Sensitive Throttle Problem

Status
Not open for further replies.
3

3Brits

Joined
Jun 28, 2016
Messages
16
Country flag
I only recently purchased my '75 MkIII after lusting after one for some time. I really love the bike. It starts, runs, shifts and rides terrifically. However, it has one quirk I want to eliminate. It has a very sensitive throttle on take-off in 1st gear. If I don't roll the throttle on VERY slowly and carefully the bike will jump off so quickly it nearly throws me off. It has a single Mikuni carburetor which I hope someone can tell me how to tame - and eliminate this problem.
Jack
 
Are you sure this isn't a clutch issue?
Does it has a quick/racing throttle installed?
 
I have a 750 with a vm34 and it is just as you say , I thought that's how they should be.

Jg

The spring in my carb is uprated as the weaker Spring had a tendency to stick open or so the bloke from Moto Carb said when I ordered a new one.
 
The Mikuni has a fairly weak throttle spring and this makes for faster reactions, I have known quite a few people who added a second spring or found a stiffer one to replace
 
I have a 75 mk3 with a mikuni and a barnett clutch. You might try holding the rpms at a set point and easing the clutch out to see if it's the clutch or the throttle springs. I agree with the additional throttle spring theory but I would check the clutch grab first. I'm running a 38 mikuni. Good luck.
Chris
 
I'll check the clutch today. If my problem seems carb related is it the piston valve spring (where the needle is) that I need to replace? Is there a recommendation for what I need to order? Part number? What if it's a clutch thing? Can I fix it?
Jack
 
It's sounds like your clutch is grabbing,have you got a belt primary drive? It may be the clutch plates,if it's the throttle that's your problem you can fit an amal mk1 concentric spring inside the mikuni spring,but my money is on the clutch being at fault cheers
 
You can fit a standard Amal, concentric, spring inside the Mikuni spring to increase the twist effort. Do check to see if your throttle cable is routed with least bends and that both the cable and the twist grip assembly are clean and properly lubricated. With the engine recently at operating temp set your throttle cable's free play to 2-3mm.

Coordinating the clutch and twist grip is just a matter of repetition and will become muscle memory before you know it; each machine is different.
 
How much "twist" does it take to reach full throttle? The bike may have been fitted with a 1/4 turn twist grip.
 
My switch was changed when the bike was rebuilt in 2012 and I now have a separate switch and twist grip assembly like the one Thomas suggested. Do you think the Mikuni cable would work with the "74 grip suggested? I'm really anxious to get this problem solved. I'm riding the bike a fair amount as it is. But it aggravates the s--- out of me. I've never had a bike I felt so positive about. I just want to make it better - for me that is. LOL
Jack
 
3Brits said:
My switch was changed when the bike was rebuilt in 2012 and I now have a separate switch and twist grip assembly
Click to expand...

Does it also have a non-standard clutch lever assembly?


3Brits said:
I'm really anxious to get this problem solved.

I just want to make it better - for me that is. LOL
Click to expand...

Although clutch engagement can be somewhat abrupt, unless there's a clutch problem of some kind then the bike shouldn't be jumping forward if you concentrate on getting the right balance between throttle and clutch control as you pull away.
 
Yes, the clutch is also separate from the light/horn switch.

I appreciate any suggestions given to me by members of this forum. And I understand the comment about there being a need for the throttle and clutch movements to be coordinated. So, I don't mean to be smart about my response - but I've been riding for enough years to have learned how to do this on a variety of bikes. That is what has led me to question the abrupt start off of my MkIII.
Jack
 
3Brits said:
Yes, the clutch is also separate from the light/horn switch.
Click to expand...

Then it's possible the clutch lever assembly may not have the correct pivot radius or lever ratio to match the clutch lifter mechanism?
 
I will say the problem is entirely in the clutch...just way too grabby. I have run into this a few times before, the plates do not slip at all, just grab. I think the only thing to do is try different bonded plates, or maybe a mix of the two.
 
CanukNortonNut said:
yea I blew over the 75 MIII and I didn't catch that.
Click to expand...

Makes no odds, as it isn't what's fitted now. :)

According to Service Release N3/25, "06-4600" was a quick-action throttle, introduced from serial 301700 in '73.
 
Status
Not open for further replies.

Similar threads

acotrel
  • Locked
Throttle position sensor
Replies
2
Views
597
kommando
kommando
windsorsmith
  • Locked
First gear problem
Replies
17
Views
1,582
Tornado
Tornado
Vicente
Norton Commando 961 electric problem
Replies
6
Views
1,461
TonyA
TonyA
jseng1
  • Locked
Cheap & easy solution for wet sumping problem
2 3
Replies
58
Views
9,100
EstuaryBoy
EstuaryBoy
Mart UK
  • Locked
Hi, New to Norton Ownership, with a Problem!
2 3 4 5
Replies
97
Views
12,045
Mart UK
Mart UK
Back
Top