was a piece of cake putting the components back together with new intakes. The sir filter fit perfectly to the carbs and the carbs to the cylinders. Foolishly, I spent hours trying to reconnect everything before I realized the rubber intakes had hardened.

  8. just one last message before i head off. one of the things that i found during the dozens of times that i removed the carbs was that the airbox has been badly designed. it overly stresses the carb mounting rubbers. these should be fitted wide side up. i modified the air box by taking about 10mm from the topside of the guide for the frame bolt to slide into, at the rear of the box.there is a moulding that helps with the size. this allows the box to sit lower and also locks the box forward in its intended position. hope this helps. cheers for now.

  9. Randal: Enjoyed your post. I have been through this on my 900 Sprint w/Mikuni’s. Did the airbox removal, re-jet and big pipes. Now I just bought a Bonneville last week. Looks like about the same routine. Mine has Sayintune pipes and is a bit lean.Thanks mostly for the photos so I know where I am going. Have to order a manual this week too.

    1. Jim,

      Welcome to the Bonnie Club! Contact Bill Gately at Bonneville Performance to get yourself a nice Mikuni 42mm carb conversion kit and lots of support. Those Saintunes look almost identical to my Predators, so you should have a very similar setup and experience to me. Let me know if I can help!

      Randy

      1. Randy: Put my gauges on the America the other day. Everything seems to be well adjusted there. I checked idle mixture and found it’s problem. Left carb was 4 turns whole the right was 1/2 turn ! Set a them both at 2 1/2 & itseems a little lean at low speed. Went up to 3 turns with out much change. Pops a lot on deceleration. (no exhaust leaks). My thinking is larger pilotjets. What do you think ? …J.D.

  10. Jim, I’ve been told and have read that some minimal popping is normal, especially on aftermarket exhausts. Bumping up the pilot may help and is certainly worth a try as long as you don’t mind the possibility of going back to the size you have now after you compare the performance of the two sizes. I’ve found that adjusting carbs requires a lot of trial and error to get things just where you want them. Your former idle mixture settings were certainly not helping… Wow!

  11. Went through this when setting up my 900 Sprint that has a set of Micron slip-ons. Tuned it out to just a sweet “growl” when you get out of it. Nice but not enought to drive the neighbors nuts.

  12. I have a 2003 T-100 that (I think) has been running a bit lean. Going to use this guide for changing the jets on mine too. Do you need replacement gasket sets for the float bowls?

    1. Scott, I don’t think you need to replace gaskets on the float bowls because they are rubber seals that sit in groves machined into the surfaces. Unless those are really bad and deteriorated, you should be fine. The part is T1241036… #14 in the diagram. Keihin CVK Exploded View Diagram

      1. Thank you Randy! I’m actually from the Chicago area too, I’ve probably seen you around at MCC, Motoworks, or Morrie’s Place, if you head that far North. Very nice page you have here! I’ll have to check it out in more depth when I get home. I love my Bonnie, just a few minor things to do with it, carbs and next is changing the leaky front sprocket seal.

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