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Author Topic: How Rich is too Rich? 1978 CB750  (Read 346 times)

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Offline Tdubsy

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How Rich is too Rich? 1978 CB750
« on: November 30, 2019, 01:38:40 pm »
Hey Everyone,

I just installed a carpy's 4-1 exhaust.

I tested it with the best jetting I could find for my stock airbox/exhaust
120mains
38 Idle.
slide needles lifted with 2 shims

With the stock airbox still on with the new exhaust the bike became extremly rich, not able to start with choke and shooting black soot out the back. even had the airbox cracked open a bit some hesitation at certain points in the throttle

See video below

[youtube]https://www.youtube.com/watch?v=VmaIfDwNbZM/[youtube]


So I threw on the pods to lean it out a bit,

It was better but still sooty, heard some backfiring out the exhaust around 3500 rpm So i dropped the 120 mains to 110.

This gave me the best throttle response I have ever felt from the bike. although still fairly rich, some soot with heavy throttle blips and even a back fire with some flame.

[youtube]https://www.youtube.com/watch?v=pEaakkDcqc4/[youtube]


I am using a color tune spark plug to see what color the flame looks like. Originally my bike would always lean out during throttle blips from idle even with accel pumps working. So i raised the needles a bit and that seemed to help. Now it feels like I should lower them back down as my ignition flame looks rich on a throttle blip and after holding it there it maintains richness and backfires. Potentially even going to 105 mains.



Now my real question Is how rich can I safely go on the bike without causing issues?

Because with its current rich state just from throttle response in neutral and a quick rip around my parking garage it feels like a completely different bike. More power better throttle response at the expense of the odd backfire and some black smoke.

Im going to set the needles back to stock position tonight and order some 105 mains. it seems strange with all the extra airflow that I have to limit the fuel supply. Is the stronger Venturi effect from more air flow causing it to pull more fuel from the carbs?

Offline Jerry Rxman Griffin aka MuthaF'er

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Re: How Rich is too Rich? 1978 CB750
« Reply #1 on: November 30, 2019, 05:18:57 pm »
Concentrate on your needles first. You are not getting into the main jet rpm range yet in the parking garage like you would at WFO on the street.

Think you should clue us in on which bike it is?!!
As of today 3/13/2012 my original owner 75 CB750F has made it through 3 wives, er EX-wives. Free at last.  ;-)

Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #2 on: November 30, 2019, 06:26:51 pm »
Ok will start with the needles and go from there.

Im more wondering about how rich can I keep the bike? as it seems way happier in this extra rich state.

1978 cb750 as per the title of the thread. K8 to be specific

Offline 1976cb750f836

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Re: How Rich is too Rich? 1978 CB750
« Reply #3 on: December 01, 2019, 08:03:59 am »
Go back to 1 shim, and check your floats , they may be too rich? With pipe and pods u should need at least 120.

Offline maxheadflow

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Re: How Rich is too Rich? 1978 CB750
« Reply #4 on: December 01, 2019, 09:06:46 am »
If you are rich enough to blow black smoke, you will have problems.  Ring wear will be quick and you need to change engine oil frequently. 

Offline kerryb

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Re: How Rich is too Rich? 1978 CB750
« Reply #5 on: December 01, 2019, 05:02:53 pm »
My k8 ran great with a no-name 4into1, stock airbox and filter, stock brass in the pd42b's.  I'm curious, did you try stock jets with the new exhaust?  110 mains, 38 pilot.  You may want to double check the accelerator pump circuit and be double sure the pilots are still clear.   

Doing a little research led me to someone's post (spanner1) that has a very appropriate tag line:
If you're sure it's a carb problem; it's ignition,
If you're sure it's an ignition problem; it's carbs....

So...could it be timing, weak advancer springs,  Or (perish the thought) carb synch?  (Yeah I know, thats back to carbs)

Just trying to help.
intrigued by the wail...seduced by the scream.

Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #6 on: December 01, 2019, 06:56:54 pm »
OK,

So I ended up pulling out 2 of the shims.

A lot less smoke but throttle response is now lacking

I still see the lightest touch of black smoke when I give the throttle a good blip. But I'm wondering if that is just due to the extra fuel from the accel pump.


I am getting some spitting from the carbs now while holding the throttle open. So I will try going back to 120 mains from the 110's


will potentially put back 1 shim depending how it responds

Offline HondaMan

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Re: How Rich is too Rich? 1978 CB750
« Reply #7 on: December 01, 2019, 07:17:10 pm »
It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #8 on: December 01, 2019, 10:38:52 pm »
It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?

Nope carb boots are about a year old, I should start having a list as well of all the other work I've done on the bike.

Wouldn't leaky carb boots lean out the mixture? When my initial problem was too rich.


Current Jetting/mixture modifications

- 38 Idle Jet
- 110 Main Jet
- 2 turns Mixture Screw
- Needles currently have no shims

- K&N Pods
- Carpys 4-1 Exhaust

Other work so far

- Dyna ignition system
- Dyna Coils
- New Carb Boots
- Full carb rebuild with new hardware
- Floats set to 12.5mm although all set differently to give matching clear tube tests
- Accel pump rebuilt
- Accel pump valve rebuilt
- Timing set
- Valves set to OEM spec
- New clutch
- solid stat regulator rectifier
- led bulb conversion
- Brand new wiring harness


« Last Edit: December 01, 2019, 10:42:26 pm by Tdubsy »

Offline kerryb

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Re: How Rich is too Rich? 1978 CB750
« Reply #9 on: December 02, 2019, 05:17:50 am »
I was so hopeful that you would try stock jets with stock airbox and needle settigs to try and set a baseline for further experimentation...but you keep throwing in 2 or more major changes instead of one at a time!  How do you know which change is having a positive effect?
Time for me to step back and watch for a conclusion.
intrigued by the wail...seduced by the scream.

Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #10 on: December 02, 2019, 10:42:11 am »
I was so hopeful that you would try stock jets with stock airbox and needle settigs to try and set a baseline for further experimentation...but you keep throwing in 2 or more major changes instead of one at a time!  How do you know which change is having a positive effect?
Time for me to step back and watch for a conclusion.


I've really only done one step at a time,  although my posts are after about 2 or 3 of those steps.


I can certainly try the stock airbox on now as the jetting is essentially stock apart from the idle @38 although the colortune has shown in the majority of the situations that it's the better size.

It didn't make sense to me at the time to use the stock airbox   thinking it would be easier to lean out the mixture by going to pods than pulling the carbs out and swapping jets.

I will try out the stock airbox before going to 120 mains and see what happens.

Offline kerryb

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Re: How Rich is too Rich? 1978 CB750
« Reply #11 on: December 02, 2019, 11:32:59 am »
I think you might be pleasantly surprised.  After trying very hard to tune a cb650sc  with cv carbs and pods, we put the stock airbox parts on and all the issues disappeared!  Of course that was after cleaning the idle jets AGAIN, arrrgh!
intrigued by the wail...seduced by the scream.

Offline seanbarney41

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Re: How Rich is too Rich? 1978 CB750
« Reply #12 on: December 02, 2019, 03:26:31 pm »
the colortune is absolutely useless for tuning any throttle position except idle. 
If it works good, it looks good...

Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #13 on: December 02, 2019, 03:46:49 pm »
That's what I've mainly used it for. Although it did help me figure out that my bike was going a bit lean with a throttle increase even with the accel pumps working and used that to add shims to the needles in the original exhaust setup. So it's actually helped me out tons in the past




Offline HondaMan

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Re: How Rich is too Rich? 1978 CB750
« Reply #14 on: December 02, 2019, 06:05:32 pm »
It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?

Nope carb boots are about a year old, I should start having a list as well of all the other work I've done on the bike.

Wouldn't leaky carb boots lean out the mixture? When my initial problem was too rich.


As I've posted many times (and is outlined in my book), a leaking hose makes that cylinder run richer below 3500 RPM. This is due to the fact that the pistons are all pulled along by one crankshaft: if one of the intakes has a vacuum leak, then the vacuum at that one will be less. These carbs mix much richer, by design, as the vacuum pulse depth becomes less (i.e. less vacuum), because it presumes that cylinder is running slower. When this is happening, the vacuum draw will not be long and consistent, like a faster-running cylinder's intake will be: faster-running intakes have a longer draw-time on the jet's venturi. At about 800 RPM the roundtop carbs mix at 8:1, by 2500 they are closer to 12:1, and by 4000 they are about 14:1 air-to-fuel mix. The PD carbs act the same way, but mix more accurately due to them being the solution for tighter emission controls in 1976 and later.

The pipes: these bikes are too short for the pipes to have any effect on mixture (the lone exception was the 750's Dunstall 4-2 setup of the 1980s, which ended almost 6" past the rear tire). The typical 4-1 and 4-2 pipes are even shorter than the 4-4 pipes were, which makes them even less effective at modifying the mixture needs. In short, the carb mixture can stay as OEM (except the 1978 PD42/b carbs, which were set very lean and can stand a 10% increase) with any and all engine mods save a longer and earlier intake cam duration: then they must go leaner below 2500 RPM or the plugs will be black in short order. The roundtops typically go from #40 to #37 (now #37.5) pilot jets for the Megacycle 125-00 cam, for example, while the mainjet for that cam should be about #108. This assumes a standard airbox.

On that latter topic: the airbox designs on the SOHC4 are to die for. There is nothing aside from cutting the frame uptubes under the seat to allow longer snorkels on the carb intakes (8-9" long on 750, 4-7" on the 500/550) and installing long, carb-bell-sized intake tubes, that can improve on the airbox performance....DESPITE what the Internet has to say about this... the Internet has not gone to schools nor thru extensive aerospace training for decades to actually learn how much Bernoulli taught the world, and his laws reach far higher and further than the Internet's rumor mills on the topic. The afore-mentioned long tubes were developed by endurance racers in the late 1980s and proved to work well, but require relocation of all that stuff under the seats in install them. This obviates nice things like electric starters, as the resulting batteries are 1/3 as large as ours.
;)
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

Link to My CB750 Book: http://forums.sohc4.net/index.php?topic=65293.0

Link to website: www.SOHC4shop.com

Offline Tdubsy

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Re: How Rich is too Rich? 1978 CB750
« Reply #15 on: December 02, 2019, 06:44:17 pm »
It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?

Nope carb boots are about a year old, I should start having a list as well of all the other work I've done on the bike.

Wouldn't leaky carb boots lean out the mixture? When my initial problem was too rich.


As I've posted many times (and is outlined in my book), a leaking hose makes that cylinder run richer below 3500 RPM. This is due to the fact that the pistons are all pulled along by one crankshaft: if one of the intakes has a vacuum leak, then the vacuum at that one will be less. These carbs mix much richer, by design, as the vacuum pulse depth becomes less (i.e. less vacuum), because it presumes that cylinder is running slower. When this is happening, the vacuum draw will not be long and consistent, like a faster-running cylinder's intake will be: faster-running intakes have a longer draw-time on the jet's venturi. At about 800 RPM the roundtop carbs mix at 8:1, by 2500 they are closer to 12:1, and by 4000 they are about 14:1 air-to-fuel mix. The PD carbs act the same way, but mix more accurately due to them being the solution for tighter emission controls in 1976 and later.

The pipes: these bikes are too short for the pipes to have any effect on mixture (the lone exception was the 750's Dunstall 4-2 setup of the 1980s, which ended almost 6" past the rear tire). The typical 4-1 and 4-2 pipes are even shorter than the 4-4 pipes were, which makes them even less effective at modifying the mixture needs. In short, the carb mixture can stay as OEM (except the 1978 PD42/b carbs, which were set very lean and can stand a 10% increase) with any and all engine mods save a longer and earlier intake cam duration: then they must go leaner below 2500 RPM or the plugs will be black in short order. The roundtops typically go from #40 to #37 (now #37.5) pilot jets for the Megacycle 125-00 cam, for example, while the mainjet for that cam should be about #108. This assumes a standard airbox.

On that latter topic: the airbox designs on the SOHC4 are to die for. There is nothing aside from cutting the frame uptubes under the seat to allow longer snorkels on the carb intakes (8-9" long on 750, 4-7" on the 500/550) and installing long, carb-bell-sized intake tubes, that can improve on the airbox performance....DESPITE what the Internet has to say about this... the Internet has not gone to schools nor thru extensive aerospace training for decades to actually learn how much Bernoulli taught the world, and his laws reach far higher and further than the Internet's rumor mills on the topic. The afore-mentioned long tubes were developed by endurance racers in the late 1980s and proved to work well, but require relocation of all that stuff under the seats in install them. This obviates nice things like electric starters, as the resulting batteries are 1/3 as large as ours.
;)


Interesting I never thought about it that way about one carb leaking to become rich.

But I have tested for vacuum leaks quite a few times already while diagnosing other issues. (faulty brand new coil and clogged accel pump valve)


With my stock pipes and intake I did have to increase the idle jets to 38 and needles up 2 shims to get the throttle to respond reasonably. And I was still ending up slightly lean on acceleration.

I did also use the colortune to verify mixtures at idle on all cylinders with mixture screws set the same. If there was a leak of some sort it should have shown up during that process as well


I did buy the pipes used and I did notice they were fairly sooty on the inside.

But how could changing just the pipes at first send me from slightly lean conditions to blowing black soot out the back? Some of it could just maybe be existing soot being blown out by I assume it would have cleared pretty quickly? I am finding other posts from other people that used the mac 4-1s where their bike went rich as well.


Even after I leaned it out a bit with pods as the 2nd step I was able to backfire flame right out the back. And that was above 3500.




« Last Edit: December 02, 2019, 07:11:04 pm by Tdubsy »

Offline maxheadflow

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Re: How Rich is too Rich? 1978 CB750
« Reply #16 on: December 02, 2019, 07:21:49 pm »
It sounds to me like the hoses between the carbs and head are leaky...probably hard as a rock?

Nope carb boots are about a year old, I should start having a list as well of all the other work I've done on the bike.

Wouldn't leaky carb boots lean out the mixture? When my initial problem was too rich.


As I've posted many times (and is outlined in my book), a leaking hose makes that cylinder run richer below 3500 RPM. This is due to the fact that the pistons are all pulled along by one crankshaft: if one of the intakes has a vacuum leak, then the vacuum at that one will be less. These carbs mix much richer, by design, as the vacuum pulse depth becomes less (i.e. less vacuum), because it presumes that cylinder is running slower. When this is happening, the vacuum draw will not be long and consistent, like a faster-running cylinder's intake will be: faster-running intakes have a longer draw-time on the jet's venturi. At about 800 RPM the roundtop carbs mix at 8:1, by 2500 they are closer to 12:1, and by 4000 they are about 14:1 air-to-fuel mix. The PD carbs act the same way, but mix more accurately due to them being the solution for tighter emission controls in 1976 and later.

The pipes: these bikes are too short for the pipes to have any effect on mixture (the lone exception was the 750's Dunstall 4-2 setup of the 1980s, which ended almost 6" past the rear tire). The typical 4-1 and 4-2 pipes are even shorter than the 4-4 pipes were, which makes them even less effective at modifying the mixture needs. In short, the carb mixture can stay as OEM (except the 1978 PD42/b carbs, which were set very lean and can stand a 10% increase) with any and all engine mods save a longer and earlier intake cam duration: then they must go leaner below 2500 RPM or the plugs will be black in short order. The roundtops typically go from #40 to #37 (now #37.5) pilot jets for the Megacycle 125-00 cam, for example, while the mainjet for that cam should be about #108. This assumes a standard airbox.

On that latter topic: the airbox designs on the SOHC4 are to die for. There is nothing aside from cutting the frame uptubes under the seat to allow longer snorkels on the carb intakes (8-9" long on 750, 4-7" on the 500/550) and installing long, carb-bell-sized intake tubes, that can improve on the airbox performance....DESPITE what the Internet has to say about this... the Internet has not gone to schools nor thru extensive aerospace training for decades to actually learn how much Bernoulli taught the world, and his laws reach far higher and further than the Internet's rumor mills on the topic. The afore-mentioned long tubes were developed by endurance racers in the late 1980s and proved to work well, but require relocation of all that stuff under the seats in install them. This obviates nice things like electric starters, as the resulting batteries are 1/3 as large as ours.
;)

I'm not sure how you can declare that on a 4 cylinder, it runs richer due to Bernoulli.  If there is an intake leak air make it into the system and less air passes through the slide and it less air passing through the carb, less fuel will flow.  Less fuel means leaner running. 

Now there may be conditions where the cylinder might run richer.  Say you synchronize the carbs at idle and adjust the pilots to correct for leanness.  In that case the cylinder with the air leak will be closed more and pulling less load while driven. Less load means lower cylinder pressures and less fuel burned. Less fuel burned means the plug looks richer.  In tuning the 4 cylinders with vacuum gauges, I used to alway check to make sure that the vacuum needles were the same at say 5000 rpm and the same at idle. The numbers will be different but they have to be the same at both locations or there is a vacuum leak.

Longer intakes are pretty much always better from what I've seen, they make low speed fueling better. I've seen it a few times on different bikes.  Putting an air cleaner right on the mouth of the carb does not seem to work as well one where the AC is mounted a few inches away.  The last case I increased a stack length inside an air cleaner. Bike ran much better. Had to lean it out over the shorter stack. I've also seen the opposite, Increased the distance to the air cleaner significantly and had to richen it up.




 

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