NEED HELP!! '81 cb750c wont fire!!

[mallen14]

There seem to be a lot of SMART guys on this forum so I figured I would ask y'all.

I recently bought a 1981 Honda cb750c and have been working on it all weekend. It shows 64000 miles but figured Honda's have always made reliable stuff. I took apart and cleaned carbs, cleaned tank, checked plugs/wires/coils, and bought a new battery.

It's getting good spark, fuel, air, and as far as I can tell just by placing my finger over spark plug holes I have compression. It will turn over all day but it wont even fire with starting fluid sprayed into the plug hole. I am stumped!! I don't know what else to check or where to even start. I wasn't expecting to be able to get it running good in one weekend but for it not to even hit a lick really bothers me!!

Any ideas would be greatly appreciated!! Thanks

[LukeM]

Welcome to our little Honda playground.

Step 1:  Go to the home page of this forum, and find the Factory Service Manual for your bike.  HERE IS THE LINK.
Step 2:  Go to the Maintenance section of that manual, and give the bike a good tune up.  This includes timing adjustments, basic setup of carbs, cam timing, etc.
Step 3:  You say it's got good compression.  The thumb test, while somewhat viable, is not a good indicator of adequate compression to get the motor to fire.  
Step 4:  Get back to us with more information: When was it running last?  How do the plugs look: wet or dry, shiny, coal black or light tan, check the gap.  How did you check for spark: Connect the plug to the plug wire, press it against the block with an insulated tool or piece of wood, and crank the engine, perhaps?  Mixture screws set to 1.5 - 2.0 turns out from lightly seated?  Is there a good ground at the green chassis wire?  

Those are just a few items to check over.  Don't get discouraged: these older machines (30+ years!) can sometimes be a challenge, but it'll be worth it when you get it going.  Keep us in the loop, and we'll help if we can.

Luke M

[mallen14]

Luke,
I got the service manual, thanks. I have adjusted timing with light and it says it where it should be. Also have cleaned stator and magneto. The previous owner said the bike was running about a year ago and has been parked since. It has a new set of plugs on the bike, and hasnt started so they still look new. I checked the gap and they are all at .025. I checked the spark on all plugs and wires but pulling plug and grounding it to engine which gives me blue spark. When I took the carb apart I cleaned every hole I could find and soaked all the small parts I could remove however didnt think to adjust any mixture screws. Since then when I turn on the gas it seems to flood the carbs and ends up leaking out. This means bad seals and what?? Where is the green chassis wire located?? I has a green wire hanging down on the left side of the batter next to all the other wires but could not locate where it went.

Thanks for the reply...

[LukeM]

Additional information received, and appreciated. :-)

If I recall (from 2 years ago), the chassis ground for this bike is a green wire with a ring terminal, and it bolts to the chassis under the seat.  There should be a crossmember there, and a threaded insert or bolt that the ground connects to.  Lack of a good chassis ground would equal lack of spark, so if you're getting spark, then your ground is good.  You'd also lose headlight, tail light, turn signals, all sorts of stuff.

Sounds like you've gone over the electrical/ignition system pretty well. Good job.

Turning the petcock to the ON position and the carbs drip from the overflow?  This means the bowls are getting too full, which usually points to a misadjusted float, a stuck open float needle, or perhaps the float needle is in backwards.  Seestheday has done a lot of work on these carbs, and perhaps he'll chime in here.

*** DISCLAIMER: DO NOT USE MEDIUM TO LARGE AMOUNTS OF STARTING FLUID WITH THESE MOTORS.  I've read it's very bad for the motor. ***  

If you choose to do so, turn the petcock to the OFF position, use the drain screws for each carb to empty the float bowls, and then use a SMALL amount of starting fluid to see if it tries to go.  If it does, then your ignition and compression (valve train, cams, crank, etc) is probably good, and the issue is with fuel delivery.
I have to let my '79 Caddy sit at the airport all week, and starting fluid is my friend when it hasn't run for 5 days.  I'll have to investigate why there's no fuel pressure after that wait.

I hope this helps.  
Luke M

[TOOLS1]

Try reversing the wires from the ignition to the coils. You might have crossed them when, you changed the coils.
TOOLS

[Hellbilly429]

If that green wire is hanging near the turn-signal relay then just leave it hanging, these bikes cames stock with a 3-wire turn-signal relay and most people replace them with a 2-wire relay because they are cheaper, green wire doesn't need to be connected to anything if this is the wire you are talking about.

As for the not firing thing I had the same problem, worked through everything and damn thing would not start. Know what it was? The f*&#ing killswitch, so I would check that, although I thought the killswitch would kill the spark so that's probably not what your problem is, doesn't hurt to look though!

[mallen14]

Yeah it is hanging right beside the signal relay, thanks, glad I know what that is. And as far as the kill switch, it doesnt have on. Apparently it broke and the two wire are twisted and taped inside the kill switch housing. That was one of the first things thought too.

[mallen14]

Still not running but have another question. Would frayed wires feeding into one of the pulse generators keep it from getting a bad spark?? When opening the left side case I found that the two wires coming out of one pulse generator were pretty badly frayed. Just wondering if that could have been it but I guess we will find out in a few days because while messing with it one broke off...fml. So that mean I have to order two new pulse generators because the wiring harness is built in. Sucks...

[Hellbilly429]

Did you check those wires yourself or are you going off of previous owner's word? If you're going to leave those wires together I would solder them to make sure you don't have any interference coming from those wires not making a good connection.

Definately would check what Tools said, make sure the wires up to the coils are not switched and make sure the plug-wires coming from the coils are not switched either, if the plugs are firing out of sequence it will not run.

[mallen14]

No I checked the wires myself. And now have to figure out what I can do about my pulse generators. Can't check anything until that's fixed. Anybody think that it would be safe to solder the wires back into the generator to ensure proper connection?? Cause I really dont wanna have to spend $140 when I'm not even sure that's the worse problem Ive got

[Rodgil]

There is an earth wire connected to the frame just in front of the coils. You may have to remove the tank to get at it. I have just replaced my failed pulse generator, bought it from Mrcycles. But if you had good spark on all four cylinders, it is unlikely to be bad earth or kill switch. Can only be low compression or fuel. Was the fuel fresh? Not been stored in a can for a while?

[mallen14]

The fuel was pretty fresh but like I said, it wouldn't even fire off starting fluid...

[mallen14]

Another question guys, looking at the left side of the bike with the crank case off, which pulse generator is for cylinders 1 and 4?? The front or the rear generator??

[seestheday]

If you're pissing fuel out of the carbs and the drain plug is screwed all the way in, more than likely your float is stuck.  You might get lucky by tapping LIGHTLY on the carbs with a mallet to get them unstuck, but more than likely you'll need to pull them and clean them.  Look up the DOHC carb cleaning thread on this board for lots of great resources.  While you still have the carbs off, fill them up with some gas and make sure that they don't leak BEFORE putting them back on your bike.  This will save you a ton of time.

That said, I don't think that this is the reason your bike isn't firing, but will be something you need to do before it'll run right.

[seestheday]

If I were you I'd buy a $30 compression tester.  Factory spec is 170, but the bike should fire with anything  over 100 (although it'll be on it's last legs).

[mallen14]

Yeah I am planning on starting the carb rebuild hopefully next week, but want to know that the engine will at least hit a few good times before I move on. Tomorrow I plan on pulling the valve cover and checking valves and gapping them correctly. Also plan on putting a compression tester on it tomorrow also. Maybe some new plug wires as well if they sell them at autozone.

I did put it in 1st earlier today and tried to roll it but couldn't. I know that's not a great compression test but maybe it's a good sign??

[Hellbilly429]

The left pickup in the pulse generator (which is towards the front of the bike) is cylinder 1&4, and the other is 2&3. Autozone unfortunately doesn't sell crap for bike parts, let alone these bikes, the wires on these bikes have to be made, not hard to do, your local bike shop should be able to do it, mine did for about $30 or $40 and they work great.

[mallen14]

Ok thanks. Well my 2&3 generator is shot. Guess I am going to order me some plug wires just to eliminate that possibility.

Also looked at my coils and they are aftermarket 3ohm coils. Idk what brand or how long it's been since they've been replaced but still trying to avoid spending too $$ just eliminating possibilities. Want a for sure answer to my problem!!  

[mallen14]

Got to thinking, that's gotta be why it's not firing is plugs. Cause after I spray starting fluid in the cylinders, try to crank it, then pull the plugs again they are still wet. So there apparently isnt even enough spark to light it!!

So coils will run me about $150 and plugs and wires about $40. Is that all I would need to replace to make sure the ignition system is working right??

[seestheday]

If I were you I'd still check the compression before buying lots of parts.  Compression testers are cheap.

Even a weak spark should give you some ignition if everything else is right, and you indicated in your original post that you were getting good spark.  Did you check all of the plugs to see if you were getting good spark or just 1?

I'm not sure how your 2&3 generator is shot if you are getting spark to 2&3.