Topic: Hesitation on Acceleration plus Excessive Backfire on Deacceleration

[YoungBlood]

P, very helpful, and informative. I now understanding what is meant.
About the advancer mechanism:
-Should I lubricate the springs with grease or motor oil?
-If the springs are restricted must I replace the unit?
-If the bolt is warped/not straight, can I temporarily straighten it out before replacement?
-How will using a strobe light diagnose the issue? I assume it's on a thread somewhere.
About the idle air screws:
They are currently all at 1 screw out from fully seated. They were tightly seated upon installation. I too have #38's installed when rebuilt and the original Keihin part was #38.
Is it okay to turn them all in approximately 1/4 turn, or should I do them cylinder by cylinder after adjusting one, testing, and seeing if another should be tweaked?
Alan, always nice to hear from you. I won't swap the coils, I'll post a pic of that grinding. I am using the honda manual: https://moto-odejda.ru/file/CB750SOHCShopManual.pdf
I've been adjusting the contact breakers according to the manual. However, when it states "turn the crankshaft...where the position is at maximum opening and check the gap is .012 -.016", I've never known that exact precise spot.
As to the rubbing I believe the contact breaker arm is of aftermarket quality and is improperly manufactured to fit against the contact breaker plate with enough clearance from the nut on the crankshaft.
To everyone: thank you, I will do one thing at a time. That is a lesson that has been lurking behind this trouble for the past few weeks. Too many ideas, too much tinkering, but I am trying to reel myself to one thing, adjust, check, continue.
So far I have not done too much other than:
-Reset timing
-Rebuilt carbs (leaning condition, setting floats to 26mm)
-Sync'd carbs
-Removed and then reassembled can exhaust
-Swapped condensers (which temporarily solved the problem )
-Removed air filter (will reinstall, after removal the hesitation issue seemed a tad better but creeped back fast)
-Checked for vacuum leaks with carb cleaner (found none!)
-New coils, caps, plugs (D8EA)

[TwoTired]

Spark and electrical do not make the carbs deliver rich mixtures.  Soot on the plug's center electrode insulator is not delivered by electricity.

Just sayin'  If the mixture lights but does not burn completely, there was too much fuel to burn in the burn cycle.

Carry on...

[YoungBlood]

Yes, but is it not also a possible on the ignition end of things I am not getting a hot enough spark, or enough spark at the exact right moment? Thus the condition appears rich when I am dealing with an ignition issue handling the gas (whether high or low in volume)?
TwoTired, if we are still dealing with a air/fuel issue that is rich then why does changing the needle size not seem to have settled the issue? I'm open to being pulled in two different directions, but at this point all opinions are equal in my eyes.

[Alan F.]

Let TT take the lead, his knowlege and experience will lead your troubleshooting in the right direction.

Those idle air screws on your carbs, they need to be seated ever so gently at the end of their rotation when turned in all the way, then when they are turned out their turns are counted and their positions written down. Your Honda manual will have the correct baseline setting, set them there and write it down with time and date. They should all be set to the manual, all the same, and all 4 at once. These mostly affect idle. Did your vacuum sync procedure include adjusting these?

There are threads about the mechanical advancer units, I want to discourage you from disassembling your advancer. It can be checked and lightly oiled as directed in specific threads.


What do you think of that sparking TT?

That modern muffler is probably not any more or less restrictive than stock, but a picture might help us to be sure.

thanks for that link to the manual you're using, I'll download a copy to follow along, maybe I'll learn something.

Edit: removed cb650 content

[TwoTired]

If the spark is not there, how does the mixture fire to leave soot, and not leave raw gas?  Enough spark is what begins the burn.  The burn is self sustaining once it has begun.  You light a match to start fire.  You don't need to keep a match on it to keep the fire going.  Spark energy is not enough to make the bike run without fuel to ignite, not by a long shot.

If it were timing, as in coming too late to complete the burn cycle, you would see it with a timing light showing the advance not working as it should.  This is normally checked during a routine tune up every 1000-1500-3000 miles (see owners manual).

The carb has three metering systems within; the pilot, the Jet needle and needle jet, and the main.

The pilot delivers all the time at any throttle position.  The jet needle and needle jet adds fuel above 1/4 throttle and above.  The main is the dominant fuel supplier at 3/4 and above throttle positions.

Changing the needle only effects one delivery mechanism, and is most dominant in cruise mode.  How much of that do you get in city driving.

The main doesn't really get involved unless you are drag racing or doing 90 and above. WOT.

Yes they all leak a bit to contaminate the pilot delivery setting.  But, it's the pilot that supplies fuel all the time at all throttle positions, even if not particularly significant at WOT.  If the pilot circuits are all too rich, that's what you will be running on in city operation.  And I'm not yet satisfied that these parts are actually what Keihin intended.

But, if you want to focus on electrical, change the condensers out.  The newer recent offerings have not been accepted as reliable or long lasting.

Lastly, you might want to elaborate on engine behavior traits you find undesirable.  As that may give us some clues regarding the problem you are trying to solve.

Cheers,

[TwoTired]

Alan F
I've been under the assumption the bike in question is a 1975 CB750F as it says Under YoungBlood's name in post author.

[Alan F.]

I just realized that too, I'll clean my posts of cb650 content and read more than I post.
Thanks for hanging around, it's good to see you here.

Stock jetting appears to be #105 & #40, but that's with a stock exhaust, can we see a picture of the whole exhaust as it sits on your bike? It looks like the PO may have jetted down to deal with this issue some?

https://www.partzilla.com/catalog/honda/motorcycle/models-with-no-year/cb750f-a/carburetor

Scroll down to items #38 and #39

I wonder, what numbers/letters can you see cast or stamped on/in to your carb bodies? Anything visible on carb 1 or 4?

[YoungBlood]

Alan, the original Keihin parts that I pulled and swapped with aftermarket rebuild kits were all stamped with the K. The slow jet (pilot I believe from what TT says), is a #40 (I was wrong when replacing), I threw #38 in all of them. All mains were 105 with K, I swapped those with 105s as well.
Before reading the thread I spoke with a relative who has a BAS in Mechanical Engineering. I explained that the timing has been thrown out within a couple days. How 2/3 have arcing across the points even though I've cleaned it up with emery paper. How 2/3's contact breaker ears rub against the crankshaft nut at times, causing a friction spark. And how I've swapped condensors before and the problem went away. Now 1/4 does not have any arcing, new condensor. So I swapped 1/4 with 2/3, 2/3 with 1/4. The arcing traveled to 1/4, but not to 2/3. So I replaced the condensor with another old condenser. The arcing is now gone. I checked the timing, it was out of funk, and I relined it up, checking my work 3x! I moved the ear away a tad, it is not rubbing, and I put a dab of hi-temp grease on the crank shaft roller. The bike no longer backfires.
I took the bike out for a spin, it is a different bike than before. It hesitates IF I give it a strong throttle twist. It hesitates if I give it even more than 1/4 of throttle twist. However, it still kinda hesitates in 1st/2nd giving barely any throttle, and it picks up speed quicker with less use of throttle. Previously, before any of this stuff, I would give the bike a good wack on the throttle and it'd scream forward. No hesitation, and I could push the rpms up as high as I wanted. Now, that is not possible. It seems once I am beyond the rpm range of 1/2/3/4 in normal rpm ratios, the bike needs to shift. If I don't shift it, it starts hesitating. This never happened before, I could shift in higher rpm ranges. Now the bike moves through 1-3 gears quickly, but without much throttle. It's just odd to me, maybe this is how it's supposed to be?
Next, I took the bike out on the highway. Everything lines up with what you're saying TwoTired. Except in 5th gear, she accelerates slowly. I got the bike up to 90, but it seemed the bike slowly creeped it's way there, again not giving nearly as much throttle as I used to. I am also in the 1/2 throttle range at around 60mph. Upon coming home from about a 15 mile trip, I checked the timing. It is still dead on. I also rerouted the gas line because I believe that is causing some problems. It was getting caught under the slider arm on the #1 carb, thus pinching it.
I am about to do a plug chop, post pictures, and reconvene with all the feedback.

[YoungBlood]

Picture of exhaust:
Picture of 2/3 bracket point war nearly rubbing on crankshaft nut 23mm
I also forgot to say that I changed location of condenser green wire, both no are directly against yellow or blue respectively in between the two small washers on the outside of bracket point ear.
Plug chop coming up.

[Alan F.]

I don't think that was a friction spark you were seeing, I think that was the points arm shorting out to the crank nut, this was causing an intermittent no-spark condition which was causing the hesitation and was intermittently letting unburned fuel into your exhaust causing the backfire.

Eliminating that arcing made a difference, I'd bet if you put all of your original jets back in you'll get your missing performance back.

Changing the position of those green wires is a good idea too, keep everything as far from your points cover as possible.

Is that a stock exhaust from the head to just before the muffler?

[TwoTired]

If something other than the rubbing block is touching the points cam, that is shorting out the spark primary, and the secondary fires when the short stops OR the points open.  This is bad and surely causes run time issues.   I've never seen that before, and seems like the points rubbing block is very worn or just badly designed.  I'd look for TEC points, were it my bike.  Kinda normal to be sparking at he point contact.  A good condenser should minimize that.  Did you recently change the points?  Or, did the PO put those in?

Glad its running better.  However, the revvs make the point cam move to a different position.  Just wondering if your short is coming back at run time.  What does your timing light say about plug firing? Is it consistent?

Are you using an in line fuel filter?  Or the stock one in the tank?

Cheers,

[Alan F.]

Did you mention that you corrected your fuel line routing?

I took a look at Partzilla and it shows 3 lengths of fuel line and a Tee fitting.
The lengths are:
#33.  80mm or  3.15"
#34 160mm or 6.30"
#35 190mm or 7.48"

And they're run on the cylinder head side of the carbs.

[YoungBlood]

Yes Alan, all stock from after the muffler to the engine exhaust manifold. I'm going to put the Keihin parts back in, but probably keep the #37 plus the 4th from top needle jet location? I can't imagine that's what's doing it, I'll be pissed if so and even try to return the garbage.
The hesitation is still there, it's better. Around town it hesitates from 1/16 - 1/8 throttle value, about 1/8 - 1/4 of a turn she kicks up quick. But it feels like I'm giving it only an 1/8 of throttle turn and it leaps from the hesitation all the way through the gears. However, anything beyond a 1/4 turn, and the hesitation returns. If I'm in fifth gear and try to go beyond a 1/4, say a 1/3, or even all the way when moving about 70mph. The bike doesn't respond, then a couple seconds later a massive feed seems to spark at the perfect time, but obviously late... When idling around town, the exhaust smells very very strong of gas.
I'm at a lost guys, I also did the plug chop, all are marshmellow except #3 is a bit white hot. I'll post pictures tomorrow, going out to see a friend and just shrug it off for the time being. The bike is functional, but something is just not right. I'll check the timing again to see if it pulled out of sync again, I'm also going to ask for contact TEC breakers for my birthday. Better yet, maybe even the electric version? Namco/Damco? Can't remember.
Right now the idle screws are set about 1 and 1/4 out, I believe. The more I turn them in, the rougher it runs. Slower idle, but rougher running. Also, Alan, no the fuel line is not set to spec. I'll post a picture soon. 1/4 inch diameter too.

[Alan F.]

Well it's hard to argue with reliable, hesitating or not.
Perhaps after a few tanks of gas it'll improve?

What fuel are you using?

[PeWe]

Try Shell V-Power. I'm sure your bike will like it

[YoungBlood]

I filled her up with 93 octane! Top of da line. I found another thread with similar problems, person never replaced the jet needles, only the needle jets and all other parts. I did the same, jet needle was too hard to remove so I said screw it. Pulling the carbs tomorrow and replacing that part. Will respond back. Best, -T

[pjlogue]

P, very helpful, and informative. I now understanding what is meant.
About the advancer mechanism:
-Should I lubricate the springs with grease or motor oil? Oui  on the springs will help prevent them from rusting
-If the springs are restricted must I replace the unit? Not sure what you mean by restricted
-If the bolt is warped/not straight, can I temporarily straighten it out before replacement? If you are referring to the rod that the advance mechanism sits upon, yes it can be straightened and should be straight.  Usa a dial gauge to check for trueness.  It often becomes bent is the PO used it to turn the engine over backwards.
-How will using a strobe light diagnose the issue? I assume it's on a thread somewhere. You can do a static timing but can't use static timing to see where the advance is set.  A strobe will show the firing point for the advance.  These are the two close lines on the advance plate to the right of the "T" and "F" marks.
 These should show when the engine RPM gets to about 2500-3000 RPM.
About the idle air screws:
They are currently all at 1 screw out from fully seated. They were tightly seated upon installation. I too have #38's installed when rebuilt and the original Keihin part was #38.
Is it okay to turn them all in approximately 1/4 turn, or should I do them cylinder by cylinder after adjusting one, testing, and seeing if another should be tweaked?  Set all your air screws to factory speck and adjust all by the same amount at the same time.  I would use 1/8 turn increments for each adjustment, perhaps 1/16th for each.  Too much adjustment at a time and you can miss the "sweat spot" where you get a balance of good starting and hesitation on acceleration.
Alan, always nice to hear from you. I won't swap the coils, I'll post a pic of that grinding. I am using the honda manual: https://moto-odejda.ru/file/CB750SOHCShopManual.pdf
I've been adjusting the contact breakers according to the manual. However, when it states "turn the crankshaft...where the position is at maximum opening and check the gap is .012 -.016", I've never known that exact precise spot.
As to the rubbing I believe the contact breaker arm is of aftermarket quality and is improperly manufactured to fit against the contact breaker plate with enough clearance from the nut on the crankshaft.
To everyone: thank you, I will do one thing at a time. That is a lesson that has been lurking behind this trouble for the past few weeks. Too many ideas, too much tinkering, but I am trying to reel myself to one thing, adjust, check, continue.
So far I have not done too much other than:
-Reset timing
-Rebuilt carbs (leaning condition, setting floats to 26mm)
-Sync'd carbs
-Removed and then reassembled can exhaust
-Swapped condensers (which temporarily solved the problem )
-Removed air filter (will reinstall, after removal the hesitation issue seemed a tad better but creeped back fast)The stock air filter is needed for proper carb function.  Pods/less restrictive filters will have you back trying to adjust mixture.  It can be done but is fairly difficult to get the engine running right through all the RPM ranges.
-Checked for vacuum leaks with carb cleaner (found none!)
-New coils, caps, plugs (D8EA)

[Alan F.]

I had a little Honda CM400C that arrived with damaged needles, they looked like they'd been whittled on. Replaced them with a better (but worn) set from spare carbs and everything was solid. It seemed like I'd fixed the issue that the PO had parked the bike for.

Good luck.

[YoungBlood]

Thanks P, I'll be sure to pick up a strobe light soon as well as hunt down a paper OEM air filter. Here's the thread I read http://forums.sohc4.net/index.php?topic=108701.0, seems the person experienced the same problems after a rebuild, and forgot the needle jet just as I did. Twotired, do you know how I can get some TEC contact breakers? Those are out of production, right? So I'd have to pick up used ones from ebay? I'd like to replace the 2/3 contact breaker ear.
Alan, thanks for the info about the fuel line. I never would have thought it would run that way! I really want to be stubborn at the moment and replace the last piece I left out from the rebuild kit because I just want to know whether the kit was low quality and causing my problems or not. Once the last piece is in (needle jet), I'll be satisfied that if it runs like crud it may be that it's not up to Keihin spec/quality.
Also, anyone have a fair estimation of what a stock exhaust would run me for the 75' Honda CB750F? I'd really like to get rid of that blasted can.

[TwoTired]

I'd give the Honda dealer(s) a chance. Might have some NOS on the shelf.  Just tell them it's TEC or nothing.  Same for ebay.  I think the Diachi brand was reported as pretty poor choice and quality.

Also used ones can be refurbished with a point file and contact burnisher.  As long as the rubbing block is not worn down too much.  Must be metal, as other abrasive devices leave behind grit embedded into the contact surface.  The grit is an insulator, which reduces life of the contact though reduced contact area and interfering with full closure of the contact.

Post a pic of yours.  Maybe the geometry can be corrected or tweaked?

Cheers,

[YoungBlood]

So the hesitation is back, and it's getting gradually worse. Now the sweat spot on the throttle for smooth pick up after a quick bogged 1-10mph has faded. I am going to check the timing, I am hoping it is off so that my culprit must be the advancer mechanism. If so, I'll pull it off, take a look, oil or grease it?, and make sure that rod is straight.
Twotired, I'll post a picture, but I'm thinking of getting the electrical ignition version (I keep forgetting the brand). My b-day is coming up, and family wants to know what I'd like!
I am desperate and may even pull the carbs and reassemble all the Keihin parts back in. I think that sounds smart, but I would like to avoid having to re-sync the carbs.

[YoungBlood]

I’ll post another.

[TwoTired]

I'd like to see the brand name on those points.

I'd also like to see where they touched the cam and sparked.

Something you might check are the power connections to the coils and the connections to the yellow and blue wires.  The theory that if these aren't clean and tight, vibration is making them intermittent.

Do you use the bar mount engine stop switch?  That's another place power can can be interrupted or intermittent.  Could make a temporary bypass.

Might also be worth getting new condensers.  Or seek out someone with a capacitance checker to test them, if you don't go electronic first.  I've always liked the idea of electronic ignition, but find so few that are properly implemented.  So, never got them for my bikes.

Things performing worse, could be plugs becoming fouled and shunting spark or the spark getting worse from condensers deteriorating.

Post pics of the plugs.  Need to see the center electrodes.

I'd still like a more detailed description of the negative symptoms you are experiencing.  Or maybe hear it when it behaves badly.

Or bring it by the house hear in AZ! ....HA HA.

Look here to get an idea of how to read plugs:
http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

[RAFster122s]

You mentioned changing your jets to aftermarket and then talk about putting the Keihin brass parts. Can you tell us what jets and needles are in the carbs right now and who made them or where they came from?
Aftermarket brass tends to lead to numerous problems trying to tune them. Same is true for several brands of electrical points and condenser parts. The Diachi /Daichi (little propellor logo) are famous for being junk out of the box where condenser is dead or dies quickly and points impossible to properly time.

[YoungBlood]

Rafster, this is the product I installed: https://4into1.com/set-of-4-carburetor-rebuild-kits-honda-cb750f-super-sport-1975-1976/ Only I did not use one of the o ring grommets, nor the needle jet. I am not sure what brand they are. Also, I swapped the 40 Keihin out for a 37 slow jet. The kit came with 37 and 40, I'm thinking of throwing the 40 in.
Twotired, I'd gladly stop by, ha - if the bike would make it and I was on a never ending vacation. I have a video I will share through google video and attach a link. The #1 plug is already sooty. I haven't checked 2-4, but assume they are all sooty because of town traffic. Will do tomorrow morning. I'll also let you know what brand those points are.
I do use the handle bar stop engine switch. I never thought of bypassing it, that's a great idea. Also great idea to check out the power at the points and at the coils. I'll check the connections and voltage tomorrow morning.
What electrical ignition would you recommend?
Symptoms:
Runs better when cold outside (was 80 degrees today, I filled up the tank with Shell V-Power 30 minutes ago, temp is 70 - bike runs a hell of a lot better)
Hesitation beyond 1/2 crank of throttle (doesn't matter what speed I'm at/gearing)
Bike doesn't accelerate like it should beyond 70. Again, I get beyond the 1/2 throttle range, it hesitates, stumbles.
Idle smells of gas strongly (I am somewhere around 1/2 to 3/4 out with idle air screw)
Bike lurches, pauses, lurches, pauses, when moving through town speed.. again problem seems to have disappeared with new tank of gas and colder night weather

I'll just get you a video tomorrow when it's hot out of what's going on.
I do have plug chop pictures to post from yesterday after running the bike at 70 for 5+ minutes. I followed the directions, and had a lot of people staring at me  on the side of the road.  Keep in mind they were new plugs, I've put another 40 miles on those plugs since (mainly town traffic).