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Francis barnett light cruiser 79 1958 (compression)

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Francis barnett light cruiser 79 1958 (compression)

Post  Martinrhyl on Mon Jul 16, 2018 8:48 am

Hi i have just done a compression test on my bike its saying 105psi after 5 kicks and stops there
I then put some oil into head and try again then im getting 115psi do you think this is to low and need new rings??its a
105 = 7bar
115=just under 8bar
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Martinrhyl

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Compression

Post  kerabo on Mon Jul 16, 2018 4:53 pm

From a 2 stroke that seems very high. I have never checked one but as the piston is part way up before it coveres the ports up I wouldn't expect much at all. I have seen cars with only 90psi and they run fine. I would be more concerned how well it holds the compression and not with oil on the piston top.

Don't expect too much.

Regards Ken
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kerabo

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Re: Francis barnett light cruiser 79 1958 (compression)

Post  Cruiser79 on Mon Jul 16, 2018 11:50 pm

If the engine runs fine with good compression working against the kick starter then I'd say just enjoy the bike. Rings and pistons are pretty much impossible to source so rebuilding the top end to factory fresh specs like on other bikes isn't normally possible on the 17T or 20T unless you're lucky enough to find the right parts. I've spent more years than I care to count trying to find +040 rings and its pretty much a case of making the best of whats there already.
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Cruiser79

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Leak down

Post  FBKev on Tue Jul 17, 2018 2:09 pm

Martinrhyl wrote:Hi i have just done a compression test on my bike its saying 105psi after 5 kicks and stops there
I then put some oil into head and try again then im getting 115psi do you think this is to low and need new rings??its a
105 = 7bar
115=just under 8bar
From my Husqvarna days IRC that a leak down test was the only true way to see if all OK (measure bore for tolerance) and remove carb/exhaust & blank off then pressurise via a modified spark plug (schraeder valve araldite'd into a plug body) piston @bdc and pump to about 5psi (gauge fitted on the exhaust bung). As long as it does not drop straight away and holds for a while all is well with the seals. As the other post says with the transfer ports there is very little area to get a reading from.
Kev
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FBKev

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Re: Francis barnett light cruiser 79 1958 (compression)

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