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CBF 250 Cam Chain Rattle - SOLVED!

Repair Date - December 2014

By Stephen Latchford/Ren Withnell

Both myself and my friend SL have Honda CBF 250's. We both agree that they are good motorcycles with excellent handling, a more than acceptable amount of grunt for a 250 and that they're a good little bike. We also agreed on some other aspects, One thing was that the motor rattles like bunch of pound coins that have escaped into a tumble drier. Both bikes improved as the engine warmed up but it never sounded quite right. What we disagreed on was the cause. 

Ren - "Piston slap, don't worry about it." 
SL - "Nah, nah, don't think so."
Ren - "It is, that's why it eases when it gets warm."
SL - "Nah, no, just doesn't seem right that. Cam chain."
Ren - "Nah, wrong, too loud, too constant, piston slap..."

And so the conversation went on. Eventually SL's bike got worse and even I had to agree the rattle was taking on that intermittent extra noise that comes from a loose chain. I still wasn't convinced and my pessimism almost had SL stripping down the whole motor to see what the issue is. He decided to start with the simplest thing, the cam chain tensioner.

cbf 250 cam chain tensioner mechanism
The cam chain tensioner - note the cross head bolt. It's there merely as a cover.

This is located at the back of the cylinder on the right hand side close to the carburettor. There's a broad cross-head bolt that can be removed and behind this deep in a recess is a slot. This can be turned with a watchmaker's screwdriver and this alters the amount of bend on the tension blade thus "tightening" the cam chain. SL turned the little slot, lo and behold the noise went away and the motor ran as sweet as a well oiled little sewing machine. Sorted. Sorted? No. As he watched the motor run he noted the slot gradually turning backwards, undoing. The rattle returned.

This is an automatic cam chain tensioner. With the use of a wound spring the tensioner should automatically twist the slotted mechanism just enough to take up the slack AND stop the slotted mechanism from unscrewing. It seems something was not up to the job, probably the amount of torque the spring could produce to keep the tension. SL faced a decision. Either purchase a new cam chain tensioner, or turn it into a manual tensioner. There's a risk a new tensioner will have the same problem, especially as he knows another CBF 250 with the same problem. And he's got no money. And he likes to tinker. So tinker he did.

the component parts of the tensioner. Spring, plunger, threaded screw and sundry bolts
The tensioner in bits. Note the spring, this is wound tight insde the tensioner.

Using the thread that the cross-head bolt went into he replaced the spring and screw tensioning system with a long bolt and a locking nut. Simply put the bolt was tightened until it pushed the tensioner face onto the blade. He ran the motor and GENTLY tightened the nut until the motor went quiet, then locked the bolt in place with the locknut. This left him with one quiet CBF 250 and a big grin on his face. While the automatic tensioning is gone it really is a 2 minute job to make occasional adjustments if the rattle returns. Sorted. Properly this time.

the converted tensioner with a long bolt
Done! Tighten the bolt and the plunger at the opposite end moves out to move the tension blade.

He sent me his instructions and some pictures as a guide and for me to use here on this website. I only had one issue with his amazing ingenuity. The threads that he used, where the cross-head bolt used to be, are not part of the casting. They are a simple push-fit bush into the casting. With the cam chain tensioner constantly pushing back against the threads and the bush, the expansion and contraction of heat and just plain bad luck I did worry the bush and threads might fall out. 

I found a captive nut from an old horn mount, cut it down, ground it down and made it fit, just a little oversize. Using a vice, brute force and some long sockets I press-fitted the captive nut on the inside of the cam chain tensioner casting. That little bleeder is going nowhere. I drilled out the old thread and inserted my own bolt and lock nut. Well...actually I didn't have a bolt long enough so I used some threaded bar and 2 nuts. Another of Ren's amazing bodges.

the camptive nut, rammed deep into the tensioner housing
The captive nut, ground just slightly oversize then rammed into the housing.

So now we both have quiet CBF 250's. I offer my deepest apologies to SL for not believing him and his superior knowledge that it was cam chain rattle. I thank SL for coming up with a most excellent solution. Now...if only I can sort out the #%&@ing tickover I will consider the CBF 250 to be the perfect motorcycle!

the final fitted manual cam chain tensioner on the bike

Reader's Comments

SL said :-
I've been trumped! But seriously Ren that is a superior modification you have done there, congrats.
1/1//2000 UTC
Nate said :-
I have a CBF250. It rattles, I was so scared that it was the main crank bearing that I took the engine out but the bearings seem fine. Is this a common problem with CBFs? is there a way of checking without putting my engine back in?
1/1//2000 UTC
Daf said :-
Wouldn't surprise me if it was the camchain Nate... To be honest CBF250 engines are pretty rock solid and long lived. The cam chain rattle isn't really a problem, it's just anoying! I'd go ahead and put the block back in the bike and see if this article works. Other thing to check is valce clearences - they can rattle. First two places to start!
1/1//2000 UTC
Ren - The Ed said :-
Daf's right. The only way to check it's the tensioner is to put the motor back in, get it running and manually tighten the cam chain tensioner. If it goes quiet, it's the tensioner alright.

Let us know what happens!
1/1//2000 UTC
Nate said :-
cheers guys I'll put the engine back in and let you know what happens. Before I loosened the tensioner I could get a good 5mm of movement with my finger. I also took it upon myself to record the engine noise before I took the engine out, did yours sound like this?
1/1//2000 UTC
Ren - The Ed said :-
It's hard to tell from a video as the sound gets messed up by your mic and my tablet, but, yeah it does sound fairly similar. Go for it bud! Worst case scenario is you have to pull the lump out again.
1/1//2000 UTC
Nate said :-
well I put the engine back in and made the cam chain tensioner manual. tightening the cam chain did reduce the noise but I had to really over tighten it and the bike started struggling.
1/1//2000 UTC
Ren - The Ed said :-
Hmmmmmm Nate...that don't sound right at all. Both Steve and myself had to just take up the slack, no pressure, barely even finger tight. It's possible the camchain is very tired or there's something else amiss.

Check you're not a tooth out on the timing. It might be worth changing the camchain. It may be worth stripping the motor to really get to the nub of the matter. How many miles has it got on the clock?
1/1//2000 UTC
Daf said :-
I'd say it's most likely that either the chain's a tooth out, or that it's badly worn... has the bike had a lubrication failure/really dirty oil in it at some point? That could account for it.

The other possibility is that it's some other part of the engine... Starter sprag clutch and the transmission clutch centre bushing/release bearing are usual suspects on these engines. Could be main bearings if you're really unlucky.
1/1//2000 UTC
Latchy said :-
Blimey Nate, sorry you have not got to the bottom of it yet. I hope you get an answer to the rattle and post back on here as to what is causing the noise.
1/1//2000 UTC
Nate said :-
well I checked the timing it was a tiny bit out less then half a tooth, I don't know how exact it should be but it took ages to even notice it, literally about a quarter tooth. taking the valve cover off also gave me the chance to check the tension on the cam chain which was fine its also not noticeably worn. But its still rattling. I'm also riding it again and like before it drives absolutely fine, still sound like the cam chain to me, that part of the engine, doesn't happen at high revs or while idling once its warmed up. Any suggestions? some kind of gypsy curse?
1/1//2000 UTC
Nate said :-
Because the timing was a little bit out do you think its worth just trying one tooth further back? Would it damage the engine?
1/1//2000 UTC
Ren - The Ed said :- careful! If you do decide to move the timing out by one tooth be sure to turn the engine over with a spanner a few times to make sure nothing clatters together. The only other thing I can think of was my initial, and incorrect diagnosis of piston slap...
1/1//2000 UTC
Jose Rosas said :-
Seems very common on this bike. I had a CBF250 that bought and returned to the second hand dealer because I thought it was an engine issue. I bought a second bike and I found out it had the same problem. I used a similar approach. I got piece of metal that fits loose between the outer screw and the tensioner so when you put the outer screew it puts some pressure on the tensioner so it gets locked in its place. Looks better from the outside and is still a fix tensioner. Jose.
1/1//2000 UTC
Ren - The Ed said :-
Cheers Jose! Seems like it is a common issue doesn't it. Sounds like you came up with a smarter solution too. Oh well, I can live with the bolt sticking out of the engine.
1/1//2000 UTC
Matt said :-
For those not wanting to do this mod, I repaired my CCT by giving the tang an extra turn or two on the mechanism... The spring is a nightmare to get back in, but it's possible if you wind it in from the side. Personally I prefer the automated method to get a lower maintenance solution.
24/12//2015 6:37:14 PM UTC
said :-

24/7//2017 9:47:29 AM UTC
Godfried said :-
Steven and Ren, you saved my bacon!
On a 12000 km solo round trip from Cape Town, South Africa to Rwanda and back my CBX 250 suddenly sounded like it was not going to take me 10 more killos. I was in the middle of Tanzania with the closest Honda dealer a thousand kilometers away. With all the diplomatic complexity I had to get the bike back to south Africa. My options were to hitch a ride on a big transport truck or to fix the bike without tools or new parts.
After reading your post I managed to get the tensioner of and bingo! The spring was broken in Mbeya I managed to do your mod my engine was puring again.
Thank you for sharing your experience, it literally saved my journey.

Simplistic solo motorcycle adventure
28/12//2018 5:50:00 AM UTC
Ren - The Ed said :-
Godfried - fabulous!! That's made me smile. When I write this stuff I never once imagined it would be helpful to a chap in the middle of Africa somewhere. I hope the rest of your trip is trouble free and your picture makes me want to be there.

Many thanks.
28/12//2018 9:45:51 AM UTC

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