1978 GS750, Carbs, Bogs at low RPM, Air Screw Prob's

[Livingston]

First, thank you all for such a great site. This bike would probably be resold out of frustration if it wasn't for all the research I've done here; and probably wouldn't have bought it if I didn't know there was a site like this for support.

The Bike:
Titled as a 1978, but the VIN plate says mfg'd in Nov '77. Odometer reads 40k miles. I don't know if it is an E, EN, EC, etc. as the title doesn't help much, perhaps if I get a picture attached, someone can help. Bought for $650. P.O.'s registration expired 3/06 so I assume he let it sit for well over a year before I got it in January. His receipts showed the carbs were rebuilt in late '05. He only put 5 miles on the bike since this receipt.

What I've done so far:
Sealed up the airbox, silicone/saran trick on end caps as the original rubber was shot.
Changed all valve shims (were at .0005" or less on all of them!).
Acid washed tank (then again as I created more rust while rinsing/drying).
Cleaned petcock and filter.
Removed and cleaned carbs, sprayed not dipped, didn't replace any parts or remove slides. Just cleaned the parts in the float bowl area. Everything in the carbs were pretty clean.
Cleaned and gapped points.
Lots of misc. parts replacements (plugs, cables, bulbs, seals, etc.)
Lots of cleaning!

Where I'm kind of stuck:
Next on the list to do is get the air/fuel mixture right and sync. While adjusting the airscrew, the 2 and 3 carb idle highest at a 1/4 turn and carb 1 and 4 doesn't change idle speed if bottomed out or turned way out. While driving it, it bogs at around 2-3k RPM (but tach readings are questionable as it is a new cable and the guage hadn't spun for an unknown number of years). So, right at the end of my clutch engagement it loses power. This makes for difficult and slow starts. I sprayed the intake boots and saw/heard no RPM change; so I don't think it is an air leak. Up over 4k RPMs it seems to have decent power, but as a newby I don't have much to gauge it against. Also, I'm in Denver (over 5000' elevation) if that matters.

I haven't checked timing or done the carb sync. yet, but I've got a guage and timing light. Would either of those be affecting the high idle mix test if off?

Thanks,
-d

[mark]

I assume your bike is stock? Since you can time it and synch it, I would start there. Have those basics covered so you know where you are. Seems like you've done most of the basic work, might as well finish it.

I'd set you airscrews for now at some where between 1 and 1/2 and 1 and 3/4's out. That's about stock configuration. Setting your air screws before the synch, timing, etc. isn't going to help you much.

Also do have you messed with your pilot screws at all?

Then, do a plug chop test at various throttle settings. Do it at 1/8 and 1/3 throttle for sure, full throttle if possible. Run the bike at those throttle settings for a minute or so, chop the throttle and check the plugs (don't ride around before you check them, that will null the test). That will tell you what your mixture settings are doing, at various throttle settings, if the bike is running lean or rich.

You may also have to do a full carb cleaning, of course.

[Livingston]

Thanks Mark. Yes, the bike has stock airbox, exhaust, and I believe electrical. If by the "pilot screws", you mean the screws in the float bowl that the Clymer says "do not adjust"? Nope, and there was still yellow paint on them so I don't think anyone else messed with them either. Should I check to see how many turns out they are (perhaps remove and give a good poke and spray?) and put them back to that same setting or is it best not to touch them?

Alright, sync'ed the carbs. The dial gauges, calibrated off #4. They were all within 2-3 psi to start, now all at 8psi at ~1100rpm.

Checked timing at <1500 and >2500 on cyl 4, it was dead on. Cyl 3 was off at both RPMs by about 2mm. Should this be concerning? Don't remember which way it was off as the strobe burnt out in the middle of my tweaking. Also, I did notice an occasional miss every couple of seconds and occured more frequently at the higher RPM. Not sure what is going on there.

Drove it around after the sync'ing last night. Still having the bogging issue at about 2500. Worse with the air screws out at 1.5 or 1.75. It gets better with cyl 2 and 3 at 1/4 to 3/4 turn out (also highest idle) but still an issue.

Plug Chop- Planning on doing this tonight. Can this be done in the garage in neutral or does it need to be driven under load?

Thanks,
-d

[Dave8338]

It has to be done under load and at the very RPM where the issues are showing up. For example, if you notice the bogging at 3500RPM's, this is where you wan't to do the chop. Get the bike out on the road and bring your plug wrench and a pair of gloves with you (I also bring a needle nose pliers with as it makes getting the plugs out from the head fins, much easier and a pen ad paper to note which plug is what color). Run the bike up through the gears until you notice the bogging issue and then hold the throttle steady at that RPM while the bike is bogging and sputtering and run it there for 20-30 seconds. Then, at the same time, pull in the clutch and hit the kill switch. When you get pulled over and stopped, pull the plugs one at a time and note the color. White = lean, light brown =just right, and dark brown to black = rich.

[Livingston]

Thanks Dave, makes sense. Weather is looking good here in Denver for the next couple days so I should get back with some updates soon.

Next question(s), after I chop and determine rich/lean, do I need to clean up the plugs before I adjust the airscrews and retest or will the plugs re-color themselves after a few minutes of run time? I guess I'm wondering if I can just scotch-brite them right there and continue ride/kill/check/adjust iterations until all are light brown with no need to write anything down. Are 1/4 turn adjustments too little or too much?

Any thoughts on the pilot screws? Tinker or no?
And should I be concerned about the timing differences between 1/4 and 2/3?

-d

[mark]

You shouldn't need to clean your plugs, unless they are badly fouled. Make your your timing for 2 and 3 is right.

Keep in mind of course that, you might have a full blown carb cleaning project on your hands. You can fool around all you want and if you've got a jet or passage that is partially blocked it won't improve until its cleaned.

1/4 adjustments are O.K.

If your pilot screws haven't been messed with, I wouldn't try to adjust them for now. They are pretty sensitive to adjustment. If you take apart the carbs make sure that you not the position of the screw and the number of turns it has been adjusted to when you remove it.

[Livingston]

Found the short in the timing light and got 2/3 timed. I didn't realize 2 and 3 were on their own separate plate from 1 and 4 (need some more light in my garage I guess). With every tweak, it seems to run a little bit better.\\/

Still a little boggy down low but I'm hoping the plug chops/airscrews fix the problem and I won't have to do a full carb rebuild.
Weather and after-work traffic in NW Denver didn't allow for the plug chop last night, but the GS got a nice shower. A little sketchy for a guy who just took his ABATE class a month ago and is on a bike that doesn't like to get moving from a stop unless I get the RPMs nice and high!

I'll post follow-ups once I can get the next step done. Thanks so much for helping out the newbie,
-d

[Livingston]

a little more progress

Upon closer inspection, the yellow paint on the pilot fuel screws have been separated from the yellow paint on the carb body. They were set at 2-3/4, 1-1/2, 3/4, and 1-3/4 out from lightly seated. Seems wrong from the searching I've done so far. I removed, cleaned and reset them all to 3/4 according to a search in the forum. I messed with the air screws for high idle and took it for a quick spin to see how this affected performance. Again, a little better but still need to dial it in with the plug chop (got too late to finish). Getting there slowly, but getting there. Now I'll follow the procedure to set these as it is well discussed here on GSR.

The only thing I'd like to ask is, when setting the "high idle" with the air screws, what range should I be seeing for idle change? It seems I can turn some all in or out without any change, some no change until fully seated, and the one that does affect RPM is so slight it is hardly noticable.

Thanks again,
-d

[mark]

Air screw range should be somewhere around 1 and 3/4 turns out as a starting point on VM carbs. By the way I never had much luck with the "high idle" approach either, I used plug color and throttle response as a guide for mine.

Your right, your pilot screws are all over the place. Shouldn't be that wide of a range. Stock settings for mine are said to be 7/8 turn, so you're pretty close. Mine are a little different because of my aftermarket pipe but not much.