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Discussion Starter #1 (Edited)
I did some searching around this thread and was unable to find anything, so sorry if there's already a thread with this answer

Anyway, the problem is that I cannot get the idle to go below 1500 rpm. I had just recently rebuilt the carbs, with 150 main and 36 (i think) pilots, and had synced both carbs using guitar string as the measurement. I also replaced the carb boots and gaskets, and replaced the felt seals on the throttle shaft and applied grease to it.

When I first put the carbs on after the sync with the guitar string, It would idle at 2000 rpm cold. As it warmed up, the idle would get higher. I could lower the idle screw (evenly on both sides) but the rpm would only drop to 1500 before the butterfly valves would just completely close, with no more room to lower it.

Any thoughts on this?'

edit: I read something about the timer advance springs would be worn and wouldn't close all the way. Took a look on mine and the springs were not pulling the assembly back together the way it should . I cut the end of the spring and bent the hole to make it shorter. It works now. I redid the timing after fixing this.

It helped with the idling issue, but did not solve it. It brought it down from 2k to 1.5k
 

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Discussion Starter #4
Tell us more about the guitar string calibration. Never heard of it.
I saw a video on youtube about it. you put a guitar string between the wall and the butterfly flap and turn out the idle screw until the string pulls out. You match the resistance n both sides and that should "sync" the carbs. It's an easy way without having to add vacuum taps on the intake.
 

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Discussion Starter #5
Try the "straw test" on your carbs. If the slides don't return it will keep the RPM's up. I couldn't get the idle down on my bike until I polished and cleaned the slides and covers. They should move up and down freely without sticking. https://www.hondatwins.net/forums/56-fuel-supply-carburation/17370-straw-test.html

View attachment 278580
Just tried it. Blowing into the two top holes resulted in the slide moving. I was unable to get it all the way up (probably because of how much larger the diameter of the straw was than the hole) but it was moving smoothly and would always set back down into position. I could flick it up with my fingers and it would come back down fine

The brass hole on the bottom had no effect on both carbs however. Blowing into that did absolutely nothing to the slide, even with a lot of air.
 

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Read your FSM. Learn how your carburetor works. You want to focus on the relationship between idle stop screws, and air/fuel mix screws.
 

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Discussion Starter #7
Read your FSM. Learn how your carburetor works. You want to focus on the relationship between idle stop screws, and air/fuel mix screws.
I understand how the carb works for the most part. The problem is that I can't lower the idle. The butterfly valve in the carb completely closes but is still idling at 1500 rpm. I'm unable to adjust anything if I can't get the idle lower
 

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Moving up and down easily is a good sign. With these carbs you want to 1st check for any air leaks, check the slides and make sure nothing is clogged. Did you put the carbs in a ultrasonic cleaner and check that all passages were clear? Here is a diagram of the carbs, the upper holes create the vacuum to move the slide (I'll confirm this on my bike tomorrow).

CB450 Carb Diagram.jpg
 

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No, the two holes in the bottom of the slide piston on the engine side allow vacuum to affect (reduce slightly) the (previously atmospheric) pressure up above the slide......Full Atmospheric pressure entering through the "vacuum port" gets under the slide and lifts it until the pressure differential equalizes..... This is why the slide does not lift much at small throttle openings......
However, as you open the throttle, the differential is more as the volume and speed of the intake air (across the slide holes in the venturi) increases the vacuum above the slide causing it to rise higher to its balance point.....
 

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That "guitar string" didn't work for my 450.. same issue, high idle.. back the idle adjustment screws untill they no longer touch, these are the screws on the side of the carb body with the spring..(not the idle MIXTURE screws). Those plates should be closed. I this is how I can tell of one carb is getting fuel to the motor, especially on early morning startups.. with the bike running, open each carb individually by the adjustment screw lever with your fingers, if RPMs increase ur good, if not .. no fuel..,my issue was the idle jet.. tiny bit of trash. C
I would also check for a vaccume leak, though rough/low idle is a symptom.
Polishing the top / plungers will help..
 

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Discussion Starter #12
That "guitar string" didn't work for my 450.. same issue, high idle.. back the idle adjustment screws untill they no longer touch, these are the screws on the side of the carb body with the spring..(not the idle MIXTURE screws). Those plates should be closed. I this is how I can tell of one carb is getting fuel to the motor, especially on early morning startups.. with the bike running, open each carb individually by the adjustment screw lever with your fingers, if RPMs increase ur good, if not .. no fuel..,my issue was the idle jet.. tiny bit of trash. C
I would also check for a vaccume leak, though rough/low idle is a symptom.
Polishing the top / plungers will help..
If I recall correctly, when I was adjusting the idle, one side would have an effect on the rpm while the other side wouldn't. I thought they had to be "balanced" before the rpm change would take place. Guess not.
I forget what side that was on so I'll check again, hopefully today. I also have to work out the left carb overflowing for some reason. But thanks for this info. I had replaced a couple parts with 4into1 replacedment ones, maybe they are not good.
 

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Do you have some slack in the cables,when the throttle plates are closed?
 

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Each cylinder has its own separate carb.. there is no crossover or ballanxe tube. There are some real good videos on adjusting cables, sync for carbs (not balance), and one I found on making sure both butterflies are even at WOT. Unless you are REAL experienced, ballencing the carbs should be done with a monometer / gauges. But unfortunitly the 450s didn't come with ports for these tools.
 

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Everyone has their preferred method. For me, with the engine hot I remove one spark plug, get it to run on the other cylinder, then reverse the process. Knowing each cylinder can now run the engine on its own, I run them together. The idle is now too high, so I back off the throttle plate screw a little on each.

I understand the guitar string method now, physically keeping the throttle butterfly open a minimum amount, the distance between the butterfly at the top or bottom to the carburetor wall to maintain an open venturi. I've not tried it. I know the gap is quite small as limited by the throttle butterfly stop screw.
 

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Discussion Starter #16
Thanks for all the advice.

I ended up letting the bike warm running on both cylinders. I then took the right spark plug out and ran it on the left cylinder.

Left side ran fine, adjusted the idle to 1100 rpm, and my idle mixture screw set on that side to highest rpm. When I took that spark plug out it was a nice brown/tan color, so the left side seems to be running just fine.

The right side however, not running as well. I can tell that’s it’s not firing every single time. Rpm was fluctuating, and I could tell audibly that it wasn’t firing every time. I had checked the timing and it was good.

I had taken the pilot and main jets out (while in the carb) and shot carb cleaner through before hand, and it still ran like crap

So I’m going to take the right carb off and take a closer look. Is there anything specifically that I should look for?
 

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Discussion Starter #17
update: I noticed that my carb was slightly crooked because the air filter is pressing up against the battery box and pushing the carb out a little. I fixed that, ran it again on the right cylinder, and sprayed with carb cleaner but there seems to have been no air leak because the problem is still there.

I did get more time to test It though on the right side.

What i noticed is that when it's idling, the rpm goes slightly up and down. Doesn't change on the needle, but it can be heard.
The main part is that whenever I would give it some throttle and let it set back down, the rpm would drop way below what it was idling at.
example: it went from idling at 1200, up to 3000 when I revved it, and dropped down to 800, and needed my throttle input to get it back at 1200, or else it would die.

Any idea on what this could be? Clogged path on the carb to the pilot jet?
 

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Discussion Starter #19
Damaged tip of mixture screw maybe?
I checked, and compared it to the left one. The tip is not damaged. I did end up finding a leftover, dried out and broken up o ring that must not have fallen out when I did the carb rebuild.
The new o-ring that I placed in was just kind of pressing up against it and there was a small piece that was right next to the hole that could have been making the fuel not go through right.

I started the bike again, and it did run better, but It still wasn't running as strong as the left. It still would have that little "dip" in rpm when I gave it throttle, and there was one time were there was a pop when I blipped the throttle.

I did my best to take out the old o-ring and visibly with a flashlight there is no old o-ring left, but maybe some bits got inside the little hole, or there is some I cannot see?
 

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Discussion Starter #20
I took both carbs off today in an effort to try and try cleans and make it run. I spent a long time making sure all of the ports are clean on both carbs. I also checked the timing and they were both spot on.

On the left carb (the good one), I had found that there too, had been an old o ring stuck inside the idle mixture screw, and even though it had been running fine, I took all the bits out and made sure the holes and paths were clear from that before putting the screw back in. Other than that, the left carb seemed to be in good shape from my understanding.

On the right carb, it looks just like the left one. I had cleaned out the same idle ports, and I had removed the old stuck o ring before but I checked the passages again and they were clear.

For both carbs I checked the float height, 20mm, used a credit card and did the wet method. I also did a quick sync with the guitar string method to set them right again

I put both carbs back on and ran it. Started right up, but still not running quite right. The idle still was wonky. I could hear the rpm going up and down, but it's almost non existent on the rpm gauge. I can see it move up and down a little bit, but it's hard to tell because the needle kinda shakes. I let the engine sit at 3k rpm for a few minutes so it can warm up. I then ran the bike on just the right cylinder, and here is a video.


it's hard to tell but it sounds like its not firing every time. Also, the rpm seems to drop below the idle whenever it comes back from higher rpm.

Another note: when both cylinders were firing, the left side was super hot. Like it almost burned my hand when I touched the exhaust. The plug was black. This was like 30 seconds after starting it cold. On the right side, I could hold my hand on the exhaust. Also, feeling the exhaust pressure, I could feel the left gas to be a lot hotter than the right.

Sorry for so much information, Im just chasing my tail with these damn carbs. the bike is so close to being up and running
 
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