Topic: 1977 cb750f2 (915cc)

[bwaller]

I'm curious too about which pipe wins the shoot-out. Thanks in advance for testing too, truth be known, seat of the pants can mislead...

I don't mean this to sound bad, but in the early seventies when there were so many pipes available I don't remember Kerker being thought of as top of the heap. Maybe I'll eat my words & that's great. Pick a pipe that you're happy with and if you can test like you will, perfect. Getting the jetting spot on completes the package and you'll have that.

Recently someone (MikeR maybe) mentioned there would be a magazine test of available CB750 pipes. I'm always interested in this stuff. I recall "in the day"both Yosh and Action Fours having pipes that delivered more power than stock, so many others actually performed poorer than OEM.

I looked forever for a Yoshimura and ended up modifying a Kerker for my racebike. I think I'm in a good place with it too.

[tweakin]

bwaller, I agree that "seat of the pants" is just that, only the dyno time can tell.  If the Yamiya was a true Yosh pipe it might be different.  One thing I have learned in all my builds is that a ton goes into pipe performance, just because a pipe looks similar you can have a HP swing of over 10HP with pipes made by different manufactures (because of build, bends, diameter and design).  No experience on cb's yet but I have done a ton of dyno testing on Harleys where this is the case.  I to look forward to the cb exhaust shoot out.  Limited pipe choices for cb's is a bit disappointing but I have to work with what is available.

Hope to have a couple runs with the Kerker soon and then I will know for sure, but the bike seems to run stronger and never lifted the front wheel as easy like it does now with the Kerker.  I only wish the dyno I use had an accurate torque reading and not just HP.

[KJ790]

I'm curious to see how the Kerker looks on the dyno as well. As for the torque curve, that is easy to calculate from the HP curve since HP=(torque*RPM)/5252.

[Doctor_D]

I'm curious to see how the Kerker looks on the dyno as well. As for the torque curve, that is easy to calculate from the HP curve since HP=(torque*RPM)/5252.

That's not the issue.  It's that the coil trigger lead on Tige's bike is so tightly wrapped into his harness, that we can't get solid rpm's on the run.  Even if we could, for some unknown reason the coil wire sensor is hit and miss with Dyna 2000's. For example, it worked perfectly on Branden's CB750, but has only worked about half the time on my bike.

The other option we have, the inductive plug-wire sensor doesn't seem to like good plug wires.  This results in our rpm-based readings skipping around a bit.

Oddly, old wires and points = perfect rpm logging.  I have a few ideas I'm gong to try this week to see if we can improve on the situation.  In any case, Tower will be making some updates to the dyno this summer should solve some of the little headaches we have with it.

[Jerry Rxman Griffin aka MuthaF'er]

Tweak,

Back to you. Not much progress today    At this point it slows considerably due to all the unknowns. Thanks to you, Branden, Steve, and others for making it easier!

I did manage to get the Dyna2000 timed at the 37.5^o mark for starters. Wasn't too bad an ordeal ONCE YOU UNDERSTAND WHAT THA HELL THEY MEAN  Look back, it's a piece of cake. Pre-set it for curve 4 based on Dyna's recommendation for SOHC4 without high compression. 10.5 is higher than 9.0 or 9.2 but it's not 12.5 either. Thoughts on this? Rev limit at 10K for now. Dyno will show me the best power spot.

Think I got that rear master mount finally installed correctly too. As you know going by the parts books exploded views is usually good enough unless it's exploded too much. lol.

Of course I just had to jump on the other bike and search for 1/8" hose connectors. lmao. You'd think a guy'd learn from others but your link doesn't work now. Hey, at least I got a little riding in. Mr Brown will be bringing them to me in a few days. At this point I ask - where did you find the rubber caps? I took a look at the throttle cables with the CR carbs in place. Damn good thing I gotta take them off for the install of those connectors. I did learn on this. I will install the cables on the carbs BEFORE installing the carbs this next time. The return will go on but the pull will not. Stock cables look like a really tight and short fit and may not fit under the front tank mounts as they did with the stock carbs. We'll see.

Kerker - looks like my baffle that Dynoman sent is 2", not your 2.5". Looks like your's is recessed more than my 2". Mine has a helluvalot bigger opening than that nice toned quiet original. It'll have to do for the price for just a friggen baffle! Hey, what's a half inch! The ex never complained. Wait, she ALWAYS complained so 1/2" didn't matter. I know our exhausts and mounts are different but take a look at mine. No way was I going to strap that big ass hanger from the top shock mount on my bike. I took the hanger and chopped it up and shaped the cuts and made some bends. It's attached behind my rear foot peg. Don't ask me why I did the 2 piece?? That's been a number of years back. It stays in place very well.





   

[tweakin]

Jerry, here are the caps I am using for the synch ports added to the carb boots.
http://www.bikebandit.com/search?q=1046965

And here are the retaining clips
http://www.bikebandit.com/search?q=1030534

Yes, I always install the throttle cables with the carbs loose, then install the carbs.  Much easier IMO.  I had my timing set at 37 degrees but scaled it back to around 32 degrees and am happy with the performance.  Next time I hit the dyno I might play around with it a bit.

I like your exhaust hanger, but I don't think it will work for my build since I went with the custom rear sets.  I am thinking of having a custom billet hanger made up.

Sounds like you are getting close, wish I was closer so I could bring over a few beers for the initial start up.

[tweakin]

Thanks for the explanation David.  I think the dyno at Twineline works fine, especially with you and Tower helping with the tune.  Not super worried about the torque figure but suspect it might go up with the exhaust change.

I'm curious to see how the Kerker looks on the dyno as well. As for the torque curve, that is easy to calculate from the HP curve since HP=(torque*RPM)/5252.

That's not the issue.  It's that the coil trigger lead on Tige's bike is so tightly wrapped into his harness, that we can't get solid rpm's on the run.  Even if we could, for some unknown reason the coil wire sensor is hit and miss with Dyna 2000's. For example, it worked perfectly on Branden's CB750, but has only worked about half the time on my bike.

The other option we have, the inductive plug-wire sensor doesn't seem to like good plug wires.  This results in our rpm-based readings skipping around a bit.

Oddly, old wires and points = perfect rpm logging.  I have a few ideas I'm gong to try this week to see if we can improve on the situation.  In any case, Tower will be making some updates to the dyno this summer should solve some of the little headaches we have with it.

[tweakin]

I decided to take out both Fs this afternoon and do a bit of comparison on the two with the different engines.  The bikes feel completely different for obvious reasons.  The 915 just wants to go and go, MUCH faster acceleration and power and the more aggressive riding position makes me feel like I am on a racer.  The 836 is an amazing all around bike.  Super comfortable and plenty of power when needed.


click on pics to make bigger










I have to say I F'n loves these bike and all cb's, its like they touch my soul...

[Retro Rocket]

Beautiful shots Tige, ignore my request for more info in the other thread....its all here....

I know you love both bikes just the way they are but if it were me i would slip the "big" engine into the red bike, i just love that bike, its overall balance is just perfect....

[brandEn]

Great pics. I think Retro is on to something.... 915 in the monkey trainer. Lego CB's.... nice.

[Jerry Rxman Griffin aka MuthaF'er]

F'n A bro. We need one more F added to a picture now. A burnt orange colored F would make a good contrast 

[andy750]

Outstanding Tige! These builds are such an inspiration! Agree with Jerry you need a sunrise orange one in there

The builds are great, bikes look good and...the run very very well. You hit all three! And you even make comstars look good! Agree the red bike is really something. The tail piece is very cool. If you ever sell the red bike put me first in line please!

thanks for sharing
Andy

[RAFster122s]

Tige, there is a third option, build another 915 motor. Maybe sell the 836 motor...
Or build another bike, putting the 836 in that new bike, with that bike's motor being built up as a 915 to swap into your burgandy bike.

Both bikes are great...

My 78 CX500 has those same Comstars with the black plastic caps at the rim. It was a one year only thing for the CX as the 79 had the wheels without the black plastic covers.
Then the next year they did the ugly reverse Comstars. Those look OK if you polish the rims and the flat edges of the stamped spokes. Seen a few sets of Comstars powder coated or nicely painted in colors like bronze, gold, and a set in yellow with the flat edge painted black rather than being polished. Yellow bikes need a decent amount of polished aluminum, steel, or chrome on the bike to look best,
I've also seen the reverse Comstar spokes stripped and polished to a high sheen before protecting with a urethane clear coat.Those took a lot of time to do and looked really good.

[tweakin]

Thanks for the complements guys, it is funny how much you guys like my quick 1978f3 build. 

I actually really like it too and it will probably get most of the saddle time but it will never get the 915 engine or any other 915 engine.    I built the 78 to be a "sport touring" bike that is comfortable and good for longer rides.  The 915 engine would totally defeat the purpose of the build, I can only get 95 miles on a tank....  Also the 78 has many upgraded parts but not nearly as many as my 77 that was built to compliment the 915 engine.

My 836 in the 78 is no slouch.  DP JE 836 pistons, heavy duty studs and chains, all new bearings, APE F2 head with SS valves, Ti retainers and mild porting, and CR29 carbs.  It will go!

[tweakin]

F'n A bro. We need one more F added to a picture now. A burnt orange colored F would make a good contrast 

Jerry, It is easier to ride one bike 1400 miles vs me bringing two.   Any time you want to make the trip up this way give me a shout, you have a place to stay.  I love the burt orange... If I lived closer I would break into your garage and finish that build up for you.

[Jerry Rxman Griffin aka MuthaF'er]

For now I was talking about that "other" builder up your way. What was his name anyway?? BrandEn?! Mine is not running yet so I couldn't do it but we'll see what this next week brings as I'll have some play time. 

[tweakin]

Yes Jerry we will get some picks with brandEns bike in the mix.  I met brandEn when I was about half way into building the 915 and its great to have another builder into the same things so close.  The majority of the work we do on all our bikes is a joint effort.

For now I was talking about that "other" builder up your way. What was his name anyway?? BrandEn?! Mine is not running yet so I couldn't do it but we'll see what this next week brings as I'll have some play time. 

[FunJimmy]

Awesome pair of 750’s Tige.

Let’s consider taking them both on the Cascade Loop and leaving my anemic little 550 at home.
Four hot 750’s……that would be so keul. 
 

[Stev-o]

Two awesome bikes, you should be very, very proud. 

What's next? (othervthan riding) any plans for another build?

[brandEn]

Im in for a photo shoot. Two 78's and a 77. Sounds like a Johnny Cash song.

[Jerry Rxman Griffin aka MuthaF'er]

Only if "you walk the line" 

[tweakin]

Jim, I am tentativly planning on riding the 78 on the trip just because the 915 gas milage sucks.  You 550 is far from anemic.

Awesome pair of 750’s Tige.

Let’s consider taking them both on the Cascade Loop and leaving my anemic little 550 at home.
Four hot 750’s……that would be so keul. 
 

Thanks Stev-o.  The real plan is to just enjoy these bikes for awhile.  WHen I get the itch again; it would be cool to have a 76f added to the line or I have always wanted to do an "over the top" 400f.

Two awesome bikes, you should be very, very proud. 

What's next? (othervthan riding) any plans for another build?

[kap384@telus.net]

Jim, I am tentativly planning on riding the 78 on the trip just because the 915 gas milage sucks.  You 550 is far from anemic.

Thanks Stev-o.  The real plan is to just enjoy these bikes for awhile.  WHen I get the itch again; it would be cool to have a 76f added to the line or I have always wanted to do an "over the top" 400f.

Sweet.  you do the "over the top" 400f and I'll do the stocker 400f and we'll do our respective photoshoots with our '750's'!

[nvr2old]

What a beautiful pair of bikes, tweakin.  I can't pick a favorite.  It's like trying to pick a favorite child.  Both are so perfect in their own respective ways.  Your attention to detail is so spot on for each of them.  Can't wait to see brandEn's next to them.  Proud to've been a small part of them all.  Thanks, man.

Larry

[madScientist]

I am pretty new to the forums and read through this entire thread (twice). To say you have inspired me is an understatement!

I have 2 78 750f's. After reading through this I have an idea of what I'm going to do with the one i'm not restoring. Thanks for this great read and the inspiration!