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4 Posts
Discussion Starter · #1 · (Edited)
hi everybody, i’m new to this forum, and also new to motorcycles. so please help me!

bought my first bike a few weeks ago, a 1971 cb350 with 722a carbs. it has aftermarket air pods + short mufflers. bike ran fine the first week i had it. then it started to have throttle issues. i would give it throttle, and the bike would struggle. the acceleration would go in and out, like a jerking feeling, and i couldn’t even get it to go 45 mph.

so i bought a carb rebuild kit from ‘4 into 1’ that also included new floats. bought a new petcock, clear fuel lines, and gas cap seal from ‘common motor collective’.

i cleaned out the rust in my gas tank, installed the new petcock and fuel lines, and cleaned the carbs and put in all the new parts (the parts were not of high quality as the emulsifier tubes wouldn’t seat and would slip right out, o-rings wouldn’t retain there shape, and the floats didn’t float as well as the ones previously in the carbs. i followed the youtube videos from ‘common motor collective’ and put the carbs back together.

all the prior carb parts i replaced were original keihin parts with the ‘k’ logo imprinted, besides the main jet. the main jet that was in the bike when i bought it was 115, secondary was 68. the kit i bought came with 100,105, and 110. i started out with 105 since i found online that 722a:

main primary jet: 105
main secondary jet: 68
slow jet: 35
pilot screw: 1 1/8 turn out
float: 26mm

so i installed all the parts to factory specs. fired her up, and noticed both float bowls were leaking out the sides. forgot to mention that before cleaning the carbs, the original floats were set to 23mm, but the floats had no spring action on the needles. but the original setup didn’t have fuel leaking out the sides of the float bowl.

rode around the neighborhood and still was struggling. same symptoms, but now the bike seamed to run lean. had a harder time setting the idle, and when i’d shut the bike off, i could hear pings inside the cylinders. engine was also noticeably hotter.

so i changed the main jet to 110. bike seemed to run better, not as much stuttering/cutting-in-and-out acceleration. the throttle ran smoother, but wasn’t getting enough speed, felt under powered.

so i thought maybe i should try the original 115s. so i replaced the old o-rings on the 115 with the 100s that came with the rebuild kit and reinstalled the 115s. back to stuttering/cutting-in-and-out acceleration.

so i checked the spark plugs, and both were black telling me the bike was running rich. so i bought new spark plugs and noticed the difference in power instantly. also,upon reinstalling the right side spark plug boot, the wire came off with ease. dunno if that had anything to do with it, but i cut the wire, and screwed it back on till it was tight.

rode around the neighborhood a bit, with the acceleration still stuttering, and checked the new plugs. the attached pic shows the spark plug on the left and the one one the right.


can someone tell me if these plugs are lean or rich?

can someone suggest how to fix this throttle stuttering problem?

i’ve been tinkering with this problem for almost 2 weeks now, and only have 2 hrs to work on it each day, since a neighbor complained and i have to be quiet in the neighborhood by 8. i’m so frustrated. i’ve probably taken off and put back on both carbs 20 times by now. i’d take it to a shop, but with the current problem, i’d struggle to make it there. towing will cost me an arm and a leg.

i just want to ride the damn thing!!! please help!


2,510 Posts
There's a new bike checklist that you should go through. It may be ignition electrical related and not carb.
Do a tune up and valve clearance check and set the ignition timing and see if that helps.

2,510 Posts
I believe outobie is the original author

here is the procedure to tune your bike
Stock CB350 Tuning Checklist

1. Check throttle cable for smooth operation and crisp return action. Throttle should snap back vigorously to full close with just a little free play through the full swing of the handle bars.
a. If it doesn’t; lube and adjust all 3 cables and ensure proper routing
2. Engine Compression should be at least 160 PSI on both cylinders with a warm engine and the throttle completely open with the choke off. If it isn’t then no amount of carb or ignition adjustments will make it right
3. check and adjust valve clearance. If the valves were out of spec then redo the compression test after properly adjusted
4. adjust cam chain tensioner. If the engine has recently been apart you should also check to ensure that the cam chain and sprocket are properly indexed with the cam and crank. This is also a good to check if the engine has more than 20,000 miles or you have other reasons to suspect the cam chain or tensioner to be out of spec.
5. replace and gap the spark plugs; they are cheap
6. high tension leads and spark plug caps should be checked for cracking and replaced. I also will trim about 1/8” back from the high tension lead and re/install the spark plug cap to ensure a good clean connection.
7. spark check make sure you are getting a good spark from both plugs. If not there are lots of electrical checks that will need to be made which I won’t go into here
8. Battery check ; Make sure you have a good battery with a full charge and your stator/regulator/rectifier are charging at between 13.8 to 14.8 volts with the engine running at 3,500 rpms.
9. ignition check; time your points or your electronic ignition to spec
10. Exhaust system check; are there any leaks from your exhaust?
11. air filter check; is your air filter clean and properly installed
12. rubber intake grommets; with the engine warmed up and idling, spray some WD-40 all around the rubber intake grommets and listen to the engine idle…if the engine idle changes then you have an air intake leak.
13. fuel supply; do you have a tank full of fresh high test gas? Have you checked the petcock bowl screen for clogging? Are the fuel lines clean and unobstructed with any inline fuel filters of the proper size and clean?
14. after everything above is done, now you are ready to begin with the carbs;
15. begin by completely disassembling both carbs and I mean everything
a. remove top of carb and withdraw the slide and inspect for any holes or tears in the slide diaphragm. Clean any gunk from the slide so it’s operating room clean. Remove the needle and clean it in carb cleaner
b. remove the float bowl, float bowl gasket, 2 main jets and pilot jet, rubber passage plug, brass floats, and fuel inlet valve, inspect and replace rubber o-rings on the two main jets, fuel inlet valve, rubber passage plug and float bowl.
c. Remove the screws, butterfly, seals, and spindle for the choke
d. Remove the linkage, spring, screws and butterfly, seals and spindle for the throttle plate
e. With a wooden dowel gently tap out the two brass emulsion tubes from the main body of the carbs. You insert the dowel from the top of the carb where the slide goes and they will both pop out of the bottom from each of the main jet holes in the carb body.
f. Thoroughly clean the carb bodies and all brass parts in a carb cleaner like Berrymans, etc.
g. Wash and dry all jets and blow out all openings in the carb body and jets with compressed air. Visually inspect all holes to ensure they are all clean and clear of all debris. Pay special attention to the holes in the pilot jet and the emulsion tubes. If you cannot clean any of the holes then you’ll need to replace that piece. Keep in mind that brass is a soft metal, so anything you use to try to scrape out the holes will leave burs on the metal and likely change the opening size and fuel metering characteristics of the jet.
h. Inspect the floats to make sure they float, are not crushed, and don’t have any pin holes in them (I always hold them under water and look for bubbles)
16. Carb assembly: after complete cleaning and drying, begin assembly
a. Install the throttle and choke butterflies with a light coating of grease on the spindle. Be sure to look at the taper on the upper and lower edges of the butterfly plate to ensure you don’t install them backwards.
b. Install the two long brass emulsion tubes. Be sure to install the secondary one (smaller with a slotted end on it) with the screwdriver slot aligned with the small crossover hole from the primary tube so as not to restrict the crossover tube with the crown of the screwdriver slot.
c. Install the fuel inlet valve with new o-ring (lightly coat the o-ring in grease) and float. Float height should be set to 26mm on the 350k4s.
d. Install the pilot jet and two main jets with new o-rings (lightly coat the o-rings in grease), install he rubber passage plug
e. Install the new bowl gasket and spring jet holder.
f. Install the needle and clip into the slide then the slide into the carb.
g. Install the pilot air mixture screw with new oring and do not overtighten…set both to 1.25 turn out
17. Carb adjustment:
a. Adjust carb throttle cables with the carb mounted to the bike
b. Set both of the idle screws so that they just barely contact their stops without opening the throttle butterfly
c. Adjust cables so that the throttle butterflies begin opening at exactly the same time (you can hear it when you open and close the throttles…it sounds like one click instead of two when they are in synch)
d. Make sure there is proper throttle cable play at the throttle on the handlebar with no binding
e. Warm up the engine to full operating temperature
f. Adjust the idle screws (not the air mixture screws) in equal turns for both carbs till the engine idles properly.
g. Set the idle air mixture screws per the manual

4 Posts
Discussion Starter · #4 ·
thanks doode, i'll try and go through that list. funny how carb adjustment is at the bottom of the list, and that's what i started out with. D'OH!

After you mentioned it might be electrical, i checked with a multimeter to see how many volts the battery was putting out with the engine off. it was reading 13.1v. it's an antigravity 8 cell lithium ion battery, and i believe a fully charged one puts out 14.8v.

i took out the battery to charge it over night, and will put it back in the bike when i get home from work. do you think maybe because the battery wasn't fully charged that was giving me the issue?

4 Posts
Discussion Starter · #5 · (Edited)
the bike idles great at 1200 rpm, with no needle bouncing around, and no stalling. for the first 30 secs to a minute, the bike will run fine, no struggling. then it will begin to struggle when i give it any kind of throttle, from barely giving it any throttle to near WOT. i haven't tried WOT because i'm afraid the throttle will catch, and the bike will go launching forward.

side note: the acceleration struggling problem started one day when i went to brunch, came back to put my key in the ignition, and noticed it was already in the on/ignition position. (something's wrong with my ignition key switch since you're not suppose to be able to remove the key when it's turn to the on/ignition position) so the bike was sitting in the parking lot with the ignition in the on position for about 45 min- 1 hour.

first thing i did when i got home that day, was check the volts with the mulitmeter, and it read around 13v, so i figured the battery was okay right? bikes run somewhere around 12v right? that's when i thought it might be a fuel problem and went thru all those steps to try to fix it.

really hope it's the battery. gonna see how it runs with it fully charged.

4 Posts
Discussion Starter · #6 ·
so i got home and the battery was fully charged at 14.3v. i put it back on the bike and hooked up the terminals, and did another reading, and it drop down to 13.4v. started the bike up and rev’d to around 3k rpm while reading it and it slowly climb up to 13.6v.

a guy on another forum said the bike should be putting out 14v -14.8v when reving it around 3k rpm.

as i’m reving it in neutral, the bike is doing it again. cutting in-and-out of acceleration. i don’t even need to ride it around the block to know it’s hesitating.

here’s a video link to what’s happening:
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