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Lean on tip-in; choke issues

  • Thread starter Thread starter gearheadE30
  • Start date Start date
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gearheadE30

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As the title says, my 1979 GS750 has a lean spot on throttle tip in, as in its pretty abrupt. RPM doesn't have much effect. It also stutters and burbles like it's running lean when at small throttle openings. Plug readings back that up. Before I open the carbs up, I want to verify: the needles control this part of the operating circuit, right? Planning on shimming the needles slightly.

Also, I have a weird issue with the choke, as in it doesn't seem to be working right. It will work pulled all the way out, but any of the middle choke ranges makes it sound like I have a vacuum leak around the choke plungers or something. All of the orings are new. It also stutters and stumbles on choke, unless it is all the way on or off. Just looking for ideas on this one, since when I have the carbs off to fix the jetting I'll pull the chokes apart as well.
 
Stock airbox and stock pipes or what? Have you rebuilt the crabs...full dip etc etc? Where are the pilots ( bottom ) and the mixture ( side ) screws set at...we need details to make an educated accessment.
 
gearheadE30 said:
As the title says, my 1979 GS750 has a lean spot on throttle tip in, as in its pretty abrupt. RPM doesn't have much effect. It also stutters and burbles like it's running lean when at small throttle openings. Plug readings back that up. Before I open the carbs up, I want to verify: the needles control this part of the operating circuit, right? Planning on shimming the needles slightly.

Also, I have a weird issue with the choke, as in it doesn't seem to be working right. It will work pulled all the way out, but any of the middle choke ranges makes it sound like I have a vacuum leak around the choke plungers or something. All of the orings are new. It also stutters and stumbles on choke, unless it is all the way on or off. Just looking for ideas on this one, since when I have the carbs off to fix the jetting I'll pull the chokes apart as well.
Click to expand...
Ok to start when were the carbs last serviced? and has the bike sat since the cleaning if there was one?
The carbs when cleaned need to be dipped and have the o-rings replaced , considering they are probably 34 years old. Along with the carb to head mounting boots and orings.
Next it sounds kind of like you are describing a plugged low speed circuit in the first part of your post , and in the 2nd part of your post sounds like you are describing mainly plugged ENRICHENER tubes. as there is no choke on our bikes.Also these bikes will not take any throttle with the enrichener in the on position. Sounds like a maintenance problem.
 
I dipped and rebuilt the carbs about 6-7 months ago with all new orings, including the ones that go to the head. It still has VM26ss carbs, as stock. K&N pods, Kerker exhaust.

I have to go back and check what my screw settings are since I haven't worked with them in awhile. If I remember right, fuel screws were at 2.5 turns out and air screws were out to high idle. Needles are in the 4th clip position, and I had 125 mains in at first. That was slightly too rich up top, but the 122.5s I have in there now are about right. Stock 15 pilots.

Good to know on the enrichener bit. I guess it makes sense that since nothing is being choked that it wouldn't be called a choke. The sucking noise that I described as a vacuum leak appears to be coming from carb #1, but again, it only is there when the enrichener circuit is active.

The carbs were vacuum synched when I reinstalled them with the new jets. I checked fuel level in all 4 carbs and it is equal and in spec.

I don't really know where the low speed circuit transitions to the needles, but if it basically determines the mixture between 0 and about 10% throttle as well as idle, then that would make sense.

Another bit of information that might help - if the enrichener circuit is on about half way and I get on the bike and try to accelerate, it sputters a bit until it hits 3k or so rpm, and then it suddenly runs just fine and tries to hang at high idle like I would expect when running the enricheners, but it eventually 'falls out' of the high idle after a short time, if that makes sense.

All of these issues were present before rebuilding the carbs, which is what prompted me to rebuild them. Other issues were fixed, but these remain. Even before the pods and exhaust (the stock one was there but rotted pretty badly) it had that lean area off idle.

EDIT: over the winter I rode the bike about every 3ish weeks since I was riding the dirt bike and it does better in the snow. My petcock works, and the carbs were drained when sitting.
 
gearheadE30 said:
I don't really know where the low speed circuit transitions to the needles, but if it basically determines the mixture between 0 and about 10% throttle as well as idle, then that would make sense.
Click to expand...

Here's the Mikuni VM chart that shows the carb circuits and components that are in-play and at what throttle position. Looks like you'd start to feel the needle effect about 1/4 throttle.

I'd say focus on your pilot circuit (pilot jet, under side fuel screw, side air screw) and if you're feeling it's good, move to your needle height.

carb.circuits.jpg
 
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OK...Set the pilot screws ( bottom ones ) out 7/8 from lightly seated and the mixture screws ( on the sides ) at 2 turns out from bottom.

Change the pilot jets to a 17.5 Main sounds about right size.

Set the clip on the needle jets at the center clip and start from there..should be a DL36-5

Be sure that the vent ports on numbers 2 and 4 ARE NOT capped off...the carbs need to vent to equalise air pressures as the fuel levels in the crabs change.



The pilot curcuit controls the idle AND thru the transistion point from the pilot curcuit onto the main jets.
 
btbarb - that is a fantastic picture; thanks for posting that. I've always wondered about the overlap.

Chuck - I'm about to go out and check the pilot settings, so I'll let you know where that gets me. I don't think I have any 17.5s lying around so I'll have to wait on those until they ship in. I did initially start with the needles in the center position. Pulling them up a position made a very noticeable difference for the better so for now I may leave those. Basically I'm trying to change one thing at a time so I can separate the responses.

The vent ports are rerouted to stable air in the battery box area since the stock location was really sensitive to crosswinds for some reason. They should be good to go now.

I'll check the screw settings and check back later. Thanks for the advice guys!
 
Just a suggestion...put the clips back in the center groove as they should be originally. Set the screws as I said and start it and run it up AFTER 3 or 4 minutes of warming up. Yes, The cylinders DO need that heat to properly vaporise and ignite the fuel fully.

You cant eliminate "one thing at a time" if everything is not at the factory settings to begin with...thats just being counter productive.

Also the pods require very very LIGHT oiling. Too much opil acts like a choking sitution and makes things rough. Clean the pods but dont oil them (for now) and do the run up.
 
No he doesnt...I had a 4 into 1 Vance and Hines on my 78 when i first bought it and pods. If he does what I tell him he will be fine.

All this "he needs a jet kit" and a stage 2 or 3 crap is just a waste of money.

What he really needs to do is get rid of the friggin pods and put a REAL intake system on the damned thing and make it run the way it was designed to!!!!!! But all these guys think "pods look cool" and then spend a thousand dollars and countless hours of down time to only realise a stock box and filter and WHAMMY..like magic the thing actually runs!!!!!
 
chuck hahn said:
No he doesnt...I had a 4 into 1 Vance and Hines on my 78 when i first bought it and pods. If he does what I tell him he will be fine.

All this "he needs a jet kit" and a stage 2 or 3 crap is just a waste of money.

What he really needs to do is get rid of the friggin pods and put a REAL intake system on the damned thing and make it run the way it was designed to!!!!!! But all these guys think "pods look cool" and then spend a thousand dollars and countless hours of down time to only realise a stock box and filter and WHAMMY..like magic the thing actually runs!!!!!
Click to expand...

hundreds of customers and countless members on here have used dynojet kits with great success and it is very easy to set up if you have reasonable knowledge and a bit of patience. its the people who just fit the kits and expect the bike to run perfectly straight away are the ones that get frustrated and slate the kits........
 
chuck hahn said:
Just a suggestion...put the clips back in the center groove as they should be originally. Set the screws as I said and start it and run it up AFTER 3 or 4 minutes of warming up. Yes, The cylinders DO need that heat to properly vaporise and ignite the fuel fully.

You cant eliminate "one thing at a time" if everything is not at the factory settings to begin with...thats just being counter productive.

Also the pods require very very LIGHT oiling. Too much opil acts like a choking sitution and makes things rough. Clean the pods but dont oil them (for now) and do the run up.
Click to expand...

I have only been making changes after riding the bike for 10 ish minutes so it is up to temp, and not heat soaked from idling in one place for awhile. I learned that one the hard way awhile back, heh.

The pods are clean and lightly oiled.

When I got the bike, it was all factory. Then I rebuilt the carbs to factory spec. Then my airbox caught fire and I switched to pods and rejetted. Then I got a free Kerker system to replace my factory exhaust, which had fist sized holes in both mufflers, and rejetted again. I've been pretty methodical about it.

Earlier I went out to check my needle settings - they were about 1 turn out on the fuel screws and a little over 2 out on the air. 45 minutes of riding got me to 1.5 turns out on the fuel and 1.75 turns on the air, and the problem is much better but still there. I couldn't get it any better than that, so I'll probably move the needles back down as per your suggestion.


SqDancerLynn1 said:
You have a pipe and pods !!!!! YOU need a jet kit !
Click to expand...

Heh, nope. VM carbs. Jet kit isn't worth the money, and I think I have the know how and the support here to do better than a jet kit on my own.

chuck hahn said:
.....What he really needs to do is get rid of the friggin pods and put a REAL intake system on the damned thing.....
Click to expand...

Do you mean going back to stock, or a modified stock airbox or something? I'm definitely willing to look into that or fab something, especially considering I ride in the rain on occasion, which pods really, really don't like. I haven't looked, but I'm sure it would be easy enough for me to find another factory airbox on Ebay or something.
 
gearheadE30 said:
I have only been making changes after riding the bike for 10 ish minutes so it is up to temp, and not heat soaked from idling in one place for awhile. I learned that one the hard way awhile back, heh.

The pods are clean and lightly oiled.

When I got the bike, it was all factory. Then I rebuilt the carbs to factory spec. Then my airbox caught fire and I switched to pods and rejetted. Then I got a free Kerker system to replace my factory exhaust, which had fist sized holes in both mufflers, and rejetted again. I've been pretty methodical about it.

Earlier I went out to check my needle settings - they were about 1 turn out on the fuel screws and a little over 2 out on the air. 45 minutes of riding got me to 1.5 turns out on the fuel and 1.75 turns on the air, and the problem is much better but still there. I couldn't get it any better than that, so I'll probably move the needles back down as per your suggestion.




Heh, nope. VM carbs. Jet kit isn't worth the money, and I think I have the know how and the support here to do better than a jet kit on my own.



Do you mean going back to stock, or a modified stock airbox or something? I'm definitely willing to look into that or fab something, especially considering I ride in the rain on occasion, which pods really, really don't like. I haven't looked, but I'm sure it would be easy enough for me to find another factory airbox on Ebay or something.
Click to expand...

good luck ;)
 
Let me explain something about VM carbs.

How does the transition off pilot to main curcuits happen..By the slides going up and changing the diameter of the venturi..which changes the air pressure to a lower pressure.
This makes a low pressure area around the air jet which sticks up in the middle of the carb throat, which in turn, causes fuel to be sipped up the hole and into the carb and then into the cylinder. ( picture a full to the rim pop bottle and you suck the pop from the rim with your mouth) Your mouth has created a low pressure around the rim of the bottle and you can sip pop (fuel )up from it. This is the basic same principle that the carb is doing.

If the jet needle is too LOW into the needle jet, then you dont get the right timing for the fuel to be sucked up..nor the right amount.

The pilot jet supplies fuel up to AND thru the transition from pilot to main curcuit. So if the pilot jet is too small for the requirements of the setup, you will get a stutter, stumble, call it what you want during the transition. So you need the jet needle setting AND the correct pilot size to compliment each other to effectively make a smooth and strong transition.

As for why the stock airbox is better. Simply put, it supplies a tad bit of resitance to the air flow ( which has been figured at the engineering level ).
This restriction also add to the low pressure inside the venturi to make the carbs suck the fuel up thru the needle jets more efficiently.

If you take the pods off and just lay a red shop rag over the carb throats and start the bike, youll see that it runs up better and with less stumble than it will without anything there at all. This is why these Suzukis always run better with the stock boxes. I will post in a follow up an article that is very informational and will support what I have just said.

Also note that fuel levels in the bowl will also effect if the mixture is lean or rich. Very HIGH fuel levels will make the carbs run richer because the require a leeser low pressure in the venturi for them to sip fuel from the bowls...just the opposite for too LOW a fuel level.
 
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Agemax - too cocky? haha

Chuck - That's super helpful. I also found this article when I was searching. It seems like a bit of an advertisement, but has some good explanations.

http://www.thunderproducts.com/AirboxArticle.htm

While the extra power is nice (and pretty noticeable) looks like I'm going to start looking for another airbox, and will move the needles back, and go from there. Thanks for all the advice.
 
Dropbox.com used to have many good OEM manufacturer articles. I had one bookmarked but now the link says error when i try to open it.
 
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