850 w/Mikunis running problem

Status
Not open for further replies.
Update: I replaced both fuel taps with new ones (the old taps were clogged and the reserve filter had broken off in the tank). Fuel appeared to flow well through the rather elaborate plumbing I rigged to allow for the crossover pipe and fuel filters to all fit between the carbs without binding.

Result: Same problem as before.

Diagnosis: The fuel tank is not venting, or not venting enough to allow the gas to flow consistently out of the taps. I have a '74 steel Roadster tank and I do not see any valve in the cap that would allow this to vent, just a spring-loaded rubber ring. I also looked at my friend's '71 glass Roadster tank and did not see any vent, but he has no such problems.

Question: How can I get my tank to vent properly? Do I have to get a modified cap, or am I missing something painfully obvious?

Thanks!!
 
nomadwarmachine said:
Question: How can I get my tank to vent properly? Do I have to get a modified cap, or am I missing something painfully obvious?
Click to expand...

There should be a vent hole?

On my cap it's drilled into the raised ring on the inner seal plate.
 
L.A.B., my ring does not have any holes in it -- does anybody have a picture of their ring? I'm happy to drill one if I know what it should look like.

The strange thing is that my friend's '71 750 Roadster (glass tank w/slightly different cap) does not have any holes drilled in the cap either, just a cork ring. Could it be that the cork ring does not seal quite as much and lets the tank breathe?

So close, I can taste it. Just need to get air into the tank and I'm good to go!
 
I agree with LAB. The cap is vented, or should be - there should be about a 1/16" hole in the cap, right on the circular ridge that runs around the metal disc that holds the rubber washer in place.

Perhaps this vent is plugged - it should be cleanable.
 
nomadwarmachine said:
does anybody have a picture of their ring?
Click to expand...

As strange as that seems....Yes I do!


850 w/Mikunis running problem
 
There is no such vent on my filler cap, which is undoubtedly an original part (and made in Wolverhampton!). Looks like I need to remove the plate and drill a 1/16" hole through it, or two. Does the hole go straight through the plate only, as I suspect it does, or does it go all the way through the cap?
 
nomadwarmachine said:
Does the hole go straight through the plate only, as I suspect it does, or does it go all the way through the cap?
Click to expand...

Actually there are two plates, another larger diameter plate also with the same diameter raised ring facing outwards which sits above the rubber seal. Both plates being fixed together at the centre.

This second plate also has an identical hole in the ring but at a different position at about 160-170 degrees from the first hole, so the two raised semi-circular rings form a tunnel shaped baffle to prevent fuel escaping.

The second hole then vents to atmosphere beneath the main chromed cap, as the underside of the cap is open to the air.
 
nomadwarmachine said:
Hmmmm....I do not remember seeing a second plate above the first.
Click to expand...


Here's the top plate.
The hole in the lower plate is approximately at the 6 o'clock position:

850 w/Mikunis running problem
 
As I delve further into the world of British bikes, I find myself increasingly distant from my own common sense. Maybe I'm just terrified of strange BSW tools and left-hand threads, but whenever I work on my glorious Norton Commando, I forget everything I've learned from years or wrenching on Hondas and Ducatis.

First of all, as you probably guessed, I DO have vent holes in the plates on my tank, and the lower one was completely plugged. Cleaned it out, blew through it, and the bike no longer dies. Thanks to all! This forum us fantastic.

Pics of the (running) bike to follow soon.

Thanks again!
 
Hello Nomad, don't feel alone with your newley found Norton experiences and problems. I too am new to the world of Norton and their different ways. I have several old Ducatis which I have rebuilt and owned for many years. ( 450 Desmo, TT2, 1980ss 95ss Darmah and the odd Pantah ) Nortons are a new experience for me also.
But I'm learning!
This site is fantastic, and the people who answer my questions are to be commended. Thank you all.
Good luck with your project.
Regards Graeme.
 
Glad you got it sport! Nortons are a repeat experience for me! I still can't remember most of it1
 
I'm running Mikuni VM34s with foam pods on my 1974 850 Commando. So far I've put about 100 miles on the Norton, including a 60-mile jaunt to the beach and back (during which I ran out of gas, but that is another story -- suffice it to say you should not ride with both taps open unless you are heading to a gas station!).

Last night I pulled the plugs off of the bike and both were a sooty black. I just balanced and tuned the carbs this last weekend, which invovled a LOT of idling, so it may be down to that, but assuming that it is running rich, I had a few quick questions:

1) Can I adjust mixture throughout the running range with the air screw, or does this only regulate idle mixture? I suspect the latter, but I wanted to make sure.

2) If I need to rejet the Mikunis, do I only need to change the main jet, or are there other parts that need to be replaced?

3) Assuming I need new parts, where is a good place to order them? I'm located in Southern California.

I will take the bike out for a snort this weekend and recheck the plugs to make sure it's not just idling too rich. Thanks in advance for the help!

-Keith
 
If it was only running rich on idle then yes you could, But if it is any other place in the range you need to re-jet. What you should do is what some call plug chops where you cruise at a few different throttle settings and then cut the engine off at each one and look at the plugs. It is hard to tell when just idle is used as there are different circuits involved, A dyno is the best way but you can get close by just going down a step at a time. Find a place where you have a long stretch of open road and then cruise at half throttle for a mile or so and cut the engine off coast to the side and pull the plugs. You can do this a few times to get and idea of what jet to change. Try first by changing the needle jet a notch. You can also download the info on the Mikuni site and this should help you thru it, Good luck.
 
hi nomad,i agree with chuck regarding plug chops on a few points,the mikunis look very similar to amals apart from the cold start system, to do a plug chop you need to get the main jet sorted first, as chuck said long straight road(slightly up hill is better cos the motor is pulling) ie full throttle,next is the needle jet and needle position between one quarter and three quarters throttle open, so go for half throttle ,upto one quarter is down the throttle valve cutaway and idleing is down to the idle jets and air or fuel mixture screw
 
That's a lot of carb you have there sport. 34s can run fine on your bike but you will lose a bit of mid range torque. As the guys say you are going to have to re-jet.
 
You will find most of your 60~70mph cruising is done on the slides, everyone says set the main jet first,as long as it is large enough I leave it untill later it only starts to get used at 80~ mph.The real problem with mikuni's is the vast arry of needle jets and needles, on amals the adjustments are quite coarse so you notice a change, you may find you need to change 2~3 needle jet sizes on a mikuni to really notice a change, pay carefull attention to how the bike cruises at a constant throttle setting,if it accelerates ok but starts to surge on a constant throttle setting it is to rich.If it spit back or doesnt really accelerate properly it is lean. At full throttle if it runs rough sounding but feels like it accelerates as you close the throttle it is to rich.To rich wears your motor quicker,lean is were the power is at but to lean and it goes pop you need to leave a happy fudge factor.
 
Status
Not open for further replies.
Back
Top