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Discussion Starter · #1 ·
Got my bike on the road and I've been trying to dial in carb. Spoke to an old triumph mechanic about the plug chop test and he said that test was only useful when gas had lead in it and with todays current unleaded/methanol fuels it would not tell you much and not to bother.

Anybody ever use a a/f gauge to get things dialed in?
 

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Got my bike on the road and I've been trying to dial in carb. Spoke to an old triumph mechanic about the plug chop test and he said that test was only useful when gas had lead in it and with todays current unleaded/methanol fuels it would not tell you much and not to bother.

Anybody ever use a a/f gauge to get things dialed in?
Because I'm the dunce around here, you'll have to explain a/f (an auto-fibbythingy?)

A plug chop remains a relevant test, and perhaps the only way to get an accurate view of mixture at any particular setting. Unfortunately, as the proof of the pudding is in the eating, then so too the reading of the plug.

You old mechanic mate is perhaps half right in his advice. Unleaded fuels do leave a black residue, but the plug can still be read from the ceramic at the very tip of the electrode. To be sure some experience comes in handy to help interpret what you see.

A picture is worth a thousand words and so can find many examples on the web where a picture of the plug is shown and interpreted by an experienced person.

It is far the best to use new plugs when doing a plug chop as an old plug, contaminated as they are, may lead to a false reading.

One assumes you understand how to properly undertake a plug chop.

By all means present photos here for us to look at, if you feel it necessary. In any case, you'll be looking for a deposit that is coloured from mid to darkish brown. You do not want to see any black.

HTH

PS. It is quite unfair that you live in Vermont, whilst I must remain in Oz
 

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Discussion Starter · #3 ·
air:fuel ratio gauge. measures via a probe in the exhaust which will tell you rich/lean without any doubt. Thanks for your reply.
 

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Tola,

If you are running a stock TR7 with stock exhaust and stock sized Amal, I'd say just go with the factory specs and don't worry. I had two 73 TR7's one brand new and only changed jetting when switching exhaust.

What is your bike's set-up like?

Joe
 

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Unless you have access to a dyno you can't really get full range readings from an a/f meter, pretty much only an idle reading.
 

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Reading spark plugs was never easy.It could be harder than ever thesedays.For every 1000 people who reckon they could read a spark plug to determine the best mixture from zero throttle up to full throttle,there'd be about 1 who could do it.I don't claim to be that good.
Read this,it's still relevant :http://www.strappe.com/plugs.html

If the timing is right (not necessarily what the book says),and if the heat range of the spark plug is right for the power output at the throttle opening being tested,and the plug is new,most of those 1000 could tell you if the mixture is grossly wrong.Looking at a used plug that's been run at every possible throttle opening will tell you almost nothing.

I don't even believe a gas analyzer will give you the best mixture.There are gas analyzers that you can take with you on the freeway to do full throttle tests.
An air-cooled engine can run as rich as 10:1air fuel mixture. Sometimes it's necessary to prevent detonation in a high compression engine.You'd have a choice to either run rich or lower the compression,whichever gives the best results.Detonation is not on the menu.If you lean it off to 12.5 or 13:1 and maybe even retard the ignition slightly,you could make more power for a short time (until you melt a piston).

It's always safer to run a little richer than maximum power.Detonation can blow a hole in a piston in 20 seconds,when things get a little bit hotter than normal.You can run a pretty rich mixture at full throttle without fouling plugs,because the plug is hot enough to burn itself clean (or clean enough).You couldn't run that rich mixture at 1/4 throttle,when the plug is cooler.

Same with ignition advance.If it's retarded slightly to make 1% less than maximum power,it's a safer place to be.What you can get away with for the first 20 seconds might not look or sound so good (death rattle) after a minute or so at full throttle when things are getting hot.

Other things are relevant when you do jet tests.Humidity or moisture in the air will make the mixture richer.Cool air will make it leaner.High altitude and low air pressure will richen the mixture.When you first fully open the throttle,the fuel level will be higher than what it is after 30 seconds at full throttle.The float needle must open more to allow more fuel flow.If fuel flow to the bowl is marginal (but still up to what the engine is using),the toe of the float could be 6mm lower than at light throttle (and the fuel level about 4mm lower).Running with both fuel taps fully open will minimize this.The latest viton tipped aluminium float needles will also give a more consistent fuel level.

I'm happy to use a main jet size 2 sizes lower than the smallest jet that causes 8-stroking at full throttle and high rpm (it may be different on each cylinder).If the needle position is too low,you're likely to get death rattle (pinking) at around 1/2 throttle.It may even go away if you open the throttle past 3/4,if the main jet is right.At around 1/4 throttle or less,you can safely run a leaner mixture.You want it rich enough to have good drivability,but not rich enough to foul plugs.Needle jet size has the most effect here (most effective from 1/8-3/8 throttle),and slide cutaway will also have effect (mostly below 1/4 throttle).The needle jet is the fastest wearing part in a Mk.1 concentric; 0.0005" wear is a lot,0.001" wear will foul plugs.

Its always better lose one or two bhp,and not have holes in pistons.
 
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