• Required reading for all forum users!!!

    Welcome!
    Register to access the full functionality of the GSResources forum. Until you register and activate your account you will not have full forum access, nor will you be able to post or reply to messages.

    A note to new registrants...
    All new forum registrations must be activated via email before you have full access to the forum.

    A Special Note about Email accounts!
    DO NOT SIGN UP USING hotmail, outlook, gmx, sbcglobal, att, bellsouth or email.com. They delete our forum signup emails.

    A note to old forum members...
    I receive numerous requests from people who can no longer log in because their accounts were deleted. As mentioned in the forum FAQ, user accounts are deleted if you haven't logged in for the past 6 months. If you can't log in, then create a new forum account. If you don't get an error message, then check your email account for an activation message. If you get a message stating that the email address is already in use, then your account still exists so follow the instructions in the forum FAQ for resetting your password.

    Have you forgotten your password or have a new email address? Then read the forum FAQ for details on how to reset it.

    Any email requests for "can't log in anymore" problems or "lost my password" problems will be deleted. Read the forum FAQ and follow the instructions there - that's what we have one for...

  • Returning Visitors

    If you are a returning visitor who never received your confirmation email, then odds are your email provider is blockinig emails from our server. The only thing that can be done to get around this is you will have to try creating another forum account using an email address from another domain.

    If you are a returning visitor to the forum and can't log in using your old forum name and password but used to be able to then chances are your account is deleted. Purges of the databases are done regularly. You will have to create a new forum account and you should be all set.

VM carburetor fine tuning

  • Thread starter Thread starter Guest
  • Start date Start date
G

Guest

Guest
Hi all,

I've recently gone through some pretty extensive work on my 1978 GS750 and it runs great but there's some small issues i'd still like to solve. I have replaced the points, did the valve adjustment, and replaced the carburetor O-rings as well as the boot O-rings. I finished installing the carburetors, set the idle fuel screws and air screws. The fuel screws liked to be just barely over 1 turn out on all 4 and the air screws are right around 2.5 out on all 4.

The bike seems to run great almost all the time. But I am getting a minor carb backfire, it seems like it's the number 2 carburetor but I can't really tell for sure. When I took the carbs off, all 4 boots were a little wet with fuel and I couldn't see any obvious signs of backfire on the intake tubes. If I ride say 80 miles in a day it'll only backfire maybe one or two times which I think is strange. It usually happens if I lightly blip the throttle for a downshift or starting out at a green light just off idle. The throttle response is a little sluggish if I go straight to full throttle off idle but unnoticeable in normal riding. I'm also getting some popping out the exhaust on deceleration although this may be normal with a 4 into 1 i'm not sure. I should probably mention the bike is entirely stock with the exception of the unknown brand 4 into 1 and a UNI filter in the stock box. It has a 15 pilot jet and plastic mains that are stamped 120 but I put in a set of 117.5's and they were way to rich so they must be stamped wrong.

The last thing is that I seem to be getting some fuel out of the bowl vents/overflows. I can't see it dripping when I put the petcock on prime and I never see any drips while riding or at a red light. But when I get back from a long ride the bottom of the vent tubes are a little wet and the bike smells like fresh fuel after parking it in the garage.

Some things I've already checked are the vacuum line to the petcock I checked with carb cleaner and by pinching the line with no change in the idle. I've also checked to make sure the new intake boot O-rings sealed. One thing to note is that when syncing the carbs there seemed to be some slight vacuum leaking past the slide into the top cover area on the #1 carburetor and not on the others. Is this normal?
 
Put your hand on top of each carbs as its running at around 2000 RPMs. When it pops youll feel the carb shudder and youll know which jug is off. Now turn the SIDE MIXTURE SCREW out 1/4 turn and see if it goes away, stay or gets worse on the carb.

If it stays or gets worse, then the jug was already lean. So turn the screw back in the 1/4 turn to where it was. Then go IN 1/8 turns till it doesn't pop anymore. Make and adjustment and wait maybe 20 seconds for the cylinder to equalize and see what happens. This is a poor mans tuning but its better a tad rich than lean.
 
With the fuel screws at just over one turn, I would expect the air screws at just under two turns, not 2.5. I agree with Chuck, you are likely a bit lean.

.
 
I'll go ahead and give that a shot. The problem is like I said it does it maybe once or twice over the course of 60 miles so i'll just have to get lucky with putting my hands on the carbs at the right time. The reason I had the side screws around 2.5 is the plugs are still showing it running a bit too rich. I have ordered a variety of jets so I can figure out whats in it and go down a size.
 
Look at the parts fiche for stock sizes. That is if you have a stock airbox and stock pipes. The problem may NOT be jets at all. The cylinder can be very much adjust through a HUGE range with the side mixture screws regardless of the pilot and main jets.,.thats the beauty of VM carbs. You can also get richness from the choke plungers not seating fully. Some things to check on the plungers are the following::

When you lift them manually with your finger ( the lifter fork is removed at this point ) do they make a "thunk" sound when you let go of the plunger and they go back in?? If so, then can you feel them seat a little more if you push down on the plunger?? If yes, then they arent seated fully.

Are any of the long thin tips bent holding them from fully seating??

Are the seats ( rubber inserts in the ends of plungers ) hard, cracked, missing a chunk??

Are the raised rims inside the hole in the carb body corroded and holding a seat up a little???

And the periodic popping may be the extra fuel from a leaking choke igniting in the cylinder from time to time.
 
chuck hahn said:
Look at the parts fiche for stock sizes. That is if you have a stock airbox and stock pipes. The problem may NOT be jets at all. The cylinder can be very much adjust through a HUGE range with the side mixture screws regardless of the pilot and main jets.,.thats the beauty of VM carbs. You can also get richness from the choke plungers not seating fully. Some things to check on the plungers are the following::

When you lift them manually with your finger ( the lifter fork is removed at this point ) do they make a "thunk" sound when you let go of the plunger and they go back in?? If so, then can you feel them seat a little more if you push down on the plunger?? If yes, then they arent seated fully.
Click to expand...

I checked all the plungers when I had the carbs apart. The rubber seal is still pliable and they move freely all the way to the seated position. I did just check to ensure I had all the levers installed in the correct direction which I do. Is there a chance the petcock could be leaking only periodically? I had originally checked it by both blocking off the vacuum line to see if the idle changed and spraying carb cleaner around the petcock seal to look for a leak but couldn't find any. The previous owner did tell me he had a problem with the petcock and installed a secondary inline shutoff valve. It also does seem to be coming from the #1 or #2 carburetor. But the petcock seems to work like it should
 
Got pictures of this secondary shut off?? The best test i have devised for petcock leak testing is to get two longer chunks of line and out one on each petcock nipple. run one hose into s dry empty soda bottle and the other into a second bottle. let it set over night..or a few days id the weathers gonna be nasty. if its leaking from eihter the main fuel feed or if the vacuum side one of the bottles will have fuel in it indicating what side of the petcocks gone bad.

If one of the bottles gets fuel in it dont wast the money or time on rebuild kits...ask anyone here thats been duped by that crap. Replace with new OEM
 
If its leaking vacuum it probably wouldnt work at all. The hose going to the petcock can develope a leak though.
 
chuck hahn said:
... dont wast the money or time on rebuild kits...ask anyone here thats been duped by that crap. Replace with new OEM
Click to expand...
Yep, have to agree with that. The success rate on rebuilding a petcock seems to be about 10-15%. That means that 85-90% of the time, you will spend money on the rebuild kit AND a new petcock.

Now, slipping into Clint Eastwood mode ... "How lucky do you feel, punk?"

.
 
I went to a Pingel valve and haven't looked back. Simple no vacuum operation. Manual on/off/reserve.
 
Are u sure?,u r valve ant a tad on the tight Side?
Just a thought?
 
81gs7501166 said:
Are u sure?,u r valve ant a tad on the tight Side?
Just a thought?
Click to expand...

About 4 of them where a little too tight when I did the adjustment. Now they're all as close as possible to the mid range of the spec.


I tested the petcock, it checks out fine. I swapped in a set of 102.5 jets (one size larger than stock). The bike has noticeably more torque but seems like the power levels off at around 6k rpm until redline. I have about 40 miles on the jets and the plugs still look like the same rich soot. Surely the bike with an aftermarket exhaust wouldn't take a jet smaller than stock?
 
On mid-80 model bikes with slip ons and CV carbs, we would drop 1 size on the main's. Manufactors ran them rich so they would burn down on extended WFO runs.
 
limeex2 said:
On mid-80 model bikes with slip ons and CV carbs, we would drop 1 size on the main's. Manufactors ran them rich so they would burn down on extended WFO runs.
Click to expand...

It's my understanding the CV carbs are very different than the VM series. Do you think this still may apply to my situation? I can get the bike to run decently well and have a nice color on the plugs by turning the air screws way out but by doing that I sacrifice idle quality
 
20171021_002200.jpg

I had a hard time getting this to show up in a picture. I went for a ride after re adjusting the mixture screws again and all four spark plugs have varying levels of this sulfur colored powder on the inside wall of the plug. This one was the worst. Anyone know what it is?
 
You must log in or register to reply here.
Back
Top