Topic: '75 CB750F won't start - please help troubleshooting (several questions/pics)

[uwjberg]

this is my first SOHC project - a 1975 CB750F. it wouldn't start when i bought it a couple of weeks ago, but i wanted a project and here we are. i pulled the carbs and gave them a thorough cleaning - it really needed it, as it was the most disgusting i've seen (dipped bodies, jets, scrubbed everything). i cleaned the fuel tank, petcock, etc. changed the oil, fresh battery and spark plugs. i tried to start, and it wouldn't catch (turns over fine). i checked the bowls, and fuel is getting in there, so i figured spark is the next place to go.

i checked the spark by pulling it and grounding against the head. i can barely see any spark, and it doesn't make the 'snap' sound when it fires. i checked the points, and 1-4 was out of spec (.43mm), and 2-3 was ok (.36mm). i adjusted 1-4 to match, and cleaned up the points when i was in there. still no difference.

i'm wondering if there is a problem with the coils. there are several wires that the PO zip tied up and left hanging, so i'm wondering if that may be a part of the problem as well. i took a pic of the points as well - are they too corroded to work properly? i plan on replacing them for piece of mind anyways.

take a look at the coils - would this corrosion prevent a strong spark?



here are the points - how bad are they?




what does this wire go to (if anything)?



where do these go?




thanks for any help - this forum has been a wealth of info to a new owner, and many projects have been very inspiring.

[number13]

what does this wire go to (if anything)?

It appears to be your coils ground connection.
Bolt the loose end to the frame and try starting the bike.

[TwoTired]

take a look at the coils - would this corrosion prevent a strong spark?

No.  The corrosion you see is just cosmetic.

here are the points - how bad are they?

Not bad. Need polishing/cleanup to ensure good electrical contact.

what does this wire go to (if anything)?

Add on by a PO, not Honda.  If Honda put it on, it would be green.

where do these go?

Another Non-Honda addition.  Where does it come from?  What wire colors in the Honda Harness does it attach to?  Did the bike have a fairing on it?  Radio?

Cheers,

[uwjberg]

the bike did have a fairing on it from the first owner, so maybe that's what those wires in the back are from.

so, if the corrosion isn't bad, and the points are ok, where do i go from here? is it the coils themselves? how can i test them?

[tonycb650]

Does the orange and yellow wires in the last pic connect to pos/neg on battery? It looks like the same type of connection for my battery tender.

[Jerry Rxman Griffin aka MuthaF'er]

where do these go?  That red and yellow harness could have been for an external battery trickle charger unless it is actually attached to somethin.

Check from the coils to the plugs:

Have you trimmed the plug wires back and reconnected to the plug caps?

Have you checked resistance across each plug cap?

Do so then look for identical sparks on 1 & 3 and 2 & 4 with fresh plugs. Lets easily eliminate this area of potential issue.

[uwjberg]

i haven't trimmed the plug wires back - is this as simple as cutting the old end off and reconnecting to the caps?

how do i check the resistance across the plug caps?

[TwoTired]

The stock plug caps unscrew from the ignition wire ends.  There is a screw post inside the cap.  Remove them and examine the wire ends.  If you can't tell if the internal wire is making firm contact with the caps' screw post, then trim the wire back until that contact is ensured.

While the caps are off measure the resistance from end to end.  Manny of these have 5K resistance, others have 10K resistance.

The voltage supplied to the coils has a direct bearing on spark.  Find out what voltage is being delivered to the coils.  And, don't overlook the point contact quality.  I said your points look OK, but they still need cleaning to ensure good electrical conductivity.  If you are unable/unwilling to clean them properly, then replace them with new.

Also, if you want a strong spark, you'll need a strong battery that doesn't have much voltage sag when using the electric starter.  But, you will never get the strong snap that a high energy ignition system typically provides.  The stock ignition doesn't do that.  And, it is debatable whether that is required.

Cheers,

[uwjberg]

so, how strong should the spark be, then? when i first looked, i didn't think i had spark. i had to look very closely to see the spark at all. it seemed that it should have been brighter, but i have nothing to compare it against. i just want to amke sure that i'm going down the right path here with the ignition side...

[shoemanII]

i picked up a spare engine that ran poorly, but had great compression.  when running, had gas shooting back into the air cleaner from #1.  found the valve adjustment was crazy!  adjusted and all was better.  gotta watch out for that PO guy - he can be real trouble.   

might want to check your valve adjustment, and good luck.
bobp

[TwoTired]

so, how strong should the spark be, then? when i first looked, i didn't think i had spark. i had to look very closely to see the spark at all.

It has to be strong enough to leap from the center electrode to the ground strap.  The proper fuel/air mixture ignites rather easily.

If you want to find out how much reserve spark potential you have, take a spark plug and keep increasing the gap until it won't leap the gap anymore.  But, with new plugs and properly set ignition, it will likely leap well beyond a .030 gap for both firing plugs.

Gotta have good points and and a fully charged battery, though.  The higher the voltage applied the higher the spark voltage out.

You could try disconnecting the condenser to see if the spark has an intensity more to your liking.  If so, replace the condensers.  Maybe, they've gotten too leaky.

Cheers,

[uwjberg]

well, i just tried the plug wire thing - cut 1/4" off the end, then reconnected while checking the screw in the cap. fired up after a few tries. running pretty rough, and doesn't like to run without the choke on, but at least it fires up now!

cam chain tensioner and valve adjustment next on deck.

thanks for the assistance, everyone.

[Jerry Rxman Griffin aka MuthaF'er]

One less component of a tune up to do now and it didn't cost anything or require any special skills. Every little thing helps. Do you have an electrical meter to test the resistance of your caps? This is simple and costs nothing too. Your meter must show a setting for ohms. If you don't have a meter one can usually be had for 10 bucks or less. Lets also eliminate this potential issue. My plug caps were ok to shot when I first checked them. Too much resistance and the electrical current will not pass through. Since you've already found one simple electrical issue you know it happens. Do the electricals before the mechanicals.

[Hush]

These old Hondas love choke, mine needs full choke and idle for 5 minutes before she settles down, after that they are all good.
Those strange wires need to be traced back to their source, if they are power wires put a female plug on the end (one with a plastic cover) just incase they had a good reason to be there.
Some will only come live after the ignition comes on so watch those, just because they aint got power now don't mean they wont be live once you are running.
Earth wires are GREEN Honda like the idea of grass/earth green so if it's only green it will be an earth unless the PO was total moron and spliced green into power.
Best of luck with your new baby....we watch with interest.