Brumby running rough, shuddering and bunny hopping

[Wonga]

I’ve recently acquired an ‘89 Brumby. I love it but there’s one weird issue I can’t get to the bottom of: Randomly it will start running very rough, not accelerating until I accelerate really hard then surging forward, juddering and bunny hopping. It feels a bit like it’s running out of fuel. The problem occurs randomly, sometimes it will be fine for a couple of weeks. When it does happen, it’s usually after a long drive or under a heavy load and usually after I’ve stopped then re-started the engine. I’ve checked the fuel filter, fuel lines and air filter and the previous owner, who was aware of the issue; rebuilt the carburettor and replaced the distributor and coil… all the usual things. Has anyone else experienced this issue? Any suggestions would be very welcome. It’s been suggested to me that it could be the head gasket, but I don’t really want to go down that route unless unless I absolutely have to.

Thanks in advance for any suggestions!

Cheers,

Will

[El_Freddo]

G’day Will,

I sat on this one for a day or so after initially reading your post as this is a perplexing issue.

I have an issue with my brumby sometimes where it feels like spark is cut as the clutch is engaged and throttle applied in the next gear up.

I don’t know what causes this issue - but I’m now planning a carby kit treatment as I have another issue to track down.

Anyway, a good friend of mine, Subarino, had a client’s brumby come it that runs fine when cold but after about 15 mins of warm up would be a pig to drive. He managed to track it down to the little vacuum “tower” sensor thing to the (vehicle’s) LHS of the carb on the manifold.

He found that blocking off the vacuum port and the now exposed port would allow the engine to rev freely.

I don’t know if there were any other adverse behaviours to this setup but it could be worth a try.

The only other thing I can think of is a sudden and intermittent cut to the ignition power to the engine. I don’t know how this could occur but anything is possible.

Please update us as to what you find changes or what solves the issue

Cheers

Bennie

[Wonga]

Thanks Bennie. The problem you describe regarding the vacuum port sounds similar to the problem I'm experiencing. It certainly seems to be worse when the engine's hot, or even on hot days. I'll do a bit of investigating down that route on the weekend. I wouldn't rule out a problem with the ignition power either. A lot of the electrics seems to have been re-done in a - to put it kindly - quick and creative manner, so it could be that.

I'll post an update once I've had a look. Again, thanks heaps for the suggestions!

Will

[Wonga]

I realise I forgot to reply with an update. I solved the problem: vapour lock. I discovered that pouring a bit of cold water over the fuel line fixed he problem immediately. I have subsequently been told that this is a known issue. I've insulated the fuel line with tape and the problem has not entirely gone away but it's not nearly as bad. Still happens a bit on very hot days. I reckon better insulation and/or a thicker fuel line should fix the problem.

[El_Freddo]

Hmmm… I’ve not heard of this being an issue before. I’d be checking the bowl vent line and the fuel cut solenoid to ensure everything is breathing and being fed properly.

Glad you’ve got it sorted though!

Cheers

Bennie