Topic: Poor Idle, Hesitating acceleration, and Pipe backfire oh my... K7 CB750

[beerme351]

Hello Everyone,

Fairly new member here, been lurking pretty heavily over the past year.  This community is amazing and incredibly helpful to so many, thought I'd share my situation, that I feel is now resolved....at least I hope so

Here's the story.  I purchased an abandoned "barn find" k7 CB750, about a year and a half ago.  She was in rough shape, sat outside in Colorado for about 21 years +(fortunately as the humidity is very low in CO as rust was minimal).  Only had 17k original miles, but did a full rebuild on her over the winter, including a engine rebuild from Hondaman.  Such a fun project, I have learned a ton and am enjoying so much of these bikes.

Since start up and break-in I have had periods where it seemed everything was going great and riding well, to periods of sudden change with idling and low rpm acceleration hesitating. 

The cycle I would get into, referencing shop manual would be to pull carbs, thoroughly clean, check all seals, slow jet passage ways, check timing, set points, and reinstall, verify vacuum synch, and everything would run great for about a week or so...then go downhill quickly.  Idling off choke was a problem, pretty ratty loud idle, occasional pipe backfire. 

On this most recent teardown, I noticed for the first time that #1 spark plug was black while the three other cylinder spark plugs looked good.  Here are the things I've learned and tried out that stand out from many threads here on the forum.

- For the 1977 CB750 the spec from Honda on float bowl height is 12.5mm..very much different from all the other bikes.  I originally had this set to low (around 14.5 mm) and the slo-jet wasn't getting enough fuel, affecting the idle circuit.  On the last rebuild, I raised to just over 12 mm to help with the stiff new valves from the carb rebuild kit.  Chatted with Hondaman about this move and it potentially leaning the bike out further.

- Suspected vacuum leak.  Original carb holder/insulator boots were stiff.  I tried the carb cleaner spray trick to see if there was a leak, but never got any response...I believe it was a very small leak and changed as the engine would get hot.  I think what was going on was when I'd finish a carb clean and reinstall, the engine and boots were cold and would seal up initially.  After a week or so of riding around town, things would loosen up from the engine heat and introduce a small vacuum leak.    Really wanted a new set from Honda, but I can't afford the inflated prices people are selling on eBay.  All other stores online were out of stock on the aftermarket replacement boots, until recently.  Purchased a set from partsnmore and arrived labeled (2) #1 and (2) #2 boots.  Had some excess rubber flaps from the molding process, trimmed and smoothed the best I could inside the rubber "flute" or throat chamber.  New clamps also installed.  ...So this is what new flexible rubber is like 

- The spec from Honda on the idle mix is 1.5 turns out, I'm currently sitting at 1.25 on all four cylinders...because of my altitude.  I might end up richening a bit with the increased bowl float height to 12 mm, but will ride for a bit and see how she rides.  (there's the very slightest lean pop on decelerations currently...probably a little lean, but will monitor)

After making the above adjustments, the bike started up great, but didn't have perfect throttle response (some bogging down) and needed some dialing in needed from some missing on cylinder #2. 

-Reset points to .014,  Points and condensers are new from partsnmore, replaced earlier in my troubleshooting.  Timing appeared good, but while checking did notice an occasional stray spark in the advancer plate assembly.  Uh oh, thought I had a complicated electrical issue.  but came across an older advancer plate post and it turns out I had my condenser wire and points wire stacked up.  This is how I found the original wires from the previous owner, and installed as found.  (I forgot to take my own pictures and stole these from a previous post, as this was exactly my situation)



As left with proper condenser wire and coil wire orientation



Started up and no observed stray spark, checked timing...which had changed quite a bit.  I had been checking and adjusting timing plenty on every carb clean and adjustment, but with the stronger spark, the timing moved (understandably) quite a bit.  Verified vacuum balanced on all cylinders.

Took her out for a few rides this weekend and smiles the whole time, I can't 100% say as time will tell, but sure feel like I got her dialed.  I've never had this throttle response and no issues at lower rpms.  Such a joy to ride, can't wait for more adventure.  I only hope these holders, hold up...pun intended.  Really wish Honda would still make these.

Cheers,

[Scootch]

Would you explain more regarding the points and condenser wiring and what symptoms created by stacking.

[PeWe]

Clogged air vent hoses to carbs have been up before as a reason for bad behavior.

[beerme351]

Would you explain more regarding the points and condenser wiring and what symptoms created by stacking.

You bet, the symptom of low rpm hesitation persisted after addressing vacuum issue (believe contributing to the exhaust pipe back fire) with new carb holders, and addressing the idling issue without choke (believe the float bowl height). 

I verified with heat on each cylinder #2 wasn't always firing.  I believe this was due to a weak spark from stray sparking occurring from the condenser and coil wires stacked.  The stray spark was seen at the points spring to advancer plate, erratically while running.  I'll take my best electrical guess at what occurs with these stacked:

-the coil steps up the voltage for a strong spark, when the points break.  My understanding is the condenser is to store some potential energy after a cycle for the next cylinder...to reduce points burning out.

-I believe when these two wires stacked, the Full voltage discharge from coil would go first to the condenser and next an unknown (lower) voltage would then move to the points.  I think by placing order of coil wire, insulator washer, points, insulator washer, point spring, condenser wire...this allows the full potential to go to the points and when broken the remaining current can then be "absorbed" and stored in the condenser.

[bryanj]

Does that bike have the pd style carbs like the F2? What you were describing is typical of a pilot jet blocking on those and as they are pressed in not screwed in they are difficult to clean

[HondaMan]

The condensor wire arrangement on the 2-3 will work OK mostly, but don't be surprised if it has a hissy-fit in wet weather: the loops that form the wire's strain relief are mounted very close to the gronded frame of those 2-3 points. This was an occasional issue in the Midwest when I lived there: on high-humidity rainy days, this could cause arcing to the points frame.

Normally the coil wire's lug and the condensor wire's lug are mounted together, flat side to flat side, and come (when new) mounted to the nut-side of the little bolts. But, so long as they are mounted to the bolt and the insulators are in place, it won't matter much in running.

Keep an eye on those plugs' colors, they will tell you what's going on. In a K7 with PD carbs, dark plugs are a rarity if all is well. 

[rotortiller]

One issue with the connections at the points is that the terminal on the wires has an up and down side for critical stinkers. If put on inverted the crimp can almost touch the points body. I have stacked them over the years but the crimp faces outboard away from the points body. Both the methods shown in my pictures work for stacking, what does not work is a missing insulation bushing, terminals installed under the insulation washer next to the points body or wire connections touching or almost touching ground.

[rotortiller]

Second stacking method.

[beerme351]

One issue with the connections at the points is that the terminal on the wires has an up and down side for critical stinkers. If put on inverted the crimp can almost touch the points body. I have stacked them over the years but the crimp faces outboard away from the points body. Both the methods shown in my pictures work for stacking, what does not work is a missing insulation bushing, terminals installed under the insulation washer next to the points body or wire connections touching or almost touching ground.

This could have been my situation.  Stray spark definitely went away when I changed the arrangement, but hearing your feedback and Hondaman's I suspect I could've had one the leads too close to the plate. 

[HondaMan]

Second stacking method.

This method is/was Honda's OEM setup. When new OEM points plates are found, they are also arranged that on those.

[beerme351]

New development.  Rode the bike few times this week, some hesitation while cold.  Once warm, ran great.  ...but now have a hanging idle situation, when warm.   And, no luck starting today, with or without choke.  I can feel #1 getting warm trying to start but, 2-4 are ice cold.  Points have spark, appears to be fuel delivery related.

[Maraakate]

I had similar problems where idle got funny and unreliable once warmed up, like a good full 10-20 minutes of riding.  It ended up being the timing on the points and plug caps.  Check the timing with a timing light.  Mine was way too advanced.  One of the caps was arcing once it got hot, and I never noticed it before until I saw it on a late night test ride when parked.  Once I replaced them I found them one of them was aftermarket cap that didn't read reliably on a tester and the other 3 were 10k instead of 5k caps.  One of the wires coming off the coil into the bullet connector was slightly corroded and had a small nest of bugs in it so I trimmed this back and put new ones on.  Make sure you set the gap properly on the points.  And re-check after you verify timing as it may move.

[Stev-o]

   And, no luck starting today, with or without choke.

These old bikes can be picky on how they are started.  My K4 [and 550F] like full choke and no throttle until engine fires, then some throttle and reduce choke as necessary.  Sounds as if you have a minor carb issue.

[beerme351]

I had similar problems where idle got funny and unreliable once warmed up, like a good full 10-20 minutes of riding.  It ended up being the timing on the points and plug caps.  Check the timing with a timing light.  Mine was way too advanced.  One of the caps was arcing once it got hot, and I never noticed it before until I saw it on a late night test ride when parked.  Once I replaced them I found them one of them was aftermarket cap that didn't read reliably on a tester and the other 3 were 10k instead of 5k caps.  One of the wires coming off the coil into the bullet connector was slightly corroded and had a small nest of bugs in it so I trimmed this back and put new ones on.  Make sure you set the gap properly on the points.  And re-check after you verify timing as it may move.

Thank you, these are new caps but didn't check the readings yet.  Will do that.   I'm confident on the points gap, have them set to .014", and timing is looking good on 1/4 and 2/3.

[beerme351]

   And, no luck starting today, with or without choke.

These old bikes can be picky on how they are started.  My K4 [and 550F] like full choke and no throttle until engine fires, then some throttle and reduce choke as necessary.  Sounds as if you have a minor carb issue.

I might.  This is current path I'm chasing.  Carbs are clean, and no vacuum leaks.  I did pull air box to see if I can see vapor/fuel getting to all four cylinders.  They all are getting fuel.  As a test, hen I block all four cylinders with choke on...it started up right away.  During my rebuild I did purchase a uni air filter (couldn't buy paper or find a k&n online).  I'm currently thinking this is too free flowing and causing a lean condition, regardless of choke, idle/air mix, etc.  I'm going to try and track down a paper or k&n to test this out...but all sources I'm aware of are out of stock

[newday777]

   And, no luck starting today, with or without choke.

These old bikes can be picky on how they are started.  My K4 [and 550F] like full choke and no throttle until engine fires, then some throttle and reduce choke as necessary.  Sounds as if you have a minor carb issue.

I might.  This is current path I'm chasing.  Carbs are clean, and no vacuum leaks.  I did pull air box to see if I can see vapor/fuel getting to all four cylinders.  They all are getting fuel.  As a test, hen I block all four cylinders with choke on...it started up right away.  During my rebuild I did purchase a uni air filter (couldn't buy paper or find a k&n online).  I'm currently thinking this is too free flowing and causing a lean condition, regardless of choke, idle/air mix, etc.  I'm going to try and track down a paper or k&n to test this out...but all sources I'm aware of are out of stock

I don't think the filter is the problem. If you had to block the intakes to get it to fire up, that says the choke panels aren't working properly(your K7 has PD carbs doesn't it?). Did you look to see if the choke cable is pulling the choke panels down in place? If the cable is broken at the mount, plastic threads break, the cable won't operate the choke when pulled.

[beerme351]

Yes PD carbs...ugh.  Brand new choke cable, choke operations operates smoothly.  This was with the air box off of the bike so I can see the choke panels moving their full range in the carb intake.

Clearly not a normal situation, I only wanted to test if changing the amount of air to the carbs would allow the bike to start and it did start on the first try, when blocking the intake of the carbs. 

With the Uni filter, I'm curious if my situation is similar to problems people see when running pods, not enough intake resistance for the pressure drop needed at low rpms.  I read through the pod sticky at the top of this forum and the symptoms sounds very close to what I'm experiencing.  Only wish I could get my hands on a stock paper air filter...

[bryanj]

You say carbs are clean but i know from experience that the pd push in pilot jets block easily and quickly, i once had to clean a set 3 times in 2 weeks because of bad starting, i believe tou cant push anything through them as there is a cast in "spider" at the top to help atomisation

[beerme351]

You say carbs are clean but i know from experience that the pd push in pilot jets block easily and quickly, i once had to clean a set 3 times in 2 weeks because of bad starting, i believe tou cant push anything through them as there is a cast in "spider" at the top to help atomisation

Agreed, yep these were pulled.  Slo jets were pulled and verified clear both visually and cleared with a High E guitar string. (it does actually fit well through the slo jet...just barely) Really wish this is what the issue was.

[HondaMan]

Yes PD carbs...ugh.  Brand new choke cable, choke operations operates smoothly.  This was with the air box off of the bike so I can see the choke panels moving their full range in the carb intake.

Clearly not a normal situation, I only wanted to test if changing the amount of air to the carbs would allow the bike to start and it did start on the first try, when blocking the intake of the carbs. 

With the Uni filter, I'm curious if my situation is similar to problems people see when running pods, not enough intake resistance for the pressure drop needed at low rpms.  I read through the pod sticky at the top of this forum and the symptoms sounds very close to what I'm experiencing.  Only wish I could get my hands on a stock paper air filter...

The air filter: the Unifoam types are highly restrictive as compared to the paper or K&N types, especially when the foam is oiled up. I used to oil them, then immediately wash them out again with soap and water, let dry (like hours and hours) and then installed them. This worked better than out-of-the-box. The CB500 twin bikes will not even start when the engine is hot, if they have foam filters.

So, the foam won't make it run lean, but rich, if they do anything out of the ordinary.

What CAN cause some issues here is the float valves, if they are the newer ones we get these days that have too-strong springs. This causes the fuel level to be slightly low. In the PD carbs I set the floats slightly low because of this (if yours are 14mm, I'd set at 13mm instead, or if 12mm, set them at 11mm instead). This is due to the slower response of the float valves with the stiffer springs. The PD carbs seldom weep fuel out the bowl seams from too-deep settings (unlike the K0/1 carbs), so that shoudln't cause any troubles. The deeper fuel level makes it run a slight bit richer across the RPM range, and mostly affects the low-speed circuit. Part of this fuel-height problem is related to having 10% ethanol in the fuel: it makes the floats ride a little higher. Add to that the stiffer springs in the new float valves, and it makes for lean low end mixing.

[newday777]

Only wish I could get my hands on a stock paper air filter...

South Sound Honda appears to have the stock paper filter.(shows green like they do when add to cart clicked and computer checks availability)
Contact them today when they open.

https://www.southsoundhonda.com/--xpartsstream#/Honda_Powersports/CB750KA_(77)_750_FOUR_K%2c_JPN%2c_VIN%23_CB750-2700009_TO_CB750-2729534/AIR_CLEANER/488d4742-027b-4b51-a833-a0b6f2ea4cd6/7bd13be5-c135-4f60-b6b1-03e7ef941ebd/y

[newday777]

[quote author=HondaMan

What CAN cause some issues here is the float valves, if they are the newer ones we get these days that have too-strong springs. This causes the fuel level to be slightly low. In the PD carbs I set the floats slightly low because of this (if yours are 14mm, I'd set at 13mm instead, or if 12mm, set them at 11mm instead). This is due to the slower response of the float valves with the stiffer springs. The PD carbs seldom weep fuel out the bowl seams from too-deep settings (unlike the K0/1 carbs), so that shoudln't cause any troubles. The deeper fuel level makes it run a slight bit richer across the RPM range, and mostly affects the low-speed circuit. Part of this fuel-height problem is related to having 10% ethanol in the fuel: it makes the floats ride a little higher. Add to that the stiffer springs in the new float valves, and it makes for lean low end mixing.
[/quote]

Mark
Is it just the aftermarket float valves you are saying have stiffer springs or the Honda float valves too??

[HondaMan]

[quote author=HondaMan

What CAN cause some issues here is the float valves, if they are the newer ones we get these days that have too-strong springs. This causes the fuel level to be slightly low. In the PD carbs I set the floats slightly low because of this (if yours are 14mm, I'd set at 13mm instead, or if 12mm, set them at 11mm instead). This is due to the slower response of the float valves with the stiffer springs. The PD carbs seldom weep fuel out the bowl seams from too-deep settings (unlike the K0/1 carbs), so that shoudln't cause any troubles. The deeper fuel level makes it run a slight bit richer across the RPM range, and mostly affects the low-speed circuit. Part of this fuel-height problem is related to having 10% ethanol in the fuel: it makes the floats ride a little higher. Add to that the stiffer springs in the new float valves, and it makes for lean low end mixing.

Mark
Is it just the aftermarket float valves you are saying have stiffer springs or the Honda float valves too??
[/quote]

I haven't seen the Honda-supplied float valves in the last 4 years, but when I did, their springs were almost as soft as the OEM ones from the 1970s. The Honda ones I last used were in a CB350F and aCB400F carb set where the owners got lucky and found some. All the others, especially those knighted as Keyster, have the stiff springs. The ones I last got from 4into1.com were stiff, but not named, for the K0-K6 type carbs.

[rotortiller]

If I were concerned with the float level spring variance, which I am not, I would run a piece of clear tubing off the float bowl drain-nipple and open the drain screw for viewing/comparing the actual level both cases. The only thing I can see stiffer springs doing is change float bounce a bit while driving especially with rubber seating minimal effort. The softer spring will be a better damper for fuel level and mechanism, like a suspension shock for more ride consistency. When I set my floats they are somewhat sideways with minimal spring compression. It takes very little force to seal and you may see no spring compression in both valve cases. All this seems to me to be theoretoric lol. I have used up to 10% alcohol and the good stuff, but observed no performance difference at all with my various carb types. 2xCB750K7, 3 x H2 triples, 1xH1 triple, 1xYZ125, 1xBeta300rr, etc.

[Airborne 82nd]

When choke is full on is your fast idle cam working as it should? You should see a stream of fuel from the accelerator pump and the cam should hold the rpms up with out you using the throttle.