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Old 11-05-2014, 11:35 PM   #1
Mr. BigWheel OP
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CV carb on a BigWheel 350 - in need of advice

I know. Most people will have at one least question that begins with "Why on earth...?" So, I'll just skip to the technical question, since most of my answers would seem really stupid to someone who doesn't suffer from the fat-tired sickness.

The 1987 BW350 came with a Tekei 28mm carb. It is horrible and totally unsupported by Tekei, so I installed a carb from a Yamaha Raptor 350. It is a CV carb and it is awesome. Up to a point.

The bike idles fantastically and is awesome until full throttle. At that point it cuts out and surges back to full throttle repeatedly at very short intervals. I've tried repeatedly to do plug chops but it is so erratic that I don't think that the results are relevant.

The carb came with a 125 main jet. It's too lean according to the plug chop. And it does the whole surging thing at full throttle.

The Raptor 350 uses a 142.5 main jet and is way too rich. The bike wouldn't even start with the 142.5 unless I pulled the choke plunger out of the carb body and left it off. It sucked air through the hole and allowed a stoichiometric mixture to occur. It does the same surge at full throttle.

I've tried a number of jets between 125 and 142.5 and they all result in the same surging at full throttle.

What don't I understand about these CV carbs? I know that they will run erratically if there is a pinhole in the diaphragm, but this is a brand new carb.

A brand new Chinese, counterfeit carb. Any advice?

Regards,

Mr. BigWheel

(PS: I've already heard the advice about getting a 'real' bike. Many times. Many, many times.)
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Old 11-06-2014, 12:34 AM   #2
FR700
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Main jet has zero, zip, zilch, to do with how it starts. You could leave it out for all the carb' cares at start-up.

What you haven't told us is, what did you do with the jet needle.
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Old 11-06-2014, 12:40 AM   #3
16VGTIDave
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You make no mention of air filter or plumbing to an airbox. My limited experience shows CV carbs to be very particular to having clean airflow into the inlet. I've used a short length of PVC plumbing pipe, about 2.5" long, to mount foam UNI filters to Mikuni CV carbs with success.

The starting issues with the large main jet sounds off to me. The main jet isn't doing anything until the throttle is wide open and the piston has risen substantially, which usually only happens at high RPM's.

Also, the "choke plunger" is actually an enricher circuit that applies engine vacuum to the top of the diaphragm, lifting the piston and needle. Similar in goal to a choke, but completely different operation. I'm not surprised it ran lean with the plunger removed, you had created a substantial vacuum leak.

I'd also suggest checking the float level, as well as the fuel supply. A partially clogged fuel filter, kinked line, petcock, etc could also cause a lean WOT mixture.

I've found the instruction at http://www.factorypro.com/tech/carbtun.html to be very helpful. Just be sure to get the engine fully warmed up first, which may be challenging this time of year...

Good luck!
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Old 11-07-2014, 04:08 AM   #4
theraymondguy
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If it's surging at WOT, first thought goes to float height.
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Old 11-09-2014, 08:53 PM   #5
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Thanks for the replies guys. Sorry about the my slow response; I got sent out of the country for work for the week.

Before I left, I tried a few experiments with no success in eliminating the surging at WOT.

I've been trying to use this document as a guide so I don't just try a million different things without any logic behind them:

http://www.factorypro.com/tech/tech_...m_engines.html

It says that finding the correct main is the first step. I went back to the 142.5 main and got the bike to run with the choke in place. This bike normally doesn't start with any throttle but now it requires it.

The idle and first part of the throttle are awesome, so I haven't messed with the pilot or the enrichening jet. Besides, I haven't made it past step one: finding the correct main.

The bike is configured with the factory airbox and manifolds. I wanted to keep the airbox since this is an offroad bike and it goes through a LOT of water crossings. The exhaust has the factory original, steel, gigantic, heavy, restrictive muffler.

The only non-stock part of the motor is a higher compression Wiseco piston (10.25:1).

I tried increasing the fuel level in the bowl by bending the float tab but didn't have the time to do a proper measurement of the the float height. The results were the same: surging at WOT.

I lowered the needle clip (raised the needle) to the only other clip on the needle. It still surged at WOT.

Hopefully someone can see a pattern in this despite my shotgun approach to the issue.
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Old 11-10-2014, 02:29 AM   #6
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Throttle cable, it does open the butterfly all the way doesn't it ?



If you are going to start messing around with float height and refuse to take note of where you set it, then stop fucking around with the thing.


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Old 11-10-2014, 06:16 PM   #7
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Originally Posted by FR700 View Post

If you are going to start messing around with float height and refuse to take note of where you set it, then stop fucking around with the thing.


.


Float height is critical on a CV carb, and is the primary reason that so many people curse them. The fuel level has to be within a milimeter or so of the required setting or you will get surging during periods of high fuel demand. Don't measure the float height (except to set a working baseline), measure the fuel level in the float bowl (bowl on, carb level) and reset as needed until you get the level correct.
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Old 11-17-2014, 01:17 PM   #8
cedric
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Originally Posted by Mr. BigWheel View Post
A brand new Chinese, counterfeit carb. Any advice?
Do you suppose the counterfeit nature of this carb might be causing your problem? I don't have any real advice but I'm curious to hear how this turns out. My old XT250 also suffers from crappy carbitis and this raptor carb could be a reasonable solution. Any chance you could tell me the ODs of where the carb connects to the boots and the length between them? Thx.
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Old 11-18-2014, 12:59 AM   #9
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cedric,

I'll get you the dimensions for the carb. After a long search, this was one of the few carbs that would work with the stock engine-side manifold and the stock airbox on the BW350.

I finally got home today and was able to set the float height properly after my scolding from FR700. I didn't realize how critical the float height was for these CV carbs and mine was at least 6mm low.

I had to surf some Raptor forums to get the skinny on the best set up, but now the carb actually does what it is supposed to do.

Thanks to K-moe and everyone else that took the time to reply.
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Old 11-18-2014, 08:24 AM   #10
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Thanks! The BW 350 and the XT250 engines are actually quite similar, so there is a reasonable chance it will work for me too.
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cedric screwed with this post 11-18-2014 at 08:39 AM
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Old 11-19-2014, 07:48 PM   #11
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I may have spoken too soon. The carb ran great when I set the fuel level at 3mm above the flange between the float bowl and the carb body. Then I attempted a plug chop and the bike ran horribly at full throttle.

When I rechecked the fuel level, it was 10 mm too high. The next time I checked it, 3mm too low. The next time the needle valve didn't seat and fuel blasted out of the overflow with the full urgency of 1.8 gallons of pressure behind it.

I think that it is obvious that I am having problems with the needle valve. Everything looks good and seems to work well with the exception that I have to blow into the clear tube to get the fuel to seek equilibrium.

These are brand new, first rate, Chinese A1 quality parts. How could this be???


cedric,

These are the dimensions for the carb:


Air box side (intake) 2.284" (58)
Cylinder side (outlet) 1.652" (41.28)
Distance between 3.741" (95)


I actually got these from Thumpertalk, but I'll confirm them the next time I pull the carb off.
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Old 11-19-2014, 08:17 PM   #12
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Don't bother with the float height until it runs right at WOT. I think your main is too small and you're doing it right, MJ first.
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Old 11-20-2014, 08:23 AM   #13
cedric
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Many thanks Sir BigWheel! I think I'll wait a bit and see how it ends up for you before I buy a China carb.
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Old 11-22-2014, 06:00 PM   #14
k-moe
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Don't bother with the float height until it runs right at WOT. I think your main is too small and you're doing it right, MJ first.
If the float height isn't set properly it will never run well at WOT.

Mr. Bigwheel, Check to be sure that the float tang isn't getting caught on the float pin perch. Is the float needle clip oriented correctly so the needle does not get cockeyed when the float moves?
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Old 11-22-2014, 07:06 PM   #15
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Originally Posted by k-moe View Post
If the float height isn't set properly it will never run well at WOT.

Float height needs will change based on MJ size and will never run right at WOT until the right main is installed. Vicious circle, but the start of that circle is the MJ.
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