Topic: Any ideas...? Could use some help...

[Holshot]

Hello guys,

I’ve been trying to figure out what’s keeping my beloved CB750 from running now for awhile and was hoping someone might be able to help or recommend some areas to check.

So, the bike doesn’t want to start but occasionally she does finally come alive after some prodding but barely and only on cylinder 3. Barely stays running then dies out. I’ve checked fuel delivery, even flushed out all the fuel and started with fresh non ethanol fuel to rule out bad fuel. Pulled plugs after and see the dead cylinders are wet so again I think fuel isn’t an issue. Even tried using starter fluid to rule out a fuel issue.

She has a newer battery, newer plugs I’ve checked timing and points which are spot on. I’m leaning toward a coil issue possibly but if 2 cylinders were dead and 2 were running that would be more plausible I think but since only cylinder 3 is running that would tell me that if it was a coil issue then cylinder 2 would be running? If I understand it correctly 1/4 have one coil and then 2/3 are on another coil. Is it possible that one coil could work on one cylinder and not the other?

Is this an issue that any one has seen before or tell tale signs pointing to one thing or another? It’s so weird to me that she will run on just one cylinder if only for a small amount time and not any other cylinder. Also, I’ve checked spark and it does seem to have spark on all 4 cylinders while trying to start. Whether or not the spark is strong enough to ignite the air fuel mixture is another story... I’m not sure how to check the strength of the spark to see if it’s up to snuff...

Again, any help and recommendations are greatly appreciated!

Thanks,
Joe

[Holshot]

And the mystery deepens... so after trying yesterday and killing the battery I threw it back on the trickle charger last night. So I just came in from the garage to give it another try and she fired up after a few cranks but again only on one cylinder. But this time and for the first time, it was running on cylinder 4 not the usual cylinder 3. Prior to today it’s always started on cylinder 3 while all the rest were dead. Now cylinders 1-3 were dead and number 4 was running... strange.

Any ideas?

[madmtnmotors]

Typically either fuel or spark so you have to diagnose. Clamp the inductive pickup from a timing light on each individual spark plug lead one at a time and check for spark while cranking to verify each cylinder is firing. Just check to see if it will light the bulb, no need to try and dynamic time it while just cranking. Once spark on all four is verified then grab a can of starting fluid and spray the intake while cranking (SPARINGLY) and see if things improve on spray. If starting fluid improves things then careful cleaning of the carbs is in order. Especially if as you say "keeping from running now for awhile". Carbs get plugged when they sit too long, you got to run it to keep it running right. 

[jgger]

Are you sure the float level is good in all carbs? I would think if the bike is on the side stand carbs 3&4 would be uphill from the others, so maybe less fuel. Too much gas will cancel out the spark plugs action.

Maybe dry the plugs, and drain a little bit of fuel from the carbs and try to start it with the petcock turned OFF and no choke. Also do the clear tube test on the carbs.

If you leave it parked with the fuel on they can leak. Check your oil for fuel odors.

[Holshot]

Typically either fuel or spark so you have to diagnose. Clamp the inductive pickup from a timing light on each individual spark plug lead one at a time and check for spark while cranking to verify each cylinder is firing. Just check to see if it will light the bulb, no need to try and dynamic time it while just cranking. Once spark on all four is verified then grab a can of starting fluid and spray the intake while cranking (SPARINGLY) and see if things improve on spray. If starting fluid improves things then careful cleaning of the carbs is in order. Especially if as you say "keeping from running now for awhile". Carbs get plugged when they sit too long, you got to run it to keep it running right. 

Thanks for the suggestions... I’ll have to get a time light and try that. But like I mentioned earlier I believe I’m getting spark at each bank because I’ve pulled one plug at a time can see the spark arc against the valve cover screws. Whether or not it’s enough spark is the question I suppose.

I did try the starting fluid trick last night and still only got it to start on cylinder 3...

I go back to thinking it maybe electrical as I mentioned on my follow up post today cylinder 4 was the only one to fire up and not the normal #3. So it’s moved over one cylinder since last night but again only one cylinder fires and runs.

[Holshot]

Are you sure the float level is good in all carbs? I would think if the bike is on the side stand carbs 3&4 would be uphill from the others, so maybe less fuel. Too much gas will cancel out the spark plugs action.

Maybe dry the plugs, and drain a little bit of fuel from the carbs and try to start it with the petcock turned OFF and no choke. Also do the clear tube test on the carbs.

If you leave it parked with the fuel on they can leak. Check your oil for fuel odors.

I have to admit I didn’t know that the floats would be something to check. When I had the bowls down they all seemed to be at the same height and moved freely with no hang ups. I actually keep the bike on the center stand usually and no leaks seem apparent.

Can you explain what the clear tube test is?

[jgger]

You connect a piece of clear fuel line to each drain screw with the hose being long enough to be above the top of the carb. Hold the ends of the lines up so the fuel doesn't just run out, then turn on the petcock.  Compare the height of the fuel to each carb it is attached to, this tells you the actual level of the fuel in the bowls. Also compare them to each other.

If the fuel level is too high it can run into the cylinder  and flood it and eventually bleed into the crankcase,  that's a bad thing! Before you make any adjustments on the floats make sure the needle is able to stop the fuel flow, there might be a small amount of debris stuck in there.

Any hoo, good luck. 

I think if you look in the tricks and tips the clear tube method is covered there better than I spliced it.

[madmtnmotors]

I believe I’m getting spark at each bank because I’ve pulled one plug at a time can see the spark arc against the valve cover screws. Whether or not it’s enough spark is the question I suppose.

That test works just as well. Just have to eliminate the possibilities one at a time. You have verified spark, so if static timing is close, then the next most likely culprit is fuel. The clear tube test is a good indicator of fuel level in the bowls.

[Holshot]

Well I have a new theory... let me know what you think. Last night right before a called it a night I pulled plug #4 just to see. It was wet/black and I left it open to air. I cleaned up the plug this afternoon stuck it back it and tried to start the bike on a fresh charged battery.

She fired up and now ran on just cylinder 4 and not the usual cylinder 3 like in the past or last night. After seeing the bike now ran on #4, I pulled the plugs from 1-3 and they were soaked and maybe why they couldn’t fire because they were flooded.

So maybe it’s getting way to much fuel on startup and just fouling out the plugs. What should I look for or do for leaning out or backing down the fuel?

Thanks!

[madmtnmotors]

What should I look for or do for leaning out or backing down the fuel?

The clear tube test is a good indicator of fuel level in the bowls.

[Don R]

 If the cylinders are wet with gas, good gas or bad gas they will foul the plugs. Let the cylinders dry out. Some new plugs can't hurt either. Drain the bowls and tank then refill with fresh fuel.

[Holshot]

What should I look for or do for leaning out or backing down the fuel?

The clear tube test is a good indicator of fuel level in the bowls.

Thanks Wilbur, I will try and find some clear tube that will fit. Just to. Be clear we are talking about the bottom of the bowls (overflow drains) correct?  Also, if see the levels are inconsistent from one to the other how do I know which one is outand how to adjust?

Thanks again for checking back and responding! That goes for everyone... much appreciated!

[Holshot]

If the cylinders are wet with gas, good gas or bad gas they will foul the plugs. Let the cylinders dry out. Some new plugs can't hurt either. Drain the bowls and tank then refill with fresh fuel.

Thanks Don, I did pull the tank yesterday and the bowls and drain all the fuel from the system. Replaced it with fresh non ethanol fuel like I do with every fill up. The bottom of the bowls were spotless, no dirt, grit or grime. I did manage to get another set of plugs today at the parts store so I figured I would swap them out as well but if the bikes running to rich im afraid the same thing is going to happen all over again. I’m going to try the clear tube test like others have mentioned. Hopefully I can find a write up of how exactly I’m supposed to do it. If anyone knows of a link to where it might be that would be greatly appreciated! I’m not a very computer savvy guy soooooo..... haha

Thanks again!

[madmtnmotors]

I will try and find some clear tube that will fit. Just to be clear, we are talking about the bottom of the bowls (overflow drains) correct?

It depends on which year and model carbs you have. The older models with "round tops" require the fluid reading be made via a customized drain screw since the drain and overflow circuits are separate. The later PD (or "keyhole") carbs were changed to combine the drain and overflow circuits so they merely require a tube attached to the drain/overflow.

[jgger]

This pic isn't real clear, but you can get the idea of what you're trying to do. I hope it comes out I'm not too good at doing this picture stuff!

[jgger]

Woo Hoo, I did it. Hope that helps there Holshot!

[Holshot]

Thanks buddy, I really appreciate the help! When I unscrew the fitting is there a barb that will accept the hose or does it require me to screw in place a special fitting to accept the tube.

Also, if I haven’t ask enough questions, do you know what size hose it requires?

[jgger]

What a lot of guys do is get vac hose fittings, the plastic ones from the auto parts store and force thread them into the hole. You can screw the plastic fittings in pretty tight so they don't leak and not damage the bowl....................OR. I think it was Scottly who said you take your clear tube and hold it over a flame and stretch it as it starts to melt. It will develop a nice taper, cut it and you have the ends for 2 tubes.

[Holshot]

Ok, I pulled the relatively new plugs and swapped them in with another new set. Did the clear tube test and saw that 1,2 and 4 were all even about a quarter inch below the base of the carb/top of the bowl. Number 3 just puked out fuel non stop. So pulled #3 bowl and pulled the float down. The needle had a slight line midway down the cone section so I cleaned it up with some 1500 grit sandpaper and spayed the internal spring with some carb cleaner. The float appeared to be bent and crooked so I pulled #4 float and bent it back to match up and reinstalled everything and she fired up on the first crank!

So, the bike is now running! Just in time for the local bike week which is one of the bigger bikes weeks here on the east coast... unfortunately, the weather is not cooperating. First thanks to everyone who has chimed in and tried to help with all the ideas!

But now, I’m wondering what caused all the problems to begin with? I’m afraid this could all happen again. I think it’s running really rich and not sure what the proper way is to determine if that is in fact the case and then if so, how to address it. Is it a jetting issue, is it those little bleed screws that need to be tuned. What way would you guys go about trying to diagnose/fix the issue?

Some back story, prior to all this the bike in order to start required me to fiddle with the main throttle plate screw. I had to open up the throttle slightly to start and then let it warm up then the idle would start to ramp up and then I would back it back. The bike would hold rpm’s after warmed up. While riding the bike it road fine but under light/part throttle in lower rpm the bike would stumble, stutter slightly but as soon as I rolled into the throttle it cleaned up immediately and pulled strong throughout the rpm range.  On decel, she pops and gurgles.

Also, I’m now also remembering that a few months before she stopped starting me a few times I tried starting her up she was running down on 1or 2 cylinders and required me ride it up the street and giving it lots of throttle input and then it would start to clear up and I could tell by the sound she was  finally running on all 4 without heavy stuttering and stumbling. Does that sound like a rich condition? Perhaps to rich to ignite the fuel?

Again, anything I’ve written sound familiar or tell tale signs of what to do next?

Thanks again to all of those who have taken the time to read and respond!

[HondaMan]

What do you have the air screws set to? They should be about 1 turn, plus or minus 1/8 turn, unless you live at high altitude (like here in Colorado). If you are above 4000 feet, set them between 3/4 and 7/8 turn instead.

[Holshot]

What do you have the air screws set to? They should be about 1 turn, plus or minus 1/8 turn, unless you live at high altitude (like here in Colorado). If you are above 4000 feet, set them between 3/4 and 7/8 turn instead.

I haven’t touched them. I was afraid to start fiddling with things and potentially making things worse. I was hoping to get some ideas first and then do some research on how to do it. So you suggest I try that next?

Thank you so much for replying and trying to help!
If you could verify the process, screw the air screw clockwise till it bottoms out, then counter clockwise one full turn? Btw, I’m at sea level.

[Holshot]

Ok, so I tried starting the bike today and she fired up fairly easily but of course I had to manipulate the throttle and play with the large throttle screw to keep her from stalling. After a few minutes the revs started ramping up and then I backed off the screw to around 1,100rpms.

Anything less then that rpm wise she will hunt a little and start to signal she’s about to stall. Took her around the block a few times brought her back and let her cool down a little bit. Found the online manual which stated to screw the air screws closed then back it out one full turn. After doing this on all 4 carbs I tried starting the bike but she wouldn’t start or even try starting. She would just crank and crank. After about 40 minutes of fiddling around and backing out the air screws almost 2 whole turns out. She now fires up again but again, she does seem to want to stay running unless I play with the throttle and throttle plate screw.

Side note, after reading the online manual it shows the incorrect float bowl heights can cause excessive rich running conditions especially off idle conditions. In the picture diagram it shows using a special measuring tool but I’ve seen some use a make shift float level tool using a credit card with a cutout of 26mm. My question is can this be performed with the carbs mounted? The drawn illustration shows the single carb at a horizontal position, slightly dipped down...

[madmtnmotors]

Ok, so I tried starting the bike today and she fired up fairly easily but of course I had to manipulate the throttle and play with the large throttle screw to keep her from stalling. After a few minutes the revs started ramping up and then I backed off the screw to around 1,100rpms.

Anything less then that rpm wise she will hunt a little and start to signal she’s about to stall. Took her around the block a few times brought her back and let her cool down a little bit. Found the online manual which stated to screw the air screws closed then back it out one full turn. After doing this on all 4 carbs I tried starting the bike but she wouldn’t start or even try starting. She would just crank and crank. After about 40 minutes of fiddling around and backing out the air screws almost 2 whole turns out. She now fires up again but again, she does seem to want to stay running unless I play with the throttle and throttle plate screw.

Side note, after reading the online manual it shows the incorrect float bowl heights can cause excessive rich running conditions especially off idle conditions. In the picture diagram it shows using a special measuring tool but I’ve seen some use a make shift float level tool using a credit card with a cutout of 26mm. My question is can this be performed with the carbs mounted? The drawn illustration shows the single carb at a horizontal position, slightly dipped down...

What choke setting are you using on startup (both when hot and when cold)?

[jgger]

If your clear tube test showed the fuel in the bowls at the proper height, then don't mess with credit cards or any other gages. The clear tube shows the actual fuel level in the carb bowls.......you can't fix something that is already fixed.

What do your spark plugs look like now, have you checked them?

[evanphi]

What you're saying about your startup procedure is normal. Remember these do not start like modern bikes. You have to work the throttle when starting and to keep them alive for a minute or two. These old gals need a bit of foreplay!

Once you have the bike running again and well warmed up, set the idle as low as it'll go before it starts to chug... so right around 1000 or 1100RPM. Remember these old gauges aren't exactly accurate either. Then don't touch the idle screw again! Use the throttle to keep it alive until it is warmed up.

My starting procedure, FWIW:
1. Gas open, choke CLOSED (lever FLAT, same as the ground). Press starter and give a gentle twist of throttle.
2. Once it catches, keep RPMs around 3K using throttle and close choke all the way.
3. Keep throttle held for a few seconds, then lower to about 1.5K, shift to first, and try riding off.
4. Don't let throttle off too far or idle to get too low as it'll probably die out. A few stop signs later you should be all warmed up.

[HondaMan]

Ok, so I tried starting the bike today and she fired up fairly easily but of course I had to manipulate the throttle and play with the large throttle screw to keep her from stalling. After a few minutes the revs started ramping up and then I backed off the screw to around 1,100rpms.

Anything less then that rpm wise she will hunt a little and start to signal she’s about to stall. Took her around the block a few times brought her back and let her cool down a little bit. Found the online manual which stated to screw the air screws closed then back it out one full turn. After doing this on all 4 carbs I tried starting the bike but she wouldn’t start or even try starting. She would just crank and crank. After about 40 minutes of fiddling around and backing out the air screws almost 2 whole turns out. She now fires up again but again, she does seem to want to stay running unless I play with the throttle and throttle plate screw.

Side note, after reading the online manual it shows the incorrect float bowl heights can cause excessive rich running conditions especially off idle conditions. In the picture diagram it shows using a special measuring tool but I’ve seen some use a make shift float level tool using a credit card with a cutout of 26mm. My question is can this be performed with the carbs mounted? The drawn illustration shows the single carb at a horizontal position, slightly dipped down...

If it won't run 'till 2 turns out, the float level is too low.