[4wheelsbad]

Folks.  I’d like a new throttle return spring for my Sixty5 (Mikcarb VM28).  Don’t seem to be available at the usual place(s).  A standard spring or  a slightly ‘heavier’ one.  Any ideas, please?

[stinkwheel]

Try mikuni part number VM28/257

Lots of mikcarb service kits on ebay, most of which include a spring. If you want it really strong you can mesh two together.

[4wheelsbad]

Thanks for the tip, I’ll have a gander.

[stinkwheel]

Although it does occurr to me, is it the throttle spring that's your problem or does the cable need some attention? I presume it's not closing fully when you release the throttle?

[allanfox]

Although it does occurr to me, is it the throttle spring that's your problem or does the cable need some attention? I presume it's not closing fully when you release the throttle?

Good call that man!

[BruLan]

I also have a sixty 5 and the Carb is as standard mounted far to close to the Tank for the Throttle slide to return smoothly, on mine the cable was in fact kinked
My solution was to remount the Carb on a shorter manifold which made a difference, got a manifold from eBay cheap as chips
https://www.ebay.co.uk/itm/314003955471
Also had the benefit of stopping the Carb jumping around as they are somewhat cantilevered as standard
Next job is to change the knackered cable and route as smoothly as possible

[stinkwheel]

Similar problem on the Mikuni I have on my 612, it's a tight bend to get it out from under the tank and into the top of the carb and takes a bit of fiddling to get a clean entry. Same goes for the decompressor. Particularly problematic on the 612 because the barrel is lifted 7.5mm on compression plates and is closer to the tank and top tube of the frame (so much so i had to file down one of the rocker cover nuts to get the tank on).

When the cables let go, I think I'll remake them using a brake noodle (that's the actual name of the thing) that they use on bicycle V-brakes ont he end of the cable.

[Raymond]

When the cables let go, I think I'll remake them using a brake noodle (that's the actual name of the thing) that they use on bicycle V-brakes on the end of the cable.

Another good call. Looks like it might be possible to adapt that brake noodle (!) to solve what can be a problem area.

[Adrian II]

you can buy ready-made carb cable elbows/ferrules that will do the job. Dell'Orto have a few at different angles, and I think they're the same thread as Mikuni but you'd have to check.

https://www.dellorto.co.uk/?s=cable+elbow&post_type=product

A.

[Raymond]

Thank you, Adrian!

[4wheelsbad]

Thanks folks.  All replies make good points and there are some good suggestions.  The cable is fine and the throttle snaps shut, and the slide returns properly to the stop with the engine not running.  Sometimes, with the engine running, I don’t get a nice, sharp return to idle.  Now, a long time ago on  Beezer with an Amal monoblock, I had a similar prob - it turned out that the slide was binding a tad due to the pressure drop across it.  Before I had the opportunity to refund’ the carb’ I fitted a slightly stronger spring which overcame the ‘stiction’.  H’s should have new springs available in a few months and I can wait until then. 🤞

[BruLan]

I appreciate these are for Dell Orto Carbs , but this snakehead seems like the answer to may problems

Snake head carburettor top for Dellorto PHBL.
This allows more easy cable routing as the cable comes in from the side not the top and it swivels and finds its own resting position.
The top swivels 360 degrees..

[Super45]

In addition to other members suggestions re cable routing/condition, it may also be worth carrying out an air leak test when the bike is running, its worth spraying some form of flammable liquid eg WD40 round the rubber carb mount (not known for their longevity)  and the exhaust where it enters the head , if the revs rise then from either end that could mean the engine is sucking in air which cause the revs to hang slightly when returning to idle. easily sorted with an exhaust re-seal or a new carb rubber.

also worth a check to see if the carb is distorted from over tightening to the carb mount, known issue on the 350's where it bolts to the head direct but potentially possible from overtightening of the clips  which could also mean the slide doesn't return correctly once warm.

[BruLan]

In addition to other members suggestions re cable routing/condition, it may also be worth carrying out an air leak test when the bike is running, its worth spraying some form of flammable liquid eg WD40 round the rubber carb mount (not known for their longevity)  and the exhaust where it enters the head , if the revs rise then from either end that could mean the engine is sucking in air which cause the revs to hang slightly when returning to idle. easily sorted with an exhaust re-seal or a new carb rubber.

also worth a check to see if the carb is distorted from over tightening to the carb mount, known issue on the 350's where it bolts to the head direct but potentially possible from overtightening of the clips  which could also mean the slide doesn't return correctly once warm.
[/quote
Thats a good point my 2003 Bullet had only covered 2500 km at the Carb mount was split, Hitchcocks do an upgraded rubber so it must be a common issue.