Regular backfire

[winter]

My CPC is running OK for a while, but after some time it is giving backfires.
At first once but after some time every minute. Takes back in rpm. but picks up again and runs until the next backfire.
Where do I have to search for the cause of this problem?

Evert

[1650bullet]

My CPC is running OK for a while, but after some time it is giving backfires.
At first once but after some time every minute. Takes back in rpm. but picks up again and runs until the next backfire.
Where do I have to search for the cause of this problem?

Evert

Thats a tricky one. Im sure someone on here in good time will give you more of an insight to what is wrong, but i have cut and pasted this from the FAQs section on here. These blokes have done there homework on putting this section together so its worth looking into.

My British Seagull runs for 15 minutes then loses power, slowing to a stop. It is not seized.

Sadly this is classic sign of the leaking head gasket, so the head is going to have to come off. I know, it says 'Do not Remove' Yes, it is for a reason, you do not have to de-coke, as we once did with two-strokes, and these screws actually go into the water-jacket, so they can be more than stubborn to remove. A gas blow lamp helps, plus some WD40. They are 5/16 BSF if you need to clean up the threads later..... I now have new S/S head bolts! £2.50 each, same price as rusty ones!

When you have the head off, check there are no faint black marks in the bore... This could be the only sign you might get that the block has cracked, giving the same symptoms. Normally the outside of the block will have gone as well, time to find a new block.... Yes I do have them, for the Forty and the silver Century, but at a price!

Assuming the block is OK, refit with a new head gasket, they are cheap enough, with heat proof silicone sealant, sold in car accessory shops as 'Instant Gasket' or 'No Gasket', orange or black in colour. British Seagull recommended it, as the head and block joints are often not smooth. Tighten bolts finger tight only, leave for 4 hours before finally tightening to 18ft lb, a good hand spanner tightness!



Try this and see how you go. It sounds like a leaking head gasket. Once you clean the gasket up, you can apply some gasket goo on both sides and refit the head.

[Stelios_Rjk]

Something more. Don't cover with RTV the whole copper gasket. That would prevent the thermal exchange. Use RTV only on the cylinder.

What I do, but I don't know if it is good. I place some RTV around the ring of the cylinder head. Always thin smears.

[Rex NZ]

Winter

A possibility is combustion within the crankcase

Basically what happens is the motor fires as usual & the blow-by flash then ignites the crankcase charge, which goes with an almighty bang, the motor faulters then recovers & the cycle repeats. This is a problem with very worn motors suffering low compression. Tends to occur at full throttle when the BMEP (brake mean effective pressure) maximises.

Rex

[winter]

Rex, what you describe looks like my problem.
I will seemif I can measure compression.

[winter]

Also, could it be that the pistonsrings are leaking?

[1650bullet]

Also, could it be that the pistonsrings are leaking?

Do a compression test first and read the cyl pressure. 50psi and up is fine, unless someone else can correct me on this. I wouldnt worry to much about the rings being worn. Next step to take would be removing the cyl head and cleaning everything including the original head gasket. Apply some Gasket goo (Very thin amount) Refit it all back toghther and see how that goes. If its still no good,Then remove the cyl. Its not all that hard to remove and refit the cyl head on those things, So its worth a try.

[Rex NZ]

Winter

Compression test would be good. 50 psi is okay

Perhaps remove the cylinder & measure the end gap in the rings.

0.004" is OEM spec. 0.008" is about your outer limit, Beyond this you'd be looking at honing & re-ringing. I'd suggest a fairly coarse hone to defer the onset of glazing.

Rex

[Collector Inspector]

Winter

A possibility is combustion within the crankcase

Basically what happens is the motor fires as usual & the blow-by flash then ignites the crankcase charge, which goes with an almighty bang, the motor faulters then recovers & the cycle repeats. This is a problem with very worn motors suffering low compression. Tends to occur at full throttle when the BMEP (brake mean effective pressure) maximises.

Rex

Backfire two stroke is above if it is a backfire. A hesitation pop is different. A pop is usually a condenser or ignition related.

You all know my thoughts on RTV by now.

B

[1650bullet]

Sorry. My bad. Thanks for reminding me C-I. I got some of that Yamaha Gasket bond threw work and i am yet to use it. But i did get some

[Vic]

My CPC is running OK for a while, but after some time it is giving backfires.
At first once but after some time every minute. Takes back in rpm. but picks up again and runs until the next backfire.
Where do I have to search for the cause of this problem?

Evert

A back fire or a "lean sneeze" due to a weak fuel/air mixture Perhaps itself due to a dirty carb or fuel flow restriction

[winter]

Thank you all for the advices so far.
Measured and cleaned everything and then, how stupid, saw that the ignition back plate was not fixed.
After tightening the screw I had the engine running half an hour and no backfires any more.
I think I will never know what the cause was but my little beauty (she is one of my favorites) runs like a young lady again.

Evert