'79 CB750K backfiring or knocking when revving

[gbwithacb]

Hello again everyone,

I'm moving onto the next phase of restoring this motorcycle. When I bought it it was completely dead. Don't think it's been started since 1991.

Well, I've fixed up some electric problems, cleaned out the carbs and I oiled up the cylinders before starting. It has the stock airbox, but the mufflers on the pipes were rusted away. I've cut those off and running free flow drag pipes. Yeah it's damn loud but I don't want to spend $300 on an exhaust.

I've spent some time adjusting the carbs. So, here are the bullet points of what's going on:

• Right now, it starts up pretty good and idles at about 1200rpm.
• When I give it gas, the revs go up but at around 1800rpm it starts making a serious knocking or backfire sound. This is the first time I'm working on an engine so I cant distinguish what the sound is.
• I thought that it might be running to rich, but when I leaned it out (a half turn on each carb) the sound came back MUCH worse, even at idle.
• I saw some posts mentioning the rubber seals. These ones seem ok but I may have put a crack in somewhere because it is really difficult getting those carbs in and out.
• I lubed the throttle cable and the mechanism on the carb snaps back. However, when I give it gas, the revs go up and stay up. It's like it's sticking.

What do you reckon the problem might be? The manual says ignition coils. I don't know a damn thing about those.

Would value any and all input - thanks everyone!

 

[amc49]

If your pipes are too short you WILL burn every exhaust valve in the motor.

Set carb idles at around 2-2 1/4 turns all the same and leave them alone, if you have problems with idle it will be carb internal like everybody else has. The revs going up is a hanging idle, the first sign of the rubbers being dead or another major air leak, or incompetence in having carbs way too far open on an engine that won't run right. The carbs need to be removed and guaranteed that all 4 bores are dead on just covering the first small hole they expose as soon as carb is opened. A bench sync in effect.

Recognize that 90% of the people who 'clean' those carbs make them work worse not better when done. They are pretty complicated and the usual conventional wisdom is worthless there.

If you oiled the engine up before starting you have likely ruined the plugs by fouling them out with oil. Your knocking and backfiring.

Recognize as well that because the valves set with shims that most of them never had it done and then compression leaks to make engine run like utter crap. Why most of those get parked for years, owners just can't get their heads around that.

 

[gbwithacb]

Hey thanks for your reply. Much appreciated. There are one or two things I'm not quite sure about:

Recognize that 90% of the people who 'clean' those carbs make them work worse not better when done. They are pretty complicated and the usual conventional wisdom is worthless there.

Yeah I'm a new mechanic, and the only other carb work I've done was on a single cylinder. Still, that one turned out absolutely perfect. I was pretty meticulous when cleaning these and not too rough, so I'm not entirely convinced the problem lies there.

If you oiled the engine up before starting you have likely ruined the plugs by fouling them out with oil. Your knocking and backfiring.

New plugs aren't expensive, and I'll happily buy another set if that's the problem. I have spent the afternoon cleaning the ignition coils and it is backfiring a lot less now.

Recognize as well that because the valves set with shims that most of them never had it done and then compression leaks to make engine run like utter crap. Why most of those get parked for years, owners just can't get their heads around that.

I'm not quite sure what this means. A valve adjustment is in order? Never done that before, haha.

 

[gbwithacb]

Hahaha damn! I put in new plugs, the sound is gone.

Look out folks, amc49 ain't messin' around.

New pipes are in order, these things are just insane.

 

[amc49]

The valves not adjusted will eventually burn at least some of them but before it does the engine will show reluctance to rev to the full 9500 rpm redline.

The carbs besides being the CV type have a seesaw type idle to primary fuel flipover deal that drives people nuts. The idle being lean (clogged) causes just off idle to go super rich and it then seems to be rich all the time when it's not. Commonly carb cleaning if the carbs are not broken apart does far more damage than good, you got some pollution control valves in there too and can't touch them with carb bank still together. Carb cleaning solvents then ruin them.

X2 on the pipes, too loud and it gets very hard to really finetune, the noise covers up many nuances that need to be heard. Although at least it's super easy to tell if all 4 are hitting at idle.

 

[gbwithacb]

Yeah I know exactly which part of the carb you are talking about. I was only able to get to it on the first one. Luckily I knew not to dump then in carb cleaner. I didn't want to break em but a full clean is in order.

Synching isn't cheap the first time. A synch kit costs $100.

 

[amc49]

You can bench sync with zero parts outlay if your eyes are sharp. Bench sync requires an engine hitting dead equal on all four though, you have no way to slightly make up for one cylinder not pulling as strong as the others, what true sync does, even them up.

You can't break the carb bank apart without removing at least two choke plates, that means removing the screws which are staked and damage to the shafts when they come out. I always grind the ends of the stake off to have the screws come out freely, they tear up no parts that way and can be reused again with a bit of loctite to make up for the stake gone.

 

[gbwithacb]

Well, I'm at a crossroads with this bike.

I replaced a headlight and figured I may as well let it idle for a few minutes.

So, started it up, no problem. But I noticed that exhaust 4 and 2 were much warmer than 1 and 3. MUCH warmer. 3 was not warm at all. So I shut if off and figured that isn't something that can be ignored.

The spark plugs were all clean, and so I figured I should test compression just to check. I mean, it's running so there should be compression but maybe something is up. Now, I used a harbor freight compression tester (brand new) with the gas tank off (does that make a difference?) and it read as follows:

Cylinder 1: 60psi
Cylinder 2: 60psi
Cylinder 3: 0psi
Cylinder 4: 60psi

How was this even running with such low compression? Is it the tool that's not so accurate? I double checked and got 60psi across the board twice.

So, I thought, oh okay, let me test the valves. Turns out that the cylinder head is only removable once the whole engine is off the frame. Is that true?

What do you think I should do? This is the first time that I've tried to restore a bike. I'm not sure if I should get stuck into this engine now, or sell it and move on.

 

[amc49]

Always highly suspect the compression gauge as faulty, most of them are. If it is not a screw in type don't even waste your time. The engines are junk at under a TRUE 100 psi. Throttle MUST be held wide open to test one and all plugs removed and the engine spun fast is better, a lackluster bike battery and your readings will be crap. I always use a car battery but NOT WITH THE CAR RUNNING!!!!

'...the cylinder head is only removable once the whole engine is off the frame.'

True.

You NEVER start up an air-cooled engine to 'let it idle for a few minutes', best way to burn one up there is.

 

[gbwithacb]

Haha, sage words right there. I know air cooled engines like to overheat quick, I had it on for less than a minute and was able to diagnose a problem.

I'll double check tomorrow with the car battery and report back. Still convinced that something is up with the third cylinder

Yeah the gauge is the screw on type.

Shit man, I hope it's just the gauge! I'm not ready to remove an engine!!

 

[gbwithacb]

Oh you sons of bitches, yes the cylinder head needs the engine to come off, but the cylinder head cover doesn't! Saved again by the FSM!

I'll be ordering a micrometer and checking the valves when it arrives.

 

[amc49]

Yes, valve cover comes off to check valve clearances, My bad, I should have said that. Checking is easy enough but special tool needed to get shims out and in, you damage tappets if you pull a shim and then rotate the engine with shim missing.

 

[gbwithacb]

Alright, so here is some goodish-badish news.

Always highly suspect the compression gauge as faulty, most of them are. If it is not a screw in type don't even waste your time. The engines are junk at under a TRUE 100 psi. Throttle MUST be held wide open to test one and all plugs removed and the engine spun fast is better, a lackluster bike battery and your readings will be crap. I always use a car battery but NOT WITH THE CAR RUNNING!!!!

I hooked her up to a car battery and ran the compression test again. I got pretty much the same results. Instead of 60 PSI on cylinders 1,2,4, I got 70 PSI.
Cylinder 3 is at 0psi.

Cylinder 3: I put a teaspoon of oil down the spark plug hole and tested it again. The PSI increased slight to maybe 10 PSI. Figure that the cylinder o-ring is probably a little bit worn, but it seems like this might be a valve adjustment on this cylinder.

As for the generally low compression, how is it possible that this engine runs and idles? Maybe it's just shitty chinesium harbor-freight gauges...Well, at least it's consistent.

To be honest, I'm not really looking forward to doing the valve adjustment. I have to buy a $50 micrometer, and probably order a whole shim kit. I have never adjusted valves before, and these shim valves seem like a very difficult task, with high stakes too. Any mistakes and it's wrecked.

This bike has fought me every step of the way! I've also been restoring an 81 GL500 over the past week and that thing took less than a week to be rideable, with very similar problems to this 750. The GL is just so much better designed. Everything is easy to get to, take off and adjust.

Still, I ain't giving up. This bike is a beast, and must be restored with love. Still, I wish it could just work with me here!!

 

[amc49]

Maybe the gauge used there and some car ones will have enough volume you can't get a good number with smaller motorcycle sized cylinders, the gauges have a volume requirement similar to a compression ratio themselves.

 

[Chris in Dorset England]

Alright, so here is some goodish-badish news.



I hooked her up to a car battery and ran the compression test again. I got pretty much the same results. Instead of 60 PSI on cylinders 1,2,4, I got 70 PSI.

Cylinder 3 is at 0psi.

Cylinder 3: I put a teaspoon of oil down the spark plug hole and tested it again. The PSI increased slight to maybe 10 PSI. Figure that the cylinder o-ring is probably a little bit worn, but it seems like this might be a valve adjustment on this cylinder.

As for the generally low compression, how is it possible that this engine runs and idles? Maybe it's just shitty chinesium harbor-freight gauges...Well, at least it's consistent.

To be honest, I'm not really looking forward to doing the valve adjustment. I have to buy a $50 micrometer, and probably order a whole shim kit. I have never adjusted valves before, and these shim valves seem like a very difficult task, with high stakes too. Any mistakes and it's wrecked.

This bike has fought me every step of the way! I've also been restoring an 81 GL500 over the past week and that thing took less than a week to be rideable, with very similar problems to this 750. The GL is just so much better designed. Everything is easy to get to, take off and adjust.

Still, I ain't giving up. This bike is a beast, and must be restored with love. Still, I wish it could just work with me here!!

Take up the challenge When it's running sweet as a nut you will be glad you did. 0 psi after oil, burnt valve, most likely.

 

[gbwithacb]

Yeah I hear that! I'll take it on!

Burnt valve means new shims, right?

 

[amc49]

I'd be casting about for another compression gauge for backup readings.

Shims are the least of your worries with a burned valve, engine comes out and apart to fix it and shim not determinable for that one until that is done.

You need more verification your compression numbers are real.

The valves were crap on these and recede very quickly with use to tighten up the clearances rather than loosen like most valves do. Couple that with owners who didn't want to check them and how most of the bikes die, then with the carbs unassembled in a box too since so many think the carbs were the issue (they often ARE adding to the problem) all along.

The service manual spec is .003", BUT it is a mistake due to what I just said, you set them all to .005" instead. Why? The spec runout says from .002"-.004" but the valves CAN and will burn at .002", I've done that myself. Why again? The cams are super loose in the heads even brand new and that lets them be pushed around in the holes to give fake clearances that are not real, a .002" is essentially the same thing as a .000" there and burn city. As a quick check on that, check one, get a number, and roll the engine around and check the exact same one again, time after time you will get variances that drive you nuts even while you swear you are checking in the exact same place and way. It is NOT your imagination, the numbers ARE changing and why you add a couple of extra thousandths there, to guarantee you have full compression so the engine runs great.

The valve clearance spec is .003", but the cam journal holes are from .005"-.008" right off the showroom floor, give that some thought and you'll understand it.

Set them at .005" if you want the job to last much longer, it works!

Don't be surprised to see plenty of the valve clearances closed up to almost zero.

 

[gbwithacb]

Ok, got a new compression gauge, one that wasn't from the ol' Hazard Fraught.

The readings are as follows:
Cylinder 1: 100psi
Cylinder 2: 100psi
Cylinder 3: 0psi
Cylinder 4: 110psi

Do you think it's still burned valve covers? Is it possible that it could be a leak on the carb boots? I took the carbs off a few times because I had problems with the float heights, and those boots were real difficult to get around. They don't look like they broke but it's possible there might be a leak in one.

 

[Chris in Dorset England]

Ok, got a new compression gauge, one that wasn't from the ol' Hazard Fraught.

The readings are as follows:
Cylinder 1: 100psi
Cylinder 2: 100psi
Cylinder 3: 0psi
Cylinder 4: 110psi

Do you think it's still burned valve covers? Is it possible that it could be a leak on the carb boots? I took the carbs off a few times because I had problems with the float heights, and those boots were real difficult to get around. They don't look like they broke but it's possible there might be a leak in one.

Zero compression is nothing to do with the carbs or the intake system at all. The problem is internal to the engine, the possibilities are: valve stuck open or burnt through the seating, blown head gasket, broken or stuck piston rings or holed piston. Any or all of the above will require engine out, cylinder head off. Prayer to the gods of engines. You may have problems getting the head and cylinder barrel off as well. Sorry but there is no way round it. Check the valve clearances for #3 as a last resort before you start pulling it all to bits. All those3 pressures are low anyway. Workshop manual says 170 +- 14 psi

 

[gbwithacb]

Well, shiiiiiiiiiiiiiiiiit.