Have you visually checked that the carbs completely close? Have you compared the original idle jets to the ones that came in the rebuild kit?
Extremely high idle problem
My '71 CB350 has been my pandemic project for almost a year, and it's ready to go but for this problem. The bike bike fires right up, but very quickly begins revving over 3,000 RPM, and climbs. I have triple checked everything on the Honda Twins site, and at a loss for a fix. No air leak at carb boots. Diaphragms are soft and pin hole free. The throttle cable connect has lots of slack, so there is no pulling there. I have removed, cleaned and lubricated the spark advance mechanism. Tried it with air filters both on and off. The odd thing is I can remove one side's spark plug, and get the motor to settle in at an idle around 1200 RPM, and repeat on the other side. However, when both plugs are connected it's 'off to the races' again, and I have to shut it down.
When I started this project the carbs were cleaned and two new carb kits installed. The frustrating part of this is that I raced these things once upon a time with a local vintage club, and never once experienced this.
I'd love to hear from anyone who has experienced this and found the source of the gremlins.
Thanks to all for reading this and your time.
...Biff
When I started this project the carbs were cleaned and two new carb kits installed. The frustrating part of this is that I raced these things once upon a time with a local vintage club, and never once experienced this.
I'd love to hear from anyone who has experienced this and found the source of the gremlins.
Thanks to all for reading this and your time.
...Biff
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Did you replace the felt seals on the throttle shafts? If so, did you properly center the butterflies on the throttle shafts when you put them back together?
No, I haven't. What is it I should be looking for? I know I cleaned the tiny 'ports' on the originals, but I wouldn't know what to look for when comparing.Have you visually checked that the carbs completely close? Have you compared the original idle jets to the ones that came in the rebuild kit?
Are those felt seals a consumable item? Are they readily available? I haven't done anything with, or to them. Where, exactly, are they located? As you can tell I've never touched them either on this bike, or my previous CB350 ownership.Did you replace the felt seals on the throttle shafts? If so, did you properly center the butterflies on the throttle shafts when you put them back together?
Thanks for the reply!
Replacing throttle shaft seals on Honda twins CB350, CB450, CB500T | This is how you replace worn or deteriorated felt throttle shaft seals with my improved seals to stop vaccum leaks on vintage Honda carburetors. My seals... | By Dude's Garage | Facebook
165 views, 6 likes, 0 loves, 1 comments, 0 shares, Facebook Watch Videos from Dude's Garage: This is how you replace worn or deteriorated felt throttle shaft seals with my improved seals to stop...
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That's funny that you should send that. I just finished watching it, and now I have a much better understanding about the importance of replacing these felts. Now to find some replacements...
Thanks, again, for your time and assistance!
Thanks, again, for your time and assistance!
I've been using 'wind instrument' (trumpet's etc) seals and cutting a centre hole. Way cheaper than the $24.00 I last saw on eBay
That sounds great. I write for a couple of local big bands, so I'll see if I can 'borrow' their horns for awhile...
So what if they're missing a bit of felt...
So what if they're missing a bit of felt...
If you have a few to choose from you may find the exact ones you need. I had to take a guess as the only thing I play is the fool.
I guess I'll have to talk to my local music instrument repair guy, and see if he's got any of the supplies I need. Wouldn't feel right to help myself to a bit of felt off a saxophone. What if it's owner never notices....?
Had a similar issue with my CB 450, turns out one of the carbs floats was upside down! ScottMy '71 CB350 has been my pandemic project for almost a year, and it's ready to go but for this problem. The bike bike fires right up, but very quickly begins revving over 3,000 RPM, and climbs. I have triple checked everything on the Honda Twins site, and at a loss for a fix. No air leak at carb boots. Diaphragms are soft and pin hole free. The throttle cable connect has lots of slack, so there is no pulling there. I have removed, cleaned and lubricated the spark advance mechanism. Tried it with air filters both on and off. The odd thing is I can remove one side's spark plug, and get the motor to settle in at an idle around 1200 RPM, and repeat on the other side. However, when both plugs are connected it's 'off to the races' again, and I have to shut it down.
When I started this project the carbs were cleaned and two new carb kits installed. The frustrating part of this is that I raced these things once upon a time with a local vintage club, and never once experienced this.
I'd love to hear from anyone who has experienced this and found the source of the gremlins.
Thanks to all for reading this and your time.
...Biff
Thanks for the input but, no, that's not my issue. I checked and triple-checked the float heights and they're bang on. Thought it might be the ignition advance mechanism, but it appears to be working well. Now I'm going to change the throttle seals, but I can't say that I'm all that optimistic about that being being the source of my problems.
Merry Christmas, and stay safe.
Merry Christmas, and stay safe.
This solved the problem for me. The main clue is when you said idle is good one cyl at a time and there was slack in the throttle cable. Sounds like a sync problem.Thanks for the input but, no, that's not my issue. I checked and triple-checked the float heights and they're bang on. Thought it might be the ignition advance mechanism, but it appears to be working well. Now I'm going to change the throttle seals, but I can't say that I'm all that optimistic about that being being the source of my problems.
Merry Christmas, and stay safe.
Thanks for taking to the time to help. What I meant by 'the slackness' comment was to say that the throttle cable isn't tight and holding the throttle open. When the bike over revs the throttle isn't 'on' at all. Syncing isn't an issue or, at least, it isn't yet. When both cylinder are running it revs like crazy. When I remove one plug at a time it'll settle down and tick over in a more normal fashion. When I put both plugs online it goes nuts again. Again, no throttle at all.
It can't rev like crazy unless it is getting air/fuel from somewhere, back off the idle stop screws, do the throttle plates actually close all the way?
The idle screws aren't touching at all. The butterfly valves appear to be closing all the way. I've done the spray test at the carb/cylinder connection, and nothing. I've examined the diaphragms using a good, bright light, and can find no pin holes. When I run the motor on one side or the other the over revving doesn't happen. When the two are working together is when I have the racing motor issue. Hopefully when my throttle seals arrive I can rebuild that part of the carbs and put this problem to bed.
Thanks for weighing in!
Thanks for weighing in!
Carb boots and clamps can be issues. I can't recall finding a used clamp that didn't need to be straightened and the towers repaired. I use a small washer on the fastener to spread the load.
Old boots can be hard and won't allow the clamp to work properly.
I don't use the original JIS screws to hold the boots in place but rather some socket head fasteners with washers. I also spray copper gasket on the gaskets.
Old boots can be hard and won't allow the clamp to work properly.
I don't use the original JIS screws to hold the boots in place but rather some socket head fasteners with washers. I also spray copper gasket on the gaskets.
Thanks for weighing in with your ideas. The odd thing is that my boots feel supple enough, and I've put hose clamps on them just to see if I notice any difference. I don't. While the bike was trying to race to its ultimate demise I sprayed WD40 all around the boots and carbs, and then switched to water. No change in engine revs, and no appearance of sucking the fluids into the intake. At this point I'm really hoping it's the throttle seals which I'll be replacing soon. I never knew that these could be a potential problem, so I hope this last ditch attempt will do the trick. After that I'm starting to wonder if I need to look for a new pair of carbs......sigh
Thanks, again, for time and suggestions. Merry Christmas.
Thanks, again, for time and suggestions. Merry Christmas.
What kit did you put in? From experience I have found that they tend to run rich especially in the idle circuit. The o-rings are thinner as well and any roughness in the carb body can allow fuel to flow past.
I have used the brass pieces with the so called gasket set from Honda.
I assume these are 3D carbs? There is no o-ring on the idle mixture screw like the 722A carbs but I have had poor fitting screws, try them without the spring to see.
I also set the floats from the OE setting of 26mm to 26.5 or 27mm. You can also check the float needle ... I have found a variety of lengths. compare the installed heights of the OE set (brass and needle) to the aftermarket set.
I have used the brass pieces with the so called gasket set from Honda.
I assume these are 3D carbs? There is no o-ring on the idle mixture screw like the 722A carbs but I have had poor fitting screws, try them without the spring to see.
I also set the floats from the OE setting of 26mm to 26.5 or 27mm. You can also check the float needle ... I have found a variety of lengths. compare the installed heights of the OE set (brass and needle) to the aftermarket set.
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