Topic: 2 Cylinders not running, completely frustrated!!!

[Fishheadbaby]

Hello all!
I'm completely stumped on this one - read a bunch of posts but none seem to match with my problem exactly-
I have a 1978 CB750 that I bought recently and has had a bunch of problems, which I'm taking care of one by one.  Up til this one!  I was driving on the highway when I noticed the engine getting really hot and losing power.  Got off the highway as fast as possible, but by that point my tachometer had somehow ceased to work, and the engine was superhot, straining, and then wouldn't start.  I let it sit for a while, and after it cooled down, it did start, but only on two cylinders.  I was close to home, so I drove it back real slow, and now it starts and runs fine, on two cylinders, the other two are useless.
1.  I'm assuming that only two are functioning because the two on the right (as you are looking at the front of the bike) get too hot to touch, and you can wrap your ungloved fingers around the two on the left, they are just a bit warm and never get hot.  This is a correct assumption, right?
2. First thing I did of course was try to determine if it was the plugs/coils/electrical, so I switched the plug boots from the working to the nonworking cylinders, hoping that the nonworking ones would then work.  No change.
3. Next I pulled the plugs and checked them, all are fine, I even put new ones in for good measure. 
4. Next I opened the float bowls on the nonfunctioning side, no problems there, throttle is fine and we're getting gas. 
5. This is where I got frustrated - if I'm getting gas through the carb, and I've taken the plug and it's boot from a functioning cylinder and switched it to the nonfunctioning one, logic says that it should work.  But it doesn't.  Whatever went wrong happened suddenly, while I was driving, and I have no idea what it was.

Suggestions greatly appreciated, no matter how basic - I'm happy to go through the steps again, perhaps I missed something? 

Thanks in advance,
Fishheadbaby

[eurban]

Well, cylinders 1-4 and cylinders 2-3 are paired in the ignition system so if you have cylinders 3&4 (sitting on the bike they are numbered 1-2-3-4 left to right) dead then it is not likely to be an ignition problem.  Sad to say that your problem doesn't sound good.  Take a look at this post:
http://forums.sohc4.net/index.php?topic=38507.0
Remove the tappet covers on the "dead cylinders" and on one of the good cylinders.  Compare how much oil is present in the good vs dead cylinder's rocker box area.  Rotate the engine and see if the rockers move up and down in the dead cylinders. They probably won't. . . My guess is that the oil passage to the 3-4 side of the top end became blocked, parts became overheated, camshaft sheared in half.  Left side is still turning.  This is why your tach stopped working.  It is driven off of the right side of the camshaft.  Hope I'm wrong!

[dusterdude]

eric,why do you have to be the bearer of bad news...but im afraid you`re right.

[mustangcar]

wow sorry to here the news.i just got my plates today for my 78k winter project pretty much finished,anyone have a good pic of what those top oil restrictors look like? what do you guys find in the oil orfices when they get plugged besides silicone, is it cam tensioner bits of plastic?I checked the strainer when i took the oil pump apart to prime it, the strainer looked clean wonder how debris gets through and plugging them up,with my luck it'll happen to me, oil changes will be top on my list, wish you luck and keep us posted

[martino1972]

Well, cylinders 1-4 and cylinders 2-3 are paired in the ignition system so if you have cylinders 3&4 (sitting on the bike they are numbered 1-2-3-4 left to right) dead then it is not likely to be an ignition problem.  Sad to say that your problem doesn't sound good.  Take a look at this post:
http://forums.sohc4.net/index.php?topic=38507.0
Remove the tappet covers on the "dead cylinders" and on one of the good cylinders.  Compare how much oil is present in the good vs dead cylinder's rocker box area.  Rotate the engine and see if the rockers move up and down in the dead cylinders. They probably won't. . . My guess is that the oil passage to the 3-4 side of the top end became blocked, parts became overheated, camshaft sheared in half.  Left side is still turning.  This is why your tach stopped working.  It is driven off of the right side of the camshaft.  Hope I'm wrong!

what he said,but then for 1-2,not for 3-4...
yeah it sounds like you had some sort of a meltdown in the engine,either cam oil starvation,and wrecked your cam,or something happend to the pistons on that side,most likely your cam for 1-2...
when you pulled the plugs out,was there a color difference between 1-2 and 3-4.

[eurban]

Martino, reread his post .  His left and right are referenced while "looking at the front of the bike."  Like I said,  the failure of the Tach at the same time is pretty much the nail in the coffin.  Tach drive gear on the cam is between #3 and #4  cam lobe pairs. Cam is sheared.

[martino1972]

yep,your absolutly right... ....i guess i was reading too fast,thanks for ticking me over my fingers...

[Hush]

Eeewww that all sounds soooo expensive, think I'll give this post a miss.

[Fishheadbaby]

Wow,
Thanks all for breaking the bad news kindly....or as kindly as possible.  That sounds totally in line with what happened, and explains everything.  Wow again.  What makes this happen, so I can avoid a repeat!
So my next question is:
I can clean carbs, deal with minor electrical problems, change oil and chains, never opened up an engine.  To fix this bike, the engine head has to be removed and the camshaft replaced, huh?  That's a major job and not something I can do on the street in front of my apartment, huh? -- If I take it in, how much am I going to have to spend on parts and labour -- if anyone is a mechanic in the Hudson Valley/NYC area of NY, here's your chance to get some business    -- Or should I cut my losses and try to sell it for parts? And if so, anyone interested

Thanks again!
Fish.

[martino1972]

cut your losses and sell it???
autch,that hurts......man,don't sell.....
theres enough people here that can talk you trough the project with their eyes closed...
maybe even a member around you that is more then willing to come over,have some beer with ya,and show ya the ropes...

[shoemanII]

i had never opened up an engine either, but when the PO left a cam chain sprocket bolt loose on a '78K i had, it took out that sprocket, cam, cam chain - then sprinkled bits of these parts everywhere.  had gone 50 miles on my "new" bike and BOOM!  found a used engine that needed a complete set of gaskets and tore that guy all apart, had pistons hangin' out!  with patience, sound advice from the members here and encouragement from a honda mechanic friend (borrowed his ring compressors too), the engine went back together.  leak-free and ran strong as hell.  you can do this.

[eurban]

Fish-
First make sure that the top end is F'd up by removing the tappet covers and inspecting / rotating the engine. . .  . The oil supplies to the head pass thru very small restrictors and can clog easily if debris is present in the oil.  Over zealous application of gasket sealant is one culprit, as are bits and pieces from the chain rollers / guides, clutch disc pieces, and sludge from infrequent oil changes.  This kind of failure does happen with some frequency in the 750 engines but I would have to say that it is not as common as some of the recent threads on the subject (top end oiler, and the one I linked for you earlier) might make you believe. . . . If your engine is indeed F'd then consider replacing your 78F's engine (I assume from you bike list that we are talking about a 78F with the black F engine?) with a used K engine. I would suggest the 77/78Ks as they have all of the lineages refinements over the years, are not particularly collectible, seem to be abundant, and make good power (slightly higher compression, decent camshaft, and the head casting has improved flow)  The F models are prone to valve guide wear, with valves and other rebuild parts expensive and or difficult to find.  Rebuilding your F engine is certainly possible but IMHO there are better choices open to you. A full rebuild would probably be required for your engine as there has likely been some piston to valve interference, and could very well be more debris laying in wait.  Even if you were only looking at the camshaft / rockers and towers, the 750 engine has to come out of the frame to do the work!. . .Whatever you do, I hope you don't give up on the bike! Put out the word that you are looking for an engine on this site and check out ebay / craigslist.   Let us know what you find when you inspect the engine . . . .