Carb jetting

S

stmike

New member
Location
SoCal
Today another forum member and I took the tank off to synchronize the carburetors on my '84 RGS 1000, and discovered that the previous owner had set the choke to be on so the bike would run richer due to the fact that there is a set of Wolfgang's free flowing stainless steel exhausts on the bike.  When we fixed the choke cables to have some play, the bike then ran quite lean, and when we took off the float bowls to check the jets it was  discovered that the jets were stock (which is probably why the P.O. set up the choke cables that way).  Now I want to change the jets to compensate, so I'll need to go up on the mains and the pilots which are 108's and 65's respectively.
I live in Los Angeles which is pretty much at sea level, but ride occasionally in our mountains at around 7,000 ft.  Do any of you with a similar setup have any recommendations for the sizes I might need?  Or should I just buy a large variety of jets and experiment?  Also, does anyone have any experiences buying jets at a reasonable price from an online merchant?
Thanks in advance for your reply.
 
There really isn't enough information to make a suggested change. Slide, atomiser, needle and position and float settings all need to be taken into perspective. How do you know it is lean. If plug chop are they the right heat range?

Is it running lean at all throttle openings?

If it is running lean and all components are standard why don't you just raise the needle a notch and see how it flies?

My experience of buying second hand bikes that have been re-jetted has shown that they are all running too rich. Whilst giving the impression of more power the unburnt fuel can wash the bores leading to premature wear. If in doubt I always head back to recommended settings. I usually find a good result not too far away.

Your experience may differ.
 
I agree, typically idle setting does not much differ between different exhausts and can be adjusted with the idle mixture screw - at least to a certain level. If you need to put the screw 3 turns out, you should change jets. But first of all make sure the float-hight of each carb is set correct to 18,5mm. Then test the idle circuit and start with the needle one position higher (should be center now, so put the clip on the lowest position). If bike runs open/open (no airbox, open exhaust), it might need larger mains, something like 112 - 118. If engine is tuned any further (large head with 36mm inlets, 41mm inlet valve) it might need even 125, but for sure not larger.

But please first start on the float settings, then adjust idle and go from there. If it feels powerless in midrange, raise the needle and go further up. Be careful not to ride it to top revs as long as your settings are not correct. I burned an inlet valve due to lean mixture in just 5 km... :-[

HTH
 
Sounds like the previous owner didn't have a clue what he was doing. I generaly find RGS's to run nice with a 60 pilot jet in them. As stated in the other posts pull the carbs off and check float levels first.
Make sure your ignition system is working correctly before you start messuing around with carburettor settings, you would be amazed how many "carb problems" are fixed with a new set of plugs and caps.



  Keith
 
...and correct ignition timing... don't trust the factory marks btw., you should check TDC with a degree-wheel and a piston stop...

Not that I have anywhere near Keith's or Piet's experience, but on all bikes that I bought I found silly settings, "interesting" modifications and partly damages that you would not believe someone could be as stupid as to do that...  :o Remember one visit to the local Laverda experts (OCT) who showed me a tooth-brush (!!!) they found in one engine... Not sure what the DPO tried to achieve with that, but it must have dropped into the engine from the oil-filler hole, as else the engine was unopened...  :o No joke (or they fooled me, but I doubt they did).

That's why typically I check EVERYTHING possible without stripping the engine completely, make a general master service (all chains exchanged, all chain tensioner reviewed and if necessary exchanged, new plugs, new oil, carbs completely overhauled, wheel-, headstock- and swingarm bearings checked and replaced, electric checked and where necessary plugs exchanged etc. etc.). The result is that I only ever had one (!) time in 20+ years of ownership where I had the bike stop working completely, and that was 100% my fault (carbs not properly installed and they blew off the rubber mounts, which got to soft by the fuel that sprayed over them so that the carbs fell off just after some miles again...). Fixed it with some more solid mounts and never had an issue ever again.

 
Correct float heights(18-18.5mm), 112-115 mains and keeping the rest stock usually works quite well for all manner of slight mods.  Keep the needle at the stock position for starters, but do make sure needles and needle jets are in good condition, if in doubt, replace.  Worn items can throw the jetting way off!

The occasional blast up the hill won't hurt the bike, only performance, as the mixture will enrichen slightly.  If you jet it for top performance at higher altitudes you will need to take it easy at sea level.

piet
 
Thanks much for the replies, as I appreciate the expertise of the list.  Prior to rejetting I'll check the float heights and needles.  If all is good there, I'll likely be back to this topic with more queries.
 
Higher float (less than 18mm distance between seal-surface and lowest point of the float) => richer at low end, risk of overflowing carbs, might result in petrol running into the inlet and wash the oil off the cylinder wall => higher wear, petrol in the sump, steaming out of the crank breather etc. etc. (only if massively to high or if the float valve does not close)

lower float (more than 18,5mm)=> leaner at low end, potential risk of being to lean all over the range with a high risk to run out of fuel at top speed.

that is how I understand the principals, but I may be wrong here.
 
gibbo-123 said:
Reading the previous posts with interest. What effect does the having the wrong or disimilar float heights give?
Click to expand...

The float height governs the fuel level in the bowls, which in turn has a direct influence on the mixture produced by the jets that are fitted.

The higher the fuel stands in the bowls, the less vacuum is necessary to draw fuel upwards through the main jet and atomiser.  The differences may be minute, but the effects can be horrendous!

Especially the 120? engines tend to wreck the float valves quite quickly.  These wear away the spring-loaded piston stop and let the valves "grow", effectively lowering fuel level and causing the mixture to lean out.  OK if the engine isn't worked too hard, but give it the berries and you're soon looking at burnt valves or a holed piston!

piet
 
With Headers I doubt if you will have to change from stock pilots but may require something like 112 or 114 for the mains. I had to change out my 750SF Delorto mains from 125 to 130's when I installed 2 into 1 pipes as it would lean out on the top end no matter where I set the needle height.
 
What main jet changes would be needed if:

(a) The standard collector (small bore) and silencers are replaced with a big bore collector and open Jota cans?
(b) A11 cams are replaced with 4C cams?

Carbs are standard 32 mm Dell Orto's: Mains are 118, pilots are 53, needles are K1 second notch.
Pistons are flat top.
Ignition is Saches.

Thanks
 
I assume you have open bellmouthes on the carbs, right? It's hard to say exact figures, I would simply follow the normal procedure (assumed all components are in good condition, sliders don't "clapper" around, jets are all in real sizes and needle / needle jet is not worn out => brand new!):

-start with the setting of the float-bowls to 18 - 18,5mm as suggested
-make sure valve-clearance and cam-chain tension is set correctly
-make sure ignition timing is correct
-make sure no air-leaks are recognizable on intake AND exhaust
-make sure choke-sliders are closing completely
-warm the bike up by riding it for at least 10 - 15 miles, don't rev. to high
-check idle circuit by adjusting the idle mixture screws to the point where you have the highest revs and center them between the point where the revs drop when you turn them out and same for when you turn them in.
-balance the carbs as good as possible
-Now check how many turns you have to do to fully turn the mixture screws IN. 1,5 turns +/- 0,25 => correct idle jet. more than 2 => go bigger (roughly you can say 1 turn is 5/100mm of jet size => 2,5 turns= go from 53 to 57, 3 turns => go to 60 etc.)
-adjust again, check synchronization
-go on the road, check transition from 1/3rd opening of the throttle to half. If it bogs or does not accelerate good, put the needle one notch higher (clip on lowest position) and try again
-if still not performing good, change needle jet one size bigger (265 => 268) and start with lowes needle position (clip on highest notch)
-change needle hight until it pulls good when you open throttle
-now focus on 1/2 opening 3/4 or 4/5th. If it pulls good, you have the right needle
-Now change the mainjet to 135 and see if it revs in 5th gear over 7.000 and how it reacts on full opening of throttle (only for moments, not for longer time at this point in time!)
-if it simply does not rev over 7.000 but also does not cause other issues, you might be slightly to lean or to rich. If it massivly drops power when you fully open, you are more far away from the optimum => go up first in both cases, as this is on the safe side...
-if problem gets bigger, go leaner. If problem goes away and your bike freely revs up to 7.500 or even 8.000, you are on the right track
-be careful!! It revs best at slightly lean mixture. Better accept a bit of to less top-speed than a burned valve and go back up one size in the main-jet.

I assume you will end up with 135 mains and - from memory - I had 60 or 65 pilot jets in my Jota. But check first that everything is in good condition, not worn out or blocked by mud or oxidation, floats are set correctly and float valves do 100% close. Else you mess around with settings and try to fix a masked problem...

HTH
 
Lothar,

Many thanks for the detailed reply and the very useful suggestions. 
Now I just need to find a quiet stretch of road to do the 7000/8000 rpm test!

The guideline for 135 mains is interesting.  When I previously used 135?s, they seemed a little rich, so I dropped down to 125?s.   
But that was a seat of the pants guess, not at all scientific.   

I am using the standard airbox and the filter is aircon gauze.  Bike breathes very well.
I was previously running 55 pilots, but the plugs seemed very sooty for low speed running.
The air screws are turned out approx 1 ?.    Anything more, and the idle drops.   
Acceleration in low gears is very good, albeit with a bit of a flat spot at about 4500rpm.

Tom
 
Thanks for the description of how the float height makes a difference to mixture, I understand it now. Trying to set up a PHM40 on a Rotax 604, sound like I've got the float too high!

Stuart
 
Hi Tom,

no problem with the description, glad if you can make use of it.

Regarding settings: your airbox might make a difference, as does valve clearance, ignititon timing etc.. Personally, I like to be on the safe side and first start with bigger jets and tune down to the right level

if your bike runs a bit rich (1,25 turns with a 53 pilot is unusual I would say) in low end, this might be either due to altitude differences (I am living at 100 meters/300 feet above sea level) or your carbs are worn (sliders, needle jet, needle) or your float-level is to high, your choke does not fully close or, or, or,... what is your bikes average consumption, please?

On the 1000 SFC's Keihins, I had an average of 9,8 liters/100km with peaks of 11 liters between two fuel stops (not sure what that makes in km/gallon), which is WAY to rich. I looked for several settings, changed all kind of jets etc. but could not make it run satisfyingly without keeping the original setting. Until one day I noticed that the chokes don't close fully...  >:( After closing the fully and adjusting the bike back to what originally was in there, the bike only used 6,5-7 Liters/100, which is roughly 30% less! Peak is now down to 8,5 liters if I really push hard.

So - check your carbs and other settings first.

Regarding the long road, you have it easier over here with some highways that still don't have any speed-limit. Some are very close, so I count me lucky...  ;)
 
You must log in or register to reply here.
Back
Top