Topic: '74 CB750K engine issues (long)

[meangreen]

 I've been lurking here for awhile, but finally joined up. I'm not new to bikes at all, but this is my first Honda 750. As a teenager, I had a green '73 Honda 450 twin that I really liked, and I've always wanted to build up a 750 from the same era.
 
 The other day, I found a '74 750K on Craigslist. I could see that a lot of work had gone into the bike at some point, but it was not currently running. The owner told me he didn't know much about it, other than the guy he bought it from said it had a bad head, he had been sitting on it for two years, it hadn't been started at all during that time, and he just kinda wanted to be rid of it. When I went to see it, I was handed a thick binder full of receipts from the previous owner, who had apparently sunk close to $5,000 into the build, and only ridden it 4800 miles before something happened to the engine. It was pretty rough from sitting, and the carbs were so gunked up that I couldn't twist the throttle.

 I bought the bike, cleaned the carbs, changed the oil, and rode it home. It runs, but barely. The #2 plug was black, and three of the four intake valves had excessive clearance (the #2 intake valve was at probably .020 or .030). It's popping out the exhaust, and has very little power, although it improved after I set the valve clearances. Removing the #2 plug wire at idle produced no change. I'm thinking the #2 exhaust valve is bad. I'm getting ready to pull the engine and tear it down, and being a total newb to these bikes, I have a few questions:

1. What do you all think could have caused this issue? I was surprised to find the intake valves loose. It's got pod air filters and a 4-1 exhaust on it, Dyna S ignition and coils, and looks to have #120 main jets...not sure which slow jets. Is there a chance it could have gone lean somewhere?

2. It's got a Web cam that appears to be pretty aggressive - .360 lift, 270 duration, and 103 degree lobe center. There are receipts for 8 new valves (although they were only $16 each in 2007), some $85 KPMI valve springs, and about $300 worth of head work that did not include porting. I was told the bike had 16k on it before the build, and since I didn't find any receipts for lower end parts, I'm assuming that is stock. Could this cam have caused my valve train issues? Did the PO go overboard on the cam and springs? Am I alright to rebuild the head and put this cam back in, or do I need to go back down to the stock cam? I realize it's not doing much for me if the head and lower end are stock.

 Thanks in advance for taking the time to read my post. Any input will be appreciated.

[harisuluv]

Probably just upped the main jet and didn't bother adjusting the needle jets (the majority of the throttle range!).  Ran lean in the middle so long that it finally had some kind of catastrophic event.

If you are saying that you removed a plug and there was no difference that you could tell then that cylinder wasn't probably firing in the first place.  If you are getting ready to pull the head and tear it down, you are absolutely jumping the gun.  A bike that isn't running on 3/4 will be a total dog.

First figure out/verify what cylinders are running.  (the ones firing will be hot)

[seanbarney41]

make damn sure you got lots of oil in the valve cover...pull an exhaust side cap with the engine running, oil should splash out

[CB750F2]

I would do a compression test and a leak down test before deciding to pull the motor just in case the problem is ignition or fuel related. If compression is low the leak down test will pin point the problem. You do not want to pull and disassemble the motor unless it is absolutely necessary. Pat

[meangreen]

make damn sure you got lots of oil in the valve cover...pull an exhaust side cap with the engine running, oil should splash out

You think it could be related to oil starvation? The oil pan drain plug was completely stripped out. It does sound valvey, even after I adjusted them.

[calj737]

You would be extremely well served to pull down a manual, buy Hondamn's book, and perform the 3,000 mile service before you start tearing the engine down.

"It sounds valvey" could easily be an indication of cam chain slap, carbs that need a full and proper clean, then to be vacuum synched. Since you say this is your first SOHC, not knowing the proper sound of a running engine is a significant hindrance. Pods will also effect the sound of the bike.

[HondaMan]

I'd suggest first a compression check to find out if a valve is bent.
Reason: these .360" lift Webcams are quite good at creating bent valves if the guides have more than 0.0040" clearance in them, which would be just about how much some of them will have at 16k miles if 10w40 oils were used in a K4 engine. What happens is: the [too] high-lift Webcam opens the rocker(s) so far that the tip of the adjuster trips over the end of the valve stem's tip, if the valve has any latitude fore-and-aft from previous wear. Then as the cam starts to release that rocker, the tip hangs up momentarily on the stem until spring pressure forces it shut again, causing it to snap as it hits the rocker tip and bangs the rocker foot against the cam lobe (this also makes for LOTS of valve noise). If this is the scenario (which I have seen too often lately), you will see small 'dents' in the cam lobes from the rocker feet hitting it, just past the full-open point of the lobe, on the closing ramp. If extra-strong valve springs were added, this will be even more pronounced. These dents will indicate a no-good cam as they will later eat the rocker arm's foot.

The only "fix" I know about for the above is: either install new bronze or Kibblewhite cast iron valve guides with 0.0008" (Honda's minimum) stem-to-guide clearance so the stems cannot rock forward-aft anymore - or, get a cam with less lift.

[meangreen]

 I suspected that there was too much cam when I read the specs, especially after finding the intake valves loose. Web doesn't even appear to offer a .360 lift cam grind anymore, so they have apparently figured it out. The valve guides were replaced as part of the original build 4800 miles ago. I have receipts for new guides, as well as for the installation when the head was rebuilt.

 I messed with the bike a little more tonight.  I went through and reset all the valves again, and was able to get it to quiet down considerably. The issue is with cylinder #3- near zero compression, wet plug, cold pipe, occasional pops as it loads up with enough fuel to fire. It's not blowing compression out the exhaust, though. I pulled the air filters, and sure enough, it's blowing back out the #3 carb.

 On closer inspection, the engine has new gaskets everywhere, and I do have invoices for the Gast undercut gears and rewelded shift dogs, plus the engine cases have been painted. I'd be surprised if the lower end is original, but who knows at this point.

 It sounds as if it'll be a ticking time bomb with the Web cam. Do you have any recommendations for a good street cam. A rough idle wouldn't hurt.

 I guess the next step is to pull the head off and see how bad it is. I'll probably tear it down over the holidays and see what I find. Thank you so much for weighing in!

[754]

Could be oil starved cam... Look around inside with valve caps off..if its not wet..ir dry on a few start looking a bit further.
Might be easier to see in darkened room.

 Check the joint on the engine cases at the front.. Any evidence of silicone squueezing out tells two things.. Its been apart.. And the more silicone squeezed out mean higher chance the cam got oilstarved....

[meangreen]

 I did see lots of oil under the valve caps. I ran the bike with one of the exhaust caps off, and got a little oil out. It wasn't squirting all over the place, but I did get some. I take it that's a  pretty common issue?

[754]

Did you check both side.. Left and right ? 2 seperate feeds to the cam.
 They can run a while while wearing sometimes, if valves keep loosening, i would supect cam wear.

 The cam in the pic  spent its last 1.5 hrs at 60-70 mph.. Then 10 minutes in town, before we decided to tear it down. If you have a frame kit, you wont have to pull the engine. I make a weldless kit that is quick to install.

[calj737]

If you have a wet plug, then id diagnose your ignition/spark issues. Getting the bike to fire on all 4 cylinders does wonders for the audio...

With new guides and HD springs, I suspect your cam is not the culprit.

[meangreen]

 What part of my last post gave you the impression that there was an ignition problem?

[meangreen]

Did you check both side.. Left and right ? 2 seperate feeds to the cam.
 They can run a while while wearing sometimes, if valves keep loosening, i would supect cam wear.

 The cam in the pic  spent its last 1.5 hrs at 60-70 mph.. Then 10 minutes in town, before we decided to tear it down. If you have a frame kit, you wont have to pull the engine. I make a weldless kit that is quick to install.

I will definitely check both sides. Thanks for the tip.

I need one of your frame kits. That looks like it makes life so much easier!

[754]

 I was four hours away from home when we looked inside... It had gotten much louder.
 What we saw when we looked closer..
Valve collar was dry on the worst ones where the rocker shaft mounts in cam holder there was a bit of aluminum dust.....ground off bits from being dry.. Then we decided to cut frame and tear cam out.
 Not saying that yours is doing that for sure, but rather, what to look for in case..at first glance you might not see it.

[HondaMan]

I pulled the air filters, and sure enough, it's blowing back out the #3 carb.

Ouch, yep. Sounds like a tweaked valve head.
If you would like a non-stock cam that won't drive you nuts with tuning issues, try the Megacycle 125-00 (probably everyone here already knows it is my favorite for these engines...).

[meangreen]

 I took it apart tonight, and found that the intake valves of three cylinders had been hitting the piston crowns. One of them is pretty bad, actually, and it may need a set of pistons as well as new intake valves. It's sad that the PO spent so much time and money on this build, just to screw the whole thing up with a dumb mistake.
 I'd love to build a monster out of it, but I think I may go down to one of the hotter stock cams and just get it running for now. With a stock rotating assembly, this thing will never support the revs it would need to be capable of to fully utilize the Web #41 cam.
 Are those pistons junk? Looks like a few of them have pretty substantial divots in the crowns from the intake valves.
 

[calj737]

Probably easier and cheaper to have the pocket milled for greater relief, or increase the gasket thickness. If the piston/bore clearance is tight and the rest of the motor is fine, then replace the damaged parts with proper clearances and be off.

Its pretty common for folks to struggle with getting the Webcam products accurately degreed. This may be the single major culprit to your issues. If the motor was built for performance, being off a few degrees on valve timing with a tight squish will achieve exactly what you see.

[meangreen]

 Thanks. The whole build has the look of being thrown together by someone without much experience with engines. I removed several bolts that were held in with JB Weld. I doubt the guy who did the assembly had ever heard of degreeing  in a cam.
 The head looks to be milled down a fair amount too...wonder if he was trying to boost up the static comp that way, and didn't consider the piston to valve clearance.
 I hate to do anything with the existing pistons if they can't be reused. With the 836 kits being so cheap these days, I would probably go that route. There was a little oil in that #3 cylinder. I hope the crown isn't cracked.

[calj737]

Well that oil could be coming from a valve seal (you did mention bent intakes) or from a badly milled/decked surface. Best to disassemble everything before you make any determinations. Dry assemble, use clay to test the clearances of the head/pistons/valves, and then decide a course of action. If the bores and pistons are good, I'd re-use and degree the cam correctly.

Or, you replace the #41 with a Megacycle, use hard weld rockers (might already have them), replace pistons/rings, hone, and assemble, test clearances, then correct as necessary. Lots of money in this path for new parts. New intakes either way you go if they're bent.

[HondaMan]

I recently (2 years ago) rebuilt an engine that went through a very similar 'trauma'.  I did a 100% teadown, because of the forces involved when valves hit pistons enough to deform them and make shiny smiles like those on it. Here's what it needed to make it run great again:

1. New pistons: it went to 0.5mm overbore. Using one of the Cruisinimage 836 kits (eBay, good stuff) would have been another option.
2. New rod bearings. They well all flattened slightly from hitting the valves, making them non-round by almost 0.0006" overall.
3. New cam. We decided on a Megacycle 125-04, hardweld rework made from the old cam (Megacycle will rework your old one, or any old used one).
4. New valve guides on those valve that had hit the pistons, as the lower ends of those guides had been forced oval, and one was cracked. The head managed to survive it, and the new bronze guides went in with no troubles.

Afterward it was a screamin' 750!

[meangreen]

 Thank you HondaMan, for yet another insightful post.

This engine has had some big bucks put into it at some point, and I feel like it's an 836 kit and a ported head away from making some pretty big power (at least for these bikes, anyway). The existing head may even be ported...not sure yet.
 It would be a waste to go into it without plans for a big bore kit and whatever head work it needs in order to really build power.

 I have a line on a complete '78 parts bike. I may use the complete engine to get Mean Green up and running for now, then take my time building the existing engine. My understanding is that the '78 will have some other goodies that will work well on my build, not to mention some other parts I'm missing (like the OE gauges and center stand).

[HondaMan]

The '78 engines are quite a bit different. The carbs, hoses, airbox, head, and pistons do not interchange directly with your engine, so maybe think twice about the exchange? The oil tanks are different (as are the hoses) and the wiring harnesses are distant 2nd cousins, even the fuseblocks have opposite genders on their plugs!

[PeWe]

Parts must match. Hotter cam need pistons with deeper valve pockets for the lift and more CR to compensate the longer duration leaking compression.
0.360" lift is rather moderate cam number. I guess that 270 duration number is total lift, not at 0.04" or 0.05" lift as usually referred to.

Test mount the head and cam. Use old head gasket or no gasket. Time cam close to the numbers.
Typical 28-22 open IN, closing around 50-55 at 0.04" lift. EX opposite numbers.
 
 Rotate crank with no plugs in and check valve to piston clearance. Rotate crank so the pistons are around 15* before and after TDC. At this position tighten the valve adjuster screw until it stop. If you can tighten about 2mm down the clearance is OK. Without head gasket 1.5mm must be OK too.

No meaning with hotter cam if head is not ported. Harder springs is a must too.

My engine has ported head, larger in valves (34mm), harder valve springs.
Cam: In .393" lift, EX.363", duration 260/257.5 at .04" lift

Important that the cam is properly timed

It is very fun with a tuned CB750! My engine will get new goodies this winter to keep up the rpm and stay there without any problems.

[PeWe]

Here are 3 of my 836 pistons.
The piston at the left is what I use now.  All 3 with valve pockets made for hotter cams.
My latest pistons had to be grinded on IN side when the valves are 2mm wider, 1mm more radius.