Pilot jet size with bigger cubes & carbs?

[Anonymous]

Just reading through the old Q&A I see that if engine capacity is increaced, the main jet size should be decreased. Does this sound right? If this is so , should the pilot jet size be decreased as well?
My engine was a 1000cc 2 valve, now it is 1150cc.
It has the non CV type carbs.
It had 26mm venturi carbs with 95 mains and 20 pilot jet sizes.
It would constantly foul plugs due to soot building up.
Would run fine carburation wise with new plugs.

Carbs were replaced with 28mm venturi (36mm towards head)
They are off a 1000cc Katana,
These had 150 main jet & 20 pilot jet.
(were used for road racing)
The stock air box has been tossed in favour of pods.
The plugs still foul.
Main jet has been replaced with size 120.
Main jet is not the problem though.
Can't even get past 1/4 throttle let alone 3/4.

Could it be that the pilot jet is too large, or small?
Engine seems to have symptoms of starving for fuel off idle, only when you go to ride it though. Just bogs down, won't take off, won't take any throttle, lots of popping and banging, but it smells rich?
I'm certain there are no air leaks and the float levels are set between 23 & 24 mm.

The fuel and air screws are set at 1 1/2 turns out each, the air screws giving the highest idle at this position. This is correct right?
Carbs have been synchronised.

Electrical system is fine. It has been blamed for ages, had everything ignition wise replaced, still the plugs would foul.

Could it be as simple as the pilot jets being too large?

Any suggestions?

This is really starting to get to me and I am at the point of considering taking it to a shop.

Very unlike me!

[Anonymous]

Have you tried raising or lowering the needles?

[Anonymous]

Yep, after reading as much as I could about jetting, it seemed to me as though it wanted more fuel, so I gave it more on the needle. Still wanted more, gave it more, still not enough. Went right through the stock needles and then went to a set that were about 3mm shorter. Started on top clip & ended up second from bottom. Seemed to still want more. This is crazy.
After thinking about it a bit more I realised the problem is before the needle would start having any effect, back in with the stock needles and put in a larger pilot jet. Seemed worse.
About 15 miles of riding and a bit of stop start testing used about 10 litres of fuel. Surely this is too rich
The carbs have been disassembled, ultrasonically bathed the bodies & bowls, blown out all passages & then carefully reassembled with all new o rings.

Maybe I've been going the wrong way, even though there is still a
lot of spitting back through the carbs, maybe it wants to be a bit leaner on the pilot jet? Does this sound sensible?

After reading that the pilot jets should be leaned for a cube increase, should the pilot be leaned also?

Does anyone know about this situation I am going through?

[Anonymous]

I've never heard of pilot jet sizing making that radical a change. Your plug
conditions indicate rich & the problems are occuring in the area of low rpm
circuits--idle circuits, slide cutaway & needle/needle jets. I'm not familiar
with the Katana carbs--do they have accelerator pumps? If so, then you
have another suspect. You might as well try going leaner on the pilot jets;
you've changed the most obvious variable, with no luck.

I'm interested in where you read about going leaner on jetting for an increase in displacement--this sounds wrong.

[Billy Ricks]

>I'm interested in where you read about going leaner on jetting for an increase in displacement--this sounds wrong.<

I may be wrong, but I've always been felt once a set of carbs is set up properly they should work on just about any bike of similar state of tune. About the only variables would be filters, exhaust, high lift cams, and big-bore kits. Even then they should be fairly close. Most tuners rarely fool with the pilot jets themselves since the pilot screws allow for adjustment. Sounds to me too that the jetting is too rich.

[Anonymous]

Carb Problems

I'm assuming the plugs are carbon fouled, and that the bike isn't burning oil. If so, carbon fouling means too rich. High fuel consumption and good running on clean plugs are also indications of over-rich mixture. Problems at idle and just above indicate that the idle settings are incorrect. Lean out the idle and get it turning smoothly and then worry about needle and main jet settings. Be methodical; change one setting at a time, one step at a time, starting with the idle. Have you checked the valve clearance and ignition timing? Both of these must be correct before you can tune the carbs.
Also why bigger carbs? 150cc's is only 15% capacity increase; bigger carbs are probably just adding to the frustration. The larger diameter carbs are more difficult to tune due to lower air velocity. They will also tend to give away power low in the power band and add on top, making the bike more difficult to drive. The same goes for the air filter pods. The airbox is more than just a container for the filter. It is a tuned resonant chamber, designed to stuff more mix into the cylinders at the proper RPM to smooth out the power curve.
As far as jetting changes for a capacity increase, leaning out doesn't sound right. More capacity will draw more air and therefore more fuel should be needed to compensate. If you still have them I would put the stock carbs and airbox back on and tune from there.

[Anonymous]

Thanks for the help so far,
I went back to the old Q&A section & found the stories about decreasing the jet sizes. They are about half way down , dated August 24 1998, titled Air filter conversion problems with carbs 1978 GS1000, and another one titled Rough running at cruise 1983 GS1100E, this one is about four questions down from the other.

The carbs do not have accelerator pumps and they are not that much bigger than stock. I think they are the same as the carbs fitted to the GS1000S models, so fitting them to an 1150 is not that bad, plus I already own them, and the stock air box will end up causing me to go mad if I have to deal with it ever again.

I took out the larger pilot jets and replaced them with the size 20 and put the 150 mains back in. This is how the carbs were when I got them. The bike can actually be ridden in this state. Still quite crappy but rideable. The fuel screws under the carbs are about 1 1/2 turns out and the air screws are just barely cracked. With the air screws opened any further the bike has no drive, to move itself off the mark. With the fuel screws opened any further it still runs, but ridiculously rich. Will run at up to six turns out but still wants the air screws shut. At this point it has a near constant flame out the back and makes this funny burning / wooshing sound as the last of the fuel burns after shutting it down. Looks and sounds cool, but I'm sure it's not right.

I think I've had enough of it. This has wholly consumed about three weekends and most of the weeknights in between.
The neighbours are starting to give me dirty looks and I think that enough is enough. I've actually arranged to have it dyno tuned.
Maybe just this once it will be ok. I have learned a heck of a lot about what goes on with carbies and should be able to make my own future adjustments once it has been properly set up. The main source of problems de
finatly seems to be the after market air filters. The dyno guy will know what to do though!

The theory about carbs going from one engine to another that is set up differently worked on my bike. The same carbs that were on the 1000cc engine worked just as great on the 1150 engine.
They got changed because they seemed to be the source of plug fouling, as well as being in pursuit of more power, they were claimed to be good for a seven percent increase in power on the "s" model over the standard models.

If anyone has any input on this question, please put it up as I will be keeping a keen eye on this story, and any that are related to it.

Thanks again,
S.Trott.

[Anonymous]

Flame from the exhaust indicates raw fuel being dumped into the pipes &
muffler. Something has to be radically wrong to get this scenario. If your
engine were being used in a four wheeled device with a fuel pump for gas delivery to the carbs without a regulator or a resevoir/recirc. system, I'd say you were pushing fuel past the inlet needles. However, since you are not running a fuel pump, you would have to have all four floats stuck or all four inlet needles leaking massive amounts--this would show up as a constant stream of fuel out of each overflow tube.

[Anonymous]

I think that all the radical type swings in performanmce are due to the fouled , or partly fouled plugs only firing occasionally.
It is very hard to tell when the plugs are fouled as it does it so quickly. In one week I have fouled three sets of plugs. The bike has has done this in less than 100km, plus a bit of back yard testing. The fouled plugs would explain tha raw fuel making it into the exhaust system.

Last night I rode it and it got me about 6km away, sat untill cool, started and got me home. Running crook but running.
Try to start it this morning and it did not want to, except for a few backfires, kind of run and then die.
Try to start it at lunch time, it starts ok but has the flames popping out the back again, probably about 1 per second.
Come home from work and put in ANOTHER new set of plugs and it started instantly, revved up to a very high idle, popping as though it were lean. With the idle speed reset, off I went.
Ran great, still a bit rough but would accelerate, would idle at lights, could lift wheel in first, just behaving really well in general.
Still poorly tuned though.

Well now it is sitting in the bike shop waiting to be dyno tuned tomorrow. That should sort out the running problems and hopefully the problems of fouling the plugs.

[Anonymous]

Pilot jets.

Trotski - me think your gas isleaking through the petcock / floatvalves.
Regards - Adam M

[Anonymous]

The extra capacity has more "suck" (for want of a better way of explaining it) and will cause the engine to richen a bit. That is why it makes sense to jet down.

[Hap Call]

I've never heard of pilot jet sizing making that radical a change. Your plug
conditions indicate rich & the problems are occuring in the area of low rpm
circuits--idle circuits, slide cutaway & needle/needle jets. I'm not familiar
with the Katana carbs--do they have accelerator pumps? If so, then you
have another suspect. You might as well try going leaner on the pilot jets;
you've changed the most obvious variable, with no luck.

I'm interested in where you read about going leaner on jetting for an increase in displacement--this sounds wrong.

I agree with John on this...if you increase displacement, you will pull more air into the cylinders. If you do not increase jet size (it should not be a radical increase if it is only 100cc or so) you will run leaner. The katana carbs are set up originally for a motor that flowed better thru the head so my gut reaction would be that the carbs are rich in the first place and you need to go with smaller jets and lower the jet needle, but only do one at a time. Tune you low speed (pilot) circuits first, then your needle and needle jet, then finally your main jet. Because the low speed jetting affects your higher speed jetting but not vice-versa, you need to do it this way.

One change at a time, test, record results, make next adjsutment. It takes time, but you will save time if you do it like this.

Hap

[Anonymous]

As you could tell by now I'm having a hard time trying to tune my carbs. I paid a guy for 5.5hrs of dyno tuning only to get it back worse than when I dropped it off. Not impressed.
He did give me a few tips though,
the emulsion tubes have been damaged,
the needle jets have been damaged,
the needles are jamming in the needle jets with the needles lowered ( 2 carbs only, only when emulsion tube is seated ),

Most of this would allow fuel to flow through when it is not supposed to. Great! that would explain the plugs fouling!

Some advice from a professional tuner/ engine builder was to replace the damaged parts and to try fitting slightly smaller needle jets.

Old parts from carbs were clearly damaged, two bent emulsion tubes, dented around the needle jet end and the needle jets were damaged also.

Replaced emulsion tubes. Replaced the needle jets , going down in size from 06 to 04 .

Reassembled and got it running. Turned air screws out by 1 1/2 turns and set fuel screws to burn the plugs cleanly. For the first time ever this bike is capable of burning cleanly at idle. New plugs with no soot or smoke. Excellent. Still sounds lean as revs are increased, usual popping and sneezing, even more so as revs are increased further.
Back to the usual old thing. Close the air screw a bit, still not enough, open the fuel screw a bit still not happy. This goes on until it sounds like a big boat engine and is blowing smoke.
Still pops and crackles though as the revs are increased.

Maybe it needs a needle that allows more fuel in at an earlier throttle position, in the area where the pilot circuit can't keep up any more, but the needle is still blocking the fuel from coming out through the needle jet? Sounds sensible to me. It would really explain the popping etc at anything around 1/8th throttle. Does this sound like somthing that may be required after going from the s
tandard air box to the individual pod filters?

Still staying sane and level headed, don't know how but.

S.Trott

[Anonymous]

Try to put original airbox and filter in place - it will make a mixture richer.
My GS 1150 has WISECO kit ( ~ 1200ccm) with original box and Uni high flow filter - works OK.
Also check your valve clearences - perhaps they are too tight.
Regards - Adam M.

[Anonymous]

I'm just starting to seriously wonder about the valve clearances, it is quite possible they could have been causing a lot of this trouble.

It looks as though my pilot jets should be a size 15 and not a size 20.
This would help too.

When you say the original box and high flow filter works ok, is it somthing that you are still not happy with? Did you try to use individual type air cleaners?

Have you come across or installed an air correcter kit ?

Carbs will not fit into airbox unfortunately.