Two stroke power delivery...

[Fil]

I went for a blast on my Mastiff earlier and it seems to have developed a bit of a two stroke style power band.

If I crack open the throttle, it chugs away reasoably well (but not as good as it ought too...) until the revs reach about 4500 - 5000 rpm, and then seems to hit it's 'power band' and takes off like a scalded cat.

Has anyone else had this problem?
Can anyone tell me what's causing the sluggish delivery below 4500 rpm?

[DAVID THOMPSON]

welcome to the wonderful world of CV carbs dual carbs and bad gas

some one had simular problem in the past keith i think in the uk
he may respond soon
think he found one of his carbs was not working correct

[keithcross]

Fil

Take a look at the diaphragm in the cv carb. Mine had a split in it. Lost a lot of power. There are 2 ways to check it.
1. remove saddle and air filter and start engine. Insert a mirror in teh air filter box, rev the engine and see if the slide in the cv carb operates.
2. Look f or the large diameter black tube that runs from teh cv carb and ends with a small plastic `bomb' (technical term as I am not sure what it does ) Suck on the end of this pipe and you should hear the slide in the cv carb operate.

Hope this helps

Keith

[Fil]

Sorry Keith, I should have said that I know the CV diaphram is ok as I took the carbs off last month to give them a clean - the float bowl & main jet were gummed up after a long lay-up.
I cleaned everything, reassembled it and the bike starts first push of the button, idles perfectly, and runs fine - apart from the lack of grunt below 4500-5000 rpm.

[keithcross]

Fair enough fil, are your sure that you located the diapragm correctly (its easy to get it wrong )

Keith

[DAVID THOMPSON]

after some hard thinking
check to see if correct spark plug is in use

http://www.mzriders.com/viewtopic.php?t=1090

NGK DPR8EA - 9


my rotax motor on the saxon tour came with d8ea when i changed
to NGK DPR8EA-9 it was like a diffrent bike

a friend with a f650 bmw uses NGK DPR8EA - 9 also and says it
cured all his problems except the name on the tank

[Fil]

I'm pretty sure the diapraghm is back in correctly (the tab was lined up with the indent in the top of the carb), and it has the correct plug (I just replaced it with a correctly gapped plug).

I'm almost certain it is a problem with the CV carb because of the very distinct 'step' in power when you hit 4500 rpm on wide open throttle.

Looks like I'll have to take the damn things off again and check everything. Maybe it's a good opportunity to buy a jet kit for it...

[keithcross]

Fil

You can just about remove the diapragm from the cv carb on a baggi with the carbs without removing them completly, just remove the mounting bilts and there is just enough room. Its tight but it can be done

Keith

[cat]

...large diameter black tube that runs from the cv carb and ends with a small plastic `bomb' (technical term as I am not sure what it does ) Suck on the end of this pipe and you should hear the slide in the cv carb operate.

It's the vacuum/diaphragm breather. The black plastic thing is a crude filter. (Apparently a bit of foam / 3M pad/mesh type stuff in it.)

It was added to the Yamaha XT at some point to provide some compensation for the enrichening effect of high altitude less dense air. When the atmospheric pressure goes below a specific point, some additional air passages are opened. (I came across this in one of the German language guides to the YDIS carbs.)

The KTM LC4 640 has the same or a similar plastic breather filter. There was a post on advrider recently showing how the guy had used Uni breather filters for the float bowl vent and diaphragm vent on his KTM LC4. Apparently some people have had problems with dirt getting into the diaphragm area and it was thought that one possible reason could be that the stock plastic filter was inadequate.

From http://www.xt600.de (WorldLingo translation via Micro$oft Outlook):

Von 1989 - 1991 (Ténéré 3AJ) verfügten die Vergaser über eine Gemischregulierung für Fahrten in großen Höhen. Eine Barometerdose (Luftkorrektor) gibt ab einem bestimmten Luftdruck zusätzliche Luftkanäle frei, um einer Gemischüberfettung entgegenzuwirken.

From 1989 - 1991 the carburetors (Ténéré 3AJ) had a mixture adjustment for travels into large heights. A barometer box (Luftkorrektor) releases additional air ducts starting from a certain air pressure, in order to work against a mixture over greasing.

Neuere Vergaser (XT600E, TT600R) haben am Sekundärvergaser rechts außen einen Luftkanal, der in den Raum unter der Membran mündet und am anderen Ende einen Filter hat. Hier wird Umgebungsluftdruck in den Raum unter der Membran geleitet (s.a. Volllastbetrieb).

Later model carburetors (XT600E, TT600R) have at the secondary carburetor on the right of outside an air duct, into the area under the diaphragm flows and at the other end a filter has. Here ambient air pressure is led into the area under the diaphragm.

The advrider thread [Mikuni BST40 "Venting"... ]
http://www.advrider.com/forums/showthread.php?t=114485
The gentleman whose work this is is renowned for his comprehensive, illustrated technical "how-to" posts.
Here are the introductory paragraphs:

There have been several threads in the past with comments on dirt getting into the Mikuni BST 40 carb via the two main vents.
One vent goes to the carb float bowl; the other vents the variable air space under the rubber diaphragm. Both allow equalization to atmospheric pressure in those areas.

The bowl vent uses a long hose that is generally routed up and along the top of the airbox, towards the battery; the intent here I believe is to have it protected from excessive exposure to dust and water.

The diaphragm vent is routed into a small plastic elbow/filter. The elbow contains a disc type filter element approximately one square inch in diameter. This filters the air going into the cavity under the diaphragm.
Older bikes, or non-US bikes may have additional emissions bells and whistles attached to this vent.

There have been numerous concerns posted regarding the disc filter on the diaphragm vent. The most common is that it allows dirt to enter the carb and accumulate in the body below the diaphragm, around and in the slide area. Some have blamed this filter for “slide sticking” issues.
Remember that air is both pulled into and pushed out of this vent.

Another concern, and one that seemed more a possibility to me, would be that the filter becomes so clogged that air cannot flow readily thru it, in effect damping, or at worst completely preventing atmospheric pressure from reaching the air chamber. Throttle response would be adversely affected.

One day I took off the carb disc filter and took a good long look at it and thought about what it had to do. And then I thought how much better it would be to have a larger area for the diaphragm to “breath” thru… not to mention something I could tell at a glance what condition it was in.