Continual Acceleration

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jsouthard

jsouthard

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Jul 26, 2008
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Hey guys,

Recently, my 69 fastback has started over accelerating. What I mean by this is that when I engage the throttle it accelerates more that normal and when I close the throttle, the RPM's stay up for a second or two before deaccelerating.

Throttle cable is fine. Carb is clean and there is not a noticeable air leak (sprayed carb cleaner on the manifold).

Single Mikuni.

Thoughts?
 
Sniff around the spark timer and verify float level though if pilot screw still affects idle then its in ball park enough not to surge and delay.

A Miki carb, hm, is the enricher lever properly positioned or still functional inside?
 
Try a splash of two stroke oil in the fuel tank, stops the slide from sticking.
 
Single miks can have weak slide springs. Seems I recall either stretching it out a bit or putting in an amal spring.
 
My Mk3 has a VM34 Mik as well. When I rebuilt it, it also was a bit lazy returning to idle on release of throttle. I installed an Amal spring inside of the Mik spring, and, as expected, the throttle return was much quicker. However, the hand torque required to keep the throttle open was much too great for comfortable riding, so, determined to find the REAL cause of the problem, I removed the Amal spring, removed the tank, and let the throttle cable assume a smooth loop to the carburetor. Upon release, the slide returned instantly to its stop with a satisfying thunk. I then remounted the tank carefully, constantly checking the throttle return while adjusting and seating the tank. It doesn't take much of a bend to constrict the cable.

When you say that the cable is ok, do you mean that the slide snaps immediately to its stop when you release the throttle? If not, try the "loosen the tank" trick. You should not need a stronger spring for the Mik to operate properly.
 
^+1 I did the same with a VM34 but left the Amal spring in. I was seeing issues when rolling off the throttle at high RPMs, the slide was lazy.

-strong
 
This is the very worst of sore subject to me as stuck Amal throttle, first time ever, caused my beloved Ms Peel to loose her power punch that made me the Norton Nut for visiting sports riders to call out then wipe smug sneers off faces.
Keep kill button in mind always and not bad Id to get rev limiter option on boyahs or other sparkers.
 
i also had to put an amal spring inside the mikuni spring may not be the best solution but it works,,,,,,,,,,,,,baz
 
Found the issue.

The slide was sticking. There was a sticky grey mass on the slide and on the interior of the carb body. Upon further inspection it's also inside the manifold. Sticky is the best way to describe it.

Cleaned it up, added the Amal spring for some extra resistance and it's better than before! Thanks for the advice guys. Your feedback motivated me to get out there and deal with it.

Actual problem.... It looks like the fiberglass tank is finally starting to erode. I have a Caswell liner that was carefully applied a couple years ago. I'm assuming it's the tank erosion but there are no signs of deterioration inside the tank.
 
thats what happened to my fastback tank it melted into the inlet manifold and then onto the inlet valve stems where it gummed them up and held the valves open gradually losing compression before i noticed!............baz
 
I decided to get a new Tank. My old friend Greg Fauth (Norbsa) is building me a new one out of an alcohol resistant resin / glass mix. I will post an update on the new tank when she arrives.

Now, I need to clean the carb and try to clean out some of the glass burn in the intake.
 
Ugh dissolving tank is about the worse cause for sticky insides. Can you or Greg report on what was noticed probing inside the fb tank with the Caswell's coat. This Implies may have to use my fb IS tank for my ashes instead of using it on the road. I've looked into 3D tank printing but ain't made contact yet to see what it'd cost.
 
An Update....

So i replaced the FB tank. I will do a separate post for that. Cleaned the carb thoroughly, cleaned the intake manifold and cleaned up any sticky brown residue i could reach with a rag.

Currently there is still an issue with a brown sticky residue getting onto the slide and all over the intake manifold. It's not coming from the tank and the float bowl has perfectly clean gas with no dirt etc.

What could be causing this build up? I've pulled the carb and manifold and cleaned it up three times and yesterday it's still building up and causing the slide to stick. It's all over the intake.

Any thoughts will be appreciated?
 
What kind of air cleaner do you have on the carb? Is it oiled? Do you have a breather connected to it? Is the new tank coated?

Although, if this has happened three times in one day, that is a lot of air degradation coming through the filter.

Now you went from a grey sticky mess to brown. My bet is on the coating on the new tank.
 
Well you have to know the source so run a gallon or more of fuel through a coffee filter and see what's in the tank. Best if it includes the bottom contents of the tank.
 
i think its a subliminal phycological subconcious thing , where youre unaware you enjoy it so much youre convinceing yourself YOU are not actually opening the throttle that much , and youre adverse to shutting it at all . :x :p :lol:

Cure would be to get the missus to give it a good polish while you sit down and relax , feet up , and have a beer while you supervise . & stick to open roads .

:D :oops: 8) :mrgreen:
 
Conclusion;

1 - Pulled the carb again, cleaned it with carb cleaner and then soaked it thoroughly in clean gas. Scrubbed, cleaned and polished every part to remove any trace of dirt, grime and any residue from the original tank corrosion.

2 - Changed the air filter.

3 - Ran a gallon of gas out of the new tank thru a coffee filter. The new tank is perfect, just clean gas coming out.

4 - Replaced the fuel lines and cleaned out all the junctions.

Took the bike out for around 300 miles yesterday with no issues. No throttle stick or valve stick. It even feels like the compression has increased since running a couple gallons of fresh clean gas. The new tank was the solution i just wasn't cleaning the carb well enough.

Thanks for all our help. Much appreciated.
 
I am another person that has put a stronger spring in to cure the throttle not shutting off properly with my VM36 on a 750 bog standard engine.
It did the job straight away without any problems since. I have a metal tank & there is no gunge getting in the carb.
I also have a quick action throttle on my commando so dont need to give it a double handfull to accelerate well.
Just fettling my 13" Norvil disc at the moment to stop it rattling. It works really well but the rattle is awful. New aluminium carrier on its way from Norvil now.
Think I will have a ride out on the Honda now as the Commando is in bits.
Cheers Don
 
Hi,
Just to shine a light on this 'sticking throttle issue': Sticking or not shutting off properly of the Mikuni VM throttle might occur if you are not using the supplied throttle cable that comes with the original Mikuni VM carb kit. That cable is in fact thinner and more flexible than the standard Norton throttle cables. Like mentioned before I recommend using an extra standard Amal Concentric spring inside the Mikuni spring to prevent sticking if you are not using the supplied Mikuni throttle cable.
Ride safe,
Constant Trossèl
http://www.hollandnortonworks.eu
 
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