Topic: My CB750K2

[newday777]

Have you tested the timing and advance with a timing light?
Have you shortened the advancer springs and cleaned and lubed the advancer?

[willbird]

Have you tested the timing and advance with a timing light?
Have you shortened the advancer springs and cleaned and lubed the advancer?

Have not Checked timing yet.

Can't see that spark advance would cause an issue with 4K rpm cruise ??

Bike ran amazing when it went to sleep in 1996.

Thinking to check spark plug cap resistance this weekend, also my new Genuine Honda float valves should be in my mailbox right now, and the new #40 Genuine Honda slow jets.

Trying to work through potential issues one by one in order to isolate what the actual issue was when it is resolved rather than throw a pile of parts all at once.

I do not yet own a compression tester so that is off the table, plus somebody well respected has said to get 100 or so miles on an engine that has set like this one did THEN check compression. Plus it makes pretty much every HP that it did when put away according to the flat out top speed dynomometer

Bill

[MauiK3]

The advancer can cause all sorts of problems

[willbird]

The advancer can cause all sorts of problems

I do not doubt that it has for some people. But I prefer to isolate a problem rather than shotgun a bunch of changes at it and then have no idea what the ACTUAL issue was to begin with.

Bill

[HondaMan]

One of the features of these SOHC engines is their 'self-healing' capability. If they were parked (unprepped) for a long time, they often get some rust on the open valves (intake and exhaust) and the rings of the piston(s) where valves were open enough to let the cylinder breathe in moist air. If this doesn't seize the bores, the rust will usually work itself off the valves and rings in 100-300 miles or so, returning the engine to a usable state. I've seen this happen dozens of times, and once to even my own 750 after I had cancer and didn't ride it for almost 6 years. When I started it back up it wouldn't idle nor reach 7000 RPM, but after 2 oil changes and 500 miles it ran nearly normally again. In other engines where someone had me 'wake them up' again I started with about 1 ounce of light (10w) oil in each bore and let it sit a couple of days, then fired it up with fresh gas and sparkplugs, and just rode it gently for 50 miles or so. Then I'd fix whatever else was needed, but those bikes are, AFAIK, still running around here today. I see one of them, a red 750F0, every now and then near the machine shop who does my machine work: the owner works in that industrial park.

[PeWe]

Missing at 4000 rpm can be a condensor.

[willbird]

Missing at 4000 rpm can be a condensor.

Last thing I did today was swap in two used condensors from a parts bike I have in my shed. No change, same behavior.

Spark advancer is working, timing looks good.

Pulled Needle Jet holders and cleaned them, did see some "stuff". test rode, no change. Removed spark plug caps, #1 was 8.4K, number 2 no matter how I try to check it is open, number 3 was 10k, number 4 was 12.3k. Swapped in NOS Honda parts. That made it idle better, was hoping it was "it"...nope.

Last thing swap in 2 condensors from a parts bike I have in the shed. No change.

Thinking I need to pull the carbs, strip, and ultrasonic clean them.

Bill

[willbird]

I don't own a points file, I think once I eat supper here I am going to run to ORielly and rectify that lacking LOL. Been up since 230AM so getting a little worn out LOL.

[willbird]

Well no good news. Sat down to chill the rest of the evening :-).

[HondaMan]

A thought: how are the fuel lines routed? There will be 2 of them. The front one should come from the forward tap of the petcock, then turn 80 degrees and go thru the upper hole in the carb mounting bracket, then cross over to the 1-2 carbs. The other one should go similarly to the 3-4 carbs in a much simpler path. If the hoses go down below the carbs and then upward to the fuel tees, the 1-2 side in particular will have a tough time trying to run above idle speeds.

[rluser]

NAPA usually has a points file in stock.

[PeWe]

Have you checked if tank get air via its fuel cap ventilation? If clogged, fuel will stop flowing, engine will stop.
It might be partly clogged.
Not that much fuel in and cap not closed can help during fault trace.

[willbird]

A thought: how are the fuel lines routed? There will be 2 of them. The front one should come from the forward tap of the petcock, then turn 80 degrees and go thru the upper hole in the carb mounting bracket, then cross over to the 1-2 carbs. The other one should go similarly to the 3-4 carbs in a much simpler path. If the hoses go down below the carbs and then upward to the fuel tees, the 1-2 side in particular will have a tough time trying to run above idle speeds.

Fuel lines are 5.5mm Honda routed as stock. No dives down and then back up. The bike runs great wide open and will reach and sustain the best top speed it has ever achieved. My local NAPA has been stumped a couple times for things folks say NAPA always has. They have a lot of stuff, but maybe not OUR stuff LOL.

I would think the fuel tank vent if it were an issue would manifest on a flat out top speed blast for sure.

I am leaning towards a bad coil maybe. I have a pair of these to install once the solid core spark plug wire and terminals show up that I had to order online from NAPA. They are 4.7 Ohm. I was checking spark with an inductive timing light and getting zip on cyl 3 yet it was firing enough to heat the exhaust piping hot. The light worked fine on 4. The wires are really old and stiff so it is a PITA to get those center cyl wires off.

https://i.ebayimg.com/images/g/i5AAAOSwcOpfT05N/s-l1600.jpg

Memorial day weekend I will take at least 2 days off so I might plan a carb clean blitz then if chasing a potential coil issue has not worked.

For sure there are periods of furstration involved here LOL.

[willbird]

Smallish local farm store had a 4cyl tractor copper core plug wire set for a mere $21 :-).

[willbird]

New coils and wires. Still runs poorly at 1/4 throttle cruise. Installed new float valves, did not expect any change, and indeed no change. One old float needle spring plunger was locked up.

[BenelliSEI]

1. Did you ever have the carbs in the ultrasonic bath? Sounds to me like somethings plugged.

2. Have you synchronized the carbs? On the bench and then at idle with vacuum gauges? Unsyn’d carbs will work on WOT but be terrible off idle.

[BenelliSEI]

I moved mine into the headlight can, just loud enough to remind me.

Just saw this. Excellent idea! I may do  this on the K3 my wife  and son ride. She occasionally goes for miles with one on……

[willbird]

1. Did you ever have the carbs in the ultrasonic bath? Sounds to me like somethings plugged.

2. Have you synchronized the carbs? On the bench and then at idle with vacuum gauges? Unsyn’d carbs will work on WOT but be terrible off idle.

Have not bathed carbs yet. Looking forward to a few days free time to devote to that.

Just got a set of vac gauges today.

Bill

[BenelliSEI]

Bill….. try the synchronization tool before you strip them all down and clean. Might be just that and you’ll be on the road. If it persists, tear them apart…..

If you split the rack, change the “0” rings on the fuel delivery cross tubes. They will leak sooner or later.

[willbird]

Bill….. try the synchronization tool before you strip them all down and clean. Might be just that and you’ll be on the road. If it persists, tear them apart…..

If you split the rack, change the “0” rings on the fuel delivery cross tubes. They will leak sooner or later.

I did get the harbor fright O ring kits that have the ones needed. I guess I'm just wondering how terrible it could run due to lack of synch. I'm pretty sure I did synch them back in the 90's when I restored the bike but memory is not 100% on that.

[BenelliSEI]

Your initial readings might surprise you on start up……

[willbird]

Synched carbs, 3-4 were pulling more vacuum initially, tweaked them then that made 1 change a bit, tweaked it and then refined 3 and 4 a little. Runs better, idles a lot nicer, but core issue remains. had this happen when removing the port adapters too.  Quick trip to the parts store to get that EZ out. Shank of EZ out measures .126, drilled a .120 hole in 1/2" deep then opened up the beginning with a .28 (1/8) drill thinking the EZ out would slip in then be tapped into the .120 section with a small hammer to allow turning the EZ out, drill decided to run a little small, so tapped it in. Brass remains came right out.



Something else that comes to mind is running a temporary jumper to the power side of coils and make sure there is not an intermittent power issue there.

I have a Hondaman ignition coming which could rule out points issue maybe. I also have one of the $57 Ebay electronic ignition setups sitting here, bought to play with, the down side of that is having to set the points back up for the Hondaman box. I don't think the $57 setup is especially "good" with it's as I recall 315 degrees of dwell, but if it ran GREAT with that I would have found something anyway.

 

[Stev-o]

Did you test your spark plug caps?   I know you replaced coils and wires, those rarely go bad

[willbird]

In the history of my bike a coil went bad at least once. I replaced caps with genuine Honda when I did coils and wires. The R values for the old caps are further up in thread. 3 were around 10k one checks totally open.

I do have new spark  plugs I have not installed yet.

Bill

[willbird]

FINALLY found the issue :-). When I first started to wake this bike up I removed all 4 plugs, squirted in some oil, and spun it over to make sure I had good oil pressure plus move some oil around in the engine that sat for 10 years since last running. I seem to recall maybe dropping a spark plug, thinking that is what happened to #3. That is a .006 gap. New NGK D8EA plugs totally resolved the weird part throttle intermittent miss coupled with an exhaust pop. I guess a very low voltage (by modern standards) waste spark breaker point ignition has a low tolerance for one of the two plugs on that coil have a .025" gap and the other one .006" gap..