Topic: Points gap issue?

[JWExperience]

So heres the deal...I have a 76 cb750 that I have put about 1500 miles on since I purchaced it last fall. When I first got it going in the spring it fired right up after sitting for a year or two and after adjusting carb settings it began running very well for the last 1500 miles and still does!

The only noticeable issue is under acceleration I notice that the engine skips like it loses spark. It mostly occurs between 3k-4k rpm's and its not too bad just one or two skips and doesn't happen every time. I started researching points on here and decided that I would atleast open things up for now and check gap. I figured my timing must be pretty good considering it idles well and runs strong up to redline. I often ride it at highway speeds with no skipping whatsoever, so its just under acceleration.

I also noticed my mileage is about 35mpg which seems a little low considering I only put a carpy pipe with k&n filter in stock airbox which is kinda why I thought spark or points. My plugs are a nice tan color and very uniform in color for every cylinder. The carbs were sonic cleaned and the floats were checked by a local shop while they had them. Jetting is 42/120 and I think the clip is in stock position. Mileage is about the same with 40/42 pilots, it just smoothed out a off idle hesitation.

The wierd part is that when I went to check my points gap I immediately noticed that both points were closed and upon spinning the larger nut to try and open them to gap neither would open. They stayed closed all the time. Also, the points will open freely if I just push them with my finger.

Do I need to replace the cam to make them open and close properly? Is there another reason they would not open?

I was under the impression that they should close and open seperately, like 180 degrees for spark. I feel like my bike should not run well at all and with the exception of the skip it runs very good.
Is there possibly a modification that my PO made to this system?
After doing hours of research/confidence building I thought I had an understanding of how this system worked and them my mind got blown
Sorry for the length, just trying to get any pertinant info out there and thanks for any input.

[seanbarney41]

If your points are not opening, you have no spark...period...bottom line,,,if you only get 35mpg, you got real problems...period

[JWExperience]

How does my bike run then with no spark?

[JWExperience]

If anyone knows how to switch a pic from jpeg to jpg then I could load a pic to show both points closed. Otherwise I will load up a video to youtube and add the link to this site showing the closed points on a running bike when everyone is awake around here.

[seanbarney41]

How does my bike run then with no spark?

that's a good question...pretty god damn sure the factory point gap spec is not 0, it's .012-.016.  Why don't you try setting your point gap to that and then adjusting your timing?, ...let me know how that works out for ya.

[JWExperience]

I'll look into that but I feel like there is a bigger issue then just setting the gap and timing if it runs now. Thanks

[Bailgang]

My guess is that your point gap is too small. I recently picked up a 73 cb500 that the PO said had recently been tuned up, whoever did it though had the timing way retarded with a miniscule point gap so small it was barely detectable when turning the engine over and much smaller than the .012" minimum gap Honda recommends. I took care of the point gap, set the timing and the engine "woke up" so to speak. The gap was so small that I couldn't tell at what point the gap was at it's highest point so I just adjusted the gap so I could actually see where its max gap was and then used my feeler gauge to set it at the gap I wanted.

Keep in mind that once you reset the point gap then you need to be checking your timing as well after you set the points gap.

[LesterPiglet]

www.photobucket.com 
Load images/videos to bucket and share the links.
I'd be interested to find out how your bike runs without a points gap.

[Bailgang]

www.photobucket.com 
Load images/videos to bucket and share the links.
I'd be interested to find out how your bike runs without a points gap.

Better yet just start the bike up with the points cover off and see if you can see the points doing something which I'm sure you'll be able to see something if the bike is actually running. In my case the gap was so small I actually couldn't see it either when turning engine over by hand at least I couldn't see it clearly (bad eyes, poor lighting) but I knew there had to be some kind of gap as miniscule as it may have been because after all there had to be some sort of gap or it wouldn't have ran at all to begin with.

[JWExperience]

Here's the link?
http://s1017.photobucket.com/user/jay_wells31/slideshow/

[JWExperience]

Wouldn't it run like crap if it wasn't gapped properly?

[Bailgang]

Wouldn't it run like crap if it wasn't gapped properly?

Probably in fact the 500 I picked up felt like it had half the power of my 550 even though they're supposed to have the same HP. Once I corrected the gap and timing the throttle was a lot snappier but haven't taken it for a spin yet to find out for sure. The bike is suffering from typical 500 clutch woes (broken push rod and adjuster) so I'm waiting for parts for that so it's down till they come in plus I found out one of the fuel lines was severely kinked which could explain why it ran so poor as well.

Oddly enough though even with the point gap set way wrong and timing way off it would still start right up and still idled smoothly. Go figure.

[JWExperience]

After running the bike I was able to see that they did barely open so I gapped the points and ran the bike again and it seemed to have a little more power but the skipping was gone from what I could tell on my short 5 mile ride. Also the bike seems to run better when cold and run a little smoother. I don't have a light to do the timing but I do have an ohm meter so I'll try to see if I can research the best way to use that. Thanks for everyone's help.

[Black 750K8]

Well as you know now the points had to be opening or the bike would not run. You really need to buy a light you can get a good enough one for under $30.00. If for no other reason you can see if the advance is working You should also get a carb balance device. You should then do a complete 3K tune up. It will run better than it ever has sense you have had it. Invest in a few tools now to make it easier to tune and trouble shoot later. When I first get a new to me bike I always do a complete tune up so I have a good base line to go buy when things happen later and the bike does not run as well later. If things like a tune up is new to you then BEFORE you make a change make a note of what the setting is BEFORE you make a change.

[JWExperience]

Well...when I first got the bike running, it seemed to run well so I just went with it and over time was able to iron what few issues I had out with just tuning and synching the carbs. Your right, now I know. Thanks everyone for your help.

[lucky]

If anyone knows how to switch a pic from jpeg to jpg then I could load a pic to show both points closed. Otherwise I will load up a video to youtube and add the link to this site showing the closed points on a running bike when everyone is awake around here.

A .JPEG and a. JPG file extensions are the same thing. It is just that the .JPEG is a much older designation and is used to keep people from using other people's photos.

Just re type the suffix to .JPG thats all. It will be recognized.

.JEPG stands for Juried Expert Pictures Group.(do not forget the period)

This is the group of people that first put the rules together for photos on the internet.

[lucky]

You may have been running the bike on two cylinders all the time.
That is why the mileage is so bad.
Only two cylinders doing all of the work.


Forget about the timing light for now.
Just statically time the bike with the volt ohm meter and make sure the advance unit is not frozen with rust.

READ THE BOOK

[JWExperience]

.

[JWExperience]

My book only has the steps using a timing light so thats why I was going research how to use an ohm meter. It definately was running on all four cylinders, albeit poorly. I have ridden a lot of different bikes and while tuning my carbs would occationally foul a plug so I know what its like to not run on all four. My plugs all had good color and I also noticed that after I adjusted the gap the bike only pulled slightly harder but not super noticeable.
The first bike I really dove into and tuned was a single cylinder 500 and I did not have a lot of the other tuning aspects of the 750 like carb synch so I am learning as I go with this one. Plus I never rode a cb750 so running good is all relative for me. Since the power was fairly smooth and my plugs appeared the same I figured I was very close with my settings. Thanks for all your help!

[seanbarney41]

READ THE BOOK

Lucky?, he is kinda strange and gets fixated on irrelevant details...but often has extremely good advice

[JWExperience]

Took it for a 25 mile ride today and noticed that a small oil leak that I had seems to have slowed or stopped since adjusting my points and obviously it runs smoother. Does this make sense that my leak would slow or even stop after this adjustment or is it most likely not related?

[lucky]

Took it for a 25 mile ride today and noticed that a small oil leak that I had seems to have slowed or stopped since adjusting my points and obviously it runs smoother. Does this make sense that my leak would slow or even stop after this adjustment or is it most likely not related?

Oil leak from where?

[HondaMan]

Set the points gap as follows:
1. Remove the sparkplugs, if you are going to turn the engine with that big nut by the points. If you pull the crankshaft backward here (CCW direction) with the plugs in, it can bend the spark advancer's mounting shaft a little bit: when this happens, you will see the spark timing on the 2-3 points "jitter" about 3-4 degrees when you shine a timing light on it later.
2. Turn the engine until you see the "T" mark for the 1-4 points, then 90 degrees CW (to your right) further. This is the "high" spot on the points cam. Set the gap at .014" right there.
3.  Turn the engine (crankshaft, that is...) another 180 degrees CW (1/2 turn) from that spot, then set the 2-3 points to the same .014" gap.

This will get you pretty close.

The mainjet in your bike is too big, if you have stock airbox, K&N filter and [any pipe, won't matter much]. It should be in the 105-110 size range. Someone was 'playing' with it before you got it, if a post-1972 750 has jets that big. The last 750 that came out with 120 mainjets was the K0 in early 1970, and THEY were too rich back then, causing fouled sparkplugs.

The "skipping" sensation you are feeling is the engine going from a decent fuel mixture in the lower speed range to a too-rich mixture when the mainjet starts working. Just back off a little on the mainjet, and the skipping will go away, and your MPG will improve.

The stock sizes are #40 pilot jet, #105 mainjet, and 7/8 to 1 turn out on the air screws of the carbs.
 

[JWExperience]

Sorry head gasket. Looks like it's weeping from the front(pucks possibly) then runs around the fins. Also notice the gasket itself looks wet all the way around but not dripping at all underneath.

Thanks HM, I'll go through everything again. Do u know what the stock clip position was on the needle? 4th down?

[Rgconner]

From what I see there is a tiny gap there. Does not take much, but any air gap will trigger the spark.

You realize that over time the points close because the cam follower, which is the part that touches the cam, is made of something soft so it does not wear the cam, but is worn down itself.

That closes the gap and means you have to readjust the points after so many miles. Less miles if you do not lube it properly or your cam is not smooth.