davidimurray
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| posted on 16/3/15 at 09:14 PM |
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Revs slow to drop
Looking for some collective wisdom from the forum before I start finding.
New engine fitted to car, running Megajolt and ZX6R carbs from my existing Pinto.
Fired the engine up the engine to get it upto temperature and prove cooling system. Carbs still need balancing but not running to badly at the
moment.
However there is one immediate concern. The engine idles nicely, but when I blip the throttle the revs rise, stay high and then slowly drop back to
idle over say 5-10 seconds. My first thought was an air leak so I need to get into hunting. But at the same time I'm surprised it idles nicely
if it was an air leak.
Any thoughts on possible causes before I jump in there?
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
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Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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scudderfish
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| posted on 16/3/15 at 09:20 PM |
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Weak throttle return spring?
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mackei23b
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| posted on 16/3/15 at 09:24 PM |
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take the throttle cable off and see what happens if you Rev it by hand?
Cheers
Ian
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kingster996
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| posted on 16/3/15 at 09:45 PM |
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Carbs could be out of balance.
I used to be indecisive, but now I'm not so sure
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sjmatthews
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| posted on 16/3/15 at 10:05 PM |
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Lean mixture or out of balance
I'll be in the garage dear!
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MP3C
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| posted on 16/3/15 at 10:06 PM |
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quote:
Originally posted by scudderfish
Weak throttle return spring?
That would be my first guess as exactly the same thing happened to my engine after a sprint, took the carbs apart and the return springs were
knackered.
Matt
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austin man
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| posted on 16/3/15 at 10:21 PM |
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I have Zx9 carbs on my zetec thats the same when letting off the throttle the revs dont drop off straight away
Life is like a bowl of fruit, funny how all the weird looking ones are left alone
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wylliezx9r
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| posted on 16/3/15 at 10:26 PM |
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Air leak in the inlet manifold can cause this.
I spent a lot of money on booze, birds and fast cars. The rest I just squandered.
George Best
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Coopz
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| posted on 16/3/15 at 10:43 PM |
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Could be something as simple as your throttle cable. Does your routing include any tight bends, cable travel free of friction?
Cheers
Toby
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perksy
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| posted on 16/3/15 at 11:00 PM |
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Check throttle cable/return mechanism
Spray some carb cleaner around the sealing face of the inlet manifold to check for an air leak
My money is on the throttle cable/returm mechanism
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MikeRJ
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| posted on 17/3/15 at 12:46 PM |
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This is a sign of a weak idle mixture. Have you set up the pilot jets to get a reasonable CO reading at idle?
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coyoteboy
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| posted on 17/3/15 at 01:07 PM |
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Weak idle and if you'd been fiddling with other stuff I'd look at idle timing too, but my first resort would be to push the throttles shut
at the mechanism to ensure it's returning fully.
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davidimurray
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| posted on 17/3/15 at 09:14 PM |
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Thanks for the ideas everyone, will have a look at the throttle linkage - it appeared to be snapping shut but will disconnect the cable and try. Idle
mixture looked ok, quick fiddle had it between an AFR of 12.5 and 14.
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
Gallery 3 http://www.facebook.com/media/set/?set=a.440671625324.232627.601005324&l=3f0d42c523
Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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coyoteboy
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| posted on 17/3/15 at 09:25 PM |
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Are the ZX6R carbs split into twos? Possible one is OK and the other not?
You say between 12.5 and 14 - in what way, it shouldn't oscillate between those extremes at idle - waving of ~0.5 is not too bad but that would
suggest misfires/imbalance to me.
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davidimurray
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| posted on 17/3/15 at 10:16 PM |
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quote:
Originally posted by coyoteboy
Are the ZX6R carbs split into twos? Possible one is OK and the other not?
You say between 12.5 and 14 - in what way, it shouldn't oscillate between those extremes at idle - waving of ~0.5 is not too bad but that would
suggest misfires/imbalance to me.
That was from me fiddling!
There is some variation but that is because the carbs are yet to be balanced. The carbs have been respaced - the were on the Pinto, but are now closer
together on the Duratec.
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
Gallery 3 http://www.facebook.com/media/set/?set=a.440671625324.232627.601005324&l=3f0d42c523
Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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kingster996
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| posted on 17/3/15 at 10:36 PM |
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quote:
Originally posted by davidimurray
There is some variation but that is because the carbs are yet to be balanced. The carbs have been respaced - the were on the Pinto, but are now closer
together on the Duratec.
There's your problem then 
I used to be indecisive, but now I'm not so sure
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coyoteboy
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| posted on 17/3/15 at 11:46 PM |
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If they're not balanced you'll be seeing an average AFR but they'll individually be all over the show. Get them balanced properly
and come back if it's still an issue  
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David Jenkins
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| posted on 18/3/15 at 04:18 PM |
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It's probably quite simple - bikes tend to have 2 throttle cables, one to open and one to shut. If you only fit one to the loud pedal then you
need something to shut the carbs down when you lift your foot off.
I fitted a very short cable and a spring to the 'close' side of my CBR600 carb mechanism:
You can see the spring just next to the filler cap, at the front end of the carbs.
I can post a better picture if necessary.
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davidimurray
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| posted on 18/3/15 at 06:46 PM |
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quote:
Originally posted by kingster996
quote:
Originally posted by davidimurray
There is some variation but that is because the carbs are yet to be balanced. The carbs have been respaced - the were on the Pinto, but are now closer
together on the Duratec.
There's your problem then
I should say they were drill bit balanced on the benche. Will be interested to see if it does make difference - never affected the throttle response
when I had them on the Pinto - just started missing and spitting back when they were out of balance
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
Gallery 3 http://www.facebook.com/media/set/?set=a.440671625324.232627.601005324&l=3f0d42c523
Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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davidimurray
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| posted on 20/3/15 at 07:15 PM |
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Had a play for an hour after work - now I'm confused!
Got my carbtune on and fired the engine up. As expected they were a bit out. Balanced them and it made no difference to the speed the revs dropped at.
Had a bit of a play and discovered that if I blip the throttle the revs drop quicker than if I hold them then release the throttle. Disconnected the
throttle cable and now difference. The wound the idle speed back at the engine ran rough but did not stall.
Suspecting an air leak I sprayed all around the manifold with carb cleaner and no change in engine speed. Tried nipping up the manifold but no real
change.
Slightly perplexed I had another look at the balance and found cylinders 1 and 2 going to maximum vacuum, with 3 and 4 slightly lower. Pull a hose off
and vac drops to nothing on the gauges, hose on back up to max. When you fire the engine up, it seems that 1 and 2 the vacuum slowly rises over
10-20seconds.
Now i'm confused!!!
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
Gallery 3 http://www.facebook.com/media/set/?set=a.440671625324.232627.601005324&l=3f0d42c523
Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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MikeRJ
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| posted on 23/3/15 at 11:09 AM |
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quote:
Originally posted by davidimurray
Had a play for an hour after work - now I'm confused!
Got my carbtune on and fired the engine up. As expected they were a bit out. Balanced them and it made no difference to the speed the revs dropped at.
Had a bit of a play and discovered that if I blip the throttle the revs drop quicker than if I hold them then release the throttle. Disconnected the
throttle cable and now difference. The wound the idle speed back at the engine ran rough but did not stall.
Suspecting an air leak I sprayed all around the manifold with carb cleaner and no change in engine speed. Tried nipping up the manifold but no real
change.
Slightly perplexed I had another look at the balance and found cylinders 1 and 2 going to maximum vacuum, with 3 and 4 slightly lower. Pull a hose off
and vac drops to nothing on the gauges, hose on back up to max. When you fire the engine up, it seems that 1 and 2 the vacuum slowly rises over
10-20seconds.
Now i'm confused!!!
Are you sure the damping restrictors aren't blocked on a couple of the gauges? This could cause the indicated vacuum to rise slowly.
Have you checked the idle mixture as mentioned previously?
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davidimurray
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| posted on 23/3/15 at 09:03 PM |
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Well I seem to have cured it - by not sure how!
Made an extra throttle return spring but that didn't seem to have any affect.
Tried balancing again with similar results. Sprayed wd40 all around the manifold and no change in revs. I also gave the manifold bolts a little nip
but they all felt pretty tight.
Decided to do a compression test and all good at 240bar.I noticed that plug no 1 was a bit oily and no 2 looked lean. Cleaned the plugs up, popped
them back in. Then screwed all the idle screws in and then back out 2 1/2 turns. Fired her back up rebalanced all seems well now. Even had a little
spin around the block
[Edited on 23/3/15 by davidimurray]
Gallery 1 http://www.facebook.com/media/set/?set=a.116893465324.130778.601005324
Gallery 2 http://www.facebook.com/media/set/?set=a.245243755324.181913.601005324&l=a9831a9319
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Gallery 4 http://www.facebook.com/media/set/?set=a.490098255324.297598.601005324&l=efb083b7df
Gallery 5 http://www.facebook.com/media/set/?set=a.10150244028550325.366987.601005324&l=583fd5cd3a
Gallery 6 http://www.facebook.com/media/set/?set=a.10150550640070325.430417.601005324&type=3&l=fe779b358c
Duratec Engine Swap https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f " target="_blank"> https://www.facebook.com/media/set/?set=a.10152527759580325.1073741828.601005324&type=1&l=40aae5e72f
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