Topic: 1972 CB750 Sluggish Throttle Response - Oversize slides possible?

[TheWiredNinja]

I've successfully rebuilt my cb750 K2 with upgrades from Mike Reick and CycleX to resolve an original bike/engine that has been sitting for over 30 years.  This included his stage 3 porting, new oversize valves, springs, RC295 camshaft and JE 836 pistons - the works!  Previous to my rebuild, the engine did run but I knew I was on borrowed time with it being slightly smokey until it became a real problem and then developed a ticking noise somewhere in the top end.  It also had somewhat poor throttle response where the RPM's would hang and the rev's would not go down quickly - I thought this was just due to needing a rebuild and worn out, leaky valves for the most part

Now that the bike is back together, with new boots and the shiny parts, the bike is obviously running much better - however the slight hanging problem still exists.  I have the Carbtune Pro 4 and upon syncing the carbs, I've noticed that I have low vacuum where the gauges are JUST showing vacuum when I have the bike at around 2000rpm (any lower and the bike will actually die out but this might be due to the lightened flywheel?).  Raising the revs further to 2500-3000rpm does help a bit raise the gauges on the Carbtune Pro but it still seems very low.  Revving the bike up and on decel the gauges will shoot up, but when it settles back down to idle the issue persists; it seems I have a vacuum leak. 

Here's where I think I messed up - since the bike was originally sitting for such a long time, the carbs needed a very thorough clean and I think I may have been a little too aggressive using a wire brush within the bores of the carbs.  Effectively "overboring" the stock roundtop carburetors which may have caused the slides to not have proper clearances against the bores for a good 'seal' - hence a lean condition and poor idle/vacuum.  Just my theory at the moment.

So my question is - is there a specification to measure this clearance?  Is there such a thing as "oversized" slides or perhaps a way to enlarge the existing slides for better clearance?  Or am I screwed and I need new carbs?

[Scootch]

Something to check.....  make sure that you don't have vacuum leaks at the Carbtune fittings. I found that the vacuum ports on my carbs were worn from years of hooking in there. One of the carbs had the threads pretty well stripped. The others would barely grab the Carbtune tubes and enable them to be securely tightened.

[TheWiredNinja]

That actually is something I should double check as I have yet to start the bike with the Carbtune disconnected and the access holes properly plugged with those 4 screws and washers.  However, something tells me that I will still have the issue as the symptoms are similar to what I had before the rebuild in terms of sluggish throttle response (the throttle hanging/increase at lower rpm's is no longer there though!)

I'll update in the next few days when I get the chance

[RAFster122s]

High revs can point to air leaks... A bit of spritzing carb cleaner near air intake boot connections between carb and motor will tell you if you have leaks. Engine will change rpm as it pulls vapor in...

[TheWiredNinja]

I got around to the bike today and sprayed brake/parts cleaner around the boots - no spike in revs at all.  Seems like I'm leaking elsewhere; possibly the carb slides/bores to be the suspect since nothing else seems loose or problematic. Or perhaps just bad tuning/jetting.

The only other items I should have mentioned is that I have had the idle and main jets changed from the previous motor configuration which seemed to work well.  Given the rebuild I may likely have to adjust things.  Currently the carbs have a 48 idle jet and a 140 main jet.  I believe stock is 120 main jet and 40 idle jets.

Before someone says "that's obviously the problem" may I remind you that the vacuum was still behaving strangely and it hasn't really changed much behaviour since I've put back the screws and washers in place of the vacuum readings for carb sync.

With that said, the bike clearly is stuttering at full throttle and doesn't seem to rev much beyond 5000rpm (all this with very little testing since the bike as just put back together).  I plan on reducing idle jet back to 40 and perhaps run a 130 main jet to see what happens.  If the bike doesn't like idling below 1800rpm-2000rpm, isn't that an indication of a lean condition?  or is that rich?  (it just cuts out, and further reminder, I have a lightened flywheel from CycleX)

Further note:  I'm running Carpy's open 4-into-1 exhaust, and open velocity stacks with the above modifications, so this bike does get alot of air.

Further note #2:  The bike doesn't like start without opening the throttle.  Perhaps further indication that idle jetting is too high?

[Jerry Rxman Griffin aka MuthaF'er]

The sync'ing is to equalize the carbs across the rack. Just try to get them all equal vacuum readings. It really doesn't have much to do with the total amount of vacuum.

Have you tried the stock jetting? Why did you go SO big? 40 slow jets is probably fine. The 48's could very well be a good part of your problem? Too much fuel = more rpm? Too big mains WILL cause it to fall flat once they come into their rpm range. You may well need to raise your needle up by lowering the clip one notch with the stock size mains. Maybe, maybe not. Most of your riding is going to be on the needles and the mains will overlap and only start to get serious at 6000 or above. My rule is to try stock first before assuming I need to go bigger.

I have a similar setup with a 915 but with CR31's and pods and a Kerker with competition baffle. Obviously different air flow vs stock 28 carbs. I'm damn near stock at 40's and 105's/110's with the exception of 6900' elevation. My dyno run, which you should do once you get it close, tells me my A/F is really close. That's what it's about, A/F ratio is the main reason to dyno. Takes the guess work out of it and gets it where it needs to be. That is the final fine tuning. Plus the extra secondary benefit is a HP reading which will not affect HOW it runs. It's just a number for bragging rights and pride of workmanship.  Make your fine tuning jetting after the dyno A/F results. So much easier and better than a seat of the pants dyno. 

[TheWiredNinja]

Well like I said, 48 idle was what worked well on my engine previous to the rebuild.  I think it was because the valves were in such a ("leaky") condition that they needed more fuel to compensate.

In regards to the vacuum and sync, I do realize that it's just a tool to equalize the slide positions on each of the carbs, but it still should pull a strong enough vacuum and not be so low as to be barely readable on the gauge.  That is why it may be an indication of an issue within the actual carbs (and perhaps due to my overly aggressive wire brushing cleanup of the carb bores)

In any case, I'll follow up after I try smaller jets and see how it performs.  I really hope that's all it is...

[Geoff Hastings]

I don’t think the slides are entirely the problem unless you really went to town with a Rotary wire brush. At full throttle the slides play no part in the running other than to hold the needle, at that stage the mixture in controlled by the main jets. The lack of vacuum at idle is more likely an air leak, either inlet manifold gaskets or intake rubber stubs. If the head has had all that work done I’d rule out leaky valves unless you’ve set the gaps too small. How is it it if you do a compression test?

[TheWiredNinja]

I don’t think the slides are entirely the problem unless you really went to town with a Rotary wire brush. At full throttle the slides play no part in the running other than to hold the needle, at that stage the mixture in controlled by the main jets. The lack of vacuum at idle is more likely an air leak, either inlet manifold gaskets or intake rubber stubs. If the head has had all that work done I’d rule out leaky valves unless you’ve set the gaps too small. How is it it if you do a compression test?

Just for clarification - the jet's were set up pre-rebuild, which included the leaky valves and other problematic area's.  The bike ran alright, but developed a ticking sound indicating imminent failure.  This made me want to rebuild and upgrade the engine where it is set up now. 


Post-rebuild, I can tell mechanically the bike is in excellent working order with great compression and power.  Since it's essentially a very different engine than before, tuning will need to be completely redone.  Although I can account for the full throttle hesitation past 5000 rpm (as it sounds like it's bogging down due to too much fuel), the idle vacuum and inability to go down below 2000rpm might be a little more difficult to diagnose.  With that said, I'll admit 48 idle jets is huge and will most likely run fine when this is back down to 40 or 42 idle jet. 

The vacuum and slight hanging issue could potentially be a timing (advance stuck at too low of an RPM) and carb issue.  I just wanted to know if anyone has dealt with worn slides/carburetor bores before.  Again, I have checked the boots using brake/carb cleaner and no rise in RPM's occured - I even made a general spray all over the carbs without any changes indicating no external leaks.

I will update as soon as get the time to swap in the smaller jets.

 Edit:  Added photo's of my setup in the meantime.

[willbird]

How far out are your idle screws ? Sorry if you said and I did not catch it on a quick scan. They look like they are sticking up above the carb casting ? Mine are below flush at one turn open from fully closed on a K2.

[TheWiredNinja]

How far out are your idle screws ? Sorry if you said and I did not catch it on a quick scan. They look like they are sticking up above the carb casting ? Mine are below flush at one turn open from fully closed on a K2.

They are about 2 180 turns out from fully closed.  I'll have to recheck that but that's where the bike responded the best when I first started her up after the rebuild.

UPDATE #2:  I installed 42 idle and 130 main jets.  It seems to start a little easier but is very much a struggle to start when cold and adding a bit of brake cleaner certainly helps.  It will start right up with partial throttle when warm/hot.  Acceleration on mid to near full throttle @ 3000-5000RPM is good and strong but I didn't get enough testing done in this area.  My primary concern is getting idle and start up properly.

Lastly, it still doesn't like to idle at low RPM's.  I would say it's lowest steady RPM is around 1700-1800 RPM - and at that point, it's rock steady with no backfire or issues (very smooth).  Any lower and it will cut out immediately.  It doesn't sputter or backfire at all during idle, even when lowering the RPM until the cut out point - I assume this means it's a bit rich?

[TheWiredNinja]

I made a video of the bike starting and idling.  In both video's, the engine is hot.  You'll notice I have to twist the throttle to get it to start.  As I search for the idle adjust screw, you'll see that it dies right away if I try to lower the RPM.

Second video shows that it can rev but the drop down is a little bit slow to react.  Again, possibly indicating an idle condition keeping the revs up slightly (ie. loss of vacuum).  I again try to lower the RPM down to 1500rpm but it cuts out.

It should also be noted again, that I have a lightened flywheel (alternator) from CycleX.  I understand that those with lightened flywheel's tend to run at slightly higher RPM, but I believe that's to keep the electricity flowing.  (and yes, I am aware of the low voltage warning; the bike still reacts the same with a topped up battery)

[newday777]

Your fuel hoses aren't running below the float bowls are they?

[TheWiredNinja]

Your fuel hoses aren't running below the float bowls are they?

They are but they loop back around to the inlet nozzle / fuel rail which are pointing upwards and are above the float bowls.  I have checked the float levels recently when changing the jets and they all seem correct.  Correct me if i'm wrong, but it's gravity fed from petcock, which is elevated above everything, so it shouldn't matter.

UPDATE #3 - I took the bike out for another ride today.  The bike does start right up when cooled off / warmish using the kickstart and partial open throttle.  Slight bog down between 2000-3500rpm at mid-throttle, but lots of power/torque.  Driving around casually the bike rides just fine.  Will NOT go past 5000rpm at all under full throttle - it's almost as if there is a rev limiter or some divine force restricting me from doing so on a freshly built motor.  I can quickly rev it passed 5000rpm, just not under load.  No popping, backfiring.  I may try going back to 120 main jet - any advice?

Idle remains at around the 1600-1700 RPM mark now when fully warm, otherwise cut out occurs.

[denward17]

What do your plugs look like now?

I think I remove all doubt and re-route the fuel supply lines so there is no loop below the float bowls.

[HondaMan]

Where to start...?
#1 - the fuel lines must NEVER go below the float valve level before they enter the carbs' fuel inlet. Having them below that level causes them to alternately vacuum-lock and then try to pressure the fuel with bike movement. If you tried a dragrace run with it, the bowls would run dry at about the 1/8 mile marker they way they are arranged now.
#2 - Where are your bowl vent lines running? They aren't visible in the pictures. Since there is no airbox, the only place you'll be able to find still, quiet air to pressurize the float bowls properly will be up underneath the seat, perhaps in a small vented bottle or tube (think: prescription bottle size).
#3 - With that cam and the oversized valves, you need a SMALLER idle jet, and likely a smaller mainjet than stock, too. I'd suggest #38 to #40 pilot and #110-#115 mainjet with the #27201 needle jets set into the middle notch for starters. However, this presumes Keihin parts. If the parts are Keyster metering jets and needle jets/jet needles, the number should be approximately #42 idle and #130-#140 mainjet. As a comparison: when I tune for the Megacycle 125-00 cam in a normal-compression engine I change the plot jets to #35 or #37.5 (aka #38) and the mainjet goes up a #5 size from the original one, and the float bowls get 1mm deeper overall. This tuning works from 736cc thru 836cc engines.

Also: drilling out the larger emulsifier holes in the emulsifiers to 0.0375"-.039" will help aerate the fuel better above 5000 RPM. Leave the smaller holes alone, though.

Another also: make sure the spark timing is spot-on and steady at speed (not jittering back-and-forth under a timing light). If the timing marks jump around, the spark advancer's shaft is bent and needs to be straightened before performing any other tuning, lest it drive you nuts with erratic "soft" throttle response troubles. The runout on this shaft should be no more than 0.004" max, 0.002" to 0.000" is best. Don't turn the engine with the big nut by the spark advancer, especially with sparkplugs in the engine - that is what bends the shaft.

[willbird]

Where to start...?
#1 - the fuel lines must NEVER go below the float valve level before they enter the carbs' fuel inlet. Having them below that level causes them to alternately vacuum-lock and then try to pressure the fuel with bike movement. If you tried a dragrace run with it, the bowls would run dry at about the 1/8 mile marker they way they are arranged now.
#2 - Where are your bowl vent lines running? They aren't visible in the pictures. Since there is no airbox, the only place you'll be able to find still, quiet air to pressurize the float bowls properly will be up underneath the seat, perhaps in a small vented bottle or tube (think: prescription bottle size).
#3 - With that cam and the oversized valves, you need a SMALLER idle jet, and likely a smaller mainjet than stock, too. I'd suggest #38 to #40 pilot and #110-#115 mainjet with the #27201 needle jets set into the middle notch for starters. However, this presumes Keihin parts. If the parts are Keyster metering jets and needle jets/jet needles, the number should be approximately #42 idle and #130-#140 mainjet. As a comparison: when I tune for the Megacycle 125-00 cam in a normal-compression engine I change the plot jets to #35 or #37.5 (aka #38) and the mainjet goes up a #5 size from the original one, and the float bowls get 1mm deeper overall. This tuning works from 736cc thru 836cc engines.

Also: drilling out the larger emulsifier holes in the emulsifiers to 0.0375"-.039" will help aerate the fuel better above 5000 RPM. Leave the smaller holes alone, though.

Another also: make sure the spark timing is spot-on and steady at speed (not jittering back-and-forth under a timing light). If the timing marks jump around, the spark advancer's shaft is bent and needs to be straightened before performing any other tuning, lest it drive you nuts with erratic "soft" throttle response troubles. The runout on this shaft should be no more than 0.004" max, 0.002" to 0.000" is best. Don't turn the engine with the big nut by the spark advancer, especially with sparkplugs in the engine - that is what bends the shaft.

I wish I could find a spare set of jet holders. Mine were not "terrible" on dia. I'm getting stuff around to be able to log fuel air vs throttle opening and it would be cool to be able to swap drilled for un drilled same day and compare them directly.

[PeWe]

My K6 with 836, ported head 34/28mm valves, Action Fours SS1 cam (milder than DP295), stock carbs with small K&N type of pods and a rather open 4-1. It had a baffle inside.

- Pilot stock 40
- Needles clips 1 step richer, 4 from top
- Main jets 147.5 Mikuni !!
Strange....

******* EDIT: *******
I remember that the 65 mm pistons used then was std Action Fours 10.25:1, replacing RC with higher domes.
Chambers opened up for the domes. Ca 24cc instead of 22.5cc
So compression was much lower but still good power. Good for long high speed not running hot touring.

The relatively low compression migh explain need of bigger main jets?
******************

Worked well in the 80's.
Aftermarket jets not good.

A friend had recently aftermarket needles and I could not understand how the engine ran that rich with a certain needle height until he told me he had replaced the good old stock with crappy carb kit needles.

[TheWiredNinja]

Thanks for all the information - this is the first time I'm reading I should have been using smaller jets with oversize valves and hot cams, despite being a an 836 bore.  Seems counterintuitive but that certainly does look like the direction the engine needs to go considering it did start up and run a bit better than my previous large jets.  The bike originally had 40 idle and 120 mains.  I'll put those in while I wait for an order for even smaller jets to come in. 

I just completed "truing" the shaft - it was off a bit @ 0.01" at the worst.  Got it within 0.001" if my gauge and setup was accurate using Hondaman's guidance and book.  (this certainly does seem key to having a smooth motor running and everyone should be doing this for accurate timing)

I will reroute the gas lines as well - I did this originally because I added inline fuel filters and they would have been in the way.  Despite the float bowls seemingly getting enough gas, I will remove all variables to get things right.

I have attached a picture of the plugs - black.  Looks rich so smaller jets are definitely in order.

Lastly, another interesting observation:  I've noticed I'm still at full advance at 1600-1700 RPM.  It is when I start to lower the RPM @ 1500rpm does the timing start to go down and then the engine cuts out.  The advance mechanism is not original but from a popular parts supplier for these bikes (and others) that are most likely replica's.  I do still have the original advancer but I do not have the springs.  I realize I can cut the springs on this advancer, but last time I attempted that, I made a mess of things...

[PeWe]

- Fuel level too high?
- Air screws not opened enough? (1 turn)
- Or the rich pilot jets?

My almost stock K2 with no airfilter, 120 will be too lean.

But more cc will create more underpressure and can use smaller jets I guess.

My K2 with stock airbox but K&N filter inside and rather good flowing Yamiya no numbers 4-4 with baffles without "wool" has recently got 120 main jets when it hesitated after 140kmh.

K6 carbs.
Needles 4:6
Pilots 140


Stock except for K7/K8 61.50mm pustons giving a little bit higher compression.
Fresh K6 head with stock valves and K8 cam. Not hot

If using no filters, at least 130 is my guess.

But why shorten engine life without filters?

My bikes got covered by a very fine powder  everywhere last fall when roads were wet.

I got thoughts about volcano ash far away by the winds raining down.

When I meet clouds of dust on the road I use the clutch, throttle off and roll until air is clean again.

I met a huge cloud of dirt dust earlier this year. I accelerated to get speed, clutch in, killed engine.
Rolled thru the difficult see thru dust until clear so I could release clutch and start.
This to not clogg my K&N pod filters on my modified K6.

No filters, no way! My expensive carbs should not like it, rest of engine same thing. 

I have had issues when those carbs had not filtered pilot air jet intakes.

That partly explained a never ending rejetting of the idle to mid range. Installed filter system that fixed it.

I doubt that engine will run better without filters independent of jetting.

[TheWiredNinja]

I share your concern with the filtration.  This bike will not be used for long hauls but very short trips and mostly around a city.  I may have to buy a whole set of carbs in the near future should my vacuum and other issues persist.  In the meantime, I'm going to see what I can do with what I've got.  The motor was just put back together so things are very much in the early stages of progress.

It most certainly seems to be a jetting issue as HondaMan has suggested.  I have put in an order for a set of small jets to experiment with.  I will look at new carbs and with filtration as a last resort, budget permitting.

[PeWe]

I wish you good luck with jetting.
Visit a dyno where the bike can be ridden slowly from idle to full with AFR in focus will help you.

Not only WAT to check power.
Well worth the money to get proper AFR not making engine dirty inside by too rich mixture.
Piston domes, chambers, export and valves full of soot.

Too rich will affect lubrication in bores too.

[HondaMan]

Most certainly a too-soon spark advance will cause blackened sparkplugs. I find that is the #1 cause in otherwise stock CB750 engines today, just due to the aging of the springs on the advancers: Honda never dreamed we'd all be riding these 50 years later(!).

On nearly every engine rebuild I do, I end up cutting off some portion(s) of the spark advancer springs, for that reason. They are now heat-annealed (some call it 'heat sacked') and advance too soon. What also doesn't help: our modern gas burns MUCH slower than 1970s gasolines, so having both slow burn and too-fast advancers causes the engine to darken plugs, quickly. (Note to readers: the 500/550 isn't any better!).

When the spark advance happens too soon, the fuel that is still unburned during the overlap cycle (in between exhaust and intake, both valves are momentarily open at once) that would normally finish burning in the exhaust pipe gets some additional ignition from the waste-spark of the coils, and this enhances that burn back up the intake tract toward the carbs just as intake begins. This both reduces the intake charge, making ignition poorer, and this 'puff' from the engine causes the air velocity in the intake tract between intake valve and carb's slide to drop. This slower airspeed makes the carb mix as if the engine was running slower, which is also richer, so the mixture overall becomes both partially burned, and partially too rich for the engine speed. Both cause dark sparkplugs and poor low-end throttle control, making the bike feel like it has a low-speed "switch" somewhere around 1800 RPM when things 'catch up' to the intake valve and it takes off, making low-speed parking lots tricky to maneuver sometimes. Delaying the spark advance fixes this like magic.

I normally cut off at LEAST 1/2 turn from both springs in the Hitachi advancers "AD125", and usually a whole turn off at least one of those springs before it is over, plus reshaping the end loops tightly enough that they cannot pop off their little anchoring posts. The TEC advancers fare a little better because they have more turns in their springs, and often losing 1/2 turn, plus re-arching the springs back to a smaller loop at the little anchor posts is enough to fix things up. In engines with 'hot' cams this isn't enough: when the intake duration is both longer and opens the intake valves earlier, it takes at least 1 full turn off both springs to civilize the low-speed performance and stop fouling sparkplugs from the too-soon spark.

[PeWe]

Both my bikes have advancer springs cut.
Had to do it on my modified K6 with TEC 300 advancer that advanced way too early.
Advancer had a huge play until springs held back the cam.
After cut, springs holds the cam from the very beginning.

It has after that got another advancer, 125-1. Springs also cut.

Engine has rather high compression, 970cc and a rather hot cam, Megacycle 125-75. Nice even idle at 1100-1200 rpm.
Can set it to 1000rpm but take offs can hick-up as the compression make it to run harder.
Very even throttle increase all the way up if handle throttle by care.

My almost stock K2 has both springs cut too.

This should be done on all CB750, an upgrade and problem solver thanks to Hondaman's early posts.

[TheWiredNinja]

Just to confirm, what RPM should I be aiming for the full advance to come "on" with a bike with my modifications?  I realize that parts don't normally have a 50+ year shelf life, but this advancer was purchased brand new!  Not sure who or where I can source proper weighted springs for these bikes...

My other concern is that I'm not entirely sure if my stalling problem (at low RPM) is primarily due to the idle jet or timing.  As described before, when I lower the RPM to around 1500, that is the point where the curve starts to go down.  It is right at this point, that the bike cuts out which makes me believe that the decrease in timing is causing the cut out.  However, the bike should still run as per HondaMan's detailed explanation, so it must be the idle jet much too high at 42... or at least I hope that's all the issue is.