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Carb questions

  • Thread starter Thread starter firetruckmechanic
  • Start date Start date
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firetruckmechanic

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I got a 85 gs550e that had a 4 into 1 header on it. I know it should have been rejetted but what too??? I dont know if it already has. Also on either side of the venturis on the out side of carb on the engine side there is a screw that looks like a idle mixture screw or something like it. What is it for and what should they be set at? ( ok did a better search and found out what the screws are but still need to know the jetting)
 
Last edited:
With just the pipe, you should be able to dial in with the mixture screws and maybe a bump to the jet needle. Lower the clip to richen the mixture.
 
Dave8338 said:
With just the pipe, you should be able to dial in with the mixture screws and maybe a bump to the jet needle. Lower the clip to richen the mixture.
Click to expand...
Not quite. :oops: That might work if he had VM carbs, but his '85 550 will have BS-series carbs.

Does your bike still have the stock airbox or does it have pods? :-k

With the stock airbox, you will need to go up about 2 sizes on the mains and possibly shim the needle. Your needle only has one groove, so you will have to get some small washers at Radio Shack (the smallest ones in the assortment pack) and reduce the thickness of the large washer above the clip on the needle.

If you also have pod filters, you will likely need to go up about 5 sizes on the mains and you will definitely need to shim the needle.

The mixture adjustment screw on the side will need to start at least 2 full turns out from lightly seated, maybe as much as 3 full turns, but you will adjust that one for best idle after the bike is warm, so that is only a starting point.

As far as what jets to use, have you looked in the 'stickies' at the very top of this forum? The very first one has the running dialog where the stock sizes for most bikes were gathered. Look for your bike to see if it's there. Actually, it would be quicker to just look in your service manual. If you don't have a service manual, park your bike until you have one. If your bike is stock, you can get by without one, but when you start modifying (header, pods, etc.), you need a manual, for sure.

.
 
firetruckmechanic said:
All that it has is a pipe. Stock air box and filter
Click to expand...
Don't just assume it needs re-jetting. As you said, it may have been done.
Is it showing any lean related problems? Surging, slight pinging under load, gas mileage that seems much better than others with the same model, etc?
If no obvious signs, then the best test (without a Dyno) is to take plug reads. Test on level or uphill surface. Fully warmed up. Test the main jets at full throttle. Bring a rag to handle hot plugs. Also, a piece of straight hose that snugly fits over the top of the plug so you can re-install the hot plugs easier. Be careful of cross-threading. Find a safe place and give her a good 1/2 mile run through the gears. Then open her up full throttle for several seconds. Then "chop off" by quickly pulling in the clutch/closing the throttle completely and coasting to a stop. Remove the plugs and see what their color is. If lean, then you can go larger on the mains. How much depends on the color and any performance issues you may notice. Show us a pic if you like. Pics can be very hard to judge but they may help.
Now, do a similar run but at only 1/3 to 1/2 throttle position. Mark the throttle housing/grip if you must. It's important to test each circuit at a particular range of throttle. This test is for the jet needle. Again, chop off and do what the plugs say. If lean, then the needle must be raised as others said. How much depends on the plug/performance reads.
For the pilot circuit, you just run at minimal throttle positions. Generally, keeping the throttle at about 1/5 or less will be a pretty good pilot jet test. Just putt around on quiet streets. Wider openings will start to get overlap effect from the jet needle. Ride around a few miles to allow the plugs to change. I find pilot circuit reading to be a bit more hard to get a true read on because there's not much load and our older bikes can have a lot of vacuum fluctuation in the carbs at lower rpm's which causes read difficulty. I really doubt you'll need to change the pilot jets but you may need to richen the mixture screws a bit (additional 1/2 turn out?). Just be careful because those screws can be stuck and the heads strip too easily. If they move freely you may want to inspect and see just how far out they currently are. Record all findings. Tell us and we'll let you know if the adjustments seem normal.
Keep in mind that if the bike needs some basic tuning such as a vaccum synch or clean carbs, then your tests can be compromised.
 
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