1974 CB360 Revving on its own(fixed) new issue: poor acceleration

My buddy and I have been working on his bike this summer (1974 cb360). We have had an on going issue with it revving on its own and not returning to idle. To give some background:

-Still happens with throttle cables disconnected
-Replaced advancer springs, they snap back properly
-no idle change in spray carb clean around boots, I've heavily inspected them too and see no cracks, they're actually still pretty rubbery and not stiff
-timing is good

Possible ideas of the issue:
-throttle plates don't appear completely flush to the carb walls even when fully closed, I've bench synced them multiple times, haven't see an improvement
-Top end head gasket appears to be a liquid dried seal - done by a shop in maine, could this be allowing extra air in or would this just cause oil to possibly leak
-valves gaps haven't been checked -? this wouldn't cause revving and slow return to idle though, would it?

going to put my carbs on his bike tomorrow and report back, in the mean time any suggestions would be helpful, thanks!

Re: 1974 CB360 Revving on its own

Did you adjust the cam chain tensioner?

Re: 1974 CB360 Revving on its own

revving at idle indicates a lean condition. double check or even triple check everything you've examined so far.

it could be the pilot jets too lean or an air leak in the carb boots. you've checked the boots, so verify the pilot jets are set right. do it with a warm engine. basic steps are:
1. close them all the way
2. open them a quarter turn
3. try to start bike
4. repeat 2->3 until the bike starts, note the # of turns
5. repeat 2->3 until the bike no longer starts, note # of turns

the right setting should be about halfway between the # of turns found in 4 and 5

notes:
- optionally start w/ pilot jets wide open and go down a quarter turn until the bike starts
- be careful not to flood the engine when starting with pilots all the way closed

next check the valves. should be fairly easy to do on the 360. make sure not to move the tappets after setting the gap when you tighten down the lock nuts. check the clearances after tightening the nuts down to be extra sure.

it looks like the CB360 has CV carbs as well. sometimes the rubber bits in the top of those can crack. they're quite fragile.

in general:
- the throttle plates should have some way of letting air past at idle. maybe a small gap or tiny hole in throttle plate?
- double check on that liquid head gasket... sounds like a problem waiting to happen

Re: 1974 CB360 Revving on its own

I had this same problem. It drove me nuts. Turned out to be a issue with the carb linkage that connected the two carbs together. I unhooked the cables and took off the tank. With the motor running I was able to push on one carb linkage with a long screwdriver and make the high idle stop.
The fix was shimming that one side with a washer made from a pop can.

Re: 1974 CB360 Revving on its own

It's likely a lean condition. I would bet you have some crud in your carbs.

Re: 1974 CB360 Revving on its own

66replica said:

It's likely a lean condition. I would bet you have some crud in your carbs.

bowls ands and jets are clean

Click to expand...

Rusnak_322 said:

I had this same problem. It drove me nuts. Turned out to be a issue with the carb linkage that connected the two carbs together. I unhooked the cables and took off the tank. With the motor running I was able to push on one carb linkage with a long screwdriver and make the high idle stop.
The fix was shimming that one side with a washer made from a pop can.

The linkage in which the throttle plates are screwed in on?

Click to expand...

TheCoffeeGuy said:

Did you adjust the cam chain tensioner?

Click to expand...

thanks for the suggestions all

1.bowls ands and jets are clean

2.The linkage in which the throttle plates are screwed in on?

3.we adjusted the cam chain a bit ago, probably should again.

Re: 1974 CB360 Revving on its own

nrcb125 said:

revving at idle indicates a lean condition. double check or even triple check everything you've examined so far.

it could be the pilot jets too lean or an air leak in the carb boots. you've checked the boots, so verify the pilot jets are set right. do it with a warm engine. basic steps are:
1. close them all the way
2. open them a quarter turn
3. try to start bike
4. repeat 2->3 until the bike starts, note the # of turns
5. repeat 2->3 until the bike no longer starts, note # of turns

the right setting should be about halfway between the # of turns found in 4 and 5

notes:
- optionally start w/ pilot jets wide open and go down a quarter turn until the bike starts
- be careful not to flood the engine when starting with pilots all the way closed

next check the valves. should be fairly easy to do on the 360. make sure not to move the tappets after setting the gap when you tighten down the lock nuts. check the clearances after tightening the nuts down to be extra sure.

it looks like the CB360 has CV carbs as well. sometimes the rubber bits in the top of those can crack. they're quite fragile.

in general:
- the throttle plates should have some way of letting air past at idle. maybe a small gap or tiny hole in throttle plate?
- double check on that liquid head gasket... sounds like a problem waiting to happen

Click to expand...

thanks for the input! boots appear to be in excellent shape and I've tested them for leaks so I'll have to try your other suggestions, the plates do sorta appear not totally flush, I'm hoping thats the issue, ill try too tinker with them tomorrow

Re: 1974 CB360 Revving on its own

The butterfly valves shouldn't be totally flush. There will be a very slight gap, but the gap needs to be equal. Set the gap on cylinder number one by adjusting the idle stop screw. The gap on cylinder two is adjusted using the springed screw on the throttle linkage. After this "bench sync" process, you should use a manometer (carb sync tool) to ensure both carbs are pulling the same vacuum. About 17" of mercury, if I recall.

Before that you should double-check your timing, though. Advanced timing will cause a bike to rev high. Because it's so easy to check and to rule out, this should be your first thing to eliminate.

If the carb sync is OK, you can always try dialing down the idle speed on the carbs. It's also worth making sure the throttle shaft block-offs are still in good condition as this will allow air leaks, too.

Re: 1974 CB360 Revving on its own

nrcb125 said:

revving at idle indicates a lean condition. double check or even triple check everything you've examined so far.

it could be the pilot jets too lean or an air leak in the carb boots. you've checked the boots, so verify the pilot jets are set right. do it with a warm engine. basic steps are:
1. close them all the way
2. open them a quarter turn
3. try to start bike
4. repeat 2->3 until the bike starts, note the # of turns
5. repeat 2->3 until the bike no longer starts, note # of turns

the right setting should be about halfway between the # of turns found in 4 and 5

notes:
- optionally start w/ pilot jets wide open and go down a quarter turn until the bike starts
- be careful not to flood the engine when starting with pilots all the way closed

next check the valves. should be fairly easy to do on the 360. make sure not to move the tappets after setting the gap when you tighten down the lock nuts. check the clearances after tightening the nuts down to be extra sure.

it looks like the CB360 has CV carbs as well. sometimes the rubber bits in the top of those can crack. they're quite fragile.

in general:
- the throttle plates should have some way of letting air past at idle. maybe a small gap or tiny hole in throttle plate?
- double check on that liquid head gasket... sounds like a problem waiting to happen

Click to expand...

some good info but wrong terminology. sounds like nrcb hasn't had a lot of cb360 experience. Im sure you have done enough reading about your own bike to know. What he is calling the pilot jets are not the pilot jets. He means to say the air/fuel mix needles. Pilot jets are located inside the carbs under the covered screw fed through the main primary jets.

Closing the air/fuel needles will cause it to go lean. Opening the needles will richen her up.

the head cover doesn't have a gasket, only Hondabond. If its not leaking oil...chances are its not leaking period. And it wouldnt matter either way if it was leaking air.

As he and sonreir stated...the butterflies will be very slightly open so the thing can get a little air at idle. Split the cost w your buddy and get a pair of gauges and check sync. you will use em on your own 360 also.

does he have decent air filters installed while the revving thing is going on?
Its getting a little extra air in there from somewhere, and it dont take much.

Re: 1974 CB360 Revving on its own

Also make absolutely certain the plates are centered just as perfectly as you can humanly get them. They shouldn't really touch the carb body at all. Any friction or wedging can cause them to hang open...enough to F up the sync. and idle.
Use a flash light shining in carb from filter (back) side and really check em close all the way around.
Especially After carbs are completely assembled w return spring and everything installed. Open and close them several times watching the action of the butterflies. Smooth not rubbing or catching on the sides of the body.

Re: 1974 CB360 Revving on its own

thanks for the suggestions sonreir and trek, as always.

So I through on my carbs and his bike ran beautifully. Further investigation of his carbs and the left throttle plate was a little off center, however even after adjusting it and throwing them back on there was little to no improvement. It appears he's missing the plug for the linkage hole above the air fuel mix screws as well. It also appears that his carb spring isn't springing back enough when letting go of the throttle. He ordered a pair of cb carbs for parts that look in great shape of eBay for pretty cheap, hopefully he'll catch the end of the riding season.

I'm going to order a pair of 360 sync plugs from crazy pj

Re: 1974 CB360 Revving on its own

lube the linkage between the carbs. remove the cable from bike and lube it til its full and running out other end like a garden hose. pull the plastic throttle twister thing out of the control and make sure its clean inside and the handle bar is smooth and free of rust or stickiness.

Re: 1974 CB360 Revving on its own

trek97 said:

lube the linkage between the carbs. remove the cable from bike and lube it til its full and running out other end like a garden hose. pull the plastic throttle twister thing out of the control and make sure its clean inside and the handle bar is smooth and free of rust or stickiness.

Click to expand...

So we fixed the revving on its own. It was due to his old carbs having some cracks in the throttle plates and the floats being too high, as well as the throttle cable being to tight :. Now we have the timing and floats on spec at 18.5 mm, it idles nice and low and doesn't rev on its own however now were dealing with hesitation and slow acceleration, as if there wasn't enough fuel in the mixture

I played around with the floats lowering them to 17 mm which had the bike returning to the high rev issue but was accelerating well ( Which makes me believe its a fuel issue as with allowing more fuel to get to the bowls the acceleration in neutral, just revving, had no hesitations to 8k rpm) but would stay at 6k rpm and not return to idle. I then set them to 20 mm and this just caused it too be even flatter. Today we found a couple tiny cracks in his diaphragms and he ordered new ones, could this cause this issue too? The fuel petcock and lines seem to be letting a good amount of fuel flow out, however I do sometimes see embolisms of air form at the bottom of the fuel lines before going up to connect to the carbs.

That would obviously be caused by a clogged breather at the cap or the petcock being clogged?

His bike is obviously running way too lean, he has pods on (shitty emgos), rejecting secondaries to 112 from 100 didn't seem to help, adjusting floats lower to allow more fuel in only caused the bike to go back to the initial problem of high revving and not returning to idle. Sick of wrenching on his bike! Just feel bad because I made him buy it too!

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

New diaphragms will make a difference. BTW I found the JBMs was the whole problem w my machine, running rich between 3 - 4500 rpm. I swapped in a pair of decent old Honda diaphragms and the issues were solved, So, I bit the bullet and picked up a a pair of Brand new Honda diaphragms, slides and springs All my troubles vanished...instantly. I beleive the JBMs are too soft (pliable) and allow the slides to lift to early (easily) dumping fuel into the system.

as far as cracks in the diaphragms...I would guess it would probably be lean above 4-5k rpm ??? simply cause, if this kept them from rising she wouldnt lift the needle to feed fuel. Or they would lift some but tend to drop quickly. EXPLAINING why the addition of 112s didnt make a difference. They only come into play when the slides raise.

Pj recommends a float height of 21mm for modified carbs. Mine are PJ modified and currently set at 19.5mm. But I have done a couple mods even beyond the work PJ does. We BOTH believe 18.5mm is a typo in the manual, or just flat out wrong info.

I had MYSTERY bubbles forming in my lines a long time ago. My issue was due to heat. The fuel was warming enough to start evaporation (forming bubbles in the lines) Now, I am no fuel expert, but I have read (on the internet) Fuel can boil as low as 90 degrees. If his lines are touching engine or frame (motor mounts)...or simply too close to engine or frame, you may want to try re-routing them. I raised my tank a bit, added heat shield, lost the inline filters, shortened lines, changed to thicker lines, and wrapped the lines in heat shield. This solved all bubble issues.

Hesitation on take off is due to lean condition in the pilot circuit. Check to make sure its clean and fully functional.
The pilot starts with fuel entering through the main primary fuel jet, then enters the pilot jet (under screw) past the A/F fuel needle, feeds to the three tiny holes under the bottom of the closed butterflies. The only way I have found to test this is to pull the A/F needles and spray carb cleaner in the hole...if you aim it just right. The spray will shoot from these three holes like a fountain. Along w and besides the A/F mix feed hole Heres some pics I posted the other day...

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

and remember, friends dont let friends buy 360s.

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

Sad times, Eric.

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

:-[

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

lol

Re: 1974 CB360 Revving on its own] - fixed, new issue: poor acceleration / hesitation

trek97 said:

New diaphragms will make a difference. BTW I found the JBMs was the whole problem w my machine, running rich between 3 - 4500 rpm. I swapped in a pair of decent old Honda diaphragms and the issues were solved, So, I bit the bullet and picked up a a pair of Brand new Honda diaphragms, slides and springs All my troubles vanished...instantly. I beleive the JBMs are too soft (pliable) and allow the slides to lift to early (easily) dumping fuel into the system.

as far as cracks in the diaphragms...I would guess it would probably be lean above 4-5k rpm ??? simply cause, if this kept them from rising she wouldnt lift the needle to feed fuel. Or they would lift some but tend to drop quickly. EXPLAINING why the addition of 112s didnt make a difference. They only come into play when the slides raise.

Pj recommends a float height of 21mm for modified carbs. Mine are PJ modified and currently set at 19.5mm. But I have done a couple mods even beyond the work PJ does. We BOTH believe 18.5mm is a typo in the manual, or just flat out wrong info.

I had MYSTERY bubbles forming in my lines a long time ago. My issue was due to heat. The fuel was warming enough to start evaporation (forming bubbles in the lines) Now, I am no fuel expert, but I have read (on the internet) Fuel can boil as low as 90 degrees. If his lines are touching engine or frame (motor mounts)...or simply too close to engine or frame, you may want to try re-routing them. I raised my tank a bit, added heat shield, lost the inline filters, shortened lines, changed to thicker lines, and wrapped the lines in heat shield. This solved all bubble issues.

Hesitation on take off is due to lean condition in the pilot circuit. Check to make sure its clean and fully functional.
The pilot starts with fuel entering through the main primary fuel jet, then enters the pilot jet (under screw) past the A/F fuel needle, feeds to the three tiny holes under the bottom of the closed butterflies. The only way I have found to test this is to pull the A/F needles and spray carb cleaner in the hole...if you aim it just right. The spray will shoot from these three holes like a fountain. Along w and besides the A/F mix feed hole Heres some pics I posted the other day...

Click to expand...

Thanks trek for all the helpful info! I'm going to make a hard assumption its gotta be the diaphragms, I've spent days making sure all his crap was right, and the needles not being able to lift to let in enough fuel correlates to the the issue that it was an overly lean mixture, as well as the bike accelerating properly when the bowls were basically overflowing.
One thing I found odd was that it accelerated beautifully when you would slowly turn the throttle but not when you would turn it similar to how you would if you were actually riding, where then it was extremely flat and stuttering through the rpms

trek97 said:

and remember, friends dont let friends buy 360s.

Click to expand...

lmao, i learned my lesson, trust me. I thought since I knew the most about the 360 it would be smooth sailing fixing up his non-runner, ended up being quite the opposite of that. Learned many new things though as far as trouble shooting goes.

I guess a problematic first bike is good in the sense that you learn a shit TON... a different kind of TON... that's what this sites title means ... right? ;D

Also trek, how are those throttle plate screws treating ya?

WOW who hacked the screws on those butterflies?

Check you timing chain tension. I know a lot on here have not had any faith in this as a solution or a problem. A loose Cam chain tensioned will result in an engine not spooling down after being revved, or spooling down very slowly.

Having run through my fair share of vintage Hondas, too much slack in the chain will advance your motor and that slack must now work its way back form the front side of the engine to the back side either going through the main crank sprocket or up top through you head. Its a quick check for what seems to be taking you a long time to solve this problem.

Sometimes its the trees, and sometimes its the forest.

JMO