Author Topic: 1978 CB750 low RPM problem (Read 5700 times)

vanhuette · « **on:** April 26, 2016, 06:29:16 PM »

So, got a bike I've been working on for the past couple of months, finally got it (mostly) road ready and I've been taking it on short rides recently. whenever i move from stopped, the bike is very jumpy and i can feel it jerk forward, and I can hear something metallic "clacking" (for lack of a better description of the sound) that occurs maybe once or twice a second. And the jerking lines up with the sound. At first i thought it was an issue with the clutch, it almost felt like people who are bad at shifting learn how to drive standard (i promise thats not the issue). but then i realized if i let the RPMs drop to around 1500 or lower in 1st or 2nd gear, i can get the bike to do it without touching the clutch lever.

I have no idea if this is unrelated or not, but at one point with the bike in first gear and the clutch lever pulled (at a standstill) I could see the drive chain jerking in time with the sound.

Things that came to mind were some sort of ignition or timing issue, cam shafts or cam chain, .... really looking for any ideas though, I don't consider myself a super knowledgable mechanic

flybox1 · « **Reply #1 on:** April 26, 2016, 06:47:48 PM »

put it up on the center stand, let it idle.
check for a rattle on the chain guard. does the chain want to jump off overworn sprockets?
do you have a 4:1 exhaust? (is the chain rubbing on the centerstand because there is no centerstand stop?)

brewsky · « **Reply #2 on:** April 27, 2016, 12:19:54 AM »

Quote from: vanhuette on April 26, 2016, 06:29:16 PM

but at one point with the bike in first gear and the clutch lever pulled (at a standstill) I could see the drive chain jerking in time with the sound.

When this happens, is the bike on the centerstand (wheel turning), or is the rear wheel stopped on the ground?

Terry in Australia · « **Reply #3 on:** April 27, 2016, 03:12:04 AM »

My 1977 CB750F2 had a stuffed starter motor, it'd turn the engine over really slowly, but it also made a clacking/gronching noise as I rode along, which took me a long time to diagnose, because it sounded like the noise was behind me as I was riding along.

If someone's over-tightened the chain they may have damaged the output shaft bearings, and possibly also the wheel bearings, which could account for the jerky feeling, same if there's a tight spot in the chain. Cheers, Terry.

vanhuette · « **Reply #4 on:** April 28, 2016, 03:33:49 PM »

Awesome thanks for all the help guys, ill try to do these responses some justice.

Quote from: brewsky on April 27, 2016, 12:19:54 AM

When this happens, is the bike on the centerstand (wheel turning), or is the rear wheel stopped on the ground?

This was with the wheel on the ground, i noticed it stopped at a light while riding. It doesnt do that all the time, but at that point it was doing it.

Quote from: flybox1 on April 26, 2016, 06:47:48 PM

put it up on the center stand, let it idle.
check for a rattle on the chain guard. does the chain want to jump off overworn sprockets?
do you have a 4:1 exhaust? (is the chain rubbing on the centerstand because there is no centerstand stop?)

Same thing here, if i start the bike and let it idle, the chain usually won't be moving / jerking. Also i took the chain guard off when i took the back tire off to clean the drum brake, and haven't put it back on so its not the guard. I don't know how to tell if the chain wants to jump off the sprockets, but I would say when it happened it didn't look like it was coming off of the back wheel sprockets, more like it was being jerked from within the engine/transmission.

Quote from: Terry in Australia on April 27, 2016, 03:12:04 AM

My 1977 CB750F2 had a stuffed starter motor, it'd turn the engine over really slowly, but it also made a clacking/gronching noise as I rode along

thats interesting, because i would say the sound that i hear when i get into low RPMs or moving from stopped is like a metallic "clacking" sound. "rocks in a metal fan" type of thing. But the bike starts up instantly with the starter, haven't had any problems with it that way.

Quote from: Terry in Australia on April 27, 2016, 03:12:04 AM

If someone's over-tightened the chain they may have damaged the output shaft bearings, and possibly also the wheel bearings, which could account for the jerky feeling, same if there's a tight spot in the chain.

How would i test this? or is there any way to? I put the back wheel back on myself and i think the chain is correctly tightened after that, but I could be wrong. The back wheel spins pretty freely so i don't think its wheel bearings, As far as the shaft bearings go i have no idea how to test that.

brewsky · « **Reply #5 on:** April 29, 2016, 02:14:58 AM »

Quote from: vanhuette on April 28, 2016, 03:33:49 PM

Awesome thanks for all the help guys, ill try to do these responses some justice.

Quote from: brewsky on April 27, 2016, 12:19:54 AM
When this happens, is the bike on the centerstand (wheel turning), or is the rear wheel stopped on the ground?

This was with the wheel on the ground, i noticed it stopped at a light while riding. It doesnt do that all the time, but at that point it was doing it.

If in fact the same noise happens when the wheel is NOT turning, then it likely is NOT wheel, chain, sprocket, chain guard related.

I would try to re-create every condition that you hear the noise, and write them down,,,,i e rpm, clutch in or out, speed, under load or coasting, engine hot or cold,,,etc. then try varying those to see if any change stops or alters the noise.

vanhuette · « **Reply #6 on:** May 01, 2016, 09:38:53 PM »

Alright will do Brewsky. Im redoing some of the wiring and ignition right now, so once the bike is back up and running i will try to be give a more thorough rundown of whats going on

brewsky · « **Reply #7 on:** May 02, 2016, 02:26:49 AM »

Hopefully it will be something simple.

I had a similar type noise I thought was transmission related, since it stopped whenever the clutch was pulled in while riding.

Turns out the chain was rubbing the guard, but only when the top was tight under load.

vanhuette · « **Reply #8 on:** May 04, 2016, 06:38:02 PM »

alright so i finally spent some wrench time tonight and got the new exhaust and ignition installed so i could get back to this problem. So i took a closer look because of your comment brew sky, and sure enough the chain was rubbing on the centre stand, and was definitely too loose. so i tightened it, and it was still rubbing so i took the centre stand off.

I didn't get to take the bike for a full ride, but just up and down the driveway kind of thing. it was still a bit jerky off the line, but i didn't notice the metallic clack sound that i was hearing before. it seemed like it might have been a bit better, but that may have just been wishful thinking. definitely need to take it out for a ride and really test it out.

Im not sure if this could be related or not, but as the bike was idling i also noticed that the RPM seemed to jump about 200 RPM every few seconds, and when it jerks at really low speed, the RPM also seem to jump a bit... I'm wondering if it could be a carb issue or if the cam chain needs to be adjusted, and if that could be causing the jerking? Ill probably do the chain on the weekend.

Airborne 82nd · « **Reply #9 on:** May 04, 2016, 07:30:00 PM »

The cam chain is part of the 3K tuneup if you do the 3K you may find your problem.

but as the bike was idling i also noticed that the RPM seemed to jump about 200 RPM every few seconds.
when this happens do you hear the motor change ?

and sure enough the chain was rubbing on the centre stand

Is the center stand going all the way up ?

vanhuette · « **Reply #10 on:** May 05, 2016, 12:24:06 PM »

Quote from: Airborne 82nd on May 04, 2016, 07:30:00 PM

but as the bike was idling i also noticed that the RPM seemed to jump about 200 RPM every few seconds.
when this happens do you hear the motor change ?

im not sure what you mean by hearing the motor change, do you mean compared to just the tach needle jumping? cause yes the actual engine will rev higher for a second, every few seconds. kind of inconsistent idling.

Quote from: Airborne 82nd on May 04, 2016, 07:30:00 PM

Is the center stand going all the way up ?

It looks like the centre stand was going too high. I don't know if there is supposed to be a bump-stop on it somewhere, but the only thing that was stopping the stand was the lower frame piece near the back wheel that connects to the drum brake on the right side. didn't look like that was supposed to be like that.

brewsky · « **Reply #11 on:** May 06, 2016, 01:56:29 AM »

I'm guessing you don't have original exhaust?

The original exhaust has the rubber stop for the centerstand.

You can add a small piece of metal from the rear rider peg with a 90 degree bend on the bottom, add a rubber buggy bumper and your good to go.

Most aftermarket 4/1 exhaust come with them.

vanhuette · « **Reply #12 on:** May 06, 2016, 01:28:31 PM »

Quote from: brewsky on May 06, 2016, 01:56:29 AM

I'm guessing you don't have original exhaust?

The original exhaust has the rubber stop for the centerstand.

You can add a small piece of metal from the rear rider peg with a 90 degree bend on the bottom, add a rubber buggy bumper and your good to go.

Most aftermarket 4/1 exhaust come with them.

Ohhhhh yup that'll be what it was, I put a new exhaust on it. It didn't come with a new bump stop but that should be pretty easy to install.

So i tightened the cam chain (in hopes that might make a difference) and took the bike out for a ride today, and as far as i can tell there are two, possibly separate, issues:

1) at low speed, and more so in lower gears, there is a sound like a "rock in a metal fan" that occurs at the clutch point. This is most noticeable moving from a stop. It does also happen as I'm slowing down to a stop when i pull the clutch lever (and in that case even when the lever is fully pulled the sound will occur, not only at the clutch point). I'm imagining this may mean i need to do a clutch rebuild.

2) at low RPM (<2000 or even <2500) the bike "stutters" as i try to accelerate, causing the jerking movement in the bike. This happens whether the clutch is in or out, and in any gear, though it does seem to be more noticeable in lower gears.

so obviously these two problems happen simultaneously when I'm moving from a stop, which is why i thought they were the same issue, but now I'm wondering if it might be two separate things.

flybox1 · « **Reply #13 on:** May 06, 2016, 01:55:04 PM »

Did you have this noise before you put the new exhaust on it?
Please take a hi-res picture of the left side of your bike and post it for us to see.
With the bike NOT on either stand, pls. Have someone hold the bike upright

vanhuette · « **Reply #14 on:** May 06, 2016, 02:02:10 PM »

yes the sound and the jerking acceleration were both before the new exhaust. What are you looking for on the picture?

flybox1 · « **Reply #15 on:** May 06, 2016, 02:07:13 PM »

Anything you havent mentioned. . .

Have to taken the clutch cover off and taken a look to see if there is something bouncing around in there?
A few weeks back someone posted a picture of a clutch basket band which surprisingly released its tack-welds

vanhuette · « **Reply #16 on:** May 06, 2016, 04:23:57 PM »

tried to open the clutch cover only to find that one of the screws is completely stripped....so that will be a job for another day. anyways, heres the pictures flybox

brewsky · « **Reply #17 on:** May 06, 2016, 06:17:47 PM »

The chain looks like it is pretty tight to me.....I would remove the front sprocket cover and rule out the chain before tackling the clutch.
Is the chain aligned?

vanhuette · « **Reply #18 on:** May 07, 2016, 05:57:47 PM »

I backed off the rear wheel a little to loosen up the chain slightly. also had a look in the clutch housing, and didn't notice anything glaringly obvious that was wrong or pieces that were . Im not sure how visually apparent it is when your clutch needs replacing, but like i said i didn't notice anything.

I've been trying to do some research on it, and it seems like people who need to replace the clutch more commonly have noises when the clutch is engaged (lever out) instead of disengaged like i am having. I also have not noticed clutch slipping. so maybe the clutch isn't the issue?

other things i have thought about: a loose primary chain, or I need to sync the carbs? and then i guess possibly an issue with the chain, didn't take a look at that today

flybox1 · « **Reply #19 on:** May 07, 2016, 07:04:22 PM »

+1 brewsky. chain looks too tight. thatll certainly cause some noise once you sit on the bike and put even more stress on the links and sprockets.
Sounds like Vanhuette needs to complete ALL of the 3k tuneup items

vanhuette · « **Reply #20 on:** May 09, 2016, 11:10:58 AM »

Quote from: flybox1 on May 07, 2016, 07:04:22 PM

Sounds like Vanhuette needs to complete ALL of the 3k tuneup items

hahahah, ahhh youre probably right. I've done a few of the items, but i don't have the equipment to sync the carbs so maybe ill take it in somewhere.

As for the chain, the noise and hesitant acceleration were there before i tightened it, as well as afterwards. so i did loosen it a bit (thanks for the recommendation) but i don't think that was the problem.

I also tried to look at the front sprocket, only to find another couple of completely stripped screws. the PO is killing me right now. so ill have to work on getting those out at some point

Brewsky, as far as i can tell the chain is aligned. ill attach a couple pics just in case you see anything that is out of order.

vanhuette · « **Reply #21 on:** May 09, 2016, 04:48:07 PM »

alright so i figured id do you guys one better and get some footage of what I'm talking about in case that helps. i also tried to take note of everything when riding it today to get a full idea of what was going on. some of it might be unrelated but ill mention it anyways.

So first off, despite only having minimal play in my clutch clutch lever before it disengages (~5-10 mm) i don't think the clutch was fully disengaging even with the lever pulled to the handle. i think thats why the chain was jerking when i stopped before - it was still slightly in gear. it happened again today, and i tightened the cable so now there is no play at all before it disengages. I know this isn't ideal, but it fixed that problem. Im not sure if this maybe indicates an issue with the clutch, i have no idea.

secondly, i found an empty road and filmed the low RPM hesitant throttle/jerking problem, and after watching the video i realized that my bike does a similar thing when its just idling. it jumps ~200 RPM. is there any chance that this is something like mistimed cylinders (cam chain?) or out of sync carbs, that are noticeable at low RPM until i rev high enough that they just work relatively in uniform?

heres the bike idling:

and here it is at low RPM cruising (if you listen close you can hear the stuttering of the engine, which also jerks the bike forward)

finally, i tried to film the sound that occurs when I'm using the clutch at low speeds. i had to shove the phone under my leg so the sound is a little muffled (in real life it sounds a little more metallic), but you can definitely hear it.
clutch sound at low speed (sorry this video is way longer than it needs to be - the sound is at the 8-11 second mark)

flybox1 · « **Reply #22 on:** May 10, 2016, 07:23:14 AM »

These engines do not like to be ridden at that low RPM.
Valve adjustment
Cam chain adjustment
Timing and advance.
Vacuum sync of the carbs.

...have ALL of these been done?

brewsky · « **Reply #23 on:** May 10, 2016, 08:36:40 AM »

Quote from: flybox1 on May 10, 2016, 07:23:14 AM

These engines do not like to be ridden at that low RPM.
Valve adjustment
Cam chain adjustment
Timing and advance.
Vacuum sync of the carbs.

...have ALL of these been done?

+1

and, have the carbs been pulled and cleaned, especially pulling the slow jet and cleaning?

vanhuette · « **Reply #24 on:** May 10, 2016, 06:06:14 PM »

I did the cam chain adjustment. Haven't touched the timing and advance, or vacuum synced the carbs (no carb syncing equipment).

looks like I'm buying some new tools!

Im sure the carbs could at the VERY least use a good clean out and rebuild. Ill try to get on that this weekend. Just for my sake - you guys suggesting these for the hesitant/shaky throttle problem, none of these could be causing the metallic clacking with the clutch?

Im wondering if i should just bite the bullet and rebuild the clutch to see if that helps. I'm kind of at a loss for that issue.

Thanks for the suggestions guys, ill try to keep you updated how it turns out.

Mr. Mike · « **Reply #25 on:** May 10, 2016, 09:48:45 PM »

Does the "clacking" or clattering sound at your clutch diminish at idle, with the clutch pulled in? Then disappear around 1300 rpm? May very well be the typical clutch clatter these bikes make at idle. The YouTube link was difficult for me to distinguish noises sorry.
Definitely get your carbs serviced. If you do it yourself, be sure to remove all jets and actually clean out the tiny holes as well as the barrel of the jet, don't just " blow them out" with compressed air. Read up on carb cleaning/adjusting if you have doubts about anything. Syncing them is a whole different thing.

vanhuette · « **Reply #26 on:** May 11, 2016, 11:33:16 AM »

Mr. Mike, ya sorry that video was not ideal. maybe ill try to take a better one at some point. its not easy to do because it never happens when its sitting in my driveway haha.

Im not totally sure whether this is or isn't the classic "clutch clatter". I thought in the descriptions i read that the clutch clatter tended to go away when the clutch was pulled in (disengaged), and would then happen again when the clutch was re-engaged.

Mine tends to occur when i pull the clutch lever in, and during the "clutch point" where its in between being in and out. And it has never occurred when i start it up and its sitting idling in the driveway (regardless of whether clutch is in or out). it seems to only occur once I've ridden around for a few minutes, and mostly happens as I'm coming to a stop/slowing down and i pull the clutch in. However, it does occur mostly at low RPM, and at low speed.

flybox1 · « **Reply #27 on:** May 11, 2016, 11:52:27 AM »

Quote from: vanhuette on May 10, 2016, 06:06:14 PM

I did the cam chain adjustment. Haven't touched the timing and advance, or vacuum synced the carbs (no carb syncing equipment).

looks like I'm buying some new tools!

Im sure the carbs could at the VERY least use a good clean out and rebuild. What are you waiting for?!?!?! Ill try to get on that this weekend. Just for my sake - you guys suggesting these for the hesitant/shaky throttle problem, none of these could be causing the metallic clacking with the clutch?
You'd be surprised how quiet your bike will run once you do ALL of the necessary maintenance, and follow it up with a nice vacuum sync. It really can go from sounding like its full of nails to a nice idle.

Im wondering if i should just bite the bullet and rebuild the clutch to see if that helps. Why repair something if you dont know if it needs it? I'm kind of at a loss for that issue.

Thanks for the suggestions guys, ill try to keep you updated how it turns out.

While you are waiting for tools....or the cows to come home.....clean and bench sync your carbs!!!
The order of maintenance items also matters as everything works together.
Please follow the steps outlined in the manual.

vanhuette · « **Reply #28 on:** May 11, 2016, 06:56:47 PM »

hahaha, i know flybox, i know. Ill get on the carbs as soon as i get a chance. To be fair, in the original post i thought that the clutch sound was causing the hesitant acceleration, so i wasn't thinking about carbs or ignition i thought there was something in the bottom end that was a problem and i was having trouble diagnosing it.

vanhuette · « **Reply #29 on:** May 23, 2016, 11:25:16 AM »

woo, finally got some time this weekend and pulled the carbs apart. Weren't actually in too bad of shape, although two of the main jets were partially plugged. I got new jets for the bike (i went with 120 mains and 38 slow jets. I got that from the fact that my 4into1 exhaust should be up two main jet sizes from 115, and then using the 2:1 jet sizing rule of thumb for main:slow, so I went up one slow jet size from 35). So Im cleaning everything out and putting the new jets in, then rebuilding and bench syncing the carbs.

CARBS!

I got a carb rebuild kit, and there are two small O-rings left over from each carb package. I tried to study the blown up carb diagram in the manual to see where they could go, and I think that one of them is for the float needle seat, but are those seats pressed in? there didn't seem to be any easy way to get them out. And the other O-ring I have no idea what its for. I attached a pic of the carb rebuild package, and then the two leftover O-rings beside the float needle for size reference if anyone has any ideas.

Hopefully Ill have it back on the bike this week

flybox1 · « **Reply #30 on:** May 23, 2016, 11:32:06 AM »

the small ones are probably for under the fuel screw, and on the drain screw.
there are NO orings in the float needle seat.
the viton tip hits the center waist of the brass float valve seat.

Keihin jets, right?

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Author Topic: 1978 CB750 low RPM problem (Read 5700 times)