main jet test

Status
Not open for further replies.
pete.v said:
Other than the .5 percenters who tend to take over threads whenever the door is ever so slightly ajar, Main Jetting at WOT is somewhat relative.
On a single 36mm VM, I have gone with a 240 all the way to a 320 successfully. For us regular people it really is more of an economy vs performance adjustment. 240 for economy, 320 for performance.

How many of us rely on WOT, rarely even going there unless it's an emergency? Tune for where you where you spend your time. That is, if you spend most of your time at WOT and need it to serve you well, then tune to where you spend your time. Otherwise it's relative, being an economy vs performance choice.

Yes, choice. There is a variance. With Amals, 180 all the way to 280 and beyond depending on your riding habits and how often you spend your time at WOT. Maybe 180 to 280 is a bit broad, but with a stock 750, 200 to 240 is not out of bounds. With an 850, 220 up to 300.

If you are a moody sort, it's a simple matter of pulling off the side of the road, screwing out the bowl plug, turn out the main with the main jet tool and pop in the main jet of choice. Go hit your Ton, submit for another patch, put your regular main jet back in and cruise on down the road.
Click to expand...

And did you actually take any real reading or are you posting this imformation on the fact that you bike kept running with this large change in jet size, max power and economy come pretty close together not 8or9 jet sizes apart
 
seattle##gs said:
What started this..on the Britbike forum there is an Amal expert who says over and over, with exclamation points, to START jetting by determining the main jet required. This seems almost an impossibility but I figured I would try here and see if anyone had the magic method.

What I have done for the past 30 some years is to accept the manual's suggested main jet size and tinker with with the other jets. This is primarily using Mikunis. I am an average street rider and never on full throttle. Unless it is a racing bike few of us ever reach full throttle.
Click to expand...

So if you start by getting the main jet right first, then what do you do when you change the pilot jet. The pilot jet never stops feeding fuel no matter what the rpm or throttle position is so it adds to the fuel from the main jet.

I guess then you go back to the main jet again....

I always start at the bottom and work up.
 
splatt said:
[And did you actually take any real reading or are you posting this imformation on the fact that you bike kept running with this large change in jet size, max power and economy come pretty close together not 8or9 jet sizes apart.
Click to expand...

I had issues while tuning is a new cam and messing with a power arch anomaly. I was trying everything and while going up and down with the mains, the difference was scant and the real issue was in needle and needle jets, which is another issue all together and probably more pertinent to discussion than mains.

When running a mikuni 36mm vm a long time ago and for a long time, I found to get better milage with a smaller main and better performance with a bigger one. Simple.

Yes, of course 8 or 9 size is extreme and was just an example. The point of the post is that unless you are at WOT all or most of the time, there is little need to split hairs when choosing a main jet. Your best guess and common sense, along with a basic manufacturers suggestion will more than likely be just fine.
 
My '70 commando has a stock cam and dual amals. For a while, I had some "early swingarm" induced weave, so I couldn't comfortably ride the bike faster than 70 mph. I was using 240 main jets and had no carburation issues at those speeds with a 21 tooth front sprocket. Once I fixed my bike's handling issues, I began going faster because the bike felt solid and stable enough to do so. Unfortunately, I couldn't get the bike to accellerate much past 80mph with the 240 mains. The engine bogged down. If I backed off the throttle a bit at 80mph it pepped up, but there was a wall at 80mph. (my timing was correct before anyone asks...)

Because of how well the bike ran, with the exception of the wall at nearly wide open throttle, I assumed it was the main jet issue. I had a pair of 220 main jets in my parts bin already, so I did a main jet quick swap through the drain plugs in the float bowls. I put the 220's in there and the bike zoomed past 80mph easily. My assumption was that I had too big of a main jet, since the 220's fixed my WOT issue.

There was ZERO detectable difference in carburation at lower RPM's with either 220 or 240 mains... so I can't see how you can select a main jet first... before you dial in the lower rpm settings first. Maybe the expert's point is that he is such an expert that he can do it backwards, but I wouldn't do it that way.
 
o0norton0o said:
My '70 commando has a stock cam and dual amals. For a while, I had some "early swingarm" induced weave, so I couldn't comfortably ride the bike faster than 70 mph. I was using 240 main jets and had no carburation issues at those speeds with a 21 tooth front sprocket. Once I fixed my bike's handling issues, I began going faster because the bike felt solid and stable enough to do so. Unfortunately, I couldn't get the bike to accellerate much past 80mph with the 240 mains. The engine bogged down. If I backed off the throttle a bit at 80mph it pepped up, but there was a wall at 80mph. (my timing was correct before anyone asks...)

Because of how well the bike ran, with the exception of the wall at nearly wide open throttle, I assumed it was the main jet issue. I had a pair of 220 main jets in my parts bin already, so I did a main jet quick swap through the drain plugs in the float bowls. I put the 220's in there and the bike zoomed past 80mph easily. My assumption was that I had too big of a main jet, since the 220's fixed my WOT issue.

.
Click to expand...

And that is exactly how it's done. Jim

Well maybe jumps of 10 instead of 20 would be safer...
 
pete.v said:
....For us regular people it really is more of an economy vs performance adjustment. 240 for economy, 320 for performance.

How many of us rely on WOT, rarely even going there unless it's an emergency? Tune for where you where you spend your time. That is, if you spend most of your time at WOT and need it to serve you well, then tune to where you spend your time. Otherwise it's relative, being an economy vs performance choice.

.....If you are a moody sort, it's a simple matter of pulling off the side of the road, screwing out the bowl plug, turn out the main with the main jet tool and pop in the main jet of choice. Go hit your Ton, submit for another patch, put your regular main jet back in and cruise on down the road.
Click to expand...

Hmmm. If you're trying to make the fuel go further, or going slow for other reasons, you don't use the last 1/3 of the throttle opening range, so your fuel is not being metered by the main jet.

So- stopping on the verge and reducing main jet size for slower running may actually be a sign of madness.
 
seattle##gs said:
What started this..on the Britbike forum there is an Amal expert who says over and over, with exclamation points, to START jetting by determining the main jet required. This seems almost an impossibility but I figured I would try here and see if anyone had the magic method.
.
Click to expand...


There is some logic in getting main jet size at full throttle right first, because you can get needle clip position wrong if fuel supply is restricted by a small main jet.

But we live in the real World and so the other settings have to be near enough to let the bike start and warm up at less than full throttle, on real roads with traffic. You can seize an engine by using the wrong needle clip. And you need an engine that is already run in. There's not much you can do about the main jet until you get a chance to test it at full throttle, except use a main jet that you think is at least big enough.
 
Setting the main jet on a rolling road under specific conditions may give you an exact size for your main jet but if you are out on the road in the height of summer and the depths of winter to be that precise you will need to change the main jet size.
For a standard bike the factory set up gives a setting that is a good approximation for most riding conditions.
On the track depending on temperature conditions you may well need to change jetting.

Four strokes are far more forgiving than 2's. I didn't change the jets in my sons racing 2 stroke and immediately had a partial seizure (the motor, not me). This was with a temperature difference from about 16c down to 8.

As has already been pointed out, how often dow e use full throttle on the road, some more than others I suppose!!
 
I had 240 mains in the SS clone ('75 850 engine) and it was a bit too fat. What really brought it in line was changing out the old needles and needle jets for new. Dropped the mains to 230s with the needle clips in the center position and the nos Champion N4G plugs looked perfect when I pulled them out yesterday. Slight black soot around the base and ceramic insulators chocolate brown all the way down. The exhaust is (ahem) quite unresricted.
 
The comment about getting the main jets correct first is actually not as silly as it sounds. If you start with main jets that are too rich, then adjust the needles and needle jets, you have the metering occurring in the correct order. While you are setting the needle jets and needles, the main jets do nothing, because you are working in the range of throttle openings from I/4 to 3/4 s. The main jets only work when you are using throttle openings above 3/4. Getting the needle jets and needles correct is essential for good all-round performance. If the mains are rich, it is pretty much irrelevant because usually when you go there the bike is revving it's tits off. If the mains are too lean, that is when you burn pistons and valves. Obviously, if you are racing, it is best to get the mixture as close as possible to lean-ness right across the rev range. The way to read a plug is not by looking at the colour of the porcelain. It is done by observing the black ring on the porcelain, right down inside the plug, where it meets the metal. Different heat range plugs conduct heat away at different rates. If you are setting the main jet size to the optimum by doing plug chops, it is better to use plugs one range hotter than usual - that keeps you away from destruction.
Note: It is possible to get the situation where the metering by the main jet never occurs at full throttle. - Can occur with methanol fuel.
I'd also point out that when I ride my Seeley, I never whack the throttle wide open. I always feed it on firmly and smoothly as you would with a two stroke. If you are doing plug chops on a long uphill road, you have to wait for the plugs to change colour, so you need a lot of room.
 
Never heard of a black ring at the base of the insulator. Post a pic and educate me.
 
Never heard of a black ring at the base of the insulator. Post a pic and educate me.
 
I'd been racing for a few years before I became aware of how to read a plug. I heard a lecture at a motorcycle club by Barry Smith who was a rider for Derbi in the late 60s. However if you think about it, it makes sense. When you ride a bike and the jetting is controlled by the main jet, if the plug is of the correct heat range and the main jet is of the right size, when you do a plug chop you will find there will be a black ring on the porcelain inside the plug, right down where it meets the metal. It tells you when the jetting is safe. It is caused by the carbon being burned off the insulator and is affected by the jetting and the plug heat range, also by the ignition advance. If your ignition timing moves it can cause the same symptoms as changing the jetting.
When you buy a bike you are told a few things - the recommended main jet size, the ignition timing and the comp. ratio. If you fix the ignition timing and the comp.ratio, you only adjust the jetting. You do that by fitting a spark plug of the recommended heat range and changing main jets until that black ring in the plug just disappears - that is if you are seeking max power. Otherwise, if the black ring is still there, you are safe. If the whole of the porcelain inside the plug is dark- coloured, you are too rich.
However main jets are not your problem. You can run rich on the mains and it usually does not matter. What really matters is whether the needles and needle jets are right. If you are even slightly too rich in that part of the range, acceleration will be sluggish. When you get it right, you can feel the difference as you ride the bike.

I use methanol fuel at low compression. A lot of guys believe you should always run methanol rich. However to get it right is as difficult (or easy) as it is with petrol. It ends up you jet it as you would a two-stroke. - Just on the unsafe side of safe. Plug readings don't tell you whether your mid-range jetting is safe. You need to be able to know that yourself when you ride the bike. If it is lean, it will cough and fart as you change up and down through the gears - raise the needles one notch and you should get best. If you don't stop the bullshit by raising the needles, fit larger needle jets - I make my own, however you cannot do that with petrol.
 
One thing I'd point out. If only one side is running rich, the carburettor might be touching the frame. Use rubber mountings for your carburettors.
 
acotrel said:
I'd been racing for a few years before I became aware of how to read a plug. I heard a lecture at a motorcycle club by Barry Smith who was a rider for Derbi in the late 60s. However if you think about it, it makes sense. When you ride a bike and the jetting is controlled by the main jet, if the plug is of the correct heat range and the main jet is of the right size, when you do a plug chop you will find there will be a black ring on the porcelain inside the plug, right down where it meets the metal. It tells you when the jetting is safe. It is caused by the carbon being burned off the insulator and is affected by the jetting and the plug heat range, also by the ignition advance. If your ignition timing moves it can cause the same symptoms as changing the jetting.
When you buy a bike you are told a few things - the recommended main jet size, the ignition timing and the comp. ratio. If you fix the ignition timing and the comp.ratio, you only adjust the jetting. You do that by fitting a spark plug of the recommended heat range and changing main jets until that black ring in the plug just disappears - that is if you are seeking max power. Otherwise, if the black ring is still there, you are safe. If the whole of the porcelain inside the plug is dark- coloured, you are too rich.
However main jets are not your problem. You can run rich on the mains and it usually does not matter. What really matters is whether the needles and needle jets are right. If you are even slightly too rich in that part of the range, acceleration will be sluggish. When you get it right, you can feel the difference as you ride the bike.

I use methanol fuel at low compression. A lot of guys believe you should always run methanol rich. However to get it right is as difficult (or easy) as it is with petrol. It ends up you jet it as you would a two-stroke. - Just on the unsafe side of safe. Plug readings don't tell you whether your mid-range jetting is safe. You need to be able to know that yourself when you ride the bike. If it is lean, it will cough and fart as you change up and down through the gears - raise the needles one notch and you should get best. If you don't stop the bullshit by raising the needles, fit larger needle jets - I make my own, however you cannot do that with petrol.
Click to expand...

I think Barry Smith's story is one of being at the extreme edge of fine tuning. Somewhere that most of us will never go, on road or track!

One difference between a road bike and a race bike in the context of this story is that a race bike accelerates through the needle jet range very quickly on its way to the main jet at WOT. So you are dead right that the needle and needle jet mixture on a race bike should be slightly lean, to ensure max power and clean running through this range.

However, on a road bike, this is where a bike spends most of its time. On a long fast run it can sit on the needle / needle jet for a long time. Here, preventing a holed piston or seizure is more important than max power. So, typically, a road bike should be slightly rich on the needle / needle jet / throttle cut away settings.
 
ando said:
Post by Danno » Wed Nov 02, 2016 1:27 pm
Well, if a picture is worth a thousand words..
http://www.braigasen.com/howtoread.htm
ando
Click to expand...

Very interesting. According the the rendering, I am running a little rich, which should just about be right for a Norton. The picture of the cutaway plugs at the end looks a bit misleading. The plug deemed "perfect" looks white all the way up from a black ring at the base. If I saw that, I would assume a lean mixture.
 
i think fast eddie nailed it with his comment

A road bike can run for long periods on the needle jet whereas a race bike will be on the needle jet for short periods so can be run leaner mid range to promote fast acceleration

This is where the benefits of the rolling road set up wins tape your twist grip draw lines at 1/4 throttle and 1/2 throttle positions and run at the marks with the gas analizer in place
you can then adjust needle position / taper & needle jet size to a safe mixture setting without the need for guess work

For normal road use on stock carbs factory settings are pretty close to optimal but when changing to bigger or alternative carbs on a modified motor initial set up can be a crap shoot
 
One thing I forgot to mention is about spark plug heat ranges. The high numbers are usually used for racing. They conduct heat better. So the black ring appears more easily - it is soot which has not been burned off. A friend of mine recently showed ne the plugs out of his Triumph 650 racer. One was perfect, the black ring was there and the rest of the porcelain was white. The other plug was brown. This might have been be due to two causes - oil, or one carburettor might have been touching the frame or a bolt.
When tuning your bike, it is preferable to use a plug which is one range too hot. i.e. if you normally use a 9, fit an 8 for tuning. That way you stay safe.
As far as road bikes are concerned, I would always stay slightly rich, right across the full range of throttle openings. So do your tuning, then go up one size or step richer, on every setting. Whatever you do make sure the ignition timing does not change. If it does, you are back to square one.
 
Status
Not open for further replies.

Similar threads

baz
Main jet thread size
Replies
3
Views
253
acotrel
acotrel
ILLF8ED
‘72 combat main jets
2 3
Replies
41
Views
1,781
acotrel
acotrel
N
Single Amal 932Carb main jet and needle position.
2
Replies
31
Views
1,679
acotrel
acotrel
ILLF8ED
‘72 combat Amal 932 main jet question
Replies
12
Views
1,164
ILLF8ED
ILLF8ED
Lovel
1977 850mk3 Pattern Blackcap Silencer Main Jet Size
2
Replies
25
Views
1,511
EstuaryBoy
EstuaryBoy
Back
Top