Topic: '76 Cb750F1 - Reviving after 15 yrs. - Low Spark/No Power

[Willy_754c]

Hi All,
Need some advice.  Bike sat 15 yrs, and I am breathing some new life into it.  I have some tune up questions for the experts.  Quick recap of what I have done:

-Cleaned carbs
-Put on pod filters, but put the original box back on after learning of how badly this affects jetting.  I will eventually want pods, but I will wait until I re-jet for pods and new exhaust all at once.  (All is stock now)
-synched carbs
-Checked timing with timing light - no adustment.
-Set points best I could, checked dwell.   Points look OK, but I am not an expert. 
-New Plugs
-new battery, fully charged.

1) I am getting dead cylinders.  It seems to be consistently in 1 and 2, which seems odd, I figured maybe I would see it more in 1&4 or 2&3.  I pull the plug to check spark, and it is barely there.  Often, it isn't even enough to get across the electrode, and I can just see it fizzling around.  Can one side of a coil go bad?
I am considering Dynatek ignition, and trying to determine if new coils in addition to the ignition is needed, or will improve anything.  I will also be getting new wires as part of this process. 

2) Following the manuals, I checked the dwell and set the points to get the dwell as close as I could.  The problem is, when I have the dwell within spec on the dwell meter (can't remember the spec right now) the manual gap is nowhere near .014"  Is this a symptom of a problem with the coils or condensors?

Billy

[Spanner 1]

Hello !..... I'm sure your really anxious to get this bike fired-up and running , but by your description
of spark problems I think you need to spend some time taking-apart some wiring connections and cleaning/ restoring good contact. Do you have a wiring diagram ?  If you have, trace the ignition wiring and clean all connections .... if you don't then polish the fuse holders and squeeze the 'clips' for better fuse contact. Clean every connection on the Red wire and every connection on the Black wire and the black/white wire too. Try to spray contact cleaner into the 'kill switch' and work it 20 times.... follow the blue/yellow from the points and clean the connection above the rear brake light switch .... just some routine stuff to do when taking a bike out of a long 'slumber'... good luck !.... Oh, and don't settle for anything less than shiny points, or poor/no contact.

[HondaMan]

Several things, Billy:
First, you need a little more step-by-step, here, after a 15-year sojourn.

1. The carbs: the pilots jets may still be plugged, especially on those 'dead' cylinders. This is due to the kind of gas we enjoy  today: if there was any old gas at all in the tank when you added new gas, it mixed with the new and made it gummy and inert. This immediately plugs the pilot jets, again, after cleaning.
2. The next thing this gas does: it gums up the little float valves where the fuel enters the float bowls. You will now find, if you look closely and carefully, rough-feeling ridges on these tiny valves. Scrape all 4 of these ridges shiny smooth, gently, with a dull pocketknife or screwdriver edge. They will then move more easily in their brass seats, and the fuel in the float bowls will be deeper. This will let the idle jets be immersed in fuel, which they must be to run.

3. The spark: remove the sparkplug caps from the wires, cut off about 1/4" of spark wire. Then measure the caps between the little brass screw (on the wire end) and the female plug contact (in the plug end) with a digital ohmmeter: they must be between 7000 and 8500 ohms if they are the original Honda plug caps, and they must be within 500 ohms of each other. If not, the spark will fizzle on one of the 2 plugs on any coil because the caps are not the same resistance. To be honest, your best bet today, after 15 years, is to go to a bike shop and get a new set of plug caps in 5000 ohm resistance (it says "5k" on the caps) from NGK. They are about $5 each and go a long way toward improving the spark.

4. The condensors: after 5 years, they are not much good. The points might still be OK, but those old condensors are definitely toast, being leaky and out of spec by now. They are inexpensive from PartsNmore online.

5. Use ND sparkplugs, the X24ES-U part number, for better performance with today's fuels. You can find them online (Sparkplugs.com is one place).

6. The points gap: it should be set to the range of 0.012" to 0.016". If it is outside this range to get your timing marks set up properly, something is wrong. Always start by setting the points to 0.014", turn the whole plate to set the left points to their "F" mark, then adjust the little subplate on the right set to get to its "LF" mark. If they are off a little between the points sets so they can't reach those marks, adjust the gap a little on the left set (open to advance, close more to retard) and repeat the process for both sets.

Your old Honda coils will give you fewer bugs than anything Dyna offers today, so long as the spark wires are not dry and cracked. (You can still buy new Honda coils, too.) If you opt to use Dyna's 3 ohm high-performance, high-voltage coils (called the "Green coils" for their color), you will likely need a Resistor Pack (I offer then at $17 plus postage) to feed them and protect the bike's Run-Off switch over time, and I also suggest using 10000 ohm sparkplug caps with those coils instead of 5000 ohm caps, to extend their short spark duration a little bit. These engines benefit from a long-duration spark, which Honda's coils provide (Dynas do not).

There are other ignition options available today: I make one type called the Transistorized Ignition, which runs from the points and provides 100% backup if it should ever fail (by unplugging it and plugging the points back in). Dyna offers their all-electronic Dyna "S" model, and Pamco has one very similar to the "S" as well. Be sure the bike runs on points before trying to install one of these, as it will confuse you otherwise by introducing another layer of complexity with their adjustments. 

[Willy_754c]

Thanks so much for the help guys.  Gives me some things to work on this week.  I think I will go ahead and get some condensors coming, and possibly some caps and different plugs. 

Need a quick clarification on the ohmmeter.  I have tried to find this info on the user's manual, but it was not much help. 
Mine is picture below.  I gather the different settings are ranges, but I don't understand them.  It's not like it's 1x, 10x, 100x, etc., so I don't fully understand the number. 

I am checking the cap from No1 now.  Here are my readings:
2000k = 012
200k   =12.1
20k     =12.08
On 2000 and 200, I can't get a reading.

How do I calculate these to see if I am between 7000 and 8500 ohms?  Is it as simple as 20k (20,000) x 12 = 24,000 Ohms? Or, from the research I have done, is it proper to say 12.08 "K" ohms, or roughly 12,000 Ohms?

Would it be safe to say that this cap is out of spec?

I'll probably have a lot of these "never a stupid question" questions.  Thanks in advance. 

[Willy_754c]

A little more info---

I did do a small amount of chasing down wiring originally.  I thought I had a bad connection at the fuse block, as I could not get the starter switch to work, so I cleaned the clips/fuses.  As it turned out, it was the starter switch itself.  Below is a pic of my (embarassing) short term work-around until I rebuild/replace the switch.  That being said, I do need to fully go through all of the connections.  I didn't realize this could affect the spark/performance so much.   

After looking closer, my deduction that the points were decent may have been premature.  I was only looking for severe detioration of the contact, but in looking closer, they do appear pitted and unclean.  I will clean these once I get a point file.  (I have heard that sandpaper is not recommended)


On the spark plugs, I got NGK D8EA in there now.  (what Honda shop recommended)  NGK D8E S-L was in there when I got it.  I'll go to the NDs. 

Thanks for the help.  These forums make this project much less daunting. 

[BeSeeingYou]

The NGK is close and will be okay but the ND'S are right on for the bike.  Unless you want to do some type of alternative stater button the only option is a complete new right side control.  The headlight is also fed through the starter switch and it kills the headlight when the starter is engaged. They are about $100 and available complete with wiring.  Looks like you are missing a throttle cable, maybe to accommodate the throttle clamp.  It is designed to have a "send and return" cable.

I would say your cap is out of spec at 12,080.  Chose the range that is the next higher to what you are measuring so you would use the 20K (0-20,000).  On my meter I can also select a range within the range I select with the dial.

[Jerry Rxman Griffin aka MuthaF'er]

I didn't read too closely but did anyone mention to swap plug wires? #1 to #4 & #2 to #3 to see if the misfire follows the plug wire/cap?

+1 with Steve, 12080 ohms is too much resistance for a weak spark to jump the cap. If you stick with stock ignition stuff get a new set of caps as Hondaman says. If you go aftermarket coils you'll be able to use new wires. You can always do the plug wire replacement trick on the Honda coils or get a splicer thing from NGK (I think it's theirs).

[HondaMan]

Thanks so much for the help guys.  Gives me some things to work on this week.  I think I will go ahead and get some condensors coming, and possibly some caps and different plugs. 

Need a quick clarification on the ohmmeter.  I have tried to find this info on the user's manual, but it was not much help. 
Mine is picture below.  I gather the different settings are ranges, but I don't understand them.  It's not like it's 1x, 10x, 100x, etc., so I don't fully understand the number. 

I am checking the cap from No1 now.  Here are my readings:
2000k = 012
200k   =12.1
20k     =12.08
On 2000 and 200, I can't get a reading.

How do I calculate these to see if I am between 7000 and 8500 ohms?  Is it as simple as 20k (20,000) x 12 = 24,000 Ohms? Or, from the research I have done, is it proper to say 12.08 "K" ohms, or roughly 12,000 Ohms?

Would it be safe to say that this cap is out of spec?

Yep, you've got it right: on a 20K scale setting, the reading you see is in x1000 ohms. So, the 12.08 = 12,080 ohms. The 20k refers to the highest reading you can get on that setting. So, if you select the lower 2k (or 2000) ohm setting, since the 12000 ohms is higher than that, there will be no reading showing: it "looks" like an open circuit at that range.

Good job!

It does look like the cap is toast. And, that is very common at this stage, after 15 years.

[BeSeeingYou]

This makes me curious to go and check the caps and condensers on my 76 750F.  They are all original parts.  The bike starts with no problem and runs like a top which must be unusual for the age of these parts so I never bothered to check them when I got the bike.

[Jerry Rxman Griffin aka MuthaF'er]

Steve, Steve, Steve, after 15 years all good F's deserve a new set of caps  Just part of the ongoing battle of loading up the credit card with another monthly installment 

[BeSeeingYou]

Steve, Steve, Steve, after 15 years all good F's deserve a new set of caps  Just part of the ongoing battle of loading up the credit card with another monthly installment 

My sheepish reply.  I just checked mine and they are in the 12200 ohm range.   Time for a run to the bike shop.

[HondaMan]

A little more info---

I did do a small amount of chasing down wiring originally.  I thought I had a bad connection at the fuse block, as I could not get the starter switch to work, so I cleaned the clips/fuses.  As it turned out, it was the starter switch itself.  Below is a pic of my (embarassing) short term work-around until I rebuild/replace the switch.  That being said, I do need to fully go through all of the connections.  I didn't realize this could affect the spark/performance so much.   

After looking closer, my deduction that the points were decent may have been premature.  I was only looking for severe detioration of the contact, but in looking closer, they do appear pitted and unclean.  I will clean these once I get a point file.  (I have heard that sandpaper is not recommended)


On the spark plugs, I got NGK D8EA in there now.  (what Honda shop recommended)  NGK D8E S-L was in there when I got it.  I'll go to the NDs. 

Thanks for the help.  These forums make this project much less daunting. 

That's a cute (if fairly backwoods) rework of the START button!
You can also modify the old switch to become a simple On/Off pushbutton if you add a relay (inside the headlight bucket) and operate it from the pushbutton. Then, the relay's NC contacts can simulate the original NC Starter contacts for the headlight, and the NO contacts can run the Starter Solenoid. I make kits for this, but you would have to make up at least a simple pushbutton of some kind to make it work.

The BLACK wire to this switch is the Ignition power circuit. When this button is released (on post-1975 CB750s) it connects the BLACK wire to the headlight, by way of the center fuse in the fuseblock on a BRN/RED (or sometimes a BLK/RED) wire. When the button is pressed, it connects the BLACK to a YELLOW/RED (or sometimes a BLK/YEL) wire to pull in the Start Solenoid. So, the little relay can mimic this with similar, but more robust, contacts for longer life. This allows you to also substitute any form of START button on the right side, either the 3-wire version from the post-1975 bikes, or the simple 2-wire version from the 1973-'75 bikes, or the 1-wire grounding version from the 1969-1973 (early) bikes. So, it provides us the best scenario.

[Willy_754c]

"The headlight is also fed through the starter switch and it kills the headlight when the starter is engaged. They are about $100 and available complete with wiring.  Looks like you are missing a throttle cable, maybe to accommodate the throttle clamp.  It is designed to have a "send and return" cable."

Aaaahhhhh......That explains the mysterious 3rd wire and why my headlight is not working.  I will get a replacement coming.  I didn't know for sure if this switch could be rebuilt like the others.  It looked less serviceable than the others I have seen in the Tips&Tricks.
---When I started cleaning up the carbs, I found that the push cable was broken and removed it for now.  I have a new one ready to go in.   

It does look like the cap is toast. And, that is very common at this stage, after 15 years.

That is interesting.  I never realized that a cap could go bad.  I figured it was just a physical way to go from wire to plug, not necessarily doing any sort of function other than that.

You can always do the plug wire replacement trick on the Honda coils or get a splicer thing from NGK (I think it's theirs).

I didn't realize that the plug wires were not replaceable on the stock coils.  I will have to look up the replacement trick. 

 

[Kickstart]

...
3. The spark: remove the sparkplug caps from the wires, cut off about 1/4" of spark wire. Then measure the caps between the little brass screw (on the wire end) and the female plug contact (in the plug end) with a digital ohmmeter: they must be between 7000 and 8500 ohms if they are the original Honda plug caps, and they must be within 500 ohms of each other. If not, the spark will fizzle on one of the 2 plugs on any coil because the caps are not the same resistance. To be honest, your best bet today, after 15 years, is to go to a bike shop and get a new set of plug caps in 5000 ohm resistance (it says "5k" on the caps) from NGK. They are about $5 each and go a long way toward improving the spark.
...

Should we get the 10k ohm or 5k ohm caps for the 75-76 CB750F?... Since they originally came with 10k (I believe you mentioned this somewhere on the forum). 

I'm also planning on getting new caps for my CB750F (I had that missing problem around 5000 RPM you mentioned in your book) but I was unsure if I should get the original 10k or go with the 5k caps.

Thanks

[HondaMan]

...
3. The spark: remove the sparkplug caps from the wires, cut off about 1/4" of spark wire. Then measure the caps between the little brass screw (on the wire end) and the female plug contact (in the plug end) with a digital ohmmeter: they must be between 7000 and 8500 ohms if they are the original Honda plug caps, and they must be within 500 ohms of each other. If not, the spark will fizzle on one of the 2 plugs on any coil because the caps are not the same resistance. To be honest, your best bet today, after 15 years, is to go to a bike shop and get a new set of plug caps in 5000 ohm resistance (it says "5k" on the caps) from NGK. They are about $5 each and go a long way toward improving the spark.
...

Should we get the 10k ohm or 5k ohm caps for the 75-76 CB750F?... Since they originally came with 10k (I believe you mentioned this somewhere on the forum). 

I'm also planning on getting new caps for my CB750F (I had that missing problem around 5000 RPM you mentioned in your book) but I was unsure if I should get the original 10k or go with the 5k caps.

Thanks

In 1976, Honda changed the combustion chamber design on the 750 to be a hemi. When it went hemi, this made the flamefront a little smaller as the mixture did not swirl past the lit spark nearly as much as it did before. To help improve this flamefront performance, Honda switched the plug cap resistance from the original 7500 ohms to 10,000 ohms to make the spark discharge last a little bit longer. They also improved the quality of the spark wire itself, to help reduce the losses from this higher resistance and make it less likely to jump out of the boots after they aged a year or two. The coils themselves remained the same. The end result of all of this: less time would occur before the plug cap resistance reached the "critical resistance" value of 11,000 ohms, where the balance between the two caps must then be almost perfect lest the spark shift away from one of the plugs, causing a late spark on one side of the coil. This is one reason why the "F" bikes showed an improvement from using the Dyna 3 ohm high-output coils: higher resistance plug caps, with the shorter-duration, higher-voltage Dyna coils, would make for a longer-duration spark (more like the fresh plug caps).

In the end, though, the Honda coils were designed for 5000-8000 ohms (x2 for the 2 caps) resistance for a 0.0012mS spark duration. As the resistance rises, the duration gets longer until it reaches the point where it gets too long and the coil losses equal the discharge loss: this happens at about 0.0015mS, or about 11,000 ohms on each coil wire. As the resistance rises, the spark gets weaker (dimmer, if you are watching it, with more yellow color instead of blue) because part of it is being lost in the coil itself. Today, we cannot get the OEM 7500 ohm plug caps that the coils were designed to use, so using the 5000 ohm caps at least puts it close to the original spec. This also allows room for the caps to slowly deteriorate again over time, but will allow more time than the original lifetime before the 8500 ohm upper limit is reached. So today, the 5000 ohm caps are the best bet on the stock coils.

If the Dyna 3 ohm coils are used, the spark is hotter but very short (0.0009 mS) if the Dyna zero-ohm plug caps are used. This does not improve anything over good OEM coils, but is often PERCEIVED to improve things only because the previous coil system was in worn condition. But, if you modify the Dyna setup a little with a Resistor Pack on the primary (1 ohm on each coil feed) and use the 10,000 ohm spark plug caps as well, the spark duration reaches about 0.0011mS, but with more than 20kV (stock coils are around 7kV) at the plugs. This can let you use things like Iridium plugs, or larger plug gaps (like 0.045"), to make a larger, hotter flamefront. This can be helpful if you have the higher-compression engines, like the "F" bikes or the 836cc kits.

In the end, "improving the spark" involves a little more than just plunking in different coils, since the power available to run the coils in these bikes is limited.

[Kickstart]

So I got my 5k ohm NGK caps the other day, and I decided to measure their resistance before putting them on the bike.  I was surprised to find that their resistance ranged between 4.8k and 5.2k. 

I guess that's within 5% of 5k each, but I was expecting better tolerances as they are already close to the 500ohm difference replacement threshold.

I'm still getting some intermittent "missing" around 5000 rpm... although not as often as before.

[Willy_754c]

I ordered 5k caps, points, condensors, and a new air filter on Sunday from Z1.  I also ordred a new start/kill switch from BikeBandit.

I tried to figure out how to check the old condensors just for reference, even though I was getting new ones.  Clymer Manual says:
"The only proper test is to measure the resistance of the insulation with an ohmmeter.  The value should be 5000 Ohms.  A make-do test is to charge the capacitor by hooking the leads, or case and lead, to a 12v battery.  After a few seconds, touch the leads together, or lead to the case, adn check for spark....."

It's sort of a moot point since I have new ones coming, but I was just interested how to perform these checks. 
I checked the condensor with my ohmmeter, and I am still sort of confused by how to read the results.  I can only get a reading on the 2000k setting.  I get various numbers, but generally around 300.  Is this 300k ohms, or 300,000 Ohms?  Seems like an out of whack number, but maybe it's that shot. 
I tried the charge/spark method, and coudn't get any spark.

I'm fairly convinced these are shot, especially since Hondaman said after 5 yrs=no good.  Just want to ensure for future reference.

I will be getting the ND plugs on the next order.  I am also deciding on the coils.  I wanted to just get OEM, but Z1 didn't have the stock coils, only various dynas, and bikebandit was too $$$ on the coils,  and I didn't feel like getting another order going somewhere else.  Looked like PartNMore was best price on stock coils.  Let me know if anyone has had good luck elsewhere for coils. 

I sort of put the coils last on the list for now anyway.  I expect that mine are OK, and the wires seem decent, and from what people are saying, coils are probably the least likely to be an issue. 

I did cut 1/4" off to get to some fresh wire.  After I did this, they seemed a little short, I wonder if somone had done this before already. 

Hopefully the items get in this week, and I can work on tuning this weekend. 

Side question - My carb boots seem OK, I have not had any issues getting the carbs on and off.  I have seen some of the post talking about softening them.  Do this softening methods work the same on the 4 rubber horns that go into the air box?  Mine are very dry and hard in certain spots, and I don't think they are sealing well to the airbox. 

[Willy_754c]

Got tune up items in.  Thanks Z1.  Ordered on Sunday, received on Wednesday morning.  Can't complain about that. 

I sat down and checked old and new caps. 
For some reason, I could not get a reading on the two old straight plug caps on any setting on my ohmmeter.  Guess that means completely shot........
My new ones were as follows:
4720
4960
4890
4980

I still can't figure out how to check the 5k ohm of the capacitors.  I can't get any reading on any setting on the ohmmeter.  Anybody know how?

Will get everything put on this weekend, and see how she rolls. 

[BeSeeingYou]

On the 2000K setting that is 2 million so the reading of 300 would be 300,000.

Softening should work on the rubber horns too.

Your new control will come with a purple kill switch so you can swap the old one in if you want the proper color.  Just do it over a piece of paper as there are some small parts in there.

I think you need a meter designed to test capacitors.

[Willy_754c]

Thanks srust,
I'll need to soften the horns this weekend as well.  I think I may have been lucky to get them apart the first time, they were pretty crusty.

New right side control came in today.  Ordered from BikeBandit on Sunday.  I was happy it came so fast.  I was worried with an OEM part that it might take a couple of weeks. 

I think I can live with the purple switch.

Got my weekend planned now.  I need to read up on how to time properly when starting from scratch. 

Just wish I had a decent tank now so I could get some riding in when I get done.....



I like how the new one has the cutout.  I was planning on cutting the notch myself so I didn't have to run through the bars when I get my clubmans.