FH 650 running problems

[peterhill]

I have a Huntmaster that starts and runs well. It pulls really well when accellerating and will run nicely at speed. But, at low speed it runs roughly. At about 20-30mph in 3rd or 4th gear it hesitates and judders like it's misfiring. It's difficult to describe but it sort of feels like your riding on a rough road when the road is perfectly smooth.

I've checked the obvious, reset the timing to the book value (11/32" BTDC at full advance) with a dial gauge through the spark plug hole, I checked the valve rocker gaps. I checked for intake air leaks. I've had the Monoblock carb rebuilt professionally and all the jets etc are correct. I set the pilot jet by the book (2.5 turns out) and then enriched and leaned it out systematically to try to find a sweet spot. I've never had difficulty starting, hot or cold so I'm assuming the magneto is in pretty good shape.

Adjusting the pilot jet did make it worse at the extreems (v. lean and v. rich) but didn't eliminate the problem. I'm assuming it's running mainly on the pilot jet because the twist grip is barely off the stop.

Anyone had a similar experience or got any suggestions for what it might be ?

Pete

[Steven.Carter]

Could be your running a bit rich. Check your plugs and try is dropping the needle one notch. It fixed a similar problem on my Huntmaster.

[peterhill]

Thanks. I'll try that.

[peterhill]

I dropped the needle a notch and it's definitely no better, possibly a bit worse. Also it doesn't pick up as well as it did before when I open the throttle.

[chris.shearwood]

What is your procedure for checking for intake air leaks?

[simon.holyfield]

So the pilot jet is the right size, it's seated correctly, it's clean, the passage in the body is clear, and the pilot jet cover is sealing?

If you push on the side of the slide through the throat of the carburetter, does it move?

[ian.scott]

Have you checked the timing is the same on both cylinders ? There are a lot of uneven slip rings around. My A7 had vastly different timing, and I need a new cam ring to correct it ?

Ian

[nevhunter]

Check for carb slide wear. just off idle. Check plug gaps. If too small they don't like running where you are experiencing problems You may also have a worn magneto slip ring which will be out of round. you can check this on the bike with one pickup removed. Nev

[peterhill]

I had the carb professionally rebuilt recently which included honing the slide bore and fitting a new slide. It's a good fit so I don't think there is an air leak past that. All the seals were replaced and the jets checked and replaced as needed and the front face skimmed. I can't remember exactly which jets needed replacing. I refitted it with new gaskets and o-ring.

I usually check for air leaks by spraying carb cleaner or similar around the carb/head gasket with the engine idling. If there is an air leak it gets drawn in and you hear a change in the engine note.

Timing is a possibility. I know there is wear in the slip ring as I can't set the points gap the same on both sides. I can't remember the difference but it's only a couple of thou'. When I set it, I split the difference.

What ignition timing are people using? The manual says 11/32" home and 3/8" export (BTDC full advance). I set mine to 11/32".

[Steven.Carter]

I use 11/32 on mine. I had a bad slip ring on one of my bikes years ago. Spot on timing on one cylinder, blew a hole through a piston on the other!!! I have electronic ignition now.