Topic: Engine Pulls Hard up to 5.5k RPM Then Not So Much

[Finnigan]

Motor - New build

Line bored cases
Balanced crank from cycle x
All coated bearings
Yoshi rods
Bored to 823cc
Daytona cam
HD valve springs
Ported head
4 into 1 Motad exhaust
K0 4-cable carbs with stock jetting (needles set to 4th position), velocity stacks until my Yamiya airbox comes in

Engine has been doing great since break in, not the easiest to start but does well when warmed up.  Put about 75 miles of highway cruising so far.  My father-in-law has a newly rebuild sandcastle and we want to see who would win from a 4th gear pull around 50mph.

Found my bike really resists revving past 5.5 - 6k RPM and I've heard that this cam comes alive around 7k.

I'm not sure how to know if this is lack of air or lack of fuel or something worse and I've never been good at dialing in carbs.  Where should I start?

[scottly]

What do your spark plugs look like?
What ignition are you running? Have you verified timing?

[Finnigan]

I don't have the original toolkit and its hard for me to get to 2 and 3 but here is 1 and 4 spark plugs.

Running charlies place electronic ignition advanced 2-3 degrees (static only) with Iridium plugs and dyna coils

[scottly]

Just set your timing so it's at the F mark for now, but verify the advancer is free to move to full advance, and returns to the F mark with the springs. Both plugs are on the rich side, but not sure if that's the problem? What Dyna coils, 3 ohm or 5 ohm?

[Finnigan]

Easy enough, I'll change to the F mark tomorrow.

Dyna coils are 5 ohm according to my emails from years ago but I can measure.

[scottly]

Do you have a timing light yet? You really should check the timing dynamically, that it is at the F mark at idle and between the two hash marks at 2500 RPM, and returns to the F at idle.

[Finnigan]

Don’t have one, is one from say Harbor freight good enough?

[Finnigan]

Just set your timing so it's at the F mark for now, but verify the advancer is free to move to full advance, and returns to the F mark with the springs. Both plugs are on the rich side, but not sure if that's the problem? What Dyna coils, 3 ohm or 5 ohm?

Spark set to 'F', made sure the advancer moves freely - snapped it a few times and confirmed it fires at 'F' again.

Got a timing light and was amazed how far off the advance timing marks were, several millimeters to the right at RPM.

I adjusted the plate to get 1/4 in and then the individual pickup for 2/3.  It still idles but the F mark for idle is far to the left. 

[Finnigan]

Took the bike for a ride after the big adjustment and can notice an immediate improvement in acceleration smoothness, opened the throttle and RPM climbed quickly to 6k and slowed down a little to 8k where I heard some stuttering but I ran out of road near my house.

I feel the engine has more to give but need to keep riding and there aren't that many places near me that I can get up to 10k RPM.  Either way the bike is running smoother and faster.

[HondaMan]

I don't have the original toolkit and its hard for me to get to 2 and 3 but here is 1 and 4 spark plugs.

Running charlies place electronic ignition advanced 2-3 degrees (static only) with Iridium plugs and dyna coils

Are those coils the 3-ohm Dynas? They have an exceedingly short spark that gets shorter above 5500 RPM. Do you have resistor plug caps on the spark wires? Those will help to stretch the short spark out a little bit.

[Finnigan]

5ohm coil I believe, need to check the resistor cap rating

[Tracksnblades1]

Took the bike for a ride after the big adjustment and can notice an immediate improvement in acceleration smoothness, opened the throttle and RPM climbed quickly to 6k and slowed down a little to 8k where I heard some stuttering but I ran out of road near my house.

I feel the engine has more to give but need to keep riding and there aren't that many places near me that I can get up to 10k RPM.  Either way the bike is running smoother and faster.

Loss of power or hesitation first, then the stuttering..?

Or..

Balls to the wall everything good, then the stuttering caused the slow down..?

[Finnigan]

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

[Tracksnblades1]

If your timing is spot on…you’ll have to determine whether your loss of acceleration between 6-8k is caused by ignition or A/F ratio..

Your slowing down when it should be waking up..It’s not uncommon for a well tuned, good condition 750 engine to happily rev right through its red line with stock components especially in its lower gears.. I think yours should be carrying the front tire at leas in first gear on just the throttle with all your mods..

[Tracksnblades1]

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

1st what size are your main jets..?

Some tuners drilled the factory jet instead of replacing them..It can cause you to scratch your head..

I would re-verify your timing first…
Determine whether the resistance to reving 6-8 takes place before the stuttering or after (during) the stuttering. Second
Third, I would make sure your throttle is wide open well before you experience the resistance to reving. So your running on your main jets..I think your sand cast carbs are harder to sync and maintain than the racked ones..IMO..

[HondaMan]

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

The needles in the K0 carbs were normally set to the middle notch. Those bikes had more-open pipes than the HM341 variety, so velocity-mix thru the carbs ramped up slightly sooner. With more restrictive pipes the needles' clips went up to the 4th notch. (Corrected 6/4)

If you pull the float bowls: check the size of the larger holes in the emulsifiers (the mainjet's holder, so to speak). If they are still the OEM size of 0.0325" (often called .033") then maybe consider opening them up with a small drill bit, or at least crank a sharp object into the 8 holes to widen them up a little bit. They were too small in the K0 carbs, which caused poor fuel aeration at midrange.

[scottly]

Just set your timing so it's at the F mark for now, but verify the advancer is free to move to full advance, and returns to the F mark with the springs. Both plugs are on the rich side, but not sure if that's the problem? What Dyna coils, 3 ohm or 5 ohm?

Spark set to 'F', made sure the advancer moves freely - snapped it a few times and confirmed it fires at 'F' again.

Got a timing light and was amazed how far off the advance timing marks were, several millimeters to the right at RPM.

I adjusted the plate to get 1/4 in and then the individual pickup for 2/3.  It still idles but the F mark for idle is far to the left.

Your original timing was retarded, causing your power loss at higher RPMs. The springs on your advancer are too weak, preventing the return to the 10* BTC point at idle. This can make it harder to set an even idle, but as long as the timing is set dynamically at the full advance marks, the top end performance won't be affected. You can try squeezing the eyes on the ends of the springs to tighten them up, or even resort to cutting a coil off each spring. The idea is to have the full range of advance the system affords. Now that you have a timing light, you can check what RPM the full advance comes in at, and tailor it to your needs. Once you get it sorted, you can try the 3* advance from the other thread.

[Finnigan]

Some updates, I got my stock airbox and new filter element from Yamiya in the mail and decided to throw it on to get closer to stock.  After doing this I tried syncing the carbs and found #1 cylinder was pulling about half the vacuum of the others and I couldn't get it higher.  Compression is dead even across each cylinder (reading 130 but it's a cheap gauge).  I went through and found the rubber connecting the head to the carb looked a little off so I replaced it.  Decided to look into the carbs one last time.

Float heights look good at 26mm, good sealing when the float lifts.  Passages and jets are clean and look very good.  Started measuring the jets after confirming sizes.   ------->

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

1st what size are your main jets..?

Some tuners drilled the factory jet instead of replacing them..It can cause you to scratch your head..

I would re-verify your timing first…
Determine whether the resistance to reving 6-8 takes place before the stuttering or after (during) the stuttering. Second
Third, I would make sure your throttle is wide open well before you experience the resistance to reving. So your running on your main jets..I think your sand cast carbs are harder to sync and maintain than the racked ones..IMO..

Main jets are 120s, I verified the size of each and found three were .051" and one was .046"...not sure if that would make any difference but for now I swapped them with a replacement set of 120s that are all .046" confirmed with a calibrated gauge set.

[Finnigan]

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

The needles in the K0 carbs were normally set to the middle notch. Those bikes had more-open pipes than the HM341 variety, so velocity-mix thru the carbs ramped up slightly sooner. With more restrictive pipes the needles' clips went up to the 4th notch. (Corrected 6/4)

If you pull the float bowls: check the size of the larger holes in the emulsifiers (the mainjet's holder, so to speak). If they are still the OEM size of 0.0325" (often called .033") then maybe consider opening them up with a small drill bit, or at least crank a sharp object into the 8 holes to widen them up a little bit. They were too small in the K0 carbs, which caused poor fuel aeration at midrange.

Decided to move the needles to the middle notch (up one from the 4th position) to see if that helps with the plug color.  Before taking the carbs apart I tried another couple passes on the open road after warmed up and at about 6 to 6.5 rpm the bike sounded like it hit a rev limiter, like the tach was bouncing but not going any higher.

On the final pass before heading into the garage I tried a lower gear and WOT early and managed to stutter up to 7k but I felt I was pushing hard and the engine didn't want to rev beyond.

Emulsifier holes measured .023" and .034".  Confirmed the idle jets were consistent value at .015" (these were marked as 40s)

[HondaMan]

Here's an 'easy' next step to try (if you have them): install a #115 mainjet. It sounds like it is just a little too rich below the cam's active RPM range, which will stop it from being able to ingest all the extra fuel that comes from being 'off-cam' until things come together.

I say 'easy' just because it is simpler to change the mainjets than to go back into the slides and drop the needles another notch. If a leaner jet size helps, it may take both steps (drop another needle notch and go to #110 mainjet) to get it to 'start' on the low end of the cam. When the higher-RPM cams "come on" the engine goes from being severely over-carb'd and 'wet' to blowing out that excess gas, then burning the mix well. The trick is to prevent the low-RPM ranges from overloading the mixture before you can get to the more-effective high RPM range. This often means making the low-end lean (even nasty lean) in order to prevent the engine from loading up when in the race and you are forced to drop RPM too low for the gear you're in, then you must act fast on the shifter to get back 'on cam' and burn off that excess fuel before the other guys pulls out in front of you.

Don't overlook this: in the K0 carbs the float-height reference ledge is NOT the flat spot on the gasket surface of the carb. There is a tiny sliver of a ledge just inside the float gasket where those notches are, and THAT is the zero reference point for the float gage. And, the K0 carbs used brass floats with one side set at 25mm (the kickstand side) and the other at 26mm. In my roadracers we used 24mm and 25mm, respectively. I still use that in my K2 today. While it doesn't have a high-RPM cam, it also has no flat spots in throttle response.

This was a common tuning issue with roadrace 750s (fixed typo here) in the early 1970s, we dealt with it a lot. It was always easier to tune up to a hotter engine by doing one thing at a time, like install new, hotter pistons of some sort and tune to those, then install the carbs or cam and tune to that, etc. We were dealing then with lots of unknowns for sure, as Yoshimura was REAL tight-lipped about tuning to the parts they sold. You might remember seeing the famous Cycle Magazine issue with the CR "kit" for a 750 on the cover: I knew of 2 guys who borrowed $$$ to get those "kits" and we never saw them on the racetracks again. They never could get them tuned right.

[Tracksnblades1]

Hmm, can't say for sure.  I'll take it on a few freeway passes tomorrow and get back to you on that.

After the run to 8k the plugs still look very dark, is it time to consider raising the clip on the needle?  As I mentioned before its in the #4 spot (second from the bottom).

The needles in the K0 carbs were normally set to the middle notch. Those bikes had more-open pipes than the HM341 variety, so velocity-mix thru the carbs ramped up slightly sooner. With more restrictive pipes the needles' clips went up to the 4th notch. (Corrected 6/4)

If you pull the float bowls: check the size of the larger holes in the emulsifiers (the mainjet's holder, so to speak). If they are still the OEM size of 0.0325" (often called .033") then maybe consider opening them up with a small drill bit, or at least crank a sharp object into the 8 holes to widen them up a little bit. They were too small in the K0 carbs, which caused poor fuel aeration at midrange.

Decided to move the needles to the middle notch (up one from the 4th position) to see if that helps with the plug color.  Before taking the carbs apart I tried another couple passes on the open road after warmed up and at about 6 to 6.5 rpm the bike sounded like it hit a rev limiter, like the tach was bouncing but not going any higher.

On the final pass before heading into the garage I tried a lower gear and WOT early and managed to stutter up to 7k but I felt I was pushing hard and the engine didn't want to rev beyond.

Emulsifier holes measured .023" and .034".  Confirmed the idle jets were consistent value at .015" (these were marked as 40s)

So, Did it slow down @ 6-6.5k before it started stuttering?

Or did the stuttering cause the slow down @ 6-6.5k..?

[Finnigan]

The stutter made it slow down, I previously thought the opposite but today I confirmed multiple times.

[Finnigan]

Here's an 'easy' next step to try (if you have them): install a #115 mainjet. It sounds like it is just a little too rich below the cam's active RPM range, which will stop it from being able to ingest all the extra fuel that comes from being 'off-cam' until things come together.

I say 'easy' just because it is simpler to change the mainjets than to go back into the slides and drop the needles another notch. If a leaner jet size helps, it may take both steps (drop another needle notch and go to #110 mainjet) to get it to 'start' on the low end of the cam. When the higher-RPM cams "come on" the engine goes from being severely over-carb'd and 'wet' to blowing out that excess gas, then burning the mix well. The trick is to prevent the low-RPM ranges from overloading the mixture before you can get to the more-effective high RPM range. This often means making the low-end lean (even nasty lean) in order to prevent the engine from loading up when in the race and you are forced to drop RPM too low for the gear you're in, then you must act fast on the shifter to get back 'on cam' and burn off that excess fuel before the other guys pulls out in front of you.

Don't overlook this: in the K0 carbs the float-height reference ledge is NOT the flat spot on the gasket surface of the carb. There is a tiny sliver of a ledge just inside the float gasket where those notches are, and THAT is the zero reference point for the float gage. And, the K0 carbs used brass floats with one side set at 25mm (the kickstand side) and the other at 26mm. In my roadracers we used 24mm and 25mm, respectively. I still use that in my K2 today. While it doesn't have a high-RPM cam, it also has no flat spots in throttle response.

This was a common tuning issue with roadrace 75 in the early 1970s, we dealt with it a lot. It was always easier to tune up to a hotter engine by doing one thing at a time, like install new, hotter pistons of some sort and tune to those, then install the carbs or cam and tune to that, etc. We were dealing then with lots of unknowns for sure, as Yoshimura was REAL tight-lipped about tuning to the parts they sold. You might remember seeing the famous Cycle Magazine issue with the CR "kit" for a 750 on the cover: I knew of 2 guys who borrowed $$$ to get those "kits" and we never saw them on the racetracks again. They never could get them tuned right.

Made some adjustments tonight and ready for timing adjustments and syncing tomorrow.

Every time I read the 750 book I find something I hadn't thought of or knew to look for, I found that the shaft the spark advance runs on was out of true by almost half a millimeter, I got it back to within .03mm - should help with the idle!

[Tracksnblades1]

Here's an 'easy' next step to try (if you have them): install a #115 mainjet. It sounds like it is just a little too rich below the cam's active RPM range, which will stop it from being able to ingest all the extra fuel that comes from being 'off-cam' until things come together.

I say 'easy' just because it is simpler to change the mainjets than to go back into the slides and drop the needles another notch. If a leaner jet size helps, it may take both steps (drop another needle notch and go to #110 mainjet) to get it to 'start' on the low end of the cam. When the higher-RPM cams "come on" the engine goes from being severely over-carb'd and 'wet' to blowing out that excess gas, then burning the mix well. The trick is to prevent the low-RPM ranges from overloading the mixture before you can get to the more-effective high RPM range. This often means making the low-end lean (even nasty lean) in order to prevent the engine from loading up when in the race and you are forced to drop RPM too low for the gear you're in, then you must act fast on the shifter to get back 'on cam' and burn off that excess fuel before the other guys pulls out in front of you.

Don't overlook this: in the K0 carbs the float-height reference ledge is NOT the flat spot on the gasket surface of the carb. There is a tiny sliver of a ledge just inside the float gasket where those notches are, and THAT is the zero reference point for the float gage. And, the K0 carbs used brass floats with one side set at 25mm (the kickstand side) and the other at 26mm. In my roadracers we used 24mm and 25mm, respectively. I still use that in my K2 today. While it doesn't have a high-RPM cam, it also has no flat spots in throttle response.

This was a common tuning issue with roadrace 75 in the early 1970s, we dealt with it a lot. It was always easier to tune up to a hotter engine by doing one thing at a time, like install new, hotter pistons of some sort and tune to those, then install the carbs or cam and tune to that, etc. We were dealing then with lots of unknowns for sure, as Yoshimura was REAL tight-lipped about tuning to the parts they sold. You might remember seeing the famous Cycle Magazine issue with the CR "kit" for a 750 on the cover: I knew of 2 guys who borrowed $$$ to get those "kits" and we never saw them on the racetracks again. They never could get them tuned right.

Made some adjustments tonight and ready for timing adjustments and syncing tomorrow.

Every time I read the 750 book I find something I hadn't thought of or knew to look for, I found that the shaft the spark advance runs on was out of true by almost half a millimeter, I got it back to within .03mm - should help with the idle!

I don’t have the 750 book, only Honda’s. But I’m pretty sure your bent shaft will have more of an effect on point adjustment than on your Charles Place electronic ignition.

Wondering out-loud…Did you ever google “Charles Place electronic ignition breaks up at high rpm” ?
I did. You may not be the only one that’s having trouble…

Do you still have a complete point plate to swap back to..?

[Finnigan]

Here's an 'easy' next step to try (if you have them): install a #115 mainjet. It sounds like it is just a little too rich below the cam's active RPM range, which will stop it from being able to ingest all the extra fuel that comes from being 'off-cam' until things come together.

I say 'easy' just because it is simpler to change the mainjets than to go back into the slides and drop the needles another notch. If a leaner jet size helps, it may take both steps (drop another needle notch and go to #110 mainjet) to get it to 'start' on the low end of the cam. When the higher-RPM cams "come on" the engine goes from being severely over-carb'd and 'wet' to blowing out that excess gas, then burning the mix well. The trick is to prevent the low-RPM ranges from overloading the mixture before you can get to the more-effective high RPM range. This often means making the low-end lean (even nasty lean) in order to prevent the engine from loading up when in the race and you are forced to drop RPM too low for the gear you're in, then you must act fast on the shifter to get back 'on cam' and burn off that excess fuel before the other guys pulls out in front of you.

Don't overlook this: in the K0 carbs the float-height reference ledge is NOT the flat spot on the gasket surface of the carb. There is a tiny sliver of a ledge just inside the float gasket where those notches are, and THAT is the zero reference point for the float gage. And, the K0 carbs used brass floats with one side set at 25mm (the kickstand side) and the other at 26mm. In my roadracers we used 24mm and 25mm, respectively. I still use that in my K2 today. While it doesn't have a high-RPM cam, it also has no flat spots in throttle response.

This was a common tuning issue with roadrace 75 in the early 1970s, we dealt with it a lot. It was always easier to tune up to a hotter engine by doing one thing at a time, like install new, hotter pistons of some sort and tune to those, then install the carbs or cam and tune to that, etc. We were dealing then with lots of unknowns for sure, as Yoshimura was REAL tight-lipped about tuning to the parts they sold. You might remember seeing the famous Cycle Magazine issue with the CR "kit" for a 750 on the cover: I knew of 2 guys who borrowed $$$ to get those "kits" and we never saw them on the racetracks again. They never could get them tuned right.

Made some adjustments tonight and ready for timing adjustments and syncing tomorrow.

Every time I read the 750 book I find something I hadn't thought of or knew to look for, I found that the shaft the spark advance runs on was out of true by almost half a millimeter, I got it back to within .03mm - should help with the idle!

I don’t have the 750 book, only Honda’s. But I’m pretty sure your bent shaft will have more of an effect on point adjustment than on your Charles Place electronic ignition.

Wondering out-loud…Did you ever google “Charles Place electronic ignition breaks up at high rpm” ?
I did. You may not be the only one that’s having trouble…

Do you still have a complete point plate to swap back to..?

Yes I still have the original points, I heard there could be issues with any type of electronic ignition - to mitigate I swapped out the coils and the plugs to get the best chance I could.  Not a bad idea to try the old points if things don't work out.  Thanks for the info!