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School me on 4th gen carb adjustment screws please!


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I've been reading multiple threads from here and other forums and to be honest I'm confused as hell. Regarding pilot screws the manual states 1 3/8 turn but when reading forums they talk about mixture screws at 4 turns and pilot and mixture screws at 2 turns and idle screws....etc. So I need some help and clarification please! My 94 is currently sitting with the tank off and air box was full of drilled holes and found writing on the frame saying "mix 1 1/2 origin". It runs incredibly rich (plugs are black) and is very hard to start when engine is cold in cooler weather with choke on full. You can smell how ridiculously rich it is. Even when moving slowly slightly above idle I can get a whiff with the wind of how rich it is. 

 

As I understand it:

 

Pilot screw. 4 of them. Control the idle and just off idle rich/lean. Can be accessed somewhere on the side with the side panels off and can be adjusted while bike is running and tank on. Factory setting of 1 3/8ths turns on my 94 and being in Canada they should be a Phillips head and not the D. 

 

Idle speed screw. I know where it is and what it does.

 

throttle (plate) adjustment screw on each carb which I was told is for syncing the carbs. Maybe people regard this as a mixture screw? I won't be touching them... where ever they are.

 

Startup enrichment screw. Manual mentions it but doesn't point out where or how many or how many turns it should be at.

 

Idle mixture screw. I have seen threads on here mention idle mixture screws separate from the pilot screws. Which are supposed to be for fine tuning as they explain it. I know nothing about this.

 

Main mixture screw. I know nothing about this other than its not the pilot and people said they had it at 4 turns.

 

 

 

As you can see I am confused as heck. I have my bike open at a friends place and ready to check the settings for idle and start up. My carb knowledge goes as far as a single 4 barrel and swapping primaries and secondaries, tuning the idle circuit for best vac, setting idle speed and the slightly more difficult accelerator pump tuning. All a lot easier than 4 carbs with multiple types of adjustments that I get mixed information on.

 

If I could get some clarification I would be most grateful!!!

 

 

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I'm no carb expert but am familiar with my ST1100 carbs which are similar to the VFR. 

 

The pilot screws adjust fuel mixture at idle and low throttle openings by metering the fuel flow through the low speed jet circuit. The jets (accessible from the float bowl) must be clean before anything else works. The pilot screws are needle jets so you seat them gently, then count out the number of rotations in the manual. There is usually an "idle-drop" method for fine tuning these with the engine running, but the factory starting point should be close.

 

I doubt the fuel enrichment circuit has any adjustment, these are usually just a plunger-type device on each carb that allows more fuel to flow when you move the choke lever on the bar. As long as they are all closed when the choke is off, and pull open evenly when you work the lever, they should be Ok. Really rich running could be a sign that the enrichment circuit is not closing e.g. seized cable.

 

There are carb balancing screws in the linkages that open the butterflies. One butterfly is fixed to the throttle cable drum, the other three work off this with linkages, and with adjusting screws/springs. The idea is to adjust the other three so the same intake vacuum is generated in each bore. If these are wildly out of whack, it is possible to do a "bench synch" before installing the carbs which is visually setting the degree of opening the same e.g. with feeler gauges, but a vacuum synch with the engine running is best.

 

The idle adjuster screw bears on the throttle cable drum and sets the stop point for this; moving the screw in clockwise opens the throttles and increases the idle speed.

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The screws you speak of are behind a plug put there by the factory to keep you from adjusting them. You can pull the plugs by drilling them and using a hook tool to prise it out (x4).  Carbs are different, but a rule of thumb is if the screw is nearer the airbox, it meters air and if it is closer to the engine, it meters fuel.  It is only a fine tuning for idle and a tiny bit of just off idle.

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Have a look at all plugs to see which ones are running right (tan) or not.  Maybe you can localize a problem to one cylinder's carb ?

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The circled screw is the pilot screw. 

 

If all the plugs are black, what is your air filter like? Blocked filter = rich running. 

Untitled.png

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The filter looked new ( I replaced it with a new filter anyways). I'll take a pic of the plugs and throw them up.

 

Ok thanks, I thought it was that but it didn't seem very accessible even if thumbscrews were on them

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It didn't idle or cruise lightly before storage. it was -10 when I stored it so I rode hard to where I store it. and shut it off as soon as I hit the drive way. Idle/just off idle is where is smells super rich and has problems starting. 

 

Also I checked the choke and everything seems to be moving smoothly. I lubed the cable and moving parts anyways since I was there.

20180304_195632.jpg

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Those plugs all look OK !!  The blackness  at the plug threads is carbon build-up and normal.  judging from the tan color on both the center and side electrodes, you're fuel mixture is  about right, maybe a 1/4 turn too rich  :happy:  Those pugs were firing OK, but color might be improved with tuning.  second from right needs help, while #3 looks best.

 

show another plug photo after you've ridden her HARD  for a few miles  and got her HOT    :491:

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Hmmm then why does it smell so rich at idle and have such a hard time starting when it's really cold?

 

I rode pretty hard to my storage (it was -10 out) and didn't do any light cruising or idling, as soon as I hit the driveway I killed it so wouldn't the colour reflect the main jet and not show if it is too rich at idle/just off idle? Which would also affect startup? 

 

Does the '94 have a catalytic converter? Maybe the PO hogged or cut it out? I know my carb'd mustangs smelled pretty strong after I removed or hogged out the cats.

 

When you say the mixture is about right maybe a 1/4 turn too rich. Isn't the only adjustment for fuel mixture on the pilot jet or slow jet? Which is for idle/just off idle/startup which I didn't do much of that on my way to storage so the plugs currently reflect the main jet which could mean there might be a jet kit installed as I read many times on here that the 750 VFR is very very lean from the factory. I think I made sense? lol

 

If there is a jet kit installed it looks like the mains are fine or (unlikely) do plugs look like that with factory jets? 

 

Which way do you screw the pilot screws for lean/rich?

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fuel idle mix screw turns ccw for richer, cw leaner.

if you do have a jetkit, maybe lower the needles one clip.  poor starting cold could be plugged choke circuit pickup tubes.

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Alright well I will wait until I clean the carbs later this spring. Probably in april. I can then check to see if it has a jet kit and should fix any plugged ports in the carbs. I will then reset the pilot screws to 1 3/8 out. 

 

If it has a jet kit should I go back to stock jets? I think the stock jets are 130 main and 40 pilot, maybe anything bigger than 132 main and 42 pilot I should replace? I'm not sure what a good jet size is for a near stock vfr 750.

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On 3/4/2018 at 6:09 PM, NateV8 said:

It didn't idle or cruise lightly before storage. it was -10 when I stored it so I rode hard to where I store it. and shut it off as soon as I hit the drive way. Idle/just off idle is where is smells super rich and has problems starting. 

 

Also I checked the choke and everything seems to be moving smoothly. I lubed the cable and moving parts anyways since I was there.

20180304_195632.jpg

Hi...  I've battled many a carb especially living at 6000', and those plugs look too rich.  Now, WHERE it is running rich is the crux here.   I would check to see that they are currently running the OEM stock jets...all the way around..including setting the idle/slow circuit air bleed needle.  You need a baseline to start with...factory setting is best, and we can work from there.  

 

Buy some new plugs so we can see what the new/factory setting is doing....plugs are going to be the major tuning indicator if we can not use a o2 sensor.   When set currently the plugs should be a light tan on the top of insulator and get darker moving further down. You need to get it hot also, complete combustion,  one thing to do is go find a clear road, run it WOT and shut it down.   Then pull the plugs..somehow, I know its hard,  and check the bottom of the insulator, the color will tell you what the main jet is doing.   Should be a light to medium tan color.  Real Dark or black the main is too rich, which will affect all throttle positions.  This has to be done on NEW plugs.  With our base line set, we can start fine tuning.  

 

Also remember to always use anti-seize on your plugs, just a tad, this will keep them from being tight/hard to get off...your going to be pulling them a lot.

 

 

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Yes they are definitely rich and the deposits on them don't help.

 

I just installed new plugs. While running WOT and pulling plugs is great for tuning WOT, I'm having problems with the slow circuit/pilot jet, so idling and just off idle.

 

I agree, when I find out what jets are installed it will be easier to figure out what to do from there. Since the bike is mainly stock it can only only take a very minor increase in jetting sizes while still being able to be fine tuned. I'm taking the carbs off in april and going to clean them, check jets and reset the pilot screws. Bike is in storage right now and I can't run it on the road until after march 15th and roads need to be dry and clear also. Anything higher than a #132 main I will change and anything higher than the stock pilot (#40) I will change.  Honda tunes the bikes very well so seeing my bike is near stock it shouldn't take much to get it dialed back in with stock jets.

 

It isn't the best excuse but I don't have the time or place to do constant WOT runs and check plugs. Plus stopping to pull plugs on a bike seems very annoying, on my mustangs it was 5 minutes, on a bike I don't even think it's possible to do in under 30min without burns. I mean I have to move the fan/rad to get at the front plugs which would be instant burn state from WOT runs and removing the tank requires removing all the fairings, at best I could leave them off and then have to DC all the fuel lines to remove tank on the side of the road. No thanks lol

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All this talk of plug chops is great, but the smallest, easiest to block jets are the low speed jets which also feed fuel to the pilot screws. There's not much point playing with the screws unless the jets are clean. 

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Yep, that's why I said I will wait until I clean the carbs and confirm jets and pilot screw settings. 

 

Question though, if jets and such get blocked wouldn't that cause a lean condition?

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9 hours ago, Terry said:

All this talk of plug chops is great, but the smallest, easiest to block jets are the low speed jets which also feed fuel to the pilot screws. There's not much point playing with the screws unless the jets are clean. 

Yup...gotta have clean ones.   I just picked up two 98 cbr600 f3 SE smoken joe, super low mileage bikes that have sat for about three years.  Owners did nothing to store them.  So I'm having to go through everything fuel and electric. AND to ad insult to injury, he tried to get it running to show it and wired the plugs wrong.  They are nice bikes, near new condition, never down, super clean, just sat too long.....number one thing that kills all machinery...  Got both for a total of $2000.00 plus two sets of matching leathers, a third Honda Line jacket.   

 

 

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8 hours ago, NateV8 said:

Yep, that's why I said I will wait until I clean the carbs and confirm jets and pilot screw settings. 

 

Question though, if jets and such get blocked wouldn't that cause a lean condition?

 

Roger that; but remember your not only tuning WOT, on those runs, your tuning the main jet which affects ALL throttle positions, including off idle, this is the only way to get it right, unless you use a O2 sensor.  Yeah, doing plug chops suck..but it really is the only way to really tell the main jet.  Idle jets are easier to deal with on fine tuning.  And I really dislike CV carbs, they are a real pain, but that's all we got on Viffers.  My inline fours I always swap for sliders.   We are here to help, when you get it back up.

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Well 'm not cleaning them in my friends garage so I just have to throw the tank and panels back on and hopefully I didn't fark anything up while I was in there. Gotta wait for the snow to take a hike and then bring it over to my place. 

 

How do you adjust those pilot screws while the bike is running though? some kind of rubber ring (carb boot?) is fully blocking the rear ones on each side.

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you can make your own (the angled bit from Ebay and the long rod to your liking or -like I did- buy one complete

 

https://www.autogereedschap.com/a-25359823/afstel-gereedschap/haakse-carburateur-schroevendraaier-laser-tools/

 

and you still want to wear mechanic gloves as not to burn your hands on the hot engine....

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i don't think it's possible to reach the (rider's) right rear or front carb's mixture screw with carbs installed on the bike.....but i could be wrong. :sleep:  Cooling  hoses are in the way, it seems.

 

with holes in the airbox top it's almost certain you have a jetkit.  read the tuning instructions online for dialing-in jetting. mixture screws are usually set at about 2 1/2 turns out if you have stock idle jets.

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  • 2 weeks later...
On 07/03/2018 at 1:25 AM, 577nitroexpress said:

Hi...  I've battled many a carb especially living at 6000', and those plugs look too rich.  Now, WHERE it is running rich is the crux here.   I would check to see that they are currently running the OEM stock jets...all the way around..including setting the idle/slow circuit air bleed needle.  You need a baseline to start with...factory setting is best, and we can work from there.  

 

Buy some new plugs so we can see what the new/factory setting is doing....plugs are going to be the major tuning indicator if we can not use a o2 sensor.   When set currently the plugs should be a light tan on the top of insulator and get darker moving further down. You need to get it hot also, complete combustion,  one thing to do is go find a clear road, run it WOT and shut it down.   Then pull the plugs..somehow, I know its hard,  and check the bottom of the insulator, the color will tell you what the main jet is doing.   Should be a light to medium tan color.  Real Dark or black the main is too rich, which will affect all throttle positions.  This has to be done on NEW plugs.  With our base line set, we can start fine tuning.  

 

Also remember to always use anti-seize on your plugs, just a tad, this will keep them from being tight/hard to get off...your going to be pulling them a lot.

 

 

my plugs in my 1997 RC36 look just like that. yes it smells rich at idle but been carbed i expect that, open the throtle and it clears up immeaditly. allso i am at sea level and your at altatude maybe its the thinner oxygen levels?

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