Help with Jetting PLEASE!!

[sudds]

Hello, As you mite tell i am new to jetting and to be honest i dont have the first clue. My problem is as follows when i pull the throttle back on my Suzuki Rm80 it begins to swallow kind of flood unless i pull the throttle back a little then a little more, I have just refitted the factory air filter as a mechanic friend said a K&N air filter could be causing it but the problem seems worse now. I was told it may be the mixture screw but i dont know where that is as i have never played with jets before or the mixture screw. How do i get at the jets/screw? do i remove the float bowl?
Thanks alot,
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[simon1221]

Have you cleaned air filter, carb etc?
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DNA50 (sorry) ->> TZR125l >> GPZ500S

[bish777]

remove the spark plug and tell us what the firing end looks like before you start twiddling.
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GSXR1100J

[Kickstart]

Hi

Chances are that it is running a touch lean at certain throttle openings.

Were any changes done to the jetting when the K&N was fitted?

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[sudds]

I have cleaned the air filter a few times, when the K&N was fitted it ran about the same maybe a bit worse. The guy i baught the bike from said it was a new plug, it looks ok to me but i do have a spare new one i was going to keep until i had sorted out all the propblems with the bike and jetting was the last one. Any ideas? Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[sudds]

Oh i just went on the bike and messed with a screw i think is the mixture screw which was located on then end of the carb near the air intake but it didnt have much affect so i returned it to its normal position but it seems the more i open the throttle the worse it get unless i open it very gradually.
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Kickstart]

Hi

I would say the most likely thing is that the mixture is leaning off as you open the throttle. As such check the needle position (think the RM is intended for competition work, so the needle position is probably adjustable), as this is what is controlling the mixture most of the time once you open the throttle. Also check for air leaks around the inlet after the carb.

All the best

Keith
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[sudds]

Hello,
I can move the needle, Would you say up or down before i go and do it? Iv herd that playing with the jetting of your bike can size your engine is it true?
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Kickstart]

Hi

Yes playing with the jetting can seize the engine, or help it burn a hole in the piston. Generally if you go too lean. Going too rich is generally fairly safe (although go far too rich and you will foul plugs and possibly cause more wear by washing the oil off the bore).

If you take the needle out of the carb you will probably find a clip at the top with half a dozen notches which it can sit it. If you move the clip down to a notch nearer the point of the needle the it will richen the mixture.

All the best

Keith
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[Dom_]

Tell us the colour of the end of your spark plug.

[sudds]

When i removed the needle it was on the top notch, (There was 5) I moved it to the middle and rebuilt the carb started and warmed up the bike and test rode it, it was alot better but no where near right so i removed the needle again and moved it down another notch and did the same with the same effect better but not right so i did it again and it is now better but not right and it is on the lowest notch. Where do i go from here? As for the plug last time i looked it was brown i think but ill check again tomorrow.
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Kickstart]

Hi

Sounds like it is definatly lean. I would start looking for air leaks, including checking the crank seals.

Hasn't started using gearbox oil has it?

All the best

Keith
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[Fatgreek]

How long have you had the bike and was it boggin out when you got it
The most common cause is air getting in where it shouldn't ergo: check the clips are tight around the carb, Turn mixture screw all the way in and then out by 1&1/2 turns The mixture screw should be adjusted in 1/4 turn increments after that.

It's difficult to diagnose a problem properly without hearing it.

If you have a crank seal gone you will have loads of smoke and unburned oil from the exhaust.

Best of luck M8
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Enjoy your sport but stay out of casualty
current bikes: EC250 YZ250 RM250

[sudds]

Its not a crank seal and the gearbox oil is fine i have checked it a few times, I am not totally sure where the mixture screw is on this carb but i think it is the screw at the back of the carb to the right of the air intake??? Any ideas? I will go check for an air leak now.
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[sudds]

Could this be the mixture screw? I removed it is the end supposd to be flat or a point?
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[sudds]

I also just removed the spark plug and its very black here is a pic, it is blacker than the pic shows.
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Odie]

Follow steps in order....First, dial in:
1. Top end (full throttle / 7.5k to redline - Best Main Jet be selected before starting step 2!
Select Best Main Jet
To get the best, most even top end power (full throttle/after 7500 rpm), select the main jet that produces the highest top speed / pulls hardest at high rpm.
If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the main jet is too large. Install a smaller main jet and retest until you find the main jet that pulls the hardest at high rpm when fully warmed up. This must be done first - before moving on to the other tuning ranges.
If the bike doesn't pull well at high rpm when cold and gets only slightly better when fully warmed up, the main jet is too small.
In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
Do not pay too much attention to the low-end richness when you are changing main jets - you still need to be using the main jets that produce the best power at high rpm. You will deal with the low-end / cruise later - after step 2.
2. Midrange (full throttle /5k-7k)
Select best needle clip position
To get the best power at full throttle / 5k-7k rpm, after you have already selected the best main jet,
If the engine pulls better on a full throttle roll-on starting at <3k, when cool but soft when at full operating temperature, it is too rich in the midrange and the needle should be lowered.
If the engine pulls better when fully warmed up but still not great between 5k-7k, try raising the needle to richen 5k-7k.
If the engine pulls equally well between 5k-7k when cooler as compared to fully warmed up, the needle height is probably properly set.
Do not pay too much attention to the low-end richness when you are changing needle clip positions - you still need to be using the clip position that produces the best full throttle / 5k-7k power in conjunction with the main jets that produce the best power at high rpm. You will deal with the low-end / cruise next.
3. Low end (full throttle / 2k-3k)
Float height (AKA fuel level & how to..)
To get best low-end power, set float height (fuel level) so that the engine will accept full throttle, without missing or stumbling, in 2nd gear from 2.5k to 3k rpm at minimum.
Float heights, unless otherwise specified in the installation guide, are measured from the "gasket surface" of the carb body to the highest part of the top of the float - with the float tang touching but not compressing the float valve spring.
If the engine has a "wet" rhythmic, soggy area at full throttle / 3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by resetting the float height 1mm greater (if the original was 13mm - go to 14mm). This will lower the fuel level, making full throttle / 2k-3k rpm leaner.
If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level.
Example: change float height from 15mm to 14mm to richen up that area.
REMEMBER, since the main jet WILL affect low speed operation, the MAIN JET has to be within 1 or 2 sizes of correct before final float setting.
Warning: If the engine is left with the fuel level too high,, the engine may foul plugs on the street and will be "soft" and boggy at part throttle operation. Adjust Floats to raise/ lower the Fuel Level.
Base settings are usually given if a particular application has a history of fuel level criticalness. The Fuel level height in the float bowl affects full throttle/low rpm and, also, richness or leanness at cruise/low rpm.
Reference: a bike that runs cleanly at small throttle openings when cold, but starts to show signs of richness as it heats up to full operating temperature, will usually be leaned out enough to be correct if the fuel level is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS EXTREMELY IMPORTANT!!!
If there are low-end richness problems, even after lowering the fuel level much more than 1.5mm from our initial settings, also check for needle wear and needle jet (part of the emulsion tube). See Worn Needle and Worn Needle Jet diagram. It is VERY common for the brass needle jets (in the top of the "emulsion tube") in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000 miles. Check them for "oblong" wear - the needle jet orifice starts out round! Factory Pro produces stock replacement needle jets / emulsion tubes for 36mm and 38mm Mikuni carbs.
4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
There is usually a machined brass or aluminum cap over the fuel screws on all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs use a special "D" shaped driver, usually supplied in the carb recal kit.
Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise operation. Set mixture screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle high rpm operation.
Pilot fuel mixture screw settings, float level AND pilot jet size are the primary sources of mixture delivery during 4000 rpm steady state cruise operation.
If lean surging is encountered, richen mixture screws (turn out) in 1/2 turn increments. Alternative pilot jets are supplied when normally required.
Pilot fuel mixture screw settings, float level and pilot jet size also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause surging problems when the engine is operated at high rpm at small throttle openings! Opening the mixture screws and/or increasing pilot jet size will usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm drops below the set idle speed, then rises up to the set idle speed, the low speed mixture screws are probably set too rich: try 1/2 turn in, to lean the idle mixture.
NOTE: A lean problem gets better as the engine heats up.
If the throttle is lightly "blipped" at idle, and the rpm "hangs up" before dropping to the set idle speed, and there are no intake leaks and the idle speed is set at less than 1000 rpm, the mixture screws are probably too lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks and the idle speed is set at less than 1000 rpm
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[bish777]

set it back to original and remove your air filter. sound like thats clogged.
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GSXR1100J

[sudds]

There are no air leeks and from what i have read i think i have the wrong main yet, Any ideas on where i go from here?
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Kickstart]

Hi

You bike can be persuaded to go to high revs, which suggests the main jet is about right. However opening the throttle hard will initially lean the mixture out (part of the reason why some 4 strokes with slide carbs have accelerator pumps, to squirt extra fuel in at this stage to richen the mixture).

Sounds like a low end problem to me.

Playing with the needle has improved the problem, but has also made it run rich (judging by the state of the plug) much of the time.

Time to check the float height I think.

All the best

Keith
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[VFR400UK]

I had a similar problem with my bike. It ran lean allthe time, i put in a bigger main jet which helped top end but mid range was useless. I replaced the needles and it ran rich top end and mid range and lean anything below that.

I adjusted my mixture screws and from then on it ran rich throughout the rev range. i went down 2 sizes of main jet and replace the needles with my originals and it ran perfect.

[Trixie]

Your plugs look like the mixture is running rich. I'd say that if there was a K&N air filter on the bike, then whoever fitted it may have upjetted to compensate for the increased air intake. Now you've got the standard filter back on, there's too much fuel and not enough air, hence the plug colour. The engine could be flooding when you open the throttle.

My suggestion is to get a workshop manual (or failing that a Haynes Book of Lies) and take everything back to standard. Then go from there.
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Dead nobodies in company cars...

[sudds]

It was running like it is now with the K&N filter on, Could i not move the needle higher up? And I think the engine is flooding when i open the throttle. Or do you recommend i put the needle back to standard and then move on the the other jets?
Thanks Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[sudds]

What do i do with the float hight?And what exactly is the float hight?
Thanks
Sudds
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Aprillia Rx50 2000 model race tuned, Aprillia RX50 Racing Late 2004 Model and Suzuki RM 80 1989 model.

[Kickstart]

Hi

The float controls the level of fuel in the float bowl, by cutting off the fuel flow into the float bowl when a particular level is reached. You adjust the float height to change this level.

If you take off the float bowl you will find the floats. Probably between them there will be a small metal tag with a small shaft attached to it. The shaft will be pointed at one end (this is the needle valve). You adjust the float height (on most carbs) by bending the tag a tiny amount, which lifts the needle valves position relative to the floats.

Note that is the needle valve which is totally seperate the the needle jet you have already played with.

You will need to check with a workshop manual to find out what the float height should be for your particular model of carb.

To a large extend the jets effects overlap on each other. So fiddling with the needle jet does (to a limited extent) affect the running at lower revs where it is mainly dependent on the air screw / mixture screw.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing