my 2002 Skorpion won't rev past 3500 rpm

[edfmaniac]

I have a Yamaha SZR660 - same engine. Check that the engine breather pipe isn't choked. The engine breather pipe (item 51) allows the crankcase to "breathe" (otherwise the oil seals would seep) and oddly feeds the secondary carb diaphragm operation.

Just curious about this one. I thought both lines were vented to atmospheric pressure. How does the crankcase vent aid in creating the vacuum that opens the slide on the CV carb? I can see how the Tunebike/Off The Road kit mod works to create more vacuum, but that is all done by the piston on the intake stroke. I didn't think the crankcase vent had anything to do with it other than allowing the lower end of the engine to deal with the constant pressure/vacuum cycle going on beneath the piston rings.

[breakwellmz]

Hi

I agree.I don`t think you could use the fluctuating pressure from the crankcase to actuate the second carb, i`m pretty sure my Mastiff crankcase vents to the airbox.
If we knew that 3,500 rpm is the point at which the second carb should come into play it`s failure to do so must be the cause of the problem surely?

[Henry74]

Hello again ,
if nothing else helps, try to remove the filter housing and both intake hoses. Next with the engine running open up the throttle Valve of the CV carb and check if the RPM rise or not. If not, squirts some fuel with a syringe and a piece of tubing ( secure the tubing firmly so it will not be sucked in ) whist opening the throttle valve again and check if RPM rise above 3500.

This will at least eliminate the possibility that you have an electrical problem!

Henry

[breakwellmz]

Hi

I agree.I don`t think you could use the fluctuating pressure from the crankcase to actuate the second carb, i`m pretty sure my Mastiff crankcase vents to the airbox.
If we knew that 3,500 rpm is the point at which the second carb should come into play it`s failure to do so must be the cause of the problem surely?

Having just had a look at my bike i was not right earlier.
One breather pipe does indeed go to the CV carb, a smaller bore goes to the intake box.I`ve looked on internet, but have failed to find a full and satisfactory explanation as to how this system operates.

[bill h]

Today I removed the secondary carb and the carb boot and valve cover to watch the valves to see if they were opening and closing smoothly as the engine cranked.
They were.
I again checked the boot and it is in good condition.
Then I hooked the secondary carb to my shop vac ( a perfect fit by the way) to see if the piston would rise as I moved the butterfly.
It did, I don't think anything is wrong with this carb.
I then marked the twist grip with white paint at full close, then full throttle then halved the distance between the marks for half throttle and then halved again for quarter throttle.
I started the bike and at 5000+ rpm the throttle position was less than 1/4 throttle.
I don't think that the butterfly opens enough to raise the piston. It is probably just transitioning from the idle circuit to the needle jet on the main carb.
Tomorrow, I'll put the tank back on and take it out and note the throttle position when it stumbles and then check the color of the spark plug.
I am now wondering if the muffler/silencer is blocked or restricted.
I am running a stock unit.
After I do my test tomorrow I will replace the muffler with a loud aftermarket one I have and see if that makes a difference.

Bill

[edfmaniac]

The vent on the CV carb is to allow the diaphragm to move, nothing more. If it weren't vented to the atmosphere, when the vacuum from the intake tried to raise the slide, a vacuum would form under the diaphragm, preventing it from opening all the way. The vent on the crank case is to allow the otherwise sealed air chamber known as the lower end of the engine to breathe when the piston moves up and down. Otherwise, when the piston was at bottom dead center, the pressure in the case would be extremely high, causing oil to leak from every possible place. They are just vents. For emissions reasons, the crank vent is rerouted through the engine just like the PCV valve on a car or truck.

[dandywarhol]

I have only recently bought my SZR so I'm still in the process of finding out it's idiosyncrasies.
The engine breather pipe feeds directly (via a coarse filter in the pipe) under the secondary carb diaphragm. This is a bizarre way of supplying what normally is atmospheric pressure to the lower part of the diaphragm. Supplying atmospheric pressure to the diaphragm allows the piston to lift when a lower pressure exists above the diaphragm, which in turn supplies more fuel/air to the cylinder. This lower pressure is drawn from the intake venturi, at the throttle butterfly and the lower pressure increases as the vacuum increases depending on the throttle position, drawing the piston up.
The Teikei Y26 carb appears to draw from the engine breather pipe when the engine is running on the primary carb. I can only assume Teikei have allowed for the pressure balance due to the air pulse from the crankcases by the size of the air feed above the diaphragm and the mass of the diaphragm and piston assembly

What does happen in reality is that the secondary piston flutters up and down (you can here it rattling, I thought it was the timing chain at first) if the idle speed is set too low. The fluttering is due to the air pulses from the crankcase breather pushing the piston up and down. None of this really matters though as the secondary butterfly should be totally closed on idle as the primary carb is supplying the mixture to the engine up until around 5mm of its opening.Apart from not fouling the air filter with crankcase oil and fumes, I really don't understand why they use this method of crankcase breathing and I really don't get operation of the secondary carb this way as the crankcase pressure will rise with engine revs but someone much cleverer than me must have an answer.
I still think the original poster should be looking around this area for his problem. Here's a pictorial on the carb cleaning................I think the oily gunge in this has been caused by overfilling the oil tank...............

http://bultaco.proboards.com/thread/502 ... ikei-y26pv

PS I hadn't read your post edfmaniac before I submitted mine - we're singing from the same songsheet
Bill H - have you checked the breather pipe is neither choked or split?

[edfmaniac]

I don't think the crank case vent is supposed to be plumbed directly to the CV diaphragm vent. Where would the oil go? And why does the carb still function normally when the CV vent and crank vent are no longer connected to anything?

After looking at the link, I'm pretty sure that guy doesn't know his ass from a hole in the ground. Never, NEVER, should you be getting burned up carbon deposits from the combustion chamber scattered throughout the mechanics of your carbs. Whoever owned the bike those carbs came off of was more than a few bricks shy of a full load if you know what I mean.

I'll have to check into the factory plumbing on the Skorpion now, just to make sure I'm not the one who is full of hot air.

[dandywarhol]

I'd be interested to hear what you find edfmaniac. I wouldn't call this a diaphragm vent, it's a source of atmospheric (or it appears, higher) pressure to counteract the depression felt above the diaphragm. That is why the carb still functions, to a point, when the pipe is disconnected.
This is the Yamaha parts diagram for the SZR660, likewise the XTZ660. If you look at the diagram in the link you'll see the breather pipe (51) has the letter "A" at the end of it - this goes to "A" on the carb body which feeds directly under the diaphragm. If I remove the breather pipe on my engine, when it is running, crankcase pressure is felt. It is entirely possible that someone has incorrectly fitted the pipe on mine - I'll need to investigate more - it's just pretty difficult in the close confines of the SZR chassis

https://www.yamahamotorcyclespares.co.u ... etor?uid=0




The amount of oil and crud in the previous link I posted is probably due to the oil tank being overfilled and possible disintegration of the insides of the breather pipe and/or the breather gauze has been removed - I dunno - certainly isn't right

[edfmaniac]

I'm just using common sense and the fact that everything I've read regarding the vent mentions it being routed to the atmosphere, not the crank case vent. How on earth is the tiny little space in the carb going to be able to mitigate the pressure pulse coming from the stroke of a 660cc piston? It doesn't make any sense and I've seen it nowhere else except for your mention of it here. Google "cv diaphragm vent" and you'll see what I'm talking about. There is no relationship between the two that I can find anywhere.

[bill h]

I took the bike out for a test run today and it ran great.
It ran strong to the redline.
While I am pleased, I don't know what the problem was or what I did to fix it.

Regarding the vent on the secondary carb, for the 3 years I owned this machine it has been vented to the atmosphere.
The crankcase is vented to the air box.

Thanks to all those who replied.
Bill

[dandywarhol]

I'm just using common sense and the fact that everything I've read regarding the vent mentions it being routed to the atmosphere, not the crank case vent. How on earth is the tiny little space in the carb going to be able to mitigate the pressure pulse coming from the stroke of a 660cc piston? It doesn't make any sense and I've seen it nowhere else except for your mention of it here. Google "cv diaphragm vent" and you'll see what I'm talking about. There is no relationship between the two that I can find anywhere.

I'll investigate my engine next time I have the tank off. I know how CV carbs work - I made the point early on that it seems a weird design on the SZR - maybe someone has cocked up at some time with the pipes on my machine - I'll investigate and keep you posted. I also know that the pulse isn't that strong because the piston causes a vacuum in the crankcase on the upward stroke which more or less equals out the pressure.

The last thing I want to do on here is cause controversy - and I'm pleased bill's problem is sorted - it would be good to know what the original cause was................

[breakwellmz]

I took the bike out for a test run today and it ran great.
It ran strong to the redline.
While I am pleased, I don't know what the problem was or what I did to fix it.

Regarding the vent on the secondary carb, for the 3 years I owned this machine it has been vented to the atmosphere.
The crankcase is vented to the air box.

Thanks to all those who replied.
Bill

Really glad you got it sorted, not so happy that you don`t know the cause!

Back to breathers-I`ve had mine from new so i know that it`s not been altered,and can confirm that one of the breathers does go to the top of the diaphragm.It still don`t seem right though!
Sorry if this sounds a rather stupid question, but what causes and how is the second carb bought into play?

[dandywarhol]

I don't understand the breather to the top of the diaphragm bit breakwell, as the top of the diaphragm is sealed, but here's an explanation of how the secondary carb works:

The piston in the secondary carb rises and falls due to an imbalance of pressures above and below the diaphragm.
The piston slide has a small hole in it and as the piston slide is located in the narrow (venturi) part of the carb, then as the air speed increases through the carb when the butterfly valve (positioned at the engine side of the carb) opens, the pressure felt in the venturi is lower than atmospheric pressure and is transferred through the small hole to above the diaphragm so the piston rises due to atmospheric pressure from the large bore breather pipe (number 51 in the pic).
https://www.yamahamotorcyclespares.co.u ... etor?uid=0 .
The rising tapered needle attached to the piston will allow more fuel into the airstream. You'll see the small hole in the 7th photo in the link http://bultaco.proboards.com/thread/502 ... ikei-y26pv
The theory and practice of the Constant Velocity carb is that the airspeed over the main fuel jet is constant, irrespective of throttle opening, keeping a fairly regular air/fuel mixture. The lower the piston slide, the faster the airspeed (like squeezing a garden hose). The higher the piston lifts, the airspeed slows until the piston is fully raised.

The debate on here is how the lower diaphragm area is supplied by atmospheric pressure. I'm totally in agreement with edfmaniac that the picture I'm painting of the engine breather supplying the lower diaphragm volume on my engine is absurd and I'm going to investigate whether someone has cocked up in the past and fitted the wrong pipe to my secondary carb - watch this space

[breakwellmz]

Hi

Thanks for all that, but that is the bit i understand!

No, my question is far more basic than that-What brings the second carb into play, is it a mechanical linkage based directly on throttle(twistgrip)position or something else?