CV carb rejetting on '80 GSL

[tirebiter]

I went nuts on her putting in the largest mains I had - 127.5. Runs great but the backfiing is moreso now but of the "too rich" ilk instead of the "too-lean" ilk. Occasional, splashy and louder now, instead of quieter and constant like popping corn.

Still has the 42.5 pilot jets and mixture screws seem happiest (lean-drop method) at 4 turns now !!! Needed to turn in the idle speed adjustment screw to raise idle speed. As the engine warmed up, the idle went down. After warmed up and a test drive of a couple miles, I had to increase the idle speed a little more again! Definitely seems too rich now but I'm not sure what's going on with the mixture screws and the idle quality except it is about 20 degrees cooler today than it was yesterday.

Maybe I should have taken Steve's (and everyone else's on here) advice and stripped the carbs to begin with and elimentate the guesswork surrounding the behavoir/backfiring, I'm experiencing now.

I looked carefully and cannot see or hear any signs of exhaust leakage anywhere from the cylinder heads to the exhaust outlet. "O"-rings are in very good condition as are the mount rubbers even if they are the wrobng ones. I think the wrong carb mounts are on the bike. These are 2mm too small, but I am certain there are no intake leaks. At least not as far as I can tell by listening through a tube all around where leaks can occur. Spraying WD-40 near the cylinder head intake ports yields no change in the idle.

Backfiring had been almost non-existant the past day, running 122 mains, 42.5 pilot jets and mixture screws afjusted to 3- 3.5 turns (lean-drop method).

Will try Eric's suggestion with 125 mains and 140 pilot screws now and report back in a couple of hours or so.

Thanks everyone for all the help !

[Steve]

Maybe I should have taken Steve's (and everyone else's on here) advice and stripped the carbs to begin with and elimentate the guesswork surrounding the behavoir/backfiring, I'm experiencing now.

Every shortcut you take is just one more opportunity to do it over.

Please trust us when we make these suggestions and recommendations. We are not just trying to keep you busy doing "stuff", we are offering what will work to get your bike going. But, ... it also depends on having the correct information, such as year/model of bike and any modifications that have been done.

Not sure if I should ask, but somebody has to do it, ... have you adjusted the valves yet?

.

[tirebiter]

At the risk of sounding terrible I am leaving out a couple of aluminum pieces, when I go for this test drive.

They were in the bottom of the carbs loosely. They have a large diameter and a small diameter so they cannot get sucked into the carb bore. The hole they go in leads directly to the carb bore. The hole they go in also has a large diameter and a small diameter. The fuel bowl gasket covers the bottom of the hole. I can see no port, inside the hole.

Parts diagrams calls it a gasket - #34 on Carburetor diagram.

At a standstill, I seem to detect a bit of the flat spot I experience when going up at slight incline. Happens right aoound 3,000 RPM.

Steve, valve clearances are greater than zero and less than 0.002" cold. Checked, did not adjust any. Around 40 MPG on every tankful and firing on all 4 even when I first start it up from cold - on the choke - in the morning.

[tirebiter]

Mainjets 125, pilot jets back to the stock 40, idle mixture screws about 3-3.5 turns. Good, solid idle. Flat spot/lean surge around 3,000 RPM is back, especially when transitioning to a slight uphill. Lean backfire is as bad as it was before I tried the 42.5 pilot jets, previously.

I'm done for the day. Will be ordering a couple of more sets (45 and 47.5) of pilot jets, this week. "Little Suzy" liked the 42.5 pilots a lot. They will go back in as soon as I have time, until the larger ones arrive. Like I said, only way I know it's too rich is to make it too rich and come back from there. Maybe someday I'll get a wideband exhaust Oxygen sensor setup but for now, I have to go by feel and experience.

The 120 and the 122 mainjets I've tried already both seemed to work pretty well. Until I get the backfiring and the 3,000 RPM lean surge 100% eradicated, I won't worry too much about which mainjet is optimum.

So far she has run best with 42.5 pilot jets and 120 or 122 mainjets.

[Steve]

At the risk of sounding terrible I am leaving out a couple of aluminum pieces, when I go for this test drive.

They were in the bottom of the carbs loosely. They have a large diameter and a small diameter so they cannot get sucked into the carb bore. The hole they go in leads directly to the carb bore. The hole they go in also has a large diameter and a small diameter. The fuel bowl gasket covers the bottom of the hole.

If you have been leaving those aluminum pieces out on previous test runs, I am surprised the bike ran at all, let alone as well as it did. YOU MUST HAVE THEM INSTALLED for the carbs to work properly.

When you install them, give them a gentle tap to seat them. There should be a square-section o-ring that goes between the aluminum plug and the bowl gasket to keep them in place.

Please put ALL the parts back in the carbs, you may find that all of a sudden you are now running too rich, because you have been compensating for a MASSIVE air leak with those missing plugs.

Steve, valve clearances are greater than zero and less than 0.002" cold. Checked, did not adjust any.

Well, at least you checked them. "Greater than zero" still leaves a LOT to be desired, though. Keep in mind that tighter valves don't spend as much time on the valve seats to cool off, so will run hotter. As clearance approaches ZERO (and it always gets smaller on these engines), you stand a good chance of burning the valves.

Apparently you are using INCH feeler gauges, that is no problem. The thinnest feeler is usually 0.0015". If that does not fit, change the shim. Yes, it might take it just over the recommended clearance, but that is far less of a problem than running too small of a clearance.

.

[tirebiter]

Oh my, WOW ! Apparently two of the Aluminum plugs have been missing the entire time I've been using the bike. Now all of them have been removed. I am currently running 120 mains and 42.5 pilot jets. Was planning to order a couple of more sets of pilot jets even larger.

The bowl gasket looks like it entirely seals the hole. I don't see that there can possibly be any air leak. I can certainly put the two aluminum plugs I have, back in and make something that will fit in the other two holes, to fill the void.

If it matters ... the bowl gaskets are not the right ones either and they do indeed cover the holes where these plugs go. I tell ya, I got a real mess here that actually runs pretty good.

[tirebiter]

I bought the metric feeler gauge. Once I'm happy that the jetting is good enough and the bike is driveable by my standards, then I will go into the valve clearances. For now, I'm aware that they are close to being at zero clearance - when cold - and am keeping a jaundiced ear on the exhaust note as I drive off in the morning.

I hope you will bear with me Steve, just a little longer. When I do make valve adustments maybe it will help others to see what difference the valve clearances can make in the jetting and driveability of these bikes.

[Steve]

Actually there is no point in doing any more jetting checks until you get the valves correct and get those plugs in the carbs.

If clearances are not correct, you might not be getting the proper amount of air past the carbs that needs to have gas added.

As an example, if you are approaching true ZERO clearance, the intake valves will be closing a bit later, allowing some air to blow back through the carbs. Since carb jets don't care very much which way the air is moving, some gas will be added, making that slug of air just a bit richer, since it already had a dose of gas added the first time around. To make things worse, the next time that valve opens, that super-rich slug of air will go past the jets a THIRD time. Guess what happens. Yep.

Get your valves adjusted while you are sorting out some sort of plug for those two carbs that are missing it. Your gasket will NOT seal it sufficiently, you need that plug.

.

[JJ]

I bought the metric feeler gauge. Once I'm happy that the jetting is good enough and the bike is driveable by my standards, then I will go into the valve clearances. For now, I'm aware that they are close to being at zero clearance - when cold - and am keeping a jaundiced ear on the exhaust note as I drive off in the morning.

I hope you will bear with me Steve, just a little longer. When I do make valve adustments maybe it will help others to see what difference the valve clearances can make in the jetting and driveability of these bikes.

Valve adjustment is the VERY FIRST thing to be done when you start tuning a GS. Get the valves right, and only then start fiddling with any carburetion issues. Oh yeah... adjust the valves with the engine dead cold. It shouldn't have been fired up for 12 hours or so.

[tirebiter]

Believe it or not, I realize the risk involved in not adjusting the valves at this time. I also realize there will likely be some more carb tuning to do after I adjust the valves. It migth be a simple mixture adjustment. It might be more involved like a jet change or two. Time will tell and I will report what I experience with this 850 engine.

As far as those pesky little aluminum plugs that are missing fom the bottom of 2 of my carbs go, I'll take a MUCH closer look into that after the weekend. "Little Suzy" will be carting me all around for work in several - 1 hour away - places, this weekend. Once I receive my next jet order (by Monday supposedly) I'll have the carbs off to put in an even richer pilot jet. Running 120 mains and 42.7 pilot jets now.

Now for the good news. The special lifter tool is on it's way also and I plan to adjust the valves sometime late next week. Yes, the way I'm going about this is backwards but so far, the gasoline I'm saving by using her regularly, as I slowly go through the bike, is worth it in my world.

[tirebiter]

I made a couple of plugs from fibrous organic cellulose to put in where I was missing the two aluminum ones. It doesn't SEEM to make any difference in how she runs though. I studied the passages and saw how a vaccuum leak could occur if the float bowl gasket was not in good shape. It seemd prudent to follow your suggestion on this one, Steve.

Presently running 120 mains and 45 pilot jets. Mixture screws are at about 2 turns. The flat spot at 3000 RPM has all but dissappeared entirely. In fact it is difficult to reproduce the slight hessitation at all, when I try. Occasionally it still rears it's head. I believe it's happening only when the throttle plates just start to uncover the low speed orifices in the carb bores.

This is where the fine tuning begins. If I can 100 % eliminate the flat spot, and still get good fuel economy, I'll be happy. Seeing about 39 MPG tankful after tankful but doing more 75 MPH driving than before.

Will be thinking along the lines of moving the needle in the air piston/slide and/or an air corrector jet change. I'm interested in finding out how much the overall operation of these carbs will be altered by changing the air corrector jets.

Expecting to do more driving than working on her carbs all this week. Will report back when I have a chance to try some other carb/jet changes.