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View Full Version : Bike bogging but fine when I remove air box?



BradBooth
07-24-2014, 08:37 PM
Hi, I have a 2009 RS50, I recently fitted an airsal 70 cast iron twin ring kit, a dellorto 21mm carb and an arrow exhaust. The bike ticks over perfect, fires up first time every time without choke. But when I come to rev the bike it bogs badly, and doesn't rev past 9.5k. I got given the carb when I bought the bike so thought I would start by cleaning it out, and it had a fair bit of sh*t in there haha. So I thought that would be the end of the problem and it would run well, however I fired the bike up and it ran exactly the same as before. I let the bike warm up again, and it continued to do the same. Then I removed the tank and opened up the airbox, and the filter was pretty clear, not enough to be bogging the bike that much. The first thing I thought was jetting, but it was supposed to be jetted to this kit anyway as that is what the previous owner was running. I then disconnected the airbox and the bike ran perfect, all the way up the rev range, seemed to bog a little in lower revs but probably just from a cold start. Does this mean the air box is restricting flow and I need an air filter instead? Or is it to do with the jetting? First time posting in the forum and a bit of a 2 stroke novice, be nice haha! Thanks a lot.

Spudgun60
07-27-2014, 05:26 AM
It appears that the jetting in your carburettor isn't matched to the standard airbox. When the standard airbox is removed, it's perfectly normal to require a larger main jet. It wouldn't surprise me if the former owner jetted the carb to run without a filter (Stupid), or run with a pod filter. It looks like your engine is running rich with the standard airbox. You need a smaller main jet. However, dropping the needle one notch may help.

emX50
07-27-2014, 12:26 PM
Hi, I have a 2009 RS50, I recently fitted an airsal 70 cast iron twin ring kit, a dellorto 21mm carb and an arrow exhaust. The bike ticks over perfect, fires up first time every time without choke. But when I come to rev the bike it bogs badly, and doesn't rev past 9.5k. I got given the carb when I bought the bike so thought I would start by cleaning it out, and it had a fair bit of sh*t in there haha. So I thought that would be the end of the problem and it would run well, however I fired the bike up and it ran exactly the same as before. I let the bike warm up again, and it continued to do the same. Then I removed the tank and opened up the airbox, and the filter was pretty clear, not enough to be bogging the bike that much. The first thing I thought was jetting, but it was supposed to be jetted to this kit anyway as that is what the previous owner was running. I then disconnected the airbox and the bike ran perfect, all the way up the rev range, seemed to bog a little in lower revs but probably just from a cold start. Does this mean the air box is restricting flow and I need an air filter instead? Or is it to do with the jetting? First time posting in the forum and a bit of a 2 stroke novice, be nice haha! Thanks a lot.

Starting without choke, from cold, is a fairly solid indication that you are running a rich setup.

From what you say, it sounds like it is substantially over-jetted, so much so that the spark plug is likely to be sooty.
Have you checked the colour of your plug?

Also..... you say it ran perfect with the air filter disconnected.
IE. Went from 'very poor running' to 'perfect'.

This may be true.... but it's highly unlikely, if the air filter is not clogged.
For certain you need to remove the airfilter for close examination, and while you are at it..... check the airbox intake holes, to ensure that nothing has made a nest in there.

Presumably you are using the oblong foam slab filter.
Has it been over-oiled...... maybe with a very thick oil, which has then clogged with dust?

The whole point of the big airbox, and big slab of sponge, is that it can feed all the air that the engine might need, with minimum restriction.
So.... either the ability off the motor to run 'was on a knife edge', and removing the air filter just took it over the edge to allow it to run.
OR
That filter needs thoroughly hand washing a few times with serious detergents in warm water.

I bought a huge slab of Twinair filter sponge, from which I cut a piece to suit, to replace the original that had gone 'kind of soggy'.
I think perhaps that the petrol vapour gradually breaks down the plastic structure of the sponge, forcing the need to replace it.

If the air filter sponge is genuinely clean and in good condition AND the intake holes are clear......... this would indicate that you will likely need to drop much more than one jet size.
The 'black' colour of the spark plug will confirm this.

If all this is true.... then you'll need to bite the bullet and buy a box of main jets, of a size range lower than what is currently fitted.

Prior to that, I would definitely agree with spuguns recommendation to lower the needle one notch (weakening the mix).
If you get a big boost in performance, then you know that you are heading in the right direction.

:)

BradBooth
07-28-2014, 07:59 PM
Yes I think the previous owner was using a pod filter, this would explain it is running rich to the airbox? The bike is at my dads at the moment so it is hard to find time to work on it haha, I think I am going to try and spend a little time on it on Wednesday. I am going to try to avoid using a pod filter, as it seems so much better just using the airbox. I'll try dropping the needle and see if it improves at all, and then buy some smaller jets. I think I will replace the panel filter thingy in the airbox also while it is off. I am also going to replace the standard reeds with some carbon reeds, will the thickness of these also affect the mixture? Thanks for the help.:)

emX50
07-29-2014, 12:03 PM
Any change has potential to effect the required mix.

I would strongly suggest that you do not dive around changing different things.
Your first task should be to get what you've got...... running well.

Dropping the needle one notch should make a dramatic difference.
You'll at least know where you're going then. ;)