If your Aisan carburetor (commonly found on older Toyota, Datsun, or other Japanese vehicles) won’t start when cold, here are the most likely causes:
Common Reasons Your Aisan Carb Won’t Start Cold:
1. Choke Not Working Properly
Automatic (electric or thermal) choke may not be engaging.
Manual choke may not be closing fully.
If the choke plate stays open, the fuel mixture is too lean for a cold start.
2. Cold Start Enrichment Circuit Issues
Some Aisan carbs have a cold start circuit that enriches the mixture. If it's clogged or not functioning, cold starts will be hard.
3. Vacuum Leaks
Vacuum leaks lean out the air-fuel mixture—especially noticeable on cold starts.
Check hoses, gaskets, and the base of the carb.
4. Fuel Delivery Problems
Weak fuel pump, clogged fuel filter, or dirty carb jets could mean not enough fuel is getting in.
If it’s been sitting a while, the carb could be gummed up.
5. Incorrect Idle Mixture or Fast Idle Setting
The idle mixture screw might be too lean.
The fast idle cam might not be engaging when cold.
6. Worn Out Spark Plugs or Weak Ignition
Cold starts need strong spark. Check plug condition, spark strength, and timing.
7. Float Level Too Low
If the float level is set too low, not enough fuel enters the bowl for startup enrichment.
Quick Diagnostic Tips:
Check if the choke closes fully when the engine is cold.
Pump the throttle a few times before cranking — if no fuel squirts into the carb, your accelerator pump may be shot, or the accelerator pump circuit is clogged.
Spray a little starting fluid in the carb throat. If it fires up briefly, it’s definitely a fuel delivery issue.
