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Carb Questions

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Did a throttle test today and come up with lean conditions across the board. Did a high speed chop test on the road and came up lean. When I decel a little after going 3/4 throttle or about 80 mph for awhile the bikes wants to die. During this I hit the kill switch and checked my plugs on the side of the highway and they were lean. Could'nt get it to start unless I gave it full choke and then it took off like nothing happened. This sounds like fuel starvation to me but how come at mid throttle and not at full throttle? This could be a float or needle jet adjustment? The lean conditions is probably why my bike likes the 100 degree F + summers.
 
If you're lean at 1/3... 1/2 throttle position, this is a lean jet needle. As I said earlier, the pipe and the filter, even with the air box lid on, will make the needle circuit run lean. If your jet needle isn't adjustable, you need a stage 1 jet kit or shim up the needles.
I also asked how you set your float levels and what part of the float did you measure at? Float levels must be set within the factory range before re-jetting.
 
The floats are set at 22 mm. The Clymers manual states 21.4 mm to 23.4 mm. The used carbs I bought have a dynojet jetting kit in it. The needle jets have 6 notches and currently they are set at 4 notches down from top. The Dynojet websight stated to start at 3 down for stage 1. If I were to put these needle jets in can I still go with the jet sizes I have now. Also, the Dynojet instructions stated to drill the slide lift holes and if this was done to the diaphragms I am using currently would this cause a leaner condition? I also noticed the diaphragm springs were much shorter then stock springs. Is this why you drill the lift holes? Should I get 120 main jets or do you think this will cure my high end woes.
 
chef1366 said:
The floats are set at 22 mm. The Clymers manual states 21.4 mm to 23.4 mm. The used carbs I bought have a dynojet jetting kit in it. The needle jets have 6 notches and currently they are set at 4 notches down from top. The Dynojet websight stated to start at 3 down for stage 1. If I were to put these needle jets in can I still go with the jet sizes I have now. Also, the Dynojet instructions stated to drill the slide lift holes and if this was done to the diaphragms I am using currently would this cause a leaner condition? I also noticed the diaphragm springs were much shorter then stock springs. Is this why you drill the lift holes? Should I get 120 main jets or do you think this will cure my high end woes.
Click to expand...
For the floats, be sure you set the level by measuring from the bowl surface (no gasket) to the top of the rounded float body...not the arm that supports the float (common mistake) or where the float "steps up".
I would use the DJ needles because you should be lean at the needle circuit. I can't say where to put the needle e-clip. If DJ suggests position 3 from the top, try it (?) and test. I find most of their "base setting" recommendations are on the lean side. I would probably try position 4 from the top, but I'm simply going by gut feelings. A 1/3 throttle test will tell you what to do. Mark the throttle housing and grip to be sure. Don't guess. Because you have 3 separate jetting circuits that regulate mixture at different throttle positions, you can stay with the current pilot and main jets but as always you have to test for each circuit. I like to get the needle perfect, then I simply install the largest main jet I can that will not create any bogging during a roll-on at 60 mph in top gear. Generally, you install the main that provides the best top end, but in some cases, this will create a bog during the roll-on. To me, the needle circuit is the most important and I won't compromise passing power/roll-on quality for a few more mph. You should not have to keep this kind of bike revving high to get good burn/acceleration. You shouldn't have to tap down a gear or two to avoid bogging. It should pull like a train from 3,500/4,000 rpm in top gear up to its top end.
When you say you tested at "3/4 throttle or 80", I'm not exactly sure what you mean. 80 mph in top gear on your bike is maybe 1/4 throttle. When testing for the main, go full throttle. Then there's no possible overlap with the jet needle. I realize these are high speeds but it's the only way I can suggest to test without a Dyno. I can't say if the 117.5 or 120, etc, is the correct main. You have to test and find out.
The purpose of drilling out the vacuum ports to the diaphragm/piston assembly is to improve throttle response. It's generally necessary when you run pods or modify the airbox. Your airbox is stock. The K&N filter doesn't change anything here. I wouldn't touch the ports or change springs.
By the way, are your two floatbowl lines clear, not kinked and run under the seat?? Some people connect them together and other stuff that will cause major fuel starvation.
 
I measured the floats on the body, the flat area, without a gasket. I tried these needles once when I first tried these diaphragms. Even though at that time I tried the 45 pilot and 112.5 mains I had a terrible flat spot at 1/4 to 1/3 throttle. I don't know if this was the jetting or the needle position. Trial and error on jets, shoot I should just pod the carbs for easier tear down.
 
Looking back It looks like I have to reset my floats. I only checked the floats but did not adjust because they appeared to be in line. Would this cause it to run lean? If the hole was already drilled in the slide pickup could this pose a problem? Man, my whole jetting plan could go in the sh***** if something as basic as my float adjustment is off.
 
If you got free flowing filter's and exhaust then 120 mains ain't going to cut it. If you using DJ mains then take off 8 points cause in Mikuni land that's like a 112 jet size.

Don't know how you guys get away with running those small jets. Must be the Texas air cause my bike will fall flat on it's face with anything smaller than a 140 and that's when it was a 1075 street motor and 34cv's :? must be those resticted exhaust systems you guys are running :D
 
chef1366 said:
Looking back It looks like I have to reset my floats. I only checked the floats but did not adjust because they appeared to be in line. Would this cause it to run lean? If the hole was already drilled in the slide pickup could this pose a problem? Man, my whole jetting plan could go in the sh***** if something as basic as my float adjustment is off.
Click to expand...
Float adjustment is a basic. It must be correct before re-jetting. I've mentioned this several times in my replies.
The purpose of drilling the slide vacuum ports is to increase/improve throttle response. This is necessary when modifying the air box or running pods. If the slides are drilled with a stock intake, I would have to think the slides are too sensitive to throttle openings. But I have no experience with this combo.
 
Pulled the carbs off again and put in the 45 pilot jet and set the floats to 22mm. Set the air screw for highest idle per cylinder and sync. The bike runs the best ever. Pulls like a train in 5th gear from 3.5 rpm to red line. Still one problem. If I run 85+ for a mile or so and back off it wants to die. If I run 70 to 80 it is fine. Pulled the petcock off and there was a broken metal tab halfway over the air port that connects to the vaccum line. Cleaned it up and took the tab out and checked the valve and it appeared to operate properly. Still same problem. Is this a bad petcock symptom or could this be a result of using the slides with the drilled ports or maybe I need to set the floats richer?
 
Pulled the carbs off again and put in the 45 pilot jet and set the floats to 22mm. Set the air screw for highest idle per cylinder and sync. The bike runs the best ever. Pulls like a train in 5th gear from 3.5 rpm to red line. Still one problem. If I run 85+ for a mile or so and back off it wants to die. If I run 70 to 80 it is fine. Pulled the petcock off and there was a broken metal tab halfway over the air port that connects to the vaccum line. Cleaned it up and took the tab out and checked the valve and it appeared to operate properly. Still same problem. Is this a bad petcock symptom or could this be a result of using the slides with the drilled ports or maybe I need to set the floats richer?
 
If you have any doubt about the petcock flow, just be sure it's clean and run the bike on prime. This will eliminate any vacuum related issues. Fuel line should be 5/16" also. If the bike still has the 85+ problem, it's not because of the petcock. Also, be sure the two floatbowl vent lines are clear/not kinked. Be sure the gas cap vent is clear.
I don't know exactly what would cause a problem at 85 but not up to 80.
Mixture or a vacuum issue? Both? Drilling out the slides with stock intake will effect the vacuum. Throttle response will be negatively effected, but I don't know why a small difference in speed would create such a drastic problem. Float level will always change things, but if the floats are set within the factory range, they should be fine.
You mentioned the same problem about a week ago and you said the plugs looked lean when you pulled over. Marking the throttle would help as to which jetting circuit the bike was using at the time of the problem. If you were going a steady 85 in top gear, you would be solidly on the jet needle (about 1/3 throttle position). If so, and the plugs were lean at this position, then it's obvious you need to raise the jet needles a position and test again. Raise the needle by lowering the e-clip one position.
Incorrect mixtures can cause all kinds of performance issues. I don't know if the stalling problem will go away, but your lean plug reads tell you the mixture must be richened.
 
The 1980 had the legendary petcock with no switch, only vaccum. Just bought a brand new one and am going to try pulling the vent lines in the morning.
 
chef1366 said:
The 1980 had the legendary petcock with no switch, only vaccum. Just bought a brand new one and am going to try pulling the vent lines in the morning.
Click to expand...
I'm familiar with that petcock. It has prime and on positions only, by turning with a slot screwdriver.
For some reason, the prime on this petcock needs the engine cranking to initiate fuel flow while on prime. Other vacuum petcocks don't require vacuum/cranking while on prime. But since you have a new one, the old one and possible problems don't matter.
 
I have read your comments about a "vortex" that can occur in your vent line hoses. Will it do any harm at all just to remove them as long as I don't spray water directly into them? The symptoms of my woes also fits the description of the vortex symptom.
 
chef1366 said:
I have read your comments about a "vortex" that can occur in your vent line hoses. Will it do any harm at all just to remove them as long as I don't spray water directly into them? The symptoms of my woes also fits the description of the vortex symptom.
Click to expand...
It won't change a thing to remove the vent lines and leave the ports open, as long as you're careful with water.
With a stock intake, removing the lines shouldn't improve fuel flow/venting. Your model is made to run with the lines on. If removing them does help you, and they're the correct size and clear, I couldn't explain it.
Without reading back, I think you said the carbs are clean, but it doesn't take much to screw up the venting. I've seen a small piece of rubber break off from the tubing between carbs and PARTIALLY block the vent passage that would vent two carbs. This partial block drove the owner nuts for months, thinking it was a surging/lean jetting problem. Only because of experience did I try blowing evenly into both vent lines. I could tell a small difference in resistance at the partially blocked vent. Found the little rubber piece by blowing it out with almost 200 psi. Bike ran fine. Turns out he pushed the carbs together during a rebuild and broke off a bit of his 1/2 rubber vent coupling hose.
A wad of spider webs will do the same thing. Doesn't take much.
 
The petcock was the issue with the highway speed meltdown. Now I'm back with the top end issue. Running great with good plug coloration from idle to 1/4 throttle, 1/4 to 3/4 but lean from 3/4 to full throttle. What higher jet would you start with, 120 or 122.5? Is the stock needle for this bike pointed or rounded at the end. The one I have is rounded and I've read that that could be an old dynojet needle. It does not have adjustable clips though. I tried the Factory brand needle but could not find a clip position that worked well. Believe it or not I tried all six( I did not have to remove the carbs to do this) If I increase the main jet size am I back to the drawing board on needle position and pilot jet size?
 
To get good plug color/performance while on the jet needle, you must have shimmed the stock needles? The Factory jet needles may be the wrong ones for your carbs, so they didn't help. I don't know the tip shape of your stock needles.
If you have lean plug reads at 3/4 to open throttle, I'd go up a full size to 122.5 and test. I don't know HOW lean the plug reads are, but it sounds to me like one step (2.5) wouldn't be enough of a change.
Hopefully the larger main won't create any bogging while rolling on full throttle from 60 mph in top gear. Sometimes there's an overlap between the needle and main circuits that forces you to decide. I like to jet a little "backwards" of what some like to do. I insist on getting the jet needle right. Then I go with the main that gives me the best top speed WITHOUT creating any roll on bog. If a bog does occur, I'll sacrifice 2-3 mph top speed to eliminate roll on bog/hesitation. Responsive/steady passing power is too important. Some decide to go with the main that gives them the best top speed, and they downshift a couple gears during a roll on to "bypass" any bogging. A motor like yours shouldn't have to be downshifted like this to have good roll on power.
 
I did not shim the needles just rebuilt the carbs with new o-rings, set the floats properly and got a new petcock. To shim the needles you would just add tiny washers to the underside of the top of the needle just above the spring? The ceramic is snow white on the plug during a full throttle chop. Can you remove the main jets without removing the carbs? Seems like you could.
 
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