woodsy
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| posted on 7/10/14 at 09:34 AM |
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Revs rising when changing gear on a BEC
Looking for some pointers on an annoying problem i have.
When i change gear on my Busa engined MK Indy the revs rise by 500-750.It doesn't always happen though.If i short shift at low rpm's,say
2,000 the revs drop as usual but if i change at say 4,000 the revs rise but then if i change at say 7,000 + revs fall as they should.
When i first got the car the tickover was set very high and would rise to 1700rpm when warm.I have dropped that down to 1150rpm and it stays at that
when cold or warm.
When the revs rise it only happens for a few seconds.
Any ideas ???
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jeffw
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| posted on 7/10/14 at 09:41 AM |
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clutch slip?
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minitici
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| posted on 7/10/14 at 09:44 AM |
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Yep - sounds like clutch slip.
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dave_424
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| posted on 7/10/14 at 09:51 AM |
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Does it happen if you just depress the clutch? Might be that your throttle return spring or your throttle cable isn't good enough causing the
throttle to be sticky and not quite fully close as you take your foot off to change gear
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adithorp
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| posted on 7/10/14 at 09:57 AM |
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If this is after you've changed and as you re-apply throttle, the revs climb but speed doesn't increase,then it's clutch slip. You
probably need a heavy duty spring kit (Barnet) and might get away with just that if you've not overheated the plates yet.
If it's that the revs lift as you disengage the clutch to change gear, then it's probably a sticking throttle cable giving throttle
hang-up. Bike throttles tend to have very light return springs. On the bike there's 2 cables one opens the throttle and the other closes it as
you roll off the power. Without the second cable you can get some throttle hang-up. If thats the case lube your cable and add another return spring at
the TB end. I found a thinner, more flexible cable, was better as well (used a cycle gear cable, rather than the more common/thicker brke cable).
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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woodsy
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| posted on 7/10/14 at 10:03 AM |
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I only get the rising revs as i push the clutch in to change gear.....so sounds like it may be my throttle cable then.
I'll need to check but i think mine only runs with the one cable.Where is the best place to get a stronger return spring from ??
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woodsy
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| posted on 7/10/14 at 10:06 AM |
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Pretty sure it's not clutch slip because as soon as i've changed gear i can boot it and it hooks up straight away
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corrado vr6
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| posted on 7/10/14 at 10:58 AM |
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Hi I had this same issue with my R1 turned out to be the throttle return spring, I managed to fashion a mini brake shoe spring to pull the throttle
back which made it so much nicer to drive!
http://r1indy7.wordpress.com/
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adithorp
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| posted on 7/10/14 at 11:25 AM |
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Sounds like cable return then. What spring to use depends on how and where you can get it to fit,which depends on how the original builder did it. A
return at the pedal end might also help reduce sticking/friction in the cable, but I'd do that in addition to one at the TB's, not
instead.
In the short term try a bit of lube on the cable, linkage and pedal.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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Slater
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| posted on 7/10/14 at 11:27 AM |
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If it helps........Here is my throttle return spring set up on the bike carbs.
Just a short cable with one end fixed to the cam then fitted some "washers" (5p coins with small hole drilled in) and suitible spring
etc... as per pic.
[img]
 
Throttle Return Spring
[/img]
Why do they call Port Harcourt "The Garden City"?...... Becauase they can't spell Stramash.
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bi22le
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| posted on 7/10/14 at 11:29 AM |
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Sounds silly but caught me out with my BEC when I first got it - I was too slow getting my foot off of the accelerator during changes. I was
depressing the clutch with a smidge of throuttle still applied. I just made sure my foot was well off of the gogo peddle and it disappeared.
I then realised how much fun clutchless upshifts were and I definatly had no problem then!! Woohooo
Track days ARE the best thing since sliced bread, until I get a supercharger that is!
Please read my ring story:
http://www.locostbuilders.co.uk/forum/13/viewthread.php?tid=139152&page=1
Me doing a sub 56sec lap around Brands Indy. I need a geo set up! http://www.youtube.com/watch?v=EHksfvIGB3I
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jossey
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| posted on 7/10/14 at 03:02 PM |
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quote:
Originally posted by Slater
If it helps........Here is my throttle return spring set up on the bike carbs.
Just a short cable with one end fixed to the cam then fitted some "washers" (5p coins with small hole drilled in) and suitible spring
etc... as per pic.
[img]
Throttle Return Spring
[/img]
Can't you still get hung for drilling a coin?
quote:
No person shall, except under the authority of a licence granted by the Treasury, melt down or break up any metal coin which is for the time being
current in the United Kingdom or which, having been current there, has at any time after 16th May 1969 ceased to be so.
Thanks
David Johnson
Building my tiger avon slowly but surely.
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Slater
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| posted on 7/10/14 at 07:59 PM |
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Oh dear i did not know that, luckily i moved to South Africa, hopefully they wont extradite me over two 5p coins!
Why do they call Port Harcourt "The Garden City"?...... Becauase they can't spell Stramash.
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kingster996
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| posted on 7/10/14 at 10:21 PM |
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Bike carbs that are out of sync can exhibit this behaviour. It feels like the revs drop off more slowly than the throttle closes, so when you shut the
throttle in between changes, it doesn't respond quick enough and revs increase with clutch disengagement.
Might be worth a check?
I used to be indecisive, but now I'm not so sure
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woodsy
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| posted on 8/10/14 at 03:30 AM |
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Had a quick look and the return spring at the pedal end does appear very flimsy so will look at fitting a better one and see what happens.
Thanks for the advice guys
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