What's on your bench today?

[David_chainsaw]

Hello can anyone help im looking for a carb rebuild kit for honda HEB423 lawn mower for a friend of mine can anyone help

 

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[RobRainford]

Time for some really anal chain sharpening

 

Can use my new vice too

 

[wisecobandit]

Its a general interest really and i have a lovely shindaiwa strimmer with a broken carb that id like to sort sometime. hutzl seems to have biggest selection on ebay.who else do you use

 

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wangluo18 user name.

Comes via royal mail but needs a signature/recorded delivery.

He does shindaiwa stuff sometimes but a bit hit and miss.

I think something shaindaiwa will be whoever you can get it fromt the time tho.

[slim reaper]

Hello can anyone help im looking for a carb rebuild kit for honda HEB423 lawn mower for a friend of mine can anyone help

 

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You may have more luck searching for HRB423 instead of HEB423

[David_chainsaw]

Thats what i ment lol

 

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[slim reaper]

Anything you fancy here ?

 

They also have parts and workshop manuals at 6 quid a pop

Edited April 5, 2014 by slim reaper

[spudulike]

Thanks, I'm still learning obviously. I'm still not too sure what this is though, if I'm understanding correctly, are you saying it it a wider diameter section at the top of the cylinder then? I always thought that when someone said they were cutting the squish band, they just meant that they were reducing the clearance gap.

 

 

 

I get what you both mean on the contribution that worn bearings or pin might make to compression. It makes sense now that you've said it. Just how bad would they need to be to make a significant difference; just worn or really bad?

 

Thanks for the info both.

 

The bearings or pin seat would need to have terminal play in them to drop the compression by 20psi, I have seen it a few times.

 

The squish band is the actual band around the outer part of the combustion chamber, the nearest point to the piston and controls how the combustion impacts the piston crown, it has changed over the years with different combustion chamber shapes and has effect on heat control, combustion and detonation.

[spudulike]

I have been in Gods Garden today, doing battle with the satanic nettles growing in it, the weapon of choice wasn't a wooden crucifix but a ported Komatsu Zenoah strimmer that gave a good show for two hours and drove back the weeds of darkness!!

 

OK - it was the help the Church, clean up the Churchyard day and judging by the save the planet bags, kneeler pads and B&Q secateurs, it was a good thing I was on hand to help:thumbup:

 

I hope that my effort will give me a ticket to access to the pearly gates but probably not:001_rolleyes:

[windy81]

What about the need for a cylinder head with a squish band?

In "Advances in the Design of Two-Stroke, High Speed, Compression Ignition Engines" it is written that with a squish band there is more heat loss due to swirl. [which brings more burning fuel in contact with metal.] Heat is what causes expansion of gases to push against the piston so it equates to say that heat loss, without regard to other factors, reduces power. The advantage of swirl is better mixing and flame propagation. But in a small engine there is usually already enough mixing due to its high intake/transfer turbulence. When I tested a head on my 55cc (w/o expansion chamber) that had a squish band the power was the same but top speed increased 1%. This most probably is only typical for small low power engines since larger engines need help with fuel/air mixing, and flame speed increase is helpful because of larger distances from spark plug to cylinder wall. Here are the two heads tested, both giving 170psi:

 

 

 

 

With small bore engines the problem with using a cylinder head with a squish band is that the squish band leaves too little room for the dome which means they usually cause high compression because of low head volume. Gordon Jennings wrote: "no [mass produced] 2 stroke engine in my experience had ever tolerated cranking pressures above 175psi". I don't worry about running 170psi on mine because I've replaced the crank and conrod bearings. But even so, that high a pressure will wear out the conrod bearings prematurely so I will try to dremel out even more metal from the dome area to reduce the compression.

 

I think my tests have proved that they don't offer enough advantage and Gordon Jennings in his Two Stroke Tuners Handbook basically backs me up here:

"But if you want to use a true (measured from top of exhaust port) compression ratio

much over 6.5:1, on a high-output engine, combustion control beyond that afforded by a

non-squish cylinder head will be necessary." (he is referring to a head w/squish band being necessary for high rpm engines with more than 150psi) The only advantage they can offer is with an expansion chamber they extend the rpm range for about 8% more top speed. That is because the exhaust temperature rises due to a longer burn which happens both from high compression and from a squish band (because 10% of the fuel mixture that was hidden from the flame under the squish band burns later).

 

What I recommend and use is a regular head with compression of around 135psi. If you go over that or have ported the engine to rev up to at least 9,000 rpm then you should use a cylinder head with a squish band. The engine I used to test had a top rpm of around 8,000. Since very few people with these engines run higher than 130psi and/or rev higher than 9000 I think my advice concerning squish bands is accurate for the majority.

 

When I presented this info on the motoredbikes forum the arrogant asshole Fred at CRMachine got really insulting, basically because he had no good argument to retaliate with. JNMotors has a good selection of cylinder heads for these engines and the owner is a nice guy. (click here) If you ignore my advice and want to get a high compression head then first you have to be willing to do what is necessary to make them work right. First you have to mill down your cylinder top till the piston at the top of its stroke is between .6mm and .8mm from the squish band. If you don't do that then too much air/fuel mixture will be in the squish area and not burn till too late. That causes a power loss. If you are like me though you don't want over 140psi because you don't want to have to change out the big connecting rod bearing once a year. So you will have to use your rotary tool to eat away at the aluminum dome to allow more head volume to reduce overall compression.

 

Cylinder compression

[Goaty]

Nah! you are a man of dark arts that are frowned upon by some and viewed suspiously by others.

Having used my dark arts 346xp that you spuderized for the last three months. Today I used my bigger untouched 357xp. I could of run on the chain like a treadmill. (hyperbole Jesus used them) Porting really does work. Im a convert.

 

You werent in Gods garden. We got kicked out of Eden.

Edited April 5, 2014 by Goaty