conversion to carbs how?????

i have decided to convert my trackday mk1 (2L 8v) to carbs and have a couple of questions. firstly where does the take off for the brake servo come from? am i right in thinking that the new inlet manifold has this take off?and secondly where does the dizzy get its vacuum from the same place? also what electrical connections do i need to the dizzy, are all the original ones for the fuel injection or do i just power up the ignition amplifier as before and it will work? you help is appreciated

 

You are indeed right! The D.C.O.E manifold should come with a stub (see pic) that you will need to drill and tap to take a fitting for the servo hose (i'm fairly sure the hose is'nt big enough to fit over the stub), obviously this needs to be done before the manifold is fitted to the engine !

The vacuum problem is easy to sort out... dont use it ! If your worried that you may lose out on fuel economy the you really should'nt be using twin carbs ! Just make sure that the ignition timing is set correctly (6 degree's @ idle should give you the 26 degree's max advance @ 3,500-4,000 rpm that should be yr starting point, swing a couple of degrees either side of that to find best power when on the rollers for the set-up that the carbs will need) and leave it at that.

Whatever you currently use to power the ignition will be fine (i use the standard 1800 DX dizzy, amp & wiring, mine is still an 1800). If your sure you wont return to K-Jet (i presume thats what you currently have) you can remove the wiring from the loom for all the injection components and still leave the ignition circuit intact.

One of the most important things you need to do is open-out the ports of the inlet manifold as the standard ports as-cast are CR%P ! (sorry, puny !) If you dont do this then the results will be less than satisfatory to say the least. Ideally, if you have access to a spare head to bolt the manifold to you will see what i mean ! If not, use a manifold gasket and scribe the manifold (head face) to show where you need to grind and a Misab plate (thats the alloy/rubber gaskets that seal the carbs to the manifold, dont use the plastic type with a recess for an "o" ring either side as they're not as good) at the carb end again to scrib the line you need to work to, then grind the manifold out. Start at the carb face end and taper down to the head end, it's easier said than done as you need to grind four ports the same size , a vernier will help here to get the size consistent. if you CAN bolt the manifold to a spare head it will help you see the shape you are trying to achieve.

You now have two choice's. Either open the ports CAREFULLY avoiding breaking into the bolt holes (which will be OK-ISH if you have a standard head, i.e, standard size ports) or grind away and break into the bolt holes, bolt the manifold in place (you can use a sealant instead of a paper gasket, honest) but smear some sealant around the head of the bolts, tighten down the manifold then smear sealer over the bolts that you will see in the port runners (its a ball-ache of a job which needs to be done with the head/manifold assembly on the bench to see what your doing), obviously wiping away all excess. The latter will be ESSENTIAL if you have a big valve/port head.

Hope this helps. What size carbs are you planning on using ? any other mods (current or planned) on the engine ?

 

thanks for such a indepth answer. so if you blank off the vacuum advance on the dizzy, how do you get the 26 degrees? does it have a mechanical advance in the dizzy? as for which carbs they are 4x40mm from a yamaha R1! head is to be flowed a little bit more manifold made to suit. already have a schrik cam (dont know what grind) and a fully flowed exhaust manifold and exhaust system.hopefully it should go and sound very well

 

All you need to do is set up the ignition timing as normal, 6 degrees should give you around the 26 mark for your starting point. The vac advance only comes into play when yr on part throttle, advancing the ignition by around 10-12 degrees to help with part throttle economy and slightly better throttle responce (allegedly !) when rolling back on the power, though i never found a real world difference on my GTi (when standard), even with a new pod... it just pinked. When you nail the throttle the vacuum drops and releases its "suck" on the vac pod, returning the dizzy to its pure mechanical advance. You dont need to blank off the vac pod on the dizzy, just disconnect the pipe... no more suck... no more vac advance. You do however need to blank off any holes left at the manifold end where the vac pipe used to be ! Its sometimes worth removing the vac pod & adding a dab of weld to stop any possible baseplate movement in the dizzy (it does'nt generally cause a problem) but its not essential.

If the carbs have a rising piston (?) inside them then you possible will get away with carbs that big, a 40mm butterfly is too big on its own (unless "choked" either via the rising piston or a removable choke) at the power level you will be at. If you were using a D.C.O.E type carb you would only be using a 34 or 36mm choke at your stage depending on the carb size/power level and would gain no more power by going bigger (unless you need the extra airflow for a race engine but thats a different ball game), an engine only needs what it needs !

Make sure that whoever sets up the carbs REALLY know what they are doing (with bike carbs) and the results could be good.

keep us posted on how you get on.

 

to get the BEST from you carb set up you want one of these.LINK

i fitted one to my single car setup and the diffrance was amazing!

 

i was thinking about something like this, ebay really does my head in though (bloody snipers!) how much is one of these set ups new?. one of the advantages of bike carbs is a tps already fitted.
p.s what does this actually do

 

it's a DIY ignition system you use a laptop to tune. being a 3D system you get the way more than a dizzy with fixed ignition curve, as you can have any amount of advance at any given RPM and engine load/TPS reading.


the unit cost me about 120 with postage and import duty from the USA. then you need the EDIS, trigger wheel, coil pack and about 5 worth of electrical components from maplin to get you rev counter working.

when i had my 2.0l 8v mk2 running a single 45mm DHLA set up on the rollers after X Mass. they had to set my dizzy at 0 degrees BTDC to stop it pinking at 3.5k RPM but now it WAY better cos i can now MAP it on the road.