Jetting problem - JD / spooge / spluttering

[Jakobi]

Don't just whack another insert in and guess. At minimum take note of where the piston edge sits in relation to the bottom of the exhaust port. That will give you some information about the base gasket stack. Then take a squish sample.

The way the S3 head inserts are made, they are all the same volume. It's just the deck height that changes which reduces squish and increases compression. This creates a situation where the ideal squish clearance very well could have too much compression or vice versa.

Also, don't get hung up on the plug unless if it was an old crusty one that has been burnt clean white. Throttle response will generally tell more of the story than the colour of the plug.

[jocser]

Originally Posted by Jakobi

Don't just whack another insert in and guess. At minimum take note of where the piston edge sits in relation to the bottom of the exhaust port. That will give you some information about the base gasket stack. Then take a squish sample.

The way the S3 head inserts are made, they are all the same volume. It's just the deck height that changes which reduces squish and increases compression. This creates a situation where the ideal squish clearance very well could have too much compression or vice versa.

Also, don't get hung up on the plug unless if it was an old crusty one that has been burnt clean white. Throttle response will generally tell more of the story than the colour of the plug.

Hello Jakobi,

Thanks for this reply. I d really like to follow your advice, but really is where my technical knowledge stops.. You confirm what I was thinking but I really dont know how to see abd measure this.

You say only the height varies, so wouldnt the Red be already better compared to the stock Grey one thats in now?

Verstuurd vanaf mijn XT1572 met Tapatalk

[barossi73]

If you dont measure squish you have 2 options-
1.assume it was set up ok with black insert and revert,then try diff needles etc to tame the storming beast.run at least 95ron(if poss,check with previous owner about setup/fuel)
2.compression test with the diff inserts,and avoid anything higher than 185-190(guages vary so allow a margin,maybe target 180) unless you go higher octane on fuel.you could finetune the cr with base gaskets if necessary,but be aware you slightly alter port timing also
If it was me id start with the previous owner and find out everything you can,what changes had been made,fuel requirements etc,before you decide which direction to head
Also if you are comfortable changing inserts youll be fine measuring squish.search and read up.its pretty simple,just be sure to avoid using kicker/starter when bringing piston to tdc

[Jakobi]

In regards to comp testing you need to know the actual gauge that you are using to see what number it blows. I have one with an adapter, and my engine that is high comp blows low psi as it's accounting for the volume of the adapter as well as the volume of the combustion chamber. Unless if you have a benchmark to work from a test alone doesn't give much insight.

With the squish there is a write-up here: http://www.gasgasrider.org/html/measuring_squish.html


The gas gas 300, by design, is lower comp than the 250. You should be able to run the black insert with squish around 1.6-1.9mm and be OK on the compression front. If you try to tighten in up to the suggested 1.3 or less you'll definitely run into issues. The point is, that regardless of which insert you use, if you bring the squish in too tight, the compression ratio will be too high as the volume of the head inserts are still the same from one to another.

[jocser]

Hey Guys,

big big big thanks to you for your extremely good help. I've been reading the articles and now I get the whole squish thing!
So my bike likely was set up for the black insert (maybe with other base gaskets), I was thinking, power is too much so I'll swap the heads, but that wasn't a good decision because I was making the squish bigger and the compression lower. So that might be the reason my bike doesn't respond well in the lower rev's, because compression is important there.
The previous owner and inter-enduro rider is an ex-MX rider who probably likes the explosive power and top power, so the jetting was set up that way.

With the JD needle the power delivery is more in the low rev's I could already feel that, so maybe I'll better switch back to the black insert, cause the bike ran fine, bet rejet with the JD red #4, fuel will burn as it's supposed to be and maybe it will be perfection for me.

Maybe to be sure I better insert the "medium" red insert first, measure squish and when thats around 1.6-1.9, leave it that way? But I am short in time for a long and heavy ride on sunday, so maybe for now just re-insert the black one and JD jet and see what happens. But with the head off, measuring squish with the different inserts isnt a big big big job so I'll see how far I get.

thanks and I'll keep you updated!

[Jakobi]

That's the approach I would take.

Drop the coolant, pull the head. Compare the inserts. You could measure the volume of each to confirm if you were that way inclined as well. Then pick the one which works best.

The base gasket stack (cylinder and port height) will also effect the power curve. A thinner gasket that drops the ports will move the power curve down to favour bottom end, where raising the ports will give a stronger top end. It's about balancing the port timing, compression ratio, and squish clearance to achieve the best outcome.

When we speak about having heads modified, it's the process of setting the port timing for purpose and preference, machining the head to tight tolerances, and then correcting the volume to achieve a particular compression ratio. It takes the balancing factor out and gives specific results. In my case I used an S3 black insert with .5mm base gasket to give 1.25mm squish, however had quite a few cc's machined out to still end up with a relatively high compression engine.

A strong engine is good. You can always gear it taller and make the power work over a larger range. It'll allow you to be a bit lazier on the shifter while also taking some of the snap out of the response.

[jocser]

Originally Posted by Jakobi

That's the approach I would take.

Drop the coolant, pull the head. Compare the inserts. You could measure the volume of each to confirm if you were that way inclined as well. Then pick the one which works best.

The base gasket stack (cylinder and port height) will also effect the power curve. A thinner gasket that drops the ports will move the power curve down to favour bottom end, where raising the ports will give a stronger top end. It's about balancing the port timing, compression ratio, and squish clearance to achieve the best outcome.

When we speak about having heads modified, it's the process of setting the port timing for purpose and preference, machining the head to tight tolerances, and then correcting the volume to achieve a particular compression ratio. It takes the balancing factor out and gives specific results. In my case I used an S3 black insert with .5mm base gasket to give 1.25mm squish, however had quite a few cc's machined out to still end up with a relatively high compression engine.

A strong engine is good. You can always gear it taller and make the power work over a larger range. It'll allow you to be a bit lazier on the shifter while also taking some of the snap out of the response.

Hi, I started and measured squish with the grey insert which was in it but the squish was bigger than the 2,7mm solder I bought. Tried the Black one, which was in when I bought the bike and that gave me a result of 1.6 to 1.85. So that should Be quite good?

I'll try it that way and try to find the time to rebuild the bike this evening, with the jd Red. Lets hope he 'll have torque as hell.

Yesterday I screwed up the m6 Bolt on top of the Head... (Dumbass too tighten...) so I went to a friend to make it An m7... So last Chance!

Another big thanks for All your help Here!

Grts

Verstuurd vanaf mijn XT1572 met Tapatalk

[Jakobi]

The M6 bolt for the coolant inlet? Those little fellas would be lucky to need 8Nm on a good day.

Based on what you found I'd say the ports have probably been lifted for top end performance. Did you note where the piston was in relation to the exhaust port at bottom dead centre?

1.6mm squish clearance is still on the large side if you were specifically aiming to set up tight tolerances and improve efficiency, however given you aren't doing that I would be quite content with 1.6mm clearance, high comp, and ports for top end. Should give a nice spread. It's actually very similar to how I had my engine setup when I was running a 300. Been on 250's for some time now though.

[jocser]

Originally Posted by Jakobi

The M6 bolt for the coolant inlet? Those little fellas would be lucky to need 8Nm on a good day.

Based on what you found I'd say the ports have probably been lifted for top end performance. Did you note where the piston was in relation to the exhaust port at bottom dead centre?

1.6mm squish clearance is still on the large side if you were specifically aiming to set up tight tolerances and improve efficiency, however given you aren't doing that I would be quite content with 1.6mm clearance, high comp, and ports for top end. Should give a nice spread. It's actually very similar to how I had my engine setup when I was running a 300. Been on 250's for some time now though.

Hi Jakobi, I ll take notice of the piston on dead End, I think i was about 1 cm under the exhaust Port. I'll take a pic!

Grts

Verstuurd vanaf mijn XT1572 met Tapatalk

[Jakobi]

The points we want to compare (simply for reference points), are the edge of the piston dome, and the bottom edge of the exhaust port, when the piston is rotated through to it's lowest point. It'll either sit flush with the bottom of the port (which is what I expect), or up to 1mm above (roughly).