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I am shopping around for my first bike and I found a running, clean titled 1972 CB350 k4 for $2500. The seller said it was a barn find that he took to a reputable mechanic to get up and running again. Cosmetically it could use some work but he said it is mechanically sound.

The problem (and the reason he's selling): it tops out around 50 mph. I was under the impression that even an older CB350 could do at least 80 mph.

Attached are the service records with a detailed description of what the mechanic did to get it up and running.
The summary: Both cylinders have decent compression (180 psi, 145psi), though the right cylinder is shy of 170. Points were adjusted and cleaned and timing corrected for both cylinders. Carbs cleaned and jets wire brushed, set to mixtures to 2 turns. Pod filters installed and new electronic system installed.

I like the bike and am willing to put in the work to get it to highway speeds, but I can't diagnose the issue. Any ideas?
 

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Note that he found the carb diaphragms 'siliconed'; it may mean you need new ones. Put in fresh spark plugs and do a plug chop, IOW, hold it at whatever speed it will reach for a couple miles, then hit the run/stop switch at the same time you pull the clutch in, and coast to the roadside. Safely, of course. Let it cool and pull the plugs out; let us know what they look like.
 
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And Trials is right about the foam filters; they often block the air jet inlets, causing a poor mixture under high flow conditions. Finding a set of OEM type filters would be better than stacks, but at least you'll know if the filters you have are the problem.
 
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Discussion Starter · #7 ·
Thanks for the quick responses. The seller has the original air filters and based on the mechanic report, it doesn't sound like the bike was rejetted to accommodate the foam filters. Do you think sticking on the stock filters would help, or are v stacks my best bet?
 

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Strongly advise you put the stock airbox back on and install any kind of air filter in it you care to maintain.
If you look at the original rubber horn shaped parts that came off the carburetors you will see they are cone shaped. They are shaped like a velocity stack, the hole on the airbox end is larger then the mouth of the carburetor. Somebody took that off and messed up the vacuum. To fix it put the velocity stacks back on and make sure your crankcase still has a clean place to breathe from and a drained place to expel blow-by gasses, oil and water into.

You don't reject the carburetors because you put new air filters into it. Ever.
 

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You have an engine problem with the difference in compression if the numbers are true. The exhaust valves commonly leak when they burn.

Compression is engine draw and that CV carb will not be opening as much as it should and the pods make it even worse.


The earlier paper filtered 350 sirbox and look at the part #2 grommet lower down on the inside. It ties both air filters to each other with a tube under battery and that allows the CVs to share the vacuum impulses to make 2X more to induce vacuum that opens the slides. The pods killed that. When you kill CV vacuum the slides at the least open SLOWER so jetting up is often backwards and why so many get lost when jetting up makes bike run even worse, the slides when carbs are rich die like a dog. If the slides have tears they are garbage.

Velocity stacks aren't spit, I've dumped them way more than once to get engines running at higher than original rpm. Nice if you can keep them but not engine killers. The shared cylinder intake impulses the OEM airboxes create are what open the CV slides up, that and the slighted restricted airboxes let the intake become rarefied even more to make it easier to build vacuum in the slides.

A CB350 better hit 90 mph or it is a dog. I've built several of them.

Someday people will figure out that ALL CV carbs like running lean far more than rich.

I wouldn't give that kind of money for that one.
 

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Don't tell people you can make CV carburetor motorcycles with a cheap pod stuck on the open mouth, you can't.
His foam air filter will become fuel soaked and it will never work right like that. Look at Any Honda engine, it will have a carefully formed velocity stack shaped air intake tract. It's Not an optional part or Mr. Honda would not have put it on there at considerable expense.

... and it's a 49 year old cheap built non restored motorcycle of course it's a dog. Somebody started to cafe yet another one.
 

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Some day people will figure out Vacuum control anything on a gasoline engine is a pollution control fuel conservation measure. and those CV carbs with a dry filter pulled over them certainly do Not have a built in venturi shape, they took that part away. and then hope the lower compression on the one side is not a damaged valve.
 

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'Don't tell people you can make CV carburetor motorcycles with a cheap pod stuck on the open mouth, you can't.'

Maybe you can't.

I've done it and got them to run faster as well, velocity stack gone. But then I've gone quite a bit deeper into them than most could dream of. Go back and look at DOHC 750 Honda tests for '79-'83 and usually around 12.40-12.50 1/4 mile. I used to bump into the 11s and 12.20s all day on one. 4 foam Unifilters on it. Knowing what I do now I could go even faster. And that's on the labyrinth type CV with no rubber which is even harder to get slides to open.

If you are fuel soaking foam filters you likely have a mechanical issue with reversion, I never once had the issue and I ran them a lot.

Actually the Honda early DOHC had the tuned stack length built into the airbox rubbers, it is not a velocity stack per se but serves the critical lengths that engine head desires, or about 50 mm. length.
 

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Actually the Unifilters shown there cannot possibly block off the port on carbs, impossible to do so as the flanges are stiff rubber.

Play at it? The length given there was used by Honda RSC when racing the bikes in Superbike racing in the days of Freddie Spencer. The RCB1000 factory racer used it too.
 

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Unifilters can, and do, block the air jet ports. Many have the step from where they fit around the end of the intake to the main airflow tube too large, and cause the main airflow to divert inside the ports. The stacks Honda provided not only increase air velocity at the intakes and reduce turbulence, they make for a smooth air flow that does reach the air jet intake ports. The increased air velocity will also lift the CV pistons more effectively, therefore providing a richer mixture at high throttle settings than without it. CV - variable venturi - carbs are used instead of fixed venturi carbs because of the wide range of rpm, therefore wider range of air volume, than carbureted auto engines. Without variable venturis, the air reaches sonic speeds, which is bad for the atomization function.
 
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'The length of what? the intake stack the bike no longer has?'

The 2 inch length difference is close to nominal, you usually incur problems at 6 inches or so. Do some math on sound waves. 2 inches will easily be in the gas temperature down length of pipe variable, it affects the actual gas speed.

Talking of turbulence is counterproductive, the carb works CV by creating much faster gas speeds. AND by likely creating a boundary layer doing it (which increases turbulence), since the carb no longer has a fully even open throat. The suction point at the slide hole is a maximum turbulence point, directional flow is crashing into right angle flow right there. The gas speed can be 3X faster under slide to wildly increase on both sides of slide when the passage opens up, and we are wasting time talking about turbulence.
 

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Post #18 where to start.......Never ever had the filters block any port if the filter was correctly chosen. I check for it. 30 seconds with a dremel fixes it too.

'The increased air velocity will also lift the CV pistons more effectively, therefore providing a richer mixture at high throttle settings than without it.'

Dead backwards, CV open slower and lower when richer, put an O2 sensor on the exhaust and prove it. Why Honda uses them, the engines run best slightly lean for emissions and look up the term 'lean best horsepower' which is the max amount that can be made, you intentionally richen past it a bit as engines tend to melt at that spec because they are making so much power. Run an engine at stoichiometric or the perfect 14.7/1 A/F ratio and race it to see how fast it burns up, and why all car engines and others go open loop at max power to go closer to high 12-13 A/F ratiowith a fixed fuel map, it is to protect the engine. It's why you no longer can add hi-perf parts to cars without a tune (software rewrite). Oh you can, but you WILL burn the engine up unless it has wideband O2s in it.

Another reason CVs get used is the male testosterone problem, or bragging about who has the biggest carbs and CVs can be much bigger than direct lift carbs. A major legal reason is because modern bikes make so much power they tend to kill idiots who simply jump on one and turn the throttle all the way open, the CVs will not open fully thus saving some d-mn fool's life.and the resultant legal wranglings over it.

'Without variable venturis, the air reaches sonic speeds, which is bad for the atomization function.'

Laughing at that one, your lawn mower will get carb gas speeds faster than sonic waves, ALL engines can do it. Do the math. You actually use sonic speeds in figuring out the intake and exhaust tract lengths. AND engines with no CV are SLOWER in overall gas speeds, again, backward.

And what to do with the CV type carbs that cars use? BTDT too.


On a 440 GTX at full weight nearly 4000lbs, and ran well into the 11s.
 
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