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Yet another pipe/pod modder panicked

  • Thread starter Thread starter mike1414
  • Start date Start date
M

mike1414

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Well everyone, I have had an unfortunate time with my carbs. As a little background, my '79 GS750 was running relatively well until one day, on my way to work, it started sputtering and cut off. I cleaned the carbs, it ran, but not well. Considering the bluing on my exhaust (bought from PO with Kerker 4-1 and K&N pods), but didn't seem to be jetted considering the poor throttle response and bluing of the exhaust.

I started with the jetting nightmare with which I am currently struggling. I have had it to the point where it would run, but would hang a bit or would bog down upon accel. I have tried essentially any combination of #15 pilots, #17.5 pilots, and #20 pilots along with #115 mains, #117.5 mains, and #120 mains. It was always the first 1/4 throttle until recently, but for some reason the mid-range has begun to lack. My symptoms have been largely rich, but with the #15 pilots in and #115 mains it seemed lean (excessive heat, hanging throttle, kind of whitish smoke).

Most recently I have tried #120 mains w/ #15 pilots, but it seemed rich. I switched my mains to #117.5, but same thing - when I say rich, it is bogging down with the 1st 1/8 - 1/4 throttle and at about half throttle with black smoke. I chose to lower the needle jet one (now 3 clicks down whereas it was 4) and put in #17.5 pilots in. With the choke on, it runs like a perfect bike. But as soon as I close the choke, it bogs down and dies. Does anyone have any input as to what to try next? I'm on my last limb here and about at my wit's end, considering taking it to the shop, but want to fix it if I can. Unfortunately, as always, I'm on a limited budget with limited time since I'm finishing up nursing school, but I just want to ride this bike. Anyone? Any help is appreciated.

Thanks!

Mike
 
You need to sort one thing at a time. You are working on three things: idle circuit (idle to 1/4 throttle), needle clip position (mid throttle), and main jet size (wide open throttle). You need to change one thing at a time and do plug chops in the throttle position for the thing you changed. Read plugs, make adjustments, repeat. You have to do it methodically, there are no shortcuts if you want it to run right. Have you read pipes, pods, panic thread? There is already lots of great advice out there where to start.
I had #15 pilots and 115 mains, with needle in 2nd from bottom position when I had a 4:1 and pods on my '78 gs750.

Edit:
Did you dip the carbs and clean out every little passage, or just do a quick spray-down? What are the settings for your gas and and air screws on the carbs (# turns out)?
You should be able to adjust the the idle circuit to work ok with #15 or 17.5 pilots, by tweaking the air and fuel adjustment screws. Consider buying a "colortune" to help in adjusting the air/fuel ratio at idle and idle/needle transition.
VM carb rebuild guide: http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf
Service manual for your bike: http://zeus.mtsac.edu/~cliff/storage/gs/GS750_Service_Manual_early8valve.pdf
Bikecliff website of awesomeness: http://members.dslextreme.com/users/bikecliff/
Pipes/Pods info: http://www.thegsresources.com/_foru...-Pods-Pipes-Panic!-(Help!-Your-Bike-has-Pods)
 
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I've been all over the PPP thread and multiple other threads. I haven't focused on one particular thing, that's forsure. This is the closest I've gotten to it running well again, albeit with full choke. I feel like that has to say something (I've read it in a million places, but can't remember exactly where) - I've learned that it essentially temporarily fixes the air/fuel mixture, but to what extent? I have been reading BassCliff's post (http://www.iwt.com.au/mikunicarb.htm) which has helped, but don't really know where to go. In tackling the pilot circuit, I've tried 15's to 20's to no avail. Is there anything else that may affect it (I know the slides play a part)? I'm also just perplexed as to why it immediately shut off the one day and hasn't run well since.
 
A piece of crud may have gotten lodged in your carbs somewhere and messed up gas flow, petcock may have malfunctioned, etc, etc..
Square one would be to check your compression to make sure your engine is sound to start with. It should start and idle with #15 or 17.5 pilots IF you have it adjusted right, regardless of your other jets and needle position. What are your current setting on the air and gas screws on the carbs?
 
I know I need to test compression forsure, but no vacuum gauge. Could probably pick one up for cheap from Harbor Freight or on sale at Ace. But it should be good; my compression has always been good. I have no air screws on my carbs, but have pilot screws set at 7/8 which, from my understanding, is factory. Correct? Everything I've read says that it is almost unnecessary to change pilot jets.
 
You have VM carbs right, 3 screws on the top of the carb? They have a pilot air screw on the side and a pilot gas screw on the bottom. The fuel screw on the bottom should be about 7/8 turn out, and the air screw on the side should be about 1 1/2 turns out (see carb rebuild link I added in second post http://members.dslextreme.com/users/bikecliff/images/vm_carb_rebuild.pdf). You should be fine with the stock #15 pilot, as you have read. But you will likely have to adjust the fuel and/or air screws to get the mixture right with your mods. Try those setting and see if you can get it to start and eventually run without the choke as it warms up. See if any of the pipes are colder than others. Check that you have spark on all 4 plugs.
 
Try feeling the exhaust to tell see which one is not firing. Did you find debris in the carb when it was pulled apart? If they were clean, when taken apart, for the bike to start suddenly running poorly points away from fuel system. If it was a sudden fuel issue, it will come out as easily as it went in. Drain some fuel through a coffee filter, see if it comes up clean.
 
Go to Harbor Freight and for less than $30 (usually) you can buy an IF thermometer. It can help you to figure out if one cylinder is running hot or not at all.
 
I found the CV carb tuning guide to be super helpful when doing this on my GS450 - I know it's a little different with the VM carbs that there are four of them, but I think the methodology is similar - do one thing at a time.

First the full throttle via the main jets, then the mid range via the needle, then the float height, then the idle jets. Again, this is for the CV's so somebody else should correct me if it's different for the VM carburetors...
 
A faulty ignition system will mimic a fuel problem. I would start by making sure it's up to snuff, then as mentioned, check the valve lash. Anything else is poking and hoping. Jerry
 
I did the clean, dip, and rebuild of the carbs according to BassCliff's instructions on these carbs, but could never hurt to clean again. My gas flows smoothly and when I open the carbs up, it's mostly clean but with slight sediment. Since the initial sudden cut-off, I developed a leak in my 4th carb, could never figure it out, got frustrated and purchased a set of Kawasaki KZ900 carbs which, from my understanding, the only difference is a lacking air screw and the throttle stop is on top along with typically larger jets (my old VM26's lacked a fuel screw as was extensively discussed in another post of mine, they were similar to a New Zealander member's carbs). Since putting these carbs on and messing with the jets, it has run, just not well. The idle and throttle response has always been a bit off, but I've ridden it around quite a bit with poor first 1/4 throttle response and rough idle. It's for that reason why I tried increasing the pilot size, but haven't made any headway yet.

So to address most things here: I have not performed a valve adjustment, I figured that it was mostly a jetting issue creating my problem, so I have been tirelessly working on this. What it is that makes it seem like it could be a valve issue? I haven't gotten a carb tuner, just been trying to do it by throttle/listening to it. Will testing how hot one cylinder runs tell me if it is misfiring or if compression is low? The exhaust temps seem to fluctuate, of course, when I switch between 15/17.5/20 pilots as I mentioned earlier it actually seemed a bit lean with 115 mains and 15 pilots (hanging idol, kind of whitish smoke).

How do I go about checking the ignition? Do you mean points and coils? The coils look like they've been worked on as there is some JB Weld or something on them, but truthfully, considering the funds issue, I haven't purchased a new set since they're so expensive. Ideally, I would get a new set of coils, get a CarbTune Pro, and take things from there, but money is a pretty grand issue.

I know that having my budget isn't really helpful when trying to get these old bikes running, I just love these old GS's and want to finally get back to riding! Thanks for the help, all. Looking forward to your responses.
 
You can test the coils and see if they have enough out put. Pretty easy, pull #4 and lay on head, crank the motor. Blue/white is good, red is dead. Check for 12V + power to the coils
Points? Get a Dyna S
definitely adjust the valves
 
So sounds like I simply need to do the valve adjustment and then check the coils - if they check out then I will explore adjusting the carbs/jets in a stepwise approach. Thanks for the help, guys. I'll check back with an update
 
This is the first time(I believe) that you mentioned you were using KZ900 carbs. they may fit but there may be lots of fuel metering differences. Its more than a pilot and main jet. Are the slide cut away's, nozzle, needle, pilot gas, pilot air the same? I'm sure you could score some coils on here from some one who has upgraded theirs.
 
Mike, I'm in jacksonville, do you still have the original carburetors? It would be best to use them if at all possible? And fix the leak.
 
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