1979 CB750K with problems

[seestheday]

Tools is right.  Check the dohc carb cleaning procedure thread for some hints.  I believe you have a sohc so it won't be exactly the same, but it should give you a decent idea of what to do.

[seestheday]

Lucky is there anywhere you can buy the FSM?  I have a high res scanner and am happy to upload a much better version if I can get my hands on a physical manual.

[SoonerRon]

I know your right. That'll be a redo i'm sure. Guess I'll spring for the rebuild kit while i'm at it.

[SoonerRon]

The 1979 is dohc engine. I will be researching further to try and learn how to do this better. Thanks for the advice.

[SoonerRon]

Ok, further testing shows strong spark at all four cylinders. Unfortunately, my borrowed compression tester is for auto plugs and hence too large. A "finger test" revealed relatively strong compression in #1, #2, #4 and relatively weak (~ 10% or less) compression as compared to the other cylinders. I wish I had real numbers, but it is clear there is not much compression in #3.

[Lucky 1]

I agree. It is not like a car carburator. The openings in the idle jets are only about .013!
You must hold the idle jet up to the light and be able to see the hole is clear.
IF the idle jet is not clear ,it will never run right.

IF it is a 77-78 and the accelerator pump and nozzles do not squirt fuel when the engine is off
and you turn the throttle ,it will never run right.

[Lucky 1]

The spark plug gap and dwell must be within limits.

Even if you take the spark plug out and look at it ,it does not meant that when the spark plug is under compression pressure that it has a strong spark.

BTW ...You can just take a DC clamp on timing light and clip it onto any of the plug wires and it will show the blinking light indicating it is working, WITHOUT taking the spark plug out of the engine.
Save yourself time and wear and tear on the spark plug threads.

[Lucky 1]

Here you go...http://www.ebay.com/itm/New-Service-Shop-Repair-Manual-69-77-CB750K-SOHC-CB750-Honda-Book-L23-/230730204838?pt=Motors_Manuals_Literature&vxp=mtr&hash=item35b89786a6#ht_3288wt_754
Just buy a Clymer manual. Lots of useful info.
OR get a used Clymers and  a Haynes manual too.
Or go to Ebay and see what is available.

Interface with reality.ie, go to a motorcycle store of big box auto store .
Many have motorcycle manuals .
Some motorcycle shops still have manuals. Believe it or not.

[Lucky 1]

"Lucky is there anywhere you can buy the FSM?  I have a high res scanner and am happy to upload a much better version if I can get my hands on a physical manual."
Seestheday


You are going to scan a 1000 double sided pages? Sounds like torture.

I would just rather buy the manual.

[Piute]

I have the Clymer manual as well as the download honda shop manual available through the link on this forum (thanks!). Would not have bought the Clymer manual from amazon had I known about the download... isn't that the way it usually works?  

  Yes,  Sure does my Clymer has some mastakes must be made by man,
 So use both need to read twice any how to be sure .then I just highlight the book,
 for next time.

[SoonerRon]

I like the Clymer manual a lot, but the shop manual seems like it has some additional info and diagrams. I like your policy of reading both before doing anything. That's what I'm going to do when I remove the engine and pull the head!

[SoonerRon]

I performed the leak down test last night on my old bike. I used a wrench to turn the crank to the 1-4 TDC mark and then the 2-3 TDC mark. I also used a screwdriver to probe through the sparkplug hole to verify each piston was at the top of its stroke before testing. #1 and #2 held good pressure with only minimal pressure loss past the rings. I could faintly hear the air escaping from the dipstick opening. Nothing from the exhaust or air breather that I could detect. #3 and #4, especially #3, both experienced rapid depressurization through both the intake and exhaust valves. I could feel the air escaping from the exhaust pipe and could hear air coming out of the air box. Couldn't really learn anything about the rings in those two cylinders, though, due to the condition of the valves. I would still like to get a compression reading on each cylinder. I hope to remove the engine from the bike in the next week or so and pull the head to get a look.

Thanks to everyone for the all the advice!

[TOOLS1]

It sounds like the valves have been adjusted too tight, or bent. Also it could be a blown head gasket. Try doing the leak down test again after removing the cams. If the air stops coming out, then it was the valves adjusted too tight. If the air is still coming out, then the valves might be bent, the valve seats damaged, or a blown head gasket.
TOOLS

[seestheday]

Do you guys not think it would be valuable for other members to have a high res copy of the fsm?  If not I won't bother.  I just thought it would be helpful.

Oh, and with many good scanners, like the one I have access to, you don't feed pages in one at a time, you just stack them all up and it'll feed them all in.  The one I will use will do the whole fsm in about 5 min (double sided - full colour - high res).

[SoonerRon]

For myself, I seem to have all the info at my disposal I can digest, between the Clymer manual I've paid for and the currently available download copy shop manual. The quality of the images is not awful. That being said, if you have in your possession a copy of the shop manual and are generous enough to make available a hi-res copy, I would certainly make use of it with thanks.

[SoonerRon]

Interesting... I don't have my manual in front of me, but I'm thinking I still must remove the engine from the bike to get the cams out. Correct?

[shinyribs]

I really appreciate the offer Sees'. It would be nice to have one in the shop to keep the grease and grime off my laptop,but i wonder if it would be cost effective compared to just buying one.Like you said,the hi-res would be nice,too. I am not personally interested at this time.I didn't even look at the manual when i first downloaded it.I just took my laptop over to my girlfriends house and told her to print it out for me.

"Okay....all 1,200 pages???"
"Whaaa?!?!? no nevermind"
 

[TOOLS1]

It's the SOHC that, you can not remove the valve cover without removing the engine. Yours will come off with out any problems. The head will also cone off without removing the engine.
TOOLS

[SoonerRon]

I ran the compression test with the following results:

1 - 60 psi dry and 96 wet (with oil)
2 - 50 psi dry and 80 wet
3 - 0 and 0
4 - 65 psi dry and 110 wet

very depressing. I think I'll have a couple shots of tequila and get busy.

[shinyribs]

If cylinder 3 is at zero pressure that suggest a valve leaking.If it had any rings at all you would get some sort of pressure.So you may just have a valve related issue rather than need to go into a huge rebuild.But you probably already knew that. I was told that 120 psi was the lowest acceptable pressure to run with but dont know that to be a fact.How many miles is on the bike?Also,if you do a compression check and do not open the throttle all the way the motor cannot ingest air effectively which will result in low readings.If you didnt test with the carbs in WOT position you may want to retry the test and see if the pressures come up.Also,i noticed that the pressure from 1-4 went higher and higher.If you tested them in that same order than the friction of turning the motor more and more as you went from cylinder to cylinder would explain the rising pressure as you went along.Just a thought.Best of luck to you.

Note:ingesting large doses of worm water while reading a compression gauge has been known to result in inaccurate "readings"