Yamaha 3.5AC hard to start

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dartmoor

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Picked up a Yamaha 3.5AC off ebay for 30 quid. It runs beautifully when started, and idles with no problems at all. Once started it re-starts with one pull. But once its cold again, its really difficult to start. The spark plug is good, and new fuel in the tank. The fact that once started, it idles so smoothly at low revs leads me to think that the fuel flow is just fine. And it responds immediately to the throttle without hesitation - so jets are clear.

Any ideas? My procedures have been to turn off fuel and run the carb empty when finishing with it. Start from cold, ensure air vent is open, fuel tap on, choke on, throttle half way open...and pull. But it needs loads and loads of pulling to get it to wake up and go.
 
dartmoor said:
Picked up a Yamaha 3.5AC off ebay for 30 quid. It runs beautifully when started, and idles with no problems at all.
But once its cold again, its really difficult to start.
- so jets are clear.
Any ideas?
Click to expand...

That engine has main, idle and starter circuits in the carb. Sounds like your starter circuit is not operating correctly.
Have you blasted the starter air inlet with carb cleaner and made sure the starter jet and starter plunger are working correctly?
View attachment 44384

From the manual "When operating the starter knob the main carburettor is kept closed, and the fuel supplied through the starter jet is mixed with the air regulated by the starter air passage. The resultant rich mixture is thus supplied to the engine."

Edit: Forgot to mention; the theory states that the throttle should be closed for the starter circuit to work properly (creates max vacuum).

Cheers
Bob
 
Last edited:
Fantastic - that's just the lead I needed ! many thanks - I will check things over. Seems a superbly engineered engine - shame its so obsolete. Never knew an air cooled engined could be so quiet!
 
If that doesn't do it you could start to suspect the seals on the main bearings. 2 strokes use the crankcase as a sealed part of the induction system. Leaking seals spoil that.
 
Mine is from 1980, and starts hot or cold at the 1st or 2nd pull. I traded it as not working, but found the filter gunged up. Cleaned that and the carb and it was fine.

Looking at Bob's drawing, I don't think mine has the same carb. And it will start with the throttle open.
 
Last edited:
RIBW said:
That engine has main, idle and starter circuits in the carb. Sounds like your starter circuit is not operating correctly.
Have you blasted the starter air inlet with carb cleaner and made sure the starter jet and starter plunger are working correctly?
View attachment 44384
From the manual "When operating the starter knob the main carburettor is kept closed, and the fuel supplied through the starter jet is mixed with the air regulated by the starter air passage. The resultant rich mixture is thus supplied to the engine."

Edit: Forgot to mention; the theory states that the throttle should be closed for the starter circuit to work properly (creates max vacuum).

Cheers
Bob
Click to expand...

Many thanks again - used some carb cleaner - and changed the starting procedure to no throttle, and it now starts first or second pull! I think I was flooding it before.
 
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