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HELP! 1980 750F suddenly developed weird metallic tapping/knocking noise

Iceman-44

CB750 Enthusiast
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San Antonio, TX
OK guys, got a 1980 Honda 750F Supersport with almost 29,000 miles on it. Recently drove bike down for the annual safety inspection we have here in Texas and noticed LOUD metallic tapping noise developed on return trip. Roundtrip mileage was probably less than 7 miles or so. Tapping was there regardless of engine rpm and got louder & LOUDER as I nursed it back home. Noise seemed more like a mechanical knock than the usual rattling camchain noise I’m familiar with from loose tensioners. Just having a hard time figuring out what could cause so sudden a loud tapping or knocking noise like that… Bike doesn’t get ridden much, and certainly not abused (hey, it’s a classic!), and the carbs were synched up just last year. Just weird that it was fine when last ridden and now it’s quickly developed that strange noise. Only performance issue I noticed was before leaving house was that the idle seemed unusually HIGH (2000-2500rpm). Other than that engine seemed to perform normally throughout the whole trip.

So, I’ve already started to examine bike for the usual suspects:
- Pulled left cover to see if starter clutch bolts were backing out (nope, all 3 tight)
- Pulled oil filter to see if engine was making metal (nope, oil & filter looked clean)
- Pulled oil drain plug to inspect for same (nope, no metal & oil perfectly clean)
- Pulled cam cover to inspect valve train & camchains (nope, quick scan shows nothing unusual)
- Checked to confirm correct crank-to-cam valve timing and everything lines up as it should

Getting ready to go thru all sixteen valves and check for correct clearances, but while I’m doing that just thought I’d throw this out there to the forum and see if anyone else has any recommendations on where to look next.

Seem to recall others have mentioned weird noises caused by bad starter gears or bushing wear?

Think mileage of 29K is too early to think about possibly replacing camchains due to excessive stretch? (Both tensioners were replaced with NOS parts at around 27K, but neither chain was replaced.)

Anything else to help narrow down my search? All constructive advice appreciated! THANKS!
 
You may have a bad rod there, yank the oil pan. Starters only make noise when working. Piston hitting head and commonly #3.

Hope not of course.
 
Pull the plugs see if there is evidence of something in the cylinders, then I would say pull the pan as above.
 
OK, dropped oil pan and inspected sump - very minor debris and nothing that looked catastrophic. Pulled plugs and inspected piston heads as best I could - nothing abnormal there either...

Finished checking valve clearances - all good except #4 (one intake & exhaust a little loose, 1 exhaust a little too tight). Gonna re-shim those.

Then the plan is to re-adjust both camchain tensioners, put everything back together, drop in some oil and test-fire to see where things stand. Frankly, I expect the noise to still be there, as none of my findings so far have offered any "Eureka" moments... Anybody got any other ideas??
 
Well, surprisingly, after reshimming the valves on #4, adjusting both camchain tensioners, & refilling with Rotella T6 15W40 synthetic, I restarted the bike and she actually sounds pretty normal - at least nothing I can detect clearly as a rod knock or anything.

However, I'm now getting a pretty obvious misfire on #4 (water spray test on exhaust shows #4 clearly running colder and exhaust has a distinct backfire & popping sound in exhaust). Dropped the bowl on #4 and checked carb bowl and jets - all good. New plug didn't seem to make any difference either. Compression is low across all 4 cylinders on this bike (and has been ever since I got it) - 115 on #1, 120 on #2, 115 on #3, and 115 on #4, so I suspect it's probably way overdue for a valve job. But, that said, it starts easily, pulls hard, and was previously running fine, so thinking it could just be a weak ignition coil or wire. Any other ideas - feel free to chime in!
 
Tensioner assembly maybe breaking up, I've seen plenty of rubber cracking on them by 30K miles here in Texas. The rubber just won't take the heat.

You heard the knock, we can't. Engines don't just knock (loud as you said) and then quit for no reason. I've seen bearings spin to make noise then they move again to not and then shortly after blow engine into pieces. #4 is telling you something. Possible carbon came loose and impacting on that, I've heard of it but never seen it but that means nothing.

One from left field..........if they run ethanol in the local fuel you could have ingested some water to knock but normally it comes with some missing as well. #4 behavior?
 
Thanks again to amc49 for continued help, as I continue to try and track down the source of this problem...

First off, good news about earlier compression test - low readings were operator error, ha! Been awhile since I did a check and forgot accurate readings only achieved with engine hot and WOT. Re-check this morning inspires mucho confidence about condition of rings/valves & overall state of cylinder sealing: #1=160, #2=165, #3=13, #4=165. Mucho better!!

That said, agree cylinder #4 is trying to tell me something. Checked exhaust temps again with IR thermometer & clearly there's a misfire there (temps are 350+ on 1-3, but only 140 on #4). Checked #4 spark with Ignition Tester & it appears to be firing, but spark looks weak… New coils/wires are cheap enough that they're worth replacing anyway on a bike that's almost 40 yrs old, but just for grins I also checked air gap on pulse generators (both good at 0.014) and swapped around ignitors to see if misfire moved but made no difference (#4 exhaust temp remained lower than rest).

As for your thoughts about ethanol, I suspect anything related to fuel would be seen across ALL cylinders - not just #4 (and its bowl showed no water contamination). And WRT possible breakup of tensioner(s), BOTH were replaced with NOS pieces from UK just a year or so ago for exactly that reason (old ones were toast!)

Good news is the motor isn't making any bad mechanical noises at all during testing but, as we both know, rod bearings on the way out can come & go if the bearings shift around (been there, done that on an old Datsun truck!). Hoping the weird noise I heard before was perhaps some exhaust harmonic related to #4 misfire? (Was wearing a helmet at the time & that can sometimes amplify/disguise noise sources...)

Plan ahead is to install new coils/wires when they arrive & see if that makes any difference. If no change, MUST be something related to carburetion, so will pull entire carb bank & go thru it nose-to-tail and re-synch.

Anybody have any other ideas or suggestions??
 
Don't discount the carbon chunk idea of making noise, most carbon in engines is soft and breaks up with no harm but some is super hard if it has really been heated for awhile, think about what diamonds are made of. I've seen it hard enough on valves that a chisel would not hardly touch it.

Engine pulling hard says possible idle issue in #4, maybe a stuck piece of trash in the idle fuel restriction or possible aircut issue which could be a small vacuum leak just to affect idle. I tend to dump the aircuts myself, they give trouble on older bikes and the replacement valves are too pricey. Harder to do without breaking the bank apart though.
 
Yep, seen some tough carbon deposits in my day, but the cylinder head and piston crowns of this bike were completely decarbonized only a year and a half ago when I had the top end apart, and I've only put a few hundred miles on it since. Seems unlikely given the low time on the engine since...

Just installed new coils, wires, and caps this afternoon- no change in misfire on #4 (exhaust temps still WAY LOWER on it compared to 1-3). So, since ignition doesn't seem to be the problem, and compression & valve clearances have all been checked good... pretty much GOTTA be narrowed down to carburetion now.

Plan ahead is to pull the entire carb bank and go thru it extensively, cleaning & checking everything (have a sneaky suspicion amc49 is on target with his comment about the pressed in idle jets - will take a hard look there for starters). Then I guess I'll synch 'em up again just to be sure.

Anybody got any other suggestions/ideas??
 
http://power-barn.com/press-fit-pil...100-xl185-cb550-cb650-cb750-cb900-cbx-trx200/

Pressed in type pilot or slow jet the '80 model uses. Note the big hole on the end, the inside restriction is very much smaller than that but you can get to it using something .013" in diameter like the E string on a guitar, the smallest one. The biggest problem being pressed in is the paired air emulsion entry holes you see on the side of the jet tube, they repeat around the jet and absolutely no way to get to those without the jet being physically out of the carb. That destroys your pressfit and new jet goes back in looser to maybe leak or fall out.

Your weeding out of the ignition as an issue may be wildly incorrect if you haven't pulled the plug to find it dead bone dry yet, the cylinder can wet out to foul the plug wet and then it will act just as you describe.

You still haven't checked the aircuts yet either, they can easily do that. The diaphragm tears or frays to leak and then you have a vacuum leak at idle and off-idle that runs fine at higher rpm but drops a cylinder out anywhere around dead idle.
 
Thanks amc49, excellent recommendations across the board! Pretty sure the plugs were bone dry when I pulled to check (both original and new one I installed afterwards), but may be worth checking again just to be sure.

Understand the limitations of cleaning the pressed-in idle jets - will clear the center orifice(s) for sure, but don't plan to remove any of them for the exact reason you mentioned.

Aircuts are a definite possibility... Haven't messed with them before other than to check and ensure diaphragms were intact. May be worth looking into that bypass procedure if I can't find anything else obviously wrong.

Appreciate the help - will keep ALCON posted as my troubleshooting continues...
 
THANKS, good to know! May have to keep an eye out for a decent set of those on eBay - sure would be nice to know all those side orifices are clear too!
 
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