engine dies beyond idling

Hi, 73 mk II with newish single carb, clean fuel, good flow delivery, starts first kick, idles at 2100 with choke on. Cannot run without choke on and any attempt to rev will cause the engine to die. Carb has been stripped and cleaned but still the same.

Not sure what to do next. Any thoughts gentlemen?

Well you would begin with pulling the plugs and fitting 2 new ones.

One throttle cable not set up properly?

Torontonian said:

Well you would begin with pulling the plugs and fitting 2 new ones.

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Indeed.

And if its not the carb or plugs etc, it must be electrical.
Remember, "99% of carb problems are electrical, and 99% of electrical problems are the carb".

You have checked though that the carb slide is lifting, and so is the needle.
And what brand is this single carb, and how suitable is the new jetting...

It's a single carb. I'd also then pull the bowl and unscrew the idle jet and hold it up to a light ,also read the jet size .

Torontonian said:

It's a single carb...

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Silly me. How'd I miss that. As you were...

If its a Mikuni with a "rubber" manifold, be very sure the is no cracks in it from old age and vibration... Sucking air through crack or faulty manifold gasket will cause it to idle fast...
What does "newish" mean? Do you know the carb?? have you ever had it, or heard it running right?...

"Newish carb"
Has this bike ever run ok with this carb?
If an Amal was just swapped for a Mikuni VM I'm wondering if you've misunderstood the enricher (choke) mechanism.
I'm not sure if I could get my Mikuni to idle at 2100 with choke on without it flooding out.
Can it be that you have the choke off when you start it and it idles at 2100 and when you think you are taking the choke off you are actually putting it on making the engine bog down when you try to go?

Hi jnash, welcome. More info please... Mikuni single? Amal? Sounds very much to be a lean bog, but need specifics to go further. A functioning idle circuit and no mid range smacks of blockage from storage or foreign body. Tell us EVERYTHING... :mrgreen:

Any backfiring or missing when attempting to rev up? Or just the firing ceases?

ensure that the two interior screws inside the carb manifold (that can only be accessed by removing the rubber carb spigot) are tight and not letting air in

You said newish. I interpret that to mean used? The first thing you need to do is find out what calibrated parts are in that carb. VM carbs with factory calibrations are set up for 2 stroke engines that need almost twice the fuel of a 4 stroke.

Lets assume that you purchased this carb from someone that had successfully run it on a similar caliber Norton, if this carb has sat unattended for over 4 months there's a good chance that the idle jet has become constricted; you may be able to see through it, but that is a meaningless observation.

Tell us the nomenclature etched on the:

Main jet, Idle jet, Needle, Needle jet and slide, and, of course, what is the bore size of the carb? The bore size is VERY lightly stamped/etched on the casting section that the choke (enrichener) is screwed into, it can be measured easily enough as well.

This problem will be a piece of cake with the data requested above.

Bill.

if its a Mikuni VM then its bound to be too small a Pilot Jet.
Commando 750 or 850 needs a .35 or .40 Pilot to run after choke off.

ML said:

if its a Mikuni VM then its bound to be too small a Pilot Jet.
Commando 750 or 850 needs a .35 or .40 Pilot to run after choke off.

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'74 850 with Dunstall mufflers and a single VM 34mm at 2400 ft altitude; 32.5 idle jet.

Nathan

gentlemen,
thank you for your input. I will try and work on this at the weekend and install new plugs, recheck the inlet manifold for leaks, and do all the other items suggested and report back with all the answers to your questions to me.
thanks again.

My sons' Norton developed this problem while we were at the rally in Utah on a ride. After all the checks for gas, swapping batteries and such, the bike would start and idle fine, as the throttle was opened it would die, just as you stated. Finally looking into and feeling into the carb as the throttle was turned, we found the slide would open but the needle stayed in the jet. The cerclip that held the needle to the slid had opened up and let the needle drop. We bent it back into shape, but it didn't look like it would last for long. His was a Spanish made Amal, so the vendors at the rally didn't have a new clip. T. C. at Sunset Motors told us what it was and had the proper clip. hth Frank Coleman

Yup ,that makes perfect sense so tear apart the carb (again).

Thanks for the needle tip.
So, I found there was a slight air leak in the 2-1 manifold at the head (loose fastening screw) . Once this was fixed with new gaskets and thread lock this time, the engine speed responded in the right direction to the throttle opening.
In the process of checking, gas flow is 8 oz in 30 secs, both taps on, measured at the float bow drain so that's good, I didn't have a gas flow issue.
Two new sparks plugs were also fitted at the same time. Heat / blow from the exhaust tips are even between cylinders when engine is running.
No back firing.
The carb was rechecked for correct slide/needle movement and it checked out OK. Carb jets rechecked for any blockages - OK.
The carb is a fairly newly installed bored out copy of the 28mm Keihn PWK" according to the seller, Jim at JSmotorsports, why the way is helping with excellent customer service.

However, idle with the choke ON are still over 2,500 RPM. The idle screw is backed out enough not to raise the slide at these revs, but not by much.
Turning the choke OFF it is hard to keep the engine running when warmed up, it will die if the throttle is not continually blipped.
Is there a used carb I can buy on eBay that fits from a Japanese bike?
Any help would be appreciated - thanks.
-Jim

jnash said:

IS there a used carb I can buy on eBay that fits from a Japanese bike?

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"Used" as in "Cheap"? There's any number of brand new Mikunis to be for on fleabay that would fit your bike, or at least be very close in jetting. Searching for "Mikuni VM 34", I find many for $100-$150 that are set up for Triumphs and BMWs that would be a virtual drop-in, based on the jetting they list. Since you already have the manifold and cable, swapping carbs is the "easy" part. Just be very wary of a used carb that was from a two-smoke, as jetting will be all wrong. Trust me; you'll get more into changing out all of the brass innards than you would by just buying a new, properly-jetted item.

Nathan

Some things you buy used and some things you buy new. Never forget that my friend. :wink:

Have you contacted jim at jseng and see if he has a solution? He speaks very highly of these carbs maybe worth a call.

J