• Enter the June CB750 Supply gift certificate giveaway! It's easy... Click here, post something, and you're entered into the drawing!

1980 CB750 Runaway Idle

You get exactly what you pay for with those cheapo pods -- you might as well be running velocity stacks for how well they "filter" the air.

UNI makes decent pod filters too, out of the same foam that comes in every dirtbike airbox. But like I said earlier any set of pods requires a carb swap to work properly. These stock carburetors require a significant amount of vacuum to raise their slides and pods simply don't provide that. To make things worse, with pods, the amount of air flowing over them while you're rolling is widely different between the outer two cylinders and the inner two cylinders. The only way to vacuum balance them so they'll run right at speed is to intentionally imbalance them.

Here's a more detailed explanation for you. In your CB750K, the stock carburetors are referred to as CV or Constant Velocity carburetors. When you pull the throttle cable in the stock carb setup, you open up a butterfly valve in all four carbs to increase airflow, and the internals are carefully designed to use this to generate vacuum that pulls the slides up to increase air volume and fuel flow. These carburetors are therefore sensitive to intake system geometry and vacuum, and in the case of our stock carbs they do not want anything but the stock airbox.
The correct carburetors for pod filters or velocity stacks are called "direct-pull" carburetors. Instead of opening up a butterfly valve with your throttle cable, direct-pull carburetors have the throttle cable attached directly to the slide. You yank on the throttle, the carb opens up and lets in more air and that pulls more fuel. Much simpler design with no fancy internal vacuum passageways, and intake geometry/vacuum changes won't keep the carb from functioning.

You can get a set of four Mikuni round-slide direct-pull carburetors in a ready-made set for your bike from Cycle X, but you'll pay a good $1200+ for them. As far as I know that's the best carb setup you can buy for these bikes off the shelf and will make you the most power.
There's a dual-carb direct-pull setup sold by a company called Murray's that is also popular, but doesn't look as pretty in my opinion because it uses a big four-into-two aluminum intake manifold. That's about half the price of the Cycle X carbs.

It's a very good idea to get that stock airbox and all its boots back on the machine if you're sticking with the stock carbs. That way you, as a novice motorcycle mechanic, have a clear and obvious roadmap of how it's supposed to look and work in the form of the factory service manual and parts diagrams. When you're just starting out and don't know what you don't know, that's the best way to avoid making big mistakes.
 
Sense Amid Madness i made the mistake of taking the 75 main jets out of my 1979 JDM 750FZ and putting 105's in . it wouldn't rev past 5000 rpm then with trial and error found it wasn't getting enough air for the bigger jets. the 75's matched the smaller air intake for emissions in japan . lesson learnt those japs know their stuff
 
You get exactly what you pay for with those cheapo pods -- you might as well be running velocity stacks for how well they "filter" the air.

UNI makes decent pod filters too, out of the same foam that comes in every dirtbike airbox. But like I said earlier any set of pods requires a carb swap to work properly. These stock carburetors require a significant amount of vacuum to raise their slides and pods simply don't provide that. To make things worse, with pods, the amount of air flowing over them while you're rolling is widely different between the outer two cylinders and the inner two cylinders. The only way to vacuum balance them so they'll run right at speed is to intentionally imbalance them.

Here's a more detailed explanation for you. In your CB750K, the stock carburetors are referred to as CV or Constant Velocity carburetors. When you pull the throttle cable in the stock carb setup, you open up a butterfly valve in all four carbs to increase airflow, and the internals are carefully designed to use this to generate vacuum that pulls the slides up to increase air volume and fuel flow. These carburetors are therefore sensitive to intake system geometry and vacuum, and in the case of our stock carbs they do not want anything but the stock airbox.
The correct carburetors for pod filters or velocity stacks are called "direct-pull" carburetors. Instead of opening up a butterfly valve with your throttle cable, direct-pull carburetors have the throttle cable attached directly to the slide. You yank on the throttle, the carb opens up and lets in more air and that pulls more fuel. Much simpler design with no fancy internal vacuum passageways, and intake geometry/vacuum changes won't keep the carb from functioning.

You can get a set of four Mikuni round-slide direct-pull carburetors in a ready-made set for your bike from Cycle X, but you'll pay a good $1200+ for them. As far as I know that's the best carb setup you can buy for these bikes off the shelf and will make you the most power.
There's a dual-carb direct-pull setup sold by a company called Murray's that is also popular, but doesn't look as pretty in my opinion because it uses a big four-into-two aluminum intake manifold. That's about half the price of the Cycle X carbs.

It's a very good idea to get that stock airbox and all its boots back on the machine if you're sticking with the stock carbs. That way you, as a novice motorcycle mechanic, have a clear and obvious roadmap of how it's supposed to look and work in the form of the factory service manual and parts diagrams. When you're just starting out and don't know what you don't know, that's the best way to avoid making big mistakes.
You are so correct, air fuel ratios are out of wack with straight pods.
CV stock carbs are very defined in a stock configuration.
I'm going to try an experiment using pods with versions external filter membrane. I will attempt to match the intake draw pressures to mimick a stock air box. If there is a will there may be a way. SenseAmidMadness, you got my wheels turning...
 
Why I'm WhiteWidow
 

Attachments

  • imageedit_3_7426294045.gif
    imageedit_3_7426294045.gif
    2.1 MB · Views: 79
Yes, the CV carbs are easier to tune with a stock airbox. And yes, there are some not so well designed pod filters that will mess up CV's because they have some turbulence that CV's (any carb actually) don't like. But the worst pods (not all) actually block critical air ports in the intake bell that mess with the vacuum chambers and impede slide operation. The filter material is not the problem. And if the pods are not interrupting those ports and they're not so short to create too much turbulence, CV carbs can be properly tuned with pods.

And CV carbs actually love velocity stacks; the right ones will actually improve flow and pull more fuel out of the jets allowing leaner settings.
Just sayin'
20220916_143400 (3).jpg
 
So, there's an old saying when troubleshooting carb problems: "everything else first". Meaning one has to eliminate combustion chamber issues (compression or leakdown tests, cam timing) and ignition issues (timing, voltage) before trying to solve a potential carb problem.

The original poster has not verified that the ignition is timed correctly. Too much advance can cause the symptoms

Another poster with the same symptoms was told it was bad CV carbs and bad pod filters causing the problems, he put Murray's on - didn't change it.

What ya gonna do?
 
Yes, the CV carbs are easier to tune with a stock airbox. And yes, there are some not so well designed pod filters that will mess up CV's because they have some turbulence that CV's (any carb actually) don't like. But the worst pods (not all) actually block critical air ports in the intake bell that mess with the vacuum chambers and impede slide operation. The filter material is not the problem. And if the pods are not interrupting those ports and they're not so short to create too much turbulence, CV carbs can be properly tuned with pods.
For a completely novice motorcycle mechanic like the original poster in this thread, I consider this to be bad advice. Someone who has no experience tuning carburetors is going to have an awful time trying to make them work in any non-stock configuration. Stock has a full set of direct printed instructions from both the manufacturer and aftermarket shop manual publishers that will tell the user exactly what to do and how things are supposed to be. For a complete novice, that's way easier than telling them "oh sure, you can do it" because that leads to this poor beginner slamming their head into a wall for 6 months trying to figure out carb tuning for their setup when none of the instructions written anywhere cover that.

Think about the thread for a second though -- OP said there have been zero jetting changes with the pods. Do you think that'll run right?
 
So, there's an old saying when troubleshooting carb problems: "everything else first". Meaning one has to eliminate combustion chamber issues (compression or leakdown tests, cam timing) and ignition issues (timing, voltage) before trying to solve a potential carb problem.

The original poster has not verified that the ignition is timed correctly. Too much advance can cause the symptoms

Another poster with the same symptoms was told it was bad CV carbs and bad pod filters causing the problems, he put Murray's on - didn't change it.

What ya gonna do?
Hey, i am the original poster. I got a used airbox off eBay with a new filter and the runaway engine went away! It still wasn't running super great because of the timing. I had another problem with not being able to adjust my timing bc I was out of rotation on my ignition timing plate. I ended up just grinding out some of the plate so that I could rotate it more. Probably not the best way to fix the problem, but now it runs better then factory! So thank you all so much for your help!
 
Hey, i am the original poster. I got a used airbox off eBay with a new filter and the runaway engine went away! It still wasn't running super great because of the timing. I had another problem with not being able to adjust my timing bc I was out of rotation on my ignition timing plate. I ended up just grinding out some of the plate so that I could rotate it more. Probably not the best way to fix the problem, but now it runs better then factory! So thank you all so much for your help!
I'm glad to hear you've made progress on the issue and without replacing your CV carbs with a direct-pull version. They may still need some tweaking on the jetting to get optimum performance. I don't spend a lot of time on The DOHC electronic ignition models, but I do suspect that a breakdown happens in the ignition components, perhaps even the mechanical advance, that causes the timing to become too advanced. Altering the pick-up plate is a novel approach, but I suspect there's another issue that the modification resolved (temporary or permanent fix: who knows).

Have you been able to verify ignition timing at idle and full advance with a strobe light and compared to spec?
 
For a completely novice motorcycle mechanic like the original poster in this thread, I consider this to be bad advice.
I acknowledged that a stock air box was an easier potential solution. The main point I was making was to verify and resolve everything else first before blaming the carbs - I think that is good, sound advice regardless of experience level.

I also think you unfairly maligned the adjustability of CV carbs and advocated switching carbs unnecessarily - any new carbs would have also required there own tuning issues. CV carbs often get an unfair rap and it often involves badly matched pods. I do agree that any CV carb, tuned for an airbox, will need correctly rejetted for even a good pod system
 
I'm thinking major vacumn leak...if carb and timing, pods are not the issue...problem occurs when engine is warm...and tolerances increase,performance is lowered. If you have gum out or any carburetor cleaner available lightly spray it around the head and see if idle picks up... could possibly be a head gasket but let's hope not
 
Sounds like you have some good advice to follow, I too would recommend that you get rid of the pods, they will give you nothing but grief, especially if you are new to the mechanical world as you say. You could rig up a temporary air intake with some rubber and a cardboard box with a proper air filter inside it. Another thing you may want to check is that the cam timing is correct, it is easy to get it out by a tooth or two and then your timing marks are off the charts. You can only set the timing when the auto advance is not engaged, so 900 to 1000 RPM after that there is no hope of getting anything right with the timing marks. You could maybe get some 3/16 clear fuel line and attach it to the carb drain and then hold it up to see what the fuel level is in the float bowl. This will not work with the engine running, what you have to do is have a secondary fuel bottle suspended above the carbs and then you can see if any one of the carbs is really out to lunch and then fix that float level. You can also use the same configuration to check the needle seats by removing the bowls and lightly holding the needle closed and then allow gas thru the line to the carb. Use a light touch, don't forget that the shutoff for the needle is only the buoyance of the float, so it is not that much. If you have any kind of leakage then you should replace the needle and seat. Let me know how it goes. Thanks
 
I performed a PODMOD to restrict air flow because the bike came with no Air Box.

A BarnFind with GoodBones and many fixable issues.

This is my technic that reduced / eliminate the dreaded Runaway Idle.

The intake restriction is performed by dye-cut 304ss screen @ 2"dia.
Oval Pods used in this modification.

1. Create a slight dome on the 2" disk for carb & pod.
2. Make sure domes touch each other.
3. Carb side seats perfectly in carb.
4. Pod side is loose but kept in place by the carb side.
5. I estimated the air flow restriction is equal to a "STOCK AIR BOX".
6. 25% to 30% restricted "AIR FLOW" is created.
7. I tried 3 variations of 304SS screen, a higher restriction and a lower restriction screen.

3 pics of the work in progress...
1) carb mod w/pod mod.
2) DragBike when 1st mods started.
3) 90% completed late fall of 2022.
This Bike will be ready to hit the streets in late May of 2023.
The "WHITE WIDOW" has been a fun project utilizing my creative expression.

Thanx for viewing...
 

Attachments

  • IMG_20230502_130650892~2.jpg
    IMG_20230502_130650892~2.jpg
    453.9 KB · Views: 70
  • IMG_20210622_202751516_HDR.jpg
    IMG_20210622_202751516_HDR.jpg
    661.7 KB · Views: 76
  • imageedit_1_9084345184.jpg
    imageedit_1_9084345184.jpg
    45.1 KB · Views: 71
Can I jump in and ask how much of a difference having incorrect valve clearances would cause? Can it actually lead to runaway idle?
i’m the original poster from a year ago. I ended up selling the bike. In my scenario I didn’t end up having to mess with the valve clearance, but guys have told me that it can cause a runaway. Usually that would be last case scenario I would believe. Other things are probably contributing to the runaway.
 
i’m the original poster from a year ago. I ended up selling the bike. In my scenario I didn’t end up having to mess with the valve clearance, but guys have told me that it can cause a runaway. Usually that would be last case scenario I would believe. Other things are probably contributing to the runaway.
Thanks for the update. I think one of the rubber connectors between the air box and carb bank has worked its way party loose so it may just be that. I had it idling fine for a few mins then suddenly it ran away. I’ll dig into it. Cheers
 
Back
Top