Topic: Weak spark advance springs & worn Needle Jets - 74 CB750

[CR125Honda]

The springs on my spark advance are a bit weak. When checking the timing with a light, the timing is spot on a higher rpm but doesn't return all the way to "F" at low rpm. The advance moves freely by hand stop to stop, no binding,etc.

Thought about cutting off one coil on the spring (bending that loop vertical, etc.) to tighten it up but haven't done that yet.

As new springs are "unobtainable", what is everyone else doing?

Regards, Ken

[andrewk]

I cut the springs and bend the loop, just like you're talking about.  That's what the HondaMan book recommends, too.

[BuffaloBill]

I had the same issue on my 550F and first tried cutting a coil off and bending a new hook.  It gave plenty of preload but I wasn't getting full advance until after 3K rpm.

At an O,Reilly auto parts store I found a distributor weight & spring kit for a small block chevy (think it was Mr. Gasket?).  There were three pairs of springs in the kit; one pair was pretty close in length, outside dia, and wire dia.  I'm using that pair on my 550F and am happy with how it performs. 

Bill

[CR125Honda]

Excellent..   Just the type of feedback I was looking for.

Think I'll try the spring cut first and see when I get full advance.

Didn't know about the small block distributor springs. Good back up plan.

I've owned this bike for 20 years or so. It's always run pretty good but never been "sewing machine smooth" on the bottom end (idle to about 4000 rpm) like previous ones owned back in the 70's & early 80's. Pretty sure I've corrected a running rich issue at lower RPM. From another post I found, I found the old needles were worn & grooved about 1/4" down from the e-clip. Bought a new set of OEM Honda needles and needle jets. Much less slop or "wiggle" in the new set.  Knowing the timing is correct will help eliminate spark issues.
Bound and determined to get this running like I know it should..

Thanks again, Ken

[HondaMan]

Use new clamps (or wider ones) on the carb boots to the head, too. They leak now, from being hard. This adds much roughness, and dark sparkplugs, and  other such issues. You can also replace them: they cost about $80 per set, new from Honda.

You can also cut one spring at a time if you wish, which will make it like automotive distributors where the early advance is faster, the later slower.

[Whaleman]

A couple of years ago I bought new rubber carb connectors and at the time Carpy sold a set of 8 very high quality connector clamps. These were way stronger than stock. This combination made an amazing difference in the way the bike started and ran. Dan

[Bootlegger56]

I wish I had me one of those spring end hook benders.  Re-bending the end of these little springs is a PITA.  Or maybe one of our spring experts will offer a simple solution......

[70CB750]

I shortened up springs, they were in the Dolly Parton stage - and now it is really snappy to go back to idle.  I like the feel of it.

[CR125Honda]

Use new clamps (or wider ones) on the carb boots to the head, too. They leak now, from being hard. This adds much roughness, and dark sparkplugs, and  other such issues. You can also replace them: they cost about $80 per set, new from Honda.

You can also cut one spring at a time if you wish, which will make it like automotive distributors where the early advance is faster, the later slower.

Thanks Hondaman & others,

I took care of the boot issue earlier this summer. When I removed the old ones, I did find evidence of a small leak (a little staining) on #3 boot.  All boots were hard but still pliable with a squeeze, no cracks, etc. Weird thing was, when I held the boots in hand and mounted one of the original 1974 clamps, I could pinch the clamp by hand and make the ends meet.. All those clamps "looked" fine (tangs not bent over from over tightening, etc.).

Replaced everything with a new set of Honda OEM intake and airbox boots. Also purchased a set of the new style Honda clamps (seem to recall the ones used on the VF750).



Also took any ignition issues out of the equation earlier this year. I thought I might have a weak spark at low rpm. When testing the coils, I couldn't get a consistent OHM check on one or two of the secondary leads so I bit the bullet and went the Dyna route (coils, CDI, & wires). Would have liked to have kept it stock but there wasn't much difference in price of a new Honda coil pack vs. the Dyna system.

All of theses repairs are an attempt to correct a rich running engine (from idle to about 4000 rpm, runs clean above that). It's been this way for a while now and I haven't been able to pinpoint the issue.

Research on this site and others recently lead me to checking my OEM needles and needle jets.
This also made good sense to me. I've tried many things but I've never been able to take the "Blurble or Hesistation" out of this bike from idle to 3500 - 4000 rpm. Never really needed choke to start and the issue became more pronounced as the bike warmed up.

Never made sense to me that the main or pilot jets could "wear out" just by gas passing through them. All my jets are original OEM's and look to be in good shape.

The needle and needle jet do rub against each other, that they could wear made sense.

What I'm assuming now is that an excessive about of gas may have been coming through the needle jet when the needle is lowered in the hole (i.e. idle to 1/4 "ish" throttle). Gas flow through the needle jet at low throttle settings should be minimal if any. The side of the needle is flat in that range and the taper doesn't come into play until the throttle is opened more.

I removed all the needles from the slides and the needle jets from the carbs. Then performed a "wiggle" test of those two pieces in the area that looked shiny / grooved.

Seemed to me to be a fairly loose fit (some worse that others) so I bought 4 new Honda OEM needles and needle jet sets. I'm  pretty sure this gamble is going to pay off. According to my "wiggle test", the new needle & jets are a much tighter fit.

Between this latest fix and the spark advance correction, I'm hoping to have this issue resolved.

Regards, Ken

[BuffaloBill]

Some spring manufactureres advertise their custom spring capabilities.  It'd be so handy if someone had the specs on the oem springs or had recommended improved specs such that we could order up a batch.  I wonder what costs and quantities would be involved?  Anyone ever dealt with these matters?

[HondaMan]

Use new clamps (or wider ones) on the carb boots to the head, too. They leak now, from being hard. This adds much roughness, and dark sparkplugs, and  other such issues. You can also replace them: they cost about $80 per set, new from Honda.

You can also cut one spring at a time if you wish, which will make it like automotive distributors where the early advance is faster, the later slower.

Thanks Hondaman & others,

I took care of the boot issue earlier this summer. When I removed the old ones, I did find evidence of a small leak (a little staining) on #3 boot.  All boots were hard but still pliable with a squeeze, no cracks, etc. Weird thing was, when I held the boots in hand and mounted one of the original 1974 clamps, I could pinch the clamp by hand and make the ends meet.. All those clamps "looked" fine (tangs not bent over from over tightening, etc.).

Replaced everything with a new set of Honda OEM intake and airbox boots. Also purchased a set of the new style Honda clamps (seem to recall the ones used on the VF750).



Also took any ignition issues out of the equation earlier this year. I thought I might have a weak spark at low rpm. When testing the coils, I couldn't get a consistent OHM check on one or two of the secondary leads so I bit the bullet and went the Dyna route (coils, CDI, & wires). Would have liked to have kept it stock but there wasn't much difference in price of a new Honda coil pack vs. the Dyna system.

All of theses repairs are an attempt to correct a rich running engine (from idle to about 4000 rpm, runs clean above that). It's been this way for a while now and I haven't been able to pinpoint the issue.

Research on this site and others recently lead me to checking my OEM needles and needle jets.
This also made good sense to me. I've tried many things but I've never been able to take the "Blurble or Hesistation" out of this bike from idle to 3500 - 4000 rpm. Never really needed choke to start and the issue became more pronounced as the bike warmed up.

Never made sense to me that the main or pilot jets could "wear out" just by gas passing through them. All my jets are original OEM's and look to be in good shape.

The needle and needle jet do rub against each other, that they could wear made sense.

What I'm assuming now is that an excessive about of gas may have been coming through the needle jet when the needle is lowered in the hole (i.e. idle to 1/4 "ish" throttle). Gas flow through the needle jet at low throttle settings should be minimal if any. The side of the needle is flat in that range and the taper doesn't come into play until the throttle is opened more.

I removed all the needles from the slides and the needle jets from the carbs. Then performed a "wiggle" test of those two pieces in the area that looked shiny / grooved.

Seemed to me to be a fairly loose fit (some worse that others) so I bought 4 new Honda OEM needles and needle jet sets. I'm  pretty sure this gamble is going to pay off. According to my "wiggle test", the new needle & jets are a much tighter fit.

Between this latest fix and the spark advance correction, I'm hoping to have this issue resolved.

Regards, Ken

The wear site in those needles is in the jets at the floor of the throat. Over many years (40+ and 140k miles) on my K2, the front side of these needle jets wore enough to be barely visibly oval (egg-shaped, really), which caused a slight richness in the 1800 RPM range (just above the airscrew adjustment range). I pulled out the needle jets in 2002 and turned them 180 degrees, problem solved. Later I got a new set of needles: they measure identically to the .0001" side-by-side, all the way down. I changed them to see if it made any difference: none. I then turned the needle jets back around in the summer of 2005 in a test: rich again. Turned them back 180 degrees, put the old needles back in, happy bike again!

To date, my own 750 is the only one I've been able to track with this many miles on it: the others seldom have even 1/3 this many miles on them when I get to see them up close. I clean a lot of carbs: the needles are occasionally bent and/or the needle jets mashed from someone attempting to remove them forcibly, but to date I have never seen another set of egg-shaped needle jets like mine.

[CR125Honda]

Use new clamps (or wider ones) on the carb boots to the head, too. They leak now, from being hard. This adds much roughness, and dark sparkplugs, and  other such issues. You can also replace them: they cost about $80 per set, new from Honda.

You can also cut one spring at a time if you wish, which will make it like automotive distributors where the early advance is faster, the later slower.

Thanks Hondaman & others,

I took care of the boot issue earlier this *******************

Between this latest fix and the spark advance correction, I'm hoping to have this issue resolved.

Regards, Ken

The wear site in those needles is in the jets at the floor of the throat. Over many years (40+ and 140k miles) on my K2, the front side of these needle jets wore enough to be barely visibly oval (egg-shaped, really), which caused a slight richness in the 1800 RPM range (just above the airscrew adjustment range). I pulled out the needle jets in 2002 and turned them 180 degrees, problem solved. Later I got a new set of needles: they measure identically to the .0001" side-by-side, all the way down. I changed them to see if it made any difference: none. I then turned the needle jets back around in the summer of 2005 in a test: rich again. Turned them back 180 degrees, put the old needles back in, happy bike again!

To date, my own 750 is the only one I've been able to track with this many miles on it: the others seldom have even 1/3 this many miles on them when I get to see them up close. I clean a lot of carbs: the needles are occasionally bent and/or the needle jets mashed from someone attempting to remove them forcibly, but to date I have never seen another set of egg-shaped needle jets like mine.

Thanks again Hondaman,

I've already installed the new needles and don't want to dis-assemble to mic and compare. Also don't have the tools (or eyes) to detect or measure any oval or enlargement in the needle jet.
The "wiggle test" and visual inspection of the needles between new and old seemed significant enough for me to believe mine were worn. The wear on the needle was about 1/4" below the E-clip and continued below that for another 1/4" or so.

Maybe the needles on the K2 and K4 were made at different plants or mine were made on Monday or?

I wont be able to get back to the repair until this weekend.
Will also try to take some close up pics of the wear / grooving on the old needles. Hope the marco mode on my camera is up to the task.

To test the needle & jet theory, I'll make this a two step repair.
Will run it after the carb needle/needle jet repair and then again after the spark advance spring repair. Will post an update when able.

Regards, Ken

[CR125Honda]

Use new clamps ***********

Thanks Hondaman & others,

Seemed to me*******

The wear site in those needles is in the jets at the floor of the throat. Over many years (40+ and 140k miles) on my K2, the front side of these needle jets wore enough to be barely visibly oval (egg-shaped, really), which caused a slight richness in the 1800 RPM range (just above the airscrew adjustment range). I pulled out the needle jets in 2002 and turned them 180 degrees, problem solved. Later I got a new set of needles: they measure identically to the .0001" side-by-side, all the way down. I changed them to see if it made any difference: none. I then turned the needle jets back around in the summer of 2005 in a test: rich again. Turned them back 180 degrees, put the old needles back in, happy bike again!

To date, my own 750 is the only one I've been able to track with this many miles on it: the others seldom have even 1/3 this many miles on them when I get to see them up close. I clean a lot of carbs: the needles are occasionally bent and/or the needle jets mashed from someone attempting to remove them forcibly, but to date I have never seen another set of egg-shaped needle jets like mine.

Hondaman,

Here are the pics of the old needles & needle jets.
My eyes aren't the best but I couldn't detect an oval shape inside the needle jet as you described.
I only did the "wiggle" test on the new needles when I got them (to compare new with old) and didn't measure with calipers before installing.
Today, I measured just above and below the grooves on the needles in the pics below.
1/1000" was the best I could do (opposed to 1/10,000).  It does show a good amount of wear.

Needle jet #1 - .096" .102"          #2 - 099" .102"          #3 - .097" .102"          #4 - .097" .102"

Largest variance being .006" and smallest being .003"

Do you think this is why my air bleed adjustments didn't make much difference?
Also, do you think the new needles & needle jets will help cure the rich running at low RPM?
Hopefully I'll be able to test it this weekend.

Thanks again, Ken

                                               

[jimbojangles]

I think you are traveling down the same path that I did over the last two years with my '73 CB750. I had the same rich issue that drove me crazy. The bike ran great other than at part throttle. First I cleaned the carbs several times. I even bought an ultra sonic cleaner. I set floats a dozen times.  Then I replaced everything ignition related (coils, wires, caps, plugs & dyna ignition). I put in a K&N air filter to try to lean things out. I even ordered a set of new needles, but not the needle jets. Nothing made a difference for me. I did so many plug chops that I ended up messing up the threads and having to helicoil two spark plug holes. I was strongly considering an entire engine rebuild because I was out of ideas.

I replaced the needle jets this summer and the bike runs and idles great now. I could have saved a ton of money and time by doing this first.

I would be willing to bet that the cylinder that corresponds with your biggest gap of "wiggle room" was probably your richest. I had one cylinder that always ran richer than the rest, but now they are all even.

The jets don't take long to press out with a dowel rod and the new ones go back in easily. I hope your able to make the repair in time to enjoy some riding before winter.

Cheers

[HondaMan]

Use new clamps ***********

Thanks Hondaman & others,

Seemed to me*******

The wear site in those needles is in the jets at the floor of the throat. Over many years (40+ and 140k miles) on my K2, the front side of these needle jets wore enough to be barely visibly oval (egg-shaped, really), which caused a slight richness in the 1800 RPM range (just above the airscrew adjustment range). I pulled out the needle jets in 2002 and turned them 180 degrees, problem solved. Later I got a new set of needles: they measure identically to the .0001" side-by-side, all the way down. I changed them to see if it made any difference: none. I then turned the needle jets back around in the summer of 2005 in a test: rich again. Turned them back 180 degrees, put the old needles back in, happy bike again!

To date, my own 750 is the only one I've been able to track with this many miles on it: the others seldom have even 1/3 this many miles on them when I get to see them up close. I clean a lot of carbs: the needles are occasionally bent and/or the needle jets mashed from someone attempting to remove them forcibly, but to date I have never seen another set of egg-shaped needle jets like mine.

Hondaman,

Here are the pics of the old needles & needle jets.
My eyes aren't the best but I couldn't detect an oval shape inside the needle jet as you described.
I only did the "wiggle" test on the new needles when I got them (to compare new with old) and didn't measure with calipers before installing.
Today, I measured just above and below the grooves on the needles in the pics below.
1/1000" was the best I could do (opposed to 1/10,000).  It does show a good amount of wear.

Needle jet #1 - .096" .102"          #2 - 099" .102"          #3 - .097" .102"          #4 - .097" .102"

Largest variance being .006" and smallest being .003"

Do you think this is why my air bleed adjustments didn't make much difference?
Also, do you think the new needles & needle jets will help cure the rich running at low RPM?
Hopefully I'll be able to test it this weekend.

Thanks again, Ken

                                               

Nice photos!
I should have you do the ones for my next book?
There's not much physical wear marking on those needles (probably more inside the needle jets), and I know from experience that engines with leaking intake valves make the needles rattle at low speed operation, on the cylinders that leak. This is a primary cause of the oval shape in the needle jet: it gets "hammered" back and forth with the loose fit of the needle in the slide. This happens most when cold, as the valves often seal better when warmed up. It's the intake tract's spitback pulses in the 1500-3000 RPM range that causes a slowdown of the air velocity in the throat, which tricks the carb into thinking the engine is running slower than it really is: the idle mixture is real rich at 12:1 on these carbs, so the leaking cylinder causes a rich-run condition on just the one. It goes away at highway speeds, unless the intake valve is totally hacked up.

Now both of you guys have be thinking about a local rider with a K3 that is having a similar issue. I remember Honda's rush and loss of quality during the K3/4 years, from the huge demand for the bike. There were quite a few things that slipped out poorly done in those days, like cams, engine cases, and clutches that were not right. I wonder: could substandard jets and needles have been a part of that 'rush', too?

[Want2Ride]

On the weak ignition advance spring topic.  For months I have worked on my rescued '72 CB750, which never had a good idle.  Cleaning and rebuilding carbs and replacing the coils and points to a Pamco system, syncing the carbs repeatedly but nothing ever completely fixed the erratic idle. 

Only by using a timing light did I see what was decribed earlier, where the advance worked fine at higher revs like 2000 rpm and above, but down low the ignition timing was jumpier.  Finally after replacing the ignition rotor I noticed that it didn't alway return to the unadvanced position.  So sometimes at idle the advance was where it should be, against the inner stops and other times it was where it shouldn't be and advanced.

I tried cutting a coil off and bending the original spring but they were pretty old and hardened and didn't like return to the fully compressed / relaxed positon.  When I cut a coil it then had too much pull and the advance would not happen until 3k RPM and made the power band very non-linear.

Pete at Pamco was very helpful telling me on my '72 full advance should happen by 2500 RPM

So from a local hardware source I matched the closest SAE spring I could find (.25 OD with a .031 wire thickness) and had to cut the spring to 10 coils and bend a new hook because it was too long

On the stand the idle is perfect now and reponds wonderfully to the throttle. The other work on carbs and ignition were probably a good exercise but the 29 cent springs were really the root cause of my idle problems.  FWIW

[HondaMan]

Looking back at this a year later, I'm having a 'duh' moment...another way to fix this unevenness at least-than 1/4 throttle range is to drill out the upper 8 air holes in the emulsifier tubes to about .038" or so. This makes a bubblier fuel mix, which tends to self-balance in the uneven airflow between adjacent cylinders. I did it to my own so may years ago that I sometimes forget to tell others about it...I have it in my book, and do something similar to all the carbs I rebuild, but forgot it here.

Must be my Partzheimer's?

[Old Scrambler]

Good one Mark.............I did not drill mine, but they are slightly larger from aggressive cleaning with a 'tip-cleaner'