Two stroke question: Backfires?

[MarJay]

Hello!

I suspect I might be being overly cautious because of the recent luck I've had with my bikes (none of them are roadworthy at the moment - the Triumph has a wiring fault, the Powervalve needs a rebore, the MZ is leaking oil over the back tyre and the LC needs an MOT). The problem in this case is with the LC. I'd like to get it MOT'd but I have the following issue:

It starts and runs fine, it feels very quick to ride (in the very short distances that I've ridden it) except it seems to like to backfire out of the right hand exhaust. I think it is only the right exhaust that is affected, but I might be wrong. I recently removed the exhausts to sand and paint them, I replaced the exhaust gaskets (which did not help - it backfired before and it still backfires now) and when I did so I discovered that the baffles in the back of the exhaust had been cut down. I also discovered that the air box lid had been drilled.

Anyway, I warmed it up at the weekend and went for a quick ride up and down my street. It still backfired a bit. When I got back to my garage I switched the ignition off and examined the plugs. They were wet with petrol and oil, and completely black. I lowered the needles one notch and did the same. The plugs seemed a bit better after I had done this, but were still wet with petrol.

Now, what concerns me is that someone on the RD forum told me that backfires are an indication of running lean - which makes sense considering the baffles have been cut and the airbox drilled. However the plugs seem to indicate it is running rich.

I originally thought that backfires were down to rich running? Does anyone know why it might only backfire out of one exhaust?

I recently replaced the float needles and float seats in the carbs, but I have not checked the float height since I've had the bike.

Any comments appreciated!
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British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Ichy]

Does it backfire on the overrun or at a particular throttle opening?

It may be that the idle circuit is lean, causing the misfire, but the main circuit is running rich, black plugs.
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[MarJay]

[EnergyInducti...]

My CZ was backfiring before, quite badly some days. I found the problem eventually, The ignition timing was WAY off. I let the garage sort that, But after, All the backfiring was gone, And what WAS a black plug, Eventually turned light tan =P
Not sure if this helps, but I hope you sort it in the end =)
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Riding: Yamaha RXS100
Aiming for : RD350 + Test
Dream bike: Suzi RG500 Gamma

[Robby]

Take it for a proper run and see. You know full well that trying to diagnose intermittent rough running by doing 2 minute runs on a bike that has been standing for ages is a poor idea.

You may find that if it's a bit of muck in a carb, getting some fuel through will clear it. If it's something electrical playing up, getting it good and hot and working will either fix it or show the fault up properly.

However...

It isn't my fault if you break it.

My valve was running like a pig on one of the cylinders when I rebuilt it - turned out I hadn't connected up the vacuum connection for the fuel tap, so it was sucking in loads of air. Do you get similar amounts of smoke from each exhaust when you rev it at a standstill?

[MarJay]

[flat spot]

Backfires are generally caused by incorrect fueling or timing issues. Easiest to do is check the timing is spot on. Also check you have a healthy blue spark.

Get a new airbox lid and put the carbs back to the standard settings. Drilling the airbox without doing anything else is LC bollocks bodgery at its best.

Check the fueling once it's all standard. If it's running lean it's sucking air in. Check the inlet rubbers on the cylinders for cracks.

[MarJay]

[Blau Zedong]

As the LC uses a wasted spark system and the problem is confined to one cylinder, I wouldn't worry about timing at the moment. You need a dial gauge and plug adaptor to check it properly anyway.
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18:54:48 Rob Fzs: jews don't give away stuff for free

18:54:59 Rob Fzs: unless its their clothes/ hair/ golden teeth

[MarJay]

I've been told on the RD forum that it could be a twisted crank causing the timing to be out. I'll give the right hand carb another clean and see what happens.
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[flat spot]

Yes it's difficult to check for cracks with them on. You need to take them off and squidge them around.

Next step, buy this.....

https://cgi.ebay.co.uk/YAMAHA-RD250-RD350-LC-HAYNES-Manual-1980-82-NEW_W0QQitemZ200337650301QQcmdZViewItemQQptZUK_Motorcycle_Parts?hash=item2ea50df27d&_trksid=p3286.c0.m14&_trkparms=72%3A1683%7C66%3A2%7C65%3A12%7C39%3A1%7C240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50

[MarJay]

[flat spot]

[MarJay]

[Blau Zedong]

[welshlad25]

im no expert on bikes but no a lil bit like others have said it cud b timing m8. my honda mtx backfired on kick over stripped it down and found the piston was knackerd

[MarJay]

[MarJay]

The plugs are fine, they are brand new. The bike backfired before it had new plugs.

I don't have a spare carb, but I reckon if I clean the carb that should prove that it is OK.

Although, one of the carbs (I think its the left one) Has a small brass tube missing on the edge of the float chamber that appears to connect the two halves of the carb together. I don't think its a major problem as it looks like it is from an overflow or something.

Anybody got any ideas what it is?
____________________
British beauty: Triumph Street Triple R; Loony stroker: KR1S; Track fun: GSXR750 L1; Commuter Missile: GSX-S1000F; Cheap project: CBR900RR FireBlade
Remember kids, bikes aren't like lego. You can't easily take a part from one bike and then fit it to another.

[Blau Zedong]

[Blau Zedong]

[MarJay]

[ms51ves3]

Sounds like the power jet fitting to me.

Does it look like this?

https://i15.photobucket.com/albums/a376/ms51ves3/DSC01039.jpg

If so, it's important. Your bike will run lean at the top end of the rev range without a tube between the two fittings pictured.

[Blau Zedong]

The only RD's that have power jet carbs are the F2 / N2 YPVS models.
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18:54:48 Rob Fzs: jews don't give away stuff for free

18:54:59 Rob Fzs: unless its their clothes/ hair/ golden teeth

[Kickstart]

Hi

I was thinking power jet pip as well. The piping on one was missing on the TZR250 which might explain why it was down on performance.

Piccy would be useful. Could be someone has fitted later carbs.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[MarJay]