Topic: Problem with Air Fuel Mixture?? Or do I need a Hotter Plug? Hard Start Cold

[warreng22]

If this has already been answered, I apologise, I just haven't been able to find anything that related directly to my problem.

I have a 1976 CB750F SS, I just restored it this year. I have done all of the "usual" maintenance as well: plugs, timing, points, condensers etc etc. It also has new coils, and a complete rebuild of the carbs. I am having a small problem with carbon built up on plugs, so naturally I figured that it was just running a little rich so I have adjusted, and adjusted the air/fuel mixture screws. I have found that on a cold start, the bike does not want to start with the standard 1 1/2 turns out on the Air/Fuel Mixture Screws (especially at this time of year in the Northern USA). It will start and idle at around 1100 rpm if I run the air screws all the way in, and then only turn the screws 1/2 of a turn out  But then after it starts, and begins to warm up it, the idle creeps up to around 1800 to 2000 rpm. Now I know that I can adjust the Idle Screw on the Bar for all 4 carbs to make it idle slower, but then i will have to put it back the next time I go to start the bike from cold...

Now Correct me if I am wrong, Since these screws are on the the air box side of the carbs, they adjust how much air is let into the carb. So the futher out you turn the screws the more air you are mixing with the fuel, Right?
If that is true, a 1/2 turn out isn't very much air...

Do I need hotter plugs to run at the standard 1 1/2 turn (+ or -)  Would this make a difference?

What else could be wrong for the bike to be so hard to start when cold?? I know these bikes are notorious for cold starts, but it takes so long turning it over, that I almost kill my battery doing this and have to charge it! Once the bike it warm it starts easy, just hit the button and boom your going. Also I should mention that when the bike is running, and you engage the choke, it does not kill the bike! But it just barely bogs it down a little... which again points to the bike running to rich, right?

Thanks in Advance
Warren

[TwoTired]

Cold engines do not atomize fuel as readily as hot ones.

You need a richer mixture when cold than when hot.  That is what the choke is for.  Some or all choke should be needed when the bike is cold.

Some bikes have a fast idle cam to increase the idle setting when the choke is applied.  Yours doesn't.  That function is expected to be provided by the operator. Earlier bikes had a friction adjust on the twist grip that can help during warm up.  The idle adjust knob is normally set for correct idle RPM when fully warmed up.   Some people report twiddling with that knob throughout their ride.

Cold operation can be improved during cold months by using a hotter plug. NGK D7EA has been reported to work well.  When weather is hot, though, you should use the colder D8EA.

You will never compensate cold operation with the idle air screws to your satisfaction.  These are set to provide good throttle response from low RPM when the bike is thoroughly warmed up.  Do not expect to snap the throttle wide open from idle and get good response. One half throttle, yes.  Your carbs do not have accelerator pumps to allow this.  While you can make the carbs very rich to do this, you may also get carbon plugs.

Newer machines have a computer and sensors to compensate for all this.  Your bike's computer is located on or above the cushy foam seat.

Cheers,

[warreng22]

Cold engines do not atomize fuel as readily as hot ones.
You need a richer mixture when cold than when hot.  That is what the choke is for.  Some or all choke should be needed when the bike is cold.

Yes I understand this, I have the Choke fully open (lever up) but it just doesn't seem to help any more than when it is off (lever down) or anywhere between. Also when the bike is warm and running, I understood that if you engage the choke fully, that it should kill the engine, that doesn't happen, it bearly makes any difference I have checked the choke linkage to make sure that it is properly adjusted, everything looks good.

Some bikes have a fast idle cam to increase the idle setting when the choke is applied.  Yours doesn't.  That function is expected to be provided by the operator. Earlier bikes had a friction adjust on the twist grip that can help during warm up. 

When I try to start the bike cold, I engage the choke fully, and I have tried to open the throttle some also to see if that helps, but it actually seems to make it sound less like it is trying to start, if that makes sense.

The idle adjust knob is normally set for correct idle RPM when fully warmed up. Some people report twiddling with that knob throughout their ride.

Yeah I have that as well, I was hoping to not have to change it like that

Cold operation can be improved during cold months by using a hotter plug. NGK D7EA has been reported to work well.  When weather is hot, though, you should use the colder D8EA.

I just bought a set of NGK D7EA, will install them tomorrow. I was running NGK D8EA before.

You will never compensate cold operation with the idle air screws to your satisfaction.  These are set to provide good throttle response from low RPM when the bike is thoroughly warmed up.  Do not expect to snap the throttle wide open from idle and get good response. One half throttle, yes.  Your carbs do not have accelerator pumps to allow this.  While you can make the carbs very rich to do this, you may also get carbon plugs.

I just couldn't figure why I could only adjust them out 1/2 turn to get the bike to start cold If I pull them out 1 1/2 turns, it doesn't even sound like it is trying to hit. Turning the air screws in is richer correct? and out is leaner? Or am I reversed

Newer machines have a computer and sensors to compensate for all this.  Your bike's computer is located on or above the cushy foam seat.
Cheers,

Nope, no computer on this bike, just the electrics; rectifier and such.

Thanks Again TwoTired!

[warreng22]

bump 

[warreng22]

Well, I installed the NGK D7EA hotter plugs. Big Improvement 

Bike starts much easier.
Its still running a little rich, see a small amount of white smoke for a sec or two if you rev it up a bit. Pops a little, when you decelerate after running 55 mph for 5 miles or so. But we some fine adjustments to the air mixture screw,
I think it will help 

Or will these screws only adjust the air for idle?? According to the claymore (yeah I hate these books...but..) this is an Idle Stop Screw. It shows an Air Fuel Mixture Screw, but There isn't one on my bike    I took a flash light and looked in the hole where it showed it was supposed to be, but there is nothing there. There is just metal filled in the back of the hole, no theads or anything, all the carbs are the same. 

Anyone know if this is normal for a 1976 CB750F SS?? 

[TwoTired]

Yes I understand this, I have the Choke fully open (lever up) but it just doesn't seem to help any more than when it is off (lever down) or anywhere between. Also when the bike is warm and running, I understood that if you engage the choke fully, that it should kill the engine, that doesn't happen, it bearly makes any difference Huh Huh I have checked the choke linkage to make sure that it is properly adjusted, everything looks good.

This is wrong.  Either your choke butterflies aren't closing all the way in each carb, or you have a serious air leak between carb and head.  The choke should create very high carb throat vacuum to draw extra fuel from the mains circuit.

The air screw adjusts air mix for the idle circuit. (out leaner; in richer)  If your air screw has a hollow tip it, has a limited range of adjustment.
The idle circuit does work over the entire throttle range.  However, it's contribution is paltry compared the mains/throttle valve fuel/air provider.

I know white smoke as water vapor/condensate/steam.  Blueish is oil, and black smoke is unburned fuel.

According to the claymore (yeah I hate these books...but..) this is an Idle Stop Screw. It shows an Air Fuel Mixture Screw, but There isn't one on my bike

I'm guessing your book (which I am not familiar with) covers the K0 carbs with the four cable slide actuators.  You have a more stable slide actuation method and the screws on the side of your carbs are air screws for the idle circuit.
You'll be way ahead by obtaining a Honda Shop Manual with the supplements that address you model bike.  I think you can down load this.  Search for Manual topics on this forum.  I bought a hard copy many years ago.

What did the electrode deposits look like on your old plugs?

Cheers,

[warreng22]

Yes I understand this, I have the Choke fully open (lever up) but it just doesn't seem to help any more than when it is off (lever down) or anywhere between. Also when the bike is warm and running, I understood that if you engage the choke fully, that it should kill the engine, that doesn't happen, it bearly makes any difference Huh Huh I have checked the choke linkage to make sure that it is properly adjusted, everything looks good.

This is wrong.  Either your choke butterflies aren't closing all the way in each carb, or you have a serious air leak between carb and head.  The choke should create very high carb throat vacuum to draw extra fuel from the mains circuit.

After the Carb rebuild a couple months ago, I made certain that the choke butterflies were closing completely, although I'm not ruling this out. There may be an air leak, I will have to check. Can I use WD-40 to do this? I really dont want to use Carb Cleaner if Possible because of engine paint. Will WD-40 increase RPM if there is a leak or  Are there any other symtoms to an air leak

The air screw adjusts air mix for the idle circuit. (out leaner; in richer)  If your air screw has a hollow tip it, has a limited range of adjustment.

The air screws do have a hollow tip, can I change these if needed to one's that are not hollow?

The idle circuit does work over the entire throttle range.  However, it's contribution is paltry compared the mains/throttle valve fuel/air provider.
I know white smoke as water vapor/condensate/steam.  Blueish is oil, and black smoke is unburned fuel.

Got it, I thought that white should have been condensation, but was not 100%

According to the claymore (yeah I hate these books...but..) this is an Idle Stop Screw. It shows an Air Fuel Mixture Screw, but There isn't one on my bike

I'm guessing your book (which I am not familiar with) covers the K0 carbs with the four cable slide actuators.  You have a more stable slide actuation method and the screws on the side of your carbs are air screws for the idle circuit.
You'll be way ahead by obtaining a Honda Shop Manual with the supplements that address you model bike.  I think you can down load this.  Search for Manual topics on this forum.  I bought a hard copy many years ago.

Sorry Clymer, stupid spell check...  I think I looked once before for the Honda Shop manual here, but I will look again.

What did the electrode deposits look like on your old plugs?

There is some carbon on the plugs, but I've seen worse. If I want to perform a Plug Chop, do I have to ride the bike to do this, or can I simply get the engine good and warm and slowly increase RPM to high range and kill it while it is still in high rev 

[warreng22]

Searched the Forum, Looks like WD-40 Does work when checking for air leaks.
I will try it tommorrow if I have time.

[DammitDan]

To perform a proper plug chop you need to be riding the bike.  The chop isn't for determining deposits at RPM ranges, it's for determining deposits at throttle positions.  You won't be able to reach wide open throttle (WOT) in neutral unless you're ready to turn your engine into scrap metal. 

The only time I've gotten a good WOT reading was when the bike was under load in 5th gear going up a long hill.  At the top of the hill I cut the engine with the killswitch (at about 75mph) and pulled the clutch at the same time.  The other readings are easier to attain, but they all pretty much have to be done while the bike is under load and moving (except for the idle circuit...  basically from no throttle to 1/8 throttle).

Did you properly sync your carbs with a manometer after the rebuild?

[warreng22]

To perform a proper plug chop you need to be riding the bike.  The chop isn't for determining deposits at RPM ranges, it's for determining deposits at throttle positions.  You won't be able to reach wide open throttle (WOT) in neutral unless you're ready to turn your engine into scrap metal. 

The only time I've gotten a good WOT reading was when the bike was under load in 5th gear going up a long hill.  At the top of the hill I cut the engine with the killswitch (at about 75mph) and pulled the clutch at the same time.  The other readings are easier to attain, but they all pretty much have to be done while the bike is under load and moving (except for the idle circuit...  basically from no throttle to 1/8 throttle).

Did you properly sync your carbs with a manometer after the rebuild?

Got It. I'll try to do the Plug Chop, Weather Permiting.
I did sync the carbs after I rebuilt them, but I may recheck them just to be sure.
I'd have to borrow my cousins Sync Gauges though... 
Anyone know where to get decent gauges for a good price 

[warreng22]

Well, I didn't have alot of time today, but I did perform the WD-40 air leak test. I sprayed WD-40 on each rubber boot one at a time, at each end of the boot, from the air box to carb, and each boot from carb to engine. I never seen or heard a difference in RPM.

Well I have a confession to make, I honestly wanted to stay away from doing this, as I was afraid I could mess it up worse. BUT...
I have not done a valve gap check/maintenance 

Could this be my problem 

If so how hard is it to perform this, and can I cause permanent engine damage ?
Also what should the gap be for intake valves and exhaust valves 

If the valve gap is wrong (too wide) will the engine make a "tapping" noise 

[TwoTired]

I have not done a valve gap check/maintenance 

Could this be my problem 

It can contribute.  Particularly if it is currently maladjusted. I it is maladjusted, it prevents proper carb vacuum sync, too.

If so how hard is it to perform this, and can I cause permanent engine damage ?

Depends on how many thumbs you have.  And, yes you can cause engine damage if you bungle it badly.  Nothing is permanent.

Also what should the gap be for intake valves and exhaust valves 

Book says 0.002 in. for exhaust, and 0.003 in. for intake.  There are other posts on this forum that make this larger, though.

If the valve gap is wrong (too wide) will the engine make a "tapping" noise 

yes, it can.

Cheers,

[motomotard]

somebody told me that NGK plugs are hotter the lower the number, like a 7 will be hotter than an 8.

is that true? i know that most plugs go the other way, from ready this post it would seem the lower the hotter on NGK plugs.

thanks

[TwoTired]

See:
http://www.kaila.net/tl125/tl125ngkcode.html

[gregimotis]

Am following this because I have the same bike with the same problem and have done the same work to try and correct it (although I did get my valves sorted before synch).

The bike always starts eventually, but it's harder than it should be.  I know my choke(s) close completely (at least they did when I had them out on my bench two months ago) but applying choke to the warm engine has only a moderate dampening effect.

I haven't anything to contribute besides sympathy though, sorry.

[HondaMan]

I'm thinking:

1. Possible vacuum leak, like TT says.
2. More than one tight intake valve (no clearance at all, typically).
3. Really dirty intake valves (try several rounds with BG44K, mixed 1/6 can to a tankfull).
4. Points gap too small (under .012"), with slow timing.
5. Float levels set too low.

[warreng22]

Well I pulled the carbs today, to re-check my floats, one was a little off, fixed that. Also checked the coke linkage, found out that 2 of the carbs were not closing all the way, then remembered when one of the linkage bolts that I had replaced, had came loose once. I had retighted it not wanting to take off the carbs at that time. So I readjusted the Choke linkage, and put mid strength thread lock on the bolt this time, that should keep it from loosing back up again. While the carbs were off, I checked the valve clearances, they were way off. Either someone adjusted them wrong, or they had never been adjusted or something  The intake valve on #1 had a gap of .008    Exhaust was about the same. So I went through and adjusted all the valves, they were all way too loose. Put it all back together, turned on the gas, pulled the choke, fired it up. Sounded much better, but then I noticed that I had gas leaking out of my pitcock valve  Turned the pitcock off, gas stopped, have to fix that in the next day or two.

Since I adjusted the valves, do I need to recheck my timing? I didn't get to run the bike long enough because of the gas leak, to really hear if it sounded off or anything.

Thanks for the Help!

[HondaMan]

The valves won't affect the timing.

[warreng22]

Well... I've just about had it with this bike. I had little time this morning, thought I would fire up the bike, to see how much help the valve adjustment did. Sounded OK, but still sounded a little off. So, I thought I would check my timing again, but my luck I have one of these stupid timing lights with an "advance" knob. So when I checked my timing, it was "off". Me being to stupid to check to make sure the advance knob was on zero, I go adjusting the timing again. Only after I got into it, did I relize that the timing knob was on like 30  So I set it back to zero, and tried to reset my timing again. I can't even get it close. 
I run out of room on the timing plate, this is even with using TwoTired's method of taking a feeler gauge to take out the slack on the breaker plate.

What is my options ? How about a DYNA Ignition System? What exactly does this do, is it worth it, and does it get rid of the breaker plate?? Best place to buy?

[TwoTired]

Well... I've just about had it with this bike.

Umm, well, (cringing) ever consider that you may be more frustrated with yourself than the bike?  Some thing's are a slower learning process than others.  Frame of mind can be an important factor.

I run out of room on the timing plate, this is even with  using TwoTired's method of taking a feeler gauge to take out the slack on the breaker plate.

There are three places to insert the feeler gauge.  Have you tried all three?  If so, perhaps the points rubbing block is worn down.  How old are the points?  Is the point cam well lubed?  If you are having issues with your timing light set up, maybe start off with a static time and a 12V light bulb? It's pretty much all I bother with, anymore.

The dyna replaces the points mechanical switch with an electronic one.  Lots of people bet their wallet on them. I'm not one of them, though.

While, as Hondaman says, the valve adjust won't effect the ignition timing, it will effect the vacuum sync of the carbs.  And, before doing that, ALL the tuneup items should be nailed, including Ign timing.

Seems like you are almost there.  Be patient.  Fine tuning is a nit pick, detail oriented process.  It's a different adventure from riding.

Cheers,

[warreng22]

Yeah, I'll try again tomorrow 
I went to work today and cooled off about the bike, just got a bit agrivated this morning.

I'll try the feeler gauge in different locations tomorrow, I didn't think about that before. I was only trying in the bottom right. That may make a difference, also what is the best point gap setting .016  That is what I had been using, but then I changed it to .015 when I went to set it this morning. Don't know if that would be enough to make a difference, I know you dont want to get less than .012, gap would be to small. I think I will go back to .016

I'll sync my carbs via vaccume, after I get this timing set.  I have bench syncd them already.

Thanks Again Two Tired 

[HondaMan]

Yeah, I'll try again tomorrow 
I went to work today and cooled off about the bike, just got a bit agrivated this morning.

I'll try the feeler gauge in different locations tomorrow, I didn't think about that before. I was only trying in the bottom right. That may make a difference, also what is the best point gap setting .016  That is what I had been using, but then I changed it to .015 when I went to set it this morning. Don't know if that would be enough to make a difference, I know you dont want to get less than .012, gap would be to small. I think I will go back to .016

I'll sync my carbs via vaccume, after I get this timing set.  I have bench syncd them already.

Thanks Again Two Tired 

Here's a couple of cents' worth of my experience with points (of different brands):
Hitachi and TEC points set right up on .014" gap.
Denso points might do it at .014 or .015, but one of them will be different from the other.
Daichi points: they are all over. I've had them set up at .016 on one, .012" on the other (plate all the way advanced), and .012" on both (plate retarded). Upon closer inspection, found the rubbing block pivots to be welded in different places on all 4 sets!

[warreng22]

Yeah, I'll try again tomorrow 
I went to work today and cooled off about the bike, just got a bit agrivated this morning.

I'll try the feeler gauge in different locations tomorrow, I didn't think about that before. I was only trying in the bottom right. That may make a difference, also what is the best point gap setting .016  That is what I had been using, but then I changed it to .015 when I went to set it this morning. Don't know if that would be enough to make a difference, I know you dont want to get less than .012, gap would be to small. I think I will go back to .016

I'll sync my carbs via vaccume, after I get this timing set.  I have bench syncd them already.

Thanks Again Two Tired 

Here's a couple of cents' worth of my experience with points (of different brands):
Hitachi and TEC points set right up on .014" gap.
Denso points might do it at .014 or .015, but one of them will be different from the other.
Daichi points: they are all over. I've had them set up at .016 on one, .012" on the other (plate all the way advanced), and .012" on both (plate retarded). Upon closer inspection, found the rubbing block pivots to be welded in different places on all 4 sets!

Well that would explain why they are so unpredictable to set.  I (of course) Bought new Daichi points     Didn't know at the time that they were the "bad" ones. Thanks for the heads up HondaMan, I'll take a closer look at these tomorrow, when I try TwoTired's suggestion about moving the feeler gauge.