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Little high speed stumbles

Chuck Hahn

CB750 Enthusiast
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Location
Norman Oklahoma
Now that Ive got the charging all up to snuff I got a chance to actually ride the bike. Its great at around town speeds but when i took it out on I35 she ran hard up to about 70 MPH ( which is now on the main fuel circuit ). Think I was at around 5,000ish RPMS.

Anyway the exhaust is the Jardine setup from back in the day. Cylinders 1 and 2 headers go into the left side muffler and 3 and 4 go into the right side muffler. When it starts to stumble I can hear a little popping coming from the right side, which tells me its either 3 or 4 carb having a problem. I set the float HTs at the prescribed 15MM per the service manual on all 4 carbs. They are all stock jetting and stock airbox and filter.

Some things that came to mind are...
One of the slides isnt sliding nice and free.
One of the carbs needs a float HT adjustment..but which way?? Tad leaner or a tad richer???
One of the floats itself may be hanging up?? New float needles installed too, by the way.

What are your thoughts and suggestions???


EDIT.... This is at wide open throttle b y the way..."tries" to catch but just seems to be a tad shy of grabbing.
 
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Why do you assume it's carb only? Once the valves begin to burn then setting them does nothing and the max rpm before banging out just gets lower and lower. It's a hallmark of the DOHC. If too rich then you will clearly see the black smoke out of tailpipe when loaded hard and it's missing.

FYI, the engine is big enough you can easily still be off main jet 'only' at 70 mph and running 50% off the primary. Until you peg the throttle wide open anyway. The sticking slide needs fixing, you run rich when they stick. Or they got mixed up at disassembly, they are matched to their caps. If worn enough to have seizure marks you can remove the rough highs of that with 600 sandpaper. If the top big OD where the labyrinth ring grooves are cut is gouged then the slide is damaged, the air passing there normally prevents any contact by centering in the play and the slide and top will not work right with any seizure there or leak. The norm is that the slide never touches anything on the top OD.
 
I havent reopened any carbs since the rebuild but i dont recall any wear mark, scrapes, dings or anything else marring up a slide or the top with the rings machined in to them. valves all checked fine with the feeler gauges.

It sort of feels like one of the carbs is sucking the fuel out and the bowl level gets low and it stumbles....slow down to maybe 65 or so and smooth as silk again. Like its trying to "catch up" but just cant quite get there. Thats about the best I can describe it. And this is with the throttle fully open. Just doesnt seem to fully catch if you know what I mean.
 
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Pulled the plugs and number 3 is the only one that look good to me. Rest look lean as all get out. Click the image addresses below to see the pics I stored on imageshack. I am thinking change the float Hts from 15 to 14 to raise the fuel level in the bowls some to make it richer??

https://imagizer.imageshack.com/v2/665x499q90/922/Eqs06O.jpg

1 and 2...

https://imagizer.imageshack.com/v2/665x499q90/923/PR3AIw.jpg

3 and 4.... guess i sat them down out of order, but 3 is the one on the right here.

https://imagizer.imageshack.com/v2/1025x769q90/923/PR3AIw.jpg
 
'... valves all checked fine with the feeler gauges.'

Means literally nothing with no numbers, the service manual will have you in a world of pickle there. As old as the engines are the valves are often in less than stellar shape anyway yet nobody questions them. Motor then runs like a dog. Owner then chases carbs over and over for nothing and why they are usually off the bikes when bike bought.

The OEM plug can run lean looking as heck and still run fantastic, they are emission engines and the CVs LOVE lean, it makes them open up all the way. Why they were used as the carb type. When you start coloring the plugs darker the carbs then will not open as far. What people still don't get.

Your compression colors the plugs as well and so does the ignition since it is weak on these even at best. The #3 could simply be valve seals there leaking slight oil and common. Or compression off a bit on that one. Or crap plug wire.
 
Looking at the troubleshooting part of the manual where its says lean mixture as a guide of things to check, my question in the previous post was if I got from 15MM float Hts to 14MM will the raised level of fuel in the bowls ( theoriectically ) richen the cylinders?? Im gonna pull the carbs anyway to check the clogged jet possibilty anyway as it is possible something got in there. I rinsed out then tank and got no rust or other chunkies when i strained the acetone i had soaking in the tank but that doesnt mean something could have happened. The in tank filter screen had no tears or holes either.


http://www.cb750c.com/publicdocs/CB...ru83CB750ServiceManualChapter04FuelSystem.pdf
 
New plug wires. I dont recall the EXACT numbers but the lowest compression number i remember was 155. And though I dont have the paper i wrote the valve clearances on I do know they fell within the numbers the service manual said they should be at. Not arguing..just letting you know what I found when following the service manuals instructions.
 
The service manual numbers for setting valves CAN burn valves, they allow for .002 as acceptable yet that number is often zero real world to either leak compression or wipe cam lobe of oil to kill it quick. I NEVER use .003, rather .005 and the bikes will last so much longer running well between valves sets it's not funny. I burned 5 valves on mine brand new in '80 using the OEM service manual specs and at less than 5K miles.

The float heights all need to be the same. I never made a big thing of it and simple check and the engines ran fine. Yes, higher float WILL richen the cylinder a bit. The plugs never color exactly the same as well, there's 4 different port layouts in the head due to the bends. Higher compression 'leans' the plug out. Weaker ignition has same effect on plug as richening the jet.
 
How about superfine rust dust? Ethanol makes it and it goes through the OEM filters like they aren't even there. Even very minute quantities can make the floats act up.
 
May just be plugs, these are very sensitive to them. I can say that if they were ever flooded on then they need to be changed, the weak ignition lets them short down the sides to bang out and with brand new plugs the engines often then run perfectly until they get slightly dirty again. I've seen perfect looking sets do it.
 
AMC.. I took the carbs off and with a little puff of air from the I noticed two didnt react as easily as the others. I took some 400 wet sand paper and dressed the slides nice and shiney and the holes in the top covers the slides go into I rolled up sand paper and scrubbed the crap out of them as well. Put back together and all the slides seemed to react the same and all fell quickly when the air wasnt hitting the carbs. Im about to take it out and see if this helped or not. Will let you know. Your info on the slides may have well been the answer.
 
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Well the clean up made it a little better...starts stumbling at 7,000 RPM and 80 MPH instead of 5,000 and 70 MPH.. Still hear the misses, "stuttering in the exhaust sound" if you will, on the right side. Clear as a bell on the left side muffler. Im done for today. May go to HF and see what a leak down tester runs tomorrow. May even take the carbs off and set the 3 and 4 carb bodies back in for anohter 24 soaking session in the Berrymans. All the bodies have already been in the sauce during the initial tear down and rebuild.

Whats the lyric in that Carpenters song?? Something like "youre so close and yet so far away".
 
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If you take the air box off the backside of the carb to allow for visibility of all 4 main Accelerator Pump Nozzles pointing towards the motor, blip the throttle and with a bright led flashlight note that all 4 accelerator pumps are squirting a steady stream of fuel into the carb. I had one plugged up with 10 year old varnish. I can only imagine the engine problems it can cause.
 
Up at the speeds (80) he's talking about the accelerator pumpshot is pretty much all used up except possibly a little bit. The engine by then is running fast enough it doesn't need it anyway as carb draw is already well established by then. The accelerator pump is for lower rpm use with low engine revs, it makes up for a lack of carb draw when you first begin to open the throttle in big chunks. At 80 that is pretty much already in place.
 
OK...so I finally got a weekend to get back to the bike.

AMC...I had got a new valve cover gasket and got shims from the local dealership and got all 16 valves set as close to the .08 MM spec as is stated in he manual. I also did the "weak advancer spring" fix. I dont recall if it was here or at the SOHC forum, but someone posted about weak advancer springs and the fix was to cut off one of the loop ends and bend a new loop, thus tightening up the springs.

Long story short, with all the valves 100% verified and the springs tweeked the bike is still the same...runs like stink till it hits around 6,000 RPMs and then gets the stutter going again. I tried adding a little choke when it starts acting up and that didnt change a thing. As for the plugs..1,2, and 4 look ok but 3 is a little ashen grey. lean for sure there.

So do you guys think the problem is with carb 3..maybe take it off and do another 24 hr dip in the berrymans?? All 4 bodies have already been dipped for 24 hrs as the initial revival. Also one other question. The bottom pilot screws...as a general rule of thumb, where do most people seem to find they get set at the best?
 
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Update..

So i used the timing light and put it on coil 1/4 and ran it up to 7 grand ( seems to rev great in N on the lift ) and pointed it at the tank and sa that there was no interuption of signal to the light..flash solidly and steady. Same results when i switched to the 2/3 coil. So I can safely "assume" that it is not loosing spark that would cause the stumbling..correct?

Next I set the timing with the " FI " line in perfect alignment with the point of the indicator on the engine case. This line is to the right of the double lines marked S ". This is the correct spot to lne up the pointer right?

Next I made a scratch mark so I knew where the pointer lined up with the " FI " mark and moved the plate jut a little counter clockwise which the manual says retards the ignition. Rode it and it seemed a little bit worse. So I moved the plate so the scratch was to the right of the initial setting which should advance the timing. Rode the bike and it seemed a little bit better but not fully removing said stumbles.

All the pipes are equally as hot to me by a quick touch with the fingers. I may just break down and by a new NOS advancer i saw on Ebay. Ive had cars and other bikes and have successfully used the light and timed them to no bad JUJU, but this bike is just kicking my rear end !!! Im gonna let the engine cool down and reset the plug gaps at the very minimum the manual states which is .024 if I recall correctly and see if that does anything. These were new NGK plugs just 62 miles ago so thats how long I have been checking, rechecking, rechecking, rechecking stuff with the manaul rihgt in front of me and still cant resolves this issue.
 
AMC..I am searching and found this old thread. I was wondering if i could bother you to call me instead of all this typing and waiting game. This seems like something i should check but not clear on how to recenter it if mines off like the one the pictures show. I will PM you my number.

http://www.cb750.com/threads/3977-Timing-adjustment-out-of-limit


EDIT.. Had a thought about being able to twist the advancer ( out of the bike of course ) to maybe move the pin back to the right spot??? I too use the big gold nut to spin the crank with a socket so maybe this is also the problem with this current bike??
 
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Yes it's a problem and why the advancer on later ones was heavily loctited in place. The retaining 'pin' there is super soft steel and bends easy as spit. Regardless, there's not enough error there to make big problems.
 
So you agree that I should get the new advancer off Ebay and see if this solves the problem? I may be way off base but it seems that as the RPMs go higher...and thus the advancer is coming into play more and more...that maybe the stumble is advancer related??? Does my 'theory' sound right??
 
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