Topic: Backfiring / Timing / Carb issue CB750 - Help!

[andyvclifford]

Dear Sohc/4,

I have been chasing this problem for a few weeks now and can’t make any headway with it and so I come to you guys - the masters -for hep!  Before I receive any “Please search the forums this is a common topic”, I have spent a lot of time reading old posts on this subject and trying what was suggested in those posts, but I feel I must be missing something.

Problem -  Backfiring / Popping in the exhaust at idle and on deceleration.  Sluggish Throttle / stumbling when opened up wide.

Bike - 1975 CB750 K5
Stock air box with new filter inside
Stock carbs with stock settings for clip, jets, air mixture screws, ect.
Stock points - although the actual points were replaced last year.
No performance modifications of any kind to the engine
15,000 miles
4-2 exhaust - unfortunately this is another bike that didn’t come with the 4-4 when i bought it. 

Solutions I have tried -  Cleaned and rebuilt carbs completely to stock spec, 26mm float bowl, 110 main, 40 slow, 1 turn out air screws, needle clip in middle.  Synced carbs with my vacuum gauges.   The only thing on the bike that is not stock is the 4-2 exhaust - however I have had this set up for about a year without any problems using stock carb settings.  Replaced all of my clamps on carb boots and air box.  Sprayed carb cleaner while running to check for vacuum leaks on both intake and carb boots.  Redone the timing by setting the points gap and timing as per manual settings.  Oil change.  New Air Filter.  Have tried adjusting the air mixture screws, but with no real modicum of success.  Pulled the plugs and found that 2-3 were black / carbon fouled (indicating rich?) and 1-4 were tan-ish color.  I assumed this meant there was something up with the timing which seemed like it could be a major reason for the popping and performance issues if it snot a vacuum leak.

Nothing seems to have worked so far - am I missing something, any help or direction would be much appreciated as I feel like I am at a dead end.


Help!

Sincerely
Andy

[calj737]

For a little clarification;

When you pulled the plugs, was it the result of a true "plug chop" or from off and on running? If the latter, that won't genuinely help isolate the issue. Best to start with an Idle plug chop. Get it right there, then proceed to the next stage of plug chop.

Also, fuel level is a recomendation, and as such, you need to verify the fuel level within your carbs with the Clear Tube method. This will be an absolute assurance on the foundation elements, and likely point towards the culprit.

Lastly, have you confirmed the plug caps are good? Measure their resistance, and if out of spec or close to it, replace them (or trim the plug wire a bit too).

These are things you can do reasonably quickly to eliminate some causes. If this doesn't help, or you've done these things, I'm sure there will be other very good ideas.

[Deltarider]

Old gas? Contaminated?
Water in the fuel? A little bit can be enough.
Maybe have a close look at your 2+3 condensor and look for possible arcing.
See this video.

In this video left condensor (1+4) is bad, right condensor (2+3) is good.
Does the advancer function as it should?

[Bankerdanny]

A saying I have seen on the board is that 95% of carb problems are electrical.

Problems with those specific cylinders would indicate electrical since they are paired together. Do you have a spare condenser that you can swap on that set of points? If not you can swap the two condensers and see if the problem transfers from 2-3 to 1-4. If it doesn't then swap the connections for the coils and do the same test.

[flybox1]

+1 to what calj said.
Clear tube fuel level check all bowls, and adjust floats as needed.
Points checked for pitting?  Clean and dress w a points file if needed.
Do you have a dwell meter?

Install a NEW set of plugs before you start it again. 
Start it and only let it idle (Adjust dwell here if you have a meter)  Pull the plugs to check the idle circuit mixture.
Post a picture of the plug tips.

[andyvclifford]

Thanks!  I will do an idle chop tonight or tomorrow and post pics.  The Points are super clean no pitting and no signs of arcing that i can see.  I have a feeling it might be electrical as well but its hard to say!

[Deltarider]

The Points are super clean no pitting and no signs of arcing that i can see. I have a feeling it might be electrical as well but its hard to say!

So the easiest first. You could look for possible voltage drop over the points when closed (easy to measure) or just move a contactfile between them (two strokes can be enough). When you're at it, you can simply check voltage when points are open. If you pry them open from closed by hand you'll hear the sparkplugs spark or at least you should. Make sure none of the connectors at the breakerpoints make ground intermittently.

[andyvclifford]

What is the best way to check the proper functioning of the advancer unit?  This is something i was also going to ask!

[andyvclifford]

Results from Idle Plug chop with new plugs - Looks like #1 is a bit grey, #2 looks new, #3 a little black soot but that could be from the outside of the engine when i was trying to get it out as it got a little stuck and i had to fish it out, #4 looks new.

I let the bike run about 5 minutes for this chop - I hope that was enough to get a correct diagnosis?

[WV_750]

Plugs look good to my less than trained eye. I check the advance with a timing light. There are two marks on the advancer for this purpose. The dot marked on your case should land in the middle of those marks on the advancer at full advance (2k + RPMs).

[andyvclifford]

Thank you!   I have never had any luck with my timing light - i fear I'm doing it improperly.  The one i have has an "induction clamp" which I assume i put around the plug wire where it rests kind of loose.  I know this is a newbie question but if I am missing something with it please let me know!

[Deltarider]

Using a timinglight there is polarity to observe. On the pick-up clamp it probably says THIS SIDE TOWARDS PLUG. Well, with our bikes this is only true for one of the two as respectively 1& 4 and 2&3 form a circuit. So if the strobe doesn't flash or intermittently, flip the clamp and you'll find it works fine. And close the clamp well.
Furthermore the HTleads should be clean (no grease, no dirt) or the timinglight will malfunction. If needed you can use a little alcohol to clean.
BTW, be careful with that clamp. It is fragile. On many of them you will read DO NOT DROP. Not only that, when dangling avoid it shocks.

[WV_750]

Yes, you just clamp to the plug wire as far from the coil end as you can. For advance, it doesn't matter which plug wire you clamp to. For timing, you can just clip to #4 to set the 1 and 4 cylinders and clip to #3 for 2 and 3 cylinders. You can do this because these bikes use a wasted spark system where the plugs ignite on both the compression and exhaust strokes.

EDIT: I forgot about the polarity of the induction clamp, thanks Delta.

[Deltarider]

Yes, you just clamp to the plug wire as far from the coil end as you can. For advance, it doesn't matter which plug wire you clamp to. For timing, you can just clip to #4 to set the 1 and 4 cylinders and clip to #3 for 2 and 3 cylinders. You can do this because these bikes use a wasted spark system where the plugs ignite on both the compression and exhaust strokes.

But flip the clamp when the strobe doesn't flash or irregular. For some reason (that I don't know) some strobe lights only work well when connected to an external 12V battery. A mechanic back then showed me this phenomenon, but couldn't explain why this was.

[DaveBarbier]

Next pictures you take of the plugs, rotate them 180°. The white insulator is what you want to look at for color. The ground electrode (L shaped tab) is in the way.

And if you want to get better pictures, shoot them on a black or dark gray surface. The white blows out and darkens the medium tones and shadows.

[andyvclifford]

Thank you for the tip (pun intended?) I will do that for my next idle chop.  I have not gotten to do the advance timing yet today but i did try to adjust the air screws for the idle adjustment based on the spark plug reading we got earlier today.  I turned in #1 to richen the mixture, and turned out #3.  Got the popping down to a smaller amount on idle and also on deceleration.

Current state - Pops on deceleration between 2k-3k rpms and not before or after.  Extremely blubbery / wobbly when just opening up the throttle, very smooth after that - almost like its getting too much air?

[calj737]

As Dave said, hard to tell from the pictures, but my inclination is all show lean at idle. At idle, our bikes tend to be on the rich side, and as such should show some color on the plugs. Its not until you get out and wind her up that the motor will lean out and fire-clean the plugs up. But a 180 angle would confirm this suspicion.

[andyvclifford]

Thank you!  That is my suspicion as well.  Would the throttle response at very low RPM also indicate a lean condition across the board?  It basically blubbers and almost dies until i get it to about 1500-2000 and then its smooth after that. 

Besides turning the air screws back in, what else can i do to remedy this?  I still have to do the clear tube Fuel check with the float bowls and then check the advancer.  I know the carbs are clean because I have gone through them 2 times now both soaking, spraying, and mechanically cleaning with carb cleaning kit (like little drill bits basically).  I can't detect any vacuum leaks with carb cleaner spraying on the intake hoses, where else could i check for leaks?

Thanks!
Andy

[calj737]

ITs all about Air/Fuel ration in my book. Clear Tube them after a plug chop. Or before. If your idle chop shows lean, then your fuel level is "questionable". But its not a guarantee that its "wrong" just suspect. There could still be numerous contributors to getting it right, but you must verify foundation stuff before proceeding as fuel level is a base requirement for combustion.

[andyvclifford]

I am trying To wrack my brain about what has changed from when it ran fine and the only new thing is the petcock. Is it possible this could be part of the problem ?  It is a brand new petcock but not the OEM one. I could try throwing the old one back on to see ?  Am I just reaching here?

Also I will defintiey do a clear tube test tomorrow. What is the best way to do this for my scenario. Should I set up all 4 together to compare ?

[flybox1]

do it on the bench, and all four at the same time.
Idle plugs should look like this....tan at the tip, white down inside


these are ok, but closer to being on the rich side.  they are closer to black than brownaround the edge of the insulator, and buildup on the end of the threads.

[flybox1]

In your picture above your idle plug chop, they are all lean. YYes, you will have poor throttle response. 
Turn all mixture screws RICHER by 1/2 turn and retest AFTER you verify the correct bowl fuel level.

[RevDoc]

Andy, you mentioned that you changed the carb boot & airbox clamps and sprayed the boots with carb cleaner to check for leaks but, did you check for leaks at the inlet runners on the head. If those o-rings are originals they are probably like hard plastic and leaking. Discovered that on a bike that I was trying to sort out for a ratty idle and acceleration. The o-rings are the same size as the ones on the tappet adjuster caps...which may be ready for replacement as well!

[andyvclifford]

Stupid question / have been searching forums to no avail for a consensus - where exactly should the fuel be on the 75 carbs when doing the clear tube test?  3-4 mm below gasket?

Thanks

[calj737]

Yes.