Champion spark plug fail

[Hughsie]

Haha, nice, those sound like good times!
Yeah sadly I don’t use this enough, but I’m building confidence with it, maybe just need to store a plug key and half a dozen new plugs in the tool box 

[Hughsie]

You shut the fuel off and let it run until it stops.

G'day Hughsie.
That way she uses all the fuel in the bowl and runs lean for the last 15-20 seconds and hopefully burns off the soot.
Try those settings but the only way to know is by doing plug chops, and with her at full operating temp (10 mile ride) not just idling in the shed.
Very nice looking bike mate. My Cafe started life as a 56-57 A7SS. I bought it to road race 500 class so was heavily modified.
Cheers

Nice one Muskrat, sounds good. Yeah I didn’t overly modify mine as I wanted something fairly standard for long term use, but added a few choice additions, thunder alloy rods, srm pump and balanced crank. Once I nail the running and hopefully start the fuel tap trick I will get more miles in.
Cheers mate

[Black Sheep]

Did you drop the needle a notch? That's the first thing I would have done. Otherwise the plugs will just keep sooting up and your mpg will be pretty rubbish.

[Hughsie]

Did you drop the needle a notch? That's the first thing I would have done. Otherwise the plugs will just keep sooting up and your mpg will be pretty rubbish.

Hi mate. Yeah, so I originally had a 250 jet, worked at the needle clip till I was all the way down, then I dropped the jet to a 220 as I was still way out.
On the new 220 jet l, the needle was in the middle, I’ve moved it one space, (so I’m bringing the needle down the way)  but still no joy, so I’m glad you said that coz 1 clip didn’t make the difference but maybe I should drop it again and see how it goes.
As I say the old carb has a 200 jet in, but the bike was very worn at that point, so hopefully I’m not far away. Still miles away from a quoted a7ss
I’ll drop the needle again I think

[groily]

You've probably been there already  . . . but what's the float level like?
With the replacement carb, could you be suffering from the old 'Does it or doesn't it need a washer between the banjo and the float chamber?'
A lot of us have earned that T shirt.

I don't think this is primarily a main jet thing, unless you are running at big throttle openings all the time. (I think Musky said that somewhere earlier in the thread too.)
Too big a pilot, wrong slide cutaway, plus position of needle groove and size of needle jet are more likely to be relevant I'd say, if the fuel level is good.

The bits you've got don't sound daft and certainly don't suggest you should need the chimney sweep to the extent your picture says he's needed!
Most boggish-standard 500 twins with a 376 Monobloc will run somewhere between 220 and 260 mains (no air filter), a .106 needle jet, and a 25 or maybe 30 pilot, with a 3, 3.5 or 4 (some marques) cutaway, and not be stupidly far off. As you go down towards 200 for the main you're entering Concentric territory, or even pre-Monobloc, where smaller is typical. Hence wondering about float level.

One other thought - is the needle stamped 'C'? There are shorter 'B' ones used on 375s and that would make a heck of a difference if it was wrong. Got that T shirt too.

[RogerSB]

. . . but what's the float level like?
With the replacement carb, could you be suffering from the old 'Does it or doesn't it need a washer between the banjo and the float chamber?'

Good point. When the Monobloc was re-introduced it acquired a fibre washer under the float needle seat. I think it must be needed to make up for the difference in using plastic floats and viton tip needles, so in this case you fit one.  However, a washer was not fitted to earlier Monoblocs so you don't fit one.
Rog.

[Black Sheep]

I assume you have a new needle jet? These things wear and can make the mixture go rich. Also, some aftermarket needle jets are suspect. Only buy from a reputable supplier.

[Hughsie]

Hi
Yes everything is new so new jets etc in the new carb.
I’ve never check the float height as it was brand new I didn’t think I would need to check this washer, but it’s on my check list now.
I’m finding it interesting that the old carb was running a 200 jet with the needle clip all the way at the bottom putting the needing all the way up.
I’m thinking i may try the old 200 jet in and if it’s close maybe offer a new 200 or 210 main.
I think everything else seems ok so this combined with leaning it out at the end of day via cutting the fuel might keep me running ok.
I will update if I have any joy.

[Slymo]

Very easy way to check the float height is to get a bit of clear plastic tube of an appropriate size 1/4" ish and push it over the pilot jet once you have removed the cover nut. You then turn on the fuel holding the pipe next to the float bowl cover and the fuel should stop at the level of the nipple in the middle of the float bowl cover. if its high there is your problem. Main jets only come into play at around 3/4 throttle so it is very clear when they are too big or small as the bike will lose power at full throttle or start fouling plugs or chucking out black smoke. The bike can run perfectly with the wrong main jet up until 3/4 throttle.

[Hughsie]

Very easy way to check the float height is to get a bit of clear plastic tube of an appropriate size 1/4" ish and push it over the pilot jet…..

Really useful information Slymo, thanks I’ve never had to do this before so I will give it a bash. Found some good visuals from Paul Henshaw on YouTube as well explaining the different float needles so I’m feeling much more in the know about this after then info.
Thanks all
I will let you know what I find.
 

[BSA_54A10]

Oh really!

Yes, only takes a minute or two, may start to run faster for a bit, coughs and splutters and then cuts out. I let it run while taking off my gloves, searching for my keys and opening the garage door.

Edit: In the day, when riding every day, we didn't have to take these precautions, just pressed the red button, shut the fuel tap, kicked the prop stand down and jumped off. . . . Oh! if it was after lighting up time we had to leave parking lights on.

Rog.

Yeah I do some thing like that
After a while you get to know exactly when it is going to stop so you can point at it or snap your fingers to the amazement of on lookers

[Hughsie]

(Edited)Just a quick update for anyone still following that could help.

So I’ve pulled the old carb down from before the rebuild and It was running the following (I am assuming the bike was being ridden with these)

Original 376 worn carb
200 main jet
30 pilot
06 needle (I think the 1 has worn off, so 106)
3 1/2 slide
Needle clip at highest setting


And now I’ve pulled the new carb and I am running
A 220 main
106 needle jet
30 pilot
Slide is a 3 1/2
Needle is just 1 down from middle

Note: I have a new carb and I have a metal shim to set the float height as per the new carb guide. It came like this? It’s not a fibre spacer as some have described???????

Based on comments I’m wondering if I should drop the pilot to a 25

Most of my riding is around my village / town so I’ve not really been at full throttle much. When I have I’ve always felt it didn’t give me much of a difference (I thought this was due to the plugs fouling so I might even need to increase the main?)

I’ve not checked float height just yet with a tube, but I have this on the cards, although as it’s a new carb I was expecting this to be correct, but again need to check this, I’ve found some tubing so I’m checking this next

Need to order a few sets of plugs

[k0337788]

Hi I had this problem (plugs failing) especially on first start up. Bike would start then immediately fail on RHS. After much head scratching I installed some short reach Champion J8 and it cured it! Im guessing that being an old engine & running rich it fouled the long reach plugs while warming up. The short reach plug doesn't extend into combustion chamber so doesn't get sooted up so easily. This is what they do in radial aero engines as they are horrible oily & sooty engines when not under load. Also something to try - when it stops running on one cylinder use some insulated pliers and pull the HT cap away from the plug about 1/4 inch & I bet it starts firing! hope this helps - good luck Andy

[RDfella]

KO's remedies work - I've used them, like when a customer in the 60's with an old Bedford truck used to pull in every day with misfiring. Those engines hated ticking over for too long and would foul plugs if left doing that. I even tried plug extenders (haven't seen any in years now) but to no avail. And so I cut the HT leads and inserted a coat button in each, meaning the spark had to jump the gap between the button holes. Looked like a firework display under the bonnet (behind the cab) and no doubt it played havoc with radios as the truck passed by, but it never fouled plugs again! Around that time a professional version was available - the Norton Spark Intensifier - which was basically the same thing in a capsule.
Having said that, beware using short reach plugs in a long-reach head. Use a plug tap to clear carbon from the threads before refitting long-reach plugs else you'll damage the threads in the head.
Also, whilst these remedies work, fixing the cause rather than the consequences is always batter. Why is the engine oiling its plugs, or running too rich mixture-wise?

[Hughsie]

Thanks for your comments.
I’ve ordered some plexiglass, x4 new plugs, a 25 pilot jet (just in case I want to try it) and a set of monobloc shims.
I’m going to check fuel level and try set this correctly, then play around with some tuning.
I can’t be far away based on the suggested jets etc, but as we’ll be a mile away