Topic: Another silly question for the experienced...

[KB02]

..Or maybe it's not. I don't know...
   Anyway, after the whole rebuild process of my 76 CB750F and getting the bike running and yada, yada, yada... I finally got around to checking engine and frame numbers to see if they matched. I knew the engine had been out of this bike before by the PO, but as it turns out, My '76 F[/i] has the engine from '75 K.[/i] (according to the engine and frame numbers listed on this site in the model guide)
   This is where my questions comes into play. I have been trying to get the bike tuned and no matter what I try, it still just seems that little bit off. Is there any chance that the Model and year difference are what's causing my the problem?
   I have Pod air filters and stock 4 into 1 pipes. I re-jetted te carbs to 130's on the mains and lowered the needle one space. (second from the bottom). The valves are set to spec as is the points gap; and the timing is spot-on according to a timing light. Reading the plugs tells me that cylnder's #1 & 4 are running just about right while 2 & 3 are a little rich (I'm going to open the air screws on those a little bit next chance I get to ride it... maybe today).
    It just feels a little sluggish. like the timing is still off - BUT - the timing plate is adjusted all ther way in one direction; AND that is where it hits "spot-on" according to the timing light.
   
   Any suggestions?

[Steve F]

I'm experiencing similar problems with one of my '76 750F's, where there seems to be a flat spot in the performace around 5000 to 6000 rpm and above.  I've come to the conclusion the the cam wasn't "dialed-in" when I had the engine out last winter to do a gasket replacement.  I'm thinking that the cam needs to be advanced a degree or two, but I won't know this until I get the engine out later this fall.  I suppose I could check for cam timing while in the frame, but time is not easily found these days.

[eldar]

Do you still have the airbox? Put that in and see what happens. I know pods look cool but many times kill your lowend and midrange performance.

Do plug chops and see what those look like. Have you done a sync? How about double check the valves?

I did my cam chain and valves and there was a power increase. Did sync not even sure if it was quite right on and there was yet another power increase.

Did you do your low end settings with a dwell? I know some can do it just by listening but that takes years or experience. Have you changed your low speed jets? Those are generally increased when pods are put on.

[Steve F]

I've been over and over that engine checking everything that I can.  Stock airbox w/ K&N filter, header with or without baffle doesn't matter.  Stock ignition (points) was set to factory specs (dwell and timing).  The timing was advancing about 2 degrees too far, so I bent the advancer stops to correct that.  Synched all the carbs with a calibrated 4-gauge setup.  Float height was measured using one of my gauges.  The carbs have been totally gone over and are clean as a whistle.  Vlave lash meticulously adjusted (cold engine).  The only thing that hasn't been verified is the cam timing and a compression check.

[TwoTired]

For KB02,
Is there a way to tell if your carbs are the ones from the 75K or the 76F?
The reason I ask, is that on the CB550 the needle taper profiles are different between the K and F corresponding to the difference in 4 into 1 and 4 into 4 exhaust.  It seems reasonable that Honda would have done something similar with the CB750 model offerings, too.
Consider the airflow around the engine.  The outer carbs are in the airflow and may get a little pressure onto the carb throats from air flowing around the cylinder bank and hitting the filter pod.  This increases the pressure in the carb throat and reduces the fuel drawn past the jets.  Conversely the inner carbs are in a partial vacuum area behind the engine with the cylinders blocking direct airflow to those carbs.  The higher vacuum in the carb throats increases fuel draw from the jets in those carbs.  The stock airbox/plenum ensures all carbs are presented the same atmospheric pressure across all carb throats.  Individual pods draw air from a common atmospheric source only when the bike is not moving forward in an air mass.
Also, remember you have directly controlled carburetor slides.  It is important to note at what throttle position you are experiencing problems as different devices within the carb dominate depending on throttle position, not engine speed.  Temporarily mark your twist grip with tape markings you can read while riding.

For Steve F.
Same comment about throttle position over RPM reference above.  Also, clean or renew your spark plugs and operate your bike at the problem throttle setting.  Do the plug chop thing and read your plugs for mixture indications.  Relate the mixture read to the throttle position and address that metering portion of the carb to make your next adjustments.
Cheers,

[Geeto67]

If you have a k engine there might be something fishy going on. Since the F engines were slightly more powerful than the K engines, most people would try and swap an F ening back into an F bike. However since most of the aftermarket goodies were for the K bikes, anytime someone modified a supersport back in the day it usually started with a k engine being put in an F frame - Why? Usually Big bore kits and cams, which were only available for the K engine. My 1976 supersport has a K engine in it when I bought it  so I opened her up and found an 811 kit and a non stock cam.

See if you can find out why the engine was swapped. Sometimes it's just because what the person had lying around that ran, other times it was well though out so many engine mods can be done.