Topic: Help with a Plug Check (1970 CB750)

[Mikey G.]

I was wondering if I could get an extra couple sets of eyes on my spark plugs.  I've got a 1970 CB750 (w/ a '71 engine, if it matters) that I'm putting the last couple finishing touches on, and I'm having trouble getting it to 100%  Here's the story best I understand it.

-K0 four-cable carbs, sync locked in.
-Airbox, air filter, etc. is K0.
-Ignition timing spot on.

When the bike is warmed up it pulls nice and strong, so no issues there.  My problems are mostly at idle, and a couple things happen.  First, the bike takes a LONG time to warm up and hold idle.  Upwards of 10-12 minutes.  Second, when warm the idle has a tendency to dip a bit after decelerating.  Sometimes it's bad enough to almost stall early in a ride.  And finally, I'm getting a quite aggressive decel pop out the exhaust whenever I slow down.  Now, I know these carbs aren't fitted with air-cutoff valves like my other bikes, so a bit of popping is normal.  But how much is "normal" for these bikes?

Anyway, on to the plugs.  1-2-3-4.



So, I can't tell if 1 is rich, or what I'm aiming for.  Not a fan of the deposits on 2, and 4 to me seems lean, especially the center electrode.  But I'm still not sure what's the ideal color to be aiming for, so any advice would be appreciated.

[HondaMan]

Welcome, Mikey!
First, let me note this: those are some MIGHTY lean-looking 750 plugs! They normally show between tan and [dark] brown in about 10 miles in these bikes, due to the 1970s-era carb calibrations.

That said, do you know the genre of the brass parts inside the carbs? Reason I ask: 100% of the non-Honda aftermarket carb kits for the 750 today are calibrated between VERY and VERY, VERY lean. This means that at the least, the mainjets need to be (considerably) larger numbers than those found on the Keihin OEM jets. For example: in the K1 engine, with its cam, and K0-style carbs, the mainet with Keihin brass had a #120 mainjet until around 1/1970, then it went to #115. This corresponds to a K&N carb's jet number of #140-#135 (same for Keyster jets).

The jet needles in aftermarket kits are also very thick, and they have their own [oversize] needle jets (this is the part in the carb where the slide needle rides up-down with the slide, metering the fuel) with accordingly oversized holes to fit them (thicker needle jets, larger holes). If the Keyster/K&N needles are used with the Keihin needle jets (in the slides) then it will be extremely lean: much like the way your sparkplugs look above?

Do you know the genre of the brass parts in your carbs?

[Mikey G.]

Brass inside the carbs is all genuine Keihin parts.  Pilots are good (whatever they need to be- I can't recall the exact number, but they were correct when I checked it at the time).  Mains are #120.  Needles are stock as well, and clip is in the middle notch.  I tried raising the needles once, but the bike ran like garbage, so I put them back and it was better.  Forgot to add I'm running HM300 pipes, for what it's worth.

I know I'm also chasing a fuel delivery issue, mainly a clogged vent in the gas cap.  It's on my to-do list, but that's more of an intermittent problems rather than a constant one, so I didn't bring it up.

[HondaMan]

I know I'm also chasing a fuel delivery issue, mainly a clogged vent in the gas cap.  It's on my to-do list, but that's more of an intermittent problems rather than a constant one, so I didn't bring it up.

I suspect you may have just quoted the cause, there? It doesn't take a lot to stop the flow, as the hydrostatic head is less than 0.2 PSI with a completely full tank.

Also: do your fuel lines run 'downhill' all the way between the fuel petcock and the carbs? I just 'fixed' [another] 750 this past week here that had the fuel lines running below the carbs, to a filter, then back up to the carbs. It wouldn't run over 40 MPH.

[scottly]

There is nothing too extreme from rich to lean that I see; you're not in immediate danger of fouling plugs or seizing. The deposits on #2 look like ash from burnt oil. How many miles on those plugs, and under what conditions?

[newday777]

I was wondering if I could get an extra couple sets of eyes on my spark plugs.  I've got a 1970 CB750 (w/ a '71 engine, if it matters) that I'm putting the last couple finishing touches on, and I'm having trouble getting it to 100%  Here's the story best I understand it.

-K0 four-cable carbs, sync locked in.
-Airbox, air filter, etc. is K0.
-Ignition timing spot on.

When the bike is warmed up it pulls nice and strong, so no issues there.  My problems are mostly at idle, and a couple things happen.  First, the bike takes a LONG time to warm up and hold idle.  Upwards of 10-12 minutes.  Second, when warm the idle has a tendency to dip a bit after decelerating.  Sometimes it's bad enough to almost stall early in a ride.  And finally, I'm getting a quite aggressive decel pop out the exhaust whenever I slow down.  Now, I know these carbs aren't fitted with air-cutoff valves like my other bikes, so a bit of popping is normal.  But how much is "normal" for these bikes?

Anyway, on to the plugs.  1-2-3-4.



So, I can't tell if 1 is rich, or what I'm aiming for.  Not a fan of the deposits on 2, and 4 to me seems lean, especially the center electrode.  But I'm still not sure what's the ideal color to be aiming for, so any advice would be appreciated.

I had the same running, decel popping and at idle earlier this summer on my K5, I had cleaned another set of carbs in the ultrasonic cleaner but hadn't run the High E string wire into the internal ports of the carb bodies through the little brass dishes with the holes that feed air in to mix with the fuel emulsion tubes like Mark lays it out in his 750 restoration book.
Spray carb cleaner and run the wire carefully to clean out the old powdered MTBE additives from the old gas that blocks the proper flow and mixtures, along with compressed air and lots of carb cleaner.
Also clean out the bowl vent ports at the same time.

[Mikey G.]

Plugs are relatively new.  A couple of cumulative days running in the garage for tuning and sync, then maybe 15-20 miles out on the road at various speeds (from sedate to.... spirited.  ).  She usually does pretty well for the first couple miles- runs well, pulls well, etc.  It seems like once it's good and hot is when the fuel starvation issues kick in, but that's mostly losing power across the whole rev range until I pop the gas cap.

(These aren't plug chops, though.  This is more of an average across my week or two of testing.)

I get the decel pop right away, though.  No matter the bike temperature.

[seanbarney41]

Try a quarter turn either way with the air screws.  That may help achieve a steady idle too.  As far as the long warm up time?...if you are stationary waiting for it to idle it may take more than twice as long as just getting on and riding and keeping your hand in the throttle for the first couple miles so it doesn't stall.

In my opinion, all final carb adjustments CANNOT be made until after about a twenty minute ride.  And then don't mess with stuff during warm up.

If the bike is performing well, I try not to concern myself with the plug reading too much.  Plug readings are gonna change slightly with different fuels even when performance does not.  I have always found these engines to perform a little better with darker readings than other engines say from Kawasaki or Suzuki.

[Mikey G.]

Alright, update time.  Got a good 30 minute ride last night, and I learned some things-

-Drilling the gas cap vent solved a lot of my fueling issues.  Nearly all of my "intermittent" issues vanished.  So that's good.

-Idle still likes to dip after a long deceleration.  If I kinda stay on the throttle it helps a little, but it'll dip really hard, plus bog and hesitate a bit when I throttle back up if I slow down by closing the throttle completely and engine braking.

-If I'm cruising at like 30 mph in 4th gear, just under 3k rpm with the throttle JUST barely cracked open, if I try to accelerate at all it'll burble and hesitate a bit before catching.  This is very repeatable.

-Funny enough, when I got home and it was idling in the garage, hot, it seems like my worst cylinder was #2.  I'll need to look into this.

I'll pull plugs again after work, but overall it's much better than it was, and the problems I have now are consistent.  I can work with that.

[HondaMan]

-Drilling the gas cap vent solved a lot of my fueling issues.  Nearly all of my "intermittent" issues vanished.  So that's good.

Good find!

-Idle still likes to dip after a long deceleration.  If I kinda stay on the throttle it helps a little, but it'll dip really hard, plus bog and hesitate a bit when I throttle back up if I slow down by closing the throttle completely and engine braking.

-If I'm cruising at like 30 mph in 4th gear, just under 3k rpm with the throttle JUST barely cracked open, if I try to accelerate at all it'll burble and hesitate a bit before catching.  This is very repeatable.

I might mention here: that's not anywhere near the engine's normal powerband? 30 MPH in 1st, 2nd or possibly 3rd gear is OK, but in 4th gear that is both lugging the little pistons and well below the linear mixing range of the carbs: at 30 MPH in 3rd gear, they are running in the normal idle-only mixing range of about 1400 RPM or so: in 4th gear it's well below that range. The SOHC4 engines don't like to run at low RPM in tall gears, nature of the beast.

[Don R]

 That higher gear at low RPM borders on engine abuse, don't expect a happy little engine being lugged down like that.

[Mark1976]

Alright, update time.  Got a good 30 minute ride last night, and I learned some things-

-Drilling the gas cap vent solved a lot of my fueling issues.  Nearly all of my "intermittent" issues vanished.  So that's good.

-Idle still likes to dip after a long deceleration.  If I kinda stay on the throttle it helps a little, but it'll dip really hard, plus bog and hesitate a bit when I throttle back up if I slow down by closing the throttle completely and engine braking.

-If I'm cruising at like 30 mph in 4th gear, just under 3k rpm with the throttle JUST barely cracked open, if I try to accelerate at all it'll burble and hesitate a bit before catching.  This is very repeatable.

-Funny enough, when I got home and it was idling in the garage, hot, it seems like my worst cylinder was #2.  I'll need to look into this.

I'll pull plugs again after work, but overall it's much better than it was, and the problems I have now are consistent.  I can work with that.

   So, as seanbarney mentioned, make sure its throughly warmed up before doing you sync, and you can recheck your mixture screws settings just to make sure they aren't a little on the lean side. 1/4 turn to the richer may have some impact at idle but beyond that, not much else in my opinion.
   But the dip in idle coming off of the throttle and the burble (stumble, I bet its really annoyingin the first 3 gears, especially when short shifting) getting back on the throttle, once warm, is a sign of an ever so slightly rich condition. My 1st inclination is a slightly high float level, have you clear tubed the rack? I didn't see that in the thread anywhere... It a process of elimination, its a good place to start, as it doesn't require much work... there's other things to look at as well but eliminating too high a float level is easy and quick...
   Wring it out a bit, get it good and warmed up. Make sure it'll pull cleanly through the entire rev range. Your pretty close...

[Mikey G.]

Thanks for the info.  I did have a couple of interesting developments.

First off, I may be over-selling how much I lugged the engine in-town.  There's a decent chance I was in third, and right now my tach is just a suggestion (that's a problem for another day).  I wasn't too far off-idle, but I definitely had the throttle cracked a bit.  Regardless.

After my last ride stated above I was able to pull the plugs and check again.  Color temp was better, but 3 and 4 still seemed a bit lean.  I enriched the idle a bit and did another trip around the block.  Now, admittedly it didn't get HOT-hot, but I did get a good 20 minute run, and you know what?  She ran pretty good!  No hesitations I could tell, no idle dip after slowing down, and a shockingly good idle.

Now, I'm not calling this done, not by a long shot.  But I definitely seem to be inching closer.