engine surge CB 750

[Gene]

My 1973 CB 750 has a very fast, rapid surge, up and down,  at speeds under 3000 rpm. Otherwise it starts fine and runs fine.  Is this a function of timing or carb synchronization?

Gene

[Re-run]

It would not be timing. It could be a sync needs to be done, it could be that the engine is running lean.
How long since the last carb cleaning? How well does the bike accelerate? Many times this is caused by dirty idle jets causing a leaner condition. Could also be air leaks around the boots between the carbs and engine.

Pull out your plugs and look at them and that will tell you. On your 73, they should be a medium brown, some go with chocolate colored.

First thing would be to spray carb cleaner or wd-40 around the boots. If the idle changes for a couple seconds, you found the probably culprit. Your boots are either loose or hardened and need to be boiled in some water and wintergreen oil or replaced. I choose replacement as boiling is only a temp fix I think.

if no change, pull the choke a bit to see if things smooth out some. You can try running some seafoam to clean things up. If that doesn't work, then a cleaning is in order.

[sgtslag]

I agree, it sounds like possibly a vacuum leak.  Check hoses.

[shinyribs]

Rpm surges are most always a result of an air/fuel issue,so more than likely not ignition related.I agree with Rerun and Sarge on the vacuum leak.A vacuum leak is the result of 90% of all rpm surge issue

[Gene]

You guys are great. Since I don't have the means to sync the carbs, I will first print out this info, go into the garage and start trouble shooting for vacuum leaks as you described.

In regard to the other statements, the bike does run great, lots of power, and above 3000 rpm is smooth, no surging.  Heck, I'm allmost 78 and wish I could surge a little bit once in a while.

Gene

[shinyribs]

Gene wrote

  Heck, I'm allmost 78 and wish I could surge a little bit once in a while.

Gene

 You can get a set of carb sync gauges fairly cheap off of E-bay.I think I paid about $65-70 for mine,shipped.They are in inches of mercury vs centimeters but they work just as well. Hondas specs are in centimeters,not inches obviously but it converts over. I think honda calls for 18-22 cm/hg   and it converts to about 8-12 inches/hg? Maybe? Anyway...

[LukeM]

78? and still riding?  More power to ya.  I do find that riding bikes does keep one feeling younger.  Works for me, and I'll be 54 this year.

You _can_ get by with a 2 gauge vacuum gauge set, if you don't mind a little extra work. I posted a while back about the trick to doing this:

Balance carbs 3 and 4.
Balance carb 2 (the reference carb) with the 3 and 4 pair, using the PAIR adjustment and not the INDIVIDUAL one for carbs 3 and 4.
Balance carbs 1 and 2.

That should get you close.  I got a pair of dial-type gauges off EBay for abour $35, so $70 for a quad set is about right. Mine came with these little in-line restrictor thingies, that help to keep the needles from bouncing too much. Hook both gauges up to a single source, adjust the restrictors so both carbs act about the same, and you're all set.

I don't know what might be causing your surge problem over 5K rpm. It's possible the carbs are so out of sync that one is in the power band and another is well above or well below it.  Difficult to say.  It's also possible that it's a fuel feed problem: maybe a carb is running dry at the higher RPM?

I'm sure with a little help, we'll figure it out.
Luke M