Topic: Shuddering while maintaining crusing speed

[winkster2005]

Hi everyone,

1976 cb750 only non stock motor parts are pods and dyna electronic ignition.

When I am crusing along not accelerating just trying to maintain my current speed the bike will shudder. It kind of reminds me of when a motor is hitting a rev limiter. If I give it a little gas and accelerate it stops doing this.

[Bodi]

Do a plug chop after a few minutes of riding with tis shuddering happening. The likely issue is carb tuning.
You'll probably get lots of posts telling you pods are no good, and why.
Anyway, I expect you'll find it is running lean. Curing that means extensive tuning work in the carbs or installation of the stock intake.

[winkster2005]

Yea I kinda figured that. I went up one with my main jets because I was a little white before but I haven't had a chance to mess with it and had to ride my bike to work because I'm having car issues. Do you think I'll be OK leaving the 1 step up with my jets and putting the breadbox back on?

[PeWe]

If you use Dynatek surpressed ign wire ( carbon) , try other common copper with NGK 5kohm caps.
I had a bad Dynatek wire that tricked my jetting for months. Intermittent problem due to one new bad wire. Replaced all with std wires.

[Eydugstr]

Winkster - Does the bike have the LH single outlet fuel petcock?  If so what may be happening is fuel starvation at highway speeds.  The bike was originally designed for a dual outlet fuel tap.

What I'd recommend is doing a clear tube tube test of the carbs and double check that the float levels are all at the correct height.  If they aren't, correct them until they are all at the correct stock level (think it's 26mm, which should put the fuel level just under the casting of where the carb bowl gasket sits, double check with the manuals) then ride the bike and see if you notice any difference.  If you feel you there's improvement but the "stumble" still happens after riding awhile at highway speeds, then try setting the floats at 25mm or 24mm (raising the fuel level in the carbs a tad bit more to compensate for the single outlet petcock.)

HTH.

[winkster2005]

It has a new oem 2 outlet petcock so that's shouldn't be the problem.

[Swoop]

Perhaps pop the gas cap while it is doing it to eliminate the tank vent issue?

[Bodi]

If it accelerates smoothly off the stumble... fuel supply and ignition are most likely OK.

[winkster2005]

I think I'm going to try using the old air box first. Between my 2 750s I was able to make a fairly decent functioning box. If that doesn't work is there a chance that my valves being too loose could cause this problem?

[Bodi]

Valves too loose would sound like a can of rocks being shaken. And not cause shuddering at steady cruise throttle but smooth accelerating.

You could just try new plugs, things go weird with crappy ones.

[Calvin]

My bike did this same exact thing before my most recent carb sync.  Still does it a little.  Basically accelerating it sounds great, cruising is pretty good, the second you let of the throttle a hair while cruising, it sounds like a rev limiter, just not really popping like a backfire but i guess, shuddering lol.

Mine is still doing it a little, hope you post what you find fixes it.

[winkster2005]

I bet this is my problem. I havent tried syncing mine yet.I know someone with a set of gauges  hopefully I can borrow them

[cb650]

Is there any air box hoses hanging down low.  I had one if my 650's do this and cam to find the carb vent hose was to long.    Shortened it and was fine.

[winkster2005]

Ok, so I decided to try the oem airbox. I put it on, took it for a ride and the problem was still there. I then decided to try a bench sync to see if that would do anything. I found #3 to be open more than all of the others so i adjusted and checked it until it was perfect. Now the shuddering is even worse than before. But on a positive note I feel like this is most likely my issue. I'm either going to get my friends gauges or buy my own and get these things all synced. Any input?

[Wingnut74]

In case it might help I was having the exact same problem at 3-4k. Just did total engine rebuild last winter including carbs and recent carb synch with old Carb Stix - this studder was a new development (not that bad but annoying). I had a worse version of this problem a month before that I traced to bad condenser causing points pitting/bad connection. I was using new condensers from one of the CB750 parts companies (saw big sparks at points), but replaced them with the old condensers that were on the bike before and it solved the problem for 1k miles. Then this lesser studder just popped up last week, still some pitting/corrosion on the new (1.5k miles)points. I just replaced the whole points plate and condensers with OEM parts last night and whammo - no studder. 

[winkster2005]

I have dynatec electronic ignition, could this still be an ignition problem?

[flybox1]

Doubt it.

Highway speeds doesnt really tell us which fuel delivery system in the carbs is being used.
You could do 60 in 3rd gear, or 60 in 5th gear.  Both use a different combination of jets.
So, instead of using speed as the marker for when/where the issue occurs, use throttle position....idle, 1/4, 2/3, WFO, etc.   

Start with doing a valve adjustment, cam chain adjustment, points gap check, and timing w a light.
Then...finish with a vacuum sync of the carbs.
While the bike is still hot, throw in a new set of D8EA plugs, start it, and let it idle again(fan on the engine) for 4-5 minutes.
Cut the engine, pull the plugs, and take a picture of the tips for us to see.
Need make sure the basics are covered well, and then to check idle mixture baseline.

[winkster2005]

It's when I'm somewhere between idle and acceleration while the bike is moving. If I'm riding at any speed and just try to maintain that speed it will happen. If I am giving it zero gas it doesn't do it if I'm accelerating at any rate it won't do it. 

[flybox1]

Incorrect timing or incorrect air fuel ratio due to your pods.