Bike carb conversion

My engine has failed. I now know why, and the reason is super lame. My engine failed because I sandblasted my engine block and my diy washing was not enough to get all the blasting media out. Laaaame. So life has given me lemons, and I have a personal moto for situations like this. When life gives you lemons…you make a 4age powered by motorcycle carbs!

I recently rebuilt my engine and I’m really not in the mood to rebuild it again and do everything the same way the first time round. It’s too soon. So I decided to use this failure as an opportunity to do something entertaining. After a bit of research I decided to do a bike carb conversion to my 4age engine.

The why

So here’s a bunch of personal and subjective, as well as a few objective reasons why I decided to do this:

  • I always wanted to drive an engine running on carburetors
  • Motorcycle carbs are cheaper, easier to tune, flow better and are more efficient than typical carburetors such as Webers.
  • Motorcycle carbs have individual throttle bodies (itbs) i.e. one carb and one throttle plate for each cylinder. That means four carbs, and four intake runners for each cylinder so the engine will look really cool.
  • Carbs in general sound great. Carbs on a 4age sound amazing.
  • There are some power gains to be made, but more importantly the engine will become even more responsive and fun to drive if the carbs are tuned right.

The how

In theory what you need to do is simple, remove the intake manifold, get a custom one made or make it yourself, slap some carbs on it and bolt it to the engine. Done! In reality it’s unfortunately a bit more complex.

ECU

The first thing you need to think about is the timing advance and the ECU. In case of the 4age in its stock form the ECU provides the timing advance at higher rpms. Without timing advance the car will perform poorly and run like crap. This means the ECU needs to stay. An alternative option is to get an aftermarket standalone ignition controller that is mappable, like MSD, or Nodiz, or Megajolt or anything else. I do plan to do this eventually but for the time being I will be using the stock ECU and save up for an aftermarket one. The trouble with the stock ECU is that it won’t run without the AFM, and the AFM needs to go away because the entire stock intake manifold needs to go away. So what’s the solution? Move to a MAP sensor that requires only vacuum from the intake manifold to work. For that you need the MAP sensor, a 4age MAP harness and MAP compatible 4age ECU. This can all be bought secondhand and is not too expensive.

Intake

After the ECU has been resolved there’s a bunch of things you need to think about when making or ordering your new custom intake manifold for your bike carbs. The ECU needs a vacuum signal from the MAP sensor to run the engine. The usual location in the stock intake manifold is gone along with the manifold. The logical choice is to tap a hole in your new intake manifold. However, it’s not as simple as that. Since the intake manifold now consists of four individual runners taping just one of them will result in a pulsing signal from MAP to the ECU and the engine will run like crap. What you need to do is tap each of the intake runners and then run the four hoses from each of the taps to a vacuum balancing canister of some sort and then from there to the MAP sensor. You can order one of these or try and make it yourself. If you have the patience and the tools it’s not that hard.
You also need to tap your new intake manifold for vacuum for your brake booster. These hoses need to be a slightly larger diameter and two taps is better than one. If you do not do this your brake pedal will behave as if a rock is sitting behind it and braking with your car will be a disgusting experience.

Fuel

Once this has been resolved you need to think about fuel delivery and fuel pressure. The stock fuel pump that runs with the fuel injection setup is a no go for carbs. It runs at a much higher psi than the carbs need. Electronic fuel injection fuel pumps usually run anywhere from 35 to 40 psi. Carbs need only 3-5 psi of fuel pressure. Here you have two options. Get a fuel pressure regulator to reduce the fuel pressure, or remove your stock fuel pump and install a motorcycle fuel pump. Solution one is more expensive and needs more parts, while solution two is more labor intensive because you need to drop your fuel tank which in the case of the mid-engined MR2 is a pain in the bum.

Carb tuning

Another major thing you need to think about is tuning the carbs. The jets of your carbs will need to be changed because they are now supplying fuel to a car engine which is a lot larger than that of a motorcycle. Once you get some approximate jetting that is good enough to fire up the car you will need an air fuel ratio gauge to properly tune the carbs. I cannot stress the importance of this enough. All the sound, performance and responsiveness you dream about with your carbs will never happen unless you tune your carbs right. So before putting everything together and firing up the engine, make sure you buy an AFR gauge and get it hooked up.

The parts

Ok now that we know how I intend to do the bike carb conversion, let’s talk about the parts that I got to make it possible.

Bike carbs, intake manifold, fuel pump and vacuum balancing for the MAP sensor

As I wrote above, you can try DIY-ing this or you can buy it all readymade. Feeling deprived that I didn’t really get to properly enjoy my first engine rebuild I am very much focused on getting this into motion quickly and having the car back on the road ASAP. If you have the time and the patience you can actually try sourcing suitable bike carbs from a junkyard or online, weld up an intake manifold yourself, make a balancing canister /bar yourself, and sort the fuel delivery any way you prefer. After doing some thinking and adding prices and numbers and taking into account my non-existent welding skills I decided to get all of this ready made by a professional. The benefits are that I will save a lot of time and get everything made professionally, which will ensure it all fits properly, is compatible with everything else and looks cool.

To that end I found a company called DanST engineering in the UK that specialized in bike carb conversions. How neat is that? Here’s pictures and links of the super sexy parts I got from DanST:

Honda CBR600 bike carbs. They come jetted and ultrasonically cleaned so you can start your car

Honda CBR600 37mm Keihin carbs. They come ultrasonically cleaned and with an approximate jetting so you can start your car. Super convenient.

Intake manifold, vacuum balancing bar, gaskets and hoses
Welding level: Automotive art

Automotive art level, TIG welded aluminum intake manifold with all the necessary taps, a high quality intake manifold gasket , a vacuum balancing bar and fluoro lines silicone hoses that will not deteriorate in contact with fuel. Here are links to the kit as a whole, and the intake manifold alone

Motorcycle fuel pump. I have decided to go with bike fuel pump approach. I am dreading removing the fuel tank but I prefer the simplicity, cost and reliability of the motorcycle fuel pump when compared to the expensive and complex fuel pressure regulators that will require more hoses, gauges etc. The bike fuel pump is an inline install with only 1 fuel hose. It’s nothing special but it will work just fine, and if it ever fails it will be super easy to replace. 

Pipercross air filter and custom baseplate to fit it to the carbs

Pipercross PX 500 air filter for the carbs. High quality, lifetime fitler that looks very sexy. The kit also includes a custom made base-plate DanST makes to fit the filter neatly to the carbs. 

Here’s all of it assembled. Once you add all the prices together this kit may seem a bit expensive, but when you take into account the time, trial and error and the looks of diy setups, the price really doesn’t seem that expensive at all.

Unboxing video:

Techno toy tuning lightweight crankshaft pulley and injector plugs

Here’s some super cool products I got from Techno toy tuning:

Lightweight crank pulley
Injector plugs

The crankshaft pulley was not a necessity at this point but I got it looking ahead into my plan to go with a standalone ignition controller later on. All standalone ignition controllers need a crankshaft position sensor. The 4age does not have one stock so you need to make it. The lightweight crank pulley from techno toy tuning makes great sense because it looks amazing, is perfectly balanced to 11.000 rpms, and you can add a trigger wheel for your crankshaft positions sensor to it very easily with just four bolts and an adapter. It also sheds unnecessary weight from your rotating assembly making your engine spin more freely.

The injector plugs are a tiny but super useful product. Once you decide to do a bike carb conversion you need to get rid of your injectors. When you get rid of your injectors you are left with four gaping holes in your cylinder head. You of course need to plug those holes because without them you are looking at a huge vacuum leak that will make your engine run like absolute crap, if at all. To plug up the holes a lot of people buy interference fit freeze plugs. They may be a cheap solution but they are not a particularly smart one because removing them is a pain after they have been in there for a while and can result in damage to your cylinder head if you are unlucky. You will remove them because you are a car guy and after a bike carb conversion you will want to do something else that requires you to put the injectors back in. The injector plugs are super easy to remove and install and do not risk any damage at all and also look really sleek.

Unboxing videos:

AEM X-series UEGO wideband air fuel ratio gauge

This is a key piece in the puzzle, because without the AFR gauge all of the pretty parts you can see above are just going to sit in my engine bay but perform miserably.
Carbs are all about tuning and they need to be tuned them at different throttle positions using a fast responding and accurate air fuel ratio gauge.

I decided to go with the AEM X-series because AEM is a reputable company with years of experience, research and development invested in their products and because the x-series is the fastest responding air fuel ratio gauge of all its competitors, which has been proved by independent tests. Fast response is super important for tuning because if your afr gauge does not respond fast enough you can easily miss a lean or rich condition at certain throttle positions. When you take into account the performance of the gauge, the price is actually very reasonable and AEM gauges are used by some very serious and successful race teams, so if they are good enough for them they are more than good enough for my street driven bike carb converted 4age.

Here’s the unboxing video:

There you have it, you have seen the plan and the parts.
Next up I will be working to assemble the engine, put all of this together and fire it up.
Stay tuned!

2 thoughts on “Bike carb conversion

  1. how are you liking the bike carbs? I’m considering the conversion for my aw11. i couldn’t find a “long term review” or anything near that on your website. great sight btw.

    1. Hi,
      There’s now long term review because I just began with the conversion. I have still to assemble I engine, and put it all back in the car. So far I’m really happy with the parts I got and I think I’ll be happy with the carbs. Of course I will do a detailed review and follow-up once it’s all up and running. You can check my YouTube channel if you want to see the bike carb conversion progress. I have also decided to go with a NODIZ pro standalone ignition controller instead of the stock ECU that I’m speaking about in this post.

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