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Round K&N filters installed now rough idle

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#1 24OUNZ


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Posted 25 April 2014 - 04:46 PM

Hi Guys

Not sure if it's in the right section.... Here goes:

Engine is L28 with twin SU (3 screw type) and stock air box

Just installed the following:
- 5/16 AN5 steel braided fuel lines from fuel rails to carbies
- a pair of SU air horns from MSA
- dual K&N round type filters (replacing stock air box and filter)
- vent cover K&N filter.
- new ryco fuel filter
- the crankcase vent is still vented to the manifold.
- new chromed dash pot covers (with original dampners from the dashpots)
- float bowl vent hoses tied down to side of float bowls.
Everything else remains stock.

Here's a similar photo of my set up:

Now I have a idle problem. Once it warms up on choke it idles but hunts between 700-1200rpm and if I give it some throttle it sometimes drops off and stalls. When I first turned on the ignition to turn on the electric fuel pump to check the hoses some fuel did come out the air intake on the rear SU carb but stopped after about 10 secs or so and was only a little bit of fuel. So I tapped the float bowl as this is usual symptom of float needle being stick due to debris.

I adjusted the top idle screw on the throttle body so it would idle around 1000rpm and stay running.

I had seen on other forums that when you do this mod you need to adjust the floats in the carbies...??

So do I need to retune the carbies now because of the setup ie fuel to air mixture??

Any help would be appreciated.

I'll be replacing all the vacuum hoses first just to make sure it's not that problem.

Here's a pic of as similar setup as mine. I'll get photos of mine in the next 24 hors.

Posted Image

#2 260Coupe


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Posted 25 April 2014 - 05:38 PM

Without sounding to harsh .............PUT THE STOCK AIRBOX BACK ON !

it has been proven on the dyno many times over that the stock 240z airbox is by far the most efficient and best performing induction filter system period...........The filters you have installed just make noise  ..............fact.

The tuning problem sounds like float height issues......Put the box back on and get it dyno tuned by an expert with Hitachi SU's

Once set correctly they should not need returning again for many years..........if ever

PM me and I'll put you only an expert who knows these carbs inside out and has a chassis dyno


#3 dat2kman


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Posted 25 April 2014 - 06:54 PM

You say you put some air horns from usa on?
How close are these to the flat face lid on the filters?

You also mentioned adjusting the rear carby due to fuel dribbling out for 10 seconds.
Thats a float height/sticking issue. But you have now upset the balance.

To get it back close, remove the filter covers, grab a 2' long piece of garden hose, and alternate getween carb horns, and listen for the air sucking sound.
The idea is to get its idle down, and set the mixture screws so it doesnt stumble.
You'll get it close, but not ideal.

#4 24OUNZ


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Posted 27 April 2014 - 01:09 PM

Hi guys

Thanks for all the good info I'll check the filter clearances with the horns. I did find last night a fouled #4 cyn spark plug only all the rest were spot on so I'll be replacing it tonight and giving it another run. All other issues like float bowl heights are all good and the bowls are clean of debris. I only had adjusted the single idle screw that's located on the throttle linkage above the carbies that's all.

I use the BPR6ES NGK SPARK PLUGS with NGK leads and have found them to be excellent.

Ill whack the stock filter again and see how she runs.....

Thanks for your help guys!



#5 phillay


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Posted 01 May 2014 - 09:57 AM

looks nice and shiny

#6 24OUNZ


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Posted 01 May 2014 - 04:43 PM

Al and Jason,

Thanks for the info heres how the engine is set up now: I gave her a quick tune using and as i thought the SUs had gone out of balance and were running slightly rich.

Posted Image

And what it was like before with the K&Ns

Posted ImageWhile i was in the bay i gave the air cleaner another coat of Chev orange……

Thanks again and ill see how i go with a dyne tune as I'm looking at the weber conversion in the coming months.



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