Open exhaust (slip-on) causes jerky engine at low RPM.

[Squishy]

Hi,

So last week i've been trying some combinations of jetting and the effect of having the slip-on silencer removed or inserted.
My 400 is stock but it now has 110 main jet. +1 rich needle and aftermarket slip-on.

With the 100 stock main jet, with or without silencer inserted didn't make any difference (even did a roll-on test with timer).
Now with 110, the difference is quite noticeable. Engine has more torque and feels more responsive.

However.. I rode it with silencer for about 3 years, and the engine is very smooth with it. I can let go of the throttle in 3rd gear and it will just roll on at idle rpm without hesitations.
But now without the silencer it bogs and stutters.
I figured this might be because it's leaner .. but I tried turning the screws to 2.5 screws out but it didn't seem to make any difference. It would now start without choke and smell like fuel so I guess it's too rich.

Is this normal behavior for the 400? Does it need a bit of back pressure to keep it smooth?

Thanks

[TJS]

Go here...
http://www.factorypro.com/Prod_Pages/prods41.html

Then goto the CV tuning page.

[Squishy]

Go here...
http://www.factorypro.com/Prod_Pages/prods41.html

Then goto the CV tuning page.

Where?
That site is one of the worst designed sites i've ever seen.
I can't find CV tuning on that site anywhere.

[El Gringo]

Squishy, i couldn't agree more, that website gives me a head ache just looking at it!

The CV tuning bit is here

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

My issue with this tuning method is, its all very well starting at the main jet and working down to the idle but if you can't get the bike to run in the first place how do you start at the top end?

[Squishy]

Squishy, i couldn't agree more, that website gives me a head ache just looking at it!

The CV tuning bit is here

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

My issue with this tuning method is, its all very well starting at the main jet and working down to the idle but if you can't get the bike to run in the first place how do you start at the top end?

What do you mean I can't get the bike to run? It runs fine.
I'm just wondering whether it's normal for the 400 to become jerky with an open exhaust.
I tried from 1.5 turns to 2.5 turns and in between but it doesn't matter.
It runs very smooth down low with silencer in the open exhaust.

You think adjusting the float level will make a difference? It's stock now (14.6mm)

[El Gringo]

Squishy, i couldn't agree more, that website gives me a head ache just looking at it!

The CV tuning bit is here

http://www.factorypro.com/tech/tech_tuning_procedures/tuning_carbtune,CV,high_rpm_engines.html

My issue with this tuning method is, its all very well starting at the main jet and working down to the idle but if you can't get the bike to run in the first place how do you start at the top end?

What do you mean I can't get the bike to run? It runs fine.
I'm just wondering whether it's normal for the 400 to become jerky with an open exhaust.
I tried from 1.5 turns to 2.5 turns and in between but it doesn't matter.
It runs very smooth down low with silencer in the open exhaust.

You think adjusting the float level will make a difference? It's stock now (14.6mm)

Sorry i didn't mean you and your bike in particular, its was just a general comment on the tuning method they describe.

For example when i was building my B4 i couldn't get it to carburate properly at the low end to the point where it was un-ridable, so how was i supposed to start with the main jet and know that i'd made the best choice.

If the bottom end is smooth i'd leave the float height alone to be honest.

If your on a straight through exhaust but you've gone up to a 110 and richened up the needles by 1 notch you've covered your bases - i'd probably go 1 3/4 on the mixtures, it does sound like you've over richened the idle circuit.

If it pulls smoothly up through the revs with no flat spots i'd say the jetting/setting is pretty close.

I'm not sure I understand the test your trying? 3rd gear and rolling off or 3rd gear and just letting it drop to idle? I'd try dropping the needle back to stock position - it sounds like a transition between higher revs and lower revs issue.

[Squishy]

Well, it's simple.
With silencer in my open slip-on, I can let go off the throttle in 3rd gear (at low speed), and it will just move forward at idle RPM (1400rpm).
Without silencer if I do the same, at 1400rpm no throttle it will start to bog a lot.

Adjusting from 1.5 to 2.5 doesn't matter, still bogs.


It runs great otherwise.. the 110 jet really improved torque. It now starts pulling from as low as 4000rpm, and wheelies in 1st gear when I snatch the throttle open.
The only thing now is when I ride 2000rpm in 2nd gear and I open the throttle it will bog (regardless of silencer btw).
I had this issue too on stock main jet (100) with needles +2 richest setting.
I guess the main jet affects the whole range a lot.

From 2500rpm I can open the throttle fully and it goes.

[El Gringo]

Sounds like a transition bog from 2000 up, I would try the needle position 1 up and 1 down from where you have it now and see if gets better or worse.

The bogging seems similar to the problem i had with ovalled emulsion tubes. Are they in good condition?

[Squishy]

Sounds like a transition bog from 2000 up, I would try the needle position 1 up and 1 down from where you have it now and see if gets better or worse.

The bogging seems similar to the problem i had with ovalled emulsion tubes. Are they in good condition?

Haven't checked the emulsion tubes yet..

As I said I had this when I tried going from +1 rich to +2 rich on the needles when I was still on 100 main jet, I had this same problem.
Now the problem is also happening on +1 rich with 110 main jet.
I can probably fix it by going to stock needle position but the mid has never been so strong as it is now.. I mean from 2500rpm it starts pulling like never before.. it's really only from ~2000rpm when I'm behind a slow car in the city and want to pull away.

I was also thinking about trying 107.5, perhaps it makes just enough difference.

[El Gringo]

I'd still try it in the stock needle position, just out of curiosities sake, but a 107.5 would be worth a go 

[Squishy]

I'd still try it in the stock needle position, just out of curiosities sake, but a 107.5 would be worth a go 

Well, that will probably fix the problem but not the one about the jerkiness without silencer ;).
As I said if I insert the silencer it's 100% smooth with +1 rich needle and 110 main jet. (which is what I'm currently using).

[Squishy]

It's also jerky at constant 1/8 - 1/6 throttle.
With silencer it's smooth.

I just wonder what they would change you would bring it to your local jetting dealership..
I know an open exhaust leans out the mixture, but with the 110 main jet and +1 rich needle it's more likely too rich than too lean.

The bogging from 2000rpm happens with or withour silencer though. That doesn't matter.
But it's exactly the same as when I tried +2 rich on the needle, and was gone at +1 rich on the needle.
This also points towards it being too rich now (because I now have 110 jet).

The above makes me think it can't really be about jetting. It's already on the rich side and adjusting mixture screws doesn't fix it.

Unless somehow adjusting the float level has a totally different effect than changing the mixture screws?
Is it common practice to adjust the float height?

To me it seems more like a backpressure thing. My silencer is closed at the end and only allows air to pass via the sides and then through some small holes. So basically the airflow hits the silencer and then is forced through the sides.