gs850g cafe racer tunning

[SocialAnomaly99]

Hey man, It sounds like youre having some of the same problems I was having with my bike when I first got it. These problems MAY stem from the carbs being gummed up. When was the last time these carbs were FULLY disassembled and cleaned/dipped? Have you at least run Seafoam through the engine yet? Give that a try??

[Big T]

Geez,

we all missed the obvious questions

Did you receive/read the Megawelcome?

http://www.thegsresources.com/_forum/showthread.php?t=196338

especially the Top 10 Newbie mistakes

and the VM carb cleaning tutorial

[loxslhelm]

Ok let me start over. I have been building this for the last year and a half. It was my grandfathers and is the first time I've ever bothered to mess with a carb. The entire engine was rebuilt by me and everything c hecked as it was reassembled. The carbs had 22yr old gas in them and were frozen solid. It took six hours with the worlds nastiest concoction just to get the slides out. Once I had them apart and cleaned I sent them to a local shop to have them cleaned again and resynched and jetted. When I got them back I threw them on and it fired right up as if it was brand new.

I did as steve suggested and put the #15 pilots back in and the #120 mains. The needle is one notch from raised all the way. The fuel screws are 1 turn out and the airs are 2 turns out.

Problems are as follows.
1. at idle there is an occasional miss in the cyl closest to the timing pick up.
2. When reved a little it will backfire in the same cyl.
3. Low throttle feels good but mid throttle sounds like its doing a lot but feels the opposite.
4. Full throttle sounds even more intense then mid but has even less power.

Hopefully this clears up some confusion.

[loxslhelm]

Steve when I clicked "here" it didnt come up with anything. It says no matches.

[Big T]

Okay

the #4 carb has a plugged pilot jet or circuit -it's skipping and popping because it's lean

Did you replace all the O rings in the carbs? The ones between the head and the intake boot?

We'll work on your symptoms one step at a time

[loxslhelm]

All new o rings. Jets are not clogged.

[Big T]

All new o rings. Jets are not clogged.

The bike is telling you otherwise

Swap the #1 plug wire and the #4 plug wire and see if the miss moves to #1.

If it does, you have an ignition issue

If it doesn't, your carb is still dirty

[Chuck78]

If you rebuilt the entire engine, I assume you used feeler gauges to check the cam to valve shim clearance, and replaced shims appropriately with (usually thicker) shims to get the proper clearances back? I think the factory spec, cold engine-not been ran for at least 12 hours) is around .02-.08mm acceptable clearance. Been since fall that I last checked mine, I can't remember the spec exactly. Shims are about $7 each, sometimes you can use discarded shims on other valves needing that size swapped in.

A little re-hash, but think of it is reinforcing the tried and true suggestions of all the ace mechanics here - Clean clean clean carbs, good sharp pointy tips on fuel screws (nothing broken off in carb bodies I hope!), new carb o-rings, new intake boot to head o-rings, GOOD CONDITION intake boots, carbs at suggested baseline settings with 120-ish mains, needles 1 clip below the middle (lowering the clip position actually raises the needle), carbs visually bench synched precisely (easy to do), get it running and set idle speed, put a good differential manometer synch gauge setup on it (not the dial type vacuum gauges), run the bike with a window fan in front of it to cool the engine, tank strapped sideways across the seat for access to the carbs (may need extended fuel hose), carb tops off, 8mm wrench I think and a good fitting flathead screwdriver, synch adapters installed, and fire it up! very little movement on the slotted flathead slide shaft will make a good difference on the gauge, and tightening down the locknut will also slightly affect the synch, so try to predict a slight difference after tightening the locknut down.

As for adjusting the pilots, I am still trying to get mine fine-tuned, but others will tell you to start with the fuel screws at about 1 turn out, very free flowing straight through exhaust may need 1-1/8, but keep in mind 1/16th turn makes a big difference on the fuel screws. Then play with the air screws to get the highest rpm. If you have any misfiring, I have found a timing light pointed at anything will help you find the range in between lean misfires and rich(fouled) misfires where you don't have any misfires.
Then.... ride it, and see how it feels power-wise when you transition from less than 1/8 throttle (pilot circuit only) to the needle jet and jet needle. If you are lacking something in a smooth continuous-power transition, you need to try tuning the fuel screws in or out and re-tune the air screws. Cruising around the neighborhood for 20 minutes at never more than 1/8 throttle, and then pulling the plugs to check the color should help you determine whether the source of your not-smooth-transition means your fuel screws are too rich or too lean.

Get the pilot tuned, then go on to WOT (wide open throttle) plug chops. To do this properly, you really really need to find a SAFE desolate road or highway entrance with no law enforcement presence, driveways, intersections, turns, etc, preferably uphill, on a set of plugs that have been in the bike for at least 30-40 miles, and start from a stop and run the bike like a drag racer at wide open throttle for at least 7 seconds, if not 10 if that is possible. It is of my opinion that you need to keep it above 4000 rpm to get the mains sized properly, if not 5000. Then the critical part - hit the kill switch, pull in the clutch, and close the throttle 100% all simultaneously. pull over, remove all 4 plugs, put them in a safe place (they are hotttt) where they won't melt anything or get all scuffed up, and take them home. install a spare set of plugs, of course.

Now, some say you can use a magnifying glass and a flashlight to inspect the color of the plugs' carbon coloring way down inside the threaded area at the base of the insulator, but most people prefer to "chop" the plugs' threads off with a cutoff wheel, hacksaw, sawsall, or bandsaw. This exposes the insulator all the way to the base. If you have a surplus of unburnt fuel, the furthest area for the flame to reach is deep down in that crevice of the spark plug, so the fuel will not be completely burnt, and you will see a dark brown or black sooty ring at the base of the exposed white porcelain insulator. Too rich on the mains. If it is too lean (makes lotsa power but runs waaaayyyy too hot), all the fuel deep down in there will be completely combusted and the heat will burn off any soot deposits that are there. You need to get it to the point of having a light brown or tan ring at the bottom. I'll see if I can dig up a handy photo. Swapping main jets in and out will be the route here. jetsrus.com sells cheap mikuni "OEM equivalent" jets if you are unsure if your sizing is in the ballpark. after you do a chop on the 120's and you need different mains, order a couple of sets in the sizes up or down that you think you will need. 122.5 or 125 would be the absolute largest I could imagine, and you might end up with as low as 110-115. Depends on the engine condition, valve condition, ring and cylinder wall condition(are they broken in yet? ride it a few hundred miles after getting a moderate state of tune and re-test), air filter cleanliness and proper amount of filter oil if K&N or UNI.

Now you can do some plug chops for about 1/2 or 5/8 throttle position to check the needle height, but you really need to have the proper mains in before you can fine tune the needle height. it helps to wrap a piece of masking tape on the right controls next to the grip, and mark a reference line on the grip, and similar line on the tape on the controls next to the grip. Then twist it to full throttle, and mark the controls tape with a line corresponding to the grip reference line, and that is WOT. Now you can put a mark at 1/2 throttle and 1/8th throttle to help you when doing plug chops and pilot adjustments.


There is also a Gunson Colortune that helps you get perfect combustion mixture on the pilots by monitoring the flame color. dark blue is perfect, light bliue or blue-white is lean, yellow flickers or solid yellow flame color is too rich. These work great on VM carbs, but not CV type vacuum slide carbs.

And another note, CV carbs were introduced to eliminate user error that makes VM carbs act up momentarily. If you instantly whack open the throttle pretty far on a VM, you immediately let so much air into the carb that you lose most of the vacuum momentarily, which kills the air velocity through the carb for an instant because the carb venturi effect relies on high velocity air siphoning fuel against gravity up into the carb. If you rip open the throttle on a Constant Velocity style carb (vacuum operated slide, not mechanical-tied-to-throttle-cable), compared to the same on a VM manual slide carb, you will think the VM isn't very good at this move, but if you open the VM throttle a little more gradually, you will reach a sweet spot where you maintain enough velocity of air in the carb to keep the fuel flowing in copious volumes to produce great acceleration as you are increasing the airflow with precise throttle twisting. The CV carbs basically are designed to do all the thinking for you, so when you whack open the throttle instantly to WOT, the slide only raises as much as it needs to in order to maintain enough air velocity, and it will raise the slide at the same rate as you would find is the best throttle twisting rate on the VM carbs anyway.

CV's have a butterfly throttle blade in the outlet of the carb that blocks the flow even somewhat at WOT, therefore you will see a 79 model with 26mm manual slide carbs and the similar 1980 model with CV's will have a 32mm vacuum slide carb. similar airflows. The VM's (and Keihin CR's, etc) are considered the best for all out speed and power, but CV's give more control with less thinking/throttle precision and are favored by road racers. Flat out drag racing, the VM's will have a slight edge. Wow, that was a lot of info, I hope I didn't overwhelm you. I've learned all of this stuff from the help of all the awesome people on GSR and a few other sources occasionally, from the past 5 years or so. Thanks to all. Please correct me if there are any discrepancies or omissions.

[Chuck78]

or if that doesn't work, a downsized version is hosted on GSR:

http://www.thegsresources.com/_forum/attachment.php?attachmentid=21310&stc=1&d=1388544811

more detailed rehash of all that info that I just dumped on you:

http://jetsrus.com/FAQs/FAQ_spark_plugs.htm

http://www.xs11.com/xs11-info/tech-tips/maintenance/carburators/68-tuning-carbs-by-reading-spark-plugsthrottle-chop.html

http://jetsrus.com/FAQs/FAQ_rejetting_101_how_to_rejet.htm

http://www.strappe.com/plugs.html

http://www.dragstuff.com/techarticles/reading-spark-plugs.html

This was from my 844cc GS750 with straight through exhaust, K&N dual oval universal filters (fit VM26's, RC2222 part number filters), decent compression on the bored 750 cylinders with 850 pistons and rings with unknown mileage.

[loxslhelm]

Thanks for all the info chuck. I found it very helpful. Yes the valves were adjusted and these carbs were so obnoxiously cleaned its not funny. Checked the intake boots agian and noticed the #4 had a crack in it. Im gonna replace all four. Im familiar woth plug reading but thats probably one of the best descriptions and diagrams I've seen. The info on the fuel screws is great very helpful. I'm borrowing a friends manometer so once I get the new boots on I'll commence tunning. Plus side I have acces to a dyno. Just needed some more info on where to start with these carbs.

Thanks.

I'll post how it goes and eventually a full build thread of the whole bike.

[Big T]

. Plus side I have acces to a dyno. Just needed some more info on where to start with these carbs.

Work your way from bottom (pilot and idle) up the RPM range

The new boots should change that lean condition you're getting at idle with #4

One more thing to check -make sure the fuel needles have an exaggerated long tip. People tend to crank these down and break off that tip in the hole, clogging and reaming out the hole

[SocialAnomaly99]

One more thing to check -make sure the fuel needles have an exaggerated long tip. People tend to crank these down and break off that tip in the hole, clogging and reaming out the hole[/QUOTE]

This happened on my bike. Was the most difficult thing I had to deal with on this whole engine. Had a hell of a time getting that tiny little chunk of needle out.

[Chuck78]

One very important thing that I forgot, that is far too often overlooked is the importance of proper float height adjustment. "Proper adjustment" on anything not brand new oem means more than just set the float height to the spec in the manual. You absolutely have to use a clear hose jammed into the fuel bowl drain screw port bent up in a u-shape next to the carb to see the actual fuel level to verify that the seats and needle valves are sealing properly fir that float height. Worn needles & seats will need taller float heights to achieve the same fuel level, as they will need more pressure to seal the needle into the seats. It's$$$$, but if in doubt, replace them all. Fuel doesnt have to be running out the overflows to warrant replacing them as a set, but a slightly off fuel level will definitely give you tuning headaches.

[Chuck78]

There are also hose adapters with that m6 thread that you can buy, but most just shave the end of a piece of clear tubing down to a taper and jam it in to seal it.

[loxslhelm]

Thanks a bunch this is all good stuff and I'll make sure to get on it all. Maybe a little off topic but there is possibly a built 1150 drag motor getting dropped in this sucker and I would like to know how a set of lectron carbs compare to the stock ones.