Topic: Spark plug caps and ignition questions 1977 Honda CB750K

[SigEpRider]

Chasing a problem starting my bike cold, running rough, and hesitating.

Here are the details:
1977 Honda CB750K
Stick PD41 carbs with stock airbox
Delkevic 4-1 exhaust with baffle
Carbs have been cleaned and sonic cleaned. Accelerator pump cleaned out using Honda chopper process.

Today I installed my Dyna S ignition thinking it would help but now idle and reffing is worse.

Took ignition measurements:
Cylinders 1 and 4
Primary 4.7 ohms
Secondary 16.2 kilo ohms
Cylinder 1 cap 9.2 kilo ohms
Cylinder 4 cap 8.4 kilo ohms

Cylinders 2 and 3
Primary 4.5 ohms
Secondary 15.7 kilo ohms
Cylinder 2 cap 8.6 kilo ohms
Cylinder 3 cap 8.9 kilo ohms

Using NGK DR8ES spark plugs.

I’ve been doing lots of research and now I believe it’s the spark plug caps. However I’ve seen that 1977 use 10K ohm resistance. If so, are my caps actually good?

Thinking of using NGK 5K ohm caps with the same plugs (as per my searches on this firum) will this combo work well with the Dyna S? Any suggestions on plugs, caps, spark plug gap for use with stock coils and Dyna S.

Any help is always appreciated.

[PeWe]

Advance might kick- in too early?
[Or ignition set wrong? T instead of F.]

I had to cut advancer springs 1/2 wound to get it to work properly.
My CB750 K6 was worse with very sloppy springs making advance to happen way before 2000rpm.
Cut the springs fixed it.
Ignition possible to adjust at idle, full advance at 2600-2700 rpm.

Work fine with points, Pamco and Dyna-S.
My K6 is modified and has good idle and fine response to >9000rpm. Good power that dyno charts has shown.

My K2 stock, work fine too.
Next CB750 will get it direct without struggling to get it right from idle and up.

I had also verified true TDC so T really will be right when 1:4 are at top dead center.
F for ignition  will then be correct too.

[newday777]

Your 77 uses the same plug caps as the earlier years. So yes, you should have NGK 5k ohm caps on your bike.
Have you snipped off 1/4" of the plug wires yet? Best to do that if you haven't to get full power again.

Good thread on caps and coils
http://forums.sohc4.net/index.php?topic=104742.0

[SigEpRider]

Your 77 uses the same plug caps as the earlier years. So yes, you should have NGK 5k ohm caps on your bike.
Have you snipped off 1/4" of the plug wires yet? Best to do that if you haven't to get full power again.

Good thread on caps and coils
http://forums.sohc4.net/index.php?topic=104742.0

Thanks for the help. I wanted to verify 5K ohm caps first. Looks like my current caps are toast. Will order NGK 5K caps first before I try anything else.

[madmtnmotors]

Have you performed the clear tube float level test? Have you checked the fuel flow from the tank? The later models have a filter in the tank up inside the petcock bung that often gets overlooked.

[SigEpRider]

Have you performed the clear tube float level test? Have you checked the fuel flow from the tank? The later models have a filter in the tank up inside the petcock bung that often gets overlooked.

I currently have the bike hooked up to a fuel bladder. Haven’t tested he fuel level yet. Can you send a link of how to do that?

Currently going to try plug caps first since I’ve confirmed they are shot. Also, when I had the air box off I noticed fuel coming back out of the carb. Like not burning properly or just spitting some back. I think that could be from the spark being weak. Thoughts?

[TwoTired]

Why are you using resistor spark plugs?  They add another 5k resistance.  Most USA models came with non resistor plugs and 5 k plug caps.  Some Canadian models were forced to use 10k caps by import requirements.  Perhaps some euro models, too.  I think using both resistor plugs and 10k plug caps is unwise, as it limits too much current during the spark event.

The Dyna ignition trigger has the coils drawing power nearly twice as long as the points do.  You might notice the coils run much hotter with the Dyna.  Heat is not good for coil longevity.

[madmtnmotors]

Why are you using resistor spark plugs?

I overlooked that. I have never felt the need for resistor plugs. At least the bladder takes the fuel restriction out of the equation.

Clear tube test:
>Remove rubber overflow tube from bottom of carb
>Connect clear tube to overflow drain at bottom of carb
>Hold tube next to carb body at top of bowl
>Open carb drain screw
>Note fuel level in relationship to top of bowl
>Close drain screw
>Remove clear tube
>Reconnect rubber overflow tube
>Repeat three more times

EZPZ

This implies that you are using the K7 PD carbs. Early ("round top") carbs require a slightly different approach.

[SigEpRider]

Why are you using resistor spark plugs?  They add another 5k resistance.  Most USA models came with non resistor plugs and 5 k plug caps.  Some Canadian models were forced to use 10k caps by import requirements.  Perhaps some euro models, too.  I think using both resistor plugs and 10k plug caps is unwise, as it limits too much current during the spark event.

The Dyna ignition trigger has the coils drawing power nearly twice as long as the points do.  You might notice the coils run much hotter with the Dyna.  Heat is not good for coil longevity.

Because I’m an idiot and didn’t do my proper research when buying plugs. Lol.

let me go back and double check that they are resistor plugs. Not you got me second guessing. I will report back.

However I’ve seen posts from Hondaman that recommends resistor plug caps with resistor plugs. But I’m not sure if that was for stock points only or for early cb750s. Hopefully he can weigh in.

[SigEpRider]

Why are you using resistor spark plugs?

I overlooked that. I have never felt the need for resistor plugs. At least the bladder takes the fuel restriction out of the equation.

Clear tube test:
>Remove rubber overflow tube from bottom of carb
>Connect clear tube to overflow drain at bottom of carb
>Hold tube next to carb body at top of bowl
>Open carb drain screw
>Note fuel level in relationship to top of bowl
>Close drain screw
>Remove clear tube
>Reconnect rubber overflow tube
>Repeat three more times

EZPZ

This implies that you are using the K7 PD carbs. Early ("round top") carbs require a slightly different approach.

Oh I can do that and report back. What am I looking for as far as the level?

Yup. 77 PD carbs.

[madmtnmotors]

Why are you using resistor spark plugs?

I overlooked that. I have never felt the need for resistor plugs. At least the bladder takes the fuel restriction out of the equation.

Clear tube test:
>Remove rubber overflow tube from bottom of carb
>Connect clear tube to overflow drain at bottom of carb
>Hold tube next to carb body at top of bowl
>Open carb drain screw
>Note fuel level in relationship to top of bowl
>Close drain screw
>Remove clear tube
>Reconnect rubber overflow tube
>Repeat three more times

EZPZ

This implies that you are using the K7 PD carbs. Early ("round top") carbs require a slightly different approach.

Oh I can do that and report back. What am I looking for as far as the level?

Yup. 77 PD carbs.

A little bit below the top of the bowl.

[SigEpRider]

Can anyone help me with the appropriate NGK part numbers for a 1977 CB750K and where to buy. I’ve been searching for VD05FP for cylinders 1/4 and SD05FP for cylinders 2/3. This is from searching in the forum. NGK part finder isn’t being helpful. I can get the VD’s for 8.90 each shipped on eBay and the VD’s for 6.98 each plus 6 shipping. It appears these are getting harder to find. Or is it just me.

Thanks again

[bryanj]

Try VB05F and SB05F

They are black coloured for 500 four , long straight and long 60 deg angle for plugs without the threaded on bit

[TwoTired]

Why are you using resistor spark plugs? 

Because I’m an idiot and didn’t do my proper research when buying plugs. Lol.

let me go back and double check that they are resistor plugs. Not you got me second guessing. I will report back.

However I’ve seen posts from Hondaman that recommends resistor plug caps with resistor plugs. But I’m not sure if that was for stock points only or for early cb750s. Hopefully he can weigh in.

Today's part stores stock what is popular and fast moving.  High energy ignition systems are pervasive in today's market and nearly all of them need resistor plugs.  So guess what get stocked and pushed by parts counter people?  Not much for 40-50 year old vehicles.

You can effectively divide the ignition system on coil boundaries, as the secondary output is isolated internally and physically from the primary circuit.  The secondary HV and resistance in the system have very little to do with the primary and energizing/triggering of the coil energy insofar as circuit power draws, as they are DC disconnected.  Nominal 5 ohm primaries are generally used with points to extend point life with the lower charge and discharge currents they experience.  But, the secondary loop of the coil output doesn't factor much in the triggering of the coil discharge.  (The dynamics are hard, and lengthy, to explain to the layman.)  The resistance of the secondary loop does have effect on the spark event current and the radiated electrical field surrounding the wire leads (which in modern high energy systems can damage sensitive computer parts that can be saved by using resistor plugs).   But for our bikes, too much resistance for the specified coil reduces the peak energy in the spark gap.  This can make the plugs difficult to self clean unless the mixture is ideal.  I recommend you look at the plugs to determine run conditions, and see if the center electrode insulator is getting a carbon build up, which may shunt away spark energy intermittently.

[PeWe]

I ordered NGK caps for my CB750. 2 of each
SD05F
XD05F

Have also seen
SD05F and VD05F

The straight one for 2:3


Difference is the angle for the caps used on 1:4
VD05F most used I guess.






Here the XD05F on one of my bikes. The red version. I have black on my other bike.
Plus some extra in stock

Good to have  extra when they can crack.

[newday777]

I just got to my shop and the 4 bikes I have, 74K4, 75K5, 76K6 and the 78K8 all have the VD05F on the 1/4 angled and the SD05F straights on 2/3.

[SigEpRider]

UPDATE: I double checked my plugs. I have NGK D8EA non resistor plugs.

Thanks for the part numbers. I’ll order a couple of sets, install and report back.

[SigEpRider]

Why are you using resistor spark plugs? 

Because I’m an idiot and didn’t do my proper research when buying plugs. Lol.

let me go back and double check that they are resistor plugs. Not you got me second guessing. I will report back.

However I’ve seen posts from Hondaman that recommends resistor plug caps with resistor plugs. But I’m not sure if that was for stock points only or for early cb750s. Hopefully he can weigh in.

Today's part stores stock what is popular and fast moving.  High energy ignition systems are pervasive in today's market and nearly all of them need resistor plugs.  So guess what get stocked and pushed by parts counter people?  Not much for 40-50 year old vehicles.

You can effectively divide the ignition system on coil boundaries, as the secondary output is isolated internally and physically from the primary circuit.  The secondary HV and resistance in the system have very little to do with the primary and energizing/triggering of the coil energy insofar as circuit power draws, as they are DC disconnected.  Nominal 5 ohm primaries are generally used with points to extend point life with the lower charge and discharge currents they experience.  But, the secondary loop of the coil output doesn't factor much in the triggering of the coil discharge.  (The dynamics are hard, and lengthy, to explain to the layman.)  The resistance of the secondary loop does have effect on the spark event current and the radiated electrical field surrounding the wire leads (which in modern high energy systems can damage sensitive computer parts that can be saved by using resistor plugs).   But for our bikes, too much resistance for the specified coil reduces the peak energy in the spark gap.  This can make the plugs difficult to self clean unless the mixture is ideal.  I recommend you look at the plugs to determine run conditions, and see if the center electrode insulator is getting a carbon build up, which may shunt away spark energy intermittently.

I pulled cylinder 1/4 spark plugs to verify that I actually have NGK D8EA non resistor plugs instead of resistor plugs. The tips of the plugs are black and the center electrode has carbon build up also. I didn’t get a chance to pull 2/3 since it was late at night.

Is this a cause because of the spark plug caps, dyna ignition?

I feel I need to address the cap issue to get better answers.

[TwoTired]

If your plugs are sooty, it’s because it can’t burn all the fuel it’s getting.  The spark only starts the burn, it doesn’t burn all the fuel for the entire power stroke.  That’s a mixture issue.  I believe you have carb problems or you are running far too long on choke.  The soot will cause ignition issues that proper spark alone can’t cure.

Clogged air filter?
Too much exhaust restriction?
 Too large fuel jets?
Wrong position/taper main needle?
Enlarged slide needle jet?
Faulty air bleeds for main or pilot circuit?
Too high float levels?

Investigate and correct.

[SigEpRider]

Clogged air filter? It’s brand new
Too much exhaust restriction? I have delkevic 4-1 exhaust with baffle.
Too large fuel jets? I think they are stock. How can I verify? I don’t remember any markings on pilot jets.
Wrong position/taper main needle? Verified correct needle markings. Clip is on first/top groove as per service manual.
Enlarged slide needle jet? I think it’s stock also. However couldn’t find markings. For sure I haven’t switched out needle jet or pilot jet size. But is there a way to verify stock with markings.
Too high float level? I set float heights to factory 12.5 MM as per service manual. Will use clear tube method to verify.
Faulty air bleeds for main or pilot circuit? How can I diagnose that?

Finally…any suggestion for setting the air mixture screw? I’m just turning them until I get max rpm. However it seems as if I turn way more than stick 1.5 turns. Could this be a function of cheap aftermarket rebuild kit parts? As I write this I realize internals can be no stock since they have no markings.

[PeWe]

I found out why I gave my bikes the XD05F caps.
They were included in an order from VintageCB750 2012.

I have continued to use that model since then.
My K6 had other caps in the 80's.
They still sell that combination.
Work fine.

[BenelliSEI]

I use the same source as PeWe and no issues.

[MauiK3]

Dumb question
Do those carbs have emulsifiers?
If so, check Hondaman’s suggested mods.
Better yet, get his book, it’s the gold standard of wisdom for the 750.

[Don R]

  I've mentioned to about a hundred guys that my F had drilled jets with the stock numbers still on them. It ran rich and was lucky to go 90 miles on a tank of gas. I went one size over stock for my glasspack muuffler and oem smog tuning. 

[TwoTired]

Dumb question
Do those carbs have emulsifiers?

Yes, they have emulsifiers in both main and pilot circuit.   I should have added that to my list of things in the carbs to investigate.  But, these should be pretty lean carbs if they have the stock components internally coupled with a low restriction exhaust.  So, either not clean or components not in stock configuration.  Possibly altered to compensate for the exhaust change?  Just a guess at this point.