100LL carb jetted quad question

[CivicHatch-CR125]

So the guy next door is selling a Honda 1986 TRX-R that has a 310CC bore kit but he said its jetted for aviation fuel, I am assuming he means 100LL cause that's what they sell around here. (My GF is thinking of buying it so we both have a bike/quad to ride)

Now that isnt cheap stuff 3.90 a gal.
I am new to all this bike stuff, would it still be safe to run 91 pump on the bike?

Is there any downside or upsides to using 100LL?

Thanks all.

Oh and BTW we are both new riders just cruising around the dunes and dirt no racing.

 

[IndyMX]

http://www.dirtrider.net/forums3/showthread.php?t=115202

Read this, draw your own conclusions..

 

[CivicHatch-CR125]

Hmm good post,

It seems I might have to get this thing re-jetted for pump gas if this quad is in fact jetted and tuned for avgas. (If i end up buying it)

thank you for the reply.

 

[IndyMX]

That's what I'd do.. You're welcome.

 

[hot125mod]

dito

 

[CivicHatch-CR125]

Is there any way to test to see if 91 is ok to use?

i would like to TRY to bypass re-jetting, I have no idea how to do it and i did not really wanna bring it into a shop.

 

[sick 96 250]

your prob going to have to change out the head on that bike to give it less compression. If he has a cool head on there already just swap the dome size.

 

[rmc_olderthandirt]

You don't adjust the jetting for the type of gas you are using, unless you are going to something exotic like alcohol.

If there was ever any reason to run a high octane gas it would be because the compression ratio of the engine dictated it. Since the engine has been modified for the big bore then there is a high probability that the compression ratio was increased at the same time.

The stock engine probably had a 9:1 compression ratio and would run on regular gas. Many "high performance" stock engines have 12:1 compression ratios and require premium pump gas. If you push the compression ratio above that then you need to run even higher octane fuels, such as aviation gas or racing fuel. You can't just "re-jet" and get away with running a lower octane gas.

Does this quad have an electric start, or is it kick start? Your girlfriend may not appreciate kicking a 310 cc high compression engine.

Rod

 

[_JOE_]

Lol, yeah the girlfriend and a 310 with forward throw kick start would most likely be a disaster. I went through junior high school with a buddy who had trx250r, it was a ported stock bore and that thing was gnarly torquey. It was down right dangerous actually. Does your girlfriend have some riding experience?

Like Rod said, you usually don't run avgas unless the compression ratio is very high. Chances are good the head will need to be reworked to run pump fuel.

 

[CivicHatch-CR125]

Ohh thanks that clears up the jetting deal, i will have to do a compression test then to be sure. Is compression tested on the bike the same way as a car?

My gf has exp riding. she can ride the 125 pretty well but she wants a quad.

I think its a kick start and now you bring it up my friend has a 400cc bike and he needs to pull a lever to start it to take away some of the compression or something just to get it started. so maybe the guy just does not know.

Good call ill surely have to look into this thing.
thanks again for the info .

 

[_JOE_]

Uhhh, the PSI isn't the same as the compression ratio. A compression test will tell you how good the ring seal is. The compression ratio is a measure of how much the combustion chamber volume decreases as the piston moves through it's stroke. A high compression engine will generally blow more PSI in a compression test and be harder to kick start. The trx250r did not have a magic button. It is forward throw kick, as in rotates the opposite direction of a bike. There is no compression release like an older 4stroke might have.

 

[Rich Rohrich]

Uhhh, the PSI isn't the same as the compression ratio. A compression test will tell you how good the ring seal is. The compression ratio is a measure of how much the combustion chamber volume decreases as the piston moves through it's stroke.

The pressure recorded when doing a cranking compression pressure test is a function of trapped compression ratio and is far more relevant to octane requirement than the geometric compression ratio you get by calculating the cylinder geometry.

On a two-stroke the exhaust port timing will determine the trapped compression ratio and on a four-stroke the intake valve close timing will determine the trapped compression ratio.

This is significant because a geometric 12:1 compression ratio engine could have a lower cranking pressure (and octane requirement) than a geometric 10:1 engine because of port/valve timing.

Here's an example:

A 10:1 four-stroke engine that closes the intake valve 30 degrees after BDC (or 150 BTDC) which is a fairly mild cam.

 

[Rich Rohrich]

Now here's the same engine with a 12:1 piston in it, but a cam shaft that closes the intake valve much later @ 75 degrees after BDC (or 105 BTDC) , which reduces the trapped compression ratio . Compare the trapped ratios and the pressure numbers between the two and you'll see that this geometric 12:1 engine has lower numbers and will actually need less octane than the 10:1 engine. :cool:

If you look at piston position at inlet close on the charts you can see how that relates in a similar fashion to the exhaust port on a two-stroke and has a similar influence on compression and cranking pressure.

If you think hard about this in relationship to two-strokes and you think about what a powervalve does at different rpm, you'll see how the power valve influences trapped compression ratio and why knock can be such a big problem when the rpm is low (power valve closed) but the load is high like in sand and mud. ;)


There is always more than first meets the eye with this stuff. ;)

One last thing, just to be clear, the math for these pressure numbers is based on absolute not gauge, but the point is still the same.


... and YES, I realize we are talking about a quad and this is likely to be a gigantic waste of words because of it, but I was sitting here waiting for an IOS image to finish loading on a wireless controller and had nothing better to do while I waited. :nener:

 

[CivicHatch-CR125]

Thanks rich that is some good info :cool: sure its a quad but its still a 2 stroke!


thanks for all the info guys.

Its going to have to come down to finding out what kinda top end it has so i know the compression. I did ask the guy and he said its very hard to kick start if your not used to it. So the compression must be high. He said the thing pulls pretty good tho ill find out tomorrow.

 

[_JOE_]

Hmmm, you really do learn something new every day.

 

[helio lucas]

hey rich, how do you know the required octane for a determined engine before any run?

is there any tables based on max or mean pressures and temperatures or it is everything by experience?

i´m asking this because of curiosity... i´m not imagining a big manufacturer built an entire engine and the test it for the required octane level...

 

[rmc_olderthandirt]

Unless your girlfriend is VERY aggressive and a great rider I would not recommend a two stroke, high compression quad. If you get out on a long ride and she starts getting tired, makes mistakes and has to start that quad on a steep hill when there just isn't anything left in her legs and she will not be happy.

I would recommend something like a Honda TRX400ex. It is a sport quad so it has a decent suspension. 400 cc four stroke has all the power you need. The power delivery is smooth and consistent, no surprise hits like the a two stroke. And it has the magic button......

Rod

 

[Rich Rohrich]

hey rich, how do you know the required octane for a determined engine before any run?

You don't. Even two engine built to the same spec could easily vary by 5 points. You build it, you test it, and then you know.

is there any tables based on max or mean pressures and temperatures or it is everything by experience?

Ask 10 people and you'll get 10 different answers. For me, experience has proven to be the most useful. Over time you'll see patterns emerge with various fuels, engine types and pressures which will prove to be a very useful guide. Good fuel blenders can use their experience to help you make these decisions.

 

[helio lucas]

You don't. Even two engine built to the same spec could easily vary by 5 points. You build it, you test it, and then you know.
.

:yikes:

thank you sir, very explicit :cool: