Author Topic: 78 cb750 cuts out at high rpm (Read 5247 times)

750Moto · « **on:** June 24, 2018, 06:34:04 PM »

I have been working on rebuilding a 78 cb750 and turning it into a cafe racer for a while now, Ive had a few issues along the way but now I have a new one that really baffles me.

For a while now my bike has had a weird dead spot at around 5000 rpm (no tach just my guess). The bike will seem like all four cylinders are not firing but it will cruze along just fine, as soon as I give it gas with no hesitation it will kick all 4 cylinders in and take off like a rocket, same goes if I lower the rpm a bit all 4 cylinders kick back in.

That went on for a while and I just rode the bike that way as I just wanted to get out and enjoy the weather. Well the other day the bike just cut out to what seems like 2 cylinders when I was accelerating toward higher rpm. I limped it home on intermittent 2 or 3 or 4 cylinders and since then the bike has been un-rideable, it idles and runs on 4 cylinders but if I wack the throttle it cuts to 2 cylinders, even if I slowly rev to higher rpm it eventually cuts out.

After cutting out the bike will run on that many cylinders for a while even if I lower the rpm, then magically all 4 will kick back in.

I'm stuck at this point I know it's not the coils, points, condensers, regulator, or rectifier as I have changed all of those. I'm leaning twoard a ground fault maybe in the ignition switch but I have no clue at this point. I believe it's not carburation because while it runs on 2 or 3 cylinders it throws flames and pops as gas is getting into the cylinders and then into the exhaust to ignite.

If anyone has anything I could check or any experience with an issue like this I would greatly appreciate it. I just want to ride and enjoy the weather

American Locomotive · « **Reply #1 on:** June 24, 2018, 08:54:19 PM »

Did you change all of those before or after the problems started? Just because you changed all of that before, doesn't mean they're still good. My bike did similar things (including flames), and it turned out to be dirty carbs. A lean misfire can still send unburnt fuel into the exhaust.

jgger · « **Reply #2 on:** June 24, 2018, 09:52:23 PM »

Have you looked at the plugs yet?

750Moto · « **Reply #3 on:** June 25, 2018, 06:27:55 AM »

Changed all of those parts after the issue started trying to remedy it with no luck, I have had the plugs out and tested them they produce spark, I assume it's enough spark as they are only a few months old

flybox1 · « **Reply #4 on:** June 25, 2018, 06:41:15 AM »

Intake, exhaust, jetting, and needle clip position(shims)?
when was the last valve adjustment, timing check/adjustment, cam chain adjustment, and carb vacuum sync?

750Moto · « **Reply #5 on:** June 25, 2018, 11:29:20 AM »

Carbs and motor were completely rebuilt, chain and valves adjustment have less than 1000 miles on them, the carbs have been rebuilt and have had everything replaced, the bike ran perfect for a while with the carbs like that, they were just bench synced for now but it ran great has a new inline fuel filter and has never sat for more then a few days without being run so doubt anything has gotten dirty. This is unlike any carburation issue I have ever seen before I think it's an electronic issue.

flybox1 · « **Reply #6 on:** June 25, 2018, 11:41:37 AM »

rebuilt by whom?
how do you know it was jetted properly based on your intake and exhaust?

..too many unanswered questions

PeWe · « **Reply #7 on:** June 25, 2018, 11:59:06 AM »

Bad 12V connection to coils? Coils bad?
I could have sudden power loss due a to a bad HT wire. Replaced them all
Some blips at high rpm, Points + Hondaman ignition module without condensers. Condensers back in fixed it.

750Moto · « **Reply #8 on:** June 25, 2018, 12:28:39 PM »

I'm leaning towards more things like pewe said, my mixture is good I have done plug chops and all is well across the rpm range. The motor was rebuilt by me and my dad who have both rebuilt multiple motors, even a few other 750s, none of that is of any worry to me

flybox1 · « **Reply #9 on:** June 25, 2018, 01:01:41 PM »

Quote from: 750Moto on June 25, 2018, 12:28:39 PM

I'm leaning towards more things like pewe said, my mixture is good I have done plug chops and all is well across the rpm range. The motor was rebuilt by me and my dad who have both rebuilt multiple motors, even a few other 750s, none of that is of any worry to me

Yeah..ok.
I'm still gonna put a plug in for in correct pilot jet size and needle (shim) position, but since you've decided to omit the pertinents (intake and exhaust config, jet size and brand, etc.), I cant be certain.
Good Luck.

750Moto · « **Reply #10 on:** June 26, 2018, 06:12:50 AM »

The pilot jet and needle shims are low rpm adjustments, if you read the title the bike cuts out at high rpm, the pilot jets are not very relevant at 6000 rpm where this issue is occurring. I had the bike running flawless with this configuration of jets prior to my current issue occurring, nothing about the issue screams fuel it is an ignition cut off almost like a coil getting grounded

flybox1 · « **Reply #11 on:** June 26, 2018, 08:46:53 AM »

Quote from: 750Moto on June 26, 2018, 06:12:50 AM

The pilot jet and needle shims are low rpm adjustments, if you read the title the bike cuts out at high rpm, the pilot jets are not very relevant at 6000 rpm where this issue is occurring.

I did read the title and this ^ statement is incorrect.
the pilot jet supplies fuel until its limit is maxed, around 1/3 throttle position, and continues to provide fuel all the way to WOT.
The needle raises as soon as you turn the throttle, but limits fuel supplied through the main and needle jet until you get to that 1/3 throttle position when the taper starts to allow more fuel through the needle jet.
Both of these are absolutely relevant around your 5000-6000 rpm range.

2wheels · « **Reply #12 on:** June 26, 2018, 10:22:50 AM »

Does it do this while sitting on the kickstand and you are just reving it?

If so you could get a timing light, the inductive pick up type. Try clipping it onto each of the spark plugs. Then you can start narrowing the problem down.

TwoTired · « **Reply #13 on:** June 26, 2018, 12:42:08 PM »

If you look at the fuel entrances into the carb bore in relation to the slide, you will see the pilot system is more forward than the main/needle exit points. This is so idle vacuum and venturi effect can have more influence on the pilot system when the slide is lowered.
When the slide is raised the pressure are applied more prevalently on the main and needle exit points, making the contribution from the pilot system mostly irrelevant. Yes, it is there. But in 1% territory. This is why I begin carb jetting with the max fuel delivery system, the main and at WOT, and then move on to the midrange or slide needle setting, length and diameter size. Then I set the pilot system for an idle that allows transition (in no accelerator pump carbs), or the minimum fuel to idle cleanly on accelerator pump carbs. Off idle stumble is dealt with accelerator pump delivery volume on carbs that have them.

Cheers,

750Moto · « **Reply #14 on:** June 27, 2018, 07:37:19 PM »

I have a regular timing light and last I checked it was spot on at idle, this was about 2 weeks ago when I changed the points. Next time I get to working on it I plan to put the light on it again to make sure the advance is working correctly.

2wheels · « **Reply #15 on:** June 28, 2018, 06:38:23 AM »

Quote from: 750Moto on June 27, 2018, 07:37:19 PM

I have a regular timing light and last I checked it was spot on at idle, this was about 2 weeks ago when I changed the points. Next time I get to working on it I plan to put the light on it again to make sure the advance is working correctly.

I wasnt clear on what i was trying to learn with the timing light test.
I want to be sure all 4 spark plugs are sparking all the way from 2000 rpm and up.
So just watch the light flash and see if one or more plugs are cutting out.
you dont even need to point the light at the timing marks.
This is a quick and easy test. To try and be sure whether it is electrical or not.

750Moto · « **Reply #16 on:** June 29, 2018, 11:34:19 AM »

I have had a break though in this case, bike was running on 2 cylinders while riding, I reached around a pulled the plugs while riding to determine it was cylinders 2 and 3 cutting out. I then stopped and removed the points cover and all of the sudden it ran on 4 cylinders. What is happening is the points for 2 and 3 are grounding out and the spark is jumping across at different rpms (sometimes even when the points cover is off). This is definitely causing my issue, now I just need to figure out how to fix it, the points are gapped to 14 and the timing is spot on, what could this be now.

2wheels · « **Reply #17 on:** June 30, 2018, 10:16:50 AM »

Looks like you are onto the problem. Thats good.
And way more fun then my approach. I can just picture removing plug wires 2 and 3 while driving down the road. Yikes.

madmtnmotors · « **Reply #18 on:** June 30, 2018, 11:51:44 AM »

Even new condensers can fail, right out of the box. Check the wiring too, I had one where the yellow wire was pinched under it's condenser bracket causing intermittent shorts.

seanbarney41 · « **Reply #19 on:** June 30, 2018, 12:13:04 PM »

Check the resistance of your spark plug caps. Should be less than 11k ohms, but more than 5k. And while you are at it be sure there is clean wire for the caps to screw onto.

750Moto · « **Reply #20 on:** June 30, 2018, 03:51:40 PM »

Ya it was a bit sketch removing the plugs will riding but I am in pure f*** it mode at this point, I just want to ride.

I have played with the condensers and swapped them between 1and4 and 2and3, that doesn't change anything so at least the condensers are good. And yes I have cut the old wire ends off and threaded the caps onto fresh wire then metered the plug caps and the entire coil system and all is within spec.

750Moto · « **Reply #21 on:** June 30, 2018, 04:12:23 PM »

Here is the main thing that let's me know it's ignition related, notice the blue spark on 2and3 (the right ones). This blue spark jumps all over the place while the bike is running on 2 cylinders, when running on all 4 it looks just like 1and4 with just a small lightning strike looking arc. On when running on 2 cylinders the spark in the points for 2and3 jumps all over the place and looks like some fireworks going off

seanbarney41 · « **Reply #22 on:** June 30, 2018, 05:38:46 PM »

I see you have a nice TEC points plate, but what brand of points are those? Daiichi are about all that can be bought new these days and are total crap. Better to replace with used points from TEC or ND off ebay. Most online aftermarket vendors will not tell you what brand you are buying and you will get Daiichi crap that is often no good right out of the package or fail soon after.

750Moto · « **Reply #23 on:** June 30, 2018, 06:08:27 PM »

Points are from 4into1 not sure what type, I have swapped them for the old ones before (also not sure brand but it ran fine at one point with them, bought new ones for no reason, they never actually failed), with the new or old points in nothing has changed so I just went back to the new ones

seanbarney41 · « **Reply #24 on:** June 30, 2018, 06:13:08 PM »

okay, pretty sure 4 into 1 sells Daiichi. Sounds like points are grounding somewhere they shouldn't, which means points are no good or you have the points leads attached incorrectly.

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Author Topic: 78 cb750 cuts out at high rpm (Read 5247 times)