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Setting mixtures on these demon carbs

Started by The Prophet of Doom, May 14, 2011, 11:08:35 PM

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The Prophet of Doom

I've been through every post here about setting mixtures on these carbs (highest RPM etc), None of this works for me...

Currently at 3 turns each, Idle dead stable at 1500 If I throttle on fast the bike dies, if I throttle on gently it drops, then picks up and runs strong.

I can screw the mixture screw in and out with NO effect on rpm or vacuum from about 1 turn to 5 turns.  Beyond that the bike suddenly dies.

I have freshly cleaned carbs, new correct jets, freshly sealed and tested YICS.  Fresh hoses & no leaks that I can find with the spray test.  I can start the bike cold, but not hot



Lucky

1982/3 XZ550 Touring Vison, Gold on Black

supervision

 Tight valves will start when cold and then because of the no clearance when warm, provide less compression, their fore refuse to start or idle poor.  I just put a 12,000 mile motor in my bike, there were 5 shims, that needed changing! 
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Cdnlouie

It sounds like a too lean condition when warm, attributable to vacuum leaks/poor vacuum in your system. If your cold start involves the choke and it starts okay cold, then you may have too much of a vacuum drain to start up warm.  This includes factors as mentioned such as compression and even a good battery and starter which tends to work better cold than warm, all which contribute to necessary vacuum for fuel/air delivery.  You can play with mixture screws forever under this scenario and not make any difference.

Do a compression test hot and cold and see if there is any significant difference for a start, so you know you have the engine fundamentals covered.

The Prophet of Doom

Thanks guys,
looks like I'm in for some work this weekend
I'll do compression test - may have to buy some gloves - I still have burn scars from last time.
I'll test float level also - did the height with the carb strip, but I know it's not the same as right side up.
And valves - I'm really trying to avoid this as PO (professional bike mechanic) claims he did it < 5,000km ago, though my faith in his abilities has eroded completely as I work on the bike.  If I don't get any joy I guess that's next.


The Prophet of Doom

And the diagnosis prize goes to....

Lucky

Thanks matey

I took off the overflow hose to test the float height and suddenly there was petrol (gas) everywhere.  That makes no sense it should be all over the floor..... f*****g bastard PO has plugged the overflow with a screw and some RTV !!!

No wonder this one has never gone quite right since I got it - I'm kind of amazed it ran at all



QBS

So, are you thinking bad float needle valve?

munkyfistfight

I'm having the same exact issue right now. I think I'll just replace all the vac hoses and do the float check before going any further. Good post.
Those who play by the book will always be beaten by those who write their own. -Travis Pastrana

The Prophet of Doom

QBS - Yes, if the float is buoyant, and the float valve is working, there is no other way to overflow the carbs that I can think of.

I can now get the carbs out, tops off and the bike running inside 45 minutes - all it needed was a sluice through with spray carb cleaner and some nice new overflow pipe.  Float levels are within 0.5mm of spec, and I measured the overflow pipe heights as I read that some were too short from factory.

I went to test compression, but found my meter broken in the box - another victim of the earthquakes we have been having - that will need to wait to another day.

Set Mixture to 3.5 turns front,  2.5 rear.  Re-synced.  I can still turn the mixture screws a full turn either way with no change in RPM, but at this setting the bike starts (nearly) first time warm or cold, and responds well to fast throttle opening with no bogging.  I think I'll leave the mixtures alone till I can get the bike out on the road - one last thing to fix, but that is the subject of another post




supervision

Their is a set screw next to each of the plastic fittings, those screws control draining of the carbs, tighten them the gas will stop coming out. in the. They are not overflows, they can be used to see fuel level, but it is not that necessary. closem
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visionbobber

every carb problem I've ever experienced was 70% due to bad float level and 30% dirty / untuned carburators or the float needles being clogged.  I do not have much experience with vaccum stuff except in classic cars with a vaccum advance on the 4 barrel carb..
quitters never win

The Prophet of Doom

Quote from: supervision on May 22, 2011, 10:48:29 PM
Their is a set screw next to each of the plastic fittings, those screws control draining of the carbs, tighten them the gas will stop coming out. in the. They are not overflows, they can be used to see fuel level, but it is not that necessary. closem
Supervision this is is WRONG
If that screw is open they will drain from the bottom of the float bowl and is used to drain, or check float heights.
If the screw is closed, they are still open to a brass column that stops about 10mm below the gasket.  If the float valve fails,  the bowls drain down this pipe rather than into your engine.

The pipes ARE necessary unless you enjoy gas spilling onto a nice hot engine.

supervision

 I stand corrected, Roro, I just blew air thru a closed set screw, and it was open !  My mistake.
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The Prophet of Doom

I should add that if the carbs ARE overflowing, and the overflow pipes are blocked, then suspect fuel in your oil.  Lucky (I think it was) posted on this ages ago, the float bowls fill up, over into your cylinder then leak past your rings.
Now I know first hand that this is the case.  $75 worth of castrol semi-synth and a brand new filter down the drain  :'(

QBS

You can probably save the filter.  It should be squeeky clean and gasoline evaporates.

jasonm.

ONe thing everyone seems to miss setting floats. The carbs MUST be PERFECTLY level. Thus checking them on the bike is NEVER accurate. They most be removed , carefully placed in a vise. ANd the phiysical level of the carbs checked in multiple directions. Then with a fuel source attached check fuel level per the manual. USe a LONG clear hose on the drains. As you can simuilate gas being used by simply lowering the drain hose lower and lower. All of the above gives accuracy. RORO,another thing...also sounds like your accel pump is dead. I had the same issue and mine was clogged.
looks aren't important, if she lets you play by your rules

musicweb

I recently installed a new accell pump diaphragm. Before I had adjustment on the front carb, now I can close the pilot screww all the way and nothing changes. The rear carb is about 4 turns out, and adjusts fine. Also get a slight gas smell when braking to a stop. Front float level is off maybe? I did not touch it at all. They both measure 37mm.
1982 XZ550 Vision
Most upgrades done... and a large pizza paint job!

jasonm.

#17
the spec on float level is 20mm below carb gasket(lower edge) +/- 1mm. Me, I set mine closer to the 21mm as the needles and seats wear and the fuel does come out the over flow on hard stops. So maybe I am at 22mm to lessen the fuel issue. I do smell gas on warm days for a second when stopped as the tank vent is crude and gas simply is evaporating out the vent at the cap. But if you say adjusting a screw does nothing. Hey, there is an o-ring around that mixture screw. They do dry out and mess up your settings a bit. It's a common o-ring for mikuni carbs. Dirt bike shop by me stocks them. Also come in kits from Sirius Canada. There is a tiny washer and spring with it. Don't lose it should you attempt to get that o-ring changed.  When changing put them on the screw...spring, washer, lastly> o-ring by tip.  LAstly .. no change at all means someting is plugged (gas jet)or leaking badly (carb gasket maybe). AKA- you  are running mostly on one cylinder.  I suggest you remove the poilot gas jet on the carb that does not adjust. It happened to me. The bike did die at stops due to this. But having my bike almsot 20 years...I knew exact what was happening. If you are new to this bike. You may not know how it's "SUPPOSE" to run. Mine idles like a swiss watch. And runs glass smooth to 80+ mph constant running on the hi-way Gets 50+ mpg. I've ridden others that are vibrators at this speed. I got both my 83 and 82 to run like this. So anything's possible...
looks aren't important, if she lets you play by your rules

musicweb

Clarifying.... my float level on a level bike is about 22mm also. The level with the carbs off and top at an angle is about 36-37mm. I also recently changed to Iridium plugs and get some surging at low speeds, but I think it's still a carb issue too. The carbs were off all winter, waiting to put the pump in and they were drained. Maybe some crud formed from sitting? I may just take them off and dip them again. I have a makeshift ultrasonic dipping setup with Berrymans.
1982 XZ550 Vision
Most upgrades done... and a large pizza paint job!

QBS

Ultrasonic with Berrymans sounds like the ultimate carb cleaner.