Topic: Hesitation on Acceleration plus Excessive Backfire on Deacceleration

[YoungBlood]

I had similar problems on my cb500k, the biggest problem was a slack in the centrifugal advance system, there was wear on the advance system, did you check if there is excessive slack in your mechanic advancer system or do you use electronic ignition with programmable advance?

The 5mm stem valves are more flexible. I was a little bit worried first.
I checked TDC without rockers or cam sprocket bolts. I noticed how the advancer unit T lined up with case mark, piston 4 at top. Lined up well with advancer at max counterclockwise position of its play. Same with 2 different crank shafts with advancers.
I was sure to bend valves with piston stop and valves lifted by cam.

I set ignition according to real TDC too which will follow advancer in correct position. Advancer at max clockwise will end up in too advanced ignition and run sh!t on low rpm and might ping if not compensated with too  rich carbs.

I still don't think this is a waste of my time. At the minimum, reducing the slack in the advancer will make it easier to adjust my timing. It may even improve the symptoms I'm experiencing.
Second, I am strongly under the impression the 300 advancer is the wrong advancer for the 75' Cb750F. A previous post shows that (apparently) all part numbers for stock advancers list part #300: http://forums.sohc4.net/index.php/topic,113338.0.html On the other hand, there were multiple advancers designed...
My hole does not appear very oval, it appears to be a perfect circle. My guess is that either the part was swapped, or, I or the PO has worn the hole. But that doesn't make very much sense if you look at the picture, it appears perfectly circular, not oval.

[PeWe]

Fixing the advancer play is one way to get a consistent setting.
Both my bikes (CB750) have advancer play.
BUT, the correct TDC is reached when advancer is in its most counterclockward position on my bikes wich seems logical for me when the play is caused turning crank clockwise.
I have used different advancers on same crank with same play so crank hole it is.

Maybe I'll do something about it next time I have it apart. Advancer pin to most counterclockwards position and fill the area to the right with something.

Maybe possible to fill with JB Weld while a rod of same size as advancer pin is in that hole. Let it cure and it will hopefully sit.
Fill the entire hole with brass, bronze or similar and drill a new hole  another thought.

This to not only to solve the irritating play, correct TDC is important too. If advancer sit too much clockward, ignition will advance.

On one of my bikes I have done an alternative case mark for advancer in most clockward position (ca: +3mm to the right). This when I had crank at TDC 1:4.
Important to remember how the advancer sat when tightened and no chance it will move.

[YoungBlood]

Thanks PeWe, I appreciate your past experience with the same problem. I look forward to seeing what the effects are with securing my advancer counter clockwise.

[seanbarney41]

Youngblood, you need to check out some of PeWe's build threads before you apply his advice to your bike...he spends lots of time and money squeezing maximum performance from highly machined and blue printed equipment.  Your stock bike does not even run right,  We need to help you fix that and all I am telling you is I seriously doubt that + or - even 4 degrees is not gonna cause you the problem you have.  How do I know?  Because I have spent hours messing with this same sloppy advancer hole and everything I did made no difference on my mildly tuned stocker.  Get some oem condenser's and double check your wires are not crossed and then start chasing unicorns.

[YoungBlood]

I need to eat my own words. Sean you’re right. I just chased that unicorn, got sheet metal and put a small piece in against the worn CW position. I used my old tinning big snips and cut a 4mm slice, and then a small tab. The advancer is now set solidly, no play, and you’re right - there’s no difference in the bike.
Saturday I have stock condensers arriving, along with the stock 2/3 points. I already have the 1/4. I’ll be installing all of them together Saturday. I also have a strobe light now, I’ll use that to dial in the timing, and check to see if the advancer is not working properly.
The bike hesitates off and on, the plugs have become sooty once again. But a significant backfire continues at deacceleration , I also get a pop at idle every three-ish seconds.
I didn’t share this earlier (shame perhaps), but a drain screw’s threads seized on the carb, half the bolt tore right off. Instead of trying to removing the seized threads I used extreme heat jb weld to seal it. It’s leaking a tad because I didn’t let the jb sit long enough. Would a very small drip leak out of that drain port cause an air leak, thus the back fire?
One of these days I’ll do a tap and dye on it. For the moment I can still do clear tube checks by swapping the leaking bowl with another.
I am also going to be able to borrow a compression gauge from someone this weekend. I’ll post the readings when that happens.

[YoungBlood]

Condensers just arrived, would it be safe to install the 2/3 points and both new condensers? My concern is that the 1/4 aftermarket contacts will be using the very valuable stock condenser. Any room for error? Also Sean, when you say that I be careful not to “cross the wires”, do you mean to run the blue yellow wires to the wrong contacts?

[TwoTired]

Would a very small drip leak out of that drain port cause an air leak, thus the back fire?

No.

[TwoTired]

Condensers just arrived, would it be safe to install the 2/3 points and both new condensers?

Yes. 

[seanbarney41]

running your engine with bad condensors ruins the points much more rapidly.  I would honestly just throw all that Daichii crap away, if thats what the 1.4 points are.  Points can sometimes be filed back to useable condition after a condensor failure.

[seanbarney41]

and yes, when a "trouble shooting frenzy" begins, it is very easy to get the blue and yellow mixed up at the points and/or the coils...even wrong spark plug wiring.

[YoungBlood]

My bike is back like a bat out of hell - hesitation wise. But the backfire still needs to be diagnosed.

[Alan F.]

My bike is back like a bat out of hell - hesitation wise. But the backfire still needs to be diagnosed.

Does that mean the hesitation is gone?


When does it backfire?

[YoungBlood]

The hesitation is completely gone. But it still backfires on deacceleration, and every few seconds at idle. The new 2/3 contact point, and two condensers made a drastic improvement getting rid of the hesitation.

[PeWe]

Backfire can be a ignition wire with bad connection. I had a few when the ignition wire glided out from the Dyna coil auto style connection. Stiff copper cable sit better.

Or one of the blue-yellow from ignition to coils
or bad coils
or bad  ignition wire with caps
 or condensor connection with its fork sit. There are isolation washers there which must sit correct vs the forks for points and capacitors.

Remove plug cap, cut wire 5mm so cap will get a fresh connection.

I had no thoughts about bad ignition back in the 80's. I was told from a friend that had a CB750 before and knew many others that  "CB750 eats ignition plates". Replace it as a complete plate easier and did not cost much more.

I used a transparent ign cover so I could see when condensors went bad when arching in the night before misfires occurred.

It was when reading this forum around 10-12 years ago I got the explanation about  Daichii quality.

I found an old box with my old ign plates, most of them Daichii except one TEC that I might have replaced just for sure.

One holiday it started to run on 2 cylinders. Open the ignition cover to see that one point did not work, the bakelite knob that glide against cam was off. We (I and a friend and lots of luggage) had to continue to ride on 2 cylinders to a nearby town with bike shop that could repair the point.
In France where english was not that common.
An english speaking guy at the gasstation where I first stopped told me how to say ignition plate in french.
Daichii it was. It was new before the tour so max 5000 km.

If I knew about TEC as a better alternative I should never had replaced it.
But the bad ones must have lasted at least 5000-8000km before misfiring.

Still sooting plugs?
- Advancer springs important, you'll see when checking with strobe lamp that full advance kick in too early with sloppy springs.

- Next can be floading carbs, floats wrong or stuck, or leaking float valves.

- If using old coils, wires etc. A good idea to replace it all. 40-50 years old coils might have bad insulation inside.  New ignition with new coils, wires, caps etc.

This is exact how my K6 started its comeback 2013-14.
The springs were so sloppy so weights could move freely rather much until springs grabbed them.

I thought my old clean VM29 carbs should have correct float settings, but not. Clear tube after help on this forum.
After this the plugs stopped to become pitch black after a  few minutes.

If advancer has a play which can cause more advanced real setting even if it perfectly matching the marks, it will not soot the plugs.
I noticed the play and had to find out how it must be set to time cam right.
So a TDC check with degree wheel  to verify the difference. Around  3-4 crank degrees if I remember correctly. Too much wrong when timing a cam.

After that was verified I set the ignition with correct TDC and affected T, F and ' '  in mind.

Correct plug is important. My bike did not run right with the Iridium plugs, like they were slightly colder. Maybe good for racing.
NGK D8EA or Denso X24ES-U work fine.

[TwoTired]

One cause of backfire on deceleration is the idle mixture being too lean.   When too lean the air fuel mixture can’t ignite in the chamber reliably. The exhaust stroke pushes that unburnt lean mixture into the exhaust, where successive cycle allow it to build up, especially in 4-1 exhaust systems with multi cylinders making contributions.  Then an ember or wasted spark event lights it off randomly.

Honda cured this problem on CV carbs by adding an enrichment valve to make the carbs juicier under high vacuum deceleration.   But, you can do a similar thing by tweaking the pilot mixture screws on your carbs.  Cheap to try, just turn them all in the same amount an eighth or quarter turn at a time.  And drive to gauge the effect.

 Cheers!

[Alan F.]

TT, so just air screws in 1/8 on all 4 and test ride/repeat?

YB bring your rain gear if you ride today, looks like you'll be dodging thunderstorms in the afternoon/evening. But after that looks like great test ride weather through Thursday. Good luck!

[YoungBlood]

Thanks everyone! I'm really looking forward to turning the air screws inward, thanks again for the great info TT. Also, thank you Pewe for the excellent info on other causes of backfire. The transparent cover is an awesome idea, cool too. How did you score one of those? Could you help elaborate on the correct positioning of the forks on the points. I currently have both the yellow/blue fork on each point conjoined directly with the green fork of the condenser. I have them mounted against one another in between both washers, not the spacer. If I understand you correctly, the condenser should sit against one of those isolation washers? The brownish/yellow discs?
J'adore les gens de France. Very cool you made it 5k on those daicchi's. The points I pulled off don't even have a brand label. Same as the condensers, my new theory is if it isn't labeled, then it's not even junk, it's trash.
I'll check the advancer with a strobe light soon. I think I'm all good after doing the clear tube method, I can send some pictures later today of when I did it a couple days back.
Bad news: last night I left work dehydrated and skipping dinner. I work for FedEx, I was not in the best mindset - physically. Went to get on the bike, lost my balance, she tipped over. I was so mad I screamed a big ****. The clutch lever broke clean in half. I made it home ~6miles, but it was the sketchiest thing I've done in my life. I ordered a replacement OEM from partzilla, probably won't be here till Wednesday. I'm embarrassed, sad, and angry at myself. Was about 90 degrees yesterday going into work, heavy traffic.
Alan, as much as I'd like to take my bike out, I can't. I'm going to stop at an old tow yard because I've seen an old Kawasaki in there. Maybe I can swap the lever for the time being. We've been needing rain, bad!
TT, is there a noticeable place I should stop if I keep turning in to get rid of the back-fire? Say 1/2 turn in is too far from 1 turn out?

[PeWe]

Good luck finding out what your bike need. All hints written here will be good to know for future issues.

The transparent cover was common in 80's. I'll bought mine ca: 1983. Smoke color like tinted visirs to helmets.
I have seen on eBay lately, not cheap.
Insane price!!
Mine costed less than stock Honda!
https://www.ebay.ca/itm/154389843495

Maybe someone can offer new for less.

[TwoTired]

Those pilot screws should be set to factory setting with a stock bike.  The muffler change makes your bike NOT stock.  I doubt that your muffler now presents the same back pressure profile as the stock one, which the stock carbs were not set to accommodate.  So, you need a new setting to compensate for the change.  As a guess, your present muffler presents less back pressure at idle than stock.  This has the effect of making factory setting deliver a bit leaner, as there is now more scavenging during exhaust cycle. So now the carbs need to deliver a bit more fuel to return engine operation to that of stock behavior.  If your pilot screws are hollow tipped and cross drilled,  any setting that yields desired engine behavior is good.  If the tips are solid, then they will be very sensitive near fully seated (and fully seated won’t allow the engine run at idle).

Do I recall correctly that you leaned the throttle valve needle during your correction attempts?  Might need to enrich that, too.  But, I’d let the plug deposits guide me regarding that adjustment.

There is also a possibility that the main jet needs alteration due to that muffler.  However, without a Dyno  or test track to do do repeated plug chop reads, all you can do is guess and hope to be lucky with those choices.   The idle screws are easy adjustments.  And as you enrich the idle your throttle twist response from idle should also improve.  But, if you take idle enrichment too far, it becomes more difficult to have the plugs self clean, especially during long periods of idle operation.

Which reminds me, a simple test to verify the idle mixture too lean is to apply partial choke to enrich the mixture, does decel backfire improve or get worse?  Of course you don’t want to ride about with choke on routinely, but it can aid in finding your best idle screw setting and pilot mixtures.

Cheers!

[Alan F.]

TT this is the final round I think, and again you're giving the best advice. I haven't fought these issues myself so I'm learning too. Thanks.
I also wanted to pat YB on the back for hanging tough and following through.

What's the name for a newb who has tools, knows how to use them, will accept and follow advice, and will report back with success or failure instead of parking his busted bike for good and pouting?

[YoungBlood]

Thank you everyone for bearing with this thread for so long. Alan, TT, PeWe, Sean, PJ you all helped get my bike back to solid performance. I can't thank you all enough, and you've all increased my understanding of my bike and my respect for this community.
TT, I'm going to keep playing around with the air screws, they're at 1/8 in from one turn out (no noticeable improvement). You're correct that I set the needle 4th from the bottom. I'll see what happens with the choke as well, and respond accordingly. I may need to get back into the carbs and replace those needles with Keihin's, not to mention inspect the needle jets.
PeWe, I'll be rereading this thread for sometime. Vast amounts of great information and technical advice. It's crazy how much a stock cover is going for!
Alan, I think you're right - this is the final round. You've been awesome with the feedback, support, and help. You kept my spirits up through all this troubleshooting. I won't forgot you all, and I hope to someday see you guys out on the road.
Final notes: by the end of summer my goal is to acquire that stock exhaust muffler. I now own the "My CB750 Book" by Hondaman aka Mark. I think I'm in good hands for the future, and if I'm ever stuck again I know you guys are here to help.

[PeWe]

It will be fine with a final conclusion about this case!
- Problem you saw and how bike behaved.
- What you changed to get it to run OK.

It can happen that new behavior is noticed after each step along the way to get it right.
This thread is very long and contains thoughts about problems that can happen and why.
Another thing what exactly helped your bike.

This forum is good at fixing the old bikes teaching all of us to become your own mechanic.

[Alan F.]

I can't help but wonder if those #40 stock jets were drilled out.

Thoughts?

[YoungBlood]

The primary problem my bike was experiencing was a hesitation when accelerating. The bike would lurch, pause, lurch, the more throttle that was given the worse the hesitation became.
The solution: swapped out the aftermarket non-labeled condensers and contact points with stock TEC. The hesitation no longer exists, bike pulls evenly and strongly.
Other problems: advancer unit had play in the Clockwise (CW) direction when positioned against the crank shaft
Solution: Found TDC using a screw driver and make shift degree wheel. Then I put a small piece of sheet metal, about 3 mm by 3mm/4x4 into the dowel hole on the the CW position because TDC was fully counterclockwise (CCW).
Other problems: rebuilt the carburetor thinking I had a fuel/air problem causing the hesitation
Solution: cleaned the carburetor, but put the damn Keihin back in or do yourself the courtesy and buy Keihin.
There is much much more in this thread about troubleshooting and possible errors with the symptoms of backfire and hesitation. My bike still has backfire but keep in mind I have an aftermarket dirt bike like muffler, incorrect fuel routing, a leaking head gasket, and possibly an air leak I cannot locate.

Update: Just used my new strobe light when the bike was at idle. F mark lines up for 1/4 2/3, but there is a bit of a jumping on the advancer wheel. Hondaman's book states this is caused by run out on the crankshaft pin/bolt that holds the advancer in place. I will borrow a dial gauge to measure it and readjust it by tapping a hammer slightly against the nut.
Also, I went to check the advancer's index marks for correct operation of the advancer. The Honda manual states to rev the bike about 2,500 rpm and check. At ~2,500 rpm the bike does not reach the first of the two index marks. It is not until 4k rpm that the bike hits the first index mark. Suggestions and thoughts? Another print out I have states the advancer should advancer around 2,500-3,000 rpm. What does this mean? Bad advancer, tighten by cutting a loop off those loose springs on the advancer? I have a 300 advancer unit. I also am uncertain the tach hour meter I was using is correct, my tachometer cable is broken (new one is in the mail).

[YoungBlood]

I can't help but wonder if those #40 stock jets were drilled out.

Thoughts?

It could be, I believe there is a way to measure the diameter. The sohc.com website has a list of specs for jet sizes and their diameter holes. Dial gauge would be needed, right? I also think this would add to a rich condition, I still need to due a plug chop and see where I am at with the new ignition parts.