Topic: How Do These Plugs Look? New Movie - Much Better Throttle Response Off Idle!

[Imago]

I'm taking a break from my job jar/barrel today (my birthday) to work on my new (to me) bike, a 1978 CB750F. I thought I'd start with the recommended 3000 mile tuneup as it's running ok, but bogs a bit when I open the throttle. I've cleaned the carbs including pulling and cleaning the slow jets. If after the tuneup it's still exhibiting these symptoms, I'll post a short video.

I fired it up, warmed it up, pulled the plugs and was going to begin with a compression check when I discovered my new Princess Auto compression checker only has adapters for 14 and 18 mm plugs, so that will need to wait for another day until I get the proper adapter.

I did, however, pull the plugs and have attached a photo. They are from left to right, cylinders 1 to 4.

To start with, do they look about right?

 

[TwoTired]

Plugs are ok.  A little on the lean side.  But, that should get you good fuel mileage.  Shame you didn't mention anything about air filter and exhaust particulars.
Good news, they are all the same deposit patterns.

Doesn't tell you anything about carb jetting particulars, unless it was only run at one throttle position for those deposits.

I'd bank on weak accelerator pump volume as cause for the stumble on throttle up.

Cheers,

[crazypj]

Plugs look good but I think they look warm because they were not tightened properly.
The crush washers look like new so may not be transferring heat from shell into head?
 

[TwoTired]

The crush washers look like new so may not be transferring heat from shell into head?

Doesn't the heat transfer through the threads?
I learned that was why the spark plug base ring was always dark,as it is too cool there to support burning off the hydrocarbons.

What you say about warm does make sense, though.  The tip transfers heat to the plug body, which is part of the path for heat to flow into the head.

[lucky]

The plugs look perfect!!!  -LUCKY

BTW...What is happening as far as  the slow jets now?
Did you install the #40's?

I would like to continue the other thread (CB750 slow jets) and
add on to the data base I am building.

Your post said:
QUOTE "Anyway, my bike is stock except for the exhaust which is a baffled MAC, 4 into 1 so perhaps that's why the stock jets work fairly well. To be honest, you're likely onto something because even when warmed up, if I hit the throttle my bike bogs and takes a second to recover. Do you think that would improve with the size 40 jets? I have a manometer I got from the PO... maybe I should sync the carbs while I'm at it as well?"

How many turns open are the mixture screws Andrew?

Thanks!!

[lucky]

Plugs look good but I think they look warm because they were not tightened properly.
The crush washers look like new so may not be transferring heat from shell into head?

That is getting very exotic.
I have been involved in motorcycle racing for 40 years and never heard
anyone looking at plug color to tell if the plugs are loose or too tight. Usually that is checked with a torque wench. I think some torque wrenches need to be calibrated.
Many times people are using torque wrenches that have not been calibrated since new.

[Imago]

Plugs look good but I think they look warm because they were not tightened properly.
The crush washers look like new so may not be transferring heat from shell into head?

That is getting very exotic.
I have been involved in motorcycle racing for 40 years and never heard
anyone looking at plug color to tell if the plugs are loose or too tight. Usually that is checked with a torque wench. I think some torque wrenches need to be calibrated.
Many times people are using torque wrenches that have not been calibrated since new.

You guys are incredible. The plugs came out VERY easily, almost as though they had been only a bit more than hand tightened... great call crazyp! I've got them back in and gave them a good seating on the way in. Perhaps I'll check them now with my torque wrench although I may not get at the center two plugs with it. I also cut 1/4 inch off each plug wire, screwed the plug caps into them and they're back on.  Two tired: stock airbox, NOS paper filter and MAC 4 into 1 exhaust. Lucky: I removed the air screws when I cleaned the carbs and replaced them by initially turning them in until gently seated, then backing them out to exactly the same # of turns to initial seating. Each was a bit different and ranged from 1.25 to 2 turns. Next... valve adjustment. I haven't done this since 1978! Much fun.

[Imago]

Oh, and Lucky: still running the #35 stock slow jets.

[Mooshie]

Plugs look good but I think they look warm because they were not tightened properly.
The crush washers look like new so may not be transferring heat from shell into head?

That is getting very exotic.
I have been involved in motorcycle racing for 40 years and never heard
anyone looking at plug color to tell if the plugs are loose or too tight.

I think he deduced that not from the color of the plugs but from the crush washer condition. 

[Lostboy Steve]

Plugs look good but I think they look warm because they were not tightened properly.
The crush washers look like new so may not be transferring heat from shell into head?

That is getting very exotic.
I have been involved in motorcycle racing for 40 years and never heard
anyone looking at plug color to tell if the plugs are loose or too tight. Usually that is checked with a torque wench. I think some torque wrenches need to be calibrated.
Many times people are using torque wrenches that have not been calibrated since new.

+1

[lucky]

Talking to IMAGO:

Lucky- This is the set up Imago has .

1978
CB750F
Intake stock Airbox and filter.
Exhaust Mac 4 into 1 with muffler.
Idle jet #35
Mainjet # 120
Needle was not adjustable.
Mixture screw turns out?
Accelerator pump and nozzles working.

IMAGO is getting everything checked out as we have been reading. Finding spark plugs that were not tightened down is very unusual and a good eye (TWO TIRED) from that photo, that could tell the washer were not crushed. WOW!!

Mixture screws were also found to be unevenly adjusted.

[TwoTired]

Finding spark plugs that were not tightened down is very unusual and a good eye (TWO TIRED) from that photo, that could tell the washer were not crushed. WOW!!

Credit --> crazypj

[Imago]

OK, onto valve adjustment:

.002 intake and .003 exhaust, or did I read somewhere that both should go up .001. Perhaps in Hondaman's book?

Reason being .003 and .004 are optimal but Honda was just trying to make things quieter?

Or am I dreaming?

[TwoTired]

I think the CB750F 77-78 valve train is touchier than the 750's that Hondaman was tweaking.  I'd stick to Honda manual settings.  The valve stems already tend to mushroom.  Extra clearance is just going to "hammer" them faster, imo.

Cheers,

[Imago]

I think the CB750F 77-78 valve train is touchier than the 750's that Hondaman was tweaking.  I'd stick to Honda manual settings.  The valve stems already tend to mushroom.  Extra clearance is just going to "hammer" them faster, imo.

Cheers,

My feelers must have been anticipating your reply as I just broke my .004 trying to see if it would go in the exhaust side using the go, no go, approach. To be honest, I broke it trying to put just the right bend in it to get in there. The .002 and .003 bent perfectly, the .004 was a bit too thick for such shenanigans.

So I only have .002 and .003! So far so good. I've done 1 and 4. The points look like new so I think I'll install Hondaman's Transistorized Ignition for Dual Points, (I bought a refurbished one from him a while back) while I'm in there.

[lucky]

OK, onto valve adjustment:

.002 intake and .003 exhaust, or did I read somewhere that both should go up .001. Perhaps in Hondaman's book?

Reason being .003 and .004 are optimal but Honda was just trying to make things quieter?

Or am I dreaming?

A quiet valve could be too tight and burn because if the valve cannot come all the way up
then the valve (exhaust) cannot dissipate the heat into the cylinder head and out the cooling fins.

An intake valve if too tight will not be sealing during combustion and you will lose power.

The valve clearances given are based on the amount of expansion and contraction anticipated.

[cgswss]

Back to the plugs...

I agree, they look good.  Only by doing a WOT plug chop would you be able to get better info (as to main jet) but I don't think it would be enough to make any real difference.

I would recommend you try one simple change and see if we can help that bogging.  I agree that this could be the accelerator pumps but before doing anything to them I would suggest moving the needles up one notch.  This will get you into the main jet just a little sooner and my cure that lean surge without having to work on the accelerator pumps.

Craig

[lucky]

Back to the plugs...

I agree, they look good.  Only by doing a WOT plug chop would you be able to get better info (as to main jet) but I don't think it would be enough to make any real difference.

I would recommend you try one simple change and see if we can help that bogging.  I agree that this could be the accelerator pumps but before doing anything to them I would suggest moving the needles up one notch.  This will get you into the main jet just a little sooner and my cure that lean surge without having to work on the accelerator pumps.

Craig

On another thread (78 CB750 idle jets) Imago said the accelerator pump and nozzles were working. 

[TwoTired]

On another thread (78 CB750 idle jets) Imago said the accelerator pump and nozzles were working.

His motorcycle is also "working".  ...Then there is "working well" as differentiation.

[lucky]

On another thread (78 CB750 idle jets) Imago said the accelerator pump and nozzles were working.

His motorcycle is also "working".  ...Then there is "working well" as differentiation.

Yes.....

[Imago]

Well, I adjusted the valves, adjusted the cam chain tensioner, (at first I tred doing it while running as per the Honda shop manual but it got rattling/noisy fast, so I shut it down and did it the original way @ 15 degrees past top center). I also adjusted the points, they were a bit too wide. It also appears the PO had set the valves a bit more open than I have, I'd say perhaps at least .003 intake and .004 exhaust as I mentioned that I might try earlier, however I did end up setting them @ .002 and .003 as per Two Tired's advice.

My thoughts are is that nothing much has changed, if anything the valve train and cam chain is noisier than before. Perhaps a function of the tighter tolerance on the valves. I took it out on the highway and it pulls hard at anything above about 3000 RPM but as you can see in the video while it idles well @ 1000 RPM, it bogs as the throttle is applied, unless choked.

Take a look at the video... I'm thinking I need to pull and repair the accelerator pump on #2 carb as Two Tired suggested, and/or clean the carbs again. When I had the carbs apart last time, I did clean out some gunk from in there, although the jets were spraying on all four when the throttle was advanced and the diaphram looked fine. I haven't checked the timing as yet, could that be part the problem?
 

[Imago]

Mixture screws were also found to be unevenly adjusted.

If/when I take the carbs out again, would it be recommended to set these mixture screws all the same? The first time I had them out I cleaned and reset them exactly as I found them.

[cgswss]

I would check each screw and then re set all 4 to where the most backed out was set.  I take back my suggestion about raising the needles one notch because its lots more fun to take the bike apart and strip down the carbs a few times.  From your vid, I would say you very much have a lean surge.  Your mixture is going dead lean just off idle.  When you put in a little choke (the mixture is very rich) you don't see it near as bad. I would only work on one thing at a time- and that would be getting the mixture right just off idle.  Now if only I could think of something that would change the mixture as you pull off idle...

[Rgconner]

I think some torque wrenches need to be calibrated.
Many times people are using torque wrenches that have not been calibrated since new.

I have a retired locomotive mechanic as a neighbor. Soon after I got the CB550, I replaced the head to stop the leaks and check the general condition of the engine.

The oil leak was worse, and he came over to see what I was swearing at. I told him and then he said he would be back.

He re-torqued the nuts using his wrench, most tightened a good 1/4 turn.

"Well, the Misses will wonder what happened to me, so I better go. Get yourself a better torque wrench, use that one as a breaker bar."

[lucky]

When you have that kind of hesitation with the throttle from idle, the mixture is not rich enough or it needs a larger idle jet.

I will say it again....
There is no way that #35 idle jet is going to work with that exhaust.
You can try richening mixture screws but I just don't think it will do the job.

The SET UP:
1978
CB750F
Intake stock Airbox and filter.
Exhaust Mac 4 into 1 with muffler.
Idle jet #35
Mainjet # 120
Needle was not adjustable.
Mixture screw turns out?
Accelerator pump and nozzles working.