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Fuel screw settings

  • Thread starter Thread starter tpetersen
  • Start date Start date
T

tpetersen

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Just out of curiosity why is there so many different start settings listed for the start of your fuel screw. I have seen 1.5 on the Mikunis site, 2 and then 2.5. Seems like a big difference. Is location a possible issue?
 
I've never seen any published data as to where the screws should be set. You found data on "the Mikunis site"? Please post a link so we can take a look at this.

My understanding is Suzuki adjusted the screws while the exhaust emissions were monitored and then caps were pressed over the screws to discourage tampering. The adjustment settings were not necessarily optimized for running, they are a compromise taking emissions into consideration.

Different markets in the world had different emissions requirements, and different carb jetting, so it stands to reason the mixture screw adjustment varied as well.

As for what you should do, please check the carb rebuild tutorial linked in my signature and then go from there. Inside the tutorial there is info on how to tune the screws - starting point is approx. 2.5 turns from lightly seated.

The running effect of tweaking these screws is minor; they only effect the idle mixture and just off idle. Some newbies think these screws have a profound effect on mixture but that just isn't true.

Oh, and while you are at it please check the Newbie Mistakes thread so you can be aware of some common pitfalls.

Good luck
 
I aree with Mr. Nessism, but with one minor change. :-k

When I rebuild a set of carbs, I start with the screws out 3 full turns, instead of 2.5 turns. I have had some carbs that simply did not run right at first until I opened up the screws a bit, so I just start with them open farther now.

The general procedure is to start the engine, warm it up, then slowly adjust the mixture screw until the engine is running its happiest. This usually involves richening the mixture to see if there is an increase in idle speed, then leaning it down to the point just before there is a drop in speed. When you detect a drop, back up about 1/8 turn. Repeat for each carb.

I have found it easier to just start with the mixture screws out in the "rich" position. Not only does it skip the first step in the fine-tuning, it helps the bike start easier with the richer mixture.

Note that the final setting will probably be in the 2 to 2.5 turn range, but you won't know until you tweak the screws and find out.

By the way, they are "mixture" screws, not "fuel" screws, if you are talking about the carbs on your 850.

The VM carbs on earlier bikes ('79 and older) did have fuel screws (and air screws), and had entirely different settings.

.
 
When looking for information I was directed here at www.IWT.com.au/mikunicarb.html

Anyway I think I will just take your advice start at 2.5 and adjust up. I think I am good at 2.75 across the board. Maybe a slight hesitation at start but very little backfire at throttle down except between 3k and 4k. I am probably looking for too perfect. I should just enjoy the ride. Seems a little rich is better than a little lean.

Just finished cleaning the pipes and so far no new bluing so maybe I am on the right path.

You guys are great.
 
tpetersen said:
When looking for information I was directed here at www.IWT.com.au/mikunicarb.html

Anyway I think I will just take your advice start at 2.5 and adjust up. I think I am good at 2.75 across the board. Maybe a slight hesitation at start but very little backfire at throttle down except between 3k and 4k. I am probably looking for too perfect. .
Click to expand...

Do the highest idle procedure on Cliff's site. Perfect is good. And easy to get.
 
I may need to redo the vac sync. Getting #2 to stay set was difficult due to the cable bracket being in the way. Will retry but good to ride for now. No change in idle above 2.75 so will leave them there for now.
 
tpetersen said:
Maybe a slight hesitation at start but very little backfire at throttle down except between 3k and 4k. I am probably looking for too perfect. I should just enjoy the ride. Seems a little rich is better than a little lean.

Just finished cleaning the pipes and so far no new bluing so maybe I am on the right path.
Click to expand...

At any throttle position above idle, the overall mixture is not affected by the idle mixture screws anymore. Depending on throttle opening, the needle and main jet determines if the mixture is lean or rich.

Plenty of good info posted on GSR on how to obtain correct mixture at different throttle openings after doing plug chops.
 
Cough out of idle

Cough out of idle

Carbs seem pretty good. Should the bike cough almost like it floods when blipping the throttle? This will happen only when the bike is really warmed up. I can smell fuel then too. I am assuming things are slightly rich at idle and the mixture screws need to be backed down. They are now set at 2.75. Engine sounds good, has that growl when going up the gears with no issues. Am I on the right track to back down the screws or is something else going on that I smell fuel. Nothing in the oil and no spilling, just hot fuel.
 
mixture screws .....

mixture screws .....

i,ve just stumbled on this now its been good reading and wourth remembering regards oldgrumpy :)
 
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