Hi all. I have a A15 engine fitted with a carb from an A14. It runs well above 2500 rpm but below this and around 2000 rpm it runs a bit lean.
What should the main jest size be as I think it's probably to small for an A15 engine.

Not at all. The jets only control the cruising a/f ratio and are primarily selected relative to the venturi size (not the engine size). If the cam or exhaust is very different it can cause dilution of a/f mixture, requiring changes to the jetting.


Quote:

At steady-state flat road cruising?
While accelerating?

For AUS, A15 is leaner jetted, according to Gregory's, possibly because the A15 has a wimpy camshaft, or because the carb is smaller.

Main jet
* primary, A14: 104
* primary, A15: 100
* secondary, A14: 150
* secondary, A15: 145


For North America, jetting varies.
* Canada had largest carb
* USA had many different size venturis
* CAL was jetted lean for Cat Convertor
* 5-speed cars were jetted leanest for fuel economy

Interesting information. Im thinking of putting twin Hitachi carbs on it to improve it as this carb is just not running the engine correctly.
the original carb was completely shot with flat spots all over the place. the shaft was worn out and the main body hole for it was not round any more. So the only carb that I could get (New) was a 306 for an A14 engine. Yes it works but not very well.
So that is why I was wondering if the gets are incorrect fort an A15 engine.

Paul the throttle position has more effect than RPM ; you'd need to know steady state, rolling on the throttle or booting it etc to sort this. 1/10th throttle at 5000 rpms is going to be on the pilot jet while full throttle at 2000 rpms is going to be on the main jet. (Note that there is more to it than this but basically you get the idea)
My guess is that your issue is in the transition between the pilot and the main, if you haven't already make sure all the passages are completely clear. It only takes a tiny bit of crud to,start causing issues..........if you have already done this, never mind.

Tom

Hi Tom.
The problem has always been there since I changed the carb. Yes it will run ok but between 2K and 2.5K it splutters and miss fires them what above this it's fine. When I look at the spark plugs they are a bit white so running lean. I have looked for air leaks and changed all the caskets many times now with no change in situation at all. I'm unable to tune the carb with the air mixture screw so it will run week at idle as well but does not miss fire. Just runs ruff.

Paul.

Runs off?


> between 2K and 2.5K it splutters and miss fires them what above this it's fine. When I look at the spark plugs they are a bit white so running lean.

Bad spark leads/wires can also cause these symptoms under very light acceleration.

Plugs a bit white is normal with modern fuel.

Paul if the mixture screw has no effect that you either have a massive air leak, which would lean the entire range or a passage is plugged. Have you sprayed all of the passages with carb cleaner to make sure they are clear? I would pay particular attention to the pilot jet.

Tom

if you have no mixture control your throttle plates might be open to far. try backing of the idle speed screw all the way then go in just enough to keep the engine from stalling and see if the mixture screw starts to take effect.

and also reset the mixture screw by screwing in all the way then back out 2 turns

does this carby have a anti diesling valve on it? if so have you checked its operation?