Back firing

[Andyt59]

Restoring an '81 CB750C Custom. I'm having issues with it backfiring. I don't know much about these carbs. That doesn't mean I am ignorant of how carbs work, but multi-carb set ups kinda scare me.

I've put the stock air box back on. The carbs have been cleaned. According to my local Honda dealer, this bike had 125/68 jetting and that is what is in it. I have a set of smaller 102 jets out of another set of carbs. I also installed stock exhaust.

It runs OK until it warms up. It makes no difference of how it runs if I choke it or not. After it is warmed up, the performance takes a dump. Literally. It's totally gutless up to ~5000RPM. It's got some cahonies above that. I have checked and rechecked the timing. Oh, and the idle is screwy too. After it warms up, the idle doesn't want to drop. It stays around 3000 RPM.

Any help would be great. I'm trying to have this bike ready for a CMA Fastlane Rally in Kamiah, ID (268 miles one way) the end of next month. Ultimately, I'm building this for my wife. I've been really lucky on the bright work (chrome) and am having the '81 tank custom painted.

-Andy

[Re-run]

What air filter are you running? Your issue is in the idle circuit so you can leave the mains alone for now.
When the carbs were cleaned, did you do it or someone else? The issue with these bikes are the idle jets like to plug up, the manifold boots between carb and engine get hard and do not like to seal or they crack.
The high idle could indicate an air leak.

So if you can, get the manual from here and check out the carb section. See if you can pull out the idle jets and clean them by hand. Check the boots and look for cracks. A common way of doing this is to spray the boots with wd-40 and listen for changes in idle.
Also, check the idle knob when warm and back it out, idle should drop but if not then it has to be an air leak.
I know these carbs also have some sort of diaphragm and you may want to check those out.

[Andyt59]

I'm running the stock filter also installed a new set of plugs.

I cleaned the carbs myself. It's amazing how clean the dishwasher can get stuff with liquid soap (Don't use the powdered or crystal type. that will definitely plug things up).  All the diaphragms looked good (not hard, brittle or cracked) and the manifold boots are still in excellent shape (again, not brittle cracked or hard). The idle will drop if I adjust the knob. I carefully cleaned the idle jets  with air and a set of welding tip cleaners (yes, I have done this many times with no ill effects.) I did the air leak check but I used non-chlorinated brake cleaner to do it (Also good for cleaning carbs) and it didn't affect a thing.

The only other thing that it could be is the mixture adjustment and that's what scares me. Isn't that what knock out the synchronization of them? or is that something totally different?

[Re-run]

Removing the slides is what kills the sync so you will be fine adjusting the idle mix.
I use a wire to clean the carbs too so no flak from me. Just do it gently of course but I am sure you know that. I have had people adamantly argue against using anything but air and spray on the jets but I still use a carb tool!

[mikey440]

 The 125 main jet is too big, ive got 102 in mine with 4 into 1 header and it runs fine, make sure your throttle cable is returning good, also sounds like a intake leak for the backfire, or the 125 mainjet is givin too much fuel and giving a flooded situation causing backfire, find which pipe is bacfiring and address that carb and change plugs, probably fouled, also turn idle mix screws out 2.5 to 3 turns from lightly seated.
 must use stock airbox for these carbs to work correctly, check timing although i doubt thats it, most of time on these bikes its the carbs, they have to be sparling clean, also check inline fuel filter for crap, if you dont have one, get one,,hope this helps, mykey440

[Andyt59]

OK. At the advice of you fine intellectual people in this group, I pulled the carbs off tonight.

Out came the 125 jets and in went the 102's

Reset the air mixture screws out to EXACTLY 3 turns each. Used the screws out of my donor carbs as they looked MUCH better than the ones I was running.

Put it all back together:
1) I actually had to CHOKE it to get it to run this time
2) The choke made a difference in how it ran (cold and warmed up)
3) The exhaust still smells a little rich but not like it did before

All of this in under an hour. I need to put some more gas in the tank and run her down the road again and see if there is improvement there.

I have a new anomaly in the electrical system. I have been running this WITHOUT the ign. switch hooked up. I'm using a 30A breaker fuse to jump the red and black wires in the ign. plug. I discovered tonight, with the ground COMPLETELY OFF THE BATTERY, the dash lights up like it is supposed to, the solenoid clicks (but the bike won't start). I reconnect the ground, and the dash goes out, the gauges light up and I have to pull the clutch to start it (no matter if it is in neutral or not). Do I need an electrician or an excorcist?

[LukeM]

Intellectual?  Me?  Nahhh...

It sounds like changing the jets helped a bunch. Maybe someone had pod filters on there before. Per the Haynes manual, they call out 1.5 turns out as a baseline for the idle mixture screws.  3 turns out sounds like it'll be running kinda rich. There's another procedure involving the idle mixture screws to get them working together: it's in the factory service manual located on this forum site.

A dishwasher as a carb cleaner...I wonder what that does for the dishes that follow...or the wife who wonders why her dinner plates smell like petroleum.

I'm not sure what's going on with your electrical issue. I do know the electric starter won't run unless the transmission is in neutral or the clutch is pulled in.  Double check all the connectors at the instrument end, and make sure nothing is touching ground.  Look for melted or nicked insulation. Also check the electrics under the seat.  If all else fails, get a good copy of the electrical diagram for this bike, and trace everything out.

Good luck, and let us know how it goes.
Luke M

[Andyt59]

Hey, in my defense, I had her permission to do that. No, there were NO DISHES involved in the cleaning of these carbs

[Andyt59]

AAAAAAAAAAAAAAAAHHHHHHHHHHHH!!!! Piece of crap!!!!

So I reset the air mixture screws, again, as suggested. 1.5 turns out from lightly seated. The bike started and ran good until I dropped it to idle and that's when all hell broke loose.

After restarting it, it idled rough and stalled when I shoved in the choke. So I backed out the screws 1/2 turn. Now the SOB won't start at all. It will catch every so often, just enough to disengage the starter. Yes, all this work has been done with the stock air box and filter. On a whim, I rechecked the timing and it's dead on what the book says it is suppose to be.

As soon as the tank comes back from paint and I have some $$$$$, she's going to Honda. I'm not giving up but I know the limits my mechanical abilities.  Needless to say I am very frustrated with it right now.

[Andyt59]

Man do I feel STUPID!!! I fought and fought and have run the battery down 2 times trying to get this thing started.

So today I thought I would mess with it for the first time since the first of LAST WEEK. I put the mixture screws back where I knew the bike ran before, 3 full turns out. Seemed like allot but, what the heck, right? I cranked and cranked on it and thought, this thing just catches and stalls, catches and stalls. I grabbed my little old gas can and dumped about half a gallon in it. Funny thing, IT STARTED!!!! At that I promptly ran the mixture screws back to 1 1/2 turns out from lightly seated. It's still back firing and comes back to idle really slow and often hangs up right around 3000RPM. If I wiggle the idle set knob, it will come on down to 1100RPM.

I'm trying to get it into the shop this next week to get it set once and for all. I think the biggest issue with this bike is all in the carburetion. It's got 3 too many carbs!!!

-Andy

[Andyt59]

Ok. Piddle farted around with it some more. Sometimes  I HATE having to recycle old parts but I got the front brakes working tonight. The caliper is a bit sticky so I intend to pull it off and give it a good cleaning. Previous owner had installed the forks backwards (i.e. the right for on the left and left fork on the right.) so I put them back. Amazing, even the speedo works now!!!

I also installed the new chain and wiped the dust off of it. I have no desire to screw with the carbs any more and fully intend to  haul it off to the shop to get them PROPERLY adjusted.

-Andy

[mitchberry]

i'll admit i didn't read everyone else's posts, but i will say that back firing does not have anything to do with carbs...

 a backfire is caused when the valves open at the wrong time, or the engine ignites at the wrong time...

 this is generally caused by timing...  in other words, you probably need to either adjust the timing chain or replace it.

 
Backfiring isn't caused by the amount of air going into the carbs... though with that said, you should make sure to jet for altitude as well as modifications to exhaust and air filters...

if it's all stock, use the stock jets.. if you have custom exhaust and air pods on it, then change the jetting.

the other thing that will cause backfiring is bad valve shims.. i assume there are shims in these motors.. i've never cracked one open...  but i can say that i don't recommend doing that yourself unless you really know wtf you're doing..

shims are spacers that adjust the space between the valve buckets and the cams within .500ths or more of a mm....   if they are off, the valve may not open or close as much as it should / BOOM  backfire....