BIG carburation problems after installing 'Hot' cam

[Mannyroad]

No Kenny rode one more like mine....

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Pi$$er is mine is behaving just like @Mannyroad bike; mine is standard 650 motor however!

Nice bike. That system is 2-2 though. One point I would say is that you have very long headers and there is a formula for ideal header volume which I understand is basically that your header volume should be approx double the cylinder volume. This can be doubled but going a volume that isn't a ratio of cylinder volume can be problematic. Maybe this is what you are experiencing??

 

[Mannyroad]

Those 'K&N style pod' filters ain't helping the bike to breathe either. UNI Pods straight foam filters work well.

They're not K&N style. They're K&N

 

[Mannyroad]

You could go with a 2into2. Similar look.
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Yes, might be my only choice. Need to look into a new collector first.

 

[Mannyroad]

Any chance of having a look at a pic of the 2-1 collector? A mate of mine has a 2-1 on an XS powered Wasp sidecar & it sounds great. I've been investigating designs/theory & am going to have a crack at building one myself but will get it running with 2 pipes first.

Best pic I have

 

[Adamc]

Nice bike. That system is 2-2 though. One point I would say is that you have very long headers and there is a formula for ideal header volume which I understand is basically that your header volume should be approx double the cylinder volume. This can be doubled but going a volume that isn't a ratio of cylinder volume can be problematic. Maybe this is what you are experiencing??

Thanks for the data.

 

[Adamc]

They're not K&N style. They're K&N

My bike had the same genuine K&N filters, but ran much better when I changed to the UNI Pods.

 

[650performance]

There was a comprehensive data sheet published in 2013 that details carb settings for high performance XS650/700/750 engines. Carbs don't care what the phasing is of an engine, so the information is relevant to your 270 engine. I really question why anyone would choose a cam that isn't well proven. The Shell #1 and Megacycle 30 and 40 cams have been run in literally thousands of XS based race/high performance engines with great success. The tuning information is well known. For example, I read that your all in ignition timing is 38 degrees. I can tell you with certainty that in the 70s and 80s, when the XS engine was in its heyday, EVERYONE set their engines at 34 degrees, with 36 the absolute maximum. And this even included the 80 horsepower Axtell OU factory race engines. Mannyroad, email me at ctweeks@aol.com and I'll send you a copy.

 

[Grimly]

Ah, right, gotcha. The scrambler is already built and had a 2-1 that suits it nicely,

My 2-1 is a Motad, with VM34s / UNI filters and afaik, standard bores. Never a trace of misfire from it, but it's not rephased.
Of course, the Motad is NLA, but it fits fairly close into the frame same as yours and the silencer is easily replaceable with something slimmer, like yours.

 

[Kevin Werner]

Nice bike. That system is 2-2 though. One point I would say is that you have very long headers and there is a formula for ideal header volume which I understand is basically that your header volume should be approx double the cylinder volume. This can be doubled but going a volume that isn't a ratio of cylinder volume can be problematic. Maybe this is what you are experiencing??

I recall an article about head pipe volume. The ideal head pipe for the application, it was a motorcycle, 18 or 20 feet but 1/2, !/4, 1/8 etc was virtually the same but a 2 piston swept volume was about the minimum because the formula used included swept volume, gas flow velocity, etc etc.

 

[Mannyroad]

That's interesting. Didn't see a Motad 2-1 on the net. Will try again. As for it being rephased, it makes no difference. Think about it, 4-1 and 3-1 motors all have collectors with pistons firing at different positions in each cycle, like a rephase and they run fine. I suspect each engine is, or is not, in its own right sensitive to the collector, which is quite a critical element.

 

[Mannyroad]

I recall an article about head pipe volume. The ideal head pipe for the application, it was a motorcycle, 18 or 20 feet but 1/2, !/4, 1/8 etc was virtually the same but a 2 piston swept volume was about the minimum because the formula used included swept volume, gas flow velocity, etc etc.

Might be the same book my mate reads at bedtime.

 

[Mannyroad]

Ok, so, after the initial mile run road testing to check if 2-2 would solve my big misfiring problems, all seemed much better, as in the misfiring seemed to all but have gone. Anyway, took it for a longer run today and it seems all isn't sorted yet. I took a ten mile run and I found that rolling on the throttle showed up a fairly broad flat spot in power / throttle response mid throttle range. Rolling on to full throttle got a marked in reasonable in response in that over 4500ish rpm it pulls like a train right through to 6000ish rpm. Infrequently I noticed some misfiring stutter mid-throttle too but not consistently.
Once home I pulled the plugs and though I know little about carburation stuff the central bridge looked a little pale to me, not a nice biscuit colour as I'd expect. So do you guys think I'm maybe a touch lean mid-range and need to lift the needles a notch?

 

[Jan_P]

I have never had so White plugs .. Having said that I don't know much about carburetors
Flat spot .. also

 

[drgonzo]

I'd raise the needles and give it a go.

 

[Mannyroad]

Out of interest, does anyone have a copy of the needle jet identification and dimensions from the Sudco manual? Be good to check my 159-P8 against it to see which jets are richer and by how much. Might end up needing to swap out.

 

[dave_a]

Out of interest, does anyone have a copy of the needle jet identification and dimensions from the Sudco manual? Be good to check my 159-P8 against it to see which jets are richer and by how much. Might end up needing to swap out.

Just want to mention that many engines these days are running/jetted correctly when the plugs are white. I've got a Yam. Injected outboard that runs on ethanol free, all plugs are stark white. A 50 y/o British car that runs on unleaded. Same color white. Modern fuel.

 

[Mannyroad]

Just want to mention that many engines these days are running/jetted correctly when the plugs are white. I've got a Yam. Injected outboard that runs on ethanol free, all plugs are stark white. A 50 y/o British car that runs on unleaded. Same color white. Modern fuel.

Interesting that. I say that because I raised the needles a notch and they suddenly (surprisingly, actually) went a lot more colourful, bordering on sooty. Seemed too big a change for such a small needle shift. Upshot was that the motor ran worse, mid-range, which was annoying. Setting these VMs is a right PITA.