con rods / imbalance

[robsgreen]

Hi Guys

I've had vibration issues since I totally rebuilt my engine. It vibrates from around 5k up-wards getting worse right up to the redline.( I know its not the transmission). My mates CbX is silky smooth all the way to the redline, nothing like mine.

Ive tried everything, re-torqued engine bolts, changed the pick up unit, the advancer, the ignitors, rebuilt and balanced the carbs, new coils and leads, six centre clutch rebuild kit.

ive now decided to strip down and have just seperated the cases. The company who rebuilt the crank and rods replaced No 1 Con rod with a spare as it was twisted. It is marked 422-031 on the bottom, the others are marked A B. Would this rod cause an imbalance and vibration at higher revs or will it be something else? Of course I'm going to go through and check everything anyway but just wondered.

Best Regards

Martin

[Mouse]

I know that race engine builders will check rods for weight and balance purposes but its unlikely yours is causing your vibration. A rod would need to have a hefty weight inbalance to cause a 500 lb engine to noticably vibrate. The rod only goes up and down and a short distance at that. Unless the rod length is different from the others and it's not opening it's valve properly which could cause unbalanced firing issue's. Also when you re-did the clutch did you balance check the unit? Check for an unbalanced crank shaft, or cam shafts, also worn bearing surfaces or worn bearings. Also check piston and con rod weights, those should be precise. Last resort, check for equal stroke distance on crank journals and con rod length. That's my
Good Luck.

[robsgreen]

Hi
Thank you for valued feedback.
I will certainly have a look at everything very closely. Everything has been replaced or refurbished in the rebuild. I have split the cases so far and nothing looks suspicious so far but will let all know what I find
best Regards

[FalldownPhil]

Rods come in A, B, or C specs. Your replacement rod should match the one that was bent.
Best,
Phil

[daves79x]

All CBX rods are marked for journal size and weight code. The manual specifies what rod is acceptable with what. There IS a significant difference in rod weight between the code pairs. You need to match the others. Weigh them on a postal scale. Not sure if that explains all of your vibration, but I'd start there.

Dave

[daves79x]

Oh, and I've seen more than one bent rod in the same engine from hydrolock. Make sure no other rod is bent or twisted. That WILL make the engine vibrate for sure. Easy way is to carefully check the height of each pair of pistons at TDC with the head off.

Dave

[robsgreen]

Thanks for the additional info, very interesting, I will check all out and post my findings.

Best Regards

martin

[Mouse]

I know that race engine builders will check rods for weight and balance purposes but its unlikely yours is causing your vibration. A rod would need to have a hefty weight inbalance to cause a 500 lb engine to noticably vibrate. The rod only goes up and down and a short distance at that. Unless the rod length is different from the others and it's not opening it's valve properly which could cause unbalanced firing issue's.

Kindly disregard this post from yesterday.
How I managed yammer on about push rods when the CBX is a push rod less overhead cam engine is beyond me.
I should drink my coffee first before I try commenting in the future.

[herdygerdy]

Hi Martin,

I am seriously thinking it could be the LH cams are installed 180 degrees out, so you have 2 cylinders actually firing together which turns it from a six cylinder with one cylinder firing at once into a (kind of) three cylinder, but with two cylinders firing at the same time.

Pull the cam cover (but mebbe you are already further in than that?) and check no lobes for any of the three cylinder pairs (ie; 1 & 6, 2 & 5, 3 & 4) are pointing in the same direction.

If your cam positions do NOT look EXACTLY like the photo below, then that will be your problem! In particular, note the positions of the exhaust cam lobes on cyls 1 & 6, and the inlet cam lobes as well.



There is an expensive trap lurking in the camshaft installation instructions of the FSM for the uninformed. The issue becomes you end up with two sets of valves operating at the same time and this doubles the load on the cam chain and valve train, usually resulting in a broken A cam chain, with a tasty side of bent valves tossed in for free, not to mention vibration like it is a Honda C90 running WFO with all the engine mount bolts not even finger tight.

If you had it running and have not yet broken a cam chain, you are one very lucky fellow.

Don't ask me how I know this, been there done that a long time ago in a Galaxy far, far away....(two decades ago, I had my engine in, running and out 9x times before I finally gave up in disgust at my ignorance & incompetence!)

Keep us posted Martin, and we are all here to help solve this mystery.

Cheers...Tony

[daves79x]

Martin has done far more miles, apparently, without breaking the cam chain for the cams to be out. 100% of them will break within 20 miles if the cams are 180 degrees out.

Done

[robsgreen]

Hi guys

I've broken no records here, my cams were set up correctly as I was made aware of the issue on here a while back.

Still appreciate the feedback though because it all helps. Cases will be apart this evening and well see what IMve got.

Best Regards

martin

[herdygerdy]

Good point & well made Dave, re miles done without breaking a chain, so it can not be that.

Any clues since splitting the cases? Primary shaft to crankshaft timing correct ? Seals seated correctly in hydraulic primary chain tensioner? Maybe use a micrometer to measure all big end and main bearing journals instead of plastiguage

Keep us posted and rest assured, we will prevail!

Cheers...Tony

[robsgreen]

Hi Guys
Cases are apart and worryingly so far nothing seems too amiss. There was very very slight scoring on a couple of shells and 1 journal but nothing else. I've taken the crank in to the specialist who originally rebuilt it. He is going to go through it again, checking carefully everything, crank straightness, con rods for straightness, small ends etc but I have asked him to also balance everything as con-rods 1 and 2 are 5 grams heavier than the others. primary chain timing was spot on, all bearings feel good with no roughness but I'm going to change Primary shaft main bearing as a matter of course as I didnt change this one. No leaks from the tensioner seals - these were replaced with repoduction ones from six centre. I will go through everything carefully and let you know if anything comes up but to be honest I thought I would see something more obvious.

I have noticed that my starter motor feels rough to turn, it wasn't when it went in, not sure what that is so will strip that as well.

Cheers

martin

[NobleHops]

Hi Guys
Cases are apart and worryingly so far nothing seems too amiss. There was very very slight scoring on a couple of shells and 1 journal but nothing else. I've taken the crank in to the specialist who originally rebuilt it. He is going to go through it again, checking carefully everything, crank straightness, con rods for straightness, small ends etc but I have asked him to also balance everything as con-rods 1 and 2 are 5 grams heavier than the others. primary chain timing was spot on, all bearings feel good with no roughness but I'm going to change Primary shaft main bearing as a matter of course as I didnt change this one. No leaks from the tensioner seals - these were replaced with repoduction ones from six centre. I will go through everything carefully and let you know if anything comes up but to be honest I thought I would see something more obvious.

I have noticed that my starter motor feels rough to turn, it wasn't when it went in, not sure what that is so will strip that as well.

Cheers

martin

Good on ya Martin, hope it gets resolved, and the way you are addressing it makes me feel like it definitely will.

[daves79x]

There is a significant weight difference in the rod codes. I'd try to match number 1 and 2 with the rest. What rod codes are they?

Dave