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850T3 Carb Rebuild - Tuning issues

Steve-H

Just got it firing!
Joined
May 4, 2010
Messages
18
All,
Just rebuild the VHB30s on my 850T3 and installed them yesterday. Bike fired right up, and as I blipped the throttle to start adjusting the idle screws the idle climbed to 4000 rpm. Thinking that I had just screwed the adjusters in too far, I shut the bike down and backed the screws out a bit. Restarted - same issue.
Ultimately believe this is symptomatic of a vacuum leak. The carbs were exceedingly difficult to remove and mount (bike has the cast metal intake manifolds and the VHBs have those plastic insulators on them). So my first thing will be to replace the insulators; as I did not remove the intake manifolds, etc. Anyone have a good source for these?

Any other thoughts? Could a poor float height be to blame as well?

Thanks!
Steve
 
You didn't accidentally put the slides in backwards did you - short side facing the engine?

Tip for easier removal and installation of these carbs: apply a little anti-seize on the inside of the plastic sleeve and on the manifold stub. Not a lot, just a little dab smeared around with your finger.

AFAIK, the plastic sleeves/insulators are no longer available anywhere. The ones for a VHB29s are, but they have a 35 mm i.d. vs. 37 mm for the VHB30 sleeve. Last time I needed some, I machined them out of Delrin tube.

Float height is very easy to check/set on the VHB series carbs - invert the carb and measure from the bowl surface to the bottom of the float. It should be 23.5-24.5 mm. What floats do you have? Hopefully not the new type white ones which are rubbish.
 
I've had similar sounding experiences when blipping after a carb rebuild. Sometimes the throttle cable sleeve will jump out of the cable adjuster at the carb and it's off to the races. It just takes 1 cable end and the engine stays at the high RPM of the blip.
Seems to only happen with my home made cables tho. Making sure the boots are on helps. On the new bike I hafta safety wire the sleeve into the adjuster to keep it there.

Alex
 
Thank you both very much! To your points:

  • Slide direction - no, double checked that a few times. Also believe that on these it isn't possible due to there being a slot where a portion of the slide rides in on one side.
  • Anti-seize - I had the same idea, so the carbs will be much easier to take off this evening ;)
  • Carb insulators - I thought I was able to find a pair, but after seeing your note requiring 37mm ones - believe I may have to make them.
  • Thanks for the help with the float heights. I did order the white plastic ones, but the existing brass ones looked fine so I left them in place. Will double check the height tonight.
  • Regarding the throttle cables - honestly they look iffy and the metal ends are loose. I've ordered two new ones.

I'll report back what I find this evening.

Thanks again,
Steve
 
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One other thought: jets in wrong positions - since both jets are the same physical size, it's all too easy to put the main jet where the idle jet belongs and vice versa.
 
Thanks again. So I took the carbs off, and here are my findings:
  • Jets are in their proper place. 50 idle and 123 main (bike has pods and exhaust).
  • Float height is 24.1mm - right in spec
  • Based on my inspection of the anti-seize pattern on the carb insulator, I struggle to believe that they were leaking
So...this leaves me with throttle cable issues. Honestly they are the originals, so can certainly be replaced without too much thought. It bothers me that they worked fine before I took the carbs off though...

Thanks again for all of your help this far. I certainly will keep this thread up to date once I get the new cables in.
 

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One way to eliminate the possibility of the cable sleeves popping out and catching on the adjusters when ya blip the engine is to check them if the problem recurs.
If, during blippage, the engine races, turn it off and look at the cables. See if the cable sleeve is stuck on the outside of the adjuster. If not, that isn't probably the cause.
Cable woes

Please ignore the rubber boot in the above picture... I gotta replace it.

Good luck,

Alex
 
OK gents - time for an update....

The new throttle cables arrived. Upon installing, I found out that there is not enough free cable (eg cable outside of the housing), regardless of routing. Thus when installed, holds the slide up:


So I began comparing the throttle cables, and found two things of note...First one being that the old throttle cable is ~6mm longer, but also has different housing ends on the carb side:


That got me thinking....And I actually do not believe that the bike had the threaded adjusters on the carbs when I disassembled. I now have 4 of them as the choke rebuild kit came with a pair, so now that they are installed these throttle cables are too short (or the housing is too long) to work with the Tommaselli throttle that is currently on the bike....Now things are starting to make sense.
Here is a shot of the amount of free cable when it is installed in the carb (with the slide fully closed) from the throttle end:

Clearly insufficient. Thinking this post is getting lengthy, will post a new reply with my path forward.
 

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Ok so first off - what are your thoughts???

If I continue down the path of assuming that all of the above is correct and I do not have the proper amount of free cable (it should be noted that I routed the cables identically to how they were before, the previous owner velcro'd them to the rear spine), then it is time to bust out the dremel. As I don't want to sacrifice the carb adjusters, I need to make some housing adjustments at the throttle boss. Taking some quick measurements, the freeplay adjuster inserts 10mm into the boss housing:

It looks like I should have enough room to cut that 10mm off, and still have proper seating into the boss. I figure I would star there, and see how far that gets me...Or should I cut out all of the threaded section of the freeplay adjuster and only use the adjustment at the carb body:

Open to all and any ideas....or feedback if Im barking up the wrong tree.

Thanks as always,
Steve
 

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Shorten the cable housing is what would do. I've had to do that in the past with replacement cables. Try to save the metal ends of the housing part of the cable. They help to seat the cable end in the adjusters. Also don't tie the cables to the spine of the bike. They need to move freely to function well.
 
John,
Thanks for the help! When you say shorten the housing - do you mean the metal adjuster barrel as I suggested, or attempt to cut the black casing in the middle somewhere? Obviously this would be the cleanest approach; but I'd need to figure out a way of doing it without severing the inner cable.

Thanks,
Steve
 
The black casing. Go slowly with dremmel and fine side cutters. Once you cut the outer spiral metal outer, there is a plastic inner sheath. Measure your distance to shorten, then go around gently with the dremmel a little at a time. Once cut, remove the black and unwind the metal outer. Then with a sharp knife or razor blade cut the plastic inner away. Try to save the end cap to put over the cut end.
 
John,
Thanks again. So are you suggesting to cut the housing with the inner cable still installed, or try to open the carb side crimped-on end to remove the cable before cutting? Sorry to be dense, but just wondering how to go about this...
 
OK gents - time for an update....

The new throttle cables arrived. Upon installing, I found out that there is not enough free cable (eg cable outside of the housing), regardless of routing. Thus when installed, holds the slide up:
2RPZV8R8gsE1Z4t3gA2u9AEAWTpBwas8q5ejHBZpLFLndtFL_uiTC4dSw4BRet0cvXHsN1uBUZ6WgaFzxfyVkcZQ9dS2dhjj5auQ7fCNwyqSy1O8zFxnEElWlTOf2zzGHpL_Xglj_6ktuwcNYNqPFRWScP5rkAFYB0_B2qsKUL0WV2ShQG7pgN4fkZATeQ2GdNHZZBpz_kevDIKBQRjv_XgT8kil31BYyJIbtjuHB2A5G-ry3Domy7mLESLdlWPhi-tBh2MFvoB4MsXmjlUVvXD6i6DdapjzjapcS4FxRbHUwrSDG2CAioH0-QCqm7dB6LctzPp0d5mx_HPmw1azBeXkrrD5RJT15IuAF1KmXZi48YntntXJQYDf6mjmi0O0odWrGdEAcq4kVKdWYSvgb2z_-HMQluFH_470aiUnLjDyiNVje3axhLcxmaZ0p2yFrzw2WssUWU4oDA-wtol87dC1cdyN89UkETbE94swzu8VNmMPwtI3BftVvD6tqWs3P3EfnaElDmv0fd7cxdMHSFd8SCtgO3FSohTAow_pTIhcUfQkkQ5-Eq5VLZmTpElUP_a9W195yLKuiTgC3lCSCbHrxpG24ANJo0nEKOFVgDySNm3s6qVf5vEaSDnz8IqXIfF5REbPPEZjKwnd65GUx5kW-T_yCQSX=w939-h1251-no


So I began comparing the throttle cables, and found two things of note...First one being that the old throttle cable is ~6mm longer, but also has different housing ends on the carb side:
h2PXv_DTiceIgRy-_HK_Ll03a6VWnFDoBiV6b1LyXF1Do1Dp68EmtWLg8_1bU2SF1M74zqzd0tGQRMWZXWrzCy5WQe4E7NBT3Nb-3LNZLfF0NLX33lD4zmOHn_8xOJoRlRPJy7NFRjbZhPgfQDw8ajFp-xlpcjp6Q8fmDCHu0MeW46P7KnJXBzyDYpVhvh8S9Fql_X5w3ywqVcZfItdPswNfzMIar3AjDwl5KPQNEZ58yQG4SR5ffvnc3F1DzW3pcaREk-jcMDqWpKeZ8th19Fn78_GsqB8nsc45v2zi1oCTtWW_3zlUbx0nv8iI3DUmZVNgr1Gn2_RQOmjOC1oE_47Ww-L2uuymhlQA74EOIPFA-lGntfvbkXUqESh2-5wnvkoj8md3_OBQK_uuKK6K3u3IPOqEkUKXkdj9ipCtbEk6Mr-jYUwjcUQG7vcprMtysrNOjXXFOmiM8cLtsPr629O29AYTx1bQg4_Ymbfrs8BWeHl4TNlsBOdrXUnmBiDwIszXXTexEa0D-DZTpE_bmNwM51cW1f-spYCuGSdsMQrJB3F2csoE2_VUMoUtM37OejFStzIx9wKBUQt_EelaqDFI2QN72h1q5LWCJux9ts2Ba-mrnrjOTkqNuFKa6-xD_IQP3GwOGti_rDIZavomVNm5dhHxYKdJ=w939-h1251-no


That got me thinking....And I actually do not believe that the bike had the threaded adjusters on the carbs when I disassembled. I now have 4 of them as the choke rebuild kit came with a pair, so now that they are installed these throttle cables are too short (or the housing is too long) to work with the Tommaselli throttle that is currently on the bike....Now things are starting to make sense.
Here is a shot of the amount of free cable when it is installed in the carb (with the slide fully closed) from the throttle end:
MXs6_GJyPCtqjex6DDUJ1RYr9l0MqbZJhehZGOB3JnmcsZnVtRu5E9eJjsI1aU1g9_2PH6oZHyWnLwbi8wuSHQWHgiNCGJAhc-MsnW6onXsn1d6PJ9Jq8me2YdikcHxCYBHPVL8K4KTScUfFIxjDtPCHlviQM83MN-D1FrhmQOCKGJ5yvcXUQ3NBdGMsCtFDBd_sRF0Fs-D8vCAg6nzZZA83BPwPz25Pu0PBJPMTAkEOc-2IaAqoQayjk0zEiMTOEV3opMbluAuvWpAKP1sP-9nsun1WjSqSNVmRjt4h7Gvsy4SNMmkgUAfP5MnNNyLZtu2-l42eCm41EWQcsQ9GteR28uAInj3sUtJpIwns42NoJImrUVvTw8G3aQhTDY84sUIN6OnFDOSoZ5kTnc3wsmVnMEYim6wNlHadFSDRmQ0VD3CkQe-Gjgo9Ol_Dnr3RFmiE5dHLWmfgT3HIZXiR9QDzeeWvtS9IJlvm0rZjQivHUM75bXJ14UeGKX8Zuq0hmzEMgLDDgcK0cgtJpWt6vnKSj1kO6l1iqFXunXp6kilxItVtbhXqzTNDjd6dvd6aJt5T29A9RDHNbiLK7nuNbOyC120kY0NcnqJyQOu7UYkXzEo1h-0cnbelhd5yQxr-HFsOqgY27HIak8vIyLcbU7JiHK3kUp1b=w939-h1251-no


Clearly insufficient. Thinking this post is getting lengthy, will post a new reply with my path forward.

I don't know about anyone else, but I can't see any of your photos.
 
John,
Thanks again. So are you suggesting to cut the housing with the inner cable still installed, or try to open the carb side crimped-on end to remove the cable before cutting? Sorry to be dense, but just wondering how to go about this...

The inner cable can't be removed. It won't pass through the sheath. I like to work from the carb end. Less to deal with after you detach from the slides. Make the distance you remove as equal as possible to make synchronizing the carbs easier.
 
With the adjuster up at the throttle, the ones on the carb tops are redundant. What I've done to (re)gain freeplay in cases like yours is to remove the locknut from the adjuster on the carb top. This will give you 4-5? mm more free length. Another thing you can do is to drill the holes where the cable elbows fit into the throttle housing deeper - not much - maybe another 1/8" or so. Best done in a drill press.

I only cut cables as a last resort.
 
Thanks again! I went ahead and cut the cables 15mm. It took me about 20mins to do so in order to assure that I very very gently cut through without touching the inner cable.
I installed the cables last night, and have more than enough free play now. So today I'll get the rest of the bike back together and will proceed with setting the idle and sync'ing.

Hopefully - this will fix my overall issue. Certainly will report back later.
 

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Well.... back to the drawing board. While I am certain the cables needed to be replaced and modified, they unfortunately did not fix my problem.

I'll remove and completely disassemble the carbs today.
 
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