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Discussion Starter · #1 ·
Hi there, first post on the board but must say I've already found lots of really useful info. Thanks Guys :) . So here's the deal. 1972 CB450 K5, 15000 mls, been stood for many years but overall in good condition. I'm going for a restore back to origional. Sorted out many electrical niggles with the bike and got her through the UK MOT test no problem but......from cold engine starts well and runs ok, as the motor warms up it seems to hang onto the revs untill eventually she revs at around 3000rpm no lower, bit embarrassing sat at the lights but no fear of stalling :lol:. So here are the things I checked so far,

Throttle cable runs freely.
Valve clearances set.
Ignition timing spot on.
Carbs removed twice and cleaned/rebuilt (Carbs are spottless inside).
Carbs carefully set up (Synch/Idle/pilot scews).
No obvious air leaks.
Compression 150/150, Yes I know it's low but at least it's the same on both pots.
Battery good.

One thing I looked at was the ignition advance unit and to me the springs do seem a little weak but I have no experience with these units. Should they snap back sharply as mine seem a bit lazy....
Is there any way to check the unit might be the problem rather than just replacing it. I have no problem replacing it but it always satisfying to be able to prove a fault. There must be some way to test it.

Well that's the score so far. Thanks for reading and hope someone can help solve this little problem.
Plaese keep up the great posts.
Hondaboy.
 

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Are the carbs balanced? Does one exhaust have more pop than the other? I have heard airleaks around the carb boots are common. I am new here also and appreciate the knowledge? I would bet a dollar on the advance.

Mine is doing it a little but for some reason if I pop the clutch a little, the revs will then drop down to an idle and avoids that crazy look from the person next to you! Maybe the same issue.
 

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Discussion Starter · #4 ·
1971 CB450 said:
Are the carbs balanced? Does one exhaust have more pop than the other? I have heard airleaks around the carb boots are common. I am new here also and appreciate the knowledge? I would bet a dollar on the advance.
Thanks I haven't used a carbtune or anything technical but have been very fussy (read paraniod) over the carb initial setup.

Znabb said:
Welcome to the site. :)
You can check the function of the advance unit by doing a strobe timing. If working ok it should advance at 3500 to 4000 rpm.
Also, have a look at Bills topic "Honda Twin Ignition Timing instruction".
[/quote]

Thanks Znabb, and tomorrow I'll be buying a shiny new strobe.....I actually set the timing using Bills very informative post and have now read the part about the advance unit...Dohhh
 

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95% probability that the carbs aren't set up perfectly.
Small chance the advancer is whacked.
Even smaller chance that the intake valves are too tight.

Verify those compression numbers. Do it again, several times, remove both spark plugs, warm engine - use electric start, throttle wide open, NO CHOKE.
Then add a little oil to the spark plug hole, check compression again.

Bill's 450 Site
 

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I have added a pdf dwg of the springs which I made to send out to spring manufacturers.
If you would like to email me I will email it to you no problem
Eventually I managed to find one that I was able to blag a couple as "samples" made out of St St they work fine.
The springs should snap back otherwise the advance will begin far too early
[attachment=1:5tpqhofz]Advance spring for 450 Model (1).pdf[/attachment:5tpqhofz]
You can see how slack the original springs are in this pic
[attachment=0:5tpqhofz]100_0779.jpg[/attachment:5tpqhofz]
 

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Discussion Starter · #7 ·
Thanks to all who have taken time to reply. Next steps will be check compression as Bill suggests, shorten/change the springs on the advance and see what if any difference that makes, add tapping points to the carb mounts then ask the wife if she'll buy me a carbtune for my birthday in three weeks.... :)

Ok back to the garage...gotta get to the bottom of this before I start the big strip down.

Thanks, Hondaboy.
 

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Discussion Starter · #8 ·
Success at last, I rechecked compresion as per Bills instructions and we are in the 180's on both pots......next found a suitable spring for the advance unit, cut it in half and made an eye at each end....swapped new springs for old....installed advance unit....timed the bike....bingo, idles well, revs well and returns to idle smoothly.
See the rather sketchy shots of the before and after.
I measured the old springs and one was around 0.5mm longer than the other and looked as though it had been over stretched at some point.
Anyhow just need to get a strobe to check the advance is coming in at the right rpm and hopefully all in the world is good.
Thanks again guys. Couldn't have done it without your input.
 

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OK, so, if I understand correctly, the whole problem was weak and/or overstretched ignition advance mechanism springs. You say you found a "suitable" spring and you were able to cut it to length and form eyes on the ends. Nice work! I would think that would be difficult. Can you comment on where you came up with a
"suitable" spring, though?

Thanks,
Mike V.
 

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I would try a few spring manufacturers on google. I did this in the UK and sent a drawing to them having searched their website and they came up trumps.
However a little caviat. I had a couple of other engines one being a 500T the advance mechanism. I found that the springs do come in slightly different lengths. I was not able to simply swap over from differeing modles asI had hoped to. Its nesseccary to count the coils and measure the closed length carefully. The hooks are set at 90 degrees from each other, which again does not make it so easy.
If the guage of the spring wire and the other dimensions are the same (best) or very similar (close), you will be very close to the original spring specification, as the charectistics of metal does not alter dramatially. That means you will be within the parameters allowed by the engine on the advance of the spark.
I know that does not answer your question about who can supply in the US but I hope that it gives you some ideas where to look for them.
 

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Discussion Starter · #12 ·
Mikevan10 said:
OK, so, if I understand correctly, the whole problem was weak and/or overstretched ignition advance mechanism springs. You say you found a "suitable" spring and you were able to cut it to length and form eyes on the ends. Nice work! I would think that would be difficult. Can you comment on where you came up with a
"suitable" spring, though?

Thanks,
Mike V.
Mike, I have a collection of springs and just happened to have one which felt about the right strength....if you notice on the pics I sent the new springs are a smaller diameter to the old but similar in tensile pull. Nothing scientific, just lucky I had one. To answer your first question last, yes it was the cause of the over revving in this case....bike returns nicely to idle now....the old springs were very weak at and just off their resting or ideling position....also had a strobe on it last night and the advance looks to be working properly. Have a look around for the little kits of springs which are sold at places like Machine Mart in the UK....or even auto jumbles etc.

Regards Kev.
 

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I had the same problem on my 72 CB450 and think I have that same issue on my 70 CL450. dont it piss you off when something wont last more than 40 years? :lol:
I lucked out and found a whole NOS assembly for a good price on ebay for the CB450. I think I am going to try the springs that I heard about on Mikes XS that he has for the XS650 but that work on our 450's as well.
Don
 

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dlslick said:
I had the same problem on my 72 CB450 and think I have that same issue on my 70 CL450. dont it piss you off when something wont last more than 40 years? :lol:
I lucked out and found a whole NOS assembly for a good price on ebay for the CB450. I think I am going to try the springs that I heard about on Mikes XS that he has for the XS650 but that work on our 450's as well.
Don
Those springs don't work for the 450 advancer. I bought a set and they were very loose and wouldn't hold the weights closed at idle. I sent them off to Bill for evaluation but I don't think he's had a chance to mess with it yet.
 

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Hi ejj,

I really was just curious about the practicality of finding correct springs to use/modify that would work well. Just so I know what I'll be up against when/if I have to replace my advance mechanism springs. For now I am in good shape. Thanks for asking!

Mike V.
 

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Code:
No obvious air leaks.
Compression 150/150, Yes I know it's low but at least it's the same on both pots.
I am new to all of this so bare with me.
I am some what confused buy everyone saying that 150 is a low compression and here is why.
I have read my Clymer manual from front to back due to the super cold weather and the need to become familar with the bike before I take pics of ever thing and tear it down for paint. Here is what the manuel say's in chapter two under Starting Difficulties.
Compression should generally read 150 lbs. per square inch or more.
Also I found this artical online at http://www.ehow.com/how_6453303_calcula ... ssure.html and it puts a compression ratio of 9:1 at 152.37 at low speed and 175.97 at a high speed. In the tips and warnings section of this article its stats that
Measuring compression pressure with a gauge using spark plug holes is a low-speed method that will read pressures lower than what your engine does at speed, and should therefore only be used for comparing cylinders.
So as a newbe I have not found a manual or any documentation that the puts it at 163 or 180 and recommends a rebuild if it is not at those numbers, but I could be and many times am wrong, am I missing something. I did a quick comp test on my bike when I got it, it was cold and i only had the throttle cracked not full open, I got 157 on both sides. I plan on following the procedure listed on this site and will do it again. Am I wrong to feel 157 or a little higher using the correct procedure is an OK comp.
Thanks
Dan
 

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I have read my Clymer manual from front to back.....Thanks
Dan
There's the problem......

We are quoting the Honda FACTORY service manual.....which states the MINIMUM spec for a 450 is 163 PSI.......
 

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like I said I many times am wrong, I will look for the manual you speak of and hope my next compression check done correctly is better than 157, would like to ride this summer and rebuild the engine next winter. Thanks Dan
 
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