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New four stroke - valves hissing

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GrahamWh05/04/2014 11:34:43
356 forum posts
53 photos

Hello All,

I am used to 2 strokes and have recently bought an ASP FS70AR. I am half way through the run in procedure.

When it arrived (the day after I bought it - excellent service at Just Engines) I noticed when turning the prop over by hand a hissing noise from the exhaust port on the compression stroke - the valve leaking.

Is this normal on a new four stroke engine? The inlet valve does not leak, unless I hold my thumb over the exhaust port tightly to block that leak, then there is a weak hiss at the inlet.

I am half way through the run in procedure given by Just Engines and there does not yet seem to be an improvement in the hissing or the compression. Will the valve eventually bed in or is there a fault?

At least I bought it from Just Engines who give a 2 year warrantee and have an excellent service record judging by other people's threads - provided I don't succumb to the temptation to take the head off and have a look at the valve seating which would invalidate this!

Is any one able to advise?



Codename-John05/04/2014 11:41:47
1087 forum posts
18 photos

have you checked the valve clearances, they are just underneath the small cover at the top of the pushrod tubes towards the front of the engine

cymaz05/04/2014 11:47:58
9273 forum posts
1200 photos

If you adjust the valves and it stops fine. It sounds like the valve is not sitting on its seat too well.

J D 805/04/2014 11:50:54
1522 forum posts
84 photos

Why not call just engines? They will help.

GrahamWh05/04/2014 11:52:20
356 forum posts
53 photos

I should have said - I have checked the valve clearances. The exhaust valve clearance was a little tight when it arrived so I adjusted it to 0.08mm, same as inlet. (acceptable range in instructions was 0.04 to 0.1 mm so 0.08 seemed sensible). Hissing was still there. Thanks.

Martin Harris05/04/2014 12:32:24
9399 forum posts
253 photos

There's a possibility that a tiny piece of swarf or other foreign object could have lodged on the exhaust valve seat which would explain the compression leak and reduced valve clearance. I think the advice to contact Just Engines is very sound.

GrahamWh05/04/2014 13:10:34
356 forum posts
53 photos

Thank you all. In view of your suggestion Martin I suppose it would be best not to continue with the running in procedure until I speak with Just Engines.

Would there normally be no hissing from valves in new four stroke engines then?

The extent of the leak with this engine is that all compression is gone within a second or so if turned by hand.

Simon Chaddock05/04/2014 13:25:57
5728 forum posts
3034 photos


The valves should be pretty gas tight, a ringed piston actually leaks more.

If there is a definite hissing sound from day one it suggests that something is not right.

A leaking exhaust valve will only get worse with running as the hot gas can actually burn away the metal. You would do well to suspend all running until Just Engines have advised a course of action.

i12fly05/04/2014 18:42:14
634 forum posts
22 photos

Don't get caught like me! I had an SC70FS. Ran it in carefully, always seemed to be quite hot but as it was my first 4 stroke didn't think too much about it. Put it aside for 3 years (it took me longer to build the plane than expected.... )

When I came to use it, it overheated even running rich so gave it another hour of running in. Still overheated, cut away lots of cowl -no difference. Examination showed the valve inserts in the head were leaking. As it was out of warranty, the model shop had closed, the importer couldn't be bothered to reply to my letter. The only solution will be to replace the head, but I can't be bothered as I don't know what other damage the really bad overheating did. So it sits on the shelf ready to use as spares -I've got 2 more.

The moral of the story is don't let the warranty run out.

GrahamWh06/04/2014 15:20:54
356 forum posts
53 photos

I have emailed Just Engines (they are not open over the weekend) and I may ring them tomorrow morning. I did find that when I was following the run in procedure on the 1/4 throttle run I needed the glow energised to keep it going, may be that was due to the leak. Thanks for all your advice.

Stevo06/04/2014 16:55:34
2699 forum posts
419 photos

Looks like you have done the right thing. On engine restorations (Ask Fly Navy!) I regrind my valves every time. There's one school of thought that on the combustion cycle the pressure in the cylinder head is very high and seats the valves better than the springs.

Not sure I like that - in my view, if they don't seat, and the engine is run for any length of time, it will burn through either the valve seat or the valve particularly if the mixture is a bit weak - eeek.

Keep us all up-to-date!

GrahamWh08/04/2014 22:52:17
356 forum posts
53 photos

Engine posted back today after speaking with Just Engines.

GrahamWh10/04/2014 14:09:42
356 forum posts
53 photos

Posted it Tuesday, got an email Wednesday from Just Engines to say they had received it and would keep me updated with progress!

GrahamWh14/04/2014 19:11:55
356 forum posts
53 photos

Just Engines have sent the engine back having examined it, run it and lapped the valves. The valve lapping has not fixed the leak, but they say to continue running it in anyway. I finished running it in today, the leak is the same, but it does run fine from around 2,500 rpm to just above 10,000 on the Master Airscrew K series 13x5 prop I am using to run it in. I will see how it goes, and return it if problems develop in the 2 year warrantee period. If the valve seat does burn out at least the problem with the leak has been registered with them.

Jon - Laser Engines28/04/2014 12:02:03
5550 forum posts
270 photos

With all Chinese engines of this type the valves/rings will leak. I have had 2 61's a 52, a 70 and the big 400 radial and all of them leak like sieves. They still run ok but after they all wore out or I sold them I replaced them with other things.

The engines are built to a price and this is one of the consequences.

That said, I recently ran in some ASP's 4 strokes for club mates (I ignored the instructions as they do not run in correctly if followed) and they have come in much better than any I have used before.

GrahamWh28/04/2014 22:17:48
356 forum posts
53 photos

Thanks Jon, good to know.

Engine Doctor29/04/2014 14:50:06
2528 forum posts
39 photos

Hi Graham. When these engines have leaky valves its usually due the head distorting when fitted to the C/case. Just engines may have lapped the valves and got a perfect seal. As soon as the head is re-fitted and tightened it distorts and the valves leak again. Its pot luck with the budget engines . Every now and again a duff one turns up. I have had many SC.and ASP engines over the years and only buy them ( new or second hand) if the comp is good . Poor or low compression equals loss of power and poor or erratic idling .Dont let just engines fob you off , if it still leaks its defective manufacture or materials and should be covered by the warranty.


Jon - Laser Engines30/04/2014 09:33:36
5550 forum posts
270 photos

I agree with E.D to a limited degree as it depends on how much of a leak you are talking about. if its a slight hiss when turned slowly then don't worry, if however its fizzing all its compression away, even when flicked fairly smartly, then clearly its no good and JEn should sort it out. A friend has an SC 70 that was leaking so badly you could not tell the difference between the valve springs being compressed and the compression stroke. needless to say it did not run very well.

if you want a quality motor then Laser are in a class of their own with (in my opinion) Enya next up. OS have been doing some strange things in recent years and I feel quality has suffered a little but the V series seem good value but not as good mechanically as the old surpass series. The Alpha's are very expensive (for the most part). Saito also make great engines but do prefer high nitro fuel which is pretty costly.

GrahamWh30/04/2014 14:19:52
356 forum posts
53 photos

Thank you guys. The hiss is more than slight in my opinion, but the compression when warm and when turned over slowly is still more than the spring resistance. It has now run for a total of at least an hour and a half and there is no more scraping sound when the engine is turned over indicating I think that the ring is properly run into the bore - you certainly can't feel the resistance that was there when new.

I flew the engine today in my puppeteer which I maidened, but it had real problems hold a tick over below 3000 rpm. Fortunately on the 13x5 I am using for running in, at a bit above 3000 rpm the plane was able to stop okay on taxiing, and just about slow down enough to land.

Ironically, I have been tinkering with an old OS fs70 surpass I had bought second hand with a leaky inlet valve, and after getting bits of advice on the various forums had managed to lap that valve and improve the compression. I had bought this ASP FS70 to replace that OS! The OS seems to have better compression, though with the leak being in the inlet side the escaping gases are going the wrong way through the engine and the carb which may be more serious generally than leaking from the exhaust.

I think if I were to buy again, I might go for an ASP from somewhere like HobbyKIng though the range of sizes they do is smaller. I doubt they provide a 2 year warrantee, but the reasonable price of spare parts and the lower initial cost may justify this. You can buy a spare head from China for the fs70 for £15 with free postage, and I have purchased one which I was going to put into the OS if the valve lapping did not work (Toothpaste with car T-cut seems to do well). I think I have got a bit of a duff ASP from JE, but in their opinion it is okay and yes, it does run. I must admit they seem a nice bunch from my conversations on the phone and emails, and indeed it can't be easy for them trading in the budget stuff with these problems. If they sorted everything out perfectly would they be financially able to provide the budget motors? Its a matter of give and take I think, and if this engine is a bit rough, maybe another they are selling to someone is really good. If I really wanted quality I would have to buy OS, and frankly I can't justify the expense with a family to provide for.

If it runs okay for the 2 years then once the warrantee is over I might use the spare head in that rather than the OS. It may be I will get the idle better as I get better at tuning the thing.

Thank you for your experience, it is good to know that a bit of a hiss at the valves is not necessarily the end. I shall plod on with it but if it all goes badly wrong I will contact JE again.

Jon - Laser Engines30/04/2014 16:02:29
5550 forum posts
270 photos

I would not consider it duff, as I said its a cheap engine an they are built to a price. it all comes down to the QC and quality of materials. You don't get what you don't pay for unfortunately.  

Anyway, I learned to fly on a puppeteer! great model, although I bet its well over gunned with a 70. the one I flew (my dads) had an old OS 48 surpass and eventually a 52 surpass. More than enough power. I currently have the legionnaire and its great fun on an ancient saito 45.

Regarding your fast idle, make sure the slow run needle is correctly set (if the idle is fast but smooth, and stops if you throttle back further, its lean, fast but lumpy and spluttering then its rich) and see if that helps any. It should idle at about 2200-2500 tops. I use mostly laser engines and any idle above 2000rpm is not tolerated with 1500-1700 being the norm. if your prop is wooden you could swap it for a nylon one as the weight will help the idle, you could also use a heavy brass prop nut to give a flywheel effect. Also given the excess of power you most likely have take a look on hobby king for the turnigy TYPE A wooden 15x5 and give that a go. My Laser 70 takes it to over 9200rpm and I have loaned it to a friend for a 6th scale 1/5 strutter. With the large diameter it works well even though its a light prop.

It must be the type A series as they run much faster than other props. I am using the 13x5 type A on my saito 45 and still get over 8000. When I put it on my laser 70 it did 13000!!! i was a little shocked but boy did it sound epic!

Edited By Jon Harper on 30/04/2014 16:05:01

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