Bonneville America Forums Forums Technical Forums Performance Mods Drilling Carb slide holes

What size was the drill bits you used to drill the carb vacuum slide hole with? Anyone that did the big bore do this yet?

I plan on drilling the hole a pubimeter bigger and putting in the CVK40 lighter springs for smoother operation.

Hey Pat, what are these holes you speak of? Just curious.
By the way, pubimeter.... I'm rollin man.

Check out STEP 3 and Cutaway reference #4, the Vacuum port here.
The article also explains the concept really well.

And oh yea, feel free to use my term "pubimeter" a very politically correct term.

Ok so what's it do?

You want to use a 7/64" drill bit to open the hole in the top of the slide, then use the next size up bit to chamfer the hole a bit, just enough to clean up the edge. Also I have installed a shim on each needle, a #45 slow jet and anywhere from a 130 and up for the main, depending on your riding style. This will get you a smooth quick responding throttle that you can crack like MAD without break up!

Member
Gary James

It gives more vacuum allowing the diaphram to react quicker which allows for more drastic and immediate throttle response.

Makes your carbs work faster. This along with a lighter spring should be a good balance with these carbs. Every DynaJet and DynaPro kit does this as well.

Very interesting. So Pat, you're not gonna go back to the aftermarket carbs, even after the big bore?

Na, gonna stick with teh CVKs until the new Amals are out, then look at them. I'm sure I can get these to work fine by dicking with the needles and jets I can flood it if I want, so getting enough gas is not an issue. I just want it faster. Supposedly the Amals are working really nice on the prototypes but not for sale yet.

Not to beat a dead horse, but what are the differences with the Amals as opposed to the Mukinis (that's what you had right?) and the Kehins?

Just because Benny, just because I want to.
The Mikunis were way to rich and I want to try something different. Not even sure if the Amals are CV, Flat side of what. No real reason other that the price sounds like it will be between the Kehins and the miks.
They were the original carbs on Triumphs and you used to have to trickle the carbs until gas came out, then they might start. You could always light you arm on fire if you lit a cig to early too. Been there, done that.

Hi Pat,
Thanks again for sharing your knowledge - this time on the drilling.
I've got one Amal off a '68 Tiger you can have. It definitely needs rebuilding again, but I think the little gland ring at the top is busted. Why did I keep this thing?
Maybe I should sell it on ebay.
Thanks Dinqua,
Dennis

Pat, if you're really feeling that nostalgic about wanting to have your arm on fire again, I could arrange that for ya:)

I hear you about "just because you want to". I mean hey, my bike rode just fine stock...

Amals are concentric round slides, not much different that the CR IIs, actually they have been around forever, old technology, good part is the tune-ability, bad part is old school technology, you loose the TPS (not really an issue)except for off idle throttle response, under good load they should perform similar to the CRs.

Pat
Is there a reason you are doing this with the big bore upgrade, or would it be the same advantage with the stock 790? Not that I need more, but hey like you said, just because I wanna.

ooooooooooooya,
Sure, do it now it will help your carb inards move easier and faster. Helps the throttle response to be more gooder.

Pat,
I know this was a pretty dumb question, but when one is dumb about something they ask dumb questions, or worse yet, don't ask anything at all. Thanks for the information, it sounds like something I might do/get help doing before spring hits.

any suggestions on what size springs to use? I think i'm gonna try this. The yellow ones gonna get fasta! trumptrash

The springs are the ones from a CVK40 carb, don't know any part #s or sizes though, sorry. Any HD shop should have them, they are stock carbs for some the HDs.

This might be where the info on the softer springs is coming from.

http://www.nightrider.com/biketech/hdcvcarbappendix.htm

btw, they are about $3 apiece at my local shop...

I never saw that one Phil. Good link, I wonder if anyone has played with any of these needles? Maybe I'll take a ride over to the new HD botique and see if they have any motorcycle parts for sale there too. I'll bring my other needles and do a micrometer comparrison. Hmmmmm.

Am I hearing this right? Are Amal carbs still out there? Talk about flashbacks...what's next, Bings? I'll admit that the old carbs were a lot more simple, but hopefully technology has improved on them. As for the starting problems of yore, I blame it on Lucas.

Though it doesn't say so at their web site here: http://www.burlen.co.uk/ Burlen is supposed to be making the new Amals. From what I saw in a web search a few days ago, there must be some pre-production units floating around in that vast wasteland to the north of us (Canada, eh!).

First models seem to be having a problem with float needle leaking, so the advisory is to shut off the fuel valve when the bike is parked. Standard fare for us old Turnip riders...

Hey Pat,

I'd love to see if the needles are anywhere close to the TBS or Thruxtons....

Pat,
I just got around to reading this thread and was wondering how you made out?

DOH!! If I would look at the dates first I would see that you just wrote this a couple of days ago.

Since we're talking slides, I was surprised to see the extent of slide wear. Should you reverse the slides to even that wear out, or is it a no-nevermind issue?

CV40 or CV44 needles will not work in the CVK carbs, totally different, however the CV40 springs are a good upgrade, allow the slide to rise faster, if you change springs you have to drill the vacuum ports also. The springs are softer, less tension, therefore will allow slide to rise quicker, but must enlarge vacuum port to make it work right. CVKs are vacuum assist slides, CVs are not same, although similar in design. The CVKs are simply a "whimpy" carb, not very responsive like the CVs.(Accel Pumps).

thank you dinqua you too bonnyusa. I'm tuning for the quickest acceleration hope this helps. agin thank you. trumptrash

Hey guys, when you are drilling out the vacuum port, how do you keep the metal shavings from getting into the carbs? Do you actually have to pull the rubber diaphram and the slide out of the carb when you drill? I imaging this is the safest way.

If so, all I have to say is "crap". I had to get that little rubber diaphram back into the groove on the carb once already... that SUCKED! I suppose it'd be worth the extra throttle response, but I'm not looking forward to it.

Triumphant,
If you're leary of pulling the slides out, as you need to if you're gonna drill them, then just get a lighter spring as mentioned above.
Another point to bring up is that this mod works the best if you have open exhaust and free flowing air, ie: freak, K&Ns and good pipes.
The spring route may be best if you're concerned about it. But remove the slide if you're gonna drill else you'll end up with crap in the carbs that will cause all sorts of trouble. Mr. Murphy guarantees this.

Hey Pat, you think lighter springs would be good for anyone regardless of setup?

Okay, I drilled them out today. As others have said, there is a noticable improvement in throttle response. I only test rode for a few minutes, and then I had to do other stuff - so I'm hoping to get in a better test ride later. In a few days I'll have the HD spring to try out too.

For anybody who is wondering how to do this, I wrote up some quick instructions with pictures:

http://frontiernet.net/~dmclec/slide.html

Gary,
OK, now I have a dumb question... How did we come up with 7/64ths?? Is this a predetermined HD thing? Why not 1/8th?
I'm just curious.....

Grump, 1/8th might actually hit the inside wall of the slide - which I don't think would be good. There is very little room to open the hole up. I suppose somebody could drill another hole... but that won't me me.

When putting the carb slides back into the cars, do the vacuum hols go in front or in back of the needles? Orientation wise...

Edit: the only go in one way... Someone was thinkin....

I sure do hessitate before drilling / enlarging holes in carbies. Has anyone out there got a long term bike that this mod was done to? Like has there been any negative effects as a result?
And the people who've made this mod, have they done it to standard bikes, or to bikes with the air injection removed and air box removed etc? I guess what I'm asking is ... can this extremely easy mod be made to a standard bike?
Madly in love (with black and silver '02 BA!)
Michael888

michael888 - I'm not sure if it would have the same effect on a stock bike. I just don't know how dramatic it would be since the stock bike is so restricted. I have my air box heavily modded (but not the freak), SS pipes, rejetted, and TBS needles.

When I did it, it was quite noticable. Throttle response is much more crisp when I crack it open.

I agree with triumphant, I have freak, and put my TBS needles in the same time I drilled. I think the drilling is really more for response than anything, so it could be done to just about any setup...

Thankyou for your response Triumphant.
Appreciated.