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Discussion Starter · #1 ·
Hi,
Honda CB450. Riding the bike and slowing down to stop or to corner the revs stay high. Stop the bike - switch off ignition - and then when I start the bike again it returns to normal idle. I have checked the throttle cable and it is fine - not getting stuck anywhere. Could this be the advance mechanism and if so would appreciate some advice and help in regard to the fix. Many thanks, Kanga
 

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Could be incorrect clutch cable routing, check to see if the clutch cable is contacting the throttle cable just above the carbs. Here's a handy video of this if that is the problem.

 

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^^^Could be that - check the clutch cable routing for sure. Could be the advance mechanism not returning (weak springs or sticky parts needing to be cleaned and lubed) or it could be a small vacuum leak at one the carb insulators. The stock 450 carbs are CV so it's less likely that a slide would stick, but it's possible. It can also be the timing is a bit retarded and when the engine revs get it up to full advance, it wants to stay there. Have you tried to slow the engine by letting the clutch out a bit in gear while stopped? If that slows it down and it stays there, it's less likely to be a vacuum leak
 

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Discussion Starter · #5 ·
thanks for the replies . the bike has been going really well . thought it may have been the throttle cable but it is reving to 4000 and climbing so I kill engine .straight away push electric start and idles fine at 1000 until I take off again(it doesn't seem to happen until I get the revs up and the only way to get it back to normal is to kill engine)).The cable is definitely free ,and I have checked fuel lines as well to make sure they were not fouling the. accelerator
.I recently did valve adjustment and didn't retime the engine .(but it was running fine ) I have a spare timing advance mechanism , but thanks Tom I will check the timing before I go ahead and replace it . thank you all very much , any other suggestions greatly appreciated and I will keep you posted. Yes Tom I can get the revs down by using the clutch to labour the engine while stopped
 

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Discussion Starter · #7 ·
Removed air filters - no difference - I have two advanced timing mechanisms - cleaned and serviced them both - also no difference. This is what is happening - while idling the bike will return to idle (1000 revs) if I don’t throttle past 2000 revs. If I throttle up to past 2000 revs the revs keep climbing and won’t come back . Have to turn bike off and then when I turn the bike back on again it will go back to normal idle.
 

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Removed air filters - no difference - I have two advanced timing mechanisms - cleaned and serviced them both - also no difference. This is what is happening - while idling the bike will return to idle (1000 revs) if I don’t throttle past 2000 revs. If I throttle up to past 2000 revs the revs keep climbing and won’t come back . Have to turn bike off and then when I turn the bike back on again it will go back to normal idle.
My bike did literally the exact same thing when i first got it. I never figured out the issue before disassembling the engine, so I will be watching this thread intently. Please let me know if you figure it out!
 

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Discussion Starter · #10 ·
no air leaks , timing correct ,advance mechanism good, carburettors cleans . valves adjusted. the bike was running well . the only clue I have, is it is ok for a few miles but when the engine gets hot its a constant....could it be valves to tight? what about coil or condenser ? appreciate any help thanks
 

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Discussion Starter · #11 ·
TOOLS I have sprayed carby cleaner around both rear and forward carburettor boots while bike is idling .. no difference .is there another way of checking?
 

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That does not always work when testing for a vacuum leak. Since it is doing it when the engine heats up it is probably small cracks in the rubber appearing as the rubber heats up and swells. One thing, have you tried retightening the clamps and screws that hold the boots onto the head and carbs while the engine was hot?
TOOLS
 

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Discussion Starter · #13 ·
thanks TOOLS N0 but I am going to do that now. Also I read some members use silicon and I have not done that ... so I will give that a try as well .I dont mine trying different options (was running out of ideas)and if this doesn't work I will pull carbs again thanks
 

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Another spot where there can be a vacuum leak is at the pilot screw. If the very tiny O-ring and washer are missing it will have a vacuum leak.
TOOLS
 

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Discussion Starter · #15 ·
Hi, The carburettors perform normally up until 3000 revs. Once the bike is revved above 3000 revs, even with the throttle shut down at carburettor (where the idle screws are) the bike continues to pull fuel and the revs keep climbing. I have checked the pilot screw for vacuum leak. I can not find a vacuum leak anywhere. Could this possibly be a needle and seat issue as there seems to be plenty of fuel going through the various jets?
 

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Is it possible for the slides to drag due to corrosion, and maybe get a bit hung up at any point in their travel? How freely do they move during a bench test? I'm not questioning the vacuum leak- I'd bet on it- but my carbs top ends needed to be cleaned inside and the slides worked much better after I hit them with mothers until they shined. Has anyone else noticed this?
 

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Discussion Starter · #17 ·
Thanks TOOLS I owe you a beer
vacuum leak - front, left cylinder boot. I sprayed CARBY CLEANER around boots numerous times and the motor did not change revs. May have been a bit timid .I think really have to spray it into boots . Ran out of CARBY CLEANER. Then i used BRAKE CLEANER and straight away the engine revved up real high from idle at left front boot. I used a small amount of silicon on boot- and all good now. Next time I pull the carbies I will clean slide and inside as well . Thanks a lot to all . Kanga
 

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no air leaks , timing correct ,advance mechanism good, carburettors cleans . valves adjusted. the bike was running well . the only clue I have, is it is ok for a few miles but when the engine gets hot its a constant....could it be valves to tight? what about coil or condenser ? appreciate any help thanks
I think Tools is right. Are the rubber stubs that the carbys slip into old and hard?
 

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Don't smear silicon on the stubs, that's gross. Buy new stubs, your old ones are are nearly 40 years old, they're stuffed, get new ones
 

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Check compression check valve adjustment. Bad or leaking valves can cause this due to idle adjustment open wider just to get it to idle

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