Low End Tuning: Pinch Test Question

[kx250ryder]

I reviewed some old posts that said to tune the engine from break in (I believe the engine that came in the car is already broken in), so I was going to do it like everyone said. First, tune the high end, then reset the idle, and then do the pinch test to set the low end. Finally, re-check high end and idle.

One thought that hit me was, how can the pinch test numbers, 3-5 seconds before revving up and stalling, be a good reference? I realized, that I cut my own length fuel line, as many others likely have done.

I have seen some cars with short fuel line, almost straight from tank to carb, and I have seen others, with extremely long fuel line, with several loops around the tank and/or engine. How can both of these stall even within 10 seconds of each other for the pinch test?

Anyone know?

 

[vbgagnon]

Fuel line length has nothing to do with the pinch test. When you pinch the fuel line it stops the flow of fuel through the line. So you are burning what is left in the carb and thats it.

 

[OFNA buggy basher]

I try to pinch it about 1 inch away from the carb. Seems to be consistant.

 

[Lessen]

VB is correct I believe. It doesn't matter how far away from the carb you pinch it. Basically all you're doing is creating a barrier so the pressure from the exhaust cannot feed the carb anymore. when you pinch the line the engine is only gonna burn what is left in the carb, not what's in the carb AND between the pinch.

 

[AC48]

Ohh I see Lesson, makes sence. Well either way it doesn't matter how long your line is.

 

[Lessen]

don't quote me. hehe, most of what I type is BS anyways, lol

 

[niggle]

Don't see the point in the pinch test myself.

What you need is a low-speed mixture that is as rich as possible (for cooling/lubrication purposes), will compensate for the extreme lean conditions seen in the transition from the LSN to HSN on snap throttle openings and most of all will allow the engine to accelerate crisply and cleanly with no bogging or misfiring.

You shouldn't expect your engine to idle cleanly for minutes on end. Expect it to load up after 10 seconds or so and clear it out by blipping the throttle every 10-15 seconds. Tweak the LSN so that the mixture is as rich as possible but will pull cleanly when accelerating from idle with no spluttering.

You need to finalise your LSN settings at optimum head temperature (200 - 220 F).

 

[mikeburgin]

the part of this meathod i dont understand is, if you tune your top end first then do the bottom end, the botom end has an effect on the top end so it will put your top end out again , when this thread was written i struggled then as well

 

[mikeburgin]

Don't see the point in the pinch test myself.

What you need is a low-speed mixture that is as rich as possible (for cooling/lubrication purposes), will compensate for the extreme lean conditions seen in the transition from the LSN to HSN on snap throttle openings and most of all will allow the engine to accelerate crisply and cleanly with no bogging or misfiring.

You shouldn't expect your engine to idle cleanly for minutes on end. Expect it to load up after 10 seconds or so and clear it out by blipping the throttle every 10-15 seconds. Tweak the LSN so that the mixture is as rich as possible but will pull cleanly when accelerating from idle with no spluttering.

You need to finalise your LSN settings at optimum head temperature (200 - 220 F).

nah you watch the guys at the track that know what there doing, if they do flip there car theres no reving to keep it going, there confident in there tuning and no its not going to die, if its to rich down low it means its going to be to lean up high, so on a long straight bang bang bang

 

[niggle]

the part of this meathod i dont understand is, if you tune your top end first then do the bottom end, the botom end has an effect on the top end so it will put your top end out again , when this thread was written i struggled then as well

How does that work? When you get to about 1/2 throttle the LSN is normally all the way out of the jet, surely the HSN has exclusive control of the mixture from this point?

 

[Lessen]

Mike I give you a lot of respect cuz I know you know your stuff but I have to argue that the LSN needle does not effect the HSN setting. The HSN controls the overall mix (high rpm and low) but the LSN only adjusts the mixture at the bottom end. when the carb is much more opened the LSN needle is completely out of it's seat, at this point the only calibration having any affect on the mixture is the HSN.

 

[Sgt Blamo]

I just spoke with an engine builder/moddifier, he told me he always sets the LSN 1st than the HSN. He recommends pinching the fuel line about 1/2" from where the fuel line attaches to the carb. 3 seconds it should begin changing the idle, 5 seconds it's dead. Only one loop of fuel line is recommended.

 

[mikeburgin]

How does a needle on a carb work???????? is it that when the needle is in is when it allows fuel to pass or is it as the needle is slowly pulle out of its seat that it allows more fuel out, the richer you run the bottom end the richer the mid throttle setting is, to counter this you have to lean your top end resulting in a lean mixture

 

[bill61]

Mikeburgin,
Lesson is correct.
As you can see in your illustration the Highspeed needle controls ALL fuel flow into the engine (that is why you set it first). After it has been set then you worry about the low speed needle.
When the needle is all the way in is when it controls the flow of fuel.

 

[mikeburgin]

u guys may be right in theory but in my limited practical exspereince, the low end does effect the top, i do understand the theory of a carb, but i also understand that you guys are forgetting the tappered needle, if there were a straight switch over from low to high end then i would agree that the low end needle has no bearing on the top end, my point is that if the bottom end is to rich it will effect the period of change over, meaning that you have to tune your top end richer to compensate, there fore running lean, remeber that high needle only has full effect at wide open throttle, well actually a little before but you know what i mean, if you want to go off theaory the fact that the level of your fuel in your tank should have no bearing on your tunning, but it does, the fact that in a carb of this style you shouldnt need to presurise the tank, but you do
can you set your top end needle first if you can't get the engine to burn clean enough down low to get top revs outta it
I have no worries tunning a engine, and i do what youguys would call horse before the cart
get it idling smooth first then do the high end, then richen the botom end a tad to improve the change over

 

[vbgagnon]

I'm going to have to agree with mike on this one. Just go out and try it, set your hsn the way you like it then set your lsn, you will notice that you are much leaner on top when you do it this way.

 

[CorradoPsi]

guys, there are different correct settings for every engine. what mike is describing is how to "race tune" your engine. which is a different setting than a "bashing tune" a race tune is typically a tad leaner up top, and richer on the bottom than a bash tune. the reason is so that the engine attains top rpms by the end of the longest straight, and the richer lsn helps control temps. a bash tune has a richer hsn so that the engine doesn't over rev when held WOT for long periods of time, and a leaner lsn for out of the hole punch.

engines can also be tuned with either needle first provided you know what your doing, just yesterday i tuned psunitros engine LSN first.

 

[niggle]

guys, there are different correct settings for every engine. what mike is describing is how to "race tune" your engine. which is a different setting than a "bashing tune" a race tune is typically a tad leaner up top, and richer on the bottom than a bash tune. the reason is so that the engine attains top rpms by the end of the longest straight, and the richer lsn helps control temps. a bash tune has a richer hsn so that the engine doesn't over rev when held WOT for long periods of time, and a leaner lsn for out of the hole punch.

engines can also be tuned with either needle first provided you know what your doing, just yesterday i tuned psunitros engine LSN first.

That's it in a nutshell. Depends on your driving style and track conditions.

If you spend most of your time in the LSN to HSN transition and very little time at WOT then Mike's method is the way to go. Where I run we have long straights and tight hairpins; we need crisp acceleration from low down out of the tight turns and safe temps on the long straights, so getting the HSN right at WOT is paramount.

 

[emu67]

When I tune, I make the LSN rich but make sure that it can idle. Gotta make sure that your carb opening is 1mm or less, too, otherwise the engine tends to run away when you do the pinch test no matter what your LSN setting is. I tune the HSN then until is seems ok, just a tad rich. I lean out the LSN until its feels good and is idling steady and low and I always notice the the HSN has become a tad leaner. I prefer to tune a slightly rich LSN and a slightly rich HSN since I race around a track but I keep on going and going and going so I have to make sure the engine isn't overheating.

 

[Lessen]

guys, there are different correct settings for every engine. what mike is describing is how to "race tune" your engine. which is a different setting than a "bashing tune" a race tune is typically a tad leaner up top, and richer on the bottom than a bash tune. the reason is so that the engine attains top rpms by the end of the longest straight, and the richer lsn helps control temps.

good clarification Corrado. That makes sense. :thumbup: