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Cx500 1978 engine probably later
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Discussion Starter · #1 ·
Had BSA B25, A65 and Ducati 860 back in the 70s and fancied another bike for fun.
Picked up what was supposed to be a rolling restoration which turned out to have been bodged together to sell to the first idiot to appear with sufficient cash!
That's where I entered the story.
Turned out the bottom end was u/s and after 20 miles in my hands was scrap!
Got lucky on eBay and found a bottom end, no heads, that had been outside with a sofa cushion on top to keep the rain out. Ever the optimist, I thought it worth a punt for £46 and it turned out a very good investment.
Having cleaned the mayonnaise out of the inside, the only rust was very light on the sides of the con rods.
Crank was seized but freed up during disassembly though it went tight at one point in the revolution.
One crank journal was black - looked like burnt oil to me but the main bearings looked ok, no wear marks but some embedded foreign matter.
Had the crank polished, the black polished out, journals measured and all within manufactured tolerance - happy days.
The bike came with boxes of bits - another engine minus the crankcases, carbs, crash bars, tail mouldings and and more.
So I rebuilt the £46 engine with some new oil seal and bearings courtesy of simply bearings, cheap and very quick - and where a part looked below par, used a better one from the "stock" I'd inherited.
The £46 motor cam chain and tensioner looked new, no wear marks on the guide and when adjusted, the tensioner was is the first 25% of its travel.
One concern was the oil pump which had some scoring on the cover plate which would reduce efficiency. I ground the scores out using wet and dry paper on a sheet of glass, other than that the pump clearances were serviceable. Well worth disassembling the pressure relief valve, it appeared to be sticking slightly as had a build up of gum - put a washer under the spring after reading some posts on the subject - thanks.
Anyway, the old girl is back on the road now having covered 300 gentle miles ( fitted new rod shells and the measured clearance with plastigauge was about 0.0008") getting more confident in my good fortune.
Fuel system was contaminated with rust from the tank - when I drained the tank the petrol was orange! Sediment did drop out in time but only good for cleaning. De-rusting and sealing the tank took several weeks but after cleaning the carbs as well, nothing fancy, just a strip down and thorough clean with a can of carb cleaner, she's running great.
I was a bit worried about performance as the best camshaft I had has some light pitting on the lobes - I decided to dress them back using combination of files and wet and dry paper to remove the pitting and then polished them. The amount of material removed was between 0.1 and 0.2mm and the refinished lobes were still well above the min service limit. She pulls pretty well at 6000 - briefly at the moment as still running in.
Once I'm happy the engine is good I'll give some thought to the cosmetics
Just like to say this community has been helpful in my build, nice to feel your not alone ( though I might be as most of the posts are 10 or more years old ), as has YouTube which was one of the reasons I went for a cx.
PS Worked out how to balance the carbs without gauges today if anyone is interested - I expect I'm not the first to do it this way but can't find any references to my method.
cheers all.
Vehicle Details
Cx500 1978 engine probably later
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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11,458 Posts
Welcome aboard, Paul! Nice intro story. Thanks for sharing that.
I'm interested to hear your carb balancing technique. I've tried it by ear before, with mixed results.
 

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Cx500 1978 engine probably later
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Discussion Starter · #3 · (Edited)
Hi Randall,
Thanks for that.
With the idle mixture screws set at factory, mixture seemed ok going by plug colour. I'd syncd the carbs by eye on the bench but the weren't quite right.
So, warmed up and wound up the idle stop on the throttle to about 1400 rpm. Then take off each spark cap in turn to see how fast it idles on one cylinder.
Adjust the throttle link until you get the same rpm on each side.
Then return the idle stop to normal. It's a bit of a faf taking the tank on and off to keep the float bowls wet but worth the effort.
Hope it works for you.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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11,458 Posts
Interesting approach. It might be good to check with gauges after to see if it really hits the mark.
 

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1978 Honda CX-500.. a work in progress
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56 Posts
Welcome, Sounds like you have some mad mech. skills. Look forward to seeing the physical changes you make to the machine.
 

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'84 CX650E that is evolving into a GL500
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17,745 Posts
Welcome to the forum. We normally ask members to add their location to their profile. It wouldn't hurt but with the forum name you have chosen anyone who can't figure out where you are from deserves to be confused ;-)

And welcome to the world of antique vehicle ownership (they own us, not the other way around). It sounds like you have a pretty good handle on things already but your bike is over 4 decades old and obviously has not had all of the maintenance necessary to keep it safe & reliable so if you haven't already done so it is highly recommended to download the Factory Shop Manual for your model (available through the CX Wiki - link in my signature) and go through all of the service procedures, regardless of whether your bike has reached the specified mileage.
I also recommend looking on all rubber parts with suspicion because rubber does not age gracefully. You probably already know to check the date codes on your tires and replace them if they are over 5 years old no matter how good they look & feel (old rubber simply cannot flow around the irregularities in the asphalt well enough to grip, especially if it is cool or wet) but a lot of people don't realize that rubber brake lines age too and should be replaced every 2 or 3 fluid changes (= 5 or 6 years) so if your bike still has the original rubber line(s) I recommend shopping for modern stainless braided ones (they last practically forever and double the life of the fluid).
And don't forget things like the rad hoses and the boot between the engine and swingarm (they can crack on the bottom where you don't see it).
 

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CX500B 1979 and 2004 BMW F650GS
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99 Posts
Pulling off the leads is okay with points, but can bugger some electronic ignitions. It’s not the sort of thing you could do on a BM with electronic ignition.
I did do the old pull ‘em off on my R65 with points. Used this technique for setting idle mixture, but still used gauges for throttle balance.
 

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'84 CX650E that is evolving into a GL500
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The manual for my Dnepr said to do it that way. It also said to put it on the stand, running in 1st gear and lock the throttle with the speedometer indicating a specific speed while doing it (no tach).
I don't think I'd want to do either on one of these bikes....
 

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Cx500 1978 engine probably later
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Discussion Starter · #9 ·
Thanks for that Bob, means the theory is on a not too far out.
I tried making my own gauge, oil in a pvc tube, one end left and the other right. Worked fine on my Kawasaki gt550 but the cx just sucked the oil out with hilarious and smokey results! Tried damping the flow but it wasn't having it and that when I had the plug lead off inspiration - and as you say, turns out I'd reinvented the wheel.
I'm very much "needs must when the devil drives".
I'm just looking to have a useable classic I can have some fun on and enjoy a bit of spannering.
Fitted a taper roller on the headstock which was interesting, care needed with spacers but results well worth the effort.
Considering the electric fan mod for the future but need to get the cam chain tension sorted out first.
With the tensioner adjusted per manual getting that sickening chopping noise at low rpm mostly when cold.
Opened her up and aluminium in the oil - that cartridge filter is worth its weight in gold!
I don't think the spring on the tensioner is man enough and the way the tensioner relies on flexing to the spring tension to work is not very satisfactory either - IMHO.
So, I'm looking at fitting a screw in the outer case that will press down onto the top of the tensioner.
The correct adjustment will be trial and error but I'm thinking start with the spring tension then add say 1/2 turn on the screw - I'll know the adjustment's right when the noise stops!
Well that's the theory.
Ps, if you want mad mech skills check out Allan millyard on you tube - I love his flying millyard bike.
Cheers all and thanks for the inspiration.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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Maybe your chain's just beyond its limit. Or it was set on the wrong stoke, so it's lax.
 

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Cx500 1978 engine probably later
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Discussion Starter · #11 ·
Thanks for your comments, I'm very capable of making a muck of these thing but in the knowledge of its importance on the cx I adjusted it several times with great care but the same result.
On removing the rear case the adjuster slot showed a half circle above the bolt and nothing below which is the early part of its travel indicating chain and tensioner condition is good.
With the engine on the bench I couldn't turn the crank to a position where the adjuster would take up any more "slack". Inverted commas because it seems tight and cosey - but it's definitely chopping.
I watched a you-tube about the balance forces in a twin m/c engine and the difference between 360, 180 and 270° cranks and the balance pros and cons of each.
The cx is close to the 270 which is the best from a balance perspective.
I was left wondering how the things don't fly apart as soon as they start!
Anyway, the power stroke crank acceleration and compression deceleration coupled with the inertia and rotational energy in the camshaft and chain results in some serious pushing and pulling between the two, most noticeable at low revs. The way the tensioner works is fine while the crank is pulling but those fractions of a rev when the crank is decelerating and the camshaft is pushing are the problem as the tensioner doesn't appear to be designed to cope with this.
I'm also considering adding some Nylatron 4.6 Stanyl to keep the chain away from the case at strategic points - if the tensioner bolt doesn't do the jobbie.
Work in progress.
 

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The way the tensioner works is fine while the crank is pulling but those fractions of a rev when the crank is decelerating and the camshaft is pushing are the problem as the tensioner doesn't appear to be designed to cope with this.
I think that the many CX500 and GL500 still in every day use indicate that the system is actually "designed to cope with this".
Is it possible that the aluminum carvings in your engine came from a previous owner? What do you know about the service history of the bike?
 

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Hi Randall,
Thanks for that.
With the idle mixture screws set at factory, mixture seemed ok going by plug colour. I'd syncd the carbs by eye on the bench but the weren't quite right.
So, warmed up and wound up the idle stop on the throttle to about 1400 rpm. Then take off each spark cap in turn to see how fast it idles on one cylinder.
Adjust the throttle link until you get the same rpm on each side.
Then return the idle stop to normal. It's a bit of a faf taking the tank on and off to keep the float bowls wet but worth the effort.
Hope it works for you.
Its a adaption of technique outlined in old Haynes manuals for CBtwins mixture, idle and synch...though they added where you'll visually see the carb linkages move for synchro. (as you had separate cables running to each carb).:)....been 40years but remember plug wire off and using the tach..

BTW-welcome.....
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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I think you've got a solution looking for a problem. If you've got chain slap, something is out of place or out of spec. Supplementing with a novel design won't address the underlying problem.
If your concern is based entirely on finding aluminum shavings in the filter, those can accumulate from running with a neglected adjuster. No real damage unless the tensioner is broken or the chain has worn through an oil gallery. Once the chain is adjusted properly, the issue has been dealt with.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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11,458 Posts
I think you've got a solution looking for a problem. If you've got chain slap, something is out of place or out of spec. Supplementing with a novel design won't address the underlying problem.
If your concern is based entirely on finding aluminum shavings in the filter, those can accumulate from running with a neglected adjuster. No real damage unless the tensioner is broken or the chain has worn through an oil gallery. Once the chain is adjusted properly, the issue has been dealt with.
 

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The camshaft has very little rotating mass and therefore not much inertia. You are over thinking most everything that is very well designed on these bikes. The twisted twin didn’t get it’s reliable reputation by accident.
 

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Cx500 1978 engine probably later
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Discussion Starter · #17 ·
Thanks for your comments.
History - I built the engine using a bottom end - no heads - from eBay.
It had been cut out of the frame, mounts still attached!
I won't bore you with all the details but examination of the tensioner and chain surprised me as no signs of wear, looked new and when adjusted all the gap was above the tensioner locking bolt.
There was gouging damage to the case that suggests it had suffered a broken chain/tensioner previously - so the new parts made some sense.
I've run it for 600 miles since the rebuild and adjusted the cam chain 3 times as per Haynes manual.

As for the design aspect, I think the amount of space on this community devoted to the subject tells a story - it's not one of the cx's strengths.
Don't get me wrong, I'm very impressed with it, 50 bhp out of a 500cc, my bsa Firebird 650 was tuned and made less.
And when I stripped the cx the difference in technology blew me away - the BSA's were positively agricultural compared to the cx!
But one thing the pushroded BSA won on was the gear driven camshaft.
Cam chains and pushrods each have their own problems, the cx has both!
I apologize if I've committed heresy, I know the people in this community love their bikes and I don't want to offend anyone.
I shall press on with the additional tensioner adjuster bolt mod and post results - good or bad.
Cheers all and take care.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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We're not offended. We just don't want to see you chasing the proverbial wild goose.
The cam chain (after being updated in 1978) isn't a CX weak point. It's a maintenance weak point. The case wear you found is 100% the result of neglect. Concocting a different means of adjustment isn't going to change that.
I will be interested to see what you come up with, but I don't think you'll see any improvement over Honda's design.
 

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Some where in the forum is a post, with pictures, of an engine to which such a modification was done.
 

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1978 CX500 "The Grub", 1983 GL650I "Nimbus"
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Honda itself tried to address the lax maintenance by introducing the automatic tensioner. Many of us consider it a step backward.
 
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