Lean running Combat

Here are the plugs after yesterday’s ride. Top plug is the left side.

I raised the needles one position this morning. I’ll take a ride this afternoon and report.

Fast Eddie said:

I totally agree htown. With the one caveat that the guesswork is greatly enhanced if the guesses are made based on an understanding of how the mixture is being affected. This diagram might help the OP with that:

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Thanks for the diagram FE. Very helpful. Ben

Well, I wouldn’t have said they look too lean to be honest!

But that doesn’t necessarily mean it’s not rich somewhere in the range.

Let’s see how your needle raising goes, hopefully that’s all there is to it.

Do keep us posted woncha ?

Have you checked the tank breather ? If it is blocked, you might be creating a vacuum in the tank which can cause irregular fuel flow and starving. Jetting the carbs is not guesswork. If you lower the needles in the carbs, you should be able to make the motor cough. When that happens, raise the needles one notch. You have a choice of 0.106 and 0.107 inch needle jets. Start with the 0.107 s . The main jets should always be slightly too rich, so start with the biggest recommended. Most Amals and Mikunis use number 3 slides and you should not have to play with them anyway.

If your motor 8-strokes, you are miles too rich. If it coughs as you ride the bike, it is too lean. When you use full throttle, you need enough fuel to stop burning valves or pistons - that usually only ever happens when using full throttle. The taper on the needles is there to compensate for loss of vacuum as you open the throttle. So if you ride aggressively or your motor is over-ported, you probably need more taper so you can open the throttle quicker.

Right plug looks okay to slightly lean. Left a touch rich but that could be due to the idle jet being a touch rich. I've never used anything but .106 needle jets in all my Brit bikes regardless of make. .107 needle jets are likely to be way too rich in any bike that is close to standard configuration.

When looking at plugs it’s important to remember that you are only looking at the colour from the last few minutes (perhaps seconds) of run time.

You can ride at WOT all day long, but if the last mile to your home is in heavy traffic, then when you remove the plugs all you are looking at is evidence of the mixture from the pilot jet and slide cutaway!

At the risk of repeating myself, this is where the marked twist grip pays dividends. Get yourself on a long open road (you’ve plenty of those over there), ride at a steady state at a marked throttle opening for a while. Then kill the engine and stop, do not let it idle. Then check the plugs, you are now seeing what the mixture is like at that exact throttle position.

A lot of folk know this and do this at WOT to get the main jet correct, which is good. But a road rider spends most of his time in the 1/4 to 3/4 throttle range, so it’s much more important to get the slide / needle arrangement correct.

Regarding your needle jet, I fully agree with htown, unless you’ve got something very wild and off-piste, a .106 needle jet is going to be correct.

You are looking at the wrong part of the plug. The heat range of the plug determines how far down the porcelain insulator the carbon burns off. If you look down inside the plug to where the porcelain meets the metal, there should be a 2mm black ring there, if the mixture is correct and you are using the recommended plug. However I only ever use plug readings to check the main jets are large enough to run the motor on full throttle. It would be pretty rare to burn anything when riding your bike using lower throttle openings than full throttle. If your motor coughs when you ride the bike changing up and down through the gears - STOP and raise the needles one notch. - It is TOO LEAN. If the jetting is too rich, the motor should be sluggish - but safe.

If you have got a cough in the motor, do not ride the bike without correcting the jetting or ignition timing.

If you fit the 0.107 needle jets, you can start lowering the needles from the top position until you get the cough, then raise them one notch . If you don't get the cough, fit the 0.106 needle jets and start again lowering the needles from the top position ( clip in bottom notch). You can detect the cough by opening the throttles when the bike is stationary, but it is more obvious when you are riding the bike. You should always know when your carburation is too lean. That is when you can do damage.

Always go from rich to lean and do the last bit carefully. Check your ignition timing first. If the ignition timing changes, it can have the same effect as changing the jetting.

I raised the needles one position - clips in the send position down. The engine still wanted some air slide to run smoothly. BUT - I rode the bike today in warmer weather (61F) for about 25 miles and it sorted out. Now it doesn’t need the air slides to run smoothly, pulls nicely on the main jet and runs up from low throttle nicely. After sitting for several hours at the office it started easily without the slides. Here are the plugs from my ride home from the office tonight, which included the last three miles at slower speeds.



It looks to me as if they are rich, with the right plug darker. I think I’ll put in new plugs, take it for a long ride and then chop it and photo the plugs. Sound ok? Thanks everyone for all of the information. I’m still a bit lost but feeling better. Ben

That's more like my plugs when I'm running mid range speeds(2900-3200rpm) and lower. If I kick it up it burns off to light tan.
Those may look pretty good after a chop.

Might try the idle mixture screw on the right a quarter turn out (leaner).

jbruney said:

That's more like my plugs when I'm running mid range speeds(2900-3200rpm) and lower. If I kick it up it burns off to light tan.
Those may look pretty good after a chop.

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That’s about where I was running coming home. Thanks also HTown. I’ll try turning the right mixture screw out.

1) you didn’t mention what fuel, not a racer... ASSUME unleaded pump gas, yes?

2) trying to read spark plugs by Coloration on pump unleaded gas is impossible.
Only when an engine is WAY OVERFUELED does it color the plugs.

3) tune by performance first. Make it run & feel strong, no misfires, no bogs. Verify economy is where expected 50+ mpg at parade speeds, 30 at go to jail speeds.

PS Expecting your (non fuel injected vintage bike) to run well at 31F without changing the carburation jetting is not a good idea. I’ve ridden in those temps several times. I just accept it’s a lean mutt until the day warms up. Like our snowmobiles at 61F, it’ll run dawg bawls, unless rejetted.

concours said:

1) you didn’t mention what fuel, not a racer... ASSUME unleaded pump gas, yes?

2) trying to read spark plugs by Coloration on pump unleaded gas is impossible.
Only when an engine is WAY OVERFUELED does it color the plugs.

3) tune by performance first. Make it run & feel strong, no misfires, no bogs. Verify economy is where expected 50+ mpg at parade speeds, 30 at go to jail speeds.

PS Expecting your (non fuel injected vintage bike) to run well at 31F without changing the carburation jetting is not a good idea. I’ve ridden in those temps several times. I just accept it’s a lean mutt until the day warms up. Like our snowmobiles at 61F, it’ll run dawg bawls, unless rejetted.

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Fuel is 93 octane pump unleaded. So it seems that I just needed to wait for a warmer day to sort the bike out. I put myself and the forum through a lot of conjecture, but I do feel that I’m gaining more insight. Thanks Concours and everyone.

On that unlead thought I know I run a little rich, but if I go leaner mine will begin doing an occasional spitting fit during acceleration. So I pretty much set where she runs the strongest without risk of white plugs. I tossed the book long ago on tuning my machine because a great part of the info just wouldn't bring it in, and this was when the wear and tear was minimal. To this day I believe it to be the cause of two PO giving up on it. Spoke with one of whom inquired if I still had & if it were running well 3 months ago, and the man was shocked to hear she was ever so tame.
Some machines have their quirks and personality differences. It took forever to learn what this one needed.

Getting the pilot air/gas mixture is easy, there is no guesswork involved if , as I have said previously on this website you buy/borrow a Gunstons Colourtune, and use it only in the dark. If you use it in daylight forget it. It will rev to 3,000 rpm, but it is not meant for that. You can, if watched carefully detect a ignition misfire at a low 1,000 revs. There are loads about on flea bay, very little used...............

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Just to rule something out.....look down the pea shooters and see how open they are. There were some made with quite a restriction, the baffle plate had a hole in the center about the size of a nickle and that was it. I fought this problem via carburation for several weeks until I discovered the muffler restriction, swapped mufflers, problem solved.
As for carburetion..low speed running, where most of us are always at, is the toughest part to tune. Most of the tuning components converge in this area...slide cutaway, pilot jet, needle jet, air screw setting, slide synchronization, worn carburetor, fuel level, and the occasional air leak. Then there is altitude and air temp variations. Try holding two Amal pilot jets side by side, a #17 and a #19. It is difficult to see a difference but the #17 produces hard starting and the #19 starts easily. So you are dealing with tiny variations that produce changes. As the throttle opens it becomes progressively less sensitive and easier to tune.
I try to tune on a warm day with the motor fully warmed up with new plugs. I might use two sets. Frequently I have found that a #107 needle jet is required, good to have a pair just to try.
Tune for the freeway or open road. Trying to tune in the city is no good. When I give the bike back to it's owner I tell him that the spark plugs will not last for a lot of city driving and when there is a drop in performance IN TOWN discard the plugs and fit a new pair. That is an inexpensive solution. Just accept that there is a difference between in town and open road riding and in town will cost some spark plugs...no big deal.