Engine swap L3 and L4

Hello All,

I bought a 1984 KZ750L4 (Sold in Canada and EU only) with a poorly running motor. I was able to buy a used 1983 L3 engine with the Ignition pick-up plate missing (the auto advancer and ignition rotor are still on the motor). I installed the L3 motor using the original L4 electrics, plus I bought a Dynatek Electronic ignition (DS2-2) to replace the missing Ignition assembly on the motor.

Problem now is the L3 motor will not fire properly.

So the question is: Can an L3 motor work in a L4 electrical system?

Comments would be greatly appreciated.

Thanks everyone!

Considering the only electrical components on the engine itself is the oil level indicator and the ignition pick-up plate, so there us nothing that stops you from doing the switch.
But mixing ignition parts can be something else. I dont have the factory manual at hand so I can't check for differences. But try to get a complete kit from one year and model.

/A

Thanks Scubaanders,

I tried using the original L4 ignition rotor and pick up plate in the L3 but it didn't fire.
I tried to find and L3 ignition pick up but they're no longer available and none on eBay.

So it looks like I may take the L3 top end from the cylinders up and install them on my L4 motor.
The L4 motor has really poor compression.

Taking the two engines apart and reassemble will not solve your ignition problem. I'd say you should try to trouble shout the ignition a-z.
According to the manual, that will save you loads of time.

Yep, 100% agree.

I'm fairly techy, but this one has me stumped.

It's at a shop right now getting looked at.

Hopefully they can solve the ignition problem for me. I gave them all the parts for both motors that I have.

Thanks!

Do NOT take the engine apart. The ignition issues can be solved.

Does the L4 engine have a centrifugal advance rotor? Or does the ignitor take care of spark advance duties? If the later then you need to install all the L4 ignition parts on your L3 engine.

Another option is to use the L3 centrifugal advance setup and use the Dyna S. I'd try to use the stock ignition as first option though to save money.

EDIT: the most common reason for poor compression is lack of valve adjustment. Did you check both engines?

I had mega fun and games with a Frankenstien 550 motor I built, minor detail differences in the crankcase castings meant that even when putting the ignition plate back in what i though was roughly the same place had the timing a mile out...I needed to slot the pickup plate. If in doubt just rotate the plate and see if it will fire, that's how i did it after trying everything else.

These are both transistorised ignitions?

Hi Nessism,

Thanks for the firm response (ha).
The L4 motor has an ignition rotor and pick up plate. The l3 has a separate ignition rotor and advancer, plus the ignition plate.

Both motors use an ignitor.
The L4 ignition rotor is one piece. It looks like they combined the advance plate and the rotor as one piece, very little in the way of markings, along with the Pick up plate.

The L3 motor has a separate rotor (looks like a cam with a lobe), advancer plate (timing marks and springs), and pick up plate.

I tried the L4 ignition parts on the L3 motor, it didn't fire. The L4 ignition parts are non-adjustable.

I'll try using the L3 rotor and advancer with the Dynatek pick up plate.

Compression in the L4 engine was bad. According to the shop all cylinders were measuring different from each other with the worst at 100psi and the best at 130psi. Normal should be around 170psi.

I'm not sure how to check my valves. I guess this would entail removing the valve cover and checking for closure?

Thanks again.

Thanks Tyrell Corp,

Yep, I may have to do that too. Funny you called your bike Frankenstein. I called my last bike Frankenbike. it was a 92 ZX7 with street fighter modifications. My wife was trying to name the KZ seeing how much trouble I was having with it for the last 2 years. I finally came up with an appropriate name.

Bride of Frankenbike.

A mechanical-advance ignition uses a mechanical-advance igniter. This is a small box, like 2/3 of a cigarette pack, with fins on it, and wires coming out.

An electronic-advance ignition uses an electronic-advance igniter. This is a larger, plain metal box with a connector.

You can't mix and match the two systems, but you may be able to swap the entire system, if it physically fits, and as Brendan said, you may have to slot the timing plate.

Also, the Dyna S system should work if you have the mechanical advance, but again, you may have to slot the timing plate more. (And Dyna Recommends 3 ohm coils as the minimum, so in the long run, you will want new 3 ohm coils if the Dyna S ends up working.)

AH, I'll have to take a look. Thanks for the explanation.

Yep, looks like the L4 use the box type ignitor and the L3 uses the one with the cooling fins.
Do you now if it would just be a matter of switching the ignitor?

Also, when installing the Dynatek it actually by passes this connection by taking power from any constant 12v feed. I connected it into the rear brake light power, with the Pick Ups connected to the negative for each coil (Coil on throttle side - cylinders 1 and 4 - coil, Coil on the horn button side - cylinders 2 and 3).

I could get cylinders 1 and 4 to fire but not 2 and 3.