Bonneville America Forums Forums Technical Forums Performance Mods air screw thumbscrews

I know some have replaced their carb slotted head or D head air screws with knurled thumbscrews for easier adjustment...where can I get these?

Pretty sure Brent and BellaCorse have em.

thanks, i'll check bella corse...i haven't found them on brent's site...

yep, bellacorse does indeed have them...they're not cheap, are they? $40 USD plus shipping! geez, eh...

I have heard several people on here complain about them for various reasons, I'm content with the D tool

Old Wolf,
Check with Grump. Last time I talked to him he may have found a supply person.

Jerry

OW,

Don't waste your money. As far as I know, the knurled thumbscrews everyone is selling are for HD Keihin carbs. I have a pair, and recently found they don't do much, if anything, due to the different taper they have from the stock screws in our carbs.
I am currently looking into getting some made by a friend of mine. It is too soon to know if he will be successful, but if he is, I will let everyone know.

thanks all for the info, gents...grump, you're right...the bellacorse site specifically states that they're from HD, and grzegorz has not been happy with his either...he has offered to send them to me at n/c if i want them, and you know, i think i will take him up on it...maybe the taper can be modified by filing or something...i believe they are brass...i'm sure you'll post something if/when you come up with an alternate, grump...i will post if i have success with modifying these HD ones

The gotcha with those is that they are tapered much more than the stock ones. The 4+ turn range you have with the stock screw will be reduced to maybe 2 turns at best. They can work, but the idle mixture adjustment will be very touchy.

I've been asked by a Forum member to comment on this subject. Yes, the Idle Mixture Thumbscrews I sell are also sold to fit H-D carbs. But H-D uses -a- Keihin CVH carb and Bonneville's use Keihin's CVK carbs. The major difference being that the CVH is intended for single carb applications and the CVK's are linked pairs.

Older non-CV Keihin carbs use a much bigger idle mixture screw, so if you are buying them from a H-D house, be sure to specify the newer CVH carbs.

Above the screws are a spring, washer and o-ring. The o-ring is extremely small and very easy not to notice if they fall out. I belive that this is the route-cause of any problems users are experiencing. I recommend placing a rag or dish under the carbs when you remove the stock screws.

It is also my opinion that if you need more than 3-1/2 turns-out to get your bike to idle correctly, your Pilot jet is too small. Under 1-1/2 turns... Pilot jet is too big. I have found that 2-1/2 turns-out (as the factory recommends) is an ideal starting point. At least, I base my jetting recommendation based on 2-1/2.

Now; before anyone goes Jihad on me, let me say that I am not presenting myself as the end-all of Bonneville experts. I base my opinions on what I personally experience with my own Bonneville's, as well as feedback from my customers including many dealers. There are many variables that go into optimum jetting and there is no such thing as one size fits all.

Michael Selman
SEL-MOTION

Michael, and all...
This is not a Jihad... just my experience.
The HD thumbscrews I have were purchased from an outfit in Arizona called Direct Parts. Their p/n for them is CP005.
They fit HD Keihin carbs CVK40 and SE CVK44.

When I bought and installed them, I never looked at them closely, I just wound them in and backed them out 3 1/2 turns. The bike started better and ran fine. My springs, o-rings, and washers are intact, btw. Recently, a method of adjusting them by disconnecting a plug wire was discussed on here. I tried that and found that the HD screws made no difference until nearly bottomed out, and even that difference was negligible. The bike was still starting and running well, so I left them adjusted at 1 1/2 turns.

Recently, I had the opportunity to have a set made, based on my stock screws. I just got them at work tonight, so I haven't tried them yet, other than to make sure they screw into the carb correctly.



Top left is the new screw. Bottom left is the stock screw. The right is the HD screw.

I could not get any closer with my camera and stay in focus, but I think you can see the difference in the taper. The tapered portion on the HD screw is also very slightly longer as well.

And that's where I'm at with the thumbscrews right now.
Once I try them, I'll report back.

I don't know how the screws that Michael or Brent sells compare to our stock screws, I just assumed (yeah I know about assuming) that the HD thumbscrews were a specially made item and therefore all the same.

My thumbscrews are from BellaCorse (P/N BCE-051) and I verified my springs, washers, and o-ring's are intact.

The taper on the thumbscrews do not match the stock CVK36 screws. The thumbscrew "point" is thinner and a bit longer.

In my case, 3 turns out on the stock screws = about 0-.25 turns out on the thumbscrews.

My current set-up is:
- Drilled Airbox
- K&N Drop-in
- Stock Pilots (142)
- 132 Mains
- TOR Longs
- Stock needles
- 2x shims with stock screws; tried shimmed and no-shimmed with the thumbscrews.

I do plan to change pipes in the future and give the BellaCorse thumbscrews another chance.

So, I'll stick to my original "observation" in another post. It appears the BellaCorse (HD) thumbscrews allow more gas to be passed and pbobably require lowering the jets.

To be fair, only a couple of Dyno runs with each type would provide empirical data.

Grump,

If successful, I'm intereseted.

Regards,

Tom

Grump,

You should adjust the carbs with both cylinders firing. It's my opinion (and just that kids, an opinion... not gospel) that the difference in taper(s) is relatively insignificant. The idle mixture screw works like a water faucet. Unscrewing it creates more of a gap between the tapered needle and the o-ring allowing more fuel to pass into the engine. Just like with the main jet needle, a longer narrow taper will give you more range of adjustability, whereas a short/blunt point would work more like an on/off switch.

But unlike the main jet needles, there is no need to re-jet when installing the thumb screws since this fuel circuit works primarily at idle. So if (for example) 2-1/4 turns out is ideal with the stock idle mixture screws, then with the thumb screws you -might- have to adjust then to 2 or 2-1/2 turns out to get the same results. The justification for installing the thumb screws is ease of adjustment.

Here in upper Mid-West, I find myself turning-in the thumb screws 1/2 turn during the hot Summer months.

Mike

Here are the results of my testing....

First of all, even though most of this info doesn't enter into the equation, here is how my bike is set up.
130 mains
Stock pilots
TBS needles
AI out
Snorkel out
1 3/4" hole drilled in airbox and stock filter
Thunderbike pipes

I removed the HD pilot screws and installed the new tscrew pilot screws at 2 turns out. Started and ran fine - popped and banged like crazy on decel. 2 1/2 turns out to 4 turns out - did the same. It got slightly better at 4 turns, but not much.
I removed the new tscrew pilot screws and installed the stock screws. Exactly the same results.
I removed the stock pilot screws and reinstalled the HD tscrews. 1 turn out. No popping at all. A pleasant rumbling on decel.
I adjusted the HD screws in and out. They made no difference in the idle until nearly bottomed out, and again it was negligible..
I left them at 1 turn out and put 100 or so miles on today - bike ran fine.

I think it's probably slightly rich at idle. I haven't checked the plugs yet. Based on all the info I've gleaned from this site, I would imagine that reducing the airflow or possibly going with a larger pilot jet would let me use the stock pilot screw again and tune out the decel popping.
I realize having a dyno to test all this on would answer all the questions, but I don't have that luxury.
Until I get the tinkerbug again, I'll stick with this setup.

If I'm all wet here, throw me a towel....
Talk amongst yourselves.......

I put the BellaCorse thumbscrews back in yesterday.

Bought a Dremel on sale at Lowes and increased the size of my airbox hole and opened up the bottom of my K&N all the way. Cleaned and lightly oiled the K&N. Bought a new pair of NGK's and side-gapped them. Re-synched the carbs. Put two shims back on the stock needles. None of this should matter much, but what the hey.

I was very careful to ensure the idle mixture screw o-ring was actually fitted to the screw just before the taper. I may have just dropped it over the taper on the original install.

At about 3/4 - 1 turn out, I'm happy and leaving the thumbscrews in. I also put about 100 miles on today and the dyna-mo-pants was happy. No back fires, and very light slight popping on decel.

Fo what it's worth, my setup with thumbscrews:
- AI Removed
- Hole in bottom of airbox all the way to "nubs"
- Bottom of K&N airbox removed
- Snorkel removed
- TOR longs (sounding pretty good actually at 6000 miles)
- 132 Mains
- Stock (142) pilots
- 2 shims (home depot brass variety) on stock needles
- BellaCorse thumbscrews
- Stock NGK's side-gapped