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Revs slow to drop
davidimurray - 16/3/15 at 09:14 PM

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?

scudderfish - 16/3/15 at 09:20 PM

Weak throttle return spring?

mackei23b - 16/3/15 at 09:24 PM

take the throttle cable off and see what happens if you Rev it by hand?

Cheers
Ian

kingster996 - 16/3/15 at 09:45 PM

Carbs could be out of balance.

sjmatthews - 16/3/15 at 10:05 PM

Lean mixture or out of balance

MP3C - 16/3/15 at 10:06 PM

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

austin man - 16/3/15 at 10:21 PM

I have Zx9 carbs on my zetec thats the same when letting off the throttle the revs dont drop off straight away

wylliezx9r - 16/3/15 at 10:26 PM

Air leak in the inlet manifold can cause this.

Coopz - 16/3/15 at 10:43 PM

Could be something as simple as your throttle cable. Does your routing include any tight bends, cable travel free of friction?

Cheers

Toby

perksy - 16/3/15 at 11:00 PM

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

MikeRJ - 17/3/15 at 12:46 PM

This is a sign of a weak idle mixture. Have you set up the pilot jets to get a reasonable CO reading at idle?

coyoteboy - 17/3/15 at 01:07 PM

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.

davidimurray - 17/3/15 at 09:14 PM

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.

coyoteboy - 17/3/15 at 09:25 PM

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.

davidimurray - 17/3/15 at 10:16 PM

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.

kingster996 - 17/3/15 at 10:36 PM

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

coyoteboy - 17/3/15 at 11:46 PM

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

David Jenkins - 18/3/15 at 04:18 PM

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.

davidimurray - 18/3/15 at 06:46 PM

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

davidimurray - 20/3/15 at 07:15 PM

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!!!

MikeRJ - 23/3/15 at 11:09 AM

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?

davidimurray - 23/3/15 at 09:03 PM

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]