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Not enough backpressure?

  • Thread starter Thread starter Grif
  • Start date Start date
G

Grif

Guest
Here's the deal. I've finally got the bike running after several carb. rebuilds and rejet attempts. Problem is, shes starving for fuel between 3k and 4k rpms if you don't roll on the throttle ever so slowly. At the moment, I've got:

Floats: 20 mm
Mains: 125
Air screws: 2 turns
Jet needles: 50

Diaphrams look good, so I can't figure out where to look. Maybe not enough backpressure, coils too weak?
Thanks!
Grif

Note: Forgot to mention I'm running pods and stock exhaust. An airbox isn't really an option at the moment.
 
Yup, thats gotta be it. We taped off half the pods to cut down the airflow, and it doesnt bog down anymore.

Guess the plan is to run to Home Despot tomorrow and get some foam lawn mower filters, cut'em down until they restrict the airflow through the pods enough to balance out the fuel.
 
Grif said:
Here's the deal. ....
.... Maybe not enough backpressure....
Thanks!
Grif
Click to expand...

feed her more beans n beer - works a treat for meh\\:D/


[QUOTE] ​


If I were you I wouldnt ask me....​


[/QUOTE]​
 
Grif said:
Here's the deal. I've finally got the bike running after several carb. rebuilds and rejet attempts. Problem is, shes starving for fuel between 3k and 4k rpms if you don't roll on the throttle ever so slowly. At the moment, I've got:

Floats: 20 mm
Mains: 125
Air screws: 2 turns
Jet needles: 50

Diaphrams look good, so I can't figure out where to look. Maybe not enough backpressure, coils too weak?
Thanks!
Grif

Note: Forgot to mention I'm running pods and stock exhaust. An airbox isn't really an option at the moment.
Click to expand...

Are you sure your engine is "starving" for fuel?? I had similar behavior and it was exactly the opposite scenario, I was drowning my engine with too much fuel. Read this:

http://www.thegsresources.com/_forum/showthread.php?t=86782&highlight=carbs

And some more jetting results:

http://www.thegsresources.com/_forum/showthread.php?t=86782&highlight=carbs
 
Positive. We got it up to 4k where it was bogging down, killed the engine, and checked the plugs. Ridiculously white. When we blocked flow to half the pods, it got all its power back across the board.

Best I can figure, the stock pipes aren't providing enough backpressure, so the carbs cant pull enough fuel to keep up with the pods.

I'll let ya know how the lawnmower air filters work out tonite...
 
I used to have a stumble off the line in my old GS500e. I finally gave up and took it in, mechanic stuck some additional foam into the middle of the air filter, and boom. No more stumble. I hope that it works out for you!
 
The filters restricted the air way too much for the 125 main jets I've got in it right now. I'll try rejetting in a couple days and see if that works it out.
 
Grif said:
Yup, thats gotta be it. We taped off half the pods to cut down the airflow, and it doesnt bog down anymore.

Guess the plan is to run to Home Despot tomorrow and get some foam lawn mower filters, cut'em down until they restrict the airflow through the pods enough to balance out the fuel.
Click to expand...
Why would you want to compromise the flow of your filters by putting foam in them? You're losing air which means you'll lose any power gains. This is an attempt to make the pod filters flow like a stock air box, which is what you'll have to equal if you want the jetting to "balance" itself. Just find a stock box if this is what you end up doing. This is "reverse" jetting.
Also, it's not about rpm, it's about throttle position. I'm guessing your lean reads were from "general" riding around, which would mostly be your jet needle position. You don't say what you did to the jet needles. You typed "jet needles... 50", but what does that mean?
Your bike is starving for fuel, probably from minimal throttle to about 3/4 throttle. Raise the jet needles. If your needles are U.S. model, you'll have to shim them. Try a spacer about .050" thick for starters and re-test. If still lean, maybe try .075". depends on plug colors/performance.
Don't neglect the jet needle. You use it more than the other jetting circuits.
 
Mostly because I'm not worried about losing the power that I've gained. The bike came with pods, but wasn't running when I bought it. I don't really want to mess with finding a decent airbox right now. So the way I see it, I can either restrict the airflow coming in, or buy new pipes thatll help the larger amount coming out. Seemed like the cheaper option.

50 is the size of the jet needle. Its a fixed jet needle, no shims except the plastic one on the top. Only thing I can to do make it shorter (and increase the flow) is to take that plastic off (losing about 1/8 inch height).
 
Man, the more I read about consistant jetting nightmares associated with pods, the more I am SO glad I kept my stock airbox. I was contemplating pods simply for the "easy carb acess" you gain with pods.
 
Road_Clam said:
Man, the more I read about consistant jetting nightmares associated with pods, the more I am SO glad I kept my stock airbox. I was contemplating pods simply for the "easy carb acess" you gain with pods.
Click to expand...
There can be exceptions, but it isn't that difficult.
Most problems are because the bike wasn't prepped right to begin with. Sometimes older bikes just have issues that the owner either chooses to neglect or doesn't understand what needs to be checked before starting the re-jet.
You also can't expect perfect street manners when you're attempting to increase power. Sometimes you have to accept certain minor "driveability" issues, as the jet kit makers call it.
 
I have an 82 850L with a Vance and Hines pipe and large holes in my air box with a k and n filter. I have 136.5 mains and dynojet needles on the 2nd from the richest clip setting. Runs pretty well.
I started with 127 mains and the needle all the way rich; the engine was so lean it wouldnt hardly run especially if it was colder than 70 degrees.

ECR
 
Keith Krause is korrect. Try shimming needles at least .05 (judging by where my clip is on my needle. Youll probly have to go bigger on the mains too.

Jetting is a little tedious but with close attention to detail you should be able to get those pods to run good. It takes some trial and error sometimes to dial them in but its not impossible.
Good Luck!
ECR
 
NO NO!!!! the plastic thing stays you need to add the shims to it to raise the needle not lower it. I don't have the part # But you used to be able to get a package of washers from Radio Shack that would work for shimming
 
Deesel said:
Keith Krause is korrect. Try shimming needles at least .05 (judging by where my clip is on my needle. Youll probly have to go bigger on the mains too.

Jetting is a little tedious but with close attention to detail you should be able to get those pods to run good. It takes some trial and error sometimes to dial them in but its not impossible.
Good Luck!
ECR
Click to expand...

.050" !!??? Thats WAY too much. Most all aftermarket needle shim kits are .020"

Heres where I got my shim kit: (scroll down the page and you will see the shim kit)

http://www.motorcyclecarbs.com/parts_W122.cfm?UserID=98530&jsessionid=7e30521133496640344
 
Road_Clam said:
.050" !!??? Thats WAY too much. Most all aftermarket needle shim kits are .020"
Click to expand...
There's always a critic.
Jetting spacers vary a bit in thickness. Most of the ones I have/or have used, are approx' .022/.023" thick. That's equivalent to 1/2 a jet needle position. This guys thread says he has pod filters. I honestly don't believe anything less than one full position up on the needle will help him get his mixture right. So one full position would be .044" or in that area. I also wanted to just add a bit because I think he'll need a little more and it's always best to error on the rich side. I'm also aware that many owners won't buy actual jetting spacers. Instead, they go to Radio Shack or wherever and try to find them.
So my ".050" suggestion was both because I think it may work and it's an easy number to remember/try to match at the store.
 
KEITH KRAUSE said:
There's always a critic.
This guys thread says he has pod filters. I honestly don't believe anything less than one full position up on the needle will help him get his mixture right. So one full position would be .044" or in that area. I also wanted to just add a bit because I think he'll need a little more and it's always best to error on the rich side. I'm also aware that many owners won't buy actual jetting spacers. Instead, they go to Radio Shack or wherever and try to find them.
So my ".050" suggestion was both because I think it may work and it's an easy number to remember/try to match at the store.
Click to expand...


Ah yes, the "pods" jetting again. I speak from my own hands on jetting experiences, but I have NO "pod" experience. So I will just politely shut my pie hole, and stick to airbox jetting advice.
 
I have a similar setup to the bike in question. I ditched my stock needles which had one clip setting for some dynojet needles which had I think 5 clip settings. The dynojet needles are very similar (maybe the same, except for the extra grooves) in profile to stock needles. In order for my bike to run decent at all, with the free flowing airbox(or pods), I have my clips 1 setting from the richest groove. This positions that needle a good .050" higher than the stock needle was. I went from 115 mains all the way to 136.5 mains as well.

Just trying to help because shooting in the dark buying tuning parts will get old. I wish I had this kinda of info when I did my bike. It has been worth it though!!!

ECR
 
Deesel said:
I have a similar setup to the bike in question. I ditched my stock needles which had one clip setting for some dynojet needles which had I think 5 clip settings. The dynojet needles are very similar (maybe the same, except for the extra grooves) in profile to stock needles. In order for my bike to run decent at all, with the free flowing airbox(or pods), I have my clips 1 setting from the richest groove. This positions that needle a good .050" higher than the stock needle was. I went from 115 mains all the way to 136.5 mains as well.

Just trying to help because shooting in the dark buying tuning parts will get old. I wish I had this kinda of info when I did my bike. It has been worth it though!!!

ECR
Click to expand...
It easier and not all that expensive to just get a kit. Rather than trying a hit or miss method where you keep buying jets and such and repeated trips for parts a kit will get you in the ballpark right off the bat.
 
I did that too. The Dynojet main jets were too rich for my setup. Those jet kits dont always apply to everyones setup perfectly. I could have bought a few sets of main jets and shimmed my needles for what I payed for that Dynojet kit, but I didnt know. Live and learn
It runs alot better now than it did in stock form- so Im happy.
 
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