Why you tune down when you tune up

When you tune an engine you are trying to get the best performance out of it easy starts fast acceleration and good mpg. Thats the goal. In order to do this you have to get all the cylinders to equal out, do the same amount of work. In order to accomplish this the best performing cylinder has to be de-tuned so its equal to the worst performing cylinder. If you can not make the poorest cylinder better you have to make the best worse. Sometimes its not easy to make a good cylinder under perform, it can be down right frustrating. Thats why its important to do things in the right order and the right way.
Heres a few things that come to mind
Do a compression check most auto parts stores will loan you a tool for free and pick up a set of plugs while your there.
Check valve clearances first, get it right then move on.
Charge the battery first and during a tuneup these bikes don't make a lot of juice when they run right as the battery gets weaker performance drops off that is the spark gets weaker so the more you tune the worse it runs
Use fresh fuel don't use old fuel or fuel thats got half a can of Sea foam in it.
Put in that set of new plugs, fouled plugs don't work so great. Spark plugs are cheap.
If your adjusting the pilot screws and they dont seem to have any effect on the engine then you probably have the idle stop screwed in to much this causes fouled plugs real fast so when you take it out for a spin you burn of the crud off the plugs so when you pull up to a stop light your engine idles at 2500 rpm and wont slow down.

A picture is worth a thousand words
Theres no such thing as a dumb question
The more info you give when asking a question the better the answers will be

 

I disagree with that statement.

When you synchronize you are adjusting the airflow so that each cylinder is breathing the same amount. You are not de-tuning any of them, you're making them all capable of doing the same amount of work.

Then you set the mixture so the proper amount of fuel is entering each cylinder. Again, this is not de-tuning, you're setting for best efficiency and performance of each cylinder.

 

Sync... Tune ... Sync... Tune.... Sync... Tune...

 

Bills got it. Sometimes thats what it takes

 

rinse... lather... repeat... You know some one is still in the shower because it doesn't say turn it off and try again tomorrow.

 

I just got mine to idle recently because I go back and
forth with the two.
I thought the idle circuits were plugged but I guess not.
It's still not running perfect. The plugs are too light.
Syncing again today to get it closer and adding fuel.

 

But the best performing cylinder will breath more than the worst performing one. So you'll have to "de-tune"(= restrict breathing of) the best cylinder so it matches the breathing of the worst.
Or something like that
That's for the synching though, the mixture is set independent for each cyl, when u forget about yics for a sec that is.

 

MiCarl said it he just does not believe it. It took me a while to come to terms with it too. It actually goes against what we think we are trying to accomplish, which is getting the best efficiency and performance of each cylinder. The problem with that is they are all connected to the same crank. If the engine is going to run smooth all the cylinders have to do the same or equal amount of work that makes sense to me. The problem comes from the fact that the cylinders are not necessarily equal there are variables.

 

If there are variations (other than sync and mixture) it doesn't need tuned, it needs repair.

They should all have the same valve timing, compression, ignition etc. within very small variances. That's why the admonition to set valve lash before syncing - so you don't have another difference affecting your readings.

When you're syncing you're adjusting the butterflies so they are open the same amount. If you're using sync to compensate for another variable (very low compression for instance) then you're not tuning, you're trying to hide a problem.

Variation between the sync gauges when you start does not necessarily indicate a "weak" cylinder (there is no such thing as a "strong" cylinder in that it over performs). If they're synced and reading much below 10"Hg then you know you have another problem. Anytime I've had a synced four cylinder pulling below 5"Hg I've had a completely dead cylinder (usually ignition).

 

No completely so though, otherwise a bench sync would be sufficient.
The variables allowed per cylinder, ie valve clearances are measured by range not the precise number, same pretty much goes for compression; within 10% of each others. So besides a bench sync where all butterflies are set equal, a final sync is needed to overcome those variables. In which case there's no other option then to adjust the butterflies to the worst performing cyl.

In addition:
De-tuning is a bit of an overstatement though, since tuning involves much more then just the last step of synching and setting mixture.

 

you are backwards with your thinking. a good performing cylinder is making more power with LESS intake charge, a poorly tuned cylinder requires more throttle application to maintain idle or equal power to the stronger cylinders.

the object is to tune the carbs and ignition (and properly clearance valves/piston rings for compression) so you can then sync the carbs, and set the idle screw so that the butterflys are letting as little air through as possible and keep a nice smooth idle, and a responsive, powerful, EFFICIENT running engine.

its all about efficiency. less is more.

 

Exactly, and since we sync in pairs, you open the butterfly of the poor performing cyl and automatically close the butterfly of the best performing cyl. Therefor restricting the best cyl to perform at its best.
Like I said before de-tuning is an overstatement, but once u get to the final step of syncing, yeah you sync to accomodate the worst performing cyl.

 

You are all wrong, #3 can't be adjusted, so all you can do is line up the other 3 cylinders to match.

 

Turn the idle speed knob down and #3 closes.

 

Correct. #3 is the "master;" turn the idle speed down and THEY ALL CLOSE. It's 'one' with the linkage so it's not individually adjusted.

Wiz is also correct, in that we don't adjust #3, we adjust the others to it. THEN we adjust them all via the main knob, which moves the entire linkage.

He was pointing out that they don't get adjusted in pairs, as suggested; we adjust 1,2 and 4 to match #3.

And the overall theory originally put forth is correct; yet so is MiCarl. Yes, you are trying to get all of the cylinders performing the same amount of work; however, some of them are "more capable" than others, and since you can't "turn up" the weaker ones, you effectively have to "de-tune" the stronger ones to match.

You guys are all unusually obstreperous of late-- is it a full moon or something? (But that wouldn't explain Wiz, he's in a different hemisphere...)

 

#3 is the key to easy tuning. It is just as adjustable as 1,2 and 4 the only difference is when you adjust the butterfly on #3 carb you also move the butterflys on 1,2 and 4 at the same time.

 

Three days away from a full moon down here in the southern hemisphere

 

i dont really understand the "restricting performance" complaint.

unless we are talking about wide open throttle, all cylinders are VERY restricted....and thats good/normal operation of any throttle bodied gas engine.

 

Wait just a sec - -
Let's say Cyl #1 is absolutely perfect as far as rings and valve performance, and Cyl #2 is a bit sketchy and is not sealed up as tightly.

Let's say you did a "perfect" bench sync. And using a water Manometer.

Are we all in agreement that the tightly sealed Cyl #1 will be pulling a harder vacuum, and the less tightly sealed #2 won't pull as hard?

Then- - you would be INCREASING the throttle angle of the "good" cylinder to DECREASE it's vacuum pull on the Manometer, therefore, the "good" cylinder will be working even harder(making more idle speed horsepower).

You're not tuning down the good Cylinders at all.
You're just lowering their idle vacuum signal.
But as MiCarl pointed out, this engine needs some mechanical work.
So I'll go on . . .

Cyl #3 & 4 have perfect rings, valve seats and identical 150 PSI readings.
Cyl #3 has tight, but in spec shims.
Cyl #4 has shims on the loose end of tolerance.
Cyl #3 ends up with marginally more lift, a couple degrees more duration and overlap. It's slightly more of a "hot-rod" for idle performance, and draws a weaker vacuum. #3 makes more HP on the top end, but throw that fact out the window for now.
Cyl #4 pulls a slightly stronger vacuum, due to less overlap, and would make more idle speed HP.

You would INCREASE the throttle angle of #4 to DECREASE it's vacuum signal, again, making the stronger running Cyl even stronger.

Therefore, A running sync does NOT make all 4 Cyl's produce the same power.

 

bad rings on one cylinder will not make it pull less vacuum (within reason). tight valves will screw anything up but for this threads argument lets just say the valves are all good. #1 has bad rings, puts out 50psi less than #2 on a comp test. they will both demand the same amount of intake charge.

when you are "synching" the carbs you arent adjusting power output of each cylinder up and down, that is a side effect of EQUALIZING VACUUM between the cylinders.

you are adjusting throttle application to each cylinder so that they are all allowed the same amount of intake charge. power output of each cylinder will still be x,y, and z for that given throttle application. synching basically keeps all 4 (or 3 or 2 or 8) cylinders "working" at idle. with a poorly synched rack of carbs, at idle it is possible to have one cylinder completely choked, and another doing all of the work which would make for a rough and horrible idle.

bench synching them gets them close using a clearance on the same point for all butterflies, but since these carbs arent exactly precision pieces there are some tolerance discrepancy's (especially after 20 years of work and weather) that allow the flow rate to be different. also take into consideration shaft seals, intake boots.

since everything cant be 100% perfect they give us the ability to compensate for these minor flaws. the adjusting screws are not meant to be used as a bandage for tight valves or other serious mechanical flaws. this is also what the pilot screws are for. if everything was perfect you wouldnt need a screw to adjust, you could put the proper sized jet in all carbs and they would all run the same. we know THAT does NOT work

think of synching your carbs as "equal opportunity employment". you are giving all of your cylinders the same amount of opportunity to perform, if your bike is in proper running order all cylinders will within reason make the same power.

of course, changing vac signal means you need to check the jetting, so the sync, tune, adjust idle, sync, tune, adjust idle comment was appropriate. if you dont like it, pay a mechanic or buy a fuel injected vehicle!

 

and if your valves are in spec, tight or loose but IN SPEC.....the difference in valve lift profile is NOMINAL at best. its a pretty tight window we are working with for that very reason.

 

But when things get in sync idle rpms will rise so you use the main to bring back down overall idle and then you go back syncing the right to the left pair.

Also completely closing the main knob doesn't necessarily close all butterflies, if the butterflies are not set equal one will hold open the other.

 

Yes they will. And if you want to get really technical about it, that's what they're supposed to do. Minute adjustments of that relationship are the point of the whole exercise.

The main knob moves the whole shebang. Number three moves with it because it has no adjustment of its own, but the big knob moves all 4.

One leans on two, they need to be equal and then go match up with three. Four is on his own to get in line with three; the only "pair" involved is #1 and #2 unless #4 is perfectly matched with #3 and so we're not screwing with it any more.

 

Zookie, I have a disagreement with the basic premise of the title of this thread.
I understand your explanations, but I'll put it this way:
If you had a "perfect" XJ 750 motor, and a "perfect" rack of carbs,

Then you installed this perfect carb rack onto an IMPERFECT XJ 750, that was within normal tolerances, and Cyl #1 happens to be "perfect" and Cyl #2 is not as perfect, but still in spec, then what would the Manometer read?

I think Cyl #1 draws more vacuum.
If you "tune it down", that implies closing the throttle blade.
I believe that is wrong.

Agree, or disagree??
Or am I mis-reading what "tune it down" means??

 

they should all pull the same vacuum. to a reasonable extent, bad rings or scored walls that bleed off compression and cause a loss of power or hard start will have no effect on vacuum.

if the valves are within spec and sealing properly, each cylinder whether new or worn will pull (X) liters of intake charge per stroke.

on paper you should be able to sync a rack of carbs on one mechanically sound engine, and put them on another and they will read the same on a manometer. in reality, just pulling the synched rack of carbs off engine #1 is enough to move the bodies and brackets and ultimately throw them out of synch. then installing them distorts them even more.

to see what i am trying to explain, put a magnetic base dial indicator on #1 carb diaphragm cover. set the dial indicator to 0 on the #4 carb cover. try to remove or install them without making the needle move around.

also, to further prove the manometer vs healthy cylinder debate.....pull a plug wire on one cylinder and see if that one cylinder drops vacuum. the rpms should drop and they all will drop vacuum, not just the one dead cylinder.

as far as the nomenclature goes....."tune down" to me means backing off timing or choking the air flow on the intake or exhaust side. with high vacuum on the manometer, the last thing you want to do is close the butterfly more in an effort to even it out. unfortunately, the english language comes in many forms. i live in CT and i cant understand what some MA people are saying when they talk. i assume tune down can get several definitions just from the people on this forum.

 

Just as an aside, Crow brought his new 'carbtune' over on Sat, to sync' my '85 900, I had previously done the sync' using the one gauge, golf tee shuffle method, Mr Crow will attest that the rack wasn't but a 'nats' out, no screw needed more than a few degrees of adjustment.
I had set the pilot mixture screws at 2 turns out from lightly bottomed, the default for these 36s & after the sync' I turned them out an extra 1/4 turn, to smooth the low end.
Result sweet as.
No YICS tool used.
If I never had a good friend, I would definitely buy one of those 'carbtunes'

 

Well, I'm glad we got that straightened out.

 

Maybe the word DOWN has caused some confusion, I only used it because its the opposite of UP as in ( tune-up). Maybe i should have said de-tune. I did not take into account the semantics of the statement. My apologies.
The point is the engine can only perform as good as the poorest performing cylinder, you cant argue that. I did not start this to argue the merits of the statement. What I was hoping for was to point out the importance of not skipping things like valve adjustments, checking float levels and such. These can have a huge effect on overall performance even though they dont seem to. Because the engine seems to run good, the effects are not easily noticed like a knock or black smoke coming from the tailpipe.
Anyone who has tried to tune a multi-carbed engine probably knows the meaning of the word FRUSTRATION. Anyone who, like me, thought they tuned thier bike then went back later and performed the task they skipped and re-tuned defiantly knows the meaning of, "HOLY SHEET"