Topic: Rode the bike today for first time! Throttle response questions.

[emlupi]

I rode my '72 CB750K2 for the first time today, had a real blast! No problems to speak of and it went very well for the most part. I did notice something while riding. The bike came off idle and pulled very smoothly on the throttle. However, it seemed to flatten out and then not want to rev any higher at:

First gear, around 5000-5500 rpm
Second gear, around 4000-4500 rpm
Third gear and up, around 3000-3500 rpm

Particulars as follows;
Stock air box (K&N replacement filter) and exhaust.
Carbs were disassembled, thoroughly cleaned and inspected.
New float needles and seats, gaskets, o-rings, etc. (all OEM parts used, Kehin brass)
Clear tube tested and synched using Carbtune.
Jets are 110 mains and 40 pilots, all Kehin.
Needle is 4th notch from the top.

As I stated, the motor would pull and accelerate smoothly up to the revs as mentioned above and then seemed to hit a wall. After that it would rev very slowly past that point. It seemed to be struggling to make any more power and rev any higher at those values. Where should I begin looking for a remedy to this situation?

Also, I would like to say thanks to all the forum members who have graciously answered my questions and provided guidance during my winter project refurbishing this old bike which had sat for many years. It finally saw the road again on this glorious summer morning and we sure had some fun. Hopefully much more to come.

Thanks much,
Ron Lupinacci

[Jimray23]

Did you test all the ignition system parts? Coils, points, plug caps, plug gaps and timing.

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Jim
I really don't see why people ask for advice if their not going to take the advice given.

[BomberMann650]

Flatspotting like that is probably from being too lean.

Do you have the original air filter still?  Swap it back in.

K&N's are nice and all.  But they offer very little pressure resistance for the carb

[emlupi]

Did you test all the ignition system parts? Coils, points, plug caps, plug gaps and timing.

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Yes;
 Installed new condensers (OEM)
 Cleaned points with points file
 Gapped points
 Static timing adjustment
 Dynamic timing adjustment with timing light.
 OEM coil primaries measured 4.9 and 4.8 ohms on multimeter
 New plugs, gapped (NGK)
 New plug caps (NGK)

I also did the 3K maintenance list items  (valve adjustments, timing chain adjustments, etc.) over the winter.

[Jimray23]

Then like BomberMann650 said, its probably a lean spot. You can either go back to original type air filter or do plug chops and make jetting changes to correct the lean.

--------------------------------------------------
Jim
I really don't see why people ask for advice if their not going to take the advice given.

[emlupi]

 I have the original air filter, I will try it and see. I did have this anomaly during my dynamic timing tests



Hello all,

I was performing a dynamic timing check today on my 1972 CB750K2 and I had some peculiar results.I thought maybe somebody here could help to explain what I found.

The ignition had been previously static timed with the gap set at .014" using feeler gauges and continuity checked at "F" marks using a multimeter.

I started and warmed up the bike and idled at 950-1000 rpm. Put the timing light on plug lead #4 and set the #1,4 "F" and advance marks to proper settings. I then did the same thing on the #2,3 marks with the pickup on the #3 plug lead. All going fine.

I then put a dwell meter, set for 4 cylinder mode,  on the points for #1,4 and set the dwell for @47 degrees. I then did the same for the #2,3 points. When I went back to check (with the timing light) the timing marks again, #1,4 was still good to go but the #2,3 marks had moved to the right of the index (advanced). So, I reset the timing marks good again and rechecked the dwell angle and now that was off again. I chased my tail back and forth with this until I ran out of adjustment in the points plate and  the frustration set in and finally I had enough. Throughout this whole ordeal, #1,4 stayed good with no problems.

So, I took it back to my "zero" point with the points gapped at .014" on the feeler gauge and checked at "F" on the multimeter. Started up again and was able to set the "F" and advance ignition to the index marks with the timing light. However, the dwell angle at this adjustment reads about 39 to 40 degrees. The bike is idling steady and running ok so I am going to leave well enough alone for now.

Any insights into why this is occurring and why only on the #2,3 ignition points? I am interested in hearing your opinions. Just to clarify, I did dress the contacts lightly with a points file before I ever started the bike.
ReplyQuoteNotify

[emlupi]

Then like BomberMann650 said, its probably a lean spot. You can either go back to original type air filter or do plug chops and make jetting changes to correct the lean.

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Jim
I really don't see why people ask for advice if their not going to take the advice given.

Tried to run it with the factory air filter. Pretty much the same performance. Still flattening out and breaking up at higher revs as noted above.

[emlupi]

Here are some plug photos. These are taken after about 100+ miles of riding at variable speeds and rev ranges. The photos are in reverse order, 4-3-2-1 and are taken from both sides of each plug (so 2 photos per plug).

[Jimray23]

Seems to me like you need to do a 3/4 throttle plug chop to see whats goin on there

--------------------------------------------------
Jim
I really don't see why people ask for advice if their not going to take the advice given.

[emlupi]

I'm definitely no expert on this, but it looks to me like the bike is running to the lean side as it comes off the idle jet. I have 110 mains, should I go up to 115's? These photos were taken using the stock air filter, but it does the same thing with the K&N replacement filter.

[pjlogue]

Make sure your fuel system is clean.  Screen in tank or screen in bowl, fuel tubing doesn't have high points with a bend and make sure the tiny screens on the fuel inlet jet are clean.  If you have inline fuel filters remove them and see if the problem goes away. 

I had fuel feed problems and the symptoms were similar to what you describe.  I have found tubing routing can cause problems if you have a high point in the tubing or if tubing is lower down behind the cylinders.  For what ever reason tubing behind the cylinders (or even worse having a filter there) can cause vapor locks and fuel starvation.  Same with partially clogged screens in the fuel system. Carbs don't get enough fuel at higher speeds.

-P.

[seanbarney41]

Have you verified the spark advancer is working smoothly

[DaveBarbier]

What happens if you apply the choke a bit? Does it pull stronger?

Are the jets and needle position the factory setting for your bike?

Very nice looking bike by the way, LOVE the orange. I'm a cafe guy but damn, great representation of the 750. 

Lastly, it's more convenient to take a picture of all the plugs in a row. Left to right 1-2-3-4. Shoot them on a darkish background so it's easier to see the plug insulator. Showing plugs after a ride with various throttle positions does nothing. It has to be a proper "plug chop" by flipping the kill switch and pulling the clutch at a specific throttle position.


Sent from my iPhone using Tapatalk

[emlupi]

Thanks for the replies.

pjlogue,
All the petcock internals were replaced with OEM new items, screen, o-rings, washers, etc. The fuel lines are new OEM 5.5mm tubing cut to factory length and routed per specs. I have no inline filters or anything other than factory items. The tank was cleaned out before installation. I will take a look at the screen and see if it is clogged.

seanbarney,

I did a dynamic ignition timing test and the advancer was operating properly with the "II" marks aligned with the timing mark on the case above 2500 rpm, returning to the "F" mark at idle.

Dave Barbier,

With some choke on it ran the same way. It would stumble and stall at idle (this I discovered completely by accident). As per Hondaman's book, the jet size and clip position are factory spec. I know I didn't do a proper plug chop, I just wanted to show what I had and see what those more knowledgable than myself could  divine from it. Thanks for the kind words. The bike looks a lot better in photos than up close and personal, in which case you can see it's patina. But, I kinda like it that way. I would call it a survivor.

[seanbarney41]

Have you done anything with the gas tank cap?...like made sure the vent is clear?

[evinrude7]

I rode my '72 CB750K2 for the first time today, had a real blast! No problems to speak of and it went very well for the most part. I did notice something while riding. The bike came off idle and pulled very smoothly on the throttle. However, it seemed to flatten out and then not want to rev any higher at:

First gear, around 5000-5500 rpm
Second gear, around 4000-4500 rpm
Third gear and up, around 3000-3500 rpm

Particulars as follows;
Stock air box (K&N replacement filter) and exhaust.
Carbs were disassembled, thoroughly cleaned and inspected.
New float needles and seats, gaskets, o-rings, etc. (all OEM parts used, Kehin brass)
Clear tube tested and synched using Carbtune.
Jets are 110 mains and 40 pilots, all Kehin.
Needle is 4th notch from the top.

As I stated, the motor would pull and accelerate smoothly up to the revs as mentioned above and then seemed to hit a wall. After that it would rev very slowly past that point. It seemed to be struggling to make any more power and rev any higher at those values. Where should I begin looking for a remedy to this situation?

Also, I would like to say thanks to all the forum members who have graciously answered my questions and provided guidance during my winter project refurbishing this old bike which had sat for many years. It finally saw the road again on this glorious summer morning and we sure had some fun. Hopefully much more to come.

Thanks much,
Ron Lupinacci

ron maybe someone mentioned this but you have listed above that the needle is "4th notch from the top".  do mean 4th from the bottom?  should be 4th or 3rd from the bottom i believe. 

[scottly]

Check the flow from the petcock, with it disconnected from the carbs, both with the gas tank cap open and closed.
BTW, nice looking bike!

[emlupi]

Here is a photo of the fuel lines and routing. The line going to carbs 1 & 2 does go uphill a little bit before going to the inlet "T". This hose was cut to 13-3/4", attached to the front petcock tube, and run through the upper mounting plate hole as per Honda specs. I have also attached the manual photos which diagram the correct fuel line routing, etc. It seems to all agree with what I have on the bike. I looked at the screen and it was clean. I don't really see anything here that would impede fuel flow.

Should I trim these fuel lines to eliminate any excess material? The factory information gives the lengths as you can see in the photos.

Evinrude,
Hondaman's book states the needle clip should be in the 4th notch from the top for the K2.

Edit: I looked at the fuel cap and vent is clear. Did a flow test with cap open and closed. Fuel flowed freely and at the same rate each way. It seems that the fuel delivery system from tank to carb bowls is working properly.

[pjlogue]

Your first photo shows the hose going to the #1 & 2 carbs rising and then going down to the fuel T.  Air (and gas vapor on a hot ride) can get caught in the high point. 

I have had a number of problems using various fuel line routing and each problem was when I had high points in the line or a fuel filter in line right behind the jugs.  Not saying this is the cause of your problem but eliminating the high point will rule out any problems it may cause.

I routed my tubing through the holes in the backing plate and down to the fuel T's between the carbs.  Installing the tubing was a pain but I shouldn't need to touch it for a long time.  This got rid of my fuel starvation issues.   

-P.

[Trav-i]

Also something to consider is the brand of the points you are using.  There is a notorious brand out there Diachi or something similar.  The backing plate on those points will not allow for proper timing adjustment (I have personal experience with them).  They have a tell tale triangle stamped on them.  The only points worth using are the original TEC brand points.

[emlupi]

Trav-i

I am using the original points which I believe are TEC and the backing plate is TEC. I ordered OEM points and received TEC items. They were out of whack right out of the package. I could not get the ignition properly static timed with them. After struggling with them for awhile, I went back to the original points and got it static timed with no issues. You can see in the photos that the contacts are not even close to being squared up or making full contact.

[evinrude7]

Trav-i

I am using the original points which I believe are TEC and the backing plate is TEC. I ordered OEM points and received TEC items. They were out of whack right out of the package. I could not get the ignition properly static timed with them. After struggling with them for awhile, I went back to the original points and got it static timed with no issues. You can see in the photos that the contacts are not even close to being squared up or making full contact.

mine are a little tweaked like photo #1 and they work pretty well.  i've found the not "square" look can be adjusted. 

[emlupi]

Yeah, I know that you can tweak the metal bracket  to bring them more into shape. I was having such a hard time with them I just went back to what was already working. I still have them and may revisit that issue another time. I believe my advancer shaft may need some straightening as well.

[flybox1]

OEM TEC and ND points have their respective logs stamped on them.
Everything else is junk.  Those certainly need to be filed flush, not rounded.

[emlupi]

As you can see that's what I have. I know they have a reputation for being the "quality" item for ignition points but from what I can see, they're crap.

I shouldn't have to take a part right out of the package and bash on it with a file to make it right.