By continuing to use this site, you agree to our use of cookies. Find out more
Forum sponsored by:
Forum sponsored by CML

Pre-ignition ?

Intermittent 'crack' on 4 stroke engine.

All Topics | Latest Posts

Search for:  in Thread Title in  
John Wagg20/09/2020 11:38:56
132 forum posts
22 photos

I have an SC 30 FS engine that this morning threw a prop. Engine running and could hear a slight intermittent "knocking" noise. Heard it before sometimes especially if the model is pointed skyward.

Using standard fuel and O.S 'F' plug. Also heard it when previously used on 5% fuel.

Any thoughts why and how to stop it ?

My initial thought is too high a compression and to fit another head gasket.

According to the diagram there should be two gaskets and I don't know how many is actually in there.

I have another SC30FS, that has lower compression but doesn't knock under the same conditions.

Cheers. John

Edited By John Wagg on 20/09/2020 11:57:45

Denis Watkins20/09/2020 12:32:11
4650 forum posts
131 photos

What size prop John ? And SC even list 10%

Is this motor in a model ?

John Wagg20/09/2020 14:17:08
132 forum posts
22 photos

Hi' Dennis - I have not seen an SC recommendation but have seen the RCM&E review that does say 10%.

http://sceptreflight.com/Model%20Engine%20Tests/SC%2030%20FS.html

I would not have thought a lesser Nitro content would cause pre-ignition ? Running a 10x4 APC prop currently and in my vintage Quaker Flash.

I have run my other SC in the same model etc. What does concern me is, if it is pre-ignition, what damage could be done such as big-end bearing or crankpin snapping off. Spears are getting hard to find now.

Cheers - john.

Keith Berriman20/09/2020 14:30:10
797 forum posts
13 photos

Hi John. How about trying two glow plug washers as a starter

Jon - Laser Engines20/09/2020 14:38:17
5632 forum posts
271 photos

Over the years i owned one ASP, two Magnum, 3 SC and 2 OS 30 4 strokes. All of the chinese ones would 'crackle' with pre ignition to varying degrees and were on often quite tricky to set up. The problem i had was that the knurl on the needle was so coarse that it didnt allow fine adjustment.

In the end i gave up and just tuned them the best i could. Some fuel tube stretched over the needle did help with tuning stability and prevented the needle leaking air when you touched it.

I would recommend 5% nitro fuel, they just dont need any more, the OS F plug is perfect, and i used a 10x5 prop if memory serves. These small engines are also quite sensitive to valve clearance so make sure that is correct as well. They also very temperature sensitive as they have low mass and are not as temperature stable as a larger engine.

John Wagg20/09/2020 15:14:02
132 forum posts
22 photos

Thanks Jon,

It was quite cool this morning and also the other times I have run this one has been in an evening, which may also have been slightly cooler.

Both my SC30s are reasonably straight forward to tune but my SC 2 strokes are 'b' awful in comparison.

Just run the SC again in the warm afternoon air and no sign of knocking. Do not understand that.

Keith - Good idea with the 2 plug washers. Very easy to try. Thanks.

Cheers. John.

 

Edited By John Wagg on 20/09/2020 15:39:36

Martin Harris20/09/2020 16:08:44
avatar
9560 forum posts
258 photos

Cold air is more dense than warm which might have tipped the balance towards knocking. I've found that a lightweight propeller can also contribute due to reduced flywheel effect but you seem to be using the same one on your other engine.

Could the one that knocks be a little carboned up? It wouldn't take much build up to cause hot spots and increase the compression ratio a little.

Engine Doctor20/09/2020 16:12:22
avatar
2629 forum posts
40 photos

The pre ignition is caused by a weakening fuel mixture . Your engine sounds as though it has good compression and if the mix goes a touch weak the temp in the cylinder rises and pre ignition and causes the prop to throw . If a small richening of the mix doesn't stop it then try as suggested a second washer under the glow plug . You could also try using fuel with a slightly lower oil content say 15 % as most fuel contains more oil than is actually needed for safe running. You are using synthetic based fuel aren't you ? If all that fails then check the head gasket / shim . I had to resort to making a shim for an Enya 60 4C while another ran perfectly without any shim ?

Different tolerances have these undesirable effects . Let us know what you have done when you sort it .

John Wagg20/09/2020 17:20:30
132 forum posts
22 photos

Thanks for all the replies.

Fuel = Irvine Standard & Laser 5%. ( Both synthetic ).

Glad I wasn't in front of the engine when the prop flew off. surprise

Will update on any outcome.thumbs up

Cheers,. John.

Jon - Laser Engines20/09/2020 18:32:05
5632 forum posts
271 photos

To confirm ED's post and clarify my comment about temperature sensitivity i dont mean external, i mean internal. If you give it the big one a small engine will quickly heat up, and equally if you slow it down it will cool very rapidly. I also had this trouble with an SC400 radial as it didnt know if it was hot or cold depending on the throttle setting. 

These temperature changes play havoc with the tuning so long full throttle blasts while trying to figure it all out will not impress them one bit. Bigger engines are better able to handle it as they have more metal to heatsink.

Of the two fuels you mention i would use the laser 5. I repaired a pair of ASP 30's for a customer (weekend job during the rona) recently and they ran well on the laser 5 when i tested them.

Edited By Jon - Laser Engines on 20/09/2020 18:33:47

cymaz20/09/2020 18:34:44
avatar
9396 forum posts
1217 photos

I have an SC 30fs. It’s been in a box for about 3 years. When it was put away, it’s run dry and well oiled with after run. Last week I needed it running for a club mate.

Stay with me on this

Put it on the stand, ran it up tuned it and it backfired and got rid of its prop very smartly. There was also pre ignition.

Are you still there....

I managed to acquire new ‘ O ‘ rings for the main needle. Hey presto! It’s a little jewel and behaves itself.

Jim Carss20/09/2020 18:43:43
avatar
2127 forum posts
81 photos

John

Try swapping the plug with the one from the other engine,a hot plug can cause the very problem of throwing the prop.

Worth a try and costs nowt.

Jim

John Wagg20/09/2020 19:45:05
132 forum posts
22 photos
Posted by Jim Carss on 20/09/2020 18:43:43:

John

Try swapping the plug with the one from the other engine,a hot plug can cause the very problem of throwing the prop.

Worth a try and costs nowt.

Jim

Thanks Jim, worth a try and I'm fairly sure they are both O.S. "F" plugs. But you never know.

Jon - noted about temperature. Usually full power for take-off (5 ft.) then trundle around for the next half hour with the odd burst back up to height. LoL.q6`.jpg

[ Yes I know that's not the APC prop'.]

Cymaz - where did you acquire the "O" rings from please. Maybe put them on my spares list.

Cheers. John.

Edited By John Wagg on 20/09/2020 19:45:33

Edited By John Wagg on 20/09/2020 19:51:55

cymaz20/09/2020 19:59:11
avatar
9396 forum posts
1217 photos

I’ve had them ages. I can’t remember, though I will search around for you.

cymaz20/09/2020 20:09:58
avatar
9396 forum posts
1217 photos

 

 

Just Engines part no. 12845. O rings. The needle O rings should fit the fs30ar as well

Edited By cymaz on 20/09/2020 20:10:37

John Wagg20/09/2020 21:28:20
132 forum posts
22 photos
Posted by cymaz on 20/09/2020 19:59:11:

I’ve had them ages. I can’t remember, though I will search around for you.

Thank you very much and Just Engines noted. thumbs up

John.

John Wagg23/09/2020 13:00:09
132 forum posts
22 photos

A bit of an update. Ran the SC30FS a couple of days ago and no "cracking" noise. Ran it on straight fuel but did richen it slightly. Same plug etc Maybe if the main needle "O" ring isn't sealing properly that may be a cause. Just have to wait until it happens again to try the various suggestions.

Cymaz - I have ordered some new "O" rings from JE. Noticed that the same part number (12845) is also the pushrod tube rings which I managed to lose one. These rings appear to be common on various SC / ASP engines so I ordered a few. I have an SC53 2 stroke that is a pig to tune. I messed about with sealing the main needle and that has made some improvement. When the O rings arrive I will fit to the 53 as well as 30FS and fingers crossed.

Cheers. John

All Topics | Latest Posts

Please login to post a reply.

Latest Forum Posts
Support Our Partners
Slec
Sussex Model Centre
electricwingman 2017
CML
Advertise With Us
Sarik
Latest "For Sale" Ads
NEW POLL - has the pandemic altered your event safety perceptions?
Q: Has the covid pandemic deterred you from attending shows and events in 2021?

 No, I'll be attending just as many as I usually do
 No, but I'll choose my event with greater care
 Yes, I'll attend fewer events going forward
 Yes, I wont attend any where previously I have

Latest Reviews
Digital Back Issues

RCM&E Digital Back Issues

Contact us

Contact us

Magazine Locator

Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!

Find RCM&E!