Author Topic: Bike won't start, please help! (Read 1305 times)

Jonm99 · « **on:** December 13, 2019, 11:04:40 AM »

Hi all,

My 1977 cb750k was running just fine until my lest session working on it 🙄. I did quite a lot so several variables to consider, would appreciate some help troubleshooting the problem.

So here goes:
Installed a new dyna electronic ignition. Bike fired up, but just for a second or so before I shut it down (didn't have the airbox in so didn't want to leave it running). I then proceeded to the carb rebuild, cleaned them thoroughly, new float needles, checked float levels (measured 12.5mm + clear tube method), cleaned the plugs (they are basically new, just cleaned them up for good measure after running the bike a few times), reinstalled everything. Bike wouldn't fire, checked voltage on the battery and noticed it was low. Water level on one of the battery plates was very low, filled it up, but couldn't get a charge above 12.2. Bought a new battery (yuasa ytx14ahl-bs) but still wouldn't fire up (12.6v after a couple of hours off the charger). Hooked up jumper cables to my car battery (car off) just to rule out the battery being the culprit, still wouldn't fire. Rolled the bike down a hill and released the clutch, this time it almost started (barely firing), but ultimately did come to life.

A few more things to note:
Electics are in order, everything is working fine with switch on.
It rained pretty heavily the other day, bike was under a cover but still was quite damp in the morning (but that was a couple of days ago so...)
I checked for spark on the plugs; they seem quite faint, but I've never seen what a good spark looks like, so hard to say.
There's definitely gas in the carbs.
I'm getting power to the dyna with switch on.
I tried switching between the yellow and blue wires connected to the dyna, but that didn't change anything.
Kill switch is working (no power to coils when off and vice versa).
Same result with the kick as with the electric starter (nothing).

So here are my questions:
Is it more likely that the problem is with the ignition or something I did wrong when rebuilding the carbs?
Before testing with my old points (pita), what else can I do to verify that the ignition is not the problem?
Could the new float needles somehow be the issue? Clear tube method shows gas at the gasket height. Is that not enough fuel?
Could this be a ground issue? Everything else (electrically speaking is OK).
Anything else I should be thinking about?

Apologies for the long rant, thanks in advance for your replies!

69cb750 · « **Reply #1 on:** December 13, 2019, 11:17:21 AM »

Install original points plate.

minimo · « **Reply #2 on:** December 13, 2019, 11:19:55 AM »

Make sure battery is at full charge. 12V seems low. 12.4V is nominal.

Try a kickstart and negate the push-start ignition. If that gets it going, get bike warmed up a little and then turn off to see if push-button starts then. If no click, clean connections.

Sent from my iPhone using Tapatalk

tlbranth · « **Reply #3 on:** December 13, 2019, 05:35:08 PM »

When I saw 'Dyna' I was ready to move on. But to determine if it is indeed a spark problem do this: Pull one or two plugs. Squirt a very small amount of fuel into the cylinder(s). Replace plugs and give it a go. If it fires momentarily, you have a fuel problem. If not, it's electrical. If it's electrical, put the original stuff back, buy a Hondaman box and throw the Dyna into the neighbor's yard.

calj737 · « **Reply #4 on:** December 13, 2019, 07:01:45 PM »

Can you be more specific as to "won't start"? Is it cranking over strong but won't fire? If you have plenty of voltage through a new battery and a jumped battery for surplus, and its cranking strong, then your issue is probably fuel related.

So, open the petcock and allow the carbs to fully fill. close the petcock. Drain each carb bowl and measure the volume of fuel. Is it equal among them? Have you verified the Clear Tube method with the carbs on the bike? Check the outer two carbs. If they're correct, likely the inner two are as well.

If the Dyna fired your bike after install, then I doubt it is the issue. You could pull a plug now and smell it. Is it wet? Smell like fuel?

Deltarider · « **Reply #5 on:** December 13, 2019, 11:28:20 PM »

Quote from: calj737 on December 13, 2019, 07:01:45 PM

So, open the petcock and allow the carbs to fully fill. close the petcock. Drain each carb bowl and measure the volume of fuel. Is it equal among them?

Not a reliable test, I'm afraid, in a situation where one fuel tube supplies gas to two carbs. When you drain the first of the two carbs, there will always be more fuel because you have also drained the fuel tube. When you then drain the second carb there's no fuel left in the tube and you collect only what was in the float bowl. This makes quite a difference in quantity.

Jonm99 · « **Reply #6 on:** December 14, 2019, 12:15:49 AM »

Bike is cranking over fine, just won't fire. I too have the impression it's a fuel issue (or maybe that's just wishful thinking...); maybe something to do with the carbs. Could it be wrong size floats that are not allowing flow? @calj737 I'll try the squirt method, and report back asap. Thanks for the suggestion. If it doesn't fire, I'll swap the old points back in and give it another go.

PeWe · « **Reply #7 on:** December 14, 2019, 12:18:03 AM »

Fuel and or ignition
Have extra spark plugs?
Connect one on the cyl 4 and start it. Any visible arc?

If not, check if coils get 12V.
The 2 wires from ignition might be disconnected from main harness. Blue, yellow.

Kill switch on handle bar can cause issues too.

Starter spray easy to use to check if no fuel is involved.

Jonm99 · « **Reply #8 on:** December 14, 2019, 04:20:41 AM »

@PeWe can you explain what you mean with the extra plug? Leave the #4 plug in and connect a 5th plug to the plug cap while grounding the plug to the engine? If so, why can you not use the plug that's in the bike for this?

Coils are getting 12v.

Kill switch is functioning properly (power to coils when on no power when off).

PeWe · « **Reply #9 on:** December 14, 2019, 04:36:34 AM »

You disconnect plug cap on cyl 4, connect it to a spare plug that you make sure has contact with head or valve cover. Easier than remove plug and do the same thing.

calj737 · « **Reply #10 on:** December 14, 2019, 11:32:43 AM »

A thought crossed my mind. Are your plug wires routed correctly to the proper cylinder from the correct coil? One coil serves cylinders 1/4 and the other 2/3. Do that incorrectly and your bike won’t fire.

robvangulik · « **Reply #11 on:** December 14, 2019, 12:58:08 PM »

Quote from: calj737 on December 14, 2019, 11:32:43 AM

A thought crossed my mind. Are your plug wires routed correctly to the proper cylinder from the correct coil? One coil serves cylinders 1/4 and the other 2/3. Do that incorrectly and your bike won’t fire.

Or just run on 2 pots....

Jonm99 · « **Reply #12 on:** December 15, 2019, 03:51:35 AM »

OK, definitely a fuel issue. Sprayed some carb cleaner (didn't have starter fluid) down the intakes, fired up momentarily (thanks for the suggestion calj737. Curiously enough, it didn't fire when I sprayed into carb 1 but worked on carb 4. Anyway, I guess focus is on the carbs now, but what did I do wrong when I rebuilt them? They were clean before I took them apart, but I did it anyway because I had a sticky float valve on carb 4 and a leak into the throat of carb 3. Both of those issues seem to be resolved with the replacement of the float valves.

Can these be wrong sized float needles (they are bike specific from 4into1 and look identical to the original ((rubber tip))?
Can my float measurements be off to the point where it won't start? I double checked with the clear tube method, so...
Something else? Maybe I installed the throttle valves backwards/wrong orientation? Can anyone confirm which side the sharp angle of the cutaway should be? Inbox side or engine side?

Thanks again for all your help.

minimo · « **Reply #13 on:** December 15, 2019, 05:45:26 AM »

Make sure the throttle valve cutouts are facing the choke valves. Also, are you sure the needle heights are consistent across all four? Third from top is stock. Also, did you maintain the use of original carb parts or is it a mix of Keyster and Keihin? The needle tip and profile of the needle are different between those brands and it’s best to keep it consistent if you’re replacing any of those parts - needles, main jets, and emulsion tube especially.

Sent from my iPhone using Tapatalk

Jonm99 · « **Reply #14 on:** December 15, 2019, 06:45:13 AM »

Cutaway facing choke valves means that the gap between the throttle valve and carb throat is larger on the choke side than it is the engine side? If so, that's how I have it set up.

Regarding needle position, I actually went up on (so it's 2nd from top now) as I swapped out pods for the original airbox. I was under impression that stock for the k7 was top position? Either way, would one position off keep me from starting the bike?

As for the swap of parts, I did not swap anything between when the bike started and nok longer started, except for the float needles.

Maybe I did not push the jets in all the way? Can anyone confirm that using the clear tube test, gasoline should be at the gasket level between the bowl and the main part of the carb? When swapping the needles I had to significantly bend the lip of the float to maintain the 12.5mm height...

calj737 · « **Reply #15 on:** December 15, 2019, 07:34:34 AM »

You have it correct for the Clear Tube method. Ideally, "just" below the gasket is ideal.

If you pull a plug please, smell it. Is fuel reaching the cylinder?

PeWe · « **Reply #16 on:** December 15, 2019, 07:37:38 AM »

Fuel flowing from tank? Maybe time for reserve?

minimo · « **Reply #17 on:** December 15, 2019, 11:14:18 AM »

Good on the cutaways. Are you sure?
Remove your pods and scan across each opening where you had your AirPods and take a peek. Make sure you got that little cutaway facing you

I worry about the replacement of just the float needles. Not the valve seats? This too may introduce a change in needle taper profile as the tip and shape of the needle valve may be different from the cavity of the valve seat which may be enough to affect how that system works. If you bought the kit that came with the corresponding float valve seats, use those. Otherwise, go back to the OEM.

Also, raising the needle clip position will create a leaner condition. If anything, especially with AirPods you would probably want to lower the clip position to allow for more passage of fuel at around 1/8-1/4 throttle. Check to see what your model is stock position.

To get a better sense of where we need to be at idle (and beyond), this chart helps:

You seem a little defiant on checking the plugs as the others have asked. Doing so will be telling of whether you are indeed getting fuel (too much or not enough) to your plugs. Check your plugs.

Sent from my iPhone using Tapatalk

Jonm99 · « **Reply #18 on:** December 16, 2019, 04:22:23 AM »

@calj737 - all 4 plugs smell of fuel. That said, plug #1 looks surprisingly clean relative to the others and plug 3 particularly funky (considering it only fired twice since cleaned, and only for a second or so). Please see attached pics...

@PeWe - fuel is definitely flowing from the tank.

@minimo - regarding the cutaway, I did have a slight hesitation regarding orientation when assembling (I remembered it reversed, but looking at the parts fische, it seems to be slit to the left with arm facing away and larg(er) cutaway facing the choke valves. I've attached pics from both sides (took carbs off for further inspection in the meantime).

As for the float needle seats, I don't think they are replaceable on the 77 carbs, hence the rubber tipped needles.

Needle clip positon did go up one from before during the carb clean (it was at middle position, now 2nd from top), but the reason for doing this was because I removed the pods and went back to the stock airbox. That said, I am running a 4:1 exhaust system. Maybe I should revert to the middle position and see if makes any difference? Although, from the chart you sent (thanks for that!), it wouldn't seem to have any impact on idle (or startup for that matter).

To be sure, if I got a momentary fire up while squirting carb cleaner down the intake, it cannot be a weak battery, right? I purchased and installed a new yuasa, so unlikely, but at this point, I feel a little lost...

Thanks again for everyone's input.

calj737 · « **Reply #19 on:** December 16, 2019, 04:30:22 AM »

If fuel is hitting the plugs, it's an ignition problem. This is not a battery problem. Scratch that off the list.

Verify your plug wires are routed correctly as mentioned. Verify that you get 12v at the coil with the RUN/KILL set to fire. Replace your plugs outright please. And check their gap.

Needle position is fine, it being a tad off is a tuning issue not a firing issue. 4:1 pipes with a stock airbox doesn't require a needle change normally. Are you operating the bike with choke to start it?

Jonm99 · « **Reply #20 on:** December 16, 2019, 11:20:24 AM »

I'm getting power to the coils when the run kill is set to fire - that said, it was sub 12v so I'm charging the battery again tonight.

I checked for continuity between the coils and the points - both OK.

I tried swapping the blue and yellow wires coming out of the ignition, that didn't change anything.

I am indeed not operating the bike with the choke on, although it was not necessary before (usually fires up very easily). Anyway, at closer look, I noticed that the fast idle was not set properly (choke does not engage the throttle, so I'll fix that while I'm in there, and try again.

Just ordered new plugs. In the meantime, I'll check the gap on what I have in the bike and report back asap.

If we are back to suspecting ignition (can the smell of fuel on the plugs not be from before I pulled the carbs?), I'll recheck the timing on the electronic ignition. If it's still accurate, I suppose next step would be to swap back in the original points and see if it fires?

calj737 · « **Reply #21 on:** December 16, 2019, 02:53:00 PM »

If you crank the bike and it doesn’t fire, pull a plug. Fuel smell should be strong and evident. Maybe even wet. Is that the case here?

Also, please confirm the plug wires are routed correctly.

Jonm99 · « **Reply #22 on:** December 18, 2019, 03:49:49 AM »

Will have new plugs tomorrow. I will also check the plug wire routing. Will update very soon.

Thanks again!

Jonm99 · « **Reply #23 on:** December 19, 2019, 07:00:29 AM »

Swapped in new plugs and checked the routing on the plugs (2,3 to the right coil, 1,4 to the left coil). Bike still won't fire. I didn't have the right size screw driver to adjust the fast cam (which is not engaging when I pull the choke), but I gently pulled the throttle when starting the bike, which I believe does the same thing essentially. Battery had 12.8v when I put it into the bike.

Any suggestions for next steps here? Could something have crawled into the bike and died when I had the carbs off (although I'm pretty sure intakes were sealed)? Should I swap out the dyno ignition for the original points?

Don R · « **Reply #24 on:** December 19, 2019, 07:38:49 AM »

If the cylinders are too wet with fuel it will wet new plugs instantly, you might try leaving them out long enough for fuel to evaporate. No water in the fuel? I bought one with water in the gas once. Assuming you see spark, it takes spark fuel and compression. If you have all three and it won't start then the timing must be occurring at the wrong time.

minimo · « **Reply #25 on:** December 19, 2019, 03:45:59 PM »

Are you sure you’re getting spark across all 4 plugs?
Sometimes I pull the plug boots off and snip off about an 1/8” off of the ends to maintain a fresh connection; spark wires to the boots to the plugs.

Did you confirm spark across all 4 plugs?

If I were you, to rule it out, I’d rebuild the carbs and insure that all Jets and Floats, seats and o-rings are fitted and adjusted nicely. Also make sure you’re consistent on your air/fuel mix screws.

Gene · « **Reply #26 on:** December 19, 2019, 03:52:50 PM »

Put the points back on and see if that solves it. Just so you can mark it off the list.

minimo · « **Reply #27 on:** December 19, 2019, 04:04:14 PM »

Also, on the Dyna S, make sure that the magnet is facing the correct direction - #7 on the following instructions:

https://articles.sohc4.net/Performance/DynaS_Ignition_Installation.PDF

Quote

A magnet approximately 1/4 inch in diameter will be noticed on one side of the rotor. The magnet must face to the left when the 1/4 side on the advance assembly is pointing up...

This magnet is usually the round casting mark on the Dyna S rotor. Put something steel on it to make sure it is magnetic.

Sent from my iPhone using Tapatalk

Jonm99 · « **Reply #28 on:** December 20, 2019, 07:25:30 AM »

And we have fire! Apparently, timing on the 1-4 cylinders was off. When I set it, I lined up with the advance mark while twisting the magnet forward, but what I did not do was check that the timing light lights up on F without me twisting the magnet. I could not get them both spot on, F is perfect now, T close enough, I suppose.

Thank you so much everyone for your help in figuring this one out.

Regarding the choke, the set screw is not engaging with the cam even when I've screwed the set bolt out to the max (anymore and it will fall out of the nut). I suppose I could simply swap the bolt for a longer one, but I wonder if I'm just tackling the symptom and not the problem. I don't think this was a problem before; is there any other adjustment I am missing that could impact distance from the set screw fitting and throttle cam?

News:

Author Topic: Bike won't start, please help! (Read 1305 times)