Engine revs take time to fall to tickover

[veloduke]

Hi
I'm having some trouble to get the 250 Road to respond correctly to a closed throttle.
The bikes starts easily, pulls away great, revs out and when moving all seems well.
It's when I shut the throttle that there's a problem, the revs don't die immediately, but take 5-10 seconds to drop to tickover.
Everything suggests an air leak somewhere, but there's no leak between the carb and the head.
To eliminate other causes I have tried both mechanical and electronic ignitions, set with a degree disk and verified with a strobe.
Checked the valve timing with a degree disk, and the timings are within 5 degrees of published spec.
I have tried two brand new carbs (Amal and Mikuni).
Cables are good and the throttle snaps shut when released.
Everything I've read about this problem said air leak.
In desparation, I removed the carb and clamped a piece of inner tube (with the valve in it) over the inlet, and pressurised it. the pressure fell from 30 to 0 psi in about 10-15 secs.
After a bit of investigation, I found that the air was escaping through the inlet valve guide.
Am I clutching at straws here thinking this might be the source of an air leak?

[Duccout]

I doubt it, but I'm not an expert. Possibly the slide is being kept open slightly by the vacuum in the inlet port at tick-over; my 250 was the same with the original Amal.

Colin

[George]

I doubt it, but I'm not an expert. Possibly the slide is being kept open slightly by the vacuum in the inlet port at tick-over; my 250 was the same with the original Amal.

Colin

I agree, slide being kept open slightly, cable inner snagging or bends to tight? weak slide return spring? could try Bill's idea on another post and add lead weight to top of slide

"Try cutting a circle of lead sheet flashing about 3mm (1/8") thick, so that it fits inside the top of the slide, then remove the centre for the throttle cable and needle. Not only does it pre-load the spring a wee bit, but also helps stabilise the throttle slide, rather than fluttering about due to the intake pulses. Works on AMAL MKI and MKII concentric throttle slides.

Good health, Bill"

[veloduke]

Well, new cable, no tight bends, throttle snaps shut audibly with adequate free play.
New carbs (Amal and Mikuni), so neither should have a weak return spring.
But I will try adding weight to the slide and preloading the spring, I heard about that only recently.
'Hope springs eternal' might be an appropriate proverb.

[Jordan]

Does it snap all the way shut, or against something that prevents that?

[themoudie]

Aye Max,

You have my commiserations!

Pages 3 to 23 of this link Desmo_450_carb_thread record my 20 months of trial and error to get my 450 MKIII running and able to ride it!

A worn AAU, as predicted by 'celes', proved to be the Achilles heel and once a Hall Effect ignition was in place, the majority of the "nonsense" didn't re-appear.

I have now been running an AMAL MKII 2932; pattern Triumph TSS rubber carb inlet pipe; 'White' cam; homemade silencer; NGK B7HS spark plug set-up and have not far short of 1,500 miles under the wheels.

If anything my valve guide to valve stem clearances were on the tight side at 0.0014" inlet and 0.0015" exhaust. So, I had them both eased to 0.002" on the advice of Nigel Lacey and the engineer who I had asked to check the clearances. Rather than fitting new valve guides, I had mine lined with bronze 'K-liners'.

If you have had new valve guides fitted, are you sure that they are sealing against the cylinder head and not leaking down the outside?

Is your valve stem to guide clearance greater than recommended (I don't have the maximum clearance figure to hand)?

Have you had new valve seats fitted? If so, are these leaking? Or, was the valve refaced along with the seat and at 90° to the valve guide? If not, this could cause problems, with the valve sealing on the seat.

My carb settings are, 220 main jet; 106 needle jet; 2A1 needle, with the clip in the 4thgroove from the top of the needle; No.3 anodised slide, standard return spring, with no additional pre-load or weighting of the slide; 25 pilot jet; standard 3 air jet; cold start jet 25; 'Stay-up' float, with fuel level adjusted to 2mm below the machined upper surface of the float bowl; air screw 1 and 3/8ths turns out from being seated and idle screw, not sure?

Fuel is E5 Unleaded, from which ever pump/brand is nearest and Castrol ValveMaster added at the recommended rate of 1ml per 1litre of fuel. This gives a grey colour to my plug insulator tip, rather than a tan/salmon pink colour.

Using a cooler plug, rather than the B6HS recommended by NGK and making my own exhaust, with back pressure, has made tuning the carb much easier, once the wayward spark timing was cured. Personally, I would advise against sticking any connecting pipe between the carb and an air cleaner if you have one. Either a quality, free breathing pod filter, or a short stubby bellmouth, with no filter seem to work best. I have the latter set-up and for me in the damp UK climate bore wear hasn't been an issue. Bearing in mind that I have owned the bike since 1976 and the first hone didn't happen until after 50,000 miles, when Velocette Venom rings were fitted, until I took the bike off the road in 2001, with over 75,000 miles on the bore, when the bike had its first proper rebore and new piston, with rings.


I hope this helps you pinpoint the cause of the problem and its not expensive to rectify.

Good health, Bill

[veloduke]

Well, after some mucking about, I changed the needle jet on my otherwise new standard spec Amal carb.
A change from .105 to .106 seems to have done the trick. I can also use a 3 slide instead of the original spec 2.

I guess that's one of the effects of modern fuel.

Once I have applied the same solution to the Mikuni I bought (thinking that would help) I will post the settings.

This is for a Spanish Road 250.

[Duccout]

Thanks Max, let us know if you find Mikuni settings that work.

Colin