Does compression affect jetting?

[edz250r]

I bumped up my compression two weeks ago from 165 to 185 by putting a thinner cylinder head gasket in. Will this affect the jetting at all? I thought I remembered reading a while ago that it makes the engine run a bit richer, but I'm not sure if thats right or not. Can someone confirm?

Thanks!!

[Iliketogofast]

I think you would want to go a little bit bigger on the jet sizes, with more compression you need more fuel and air to cram in or else it doesen't work.

[BushManRebel]

REally??

Compression has nothing to do with air/fuel mixture... your simply compressing the mixture more thereby creating more HP.

[Iliketogofast]

I see... <-----Idiot. I had the understanding that you needed more, because you cram it up more, thereby using more fuel. Oh well...

[wilkin250r]

Good question.

It's entirely possible that it will make a difference, especially on a 2-stroke because it will change the intensity of the exhaust pulse going through the pipe, therefore changing the characteristics of the reflected pressure waves.

However, I can't tell you if it will get richer or leaner, and what areas (low rpm, mid rpm, or high rpm) it will effect. It may have no effect at all.

However, you should assume as a general rule that ANY time you add an engine modification that affects the combustion process, whether a filter, pipe, porting, carb, or whatever, you should always check your jetting.

Yes naturally there will be some tuning involved as the exhaust temperature will go up which affects the returning pressure wave, but i doubt very highly you have to change out the jet itself.

I have been doing 2 stroke now for many many years from R/C to Bikes and in R/C nitro or regular gas, increasing the compression ratio by removing a gasket affected no tuning other than the low end mixture but greatly improved HP.

[edz250r]

Can Wilkens or Dier tell me their exact process when checking plugs during jetting? I'd like to know if you warm up the engine, cut the engine, put a brand new plug in and do a wide open run immediately from the spot you shut down the quad in the first place or do you put the new plug in and then do some normal riding first to get some color on the plug. I'm curious because I put a new plug in after warming up my motor (this was right after the gasket replacement) and did a WOT run without any riding on the new plug at all and the plug looked very white and dry. I could see a little brown deep in the plug, but I didnt do a chop. I dont think the gasket made the engine that lean because it was very rich before the gasket replacement, but do you think that a quick 7-10 second WOT without ANY riding before the new plug would give an accurate reading or wont the plug change color that fast?

Believe i answered this already.... a quick WOT will not change the new plug color right away, the color change starts at the very base of the ceramic and moves up as you ride more and more.

So only way you can tell right now is to hack off the threads covering the ceramic and then putting a new one...

I suggested you just go riding for a while, dont overdue it going crazy with WOT for long periods just in case it is lean, which i doubt it is.. Gasket should not effect anything but the compression ratio.

if you see a little coffee color at the base already , it should be fine.

[machwon]

A change in compression will have no effect on your jetting, if it was done properly to begin with.

[wilkin250r]

Originally posted by edz250r
Can Wilkens or Dier tell me their exact process when checking plugs during jetting?

Plugs are more complex than you think. To be honest, I haven't gotten it all figured out, but here is my take on things so far.

The whole "light brown, coffee colored" technique is bogus, in my opinion. It only applies to an old plug. If your plug has been in your engine for two months, and you pull it out and it's coffee colored, you're probably good on your jetting, but that certainly doesn't apply to a brand-new plug, plug chop, and rejetting procedures. Nobody takes two months to rejet.

It is IMPERITIVE to have the correct heat range plug. If your plug doesn't get hot enough to burn off deposits correctly, you can't get an accurate reading. If you're lean, but your plug is cold so it doesn't burn off deposts, it will look rich. Likewise, even if you're rich, but your plug is so hot it burns off all deposits, it will look lean.

The tip of the sparkplug is meaningless, except possibly at idle (but there are easier ways to check idle jetting). The tip of the porcelain should be clean and fairly white. The mixture is read down inside the spark plug, almost where the porcelain meets the metal. If it's white all the way down, you either have too hot a plug, or too lean a mixture. If its dark all the way up to the tip of the porcelain, you are either too rich, or have too cold a plug. Some people will cut the threads off a spark plug to read it properly, some people will get a lighted magnifier (almost identical to the thing a doctor uses to look inside your ear).