Carb jetting vs. Tuning nightmares By jasoninnaples (Read 873 times)

[jasoninnaples]

50mm bbk with the big valve cylinder head, racing camshaft, racing cdi a performance glass pack exhaust with a 19mm Keihin CVK Carb with UNI air filter directly attached to the carb. Tried everything from a 100 to 85 main with a 30-45 main. Still running like crap.
In the meantime I think I have gotten 3 bad petcock in a row in trying so many carb adjustments I developed a new problem, the bike keeps cutting out at most throttle positions, acts like a float problem but it is happening as low as 1/3 throttle. Can't figure it out. Stock cdi didn't help . Bypassed petcock and ran gas directly to carb with a 2 way off switch then plugged vacuum port on intake manifold . Ready to park the thing downrange and blow it up just to feel vindicated. Please help

"Chinese scooter nightmares, I've never had so much fun with frustration."


"Chinese scooter nightmares, I've never had so much fun with frustration."

[Bashan]

Do you have experience with previous upgrades specifically high flow air filters and exhaust? There's lots of discussion on this topic on the motorcycle forums. I know quite a few engineers from the (alas closed) Harley R&D facility at the Talladega super speedway. They have told me that an engine, any engine, with CV carbs needs a certain amount of resistance on the intake side of the carb. It also needs resistance from the exhaust header. It's true that the main vacuum for the CV carb is created by the air flow through the venturi according to the Bernoulli effect. However, some resistance is needed at the airbox to facilitate the diaphragm pulling up.

So, if your needle isn't pulling up due to compromised vacuum from the upgrades it can lead to just the symptoms you described. I've been there. I would suggest taping off part of the air filter and see if it makes any difference. Any positive change in how it runs will indicate it's the new filter.

In regards to your petcock concerns, install a Briggs & Stratton 1'4" valve from Tractor Supply, or the lawn equipment section at Lowes. I hate the vacuum valves and petcocks. They're just a vacuum leak waiting to happen. Oh yeah, I forgot to mention, you should also look for vacuum leaks.

[oldscootcoot]

Also check the valve clearances, could have changed after the initial start up. I just recently installed what sounds like the same kit. Came with an 85 main which I installed. It ran like the hounds of hell from the get go. I was amazed! I agree on back pressures. I put a shorty muffler on a 200cc M/C to replace the expensive stock unit. From the get go it was boggy and just ran terrible. A Chore Boy cleaning pad stuffed down and safety wired into the muffler did the trick and it ran like it always did. Did you do the install?

[jasoninnaples]

Thanks to the both of you. Last night I was wondering about the resistance /back pressure things. I saved a length of tubing from the old airbox with a nice bend in it for just this reason. I will be rebuilding the top end today mainly because I have 4 days off and I am bored and also because I just received some HOCA rings and the one's from the bbk are junk. I will try an 85 jet in the carb but I'm leaning towards the pressure idea being a good fix. That and most of the bogging down problem I have had previously have been all idle jet in nature. What size idle were you running with the 85?
As for the exhaust, I think that just because of the angles on the header should be able to create enough back pressure. We'll see how things work out here later today. I ve been rebuilding this thing since about 1500 miles and it's down more than it's running. Thought about replacing the chain and guides for the timing /cam. Any suggestions on how to get that done without splitting the whole case?

"Chinese scooter nightmares, I've never had so much fun with frustration."

[Alleyoop]

Yep the guys are right on to MUCH AIR, the Uni Filter directly attached to the carb sucks in to much air. Many what they do is leave the AIR BOX on and that settles it down. But put on a tubing on the Uni and that will help a lot to slow down the amount of air. The other thing is make sure you oil the uni the Oil will help cut down the amount of air.

Also to find the right PILOT JET the FUEL RATIO screw should be out no more than 2- 31/2 turns out also when Leaning it out turning it CLOCKWISE it should kill the motor when you have about 1 turn before completely closed. If it keeps on running closed the PILOT JET IS TO BIG. If you have to turn RICHEN it past 3 1/2 turns for it to idle good it is to BIG.
Alleyoop
Alleyoop

[jasoninnaples]

Thanks Alleyoop and others. I am regretfull to report that I rebuilt the motor and went through the entire top end with a diligence. All things are within spec. I'm baffled. It now starts with the kick-start much better than with the electronic one and I can tell it has somewhat better compression. Now, after adding the length of pipe to the air filter, it idles great but dies with any movement of the throttle. I'm beginning to think that w
Either the big valve head and /or the exhaust is just too much air being moved. Going to try to restrict it some more now. Also the exhaust came with a screw in restrictor on the end of the tail pipe that came out along the way somewhere, maybe I should try to restrict that as well? Maybe it needs a bigger carb to handle more fuel? Going to tape off part of the air pipe into the carb and see, I may oil the filter first also. It ran the same with the vacuum port on the intake manifold open as it did closed, I'm hoping to find the problem here shortly.

"Chinese scooter nightmares, I've never had so much fun with frustration."

[Alleyoop]

Here is a little trick that works when to much air is being sucked in when a Free Flow Filter is being used. Get some tape and start at the end of it and tape around the uni try one strip of tape then add another until you find the amount of air restricted enough that the motor will except.
Alleyoop

[Alleyoop]

How many turns out is the FUEL RATIO SCREW where it starts and idles pretty good? That will tell you which way you have to go.
Also have you adjusted the Fuel Ratio Mixture while on the Center Stand to get the HIGHEST IDLE after IT WARMS up for at least 5 minutes. The problem with not able to start with the electric is now that you have a lot more compression the battery is NOT STRONG enough to turn it over fast enough. That problem you can solve by getting a 7AMP battery with HIGH COLD CRANKING AMPS close to 200 CCAs or more.
Alleyoop

[jasoninnaples]

Short answer is that it won't run long enough to figure out. Currently I have no spark so I will have to figure that out first. Working 14 hours shifts the next few days. Stay tuned and thanks for the insight, it is a huge help.

"Chinese scooter nightmares, I've never had so much fun with frustration."

[Alleyoop]

Nov 6, 2013 16:15:07 GMT -6 Alleyoop said:

Here is a little trick that works when to much air is being sucked in when a Free Flow Filter is being used. Get some tape and start at the end of it and tape around the uni try one strip of tape then add another until you find the amount of air restricted enough that the motor will except.
Alleyoop

Did you do this?

[jasoninnaples]

Never had the chance, started it and let it warm up, shut it off and an hour later went out to attempt your suggestion and wouldn't start, No spark. Gonna try and chase down the problem for a bit here.

"Chinese scooter nightmares, I've never had so much fun with frustration."

[beebo]

When I read your first post, I was thinking valves, or valve timing. Since you have gone in and re-done the top end, if the valve timing was off, you probably corrected that. Noting that you seem to have better compression and it will idle smoothly, I think you have it together right. I was wondering if you still have your original carb--you might try it out to see if your new carb is the culprit or not. If your old carb works, I would be looking at that new one-the jetting, main jet needle, diaphragm spring tension, etc.
Some of the members here mentioned restricting the intake/exhaust systems. I agree that on a stock system, there is a certain amount of restriction in the intake as well as the exhaust systems, not so much for efficiency as an effect of emissions requirements. Not just organic but for sound control. The carb has been tuned with these requirements taken into consideration. Seeing as you have put a high-flow head with big valves in it, I assume that more flow is what you are after, not less. The bike really should run with your stock carb, although probably not at its best. In my experience, 2 or 3 jet sizes is usually all you might have to move up. If you changed to a race type cam, you might need it to be a little richer on the low speed jet. Big valves+big cam+big carb= low vacum signal at the venturi, especially on a one-carb-per-cylinder setup. That problem usually goes away as rpm goes up.
If you have problems with either carb, you might look for one with an accelerator pump. Most of these CV carbs don't have one, because they meter fuel so well they don't need one. Some do, though- the CV carb(Keihin) on my Harley has a pump on it, and I have seen some others that did.
Anyway, these carbs are very tuneable, it might take some trial and error.
I hope you have mure fun than frustration!