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miniseven

A Series 7 Port Head Need Some Help

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Hi everyone

just joined having problems setting up an A series engine with twin dellorto Dhla 40s no numbers with three progression holes on a 7 port cross flow head. the carbs were bought on ebay and look really good, all jets have been removed and carb blown out is very clean and a matching pair. i need a starting point with jetting and choke size. Can anyone help?

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Think those might be based on single carbs used with the standard 5 port head? Bit of care needed. Well remembered though Steve - I've got a copy of that book also and had forgotten that page!

 

Cheers

 

Nick

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Nick,

 

I think you might be right about that information I've posted, so miniseven do be careful. D.Vizard says in his book he wasn't involved a lot with the 7 port heads but did mention who was. I'll check who and post that info.

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As smallest chokes are 28mm they should be used first.

As we do not know anything about the tuning stage

this can be the only recommendation.

Main jet and air correction depends on cam,

the hotter the cam the closer they walk together, I found out.

 

Close to stock I would start with around 115 main and 150 air.

Idle jets keep as it is and try out first.

The number of rotations for the idle mixture screw set for proper idle

gives an indication where you are.

Nearly closed means idle fuel jet needs to be smaller.

Opposite you may check size and drill with a proper driller larger if too small.

Do not hesitate to do so if someone else tells you that is scrap!

 

When idle is established try if the engine runs at 2000rpm with no load

and check AFR at that rpm, should be between 12 and 14.5

best around 13.5 to 14.

Again this is changed with the idle jet!

 

Innovate LC-1 or similar should be employed,

otherwise you might spend hours of useless time!

 

Grind out the numbers on drilled jets that they are not mixed up.

 

A second person should check the LC-1, never the driver!

12.5-12.8 is perfect for full load

13.5 to 15 depending on the engine is good for part throttle.

>16 misfire will occure

<12 power loss and excessive fuel consumtion will happen.

 

As parts for DHLA are rare and expensive I would try out

and buy proper jets at last when you are sure.

 

Check the height of the fuel level and size of fuel valve.

Otherwise you might do the work twice....

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When I received my crossflow head from AKM Racing back in 2004 it came with the info below. Sorry several years have passed since I've worked on my Sprite so I don't remember what we ended up with, but this is a good start, and these numbers come from Georg Mikkelsen himself:

 

For road 1300cc try:

Dual Weber 40 DCOE

28 to 32 choke

125 main jet

180 air corrector

40 pump jet

F16 emulsion tube

50F8 idle jet

4.5 aux venturi

 

Hope that helps!

 

- Tom

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