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Author Topic: Jetting for a 6a engined mk1 on 45s?  (Read 261 times)

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Offline Tristan

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Jetting for a 6a engined mk1 on 45s?
« on: May 25, 2014, 11:21:34 pm »
My mate has a 2.0 16v mk1 , "stage 2" (?) head , stock compression , and  mild cams with a vernier pulley set at 0, new 45s and a 4 branch exhaust , new KnN filters .  It runs a KR dizzy , timing set to 6 degrees advance . Carbs were supplied from the localish Weber agent , with jets supposed to suit . It seems very flat to me , not particularly quick at all , no pull , sluggish to rev . It's sooting up the plugs a lot and on cruise and overrun  it;s back firing a bit , and popping into the exhaust . It starts easy and idles fine .  any suggestions for jet sizes? Apart from running too rich , what else can cause these symptoms?  We'd like to have it running as good as possible before it sees a dyno.

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Offline JMR

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #1 on: May 27, 2014, 02:06:10 am »
If they're new Webers they would have come with 36mm chokes unless changed...have a look. If 36's, start with F16 tubes, 145 mains, 175 airs, 50 F8 idles & 40 pumps. If too lean at the top end drop the airs to 165...maybe try 45 pumps if there's a flat spot on initial response.

Make sure the carbs are balanced, set the idle mixture 1 to 1-1/2 turns out each.

What cams exactly?...std c.r is lower than published so most cams benefit from a c.r increase. Try the ignition at 10.

Let me know the cams and I'll give you some timing suggestions.

Offline Mycroft

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #2 on: May 28, 2014, 12:24:18 pm »
Hi, I'm the owner of the offending car!!!!



These are the jets as is.

The Cams are from the 1.8 KR engine as the 6A are reportedly too mild.

After that I'm open to suggestions.

Offline Tristan

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #3 on: May 28, 2014, 11:06:12 pm »
Hey Andy , would you throw up a few pics of the car , the lads here love a nice mk1.

Offline Mycroft

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #4 on: May 28, 2014, 11:32:22 pm »
Well this is what I started with....



Classic case of...Two steps backward to take one step forward...



Couple a years later I ended up here...



and another year after that....



and the offending articles....




Offline Tristan

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #5 on: May 30, 2014, 08:16:59 am »
In case there are horrified gasps , he DOES run filters!  ;D

Offline JMR

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #6 on: May 31, 2014, 06:39:19 pm »
Hi Andy...welcome aboard!

Car looks cool...fair bit of work gone into that...top effort.

If the chokes are still the std issue 36mm then the F16 tube and 145 mains should be fine. I'd back off the air corrector jets tho...155 will make it rich at the top end of the power band...id try 175's to start with.

The idles @ 55F8 could be too rich too...tho it depends on the idle screw positions.

Start by turning all the idle screws in, counting each half a turn. Then turn them all back out 1-1/2 turns each and fire up the engine. Turn one idle 1/2 turn back in and note the idle..if it goes up do the others the same...you will no doubt need to reduce or increase the idle speed while doing this if it races away or trys to stall. The aim is to get all the idle screws roughly in the same place within a window of 1-1/2 turns out each...if best idle is on less than 1 turn out the idle jets are too rich...more than 2 turns out and they're too lean.

When you drive on part throttle your driving on the idle jets...too rich or too lean and the response to gentle throttle use will be pants. It sounds to me like it's too rich...50F8's could be a better option or even 45 F8 or F9.

You also need to make sure the carbs are balanced...either use a proper balance airflow measure or a bit of heater hose and listen too the middle 2 trumpets...the "hiss" needs to be the same...again idle speed will need tweeking while doing this.

Check your timing off the correct mark...it's not the v groove or raised square boss...get number 1 piston to true tdc and look for the little dimple in the flywheel...thats the tdc mark...set the timing light to 8 degrees and line up the dimples with the arrow in the gearbox case timing hole...don't use the front pulley as they twist on the rubber damper with age putting the timing mark out.

Have a go at that first.

What filters are normally used?...if it's foam socks throw them away. Also...you'll need a shield in front of the carbs...direct air being forced into them upsets the signal to the auxiliary venturi and buggers up any settings at speed that may be fine at idle or on a rolling road.

Offline Interstellar50

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Re: Jetting for a 6a engined mk1 on 45s?
« Reply #7 on: September 13, 2021, 10:09:31 am »
Thank you for making me understand about car engines.

 

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