Wide open throttle: how long can you hold

[Dennish87]

This is a newb question but how long can you hold the bike at wide open throttle? I ride on the stone roads around my house alot and wide open like a 4 stroke would be ideal but I never run my 2 stroke wide open for extended periods of time. I usually give it 3-4 seconds of wide open then go back to about half throttle and repeat. I've never really heard this discussed before and would like to know your opinion.

[s14 2510s]

If you have the bike jetted correctly you should be able to go wide open untill you run out of gas.

[SEOINAGE]

well you obviously don't want to sit there wide open over-revving the bike past what it is capable, you obviously back off the throttle when you get to that point.

[sykal]

+1 on the jetting.

if it is jetted correctly, and your cooling system is functioning, it can rev until you fall asleep.

[Dennish87]

If you have the bike jetted correctly you should be able to go wide open untill you run out of gas

.

if it is jetted correctly, and your cooling system is functioning, it can rev until you fall asleep.

Really?!? I've allways been told (by people that don't own 2T's mind you) that you can't hold it wide open like while riding on the road.

Now about jetting, The plug was a very light chocolate color and more toward metalic/tan chocolate and dry. Would you consider that correct jetting? I know there's threads all over about the correct jetting but a simple yes or no with a short explanation would work here.

[arnego2]

+1 on the jetting. If it is jetted correctly, and your cooling system is functioning, it can rev until you fall asleep.

I wonder if you have tried that yourself?

regards

Arnego2

[sykal]

I wonder if you have tried that yourself?

regards

Arnego2

you would actually be surprised.

what you would be more surprised at, is how many people have incorrectly jetted bikes.

[sykal]

Really?!? I've allways been told (by people that don't own 2T's mind you) that you can't hold it wide open like while riding on the road.

Now about jetting, The plug was a very light chocolate color and more toward metalic/tan chocolate and dry. Would you consider that correct jetting? I know there's threads all over about the correct jetting but a simple yes or no with a short explanation would work here.

dark brown/tan and dry is ideal for a plug.

did you do a plug chop test for this though? or did you simply pull the plug?

plug chop is the only way to tell how your bike is running when WOT.

[mliebal]

Actually the end of the wide open run can be more critical. The engine is really hot, then you cut almost all air flow and fuel/lube. If you are running lean at idle, you are at risk of seizure at the end of wide open run.

I always blip the throttle a bit after a long fast run to keep the piston/cylinder lubed.

Overkill?

[Dennish87]

No I didn't do the chop test yet. I just road it then pulled the plug. How do you do a chop test? Don't you run it full throttle for like 1/4 mile then while at full throttle shut the bike off while keeping the throttle open?

I ussually do blip it after a long full throttle run like you said.

[adam728]

There are a ton of factors that come into play here. It's all about controlling temperatures. 2 strokes have I worse than 4 strokes since they fire every revolution and give the piston crown much less time to cool. Of course, they are some what aided by the fact that there is a cool intake charge right under the piston.

I got to talk to an engine builder one time that said his biggest challenge with 2 stroke desert bikes was keeping piston crown temps in check. Says by the time you get enough fuel in the engine to keep it cool it runs like crap. This guy is a mechanical engineer, ex-factory wrench, and build some killer engines. Been in the game since the early 80's.

On my old kdx I'd go rediculasly rich in the dunes to keep detonation and overheating at bay. One up on the pilot, and from a 152 to a 175 main. It was pig rich, would be unrideable on trails, but once "cleaned out" I could hold it WOT indefinitely in the sands (and rising with two cr500's I needed to be wot 100% of the time).

So the answer isn't just jetting, but also porting, exhaust design, etc. Engines like a string trimmer can run wide open indefinitely. I wouldn't expect my yz to do that unless I went overboard on jetting. Cylinder temps ridin trails just don't compare to what is seen under sustained load.

[The Spanky]

There are a ton of factors that come into play here. It's all about controlling temperatures. 2 strokes have I worse than 4 strokes since they fire every revolution and give the piston crown much less time to cool. Of course, they are some what aided by the fact that there is a cool intake charge right under the piston.

I got to talk to an engine builder one time that said his biggest challenge with 2 stroke desert bikes was keeping piston crown temps in check. Says by the time you get enough fuel in the engine to keep it cool it runs like crap. This guy is a mechanical engineer, ex-factory wrench, and build some killer engines. Been in the game since the early 80's.

On my old kdx I'd go rediculasly rich in the dunes to keep detonation and overheating at bay. One up on the pilot, and from a 152 to a 175 main. It was pig rich, would be unrideable on trails, but once "cleaned out" I could hold it WOT indefinitely in the sands (and rising with two cr500's I needed to be wot 100% of the time).

So the answer isn't just jetting, but also porting, exhaust design, etc. Engines like a string trimmer can run wide open indefinitely. I wouldn't expect my yz to do that unless I went overboard on jetting. Cylinder temps ridin trails just don't compare to what is seen under sustained load.

For riding conditions like you describe, choosing a fuel with a high 90% and end point evaporation points can provide enough additional cooling to the piston to negate the need for excessively rich jetting.

[RCannon]

For riding conditions like you describe, choosing a fuel with a high 90% and end point evaporation points can provide enough additional cooling to the piston to negate the need for excessively rich jetting.

Chokey, is there a gas, thats easy to find, that meets the requirement? The idea sounds reasonable, for sure...

[The Spanky]

Chokey, is there a gas, thats easy to find, that meets the requirement? The idea sounds reasonable, for sure...

Actually, pump gas already has a very high %90 (@ 305F) and end point (@ 360F) compared to the most common race fuels that we use such as VP C12 (228F and 233F respectively) or U4.4 (225F and 225F). So running pump gas will provide more piston cooling in our engines than these fuels. But of course if detonation is an issue then that isn't an option. VP's Motorsport line of fuels (103 and 109) both have high 90% and end points similar to pump fuel, but with higher octane and much more consistent formulation, but the 103 has a very low RVP, so it probably wouldn't be very suitable for hot weather use in our engines.

Sunoco Standard has an end point of 380F, so it would offer pretty good cooling properties, and it also has a 90% point of 253F so it would still offer good throttle response. But it also has an RVP that is a bit low for really hot weather riding.

I have no experience with desert conditions, so I have no first-hand knowledge on this subject, only what I have read and been told. But surely there are some knowledgeable desert racers on TT that could shed some light on this subject?

For what it's worth, here in the Florida heat I really like C-12, but the riding here is very different than dune riding.

[jsantapau]

chokey, are you around Clay County or Jacksonville? If so where is c-12 around here

[Dennish87]

ummm so after all that, can someone tell me how to do a chop test lol. I think you guys got carried away on the fuel types. :/

[The Spanky]

chokey, are you around Clay County or Jacksonville? If so where is c-12 around here

I ordered mine online. You can go talk to your local dealer and see if they carry it.

[The Spanky]

ummm so after all that, can someone tell me how to do a chop test lol. I think you guys got carried away on the fuel types. :/

Make sure the bike is at full operating temperature. Install a fresh plug. Start the bike, and do several passes through the gears at wide-open throttle. Kill the bike, don't idle it or putt-putt it back to the truck, as this will affect the reading. Remove the plug. Then you either have to remove the threads, or use a bright light and magnifying glass to look all the way down at the bottom of the insulator nose, where the threads meet the porcelain.

This plug is too lean, but you can clearly see the mixture ring at the base of the insulator:

[ben williams]

I asked that question to an old desert racer and he told me that when he used to run it wide open say across a lake bed, he would hold it wide open and with the throttle still wide open he would hit the kill button for a second or two, then let off the button. Don't know if it is right or wrong, but he was fast.

[Dennish87]

This is what the plug looked like after the main bearings went out. Before they went out it looked more of a chocolate color without the metallic tint that it has to it now. It didn't change significantly in tone or color though. Just a little. Note I wasn't running it at full throttle nor is this a result of a chop test. I just saw the pic of a spark plug and it reminded me that I need to ask about this.

What do you think about this plug? Too lean or rich?