Topic: Flaming backfires!

[ic455]

Ok, now that the '76 CB750K is running, and the points are adjusted pretty close, she has got gobs of torque.  Test ride produced watered eyes in 2nd gear at about 45 MPH, almost no brakes so went no faster. 

Details: Seems to be factory exhaust with baffle removed (and missing, unfortunately), carbs are obviously in desperate need of rebuild (#4 leaks like crazy) rear tire size incorrect, rear sprocket size incorrect, chain worn out, wiring hacked to bits, front caliper is locked up and rear brake is SOFT. 

The current stumper: Idle remains very high (tach cable broken so not sure exactly how high) even when idle screw is backed off of the rack all the way.  At idle bike will backfire/miss every 20 seconds or so.  When the throttle is snapped open past 1/4 the bike will produce an ear-splitting backfire with a 4 foot flame(orange).  Other than these problems the bike seems to run good going down the road, will backfire (sounds like a burp?) if coasting in gear.  Rumble,rumble,rumble POP! like a firecracker in a Coke can.  The plugs are resistor type Autolites, so I know I need to get D8EA's in it.  Tried to set timing, but erratic idle and no tach prevented doing so. 

Thought I'd describe the specific problem and see if there's a Doctor in the house, so to speak.  Haven't been able to piece together a good answer from other articles, since there seems to possibly be more than one problem.

Any help is appreciated!!  Ride on!!

[Tim.]

Problem?  What problem?  I'd love 3 feet of orange flame kicking out my pipe every once in a while to let people know I mean business 

[jtb]

Tintin, when you get that turbo ss going, you can send all kinds of flame out the back!

[750goes]

1. check for air leaks at rubber boots between carbs and engine - do them up firmly - and hopefully there are no cracks or splits.
2. check the fuel bowls for sediment - if dirty  - clean them out
3. in-line filter?? if not and the bowls were dirty, think about putting one on..
4. pull the idle jets, and clean them
5. pull the main jets and clean them
6. set all carbs back to whatever the stock setting should be - 1 turn out +/- a little bit..
7. check the valve gap if possible
8. adjust the timing chain
9.check the points and timing again
10. check the operation of the advance unit (need your strobe timing light for this) - this was a real problem on my bike as the advance unit had frozen, and was not working......when lubricated - completely different feel and power...
11. check all plug leads and plug caps for cracks, splits and plugs for proper gap

bet it is diiferent now ??/ and it gets better every time you do it (just like riding a bike)

[ic455]

Problem?  What problem?  I'd love 3 feet of orange flame kicking out my pipe every once in a while to let people know I mean business 

Little white car behind me at stop sign during test ride, didn't know it was there (mirrors are gone) until I throttled it and they honked at me(I guess the flames are scary if you're looking right into them) 

[ic455]

1. check for air leaks at rubber boots between carbs and engine - do them up firmly - and hopefully there are no cracks or splits.
2. check the fuel bowls for sediment - if dirty  - clean them out
3. in-line filter?? if not and the bowls were dirty, think about putting one on..
4. pull the idle jets, and clean them
5. pull the main jets and clean them
6. set all carbs back to whatever the stock setting should be - 1 turn out +/- a little bit..
7. check the valve gap if possible
8. adjust the timing chain
9.check the points and timing again
10. check the operation of the advance unit (need your strobe timing light for this) - this was a real problem on my bike as the advance unit had frozen, and was not working......when lubricated - completely different feel and power...
11. check all plug leads and plug caps for cracks, splits and plugs for proper gap

bet it is diiferent now ??/ and it gets better every time you do it (just like riding a bike)

Thanks, will do!!

[Jerry Rxman Griffin aka MuthaF'er]

You running moonshine from LBL in your tank?

[kghost]

Carbs overflowing and flame tends to make me think rich mixture.

[theunrulychef]

I had this going on a while back after I had messed up my carbs.  Turns out I was actually running really lean - seemed strange.  I had to turn out all of my pilot screws something like 5 turns to get it running ok until I pulled the carbies again & got new plugs.  I may have had an air leak which compounded the dirty carbs.

The only fun part was that I had flames shooting out of my tailpipe.  The rest of the situation sucked balls as the bike was performing like crap.

BTW, personally, I'd get the brakes working before I got the bike running strong.  No need to temp yourself to ride around with just a soft rear brake.  I'd want to live long enough to get to fix all the other stuff.

-Jay in Philly

[sparty]

When the throttle is snapped open past 1/4 the bike will produce an ear-splitting backfire with a 4 foot flame(orange).

Sweet!  Maybe you can take your bike over to Afghanistan and fight terrorism.

There is a local guy who has modified his hotrod and motorcycle to shoot flames out of the exhaust.  He came to bike night earlier in the summer and scared the living crap out of everyone when he shot flames from his drag pipes.

You could be that guy in your area.  You can also rent out your bike for the next James Bond film.

Sparty

[ic455]

You running moonshine from LBL in your tank?

ooohh, White Lightnin'!

[ic455]

When the throttle is snapped open past 1/4 the bike will produce an ear-splitting backfire with a 4 foot flame(orange).

Sweet!  Maybe you can take your bike over to Afghanistan and fight terrorism.

There is a local guy who has modified his hotrod and motorcycle to shoot flames out of the exhaust.  He came to bike night earlier in the summer and scared the living crap out of everyone when he shot flames from his drag pipes.

You could be that guy in your area.  You can also rent out your bike for the next James Bond film.

Sparty

Not a bad idea, but Gawd, my head still hurts!

[ic455]

Carbs overflowing and flame tends to make me think rich mixture.

I did notice alot of sooting of the tailpipe, and I caught a glimpse of a puff of smoke before the afterburn  I think I'll just start by rebuilding the carbs, after the brakes that is

[ic455]

I had this going on a while back after I had messed up my carbs.  Turns out I was actually running really lean - seemed strange.  I had to turn out all of my pilot screws something like 5 turns to get it running ok until I pulled the carbies again & got new plugs.  I may have had an air leak which compounded the dirty carbs.

The only fun part was that I had flames shooting out of my tailpipe.  The rest of the situation sucked balls as the bike was performing like crap.

BTW, personally, I'd get the brakes working before I got the bike running strong.  No need to temp yourself to ride around with just a soft rear brake.  I'd want to live long enough to get to fix all the other stuff.

-Jay in Philly

Good point, see post just below.  It is becoming increasingly tempting to go tear-assing around on it, especially since I live out in the sticks   I better go ahead and pull the rear wheel off and yank the carbs, then I'll be able to control myself

[ic455]

UPDATE:  Pulled carbs and disassembled/cleaned.  Found jet needle clip to set in fourth hole on all carbs, changed to middle.  Slow jets were filthy, one completely clogged.  Checked/reset float levels.  Bench-sync carbs.  Checked carb boots, saw no tears or cracks but found clamps were loose.  #1 carb vac. opening (for gauge) screw was missing seal, made seal out of brass wiring ring terminal.  Reinstalled carbs, checked advance weights and point gap.  Fired it up and.....damn!  Still sucks.  Checked headers and found #2 too hot to hold on to, #1 touchable but still a bit warm to grab, and #3 and #4 barely warm, wrapped hand around each and held on with engine running.  Pulled plugs, found #3&4 sooted (ceramic insulators blackened) and wet with fuel.  #2 slightly sooted and dry (ceramic insulator still white).  #1 sooted a bit more than #2 but still dry.  Re-gapped plugs.  Re-checked point gap.  Fiddled with air screws.  #4 header finally getting pretty hot (after running with vac port screw removed for a minute, dunno if that had anything to do with it or not).  Rode about 1/2 mile and rechecked headers, still able to grab #3 without a burn.  Still seems to be missing.  I feel the miss is caused by #3.  Idle surging. 
I'm about ready to just quit messing with it until I get carb kits and the new Accel coil kit.  Have not yet done a valve adjustment as I still can't find my friggin' feeler gauges.

Any ideas welcomed!

[750goes]

Think you still have fuel issues, check that all carbs are getting fuel through them.. drop the bowls off one at a time and open the fuel tap.....if it flows freely thats good...if not investigate upstream of this...when flowing freely, lift the floats gently and see if this stops fuel flowing as well...then you will know the following
1- getting fuel to all carburettors
2- when fuel bowls full - hopefully they are stopping any going out the overflow tubes.

why did you change the needle clip position ?? was this a standard setting to put it back in the middle..

check the idle passages as well as the jets - take out the idle screw/spring/seal and spray some carb cleaner down the holes - are they clean - reinstall the screws, and set them to standard setting for wahtever the carb models are - check out this link for your carb model specific settings

http://hondachopper.com/carb.specs.html

hope this gets you a bit further down the track... keep at it

tighten up the boots carefully when the bike is warm- may also help the idle problem..

[Jerry Rxman Griffin aka MuthaF'er]

OK, you reset your needle positions to middle. Most likely a good thing.

What size mains you running? That should show up on all 4.

Check the resistance across your plug caps. Old caps suck. That could show up rich/wet on 1 to 4 cylinders if not firing good.

Plug wires would show up on individual cylinders as rich/wet if not firing properly. New coils will take care of potential coil, wires, caps issues and that potential will be eliminated once installed. Next expenditure is a Dyna S ignition.

Find those feeler gauges and set your valves!  That could show up on any cylinder if not set correctly. Too tight could show up as lean.

How'd you check points gap without feeler gauges?

Run a fine wire down the small passage from the intake side of your carbs to where it exits inside the carb. Spray cleaner through afterwards. Wish I could describe it better. Search Hondaman's posts for this issue.

Refer to previous post by 750goes.

Spray some WD40 etc around the intake boots with it running and see if it sucks it in to look for leakages. Performance should change if so.

Ride down to Leder Bottoms or head South and get a half pint if all fails 

If it were coils or points or condensers it would show up on pairs of cylinders ie 1 & 3 and/or 2 & 4

[Honda CBer]

check your valves...

a poorly seating exhaust valve (which will show up as low compression) causes poor ignition of the mixture, which dumps unburned mixture into your header, which can then be ignited, causing flaming backfires... hopefully, the valve and seat area are not damaged (if that turns out to be the case)... i have also heard stories of headers being destroyed, but have never seen that for myself...

[ic455]

Think you still have fuel issues, check that all carbs are getting fuel through them.. drop the bowls off one at a time and open the fuel tap.....if it flows freely thats good...if not investigate upstream of this...when flowing freely, lift the floats gently and see if this stops fuel flowing as well...then you will know the following
1- getting fuel to all carburettors
2- when fuel bowls full - hopefully they are stopping any going out the overflow tubes.

why did you change the needle clip position ?? was this a standard setting to put it back in the middle..

check the idle passages as well as the jets - take out the idle screw/spring/seal and spray some carb cleaner down the holes - are they clean - reinstall the screws, and set them to standard setting for wahtever the carb models are - check out this link for your carb model specific settings

http://hondachopper.com/carb.specs.html

hope this gets you a bit further down the track... keep at it

tighten up the boots carefully when the bike is warm- may also help the idle problem..

Needle clip position changed due to the statement that the center(third) slot is factory, as seen here:http://www.salocal.com/sohc/tech/carb/dspg_mgs/rmvjtndl.htm

dunno why I hadn't thought of tightening boots when warm, will try it.  Already pulled bowl drain screw #3 and fuel is getting there.  Will try pulling bowl and check float needle.

Air passages are definitely clean   As for any idle circuit passages: I did not dip the carbs, so there may be deposits left that my spray didn't get.

Thanks!

[ic455]

OK, you reset your needle positions to middle. Most likely a good thing.

What size mains you running? That should show up on all 4.

Check the resistance across your plug caps. Old caps suck. That could show up rich/wet on 1 to 4 cylinders if not firing good.

Plug wires would show up on individual cylinders as rich/wet if not firing properly. New coils will take care of potential coil, wires, caps issues and that potential will be eliminated once installed. Next expenditure is a Dyna S ignition.

Find those feeler gauges and set your valves!  That could show up on any cylinder if not set correctly. Too tight could show up as lean.

How'd you check points gap without feeler gauges?

Run a fine wire down the small passage from the intake side of your carbs to where it exits inside the carb. Spray cleaner through afterwards. Wish I could describe it better. Search Hondaman's posts for this issue.

Refer to previous post by 750goes.

Spray some WD40 etc around the intake boots with it running and see if it sucks it in to look for leakages. Performance should change if so.

Ride down to Leder Bottoms or head South and get a half pint if all fails 

If it were coils or points or condensers it would show up on pairs of cylinders ie 1 & 3 and/or 2 & 4

I set the point gap by using a spark plug gap gauge and a pocket screwdriver   I found that the very tip of the (flathead) screwdriver is about .015 to .017 thick.  I cheated bigtime

Plug boots are in seriously poor shape so I'm betting that's part of the problem.  Will have to wait for funds to stabilize and acquire an Accel coil/wire kit.

As you mentioned about coil/ignition problems showing up on paired cyl:  my first thought on the cold #3 and #4 was coil, but since #1 and #4 are on the same coil I knew that was a dead end.   

As for the 1/2 pint, I'd say that's an underestimate!  You know it takes a gallon of KY Gentleman to get anything done!

[ic455]

check your valves...

a poorly seating exhaust valve (which will show up as low compression) causes poor ignition of the mixture, which dumps unburned mixture into your header, which can then be ignited, causing flaming backfires... hopefully, the valve and seat area are not damaged (if that turns out to be the case)... i have also heard stories of headers being destroyed, but have never seen that for myself...

I'm going to just swallow my pride and borrow mu friend's feeler gauges.  I think I'll drag out the ole compression tester first, though......   There's another one I should've already done, I guess I have been afraid of the result

[mrbreeze]

Make sure you have the right adaptor for that compression tester.These bikes have a goofy thread in them (What is it??...like 12mm or some #$%* like that?) I acually haven't done mine yet but heard in here somewhere that its a different thread. Good luck.

[ic455]

Make sure you have the right adaptor for that compression tester.These bikes have a goofy thread in them (What is it??...like 12mm or some #$%* like that?) I acually haven't done mine yet but heard in here somewhere that its a different thread. Good luck.

Yeah, I believe that it is a 12mm.  I haven't dug out all my adaptors yet, but I'm pretty sure I have a 12mm somewhere.  I know I've got a 14mm....dammit now I'm gonna go hunt for all of 'em. 

[ic455]

OK, swallowed pride and bought new set of feeler gauges.  Armed with printouts from all over this site I went to work on the bike today.  Adjusted cam chain tension (although I think the tensioner is bad or the chain needs replaced), set point gap to .012, set static timing.  Adjusted valves:  #3 (which still has a really cool header, cool enough to grab while running) exhaust valve was way too tight, probably .001 or thereabouts, and the intake was about the same.  Adjusted all intake to .002 and all exhaust to .003.  The good news is that even though #3 header is still not heating up correctly, the bike is running extremely well.  When I first started it, I had the choke closed, but it would only catch and run a second then die.  Opened choke all the way and voila!  Fires up and runs first kick.  Odd considering bike hadn't been started since last Saturday, and weather is cooling off......Anyway, the exhaust note has changed considerably, no more flaming backfires, but still loud as hell (still haven't found the diffuser for the muffler).  idles really good and starting from a dead stop is no longer a clutch-killer.  Was only able to test ride in 1st gear in the front yard since the rear brake is even worse after I tried to tighten up the pedal engagement, but the power is definitely there.  I'm thinking the #3 problem is going to require rebuilding the carb as I'm sure, even though I gave all carbs a good cleaning and nothing looked out of place, that new parts are going to be required.  Many thanks for all the info and advice!  Now how about some rear brake info

[750goes]

ic

if #3 is still not getting hot - swap out with another plug say 3 & 4, and see if that follows to #4 header....also if the plug caps are toast (is it #3 that is bad) you may be able to get by with wrapping some insulating tape around the bad caps.... that may also help to fix any intermittent zaps from plug leads to head etc..

tensioner may just need a bit of lubrication - if you can get it out easily enough - only held in with 3 bolts - but awkward..
take it out and give it a service - it's only a bolt and lock nut holding a spring loaded rod... - but it could be catching or been tightened too much and not moving or be able to be adjusted properly.. just undo the three bolts and take it out... don't undo the bolt and lock nut until you have it on your workbench...

simple to do and then re-do your cam chain tension (measure how far in the little rod is in the back of the tensioner first) then measure again after installing and adjusting.

do you have rear disc or drum ??

glad to hear it's running better...hard work pays off.