Mikuni needle shims on an Amal

These are pics are the plugs from my 72 Commando.
Before sectioning

With flash

Without flash

Plugs have about 200 miles on them are not from a plug chop, just around town riding. Very little riding at WOT throttle so I assume most of what I'm seeing is needle jet/needle influence at 1/4 to 3/4 throttle. Carbs are 932 Premiers with 220 main jets, #3 slides, .106 needle jets and needle in the bottom groove (richest setting). Float set so fuel level is .21" below top of bowl. Bike starts one kick, pulls hard and idles well.
Based on charts I've seen the right plug looks like the jetting is pretty close as a fuel ring is starting to become visible and extend about a 1/4 down the insulator. It's more apparent looking at the plug than the photo shows.
However, the other plug concerns me as it may be on the verge of being too lean with no clear cut fuel ring. There's a white horizontal line about a 1/3 up but little darkening above it. Again these plugs only have about 200 miles on them. The exhaust header pipes also confirm the plugs with the right being mostly golden with a little bit of blue and the leaner left side being more solid blue to purple.
The question is how to get the left side a little bit richer. I can't raise the needle as it is already in the lowest groove. I had a couple of ideas.
1) I have the needle jets from the original carbs. Assuming these have some wear they might be slightly richer than the almost new jet. John Healy's mantra about "it's the needle jet", usually in the case of an over-rich problem might work in reverse here.
2) Reading through John's TIOC writeup on Amal tuning he mentions using Mikuni needle jet shims in an Amal to fine tune the needle. I've seen kits on fleabay that are .020 thick. Anyone tried this approach?
3) Go to a .107 needle jet and lower the needle.
Any and all input would be appreciated.

Start with the simple ones; carbs balanced so that the left isn't carrying more than its share? Timing equal left to right? Keep in mind that a slight variance in timing can affect plug readings. Once those are addressed, then move onto jetting.

Idle (pilot) jets carry an amazing amount of influence for the large majority of street bike operation, even as seemingly far-removed as sustained freeway travel. This is followed closely by the slide cut-away.

I'd start with a slightly larger idle jet in the left side. Next would be a lower slide cutaway (2-1/2?). Even if it wouldn't be the same as the right (presumably), the proof of the pudding is in the eating.

Nathan

Pazon surefire so timing should be same on both sides. Carb's synched recently.

your main jets seem on the small side.. and at one third throttle up will begin to have an effect. Wondered whether you had tried running with both fuel taps open... why are you running with Resistor plugs?

Yes, both taps open. I have a Sparx 3 phase alternator and they recommend either resistor plugs or caps.

Plug chop is a great tool, WOT and using leaded fuel.
At low/mid range, power, response and expected fuel consumption are all much more useful than plug coloration. Assuming unleaded pump gas, no useful info, except a near flooding rich condition.
JMWO :mrgreen:

These things are tricky because modern fuel can be very variable in terms of how it colors plug insulators. Had thought of fuel starvation and obviously of weak mixture , does it spit or is it getting hotter than usual? If not would suspect something ignition related. Pazon do not recommend resistor plugs for the Smartfire, the Surefire maybe different. What plug gaps are you running? You need to do plug chops right up the range and to cut the motor straight away. Just a few firing cycles is enough to mask the real result.

Got some shims and a .107 needle jet on the way. Will try the shims first as that seems like the smallest amount of change. Trying to think this through. Does raising the needle only have an effect at throttle openings when the tapered part of the needle is in the jet? To have an effect at smaller throttle openings would the jet need to be changed?

I only use plug readings to set the main jets. The rest is as lean as I can get without getting the cough or flat spot, when the motor is hot. It is possible to get metering on the needle even at full throttle. If fitting bigger mains doesn't richen the mixture at full throttle, you have that problem. I don't know what the shims are which you are referring to. I use 34mm Mk2 Amals with Mikuni needles, however I use methanol fuel, so I can't really comment on what you are doing. The tuning routine is the same except the jets are much larger.

Bike is street ridden so I estimate probably 95% of riding I do is between 1/4 to 3/4 open throttle as opposed to race bike which is running WOT a lot of the time. My understanding is 1/4 to 3/4 throttle is mainly controlled by needle and needle jet so that was where I was focusing. Shims are .10 to .30 in size and fit under the needle clip to raise the needle. The standard gap in needle grooves is .40 so these allow a little finer adjustment. I simply can't raise the needle one groove because it is the bottom one already.

Re read the comment about the pilot jet, you are probably on it more than you think and it has influence.

I suspect that shimming your needle will give the fine tuning you want. It is done on Mikunis because they respond to this type of very fine tuning (meaning they have more variations than most of us ever need). Most of us feel Amals don't (meaning that the limited range of Amal adjustment suits most applications.

I suspect the 107 needle will show too rich whatever you do. A whole cutaway size on the carb slide is probably too much! More likely progressively filing the edge from too rich a slide could get there....Jeez....

But, why not use a spot thermometer and see if one side is actually running hotter than the other. (or fit thermometer devices under the plugs if you want to spend more money and worry yourself through every ride)

If not...stop messing! (meant politely)

The other questions on your plugs is....why 8 not 7?

And why BP not B

(yes I know some prefer them, but why are you using these particular plugs?, what influenced your choice? is it best for your ignition set up?)

Racing or not the advice to actually do a plug chop on fresh plugs with leaded fuel (if you can get it!) is a good one just so you have some idea what happens on WOT. Dyno run is the best way to see mixtures these days. Might tell you more than you need to know :wink:

If you are racing the finest tuning possible is good even with methanol fuel. With petrol the fine tuning is much more critical to get best performance, however on a road bike I would always run slightly rich both in midrange and when operating on the mains. In road racing the midrange tuning is much more critical on tight circuits. If the midrange is suitably lean, as you wind the throttle on coming out of corners, the motor spins up quicker and the bike accelerates better. I still haven't bothered to lean off the main jets beyond what I originally set them at (slightly too rich). I ride the bike like a two stroke - feeding the throttle on quickly coming out of corners in one smooth action. However it never runs wide. A lot depends on how you've got your bike's handling set up, and the gearing you use. My bike oversteers and it is very high geared.
On a racer you are unlikely to burn a piston through leaning off the midrange too much - the bike just becomes difficult to ride because of the cough. On a road bike you might have a problem if you rarely use full throttle.

My first thought would be to swap the needles and see if the problem swaps too. A long time ago I drilled a small flat piece of aluminum so that the needles would catch on the taper and not fall through. The holes were very close together so I could compare needle groove position. As expected, the needle grooves were all over the place...this is how I would select a matching pair.

I believe the straight part of the needle is in effect for perhaps 3/16 throttle.

One problem I have run into is the phenolic spacer is not the greatest for being a gasket, even if sealer is used. If you have ever tried yamabond on it and later remove the part you will see that there is no trace of it left. The motor has sucked all of it off the phenolic and run it through the motor. Which means you have a small air leak in this dept. Unregulated. Is it possible that this is happening to you?

With much controversy, I throw away the phenolic and use the 1/8" asbestos type gasket. With Yamabond ( 1104). I use light pressure on the two mounting bolts as the Yamabond is also a glue and holds VERY WELL. so well that it splits the gasket rather than having it fall off during removal. No air leak at all.

As Steve said, Why are you running bpr8es instead of bpr7es's?? Do you have a higher than stock compression engine?

and as acotrel said, I would rather be in the rich side of mixture adjustment if I was tuning a street bike...

Both of these comments taken together means you are running a lean mixture on a colder plug judging by the looks of the picture you posted. If you replaced the bpr8es with the standard, hotter bpr7es plug, it would look even leaner when you inspected them.... You might want to try the next larger needle jet size, if you are at the end of your adjustment by moving the needle upward.

I have 930's and run a 220 main, 106 needle jet, maybe your 932's could work better with 107 needle jets... just a thought...

*Is there a reason for the bpr8es??

....And yes, I think you could use shims to microadjust your needle height. You could also index your plugs so they both point in the similar direction when they are threaded into your head, so the fuel has the exact same exposure to the spark in each jug...

This bike is a very late 72. It has a standard compression head not a Combat but it uses 32mm carbs similar to a 73 rh5 head, but it doesn't have any stamp. Both the head and top of the barrel was skimmed to true but I doubt the compression was raised that much. The right hand plug thread stripped and a time-cert insert was installed. Comnoz on this forum suggested I run an 8 on that side as the heat may not dissipated quite as easily with the insert as the the plug running directly in the head. Jim said I could run a 7 on the left but I had another 8 so I used it on the left side also and yes I am concerned that the cooler plug is showing lean. What I'm trying to accomplish is richening that side up.
One other wrinkle, when I ordered the new Premieres for this bike they came with the stepped spray tube and 4 ring needle like the 850's have. I guess the vendor assumed that since that since they were 32mm they were for an 850. I assumed the tube and needle would work okay in a 750 but that might not be so. I understand the spray tube was to correct a bog on rolling on the throttle by temporaily enriching the mixture. What concerns me more is the needle shape. The four ring needle has a longer straight part before the taper begins and the taper section itself is longer so that the overall needle length is about a 1/4 inch more. The four ring needle is also slightly thicker at the end of the taper. I'm wondering if the 4 ring needle might be causing some leaness in the mid range. So the first thing I plan to do is replace the spray tubes and needles with the flat top and 2 ring types. These are what the original carbs that were on the bike had.

I've run bpr8es in my 750 before. They worked fine. I don't recall if the plug looked richer as a result, but if you can tune your bike to run every bit as well on a cooler plug then I wouldn't think that's a bad thing.

You sound like you are logically working through a sequence of carburator parts changes to get your mixture right. lt may take a lot of time disassembling, reassembling, and then taking test rides to inspect your plugs, but you sound like you are on the right track to me.... good luck.