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Revving at speed issue

MaDhATt3r

Member
Joined
May 7, 2022
Total Posts
46
Total likes
0
Location
Chattanooga, TN USA
Took the cb450 out for a ride around the block today and it acted a little funny. if i went to give it throttle after idle it would rev fine but if say i had it at 1/4 throttle then went to give more at speed (15 mph) it would act like it was trying to rev but unable to. immediately letting off throttle to idle then back it would rev fine again. Not sure if a mixture thing or maybe a spring loose on the mechanical advance? I did notice one of the springs were a little loose on the advance but had read it was best to leave them alone.
 
If a spring came off the advancer unit it would advance the timing sooner so retarded timing is not likely the issue. Since you didn't post this in your project thread I can't recall or re-read whether or not you have the stock air filters on the carbs, or whether the carbs were cleaned/rebuilt. IIRC you got it running with a simple points plate change and may not have set the timing or points gaps. Before you wonder about what might be wrong, it's best to work out all the standard tune-up adjustments to be sure of what is set properly and what is still unknown.
 
. if i went to give it throttle after idle it would rev fine but if say i had it at 1/4 throttle then went to give more at speed (15 mph) it would act like it was trying to rev but unable to.

Did the RPMs initially drop in that instance as you tried to open the throttle from 1/4? What gear were you in? Maybe try a more gentle roll on to see if that makes a difference?

If you're worried about the advance springs it might be a good idea to check the advance with a timing light. I just bought the cheapo light from Harbor Freight last week and it worked okay for me. It's about $30.
 
Rebuilt carbs and replaced all ignition electronics. Only part left to install is the new intake rubber boots between engine and carbs. It will do anything you ask with throttle while sitting in neutral, just while moving, say if you rode in second at 15mph for 5 seconds then went to open up throttle more, it stays the same rpm but acts like it is trying to rev, pulling in clutch and letting it settle to idle while still rolling then letting clutch back out and then giving gas it then revs properly throughout the rpm range and your on your way again.
 
Rebuilt carbs and replaced all ignition electronics. Only part left to install is the new intake rubber boots between engine and carbs. It will do anything you ask with throttle while sitting in neutral, just while moving, say if you rode in second at 15mph for 5 seconds then went to open up throttle more, it stays the same rpm but acts like it is trying to rev, pulling in clutch and letting it settle to idle while still rolling then letting clutch back out and then giving gas it then revs properly throughout the rpm range and your on your way again.

Does that mean the RPMs increase but your actual speed does not? If so, it sounds like your clutch is slipping.
 
Air leaks at the carb boots on the head will only dramatically affect idle and return to idle, not acceleration typically. Stock air cleaners?
 
21YyQt0nNcL._AC_.jpgNo increase in RPM. Stays same rpm and acts like it is trying to rev but cannot. definitely not a clutch issue. Air filters were just put on but it did it even before I put them on. They are aftermarket ones from 4into1 for the cb450.
 
The CV carbs from Honda require the proper intake flow created by the OEM style air cleaners so it would never run properly without them, but if it is doing what you've described with everything assembled then it points to either timing significantly off or the carbs are not yet fully clean. Did you set the points gaps to .012"-.016" and then set the timing, preferably with a strobe light but at least with a test light at LF and F on the alternator rotor?
 
Yes, gaps set with a light and come on/off exactly as they should with timing marks. Carbs were cleaned extensively and only thing not replaced from kit were the jets because the kit came with 140 and 150 jets and mine being a 14h has 130 jet. Also to note would be timing was set after cam chain tension set, and then valves set properly. again at sitting on stand it will properly do anything you ask of it when revving and returns to idle promptly as it should. will idle all day at 1200 rpm.
 
I believe 15mph and 2nd gear puts you right around 2500 RPM, which is right in the middle of the advance region. I wonder if it would do the same thing in other gears at 2500 RPM, which is a bit low for a cruising engine speed on this bike.
 
Did you replace the float needles and seats (if they came in the carb kits)? It's best to not use the brass parts in most kits as they are usually incorrectly sized and often cause more trouble than it had previously. that said, did you check or set the float levels?
 
https://www.youtube.com/shorts/g7KUWfWGTqM

Yes seats, needles, floats all new. I didnt have a good method of setting float level other than letting the bowl fill, then shutting off gas and pulling bowl and seeing how high it was, and adjusting accordingly.

Float height thread

Sounds rich to my ear. I wonder if the fuel level in the bowl is too high. It's best to set the actual float height, but if you're judging by the level in the bowl, I think it should be high enough to reach the "step" in the bowl, but not too much more than that.
 
Ah. It is currently half way between the step and top.

I think it should definitely cover the step, but not by too much. The float will raise the bowl level once it sits on top, partially submerged. Lower a little and see if it improves.

I believe the starting position for the mixture screw is 1.25 turns out.
 
on mine, (14H carb) it is supposed to be 3/4 but surprisingly it likes it more around 1/2 than 3/4 of a turn. by the time i get to a full turn the rpm's are dropping. I'll put it on center stand and re do the floats and try again and see if it fixes. I was definitely wondering if maybe it was being fed too rich when under throttle and moving and if that might have been the cause when you try to give more throttle while moving.
 
on mine, (14H carb) it is supposed to be 3/4 but surprisingly it likes it more around 1/2 than 3/4 of a turn. by the time i get to a full turn the rpm's are dropping.

Oh, right, I forgot that the 14H has a different screw setting. The float height will affect the mixture, so that is probably part of why you've found a leaner setting on the mixture screw to be best thus far.

I'd recommend setting the float height properly at the next opportunity and then readjusting the mixture screws.
 
^^^Which would make some sense if the float level is high, since they are fuel screws - in = leaner, out = richer
 
same result so far. someone else suggested that i open the gas cap and see if that changed anything. interesting thought. dont know where it pulls recovery air from in tank.
 
same result so far. someone else suggested that i open the gas cap and see if that changed anything. interesting thought. dont know where it pulls recovery air from in tank.

The vent is in the cap, but if it were blocked, it would slow/stop the flow of gas to the carbs, which would cause a lean mixture. It also doesn't fit with the fact that you can accelerate from idle without issue.

Can you try the same experiment at 2500rpm in 3rd? I'm still wondering if it's related to the advance springs possibly being loose. The timing advance may "bounce" when you pull on the throttle at 2500rpm.

What size jets are you running?
 
There's always the possibility that those 130s are not OEM jets and therefore might not be properly sized, so they could actually be larger than what a Keihin 130 really is. If you take a plug socket with you, ride the bike steady at the point it runs poorly for about a quarter mile in 3rd gear, then pull the clutch, hit the kill switch, coast to a stop and check the plugs for color, you'll find out if it's rich or lean at the problem area.

Also, what about your plugs? Can't recall if you're running B8ES or BR8ES and since you have new coils/wires/caps you already have resistor caps in place, so if the plugs are BR8ES then you've doubled the resistance which can't help.
 
A couple questions, do you know the compression?

What's the fastest you've driven the bike so far?

Will the bike drive to 6000rpm in 1st gear? If so, is it quick or slow?

10 seconds after start up, are both headers hot and the same temperature, or is one cooler than the other?
 
Both are hot. as well as same output flow coming out of tailpipes. it is as though it runs perfect to a certain rpm then falls flat on its face. Set the floats to spec today and still same result. Bought a timing light and have bike leaned over to right side. Thinking about pulling stator cover off and checking timing to make sure it is advancing. oh, and i pulled the advance unit out yesterdaY and from what i can tell, it is working as it should. as well as both marks on the end where bolt goes are lining up. I also cleaned and lubricated it. I could ride it as long as i wanted to as long as i dont expect it to rev past about 5k. hell i even pulled the carbs back off today just to make sure something hadn't recently become clogged. and nope. every carb passage still clear when checked by spraying carb cleaner through it. Idles like a dream but falls on its face past 5k rpm. I have noticed it get warm enough to see light smoke emitting from the cooling fin area. then again it isnt really moving when idling. bought some clear fuel lines today just so i could see what was going on underneath. It has an issue where it jumps from second to neutral the first time i go to second from first, but i will deal with that one later as it never does after the first time and i pull up for second again. (pulls second for about 4 seconds then jumps to neutral, then once back to second fine till 3rd, etc.)
 
I'd be curious to hear from other dohc 450 riders (with stock carbs) on this, but I tried rolling on the throttle from 15mph in second yesterday and was able to do it, although it wasn't like I could just take off and go. I think the RPMs are so low that it's not really in the power band, to say the least. I would probably feather the clutch if taking off at that low a speed in second gear in order to keep the engine speed a little higher during initial acceleration.

I was trying to pay attention to my downshifting and I was typically shifting down around 3000 RPM. As I mentioned above, I think 15mph in second is a little under 2500 RPM, so that's not a combination I would typically experience naturally.

How does it respond at higher RPM? Are you experiencing sluggishness in other conditions?

You ought to be able to pull the stator cover off with the bike in the center stand. A little oil might come out while checking timing, but not too much.
 
What i mean by it is it starts to pull then hangs and falls on its face. Not that it doesnt at all. I added to my last post so it might help answer some. I have set timing without running, on center stand, with a dummy light, but have it leaned to right now and want to check again with a real timing light while running.
 
If you could get a video of it acting up while riding it would give us a better idea of what you're experiencing. I'm pretty much out of ideas given that all you've described is happening as it is. Still seems like carbs but obviously without being there it would be impossible to say. Have you tried to close the choke just a bit when it bogs down to see if it makes it better or worse?
 
so if your looking at points plate currently, and the top notch that is normally centered. it acts like this.. Centered=lower rpm, left of center=higher rpm, right of center higher rpm. so advancing or retarding outside of centered results in higher rpm at idle.
 
What i mean by it is it starts to pull then hangs and falls on its face. Not that it doesnt at all. I added to my last post so it might help answer some. I have set timing without running, on center stand, with a dummy light, but have it leaned to right now and want to check again with a real timing light while running.

Okay, I was focusing too much on the first post - sorry. What you're describing (in post #26) sounds to me like [maybe] the motor is exhausting the fuel in the bowls faster than they can refill. That could be related to the cap vent that you brought up before or restrictions in the needle valve, fuel line, or petcock. It could also happen if the throttle cables are not properly synched, which could cause one bowl to go dry while the other has fuel remaining. This would feel like losing one cylinder. Then, when you let off the throttle, the bowl gathers a fresh supply and you can go again, at least for a while.
 
yeah i will try to do something tomorrow which shows. its just hard without a gopro or something and a mount. my riding jacket doesnt really have a pocket but ill see what i can do. I might fix something up with some zipties or something.
 
so if your looking at points plate currently, and the top notch that is normally centered. it acts like this.. Centered=lower rpm, left of center=higher rpm, right of center higher rpm. so advancing or retarding outside of centered results in higher rpm.

Timing isn't necessarily about achieving max RPM. I would be focusing on the timing marks only and making adjustments to get the cylinders firing on the marks.
 
yeah i will try to do something tomorrow which shows. its just hard without a gopro or something and a mount. my riding jacket doesnt really have a pocket but ill see what i can do.

I sometimes record audio with my phone in a shirt or jacket pocket. Audio can tell a lot.
 
Video incoming. Watch Tach i live on a steep hill lol. When it hangs i am still asking more than that of it. jumped from second to neutral once.
 
Hard for me to conclude anything, but it doesn't sound like it's running low on fuel in the bowls.

Sounds more like it's bogging down from being too rich, but I'm not certain. Did you do anything with the needle in the carburetor slide?

What's the status of valve timing (clearances) and compression? Have those been checked recently?
 
Nice vid, thanks!

Are your choke flaps down?

The carb pistons clean and carb bore clean and slide smoothly?

What do your plugs look like?
 
Carb was just redone, slide moves as it should and covering one hole and blowing in other the slide moves up and then back down freely without an issue. yes cam chain tension was set at 90 degrees past TDC for left side, then valves set with correct feeler gauge then timing set. Needle on slide looks brand new and was untouched. Choke was off as bike would stall if choke was on.
 
Sounds like it flattens out to me, as if it's rich. My question about the choke was to try closing it when it's running poorly to see if it makes things worse or better, at this point I'm guessing it would make it worse. I wonder if the main jets were changed with aftermarket jets before you got it and they aren't correctly sized. Sounds like the timing might be a bit retarded too. You should know that the points plate generally fits a bit loosely in the base and turning it to change timing can also affect the points gaps due to a less than perfect fit in the base which allows the plate to move laterally. The points should always be checked for proper gap after any plate rotation, if the gap changes because of plate movement and gets smaller it retards the timing. It isn't as precision as an old V8 with rotating the distributor and all else stays the same except the timing adjustment.
 
I've found on initial start up the bike will chug along at lower rpm with choke on until you give it gas... stumbles similar to your vid which is why I asked about the choke and carb piston

I am curious about the plug color and perhaps trying new plugs
 
It might also be worth testing the bike with the side covers and air boxes removed. This would lean things out some and you may notice a slight improvement that would support the theory that it's running too rich right now.

I once had a main jet in my 360 come loose in the bowl. While I was limping it home (not knowing what was wrong) I couldn't open the throttle very far without it bogging down, so I accelerated very slowly to avoid that.
 
You mentioned "carb was just redone" in an earlier post. If true and a rebuild kit was used, the jet calibration may be off. That is, a 135 main jet from the kit (non-Keihin), may perform much like a 160 main (genuine Keihin).

I had a 500T that acted just like that and the jetting was root cause, I used the kit jets. once the original brass was refurbished/reinstalled, the bike ran fine. Not an uncommon story from what I see around the interwebs.

just a thought....
 
Just saw the vid......
1600 RPM is way too high for idle speed, and shifting below or near 5K is not even close to getting where the engine runs best/efficiently.....
Seems as if it's running rich (float level or other carb setting ?) and/or advance/timing is off.....
 
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