For those of you running an EGT--> If I were to compare temperatures at 1/2 throttle and 3/5 throttle...would there be a significant difference? When I was doing my midrange jetting tests I may have been a bit over 1/2...just a bit. I mesured the bottom of the slide this time to be sure and I may have been 10% more open than I thought. Could this have effected my results by 150 degrees?

Last thought: If running at 1250 degrees at WOT is O.K. then why would it hurt the engine to run at 1150 or 1200 at 1/2 throttle? Whats the science behind this? Just curious...

[img]i/expressions/face-icon-small-smile.gif[/img]

 

I am no expert, but here are my findings/opinions. Under load, the EGT should read somewhere in the 1200-1300 degree range. If you are reading 1200 at 1/2 throttle, I think it should be OK as long as there is load. If you are just tooling around, you should be running around 1000-1100 (give or take depending on load). If you read 1200 without load, you are still a little lean in the midrange. How to cure it is best up to trial and error. Can you raise the needle another notch (or shim it up) to see if that does any good? I have even heard of people running the idle a little fat when they couldn't get the needle set just right. It isn't the 'right' way to do things, but it beats running too lean.
At 3/5 throttle opening, the needle jet starts to come into play also, more than it would at 1/2.
If you want the most comprehensive guidebook I have ever seen on dialing in carbs for quads/sleds (or 2 cycle motors in general) get the carb tuning handbook from Olav Aaen. It goes into GREAT detail over every aspect of jetting.

Of course, if you are the maniac type, just keep it wide open and you will be fine LOL [img]i/expressions/face-icon-small-happy.gif[/img]
I don't know if I helped or not, but I at least kept out of trouble for a few minutes.

 

Sounds like a great book, what is Aaens web site?

 

<< ...Last thought: If running at 1250 degrees at WOT is O.K. then why would it hurt the engine to run at 1150 or 1200 at 1/2 throttle? Whats the science behind this? Just curious...[img]i/expressions/face-icon-small-smile.gif[/img] >>



Like farmer said engine load is important. The greater the load, the hotter the engine will run. Have you ever watched your car's temperature gauge at 55 vs 85? Same principle.

Most maximum power specs on dyno runs with lean jetting are only for very short durations of time. If you took that same engine with the same settings and wound it out on a rural asphalt road with an atv it would melt down. If you had a snowmobile and held it wide open on a long lake/river it will melt down if the main jets are lean. The same snowmobile would probably never seize under normal trail conditions if the mid range settings were &quot;fat&quot;

The leaner the jetting on the top end the shorter the power bursts have to be.

In the midrange, exhaust temperatures are even more important because that is where the vast majority of the driving is being done. The normal operating range of the engine. That is why so many people are confused when their engines burn holes in pistons while riding down the trail at reasonable trail speeds. You don't think of it as engine abuse.

In the long run you will be a lot happier if you don't tune the jetting for that last ounce of power. Remember, two strokes put out most of their power and run like they never ran before... right before they melt down.

Are you still trying to tune the &quot;carb from hell?&quot; or is this back on you stock carburetor?

Was this engine built to a particular, proven recipe by a major parts supplier?

 

What exactly does &quot;load&quot; mean? (I am a frenchy now!) I built this 400 to HPD specs, their pipe, porting and carb set-up...except for the HotSeat Billet Head. I removed that this weekend just in case it was defective. The 39mm was just checked by my local dealer also and it looks in perfect working order.

I have tried everything humanly possible to jet the midrange. I richened up the pilot jet, changed main jets, changed needles, shaved needles, tried every possible mix and match between these over the past month. Nothing but lean midrange and 1150 to 1200 degree readings.(instead of 1000) If I could get the midrange to stick around 1100 degrees I would be satisfied but when it hits the 1200 mark I start getting worried. 1250 should be maximum performance and meltdown at 1300. I am testing her out this week with less oil in my premix. I was running about 32:1 and am going to try 50:1. More gas = richer jetting...it may help a bit?!?! [img]i/expressions/face-icon-small-confused.gif[/img] What about a cooler plug?

 

By load we mean how hard the engine is pulling. Try holding the throttle open to the halfway point, drag the brake until the engine starts to pull down, and then see what the temperature says. If it rises to 1300+, you are still too lean.

This may sound stupid, but maybe you have too LITTLE intake restriction. I have seen this when people have opened up the airboxes (or taken the lids off), installed a high-flow carb &amp; put in a K&amp;N filter in a bike. They just couldn't get enough vacuum to draw enough fuel around the needle. If the temp reading is high, try putting your stock air filter back in. If the temp drops down, that may be part of it.

It will also be hard to jet midrange if the carb is too big for the amount of airflow of the engine, especially with low vacuum.
I don't know what carb you have, and I couldn't help you if I did, it has been too long since I have tinkered with this to ever remember carb #s, let alone jet &amp; needle #s, as well. I am just trying to throw you a bone and see if you can run with it. There are too many variables to answer anything specific over the 'net, but I can give you the generalizations that I do know.

 

Farmr123, I have the usual air-box mod and Uni-Filter. It flows a bit more than the suggested amount but from what I have heard it only affects top-end power. I may still try to restrict it again just to be sure. I would rather loose a few on top and get the midrange jetted right. I will put the restrictor plate back on tomorrow...just to put every advantage on my side. [img]i/expressions/face-icon-small-happy.gif[/img]

 

Just thinking out loud, have you checked the ignition timing or bumped it down a few degrees? Timing greatly affects combustion temperatures.

Rainman

 

I removed the flywheel and checked the line-alignment behind it. It was off by about 1/16 inch. I re-adjusted it.

Just got back from testing. I had removed the 12:1 head, added restriction to the airbox, siliconed everything. It is worse than ever. It is going to be ripped apart and sold for parts. Piece of ****!![img]i/expressions/face-icon-small-mad.gif[/img]

 

Brian,

Sorry to hear that your problems continue.

If it would be any help at all, I have a friend down here that might be able to help. He works mainly on sleds, but I guess when it comes to carbs it's basically the same thing.

I'm sure you guys have people who know what they are doing up there, but if you ever want to take a trip down here, we could give it a shot together.

Just thought I'd try to help...wish I knew more.

-Josh