Topic: Backfiring, vibrating and rattling in engine (video)

[MJL]

Undo that locknut with a 10mm wrench and back the screw off with a flat blade screwdriver. At this point you should be able to wiggle the rocker arm around and tell that there's a gap between the bottom of the screw and the top of the valve.

Noooooo..... 

Check the clearance first before you loosen anything. You may not have to adjust it.

[IAmCitizenMe]

So my tappet cover came in today (quick turn around, Bike Bandit!) and I took her for a test ride.

She was a bit louder since I gapped the exhaust valves wider (though I may go back since winter is coming on so soon).

However, the low/deep popping sound is still there. But I think I reduced it somewhat because I only hear it when accelerating out of first gear instead of before when it was first, second and possibly third. I know I still need to do a carb sync. So could having my exhaust valves wider be a problem?

She sure runs better now, that's for sure!

[Johnny5]

Did you time the bike with a strobe light? Or static time it? Before you sync the carbs, see if you can time the bike with a strobe light ($25 at Autozone). That weird noise you're hearing might go away after it's timed and then sync'd.

[IAmCitizenMe]

I did a static timing. I guess I know what I'm doing for my day off tomorrow. I can hardly complain 

[Johnny5]

Yeah, with static, you'll be close, but from all my times timing my bikes, static always just got me in the ballpark. Kinda like bench syncing the carbs. But make sure you do all the regular maintenance first before the carb sync.

Have fun tomorrow!

[Duke McDukiedook]

Have you checked your coils, spark plug wires and caps?
I got a bad clicking noise on my 750 when I had a bad wire to cap connection.

[IAmCitizenMe]

Have you checked your coils, spark plug wires and caps?
I got a bad clicking noise on my 750 when I had a bad wire to cap connection.

It's not a clicking noise. I fixed the vibrating noise (I forget who said it, but they advised me to adjust the cam chain. fixed it). The only problems now are that low, almost a thud sound coming from who knows where, and it feeling like it's always running out of gas in low RPMs.

[IAmCitizenMe]

OK. So the problem has persisted. The backfiring, I believe, is coming from the carb. Is that possible? Valve clearances are spot-on. Everything is timed. I thought maybe my carbs needed to be cleaned and I was running too lean.

I checked my spark plugs (1, 2, 3, 4)




Then I noticed some blackening around my number 4 carb.


It wasn't there before and it is only on number 4.

I do a lot of highway traveling (school and work and home) so should I move up to a D8?

[the technological J]

looks loose u prob have a air leak to check when shes running spray something like starting fluid around that carb boot( and others)  see if ur idle moves  at all. u prob need to loosen up those clamps and push that boot the whole way on(i use a piece of wood the lever it on BE CAREFULL!!!).. when timing my bike i adjusted the timing plate (after i did the points) to the point when the bike idled the highest(that that adjustment allowed) i anchored it down then adjusted the idle to  a level that i liked... and just incase i missed it did u adjust ur cam chain? i adjusted mine when it was running and left it where it was quietest(its a wierd springy adjustment)   and have u messed with ur air mixure screws at all? screw them all the way in and bring them out about 1 and a half turns ( the book calls for 1 and 3/8) if u can  it should only take an hour to do all of these... i used a double sided bit to adjust the air screws (i dont have little hands and theyre a challenge) theres my  2 cents... good luck

[Pinhead]

Sounds to me like the starter never disengaged after the engine started....

[IAmCitizenMe]

looks loose u prob have a air leak

That's what I was thinking, as well. Thanks for the input!

Sounds to me like the starter never disengaged after the engine started....

How do I check/fix that?

[Pinhead]

Not sure... Maybe try to listen to the difference; start the engine and hold the starter button for another couple of seconds to see if it sounds any different when you let off the button. Don't hold it for too long, though, 'cause it can be pretty hard on the starter gears (if that's not what the problem is).

[hummbug]

I read the start of your thread and it sounds similar to a problem I had a couple of weeks back. I was riding on a windy day and experiencing a loss of power at a certain RPM. I had a few suggestions from some helpful members and it turned out to be my air filter (a foam uni-filter) was too thick with filter oil. I removed it and went for a ride and things got better straight away. I then put the old paper one back in and the problem has gone away.
I also have a pronounced pop from the exhaust on decel and don't have any other indications of lean running. This pop could be a result of exhaust header leak OR as it looks to be in your case, an air leak around the carbs. I have bought a set of O-rings which go between the carbs and the header but not put them in yet. I am hoping it will solve the problem. The O-ring part number is 91302-001-020.

I know it's not much help but it may point you in the direction of a couple of areas to look at...

[IAmCitizenMe]

I read the start of your thread and it sounds similar to a problem I had a couple of weeks back. I was riding on a windy day and experiencing a loss of power at a certain RPM. I had a few suggestions from some helpful members and it turned out to be my air filter (a foam uni-filter) was too thick with filter oil. I removed it and went for a ride and things got better straight away. I then put the old paper one back in and the problem has gone away.
I also have a pronounced pop from the exhaust on decel and don't have any other indications of lean running. This pop could be a result of exhaust header leak OR as it looks to be in your case, an air leak around the carbs. I have bought a set of O-rings which go between the carbs and the header but not put them in yet. I am hoping it will solve the problem. The O-ring part number is 91302-001-020.

I know it's not much help but it may point you in the direction of a couple of areas to look at...

I took out my filter and went for a ride and the problem definitely got worse. But, this is a good thing, because now I know that my filter isn't the problem!

Judging by my sparkplugs in the picture I posted, would you say that I am running lean? Because it pops on accel AND decel. The boot around my number four carb looks like it has an air leak, yes. But all my spark plugs look exactly the same. Shouln't my number 4 plug look like it has been running more lean than the other three?

[TwoTired]

Do you have all the stock intake and exhaust components?
Have you done the entire 3000 mile tune up check list?
Have you vacuum synced the carbs?

Is it "backfiring" in the exhaust or the intake?

[IAmCitizenMe]

Do you have all the stock intake and exhaust components?
Have you done the entire 3000 mile tune up check list?
Have you vacuum synced the carbs?

Is it "backfiring" in the exhaust or the intake?

The exhaust it not stock. I'm not confident enough to take my carbs apart so I don't know what jets I'm running. (This problem has come up recently. When I bought it, it ran beautifully).
You know, I've heard of this check list, but I have never seen it. Could I find it somewhere or is it something one learns?
No, I have not synced the carbs... yet.

I'm pretty sure it is backfiring through the intake.

As I looked closer into it, I realized that the previous owner somehow ripped open the air chamber and then rubber-welded it back together. The picture is on the chamber side of the number 4 carb, interestingly enough. Wonderful. I would much rather buy a new air chamber than temporarily fix this one. But something tells me they are going to be hard to come by.

[BVCB650]

Any chance that repair job has broken open allowing more air in making it run lean?

[TwoTired]

Below is from the 74 550 owner's manual, my 77 manual has some formatting changes, but is otherwise the same.

My 78 550k had carb spitting/popping issues that a vacuum sync of the carbs cured (after a tune-up, of course).   It seems the previous sync was done by a blind man without braille gauges.

back fire though carbs can be caused by:
early ignition timing
A lean mixture condition (This includes a carb slide lower than the others.)
Intake valve burnt, leaking, or adjusted too tight.

[IAmCitizenMe]

Any chance that repair job has broken open allowing more air in making it run lean?

I'm beginning to believe that is what is causing the problem. I re-snugged the ruined portion of the air chamber and clamped down the collar clamp and took it for a test run. Ran smooth as a baby for a little while, then started popping again. When I got back the collar clamp had slipped back again to where it was before.

Below is from the 74 550 owner's manual, my 77 manual has some formatting changes, but is otherwise the same.

Thanks! I had always wondered about that. But I was too shy to ask. I guess it's hard to hide what you don't know haha.

[BVCB650]

Any chance that repair job has broken open allowing more air in making it run lean?

I'm beginning to believe that is what is causing the problem. I re-snugged the ruined portion of the air chamber and clamped down the collar clamp and took it for a test run. Ran smooth as a baby for a little while, then started popping again. When I got back the collar clamp had slipped back again to where it was before.

Below is from the 74 550 owner's manual, my 77 manual has some formatting changes, but is otherwise the same.

Thanks! I had always wondered about that. But I was too shy to ask. I guess it's hard to hide what you don't know haha.

Mine was butchered up when I got it. This is what I did until I got a new one....
I used duct tape (yes, I know) and on the inside of the chamber, wrapped it around the collar towards the carbs and made a sort of cast out of it. Then, I sealed it with 100% silicon from a calking tube. Let that harden. That repair was air tight, and lasted until I tore it out replacing the old part. It probably would have still been working properly today. Maybe better than just a clamp.

[IAmCitizenMe]

Mine was butchered up when I got it. This is what I did until I got a new one....
I used duct tape (yes, I know) and on the inside of the chamber, wrapped it around the collar towards the carbs and made a sort of cast out of it. Then, I sealed it with 100% silicon from a calking tube. Let that harden. That repair was air tight, and lasted until I tore it out replacing the old part. It probably would have still been working properly today. Maybe better than just a clamp.

What a fantastic idea! Hahaha. I'm going to look into something like this (perhaps black duct tape, though).

[BVCB650]

Mine was butchered up when I got it. This is what I did until I got a new one....
I used duct tape (yes, I know) and on the inside of the chamber, wrapped it around the collar towards the carbs and made a sort of cast out of it. Then, I sealed it with 100% silicon from a calking tube. Let that harden. That repair was air tight, and lasted until I tore it out replacing the old part. It probably would have still been working properly today. Maybe better than just a clamp.

What a fantastic idea! Hahaha. I'm going to look into something like this (perhaps black duct tape, though).

It was tight getting my hand in there, but I used my hand to smear it all over. I should have used a glove. I used long strips of tape and ran the tape from the back towards the carb, in through the collar and taped it to the flat side. I overlapped the tape on each layer, working my way around the collar. Then plaster away with the silicon.

[IAmCitizenMe]

This is my project for tomorrow night. Maybe I'll start another thread for it.

[BVCB650]

Good luck with it. You may need to press the air filter in to make a "home" for it in the silicone if you get it back that far. Otherwise, when it hardens, it may not seat correctly.

[IAmCitizenMe]

So, I fixed the air coupler. I even pulled the carbs off and made sure the jets were clear. They were. Put them back on and fit the couplers back on the carbs (better than they were previously). She fired right up. I took her for a spin down the street and she STILL BACKFIRES! I am lost. Everything is timed, cleaned, gapped. What am I not doing?!