Topic: 1978 CB750F Carbs Spitting Gas From Intake?

[ViperGravyDavey]

Hey Everyone,

I'd like to hear your thoughts on what is causing this issue on my 1978 CB750F. Here is a video of the gas mist blowing back from my carbs:

https://m.youtube.com/shorts/xug2FpNUhqs

You can also hear a couple of pronounced "pffts" accompanied by a larger mist of gas blowing out of them. Why is that?

My brain tells me this is due to one or more of the following:
A) Cam chain tensioner has a weak spring
B) Incorrect tappet adjustment procedure
C) Camshaft installed one tooth off
D) This is normal for a high-duration performance camshaft at low-ish rpms? (Yeah she got cam'd 😀)
E) I did a bad job at lapping my intake valves (the combustion chambers held water though when testing whether or not they sealed...)
F) My bike (her name is Pearl) just likes giving me a hard time
G) Cam chain is stretched and needs to be replaced

The whole reason I'm asking this is because my pod filters are all (especially cylinder #2) dripping with gas after a few minutes of riding. I'd prefer not to turn into a flaming ball of Honda parts while riding my favorite routes.

Let me know what you think!

-Cole

[bryanj]

A no effect
B procedure or clearance
C with big cam would probably hit
D probably
E possibly
F almost certainly
G unlikeley

Most big cams require different ignition timing have you reset to cam makers specs?

[PeWe]

My CB750K6 could do that when carbs were too lean, pilot circuit.

And a lot when advancer springs were too sloppy so full advance kicked in too early.  See my thread about it below.

It has rather hot cam, Megacycle 125-75.
Dyna-S ignition timed spot on the F mark at 1100-1200rpm.
Cut both advancer springs almost 1 wound is a must.

Full advance kick in at around 2700 rpm.
It idles really nice at 1000-1100 rpm despite cam and rather high compression. (Balanced APE crank part of it)

Will increase to 1200rpm for easier take offs.

My almost stock K2 has its springs cut too.

Here when I fixed my K6
http://forums.sohc4.net/index.php/topic,131339.msg1464986.html#msg1464986

Then with Pamco ignition and its stock 300 advancer.
Worked fine with points too.
Later changed to Dyna-S and AD125-01 advancer.

My K2 got cut springs too with points to delay ign advance enough to be spot on F at idle.
I tested Yamiya "race springs" that did not make much difference.
Cut stock springs fixed it.
It has 300 advancer.

If advancer has a play when the small nut holding it is released:
Move it most counterclockwise and lock it there.
T will then align correctly at TDC.
F will be correct too.

Same on both my bikes. I have checked with piston stop.
Also measured on pistons when head was off.

Rotate engine with the big nut on advancer will make it move forward even if plugs are out.

[Tracksnblades1]

What cam.
What intake clearance?
Did you degree the cam and check clearances?

I would do a leak down test on it as it is in the video.
Lightly leaking intake valves spit back through the carbs too. However it is rare that all 4 would leak, unless bent slightly. Long duration cams have some reversion. Fuel can exit the exhaust unburnt at idle speeds. The cam card will tell “the rest of the story”

[ViperGravyDavey]

Thanks for all the responses so far, man the people on this forum are awesome  I just attached an image of the cam card.

This was my first attempt at putting a new cam into anything (yes I used zinc additive and breakin oil  ) and when installing it I simply followed the Honda manual's procedure for resinstalling a camshaft. Looking back perhaps that was a big no-no since "hey dummy, that's not Honda's cam so of course you gotta degree it durrr..." but I'm curious to hear what y'all have to say.

I did adjust my intake and exhaust tappets to have the clearance on the cam card too, .005 inches of lash on both.
After a carb sync, she still does not want to idle below 2,000 rpms. Perhaps this is related to PeWe's comments about the advance mechanism springs?

I'm running a Dyna S ignition too, so we can rule out any mechanical points and condensers as variables. New coils, new gapped spark plugs, 5k resistance on spark plug caps with new 5 ohm coils and new spark plug wires too.

My idle circuit would turn the plugs black and sooty with 40 idle jets, same when I tested them with 35's. Tried multiple air screw adjustments and still ran a little rich at idle according to the plugs being black. I'd like to get this gas spitting issue sorted out first before seriously dialing in the carbs, unless of course this behavior is normal for this cam.

Super long post, but I'd rather give too much info than not enough. Thanks all!

[newday777]

Thanks for all the responses so far, man the people on this forum are awesome  I just attached an image of the cam card.

This was my first attempt at putting a new cam into anything (yes I used zinc additive and breakin oil  ) and when installing it I simply followed the Honda manual's procedure for resinstalling a camshaft. Looking back perhaps that was a big no-no since "hey dummy, that's not Honda's cam so of course you gotta degree it durrr..." but I'm curious to hear what y'all have to say.

I did adjust my intake and exhaust tappets to have the clearance on the cam card too, .005 inches of lash on both.
After a carb sync, she still does not want to idle below 2,000 rpms. Perhaps this is related to PeWe's comments about the advance mechanism springs?

I'm running a Dyna S ignition too, so we can rule out any mechanical points and condensers as variables. New coils, new gapped spark plugs, 5k resistance on spark plug caps with new 5 ohm coils and new spark plug wires too.

My idle circuit would turn the plugs black and sooty with 40 idle jets, same when I tested them with 35's. Tried multiple air screw adjustments and still ran a little rich at idle according to the plugs being black. I'd like to get this gas spitting issue sorted out first before seriously dialing in the carbs, unless of course this behavior is normal for this cam.

Super long post, but I'd rather give too much info than not enough. Thanks all!

It is because of not using a degree wheel to time the cam and so you have valves open at the wrong time, thus spitting raw gas into your filters.

[PeWe]

Kenny Harmon D cam.
When Action Fours made them they claimed in their catalog that they could be installed right in without timing.
Their SS-1 cam too. But I timed it to be sure.

I guess that Webcam take for granted that adjustable sprocket is used and timed according to cam card. Ca 2 degrees advanced is generally ok.

Before fiddling with the cam, cut the advancer springs as I described.
If they are as mine were, engine will run as crap spitting.

[HondaMan]

Before fiddling with the cam, cut the advancer springs as I described.

^^ What Per said...
When any cam is installed in the 750 that opens more than about 10-12 degrees BTC intake and the spark advancer is not slowed down, the engine spits back like yours. At dragstrips in the 1970s this sometimes made for flaming spitback from intake velocity stacks when running the Saturday Night Drags.

I'd suggest slowing the spark advance to reach maximum somewhere north of 3200 RPM with that cam, maybe a little more.

[Tracksnblades1]

Viper,
Tell us if you’re pretty handy with engines..?
Can we be sure your 0.005 thousandths lash is 0.005…?
Did you have the valves and guides renewed…?

Yes your dyna rules out poorly adjusted points and bad condensers…
But it does nothing for scattering timing..?
Or poorly performing idle circuits in the carbs…?

The 2000 rpm idle with less than a off road drag race only camshaft is giving you a hint…
Follow PeWe's springs advice. It would be better if it could idle at 1200 rpm and watch the timing with a timing light..

You could always run 0.010 thousandths or more intake lash for a short time to see if it’s actually the cam timing.
Remember these fire on the exhaust stroke too…you don’t want nothing lighting off during valve overlap …?

[seanbarney41]

My 836 with KHD cam idles at 1200rpm nearly as smooth as a stocker, no crazy jet changes either, just 130 mains that may only be necessary because I need to raise the needle.  I did not degree the cam, just stabbed in with the stock cam gear just as it had been from previous owner.

What I would look at...
    float operation/level
    Vacuum leaks
    Ignitiom timimg

[ViperGravyDavey]

Based on all the feedback this appears to be a timing advance issue that can be solved by tightening up my advance's springs (shortening them). I assume this gas "spitting" is due to the wasted spark igniting some leftover fuel during the exhaust stroke where my cam has some overlap going on? That's what PeWe's post linked earlier in this thread seemed to be anyway.

Hopefully newday777's scenario is not the case as I do not have a frame kit to work on top-end components in-frame and would have to drop the engine again (what a pleasure that is   ).

Seanbarney41, I have checked for vaccum leaks via the WD-40 spray test and saw no change in idle speed, as well as the clear tube test for float levels (which were about 2mm below the float bowl gasket on the side closest to the engine). Looks like ignition timing is all that's left!

Tracksnblades1, I am no engine expert but I have done a couple of top-end rebuilds on other Hondas (Rebel 250) and now this CB750 SOHC. I am confident in the .005 measurement  And no, my valve guides are original to the best of my knowledge, but I did replace the valve seals and lapped all of them earlier in this build.

Hondaman, any advice on how far to chop my advancer's springs? Just a little bit at a time until the advance activates around the range you suggested? Or until the gas spitting stops I guess (without getting crazy late advance activation).

Much appreciated!
-Cole

[Don R]

 Most of the end coil seems to be enough, I like to use two needle nose pliers to make the new loop. It won't be perfect like what's already there, but you can get it good enough to stay on and work.

[Tracksnblades1]

Ok Viper, that’s good to know.

Just worried about your guides, retainers, locks, and valves..

78F’s had their reputation in case you don’t know about it..

[Tracksnblades1]

My 836 with KHD cam idles at 1200rpm nearly as smooth as a stocker, no crazy jet changes either, just 130 mains that may only be necessary because I need to raise the needle.  I did not degree the cam, just stabbed in with the stock cam gear just as it had been from previous owner.

What I would look at...
    float operation/level
    Vacuum leaks
    Ignitiom timimg

Yeh, down here it was 125s in about every 836 with a bread box…😁
Some read 105 but the local guy drilled them, so following his lead you had to gauge everything so you knew what you had before making a change..

[HondaMan]

Hondaman, any advice on how far to chop my advancer's springs? Just a little bit at a time until the advance activates around the range you suggested? Or until the gas spitting stops I guess (without getting crazy late advance activation).

With that much early opening on the intakes it might take more than 'usual', but 'usual' involves cutting off the end loop where it hooks into the weight, then bending the next loop out to make it fit into the little hole on the advancer weight(s). On most of these I rebuild I find they need at least 1/2 turn cut off both springs due to the springs themselves just being softened (annealed) by the engine's heating cycles. That only restores the OEM advance rate in most cases, With such early-opening intake valves I'd suggest cutting of a whole coil from each spring to slow down the advance more.

In the last century (when the springs were still new-ish) we would sometimes grind back the stops for the weights (on the advancer's baseplate) to half-thickness or less to make a longer advance angle, then set the points baseplate back so the idle advance was closer to 0 degrees. It would still advance to the normal full advance (at the OEM advance rate, the springs being new) at a little later RPM (like 2800-3000) but having less advance at idle speeds let those engines idle for street use. IIRC those cams were opening the intakes at around 10 degrees BTC (they came from Action Fours), and this change let the engines behave well enough to run the Saturday Night Main Street circuits and even commute to work pretty well.

[bryanj]

Many years ago i fitted a Yoshi street cam to a 500 and it was a real dog to ride, when we got onto suppliers and they sent the full fitting instructions it said increase timing advance(sorry cant remenber what to, but i do remember the advace mark was only just on the advancer unit) and also add 3 teeth to the rear sprocket, this brought it back to just over standard 500 but reving higher and "power band" was much tighter and higher.imo not worth the expense

[PeWe]

Check the link in post #3.
Photos how I cut the springs. You cannot do it wrong.

That advancer worked well after that with various cams.
- CX-7 (when I got and solved the problem)
- Megacycle 125-20 with similar cam open- close timing as KHD.
- Action Fours SS-1
- DP 315 (open very early- close later with less valve clearance, as other cams)
- RC 295

[ViperGravyDavey]

Lots of helpful info from everyone, I finally have some free time tonight and I will be cutting down my advance springs and triple checking timing with my timing light. I will report back with my results.

[ViperGravyDavey]

Ran a little short on time tonight so I didn't bust out the timing light  but I was able to cut both advance springs down, reinstall ignition, and give her a couple minutes of neutral revving in the garage. I still have a touch of spitting going on, though it gets better as she warms up. Overall, I'd say there was a 50% improvement from what my video in my original post showed once the engine was warm.

I'm hopeful that doing a valve lash adjustment tomorrow night or maybe the next (as well as fine-tuning the idle circuit so she isn't too lean) will take care of the remaining spitting.

Still does not like idling below 2,000, though I synced the carbs a few weeks ago. Probably an idle circuit thing still. Maybe I should cut the advance springs even further? I'll use the timing light in the near future to see where she reaches full advance to get a better gauge of whether I should cut again or not.

Definitely an improvement, but still have some work to do. My pod filters were not dripping fuel like they were before the spring modification.

Pic below is how much material from each advance spring I removed (Just the original end loop, probably best I cut a bit more back. Didn't want to get too carried away.)

[PeWe]

Not more than that needed.
It must be something else.
Verify that carbs are OK.

It's a pity that cam can not be timed or only measured without pulling the engine or cut frame for a frame kit.
Both my CB750's have frame kits.
Time cam is easy, even change cam or head or cylinder is easier when engine in frame.
Franks weldless.

[denward17]

Just curious, but is your fuel routing the same on the bike as with the video?

If so, could be fuel starvation affecting the running, but I don't see how it could affect fuel spitting out the back.

[ViperGravyDavey]

Just curious, but is your fuel routing the same on the bike as with the video?

If so, could be fuel starvation affecting the running, but I don't see how it could affect fuel spitting out the back.

This is my setup currently. (Thanks for giving me a chance to show her off
No fuel starvation here. I did make an interesting discovery today though. First off, with the advance springs cut she spits back much less whch is great. Secondly, she only wants to idle at 2,000 rpms which according to my timing light is when my advance just finishes flaring out. Sounds like 2k rpms by ear, but could be a wee bit off. Seems like I need to cut my advance more. For context, she runs great on the street at higher rpms. Around 4k to 7k she pulls and seems to be asking for more, but 3k and below seems really sluggish like I'm trying to get her to ride through mud from a stop. Also doesn't spit fuel back at high rpms.

Seems odd that she wants to idle right where my timing advance finishes flaring out. Recommend advancing my initial spark timing so she can (maybe) idle lower as well as cutting springs back more?

Thanks again,
- Cole and Pearl

[Tracksnblades1]

2000rpm is not in the idle circuit…it’s already handing off or at best overlapping with the intermediate circuit..

Were you able to verify your timing is no longer scattering about with a timing light..

[Tracksnblades1]

New video with it running and touching each exhaust pipe with a wet rag…please..🤞

[HondaMan]

2000 RPM is far too soon for full spark advance, even with a stock cam(!). Even my own stock-cam 750 reaches full advance around 3000-3200 RPM for its best results.
About idle jets: when I install the Megacycle `125-00 cam (which is much milder than yours) I also change the idle jet in the carbs to #35 instead of the normal #40. This lets the idle-mix adjust screw work in the 3/4 to 1-3/4 turns range. With the stock #40 jet and that cam the idle-mix adjust screw has to be turned in to 1/2 turn, which leaves little to no room for future wear issues. This might help with your adjustablility, too? If you have some Keyster brand idle jets (I have MANY of these) you can think of them as #38 pilot jets: that might lean things out a bit, too. They are set to different values from the Keihin versions of the same number.