Engine running lean?

[surfish95747]

Okay guys, I rebuilt the top end with new rings, new exhaust valves and set the valve clearances to within spec, most are right at .003", a couple are at .0025" and one is at .005 and .006". The ones that were really out of spec I replace to bring them within acceptable ranges. I just synced the carbs and tested compression. I got 135, 130, 125 and 135 for compression. I have riden it around for the last week and took a picture of the plugs.





This was from cylinder one but all four look exactly the same. I'll get a picture of all for together. I have a MAC 4-2-1 exhaust. Does this look lean to you? I get a little popping on decelleration and at high rpms, like around 7000 or 8000 I fell like the bike doesn't get enough gas to it like it chokes very slightly. There is a slight rattle as well. What do you think? 1980 cb750k.

[surfish95747]

Well, I did some more research on my own and these look slightly lean compared to the picture:



So, I have a Dynojet jet kit and according to them I will only use their Stage 1 kit, since I have stock airbox and stock type K&N filter and stock carbs with a MAC 4-2-1 exhaust. Do you guys recommend using this kit or just simply going up a size on the jets?

I have popping at times (not all the time) on deceleration only and when getting into the higher RPMs sometimes it feels like the engine chokes a little or cannot get the juice it needs to be there. It is just very very subtle. Other than that the bike starts right up, warms up and idles right at 1100 no problem. Thanks guys.

[shinyribs]

The 'optimal' plug may be accurate for an electronically controlled fuel injected vehicle. But for a carbureted ride I'd consider that dangerously lean. Odd the the ''lean'' plug on the chart is not a lean as the ''optimal''.  

Coffee with cream in it is the best description for a good plug color. The color of your ground strap ( the part you bend to adjust the plug gap) looks like overly advanced ignition timing, or wrong heat range of plug. That strap should stay shiny. When the coating burns off and it turns flat looking that's a sign of an overheated plug. That ''optimal'' plug has been so hot you can see the heat discoloration working it's way down the threads.

Actually, now that I think about it, I'm pretty sure whoever labeled that pic accidentally confused the lean and optimal plugs. That ''lean'' plug looks real good IMO. I bet it you were to wipe the carbon off the ground strap it'd still be shiny under there.

Your plug really looks OK. Maybe a tad lean, but not bad at all. Hard to say how accurate the ground strap color is in the pic. Will the bike rev out to redline cleanly?


p.s.- DONT run any valves too tight. Valves have a tough life. Especially in an air-cooled engine. The only time they have to be cooled/ dissipate heat is the split moment they are in contact with the seats. Running a too tight valve can burn one in a heartbeat. You're better off running a tad loose. That way they have a bit more time to transfer their heat in to the cylinder head,and eventually out through the cooling fins. This is even more important considering today's hotter burning ethanol junk they call gas

[surfish95747]

So are you saying mine looks okay? Just need to check the timing?

On May 2, 2016 12:59 AM, "shinyribs [via Honda CB750'S]" <[hidden email]> wrote:

The 'optimal' plug may be accurate for an electronically controlled fuel injected vehicle. But for a carbureted ride I'd consider that dangerously lean. Odd the the ''lean'' plug on the chart is not a lean as the ''optimal''.  

Coffee with cream in it is the best description for a good plug color. The color of your ground strap ( the part you bend to adjust the plug gap) looks like overly advanced ignition timing, or wrong heat range of plug. That strap should stay shiny. When the coating burns off and it turns flat looking that's a sign of an overheated plug. That ''optimal'' plug has been so hot you can see the heat discoloration working it's way down the threads.

Actually, now that I think about it, I'm pretty sure whoever labeled that pic accidentally confused the lean and optimal plugs. That ''lean'' plug looks real good IMO. I bet it you were to wipe the carbon off the ground strap it'd still be shiny under there.

A free people ought not only be armed and disciplined, but they should have sufficient arms and ammunition to maintain a status of independence from any who might attempt to abuse them, which would include their own government.~George Washington

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[TOOLS1]

I have agree with Shiny on the plug labels being switched. There is no way that that "optimal" plug should have the ground strap burnt off at the end. As for the popping on deceleration, have you checked the aircut valves? They will make a bike sound like a machine gun.
TOOLS

[Re-run]

I have to say, I don't go with the flow on plug colors. In my 78, I go for about what the "lean" plug looks like, slightly whiter even. Darker plugs might be fine for older carbs without accel pumps and such, but if I were to run my bike at "coffee with cream", it reeks of gas in the exhaust, and I smell of it all day after 5 minutes.

Also, if you are still using a paper filter, changing your exhaust will almost certain  NOT require a rejet. Jet companies like to tell you that garbage because it makes them money. Before you ever rejet from an exhaust change, try tuning the bike first. I put in a k&n filter in my box, slight retuning is all that was needed.

When tuning, don't try to go by ear, you will never get very accurate results. I have seen too many people do it that way and have their mix way off. Never have I seen that be a successful method. Do it right, get a decent electronic tach. They can be had for relatively cheap and can be used on other bikes too. Hook it up and follow the manual. Start with #1 and tune your idle mix till the rpm maxes out, then back it down the amount the manual recommends, maybe just slightly more. You will basically do that across all 4 carbs.

When done right, you will have a responsive bike with good power.

[surfish95747]

Okay, so you are saying mine looks too rich? So I followed the manual as far as syncing the carbs, but in a lurker confused about the electronic tach? My bike only has a mechanic tach with cable. But your also talking about adjusting the pilot screw right? In just trying to wrap my head around all this fine tuning.

Tools, I will check my accelerator pumps.

Thanks guys.

On May 2, 2016 8:28 AM, "Re-run [via Honda CB750'S]" <[hidden email]> wrote:

I have to say, I don't go with the flow on plug colors. In my 78, I go for about what the "lean" plug looks like, slightly whiter even. Darker plugs might be fine for older carbs without accel pumps and such, but if I were to run my bike at "coffee with cream", it reeks of gas in the exhaust, and I smell of it all day after 5 minutes.

Also, if you are still using a paper filter, changing your exhaust will almost certain  NOT require a rejet. Jet companies like to tell you that garbage because it makes them money. Before you ever rejet from an exhaust change, try tuning the bike first. I put in a k&n filter in my box, slight retuning is all that was needed.

When tuning, don't try to go by ear, you will never get very accurate results. I have seen too many people do it that way and have their mix way off. Never have I seen that be a successful method. Do it right, get a decent electronic tach. They can be had for relatively cheap and can be used on other bikes too. Hook it up and follow the manual. Start with #1 and tune your idle mix till the rpm maxes out, then back it down the amount the manual recommends, maybe just slightly more. You will basically do that across all 4 carbs.

When done right, you will have a responsive bike with good power.

The ride IS the adventure. The destination is just to get gas!

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[Re-run]

The bike tach sucks. No bones about it. It is only good as a guesstimate. They can be over 100 rpm off. So you get a good electronic tach, one that runs off the spark impulses. You hook it to your battery for power. Many are accurate to about 30 rpm.

Yes, the pilot is also called the idle mix screw. The idle jets actually affect a large rpm range, up to about 60-70% throttle. On these later carbs, they meter fuel, not air like the early carbs did. So, turning the screw out makes the mix richer, in makes it leaner. Think of it like a fuel valve.

As for your plugs, if they were i MY bike, I would call that slightly rich. YOUR bike may be different. That is the issue with using pics from other sources. They are not YOUR bike. That is why proper tuning is important.
I mean, you can tune it so it runs "well enough", or you can tune it to get good performance.

[surfish95747]

Okay I will get an electronic tach.

On May 2, 2016 1:05 PM, "Re-run [via Honda CB750'S]" <[hidden email]> wrote:

The bike tach sucks. No bones about it. It is only good as a guesstimate. They can be over 100 rpm off. So you get a good electronic tach, one that runs off the spark impulses. You hook it to your battery for power. Many are accurate to about 30 rpm.

Yes, the pilot is also called the idle mix screw. The idle jets actually affect a large rpm range, up to about 60-70% throttle. On these later carbs, they meter fuel, not air like the early carbs did. So, turning the screw out makes the mix richer, in makes it leaner. Think of it like a fuel valve.

As for your plugs, if they were i MY bike, I would call that slightly rich. YOUR bike may be different. That is the issue with using pics from other sources. They are not YOUR bike. That is why proper tuning is important.
I mean, you can tune it so it runs "well enough", or you can tune it to get good performance.

The ride IS the adventure. The destination is just to get gas!

---

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http://honda-cb750-s.456789.n3.nabble.com/Engine-running-lean-tp4060241p4060261.html

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NAML

[surfish95747]

http://pages.ebay.com/link/?nav=item.view&id=161885080877&alt=web

Is this what you are talking about?

On May 2, 2016 1:05 PM, "Re-run [via Honda CB750'S]" <[hidden email]> wrote:

The bike tach sucks. No bones about it. It is only good as a guesstimate. They can be over 100 rpm off. So you get a good electronic tach, one that runs off the spark impulses. You hook it to your battery for power. Many are accurate to about 30 rpm.

Yes, the pilot is also called the idle mix screw. The idle jets actually affect a large rpm range, up to about 60-70% throttle. On these later carbs, they meter fuel, not air like the early carbs did. So, turning the screw out makes the mix richer, in makes it leaner. Think of it like a fuel valve.

As for your plugs, if they were i MY bike, I would call that slightly rich. YOUR bike may be different. That is the issue with using pics from other sources. They are not YOUR bike. That is why proper tuning is important.
I mean, you can tune it so it runs "well enough", or you can tune it to get good performance.

The ride IS the adventure. The destination is just to get gas!

---

If you reply to this email, your message will be added to the discussion below:

http://honda-cb750-s.456789.n3.nabble.com/Engine-running-lean-tp4060241p4060261.html

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NAML

[Re-run]

I'd try to find more info about it, but essentially, yes. Years back, I got a dwell meter when I still ran points. Much more accurate than simply gapping the points. Anyways, it also has a tach feature, and I use that. Most electronic tachs are pretty cheap these days, and pretty accurate.

[surfish95747]

Okay. I'll look into it. I'm going to start this weekend by taking the carbs back off and checking the air cut off valves. I don't think I replaced those last year when I rebuilt them, and there is something nagging the back of my brain saying that one is missing a spring... I can't remember, hence why I need to get back in them. I'll double check and make sure the accelerator pump is working and then reset the pilot screws to what factory says (they should be there already, just making sure). Then once back on I will check the timing, tune the air/fuel mixture like you explained and follow the manual and resync the carbs. After that I will let you all know my results.

Is there any damage that can be done to the engine riding this week while I wait to tear into it again? Thanks.

[Re-run]

I don't see why there would be anything wrong with a bit of riding. I probably would. One thing you could do to see if it is lack of fuel is ride with the choke on at various positions and see if that high speed issue occurs.

[surfish95747]

Okay. So, I tried opening the choke a little, and with it pulled out just barely, the rpms increase a tiny bit and I get popping in gears 1-3 and then when I get into 4 and 5 it is noticeably smoother and less popping on decel.

[surfish95747]

Also I just pulled off one of the Air cutoff valve covers and the valve looks very deteriorated. I am thinking I had planned to replace those when I rebuilt the carbs but then forgot and just put the olds ones back in. I think, if memory serves, one might even be missing a spring.

[surfish95747]

I will be buying new ones.

[Re-run]

Yeah, it's probably best to have fully working carbs before attempting to adjust anything.

[surfish95747]

So testing the timing I got this.



The fixed index mark line up just after the 11 mark and before the I mark. Is that acceptable or does it need to be right on with the 11 mark? And when shooting the timing light at it, should that light be going off consistently or not, because mine was not consistent. It did seem there was a pattern to it, but I couldn't tell. Thanks!