Topic: Question for you Tuning Wizards

[stevenmac]

Hey everyone,

I've got a pretty sweet running '73 CB750, except for a couple of tuning issues that I think may relate to the jets/plugs/exhaust combination. I thought some of you guys might be able to lead me in the right direction, seeing as I know very little about jetting and tuning.

Here are the main symptoms:

1. Bike runs fine, but it tends to pop ("backfire") on decel. (A lot.) The popping is more pronounced as the bike gets warmer, and it's at its worst when decelerating down a hill.

2. The bike doesn't respond well to a sudden crank on the throttle. If I gun it, the engine sort of hiccups (like choke, delay, then vroom). If I really gun it, the engine pretty much shuts down, and will actually die if I don't let up on the throttle.

3. I always twist the idle adjust screw before I start the bike, and then I have to keep lowering it as the bike warms up. That seems normal, except that I have a mess with it quite a bit. The idle will shoot up to 3000 rpm after it's been running for about a minute, then I drop it to 1000, drive for a bit, and it'll be back up at 2000, drop it again, repeat. I talked to one guy who told me he never missed with the idle screw on his bike and I was shocked.

The set-up:

Pod filters
4-2 exhaust
Rejetted by PO--jet size unknown
Stock ignition

So...my question is whether all these characteristics suggest that I should check out the jet size, go to hotter/colder plug, or what? Ideally, I'd like to reduce the backfiring, improve throttle response, and have the bike start more readily.

I appreciate any info ya'll have. Let me know if you have any questions. Thanks.

[cobrajunkie]

uh oh. You'd better run and hide.  The POD police are gonna have a field day at your expense.

[TwoTired]

The set-up:
Pod filters (makes the carb provide lean mixtures)
4-2 exhaust (If original baffles, not much to be concerned about.  If baffles changed/removed it make the carb provide lean mixtures.)
Rejetted by PO--jet size unknown  (assume he didn't know crap and that's why he sold the problem to you.)
Stock ignition (Not an issue if adjusted per 3000 mile tune up schedule.)


Ideally, I'd like to reduce the backfiring, improve throttle response, and have the bike start more readily.

You need a better air induction set up (matched to carb settings). Or, make the pilot circuit richer, the throttle valve circuit richer, and probably the main circuit richer to compensate for the loss of vacuum cause by the pods.

If your exhaust doesn't have baffles, you may never get the backfire on decell problem solved.

2. The bike doesn't respond well to a sudden crank on the throttle. If I gun it, the engine sort of hiccups (like choke, delay, then vroom). If I really gun it, the engine pretty much shuts down, and will actually die if I don't let up on the throttle.

It doesn't have fuel injection or an accelerator pump.  But, you should be able to snap the throttle to 1/2 total throttle travel and get brisk, reliable response.

Cheers,

[daley]

I'd start by cleaning the carbs. The basics first.

[Cvillechopper]

You have to at very least drop the bowls and pull the jets to see what size to PO put in there.  Since you have to do that and it doesn't sound like you've worked on the carbs before, I would recommend pulling the carbs and giving them a complete cleaning making sure all the little orifices are clear and make note of the jet sizes.  Head over to the FAQ section to get a general idea of what jetting changes should have been done based on the pods and exhaust so you don't put the carbs back on and have the same problem.  check the manual for the stock size jets and figure out what size you need based on the posts in the carb FAQ and order them.
If you do pull the carbs know that you'll have to synch them when you get them back on the bike. 

Good luck

[mrbreeze]

I would say that your carbs are out of synch by what you are describing (way high idle when warmed up). I second the idea of removing and cleaning carbs just so you know what you are starting with. It isn't hard to do although it will cost you some money for carb kits. Once you have the carbs back together and have your needles set at the right clip... proper main jets ..... you need to bench synch the carbs. Then after all tune up stuff is done (points,timing,valve adjust, etc.)..... reinstall the carbs and it should run good enough to synch the carbs with a manometer. That is the icing on the cake!!!! (you will be amazed at how much better it will run). Have fun with it!!

[stevenmac]

The set-up:
Pod filters (makes the carb provide lean mixtures)
4-2 exhaust (If original baffles, not much to be concerned about.  If baffles changed/removed it make the carb provide lean mixtures.)
The exhaust definitely has baffles, I would guess original to that exhaust setup.
Rejetted by PO--jet size unknown  (assume he didn't know crap and that's why he sold the problem to you.)
The PO wasn't too bad--he had a bad-enough Honda habit that he had a couple of extras stored at his neighbor's house so his wife wouldn't know.
Stock ignition (Not an issue if adjusted per 3000 mile tune up schedule.)


Ideally, I'd like to reduce the backfiring, improve throttle response, and have the bike start more readily.

You need a better air induction set up (matched to carb settings). Or, make the pilot circuit richer, the throttle valve circuit richer, and probably the main circuit richer to compensate for the loss of vacuum cause by the pods.

Man, I kept thinking it was too lean. I figured that if the bike choked from heavy throttle it was because it couldn't get the air intake to match the influx of fuel. But from what you're saying it sounds like I have it backwards.

Thanks for the info.

[stevenmac]

I would say that your carbs are out of synch by what you are describing (way high idle when warmed up). I second the idea of removing and cleaning carbs just so you know what you are starting with. It isn't hard to do although it will cost you some money for carb kits. Once you have the carbs back together and have your needles set at the right clip... proper main jets ..... you need to bench synch the carbs. Then after all tune up stuff is done (points,timing,valve adjust, etc.)..... reinstall the carbs and it should run good enough to synch the carbs with a manometer. That is the icing on the cake!!!! (you will be amazed at how much better it will run). Have fun with it!!

Well, in theory the carbs have been synched, and the valves adjusted, last year. But, I've never pulled, cleaned, and bench-synched the carbs. The deal with the idle screw is that I have to crank it up when the bike is cold in order to start it in the first place. If I follow you, would a thorough carb service result in being able to start it with the idle screw in a lower initial position?

[Roach]

I would say that your carbs are out of synch by what you are describing (way high idle when warmed up). I second the idea of removing and cleaning carbs just so you know what you are starting with. It isn't hard to do although it will cost you some money for carb kits. Once you have the carbs back together and have your needles set at the right clip... proper main jets ..... you need to bench synch the carbs. Then after all tune up stuff is done (points,timing,valve adjust, etc.)..... reinstall the carbs and it should run good enough to synch the carbs with a manometer. That is the icing on the cake!!!! (you will be amazed at how much better it will run). Have fun with it!!

Well, in theory the carbs have been synched, and the valves adjusted, last year. But, I've never pulled, cleaned, and bench-synched the carbs. The deal with the idle screw is that I have to crank it up when the bike is cold in order to start it in the first place. If I follow you, would a thorough carb service result in being able to start it with the idle screw in a lower initial position?

always assume that the PO never touched a thing they are well known for saying stuff and doing the opposite

[Lavis500]

When you say you're cranking up the idle adjust screw...  you mean the single screw that adjusts all 4 carbs?  You know that is the exact same thing as giving it just a little bit of throttle, right?  It's just increasing your minimum throttle, but all you have to do to accomplish the same thing blip the throttle a bit during start up.  Then after warm up you can let off the throttle and it will settle to it's regular idle.

I thought backfire and reluctant throttle were symptoms of running rich, but I'm no expert by any means.  Why not do a plug chop to see exactly where you sit?

[stevenmac]

When you say you're cranking up the idle adjust screw...  you mean the single screw that adjusts all 4 carbs?  You know that is the exact same thing as giving it just a little bit of throttle, right?  It's just increasing your minimum throttle, but all you have to do to accomplish the same thing blip the throttle a bit during start up.  Then after warm up you can let off the throttle and it will settle to it's regular idle.

I thought backfire and reluctant throttle were symptoms of running rich, but I'm no expert by any means.  Why not do a plug chop to see exactly where you sit?

Yeah, that is what I mean, the single screw that attaches to the throttle cable. The reason I crank it up is because otherwise I would have to stand there holding the grip for a couple minutes at least before the bike would idle. I also presumed that backfire and reluctant throttle indicate running rich. What's a plug chop? The plugs are black and dry. 

[TwoTired]

Man, I kept thinking it was too lean. I figured that if the bike choked from heavy throttle it was because it couldn't get the air intake to match the influx of fuel. But from what you're saying it sounds like I have it backwards.

Thanks for the info.

It IS too lean.  When the slide suddenly opens, there is an inrush of air, but the throat vacuum is also lost.  Since the vacuum is the only thing that pulls fuel up from the bowls, you get plenty of air but little fuel.  The engine simply wheezes and balks.  Some carbs have accelerator pumps to shoot a stream of gas into the throats when the slide is opened, so there is fuel for acceleration. These pumps were found on the 77-78 PD carbs for the Cb750s, and some CB650s.  Alas, your 73 didn't get accelerator pumps.

The 73 carbs were originally set over rich at idle, so when the throttle is opened, it doesn't get so lean as to make the engine quit.  POD filters cause a loss of vacuum in their own right due to their inherent design, and certainly more than the stock induction does.  So, they lean the mixture over all operating ranges, simply due to less vacuum in the carb throats pulling fuel up from the carb bowls.  If you check your spark plug deposits, you will probably find white insulators with very little deposits. (...unless only the main jet was sized upward without other carb adjustments.  Then the plug deposits will be confusing after some time in general run conditions.)

On decell, with the slides closed, the mixture is too lean for several firing cycles to actually support combustion in the chambers.  So, the mixture goes through the engine and collects in the exhaust unburned, where eventually a hot ember comes along and lights if off; bang/pop.  There are carb designs that have decell enrichment valves just to prevent the exhaust backfire on decell.  Some CB650 carbs for example.  Your carbs don't have these either.

For your bike, you may see some improvement, if you turn inward the pilot air bleed screws a bit. Try 1/4 turn in from where they are now.  None of the stock, book settings apply to factory carbs without the factory induction set up (and to some extent the exhaust, too).  It is rather a pain (or expense at the dyno) to retune them well over the entire throttle range.  Many pod users just tweak them until they can tolerate how the engine runs, with wide open throttle being the primary test criteria. (Even though most operation is done in the midrange where it is the most difficult to optimize proper mixtures.)  I think that most of them are re-tuned way over rich, which runs ok, but doesn't actually get best power, and almost certainly, worse fuel optimization.

Cheers,

[Lavis500]

It's a way to judge the fuel/air mixture running through your engine so that you can determine if you are too rich or lean.  I have yet to be able to do one as my engine is still under the knife, but from everything I can find the process is something like this:

First, mark on the throttle where 1/4, 1/2, 3/4, and full are.  This is commonly done with a piece of tape.  Then, take the bike and run it under load (perhaps up a slight grade or on an even surface in 4th gear) at one of the 4 throttle positions for about 1/2 mile.  At the end, pull in the clutch, kill the engine, and come to a safe stop.  Immediately remove your spark plugs and compare them to charts to find if you are running rich or lean.  This process is repeated for every throttle position.  I know TwoTired has posted somewhere a chart that tells you exactly which fuel circuits are used depending on the throttle position.

If you do anything, listen to TT.

Thanks, TT, for another great in depth explanation.  That makes perfect sense!

[stevenmac]

Man, I kept thinking it was too lean. I figured that if the bike choked from heavy throttle it was because it couldn't get the air intake to match the influx of fuel. But from what you're saying it sounds like I have it backwards.

Thanks for the info.

It IS too lean.  When the slide suddenly opens, there is an inrush of air, but the throat vacuum is also lost.  Since the vacuum is the only thing that pulls fuel up from the bowls, you get plenty of air but little fuel.  The engine simply wheezes and balks.  Some carbs have accelerator pumps to shoot a stream of gas into the throats when the slide is opened, so there is fuel for acceleration. These pumps were found on the 77-78 PD carbs for the Cb750s, and some CB650s.  Alas, your 73 didn't get accelerator pumps.

The 73 carbs were originally set over rich at idle, so when the throttle is opened, it doesn't get so lean as to make the engine quit.  POD filters cause a loss of vacuum in their own right due to their inherent design, and certainly more than the stock induction does.  So, they lean the mixture over all operating ranges, simply due to less vacuum in the carb throats pulling fuel up from the carb bowls.  If you check your spark plug deposits, you will probably find white insulators with very little deposits. (...unless only the main jet was sized upward without other carb adjustments.  Then the plug deposits will be confusing after some time in general run conditions.)

On decell, with the slides closed, the mixture is too lean for several firing cycles to actually support combustion in the chambers.  So, the mixture goes through the engine and collects in the exhaust unburned, where eventually a hot ember comes along and lights if off; bang/pop.  There are carb designs that have decell enrichment valves just to prevent the exhaust backfire on decell.  Some CB650 carbs for example.  Your carbs don't have these either.

For your bike, you may see some improvement, if you turn inward the pilot air bleed screws a bit. Try 1/4 turn in from where they are now.  None of the stock, book settings apply to factory carbs without the factory induction set up (and to some extent the exhaust, too).  It is rather a pain (or expense at the dyno) to retune them well over the entire throttle range.  Many pod users just tweak them until they can tolerate how the engine runs, with wide open throttle being the primary test criteria. (Even though most operation is done in the midrange where it is the most difficult to optimize proper mixtures.)  I think that most of them are re-tuned way over rich, which runs ok, but doesn't actually get best power, and almost certainly, worse fuel optimization.

Cheers,

TwoTired: You're awesome. I meant to write "rich" in that earlier comment. Everything you're saying here makes sense, and it also makes me wonder if the pods are simply a bad idea. I'll give the carbs a good clean, as others suggested, and then try 1/4 turn on the pilot screws. Thanks. 

[edbikerii]

Perhaps the simplest test of all:  if it is too lean, putting on the choke will cause some improvement.