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Discussion Starter · #1 ·
I have rebuilt the VB22 carbs as instructed in the sticky, they seem to be working well with good idle, smooth acceleration but left sided backfire on deceleration. I checked the plugs (plug chop) and the left plug is blackened a bit, the right is a nice cardboard color. Temperature checks show the right about 150 deg F cooler than the right so I'm inclined to think the right is correct as the plug looks good. So why is the left too rich and cooler. I synced the carbs as best I could by sight while they were on the bench (I don't have a vacuum tester) and they looked exactly the same. Mains are 70/110 both sides. Pilot is 1 and 3/4 out for idling. Accelerator pump squirts well. Does the extra oil rich air from the crank make a difference here? Should I look at jetting or should I go get the carbs synced by someone with the right tools? Because I have had two stroke twins in the past I checked the TDC on both sides and the cranks looks to be true (I had an RD400 which was 9 degrees out once-not good for pistons) and the compression is normal range. Any help?
Geordie
 

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Rich conditions will cause cooler engine temps and blacken plugs. The plugs should look very lightly tanned, almost white.
How were the float levels? Did you replace the float needles with aftermarket ones?
A very carefully done bench sync will get you close, otherwise it needs to be done with special tools to get correct. Then mixtures can be set correctly.
I'd start by replacing the plugs so you got new clean ones and go ride it for a few miles to recheck what the plugs look like.
 

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Discussion Starter · #3 ·
Float levels were correct, I originally replaced the float needles but they were jamming as the end of the needle was too fat so went back to the old originals which don't jam or leak (The carb rebuild kit wasn't good I think). I should mention that when I took the engine apart the left side certainly had more carbon than the right so I guess this is a long term problem. Could the float needles cause this? I'll replace the plugs and do another chop and see if it is any different but as I haven't changed anything yet I suspect it will be the same. I might change the plugs to the opposite cylinders and see if that changes anything too. Thanks!
 

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The aftermarket float needles are too short or long which causes mixture problems, since you're back to OEM's then that's not an issue.
Try swapping sparkplug wires side to side. Might be a weak spark issue. There's a sticky in the electrical section on diagnosing the ignition system that might be good to run thru to verify it's not an ignition problem, symptoms can be very close to each other.
 

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Discussion Starter · #5 ·
I went through the sticky and there are no electrical issues, the stator is good and it's a new coil so the spark is good, resistance is very low across the coil. I guess the carb is just not set up right even though I took pains to make sure the flutter valves were the same on the bench. I think I will have to have the local mechanic test the vacuum and see. If that is equal then I'm stumped as to why it would run cooler on one side, I reckon it would have to be from extra gas or slightly restricted air flow.......but how?
G
 

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The cylinder temps are dependent on the air/fuel mixture. Hotter is leaner, colder is richer.
Get the carbs sync'd and then you can set the mixtures correctly.
Process is to ride 2-3 miles or more to get the engine completely hot, sitting at idle doesn't do it. Park on the center stand and set the idle speed to 1200 rpm. Pick a carb and adjust the mixture screw to get the fastest idle speed, you have to readjust the idle down if it gets to 1300. Once that carb is done repeat with the other one. Repeat this one more time and go ride it for a few miles before rechecking.
Note that once a sparkplug fouls out it's garbage.
 

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Did you replace the brass OEM Honda parts with brass parts from the carb kits you used? If so the non OEM brass parts are not up to the same spec tolerances as Honda parts. Just like the float needles as an example, so it could also be an issue caused by the specs on these parts.
Best practice is to always clean and reuse the Honda brass parts, or buy OEM brass if you can't reuse for some reason.
 

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Discussion Starter · #8 ·
Unfortunately I did use the new brass in the carb kit and you may be right that it isn't up to Honda specs. It looked the same but like the float needles looks can be deceiving. Does anyone have a rough idea what the normal operating temperature for a CB Twin should be at the first bend in the down pipe? I had 620 left and 440 right after about a 5 minute ride. Hard to know if the one side should come up or the other go down. The better looking plug was on the hotter side but again looks can be deceiving.
 

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I don't know the temps of the pipes but 650 sounds better than the 440, burn temps on top of the piston are 1400-2100 range typically.
The sync of the carbs is important since that controls the air/fuel mixture available to burn and off a little bit can have an effect. If you have the old brass I would start by cleaning that up and re-installing it, particularly the piston needle and corresponding jet/orifice.
If the carbs have been sync'd there'll be no need to repeat that unless you split the carbs.
How were the air cut valves and the carb insulators? New or in really good condition?
I'm assuming that the compression is fairly even and within reason, over 165, and that you're using the NGK D8EA plugs.
 

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Discussion Starter · #10 · (Edited)
Compression is 125 each side (I replaced the rings only) the air cut valves were okay but not great the rubber was stiff but still pliable. The carb insulators are the originals they too were stiff but I soaked them for a while in oil of wintergreen and they softened right up. I just replaced the pilot screws(fuel mixture) with the originals. They look almost exactly the same but you can see minute differences in length and taper on the needle end of the pilot. I don't know if that would affect the motor too much. I ran it and it's still hot on the right cool on the left and I notice a slight backfire on the left but not on the right so I'm assuming it might be getting a little too much fuel? The tach on this old bike looks to be on its last legs, it will pick up the revs okay but when the bike's revs drop, it doesn't. it takes its own sweet time to drop down to the correct level which makes it hard to rely on what I see on the rev counter.
I guess I just have to have the carbs balanced; when I asked my local bike shop about doing that there was some significant looking skywards and scratching of furrowed brows. I think the age of the bike might scare them a little. I suggested they let me have the equipment and I'd do it but they didn't want to do that either. I'll fiddle with it some more and see if I can remove the backfire and potential richness on the left with the fuel screw adjustment. It certainly is giving me a bit of a run-around. As a foot note I also tried swapping the spark plug wires over, no difference and also tried spraying started fluid round the carbs with no difference so it is air tight. I has to be an air-fuel mixture imbalance which I am just not able to pinpoint ...yet.
Thanks for all the help and suggestions, it is much appreciated.
 

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Discussion Starter · #11 ·
I just got the back back from the local bike shop. The carbs are now synced, it seems they were off by quite some way even though they were even on the bench and I was comfortable with them being set correctly. They are now set up so the vacuum is equal and when I got home I tested the temp and they were within 10 degrees at about 580 each side. Now I'll work on getting the fuel mix just right and off we go. I would never have thought there could be so much discrepancy between the bench set up and the working in situ effect. It goes to show that you think you are doing the thing right but a small variation can make a huge change further down the road.
Thanks so much for all the help, I really appreciate it!
Geordie.
 

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I've only been successful in 1 bench sync matching actual vacuum sync and totally shocked when that happened. Thought to myself, "Damn I'm getting good at this", hasn't happened again.
 
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