TPS suggestions, or MAPS

Nige · Post by **Nige** » Tue Oct 27, 2009 7:40 pm

Hi,

I'm strongly considering getting a megajolt install started, but need to figure out whether to go for TPS, or MAPS.

I have a very early 63 spitfire with the original, but now modified 1147cc engine. It has a pair of Keihin CR Specials attached, but the sticking point is the advance curve is now completely wrong as I have set it by max advance without the vacuum pipe. This means that it is o.k at full tilt, but pinks like mad at different loads. Plus due to the modifications the original curve wouldn't be of much use anyway. It would have originally used the vacuum advance from the SU carbs, but cannot with the keihins as they don't have a vacuum pipe. This isn't a problem as I plan to tap into the manifold, or fit a TPS in order to fit the megajolt.

The problem is which should I go for. The Keihins would be pretty awkward to fit a TPS to as there isn't anywhere on the rotation of the mechanism to mount a TPS (see images). I did see somewhere that the preferred way to fit the megajolt kit with bike carbs is using a TPS. Would this be correct? If it is would there be any disadvantage to tapping into the manifold with this set up?

If you'd recommend tapping into the manifold, where would I get hold the correct size grommets to screw into the manifold? I've scoured fleabay and googled, but I'm not really coming up with anything useful.

Thanks in advance,

Nige

NITROPIXIE · Post by **NITROPIXIE** » Tue Oct 27, 2009 8:50 pm

Sounds like you have a really fun project on your hands there. What size choke are those carbs?? they look a little big for my liking for that size engine.

Personally i would use MAP as a source of advancing unless your using a whacky cam in this setup, as load is better detected through the MAP than TPS. Too add TPS if this was the route you wanted to go down, then you don't have to actually fit the TPS to the spindle, you can always attach it to the throttle linkage on the carb somewhere. You can think out of the box a bit for this.

The best way to tap this, hmmm?? If your manifold has thick enough material then drill it and fit in a piece of pipe with a slightly thicker diameter, for an interference fit, maybe with some araldite/epoxy to ensure a leak free fit. Some brass pipe would be ideal with an approx 1mm internal diameter, generally the smaller the better but not miuscule. Will have to have a look to see what else could be used.

Maybe you could tap the rubber sleeve joining the carb to manifold??

Do you have a brake servo connected up??? I'm guessing not.

You will need to tap both port/carb for MAP, then run 2 long lines prior to joining a y piece, maybe a small reservoir to smooth any pulsing then to the MAP sensor.

Ryan

Nige · Post by **Nige** » Tue Oct 27, 2009 9:18 pm

Thanks for the advice Ryan,

The carbs are custom built and jetted for this engine size and modification amount by PRI Racing http://prirace.com/spitfire-carb-kit.htm They are great to drive with and run it very well at the moment considering the advance is all over the place.

The cam is a simple piper fast road cam as I'm not racing it. No servo, just stand on the pedal.

I think I'll get a pair of tubes into the rubber sleeves as there is a few mm gap between the carb and the manifold. I had a Doh! moment when you mentioned a simple pipe for the vacuum. I had a threaded grommet in my head and didn't stop to think about a simple piece of brass pipe.

Thanks again, that is the last piece of the puzzle that I needed before ordering the megajolt and sourcing the parts.