Author Topic: separate part number available for the vacuum piston diaphragm on a CV carb?  (Read 3300 times)

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Offline Xanadu

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I have a 93 fatboy with the stock CV carb.  I installed the thunderslide kit including the plastic slide and the thunder nozzle, needle and main jet holder many years ago.  I would like to remove the kit due to horrible fuel economy.  I have been told by Dnya (the OEM) that once you remove the rubber diaphragm from the stock aluminum slide and install it on the plastic slide you cant go back due to the installation process.

The rest of the kit looks to be removable without too much difficulty. 

My parts book shows the vacuum piston (the slide) being a single part number ?????-88 but the rubber diaphragm is not listed as a separate part number. 

Does anybody know if the rubber diaphragm that fits over that vacuum piston can be purchased separately sparing the cost of the aluminum slide?  I would need a part number for the rubber diaphragm that would work with my 93 model year CV carb on a softail.

Again, this is a 1993 fatboy, stock CV carb.  I installed the thunder slide kit, including the thunder nozzle.

Any suggestions on something that will give descent performance without killing my fuel economy.  I am currently at 36-40 mpg,

the bike has SE heads, SE breather, EVL-5005 cam, D&D slip on's attached to 08 headpipes. 

I am trying not to change carbs to Mikuni, or S&S due to cost.  Any suggestions?

Thanks,

XMAN

Offline ViennaHog

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #1 on: Saturday, August 13, 2011. 04:43:13 AM. »
the diaphragm has no part number.

Offline FTBY55

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #2 on: Sunday, September 04, 2011. 12:53:27 PM. »
When I replaced mine it came as an assembly, slide and diaphragm together.

Offline hdbikedoc

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #3 on: Sunday, September 04, 2011. 05:41:04 PM. »
 :cry: Have to bite the bullet and by a diaphram slide unit not a seperate item,I have bought carbs on e-bay that were damaged for 5 dollars maybe try this

Offline macevo

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #4 on: Friday, September 09, 2011. 06:40:20 AM. »
kinda late but...Vacuum Piston #27585-88 ('91 flhtc)

Hope this helps

Mac

Offline FSG

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #5 on: Friday, September 09, 2011. 01:05:52 PM. »
27585-88    VACUUM PISTON, CARB,  XL     MRRP   46.00

« Last Edit: Saturday, September 10, 2011. 09:51:44 AM. by FSG »

Offline Xanadu

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #6 on: Tuesday, October 04, 2011. 11:05:48 PM. »
I ended up buying a new slide zanotti's I believe, the shipping at that place hardly makes the 20% parts discount worth anything as the end cost is as much as at your local "dealer".

Anyway, I polished the slide, drilled out the vacuum hole to 1/8th inch, installed the N65C sportster needle, with a shorter dyna jet spring, installed the stock jet needle holder and the 185 main jet.  I pulled the thunder nozzle and replaced it with my stock parts. 

The bike was running great with the thunderslide kit, only issue was the poor fuel economy, only 36-39 mpg.

took the bike out for a ride, it seemed to do just fine until I try to go past 1/2 throttle.  Past 1/2 throttle the bike has no throttle response, WOT in 5th gear it will not climb past 80mph.  Before I could get to 110 with no problems.  I am not sure if I have a problem with the slide or if there is an issue with the main jet. 

I recall reading something here from Craig Arizona about blasting compressed air into the throttle and the slide should rise, it doesnt.  I tried running the bike with the air cleaner off in the garage and inspected the slide as I revved up the bike, the slide does not move.

any suggestions on how I can get this bike to perform short of putting the thunderslide back in?  thanks for your suggestions.

Offline macevo

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #7 on: Wednesday, October 05, 2011. 04:57:26 AM. »
I had similar issue after changing needle on my FLHTC...in my case needle was tooooo rich.  However, in your case, if I remember correclty, you need to reduce main jet size (165?) when running the XL needle.

In any case it seems like it might be too rich for one reason or the other.

Good LUck!
Mac

Offline moosemush

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #8 on: Wednesday, October 05, 2011. 11:20:49 AM. »
You want to blow air into the opening above the venturi to check if the slide is working properly. It doesn't take much air pressure to get the slide to move. If it doesn't move then I'd concentrate on solving that problem before I'd start messing with jets.

Offline Xanadu

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #9 on: Wednesday, October 05, 2011. 11:26:37 AM. »
thank you for the picture of where to shoot the air, I was blowing it into the carb itself.  I tried to run the bike with the air filter off looking at that slide, couldnt get it to move, but couldnt get past 1/2 throttle in neutral either, the bike revs up very quickly in neutral.  I will try the air later today and report back. 

Offline tomfiii

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #10 on: Wednesday, October 05, 2011. 11:38:26 AM. »
Hi, I tried that thunder gas guzzler set-up and changed the nozzle&spring &needle back to stock. what I found that works is a washer or two under needle,richen up low speed ,but not too much,cut 1 loop off diaphragm spring. keeping the thunder slide will work fine &larger hole in stock slide ok.Note: a heavy bike will like stock diaphragm spring  afterwards had a small spot at low speed that felt strange and I used advice someone had in a magazine about drilling out carb holes/since he charged for work I drilled out all holes in carb(air bleeds ,nozzle) to next drill size which eliminated my small flat spot/also rounded and polished front edge of slide.

Offline remington007

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #11 on: Wednesday, October 05, 2011. 02:10:21 PM. »
Make sure the diaphram is not out of place. Its easy to do when the cap is installed.

Offline Xanadu

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #12 on: Wednesday, October 05, 2011. 07:23:58 PM. »
thanks tomfii for your inputs, to recap this is what I had:

CV carb for 93 fatboy
Thunderslide kit including the following:
1)black plastic thunderslide that I removed the diaphragm from my stock slide to fit onto.
2)thunderslide spring, and needle
3)thunder nozzle with weight and small BB.
4)  Thunderslide emulsion tube, (the part that holds the main jet), it was identical to stock with number of holes in same location as stock.


Performance was great, great throttle response, bike had no limit to the top end speed, it just kept going and going.

gas mileage was 36-39mpg.  even riding very conservatively I rarely got above 40mpg.  I had this configuration for many many years having it dyno tuned several times.  the main jet is 185.


I am reading of everybody getting much better mileage and people like Joe Minton stating stock performance can be as good as anything. 

So...

I made the following modifications:

1) removed the thunder nozzle with the weight and BB, reinstalled the OEM nozzle with the stock weight with the pointed tip.
2) bought a new slide with diaphragm installed, by the way once you remove the diaphragm from a stock slide, the slide cant be reused.  the attachment of the diaphragm to the stock slide is with a riveted connection, it does not come apart to reinstall that diaphragm, so I got a new slide from zanotti.  Have you checked their shipping prices lately?  I rounded the front edge of the stock slide, drilled out the vacuum hole to 1/8th inch per literature such as kip woodrings book, then polished the slide so it looks like chrome.
3)Installed a N65C sportster needle in the stock slide and mounted it on the bike, no washers, or other parts.
4) reinstalled the stock main jet holder with my 185 main jet.

put the bike back together.

took it for a test ride and it runs great up to about 1/2 throttle, then nothing.. it will not go any faster than 80 and there is very little acceleration.

When I installed the slide I could not get that screw off the side of the carb that holds that bracket over the top of the carb cover, I had to finagle it to get it together.

I blasted compressed air into the hole in the carb as shown by a previous post.  the slide did not move at all. 

I am going to pull the top off and get that bracket out of the way and try again.  how much air pressure or volume is needed to see that slide move?

I will report back as to what I find out.

xman



Offline moosemush

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #13 on: Wednesday, October 05, 2011. 08:00:40 PM. »
Not much. You should be able to blow into it with your mouth and make the slide move.

Offline CraigArizona85248

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #14 on: Wednesday, October 05, 2011. 08:18:25 PM. »
Yep... just blow into that opening at the top of the carb inlet and the slide should rise.  If not, something is wrong.  I'm not a big fan of drilling the carb slides.  That should actually make it rise easier though.
 
If you get stuck, bring the carb by some evening and we can disassemble it and look it over.
 
-Craig

Offline Xanadu

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #15 on: Thursday, October 06, 2011. 02:33:48 PM. »
A problem I had was not being able to get the screw loose that holds that throttle cable bracket to the side of the carb.  I got a T-handle screwdriver and that worked. 

I tested the slide movement with the compressed air, nothing.  I would not move with air, no matter how much I blasted in there.

I took the top off the carb and cleaned out everything, reinspected, found no damage to the diaphragm, and all the parts seemed to be there.  I put it back together and whala, the air test moved the slide no problem.  I am using the shorter dyna jet spring, a N65C needle, no washers, and a stock metal slide with the vacuum hole drilled to 1/8 inch (just slightly larger than what it came with).  put it back together and it ran great.  got up to around 105 on the freeway, I think that is about all it will do.  Now to work on the jetting.  the 185 may be too rich?  It is not blubbering, or missing at WOT conditions, and maybe 105 is all I can expect.  I never ride there by the way, just checking the main jet performance.

thanks for your suggestions, it was a slide that was not moving.  I dont know what was wrong only the slide would not move up under normal operation.  I could physically move it just fine, it just would not move when the bike was on the road.

Now, to see what it does to the MPG.....

Offline -deuced-

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Re: separate part number available for the vacuum piston diaphragm on a CV carb?
« Reply #16 on: Friday, October 07, 2011. 05:38:31 PM. »
the slide would not move up under normal operation
The rubber diaphragm wasn't seated properly. It's like someone cut your rev limit in half.