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Discussion Starter · #1 ·
Hi all,
I'm getting my ass handed to me on my daughter's bike.

Bike starts and idles great, but rough as a cob when I open the throttle.

TPS checks good

Carbs have been cleaned thoroughly. Several times. Stock jets. Float levels correct. No intake leaks. Good compression.

Valve timing makes me wonder, as the lines seem a couple of degrees off, but there's no adjustment for valve timing, as far as I can tell. (tensioners are good).

If I cover the oval ports (adjacent the main body inlet) with tape, it runs better, but not good.

This started as an intermittent problem, but now constant.

ANY pointers appreciated.

Thanks so much,

Chris
 

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If you partially close the choke, will it rev up better ?
Is the intake and pipes stock or has it been messed with ??
Is the idle speed up where it should be ??

Have you checked for vacuum leaks.....around the carbs, manifold and the vacuum lines.
Could even be sucking air through a cracked petcock diaphragm.
I assume you have drained the tank and fuel lines and put in fresh fuel ?

You can't ever clean a bike carb "thoroughly" because the low speed jets are tiny little passages that clog easily and are difficult or impossible to get to manually.

Berrymans B12 Chemtool is the best "consumer" carb cleaner you can get.
I suggest putting 2X the recommended amount in the gas and then letting it run until hot.
Let it sit overnight.
Repeat that cycle for a few days.

It is possible that you need new carbs........but I wouldn't go there just yet.
 

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Discussion Starter · #3 ·
Choke doesn't help. Everything is stock. Idle speed is where it should be, though I've tried higher and lower with no luck.
I capped the boots and pressurized to 10 PSI, and checked with soapy water: zero leakage.
I stripped the carbs completely and ran them through the ultrasonic cleaner twice.
I'll look into Berryman's.
I'm not convinced it's a carb issue, though. It started happening intermittently; would run like crap for an hour, then go back to 100% normal, like at the flick of a switch. But then it became permament.
I also get a cloud of fuel vapor above the carb inlets when I open the throttle, and the plugs are getting wet.
I notice the valve timing marks seem about 4° off, but see nowhere in the manual where valve timing can be adjusted. I even tried increasing valve-lash in the intakes, which seems to help a bit.
I'm an aircraft mechanic, but our carbs are simpler ;-)
Love your tag line; I agree completely, which is why I'm reaching out.
Appreciate the feedback :)
 

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I also get a cloud of fuel vapor above the carb inlets when I open the throttle, and the plugs are getting wet.
This has GOT to be a significant clue.

Do you have an automatic fuel shutoff petcock ?
Do you know how it works ?

It really sounds to me like your float(s) are not set right or the needle valve isn't closing, allowing the fuel level in the bowl(s) to be too high.
 

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Discussion Starter · #5 ·
This has GOT to be a significant clue.

Do you have an automatic fuel shutoff petcock ?
Do you know how it works ?

It really sounds to me like your float(s) are not set right or the needle valve isn't closing, allowing the fuel level in the bowl(s) to be too high.
Fuel tank is off; I'm supplying the pump from a funnel. Float level I set per page 196 of the manual I have.
I still get the impression that valve timing is off; like, the intake valves are still open WAY too long after the compression stroke starts. This would explain the fuel vapor cloud when I gun it. But then again, i've been around long enough to not believe everything I think :)
Which is why I SINCERELY appreciate your input.
My daughter laid the bike over a few months ago, and bought another to get to work. Somebody made a stupid U-turn, in the rain. Cop happened to see it. Not bad, just a little spill, but now she trusts dad when he always made her promise to ALWAYS wear a jacket. Yeah, in Miami. A little road rash can serve a valuable lesson ;) Anyway, I fixed the bike up, painted everything, detailed it so she could sell it, and now... well, I'm frustrated. Can't afford to put more money in it, and can't sell a bike that's not running right.
Again truly appreciate your input.
 

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What bike is this? You mentioned a fuel pump....has it been cleaned out...there should be a low pressure filter....similar to a tea bag that can be replaced on many pumps. The internal filters are supposed to be non serviciable but can be removed....wont go into that as its quiet involved but many have luck by reverse flushing the pump with fresh fuel.

I cant see the valve timing being affected by the drop....maybe a valve is sticking?...does the bike have a tip over sensor....long shot....

Its difficult to understand what you say about where the fuel vapour comes from...I know years ago I had a bike that would back fire fuel from the intake if the ignition timing was off...yours could be a misfire being caused by a very advanced timing issue. Also I assume this is this a CV setup...I would be checking the carb vacuum diaphragm for any pinholes etc....this would cause the carbs to fail to lift the slides effectivley or smoothly.

Check the springs are not tangled or cocked over causing the slides to hang closed...I always fit the spring to the top....into the groove then slowly lower the top onto the carb body while holding the slide in the up position.

One final thing check any small 'O' rings are in place under the caps...CV carbs often have a small 'O' ring in the air inlet circuit for cold starting.

Good luck
 

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I still get the impression that valve timing is off;
You need to GET OVER that obsession.
If the valve timing was off a significant amount, it wouldn't idle good either.

AND.....I don't remember ever setting a float height when I didn't pause for a second and think "Is that really what it means"......!!!
 

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Discussion Starter · #8 ·
You need to GET OVER that obsession.
If the valve timing was off a significant amount, it wouldn't idle good either.

AND.....I don't remember ever setting a float height when I didn't pause for a second and think "Is that really what it means"......!!!
Appreciate that; I'm happy to get over that obsession :)
So, recheck float levels?
 

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Discussion Starter · #9 ·
What bike is this? You mentioned a fuel pump....has it been cleaned out...there should be a low pressure filter....similar to a tea bag that can be replaced on many pumps. The internal filters are supposed to be non serviciable but can be removed....wont go into that as its quiet involved but many have luck by reverse flushing the pump with fresh fuel.

I cant see the valve timing being affected by the drop....maybe a valve is sticking?...does the bike have a tip over sensor....long shot....

Its difficult to understand what you say about where the fuel vapour comes from...I know years ago I had a bike that would back fire fuel from the intake if the ignition timing was off...yours could be a misfire being caused by a very advanced timing issue. Also I assume this is this a CV setup...I would be checking the carb vacuum diaphragm for any pinholes etc....this would cause the carbs to fail to lift the slides effectivley or smoothly.

Check the springs are not tangled or cocked over causing the slides to hang closed...I always fit the spring to the top....into the groove then slowly lower the top onto the carb body while holding the slide in the up position.

One final thing check any small 'O' rings are in place under the caps...CV carbs often have a small 'O' ring in the air inlet circuit for cold starting.

Good luck
Hey Steve, thanks again...
I replaced the fuel pump, just in case.
I don't think the bike has a tip-over sensor; searched the manual for "tip" and Sensor, and found nothing.
I'll check ignition timing again. Don't know what CV is.
The slides seem to open smoothly.
There is an enrichment circuit, but after going through THREE kits and rebuilds, and carefully looking at everything, I have yet to see a problem. Also, what are the odds of BOTH carbs going south at the same time? I could see if one of them was acting up, but, hell... both? At the same time? Intermittently, at first? Then suddenly both are fine again? Then back to sudden crap?
Still pulling my hair out, but timing may well be an issue. And, as i understand, it's controlled by the TPS. Which ohms-out according to spec, but, it's not vibrating on the bench, either ;)
Thanks again
 

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Right CV stands for constant vacuum....almost all bikes that have carbs will have CV ones. A vacuum is used to lift the slides which increases with the engine speed....more vacuum = higher slide... I think Er may be onto something...if the floats are too bigh then there certainly will be too much fuel delivery and operating the enricher will only make things worse.

It might only take one carb problem to make a twin refuse to run....one dead cylinder will drag the other one down slow enough to fail to fire.

good luck.
 

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Appreciate that; I'm happy to get over that obsession :)
So, recheck float levels?
Not only the float levels but the needle valves too.
Did you replace the floats or valves in any of your "rebuilds" ??

AND.....if you check the float levels and it appears that they are right......you need to consider that maybe you didn't fully understand the procedure. I say that because of personal experience.
 
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