Topic: 1978 CB750A Turbo Build

[BenelliSEI]

My comment got buried in the original post. Those head nuts only 1/2 engaged look dicy to me. Why not eliminate the washer and get full bite?

[LiveeviL81]

Thoughts?
Kaleb

  I've always liked that color.  I've used it on several engines I've painted.  Depending on how much you want to do, it would look really nice if you removed one screw at a time from the covers and cleaned the head and re-inserted it, move on to the next one, and so on.  An hour or two of cleaning parts like those and the valve covers and maybe the Honda on the alternator cover would really make it pop.
[/quote]

I purchased a kit for converting all of the cover bolts with JIS screws to hex head stainless, It looks great! Though it makes me want to do that with the case half bolts as well, or at least polishing their heads. But I worry about untorquing some of the important bolts, even if its one at a time. My big worry is this paint has not seemed to be very good at adhering so far, granted it hasn't been baked yet, but some of the bolts have had their paint flake, and a few small scratches have appeared. I even had some paint removed when using painter tape!

Kaleb

[LiveeviL81]

Was able to get the engine fully assembled and painted this last weekend. Left the ARP Bolts and Nuts stay torqued for a week before loosening and retorquing them. I noticed that some of them have a lot less thread engagement than others, All studs were torqued to 15ftlbs and the nuts were torqued to 22ftlbs. Running a Cometic Head gasket and normal base gasket. I am using the original steel washers with the ARP nuts, and notices the APR nuts are taller. I think it'll be okay, but it was a little worrying.

Kaleb

Why not take out the washer? It serves not purpose and you’ll have full engagement.

I had thought the washers were used so the steel nut wouldn't "bite" into the aluminum casting when torquing the head down, since the stock nuts and the ARP nuts both had the same diameter flange.

[MRieck]

Was able to get the engine fully assembled and painted this last weekend. Left the ARP Bolts and Nuts stay torqued for a week before loosening and retorquing them. I noticed that some of them have a lot less thread engagement than others, All studs were torqued to 15ftlbs and the nuts were torqued to 22ftlbs. Running a Cometic Head gasket and normal base gasket. I am using the original steel washers with the ARP nuts, and notices the APR nuts are taller. I think it'll be okay, but it was a little worrying.

Kaleb

Why not take out the washer? It serves not purpose and you’ll have full engagement.

I had thought the washers were used so the steel nut wouldn't "bite" into the aluminum casting when torquing the head down, since the stock nuts and the ARP nuts both had the same diameter flange.

I'd eliminate the cylinder stud washers too....you want to grab as much thread as possible especially with the puny 8mm head studs. Lube the threads and bottom of the nut with light engine oil

[newday777]

 My big worry is this paint has not seemed to be very good at adhering so far, granted it hasn't been baked yet, but some of the bolts have had their paint flake, and a few small scratches have appeared. I even had some paint removed when using painter tape!

Kaleb

Kaleb
Did you wash the motor, then sand the old paint, then wash again, final step is to wipe down well with denatured alcohol and let dry before painting?

[LiveeviL81]

 My big worry is this paint has not seemed to be very good at adhering so far, granted it hasn't been baked yet, but some of the bolts have had their paint flake, and a few small scratches have appeared. I even had some paint removed when using painter tape!

Kaleb

Kaleb
Did you wash the motor, then sand the old paint, then wash again, final step is to wipe down well with denatured alcohol and let dry before painting?

I had the motor media blasted, cleaned with brakeclean and rags until no more dirt appears, then after assembly before painting I again used brake clean and a rag to wipe down.

[LiveeviL81]

Was able to get the engine fully assembled and painted this last weekend. Left the ARP Bolts and Nuts stay torqued for a week before loosening and retorquing them. I noticed that some of them have a lot less thread engagement than others, All studs were torqued to 15ftlbs and the nuts were torqued to 22ftlbs. Running a Cometic Head gasket and normal base gasket. I am using the original steel washers with the ARP nuts, and notices the APR nuts are taller. I think it'll be okay, but it was a little worrying.

Kaleb

Why not take out the washer? It serves not purpose and you’ll have full engagement.

I had thought the washers were used so the steel nut wouldn't "bite" into the aluminum casting when torquing the head down, since the stock nuts and the ARP nuts both had the same diameter flange.

I'd eliminate the cylinder stud washers too....you want to grab as much thread as possible especially with the puny 8mm head studs. Lube the threads and bottom of the nut with light engine oil

I appreciate your guy's insight. The engine was actually installed in the frame last night... I know the Hondamatic's are able to have the valve cover removed in frame, but I wonder if I can take all of the valvetrain out enough to get to those head nuts in order to remove the washers, or if I'll have to get some buddies to help remove the engine as well... Shoot you guys got me worried.

[BenelliSEI]

Those nuts already have a flat on the btm. face. Take out those washers.

[LiveeviL81]

Ended up taking the top end apart and helicoiling the cam tower threads while it was all apart as well. Scratched the paint a bit getting the valve cover out, but totally possible to do the entire top end in frame. Only real issue was torqueing the cam sprocket bolts with the torque wrench. Took the washers off and covered the threads with oil, tightened to torque spec, then checked again a day later. Got the whole engine back together so now I'll be focusing on getting the exhaust, turbo, and intake mounted up along with the wiring and plumbing for fuel.

I purchased a larger petcock and flange, converting to a Harley 22mm with a 5/16" petcock ID. Found out the stock petcock flange was brazed, so I will also try to TIG braze the fuel tank when I put the Harley bung on it. Any tips for cleaning out light surface rust and draining the tank? I had a ton of issues trying to get all the old gas out where no matter how I shook it, something still sloshed.

I also got ahold of some reverse comstar wheels from a buddy at work, and I was happy to find out they are easy swaps. Bolted right up!

If I push hard, I should have first start this weekend.

[LiveeviL81]

Busy weekend! Didn't make first start, mainly due to the pulled out exhaust stud I wrote about in another thread. Ended up getting a lot of other things done that I was pushing off till "After first start". Fresh rubbers are getting put on the reverse comstar wheels, brazed in the larger petcock bung to use 22mm Harley petcocks with 5/16" I.D. Got the new exhaust stud in, and manifold and preturbo pipe installed.

Thinking about painting my turbine housing for the turbo with some "2000F" paint I have, it has some flash rust on it from sitting, and I think it would look pretty good!

Kaleb

[LiveeviL81]

Is there a good method for priming the oil pump on the CB750A's? Since its a different oil pump system I tried to follow the documentation before assembly, but it made a mess! Was thinking about using a syringe to push oil up in the oil filter area to try to put a "head" on the pump.

[LiveeviL81]

First Start!
Was able to get first start Sunday Night, she popped right off with a little coaxing from the accelerator pump and choke. I am surprised it runs as well as it does with no tuning on the carb yet since the big bore. Plan to install the Lambda Sensor here after a few more minutes of idling and reving to clean out the assembly lube from the cylinders. Everything sounds good to me, a little excess cam chain noise at idle, but I followed a method of HondaMan by releasing the tensioner hold bolt while Cyl 4 Intake Rocker was starting to head down, light pressure in the back to make sure it adjusted, then tightened back up. Will probably need to do it again as the brand new cam chain stretches a bit.

https://www.youtube.com/watch?v=H97wL0DKJIc

Now to the bad part. You can see some oil leak out of the torque converter case cap, tried to RTV it last night and see if I could get a drive in but its still leaking. So I'll wait for the gasket to come in. I also idled it for a few minutes today and saw another leak I'm asssuming from the rubber grommet for the 3 stator wires, So I'll be digging in there as well to RTV.

I also was not able to get the engine to provide torque to the wheel in 1st gear, revving up wouldn't spin the rear wheel anymore than the bike in neutral on the stand. 2nd gear seems to work fine, I did a bit of brake torquing, but didn't want to drive it with the leak on the torque converter cap. Going to double check oil level since the troubleshooting manual says it could be low line pressure in 1. Really hoping no major engine removal issue.

I have attached the a photo of my shifting mechanism, which lets me correctly shift through all the gears, and the kickstand return works as well, but maybe I missed something obvious. More photos to come if the oil level isn't low and I need to open that cover.

Thanks!
Kaleb

[LiveeviL81]

Ended up having to pull the engine from the frame, which sucked seeing all my work go back on the shelves. Engine paint didn't get too badly scratched on the way out though. Took the time with the engine out to try to fabricate my own engine stand/rotisserie. Sadly I didn't realize engine stands are canted back about 5 degrees and doesn't allow the two bore axis to be aligned and rotate the engine. Made mounts to hold the engine upside down for me, but I won't be able to rotate it. Planning to cut some sections out and align the bores in the future for other projects.

Figured out pretty quickly the issue with the engine after splitting the case-halves. The bearing race/oil feed piece for the low clutch rotated in the case and blocked the oil feed to the low clutch. Easy fix, but I am surprised it happened since I can feel the pin and hole align so hopefully that doesn't happen again. Time to clean all the RTV and get ready put the engine back together.

Found another issue. The main oil gallery feed from the filter through the case halves seems to have cracked on the o-ring seal, and while I was seeing 60+psi oil pressure in the main gallery when the engine was running, it worries me enough to try to fix it before putting the case halves back together. Currently discussing in the main SOHC forum about solutions but I think I've settled on attempting to weld it back up with the engine still assembled, and seal it off with some RTV/Hondabond along the steel insert and monitor oil gallery pressure more often in the future.

[LiveeviL81]

Wow, I really forgot to update this build. Let's do a wall of text to catch everyone up since I got some interest on a previous post. 1978 CB750A I got for $500 non-running. Mostly original outside of the sidecovers. After riding the bike in the summer of 2023 I got the idea to do a draw through carb turbo setup because of the factory low compression 8.8:1 I think? I ran the setup with an HSR42 carb with some wildly wrong jets and a GT15 for the first year, eventually blew every cylinders head gasket, but the bike was still healthy and I enjoy the automatic of it for cruising around so I got some more mods for the engine during the rebuild since I wanted to keep it as my project bike. 2024 the engine got 838 big bore, flat head pistons (8.5:1 CR) along with rods, bore alignment from cycleX and a cryo cam chain, a K cam and a copper base gasket to put me closer to 7.7:1 CR. I also added an oil cooler, dropped the turbo to a TD025 (30ish% smaller) to build boost before 5krpm and did some other small mods to the bike. Ran most of 2024 without issue but could never get the carb quite right. Ended up breaking my wrist in a different motorcycle accident and worked on a Suzuki GT550 for the 2024 Winter. 2025 I focused hard on getting the carb adjusted right and through the miraculous mistake of forgetting to install a main jet when swapping them, I learned the bike didn't even really want a jet. Ended up drilling a straight bore on a spare jet and would run 10.5 cruising AFR and 13.5 WOT... but I could never fix the low rpm throttle response, or how rich it was when cruising around town. Ended up going to a local dyno and getting only 70HP at 7,000 RPM. This confirmed that while the TD025 would build full boost around 4000 RPM, It was stalling out the bike by 5500, and torque was falling like a rock. This sets us up to this winter project.

I am doing an EFI swap, individual port injection with the original larger GT15. I may get it un-carbon seal swapped at some point, but for now it'll do. Using a Microsquirt MS3Pro Mini, some 48lb/hr injectors I got from a coworker for a Mustang, Cam and crank sensors and a throttle body from a Ducati Monster 1200 (about 45mm through bore, which is just slightly bigger than the compressor outlet of the turbo). I am in the process of getting new valve springs for the bike, and maybe a cam, Opinions appreciated. I upgraded to Ricks's rectifier on the stock stator using a lithium battery to try to get all the power out of as I can, along with swapping all the lights on the bike to LED replacements. My goal is to still be able to hit max boost around 4500-5000 RPM, have much better drivability when doing a WOT from a low rpm, and touch the 100 hp mark (15lbs of boost) I would love to talk to the guys who worked on SAMAUTO, cause I am already seeing like 75% lockup on the torque converter at only 56 ftlbs of torque, not to mention with such a low stall speed, it makes it hard for me to get into my powerband from a dig I remember reading of a post where the TC was sent to a company in like Alabama to be rewelded, but it was upwards of a few grand?

Now:
I have made a mounting plate for all the electronics, kinda in the same spot as original, but everything is shifted around. Added a fuel pump, the ECU, a Data logger, an EGT box, and will be adding a tachometer from KOSO (TNT-01) for the retro styling, and a small screen that uses CAN to display the ECU parameters while riding. I designed and had my intake 3D printed out of aluminum, and welded and modified the Ducati throttlebody, along with using SendCutSend for the crank wheel and some mounts. Currently trying to figure out where to put the cam pickup, since my original idea of shoving the sensor through the cable tach hole won't work, along with starting the fully custom wire harness here soon. Photo dump!

Future:
This is still my project bike, and unless I absolutely love the turbo and everything about it, it will probably change again at some point. I have been thinking about a supercharger for years, but I love the look of my pipes, and no longer have originals to put back on it. I also won't ever be in the search of record breaking power. This bike is scary enough around 100mph, I don't need to go much faster, or break it more than once every 2 years. But that really brings the question of, how many miles can one of these bikes do if they are putting out 100hp or double factory! before rebuilds? My biggest worry is a weak link on some un-obtainable part or a totaled engine.

[LiveeviL81]

Dyno Graph and my hand calcs for the stall speed of the turbo's I was optioning out. Ended up ballparking a volumetric efficiency of like .75 and calculated a volumetric flow rate to determine the speeds of the compressors and the boost produced, along with power numbers. Pretty much sure that the TD025 was wildly past the stall line of the turbo graph, and the GT15 is about perfect, if not a tad large for my expectations.

From the dyno graph you can see how I told the guy to go to WOT from about 4k RPM, or else it would normally fall on its face,  and it was making boost almost immediately, but very soon started choking out. There has been no headwork done to this, stock valves and springs, so I can't say I am too surprised, hoping the bigger turbo will make it happen. Headwork and valves are expensive!

Here is a link to the dyno pull, listen to that turbo scream! He was in second gear and doing almost 130mph wheel speed!

[LiveeviL81]

More Photo Dumps, crashed my first post when I added a bunch.

[LiveeviL81]

More Photos of the 3D printed aluminum intake and fuel rail/throttle body combo. Thing packages perfectly and should also allow for the the MAP, air temp and boost reference sensors. Only cost me a weekend of design and $300 from China.

[LiveeviL81]

Last one, More posts of the plenum and setup in the bike! It's been tough figuring out where to package an ECU, data logger, EGT box, and fuel pump where the stock electronics used to go!

[newday777]

Dyno Graph and my hand calcs for the stall speed of the turbo's I was optioning out. Ended up ballparking a volumetric efficiency of like .75 and calculated a volumetric flow rate to determine the speeds of the compressors and the boost produced, along with power numbers. Pretty much sure that the TD025 was wildly past the stall line of the turbo graph, and the GT15 is about perfect, if not a tad large for my expectations.

From the dyno graph you can see how I told the guy to go to WOT from about 4k RPM, or else it would normally fall on its face,  and it was making boost almost immediately, but very soon started choking out. There has been no headwork done to this, stock valves and springs, so I can't say I am too surprised, hoping the bigger turbo will make it happen. Headwork and valves are expensive!

Here is a link to the dyno pull, listen to that turbo scream! He was in second gear and doing almost 130mph wheel speed!

You didn't upgrade your valve springs, retainers and keepers yet? Those are a fairly cheap upgrade and gain (even if you don't go bigger valves yet). I would have thought that Kenny would have advised you to do that already.
Yes headwork and valves are expensive today. You would be blow away at the increase of costs compared to what it cost us in the mid 70s.

[MauiK3]

Amazing project, who knew the transmission could take that!!
Great work figuring it all out.

[LiveeviL81]

You didn't upgrade your valve springs, retainers and keepers yet? Those are a fairly cheap upgrade and gain (even if you don't go bigger valves yet). I would have thought that Kenny would have advised you to do that already.
Yes headwork and valves are expensive today. You would be blow away at the increase of costs compared to what it cost us in the mid 70s.

Yea, Kenny and I had a few conversations about the bike, and with the original budget I only had him do bottom end stuff, and had a local guy I trust deck the head. I realize now that I am pretty lucky to still have the stock springs and valves hanging on, just tough to not get scope creep when buying new valves, to not buy bigger valves and new seats and guides.
I always struggle to remember what head my bike uses, because I think that even though it is a 78, it uses all the parts of the <78' K model for the head. Are oversized valves worth it? Is there anyway to do the valve springs without removing the head to get the spring compressor tool under the valves?

[scottly]

What Mustang used 48lb/hr injectors? The late '80s 5.0 used 19lb/hr injectors, which I'm using on a 413cu.in. Chrysler with a home brew EFI system. The rule of thumb for sizing injectors is 1/2lb/hr per HP, so 4 48lb injectors will support 384 HP. If the injectors are too small, like in my case, they "run of of gas" once they get to 100% duty cycle, and if too large, they are barely opening and hard to control accurately.

There are two ways to change valve springs with the head installed that I know of:
1. Fill the cylinder with compressed air to hold the valves closed. Be aware that the crank will turn if the piston is between TDC and BDC.
2. Stuff a small rope into the chamber through the spark plug hole and bring the piston up until the wad of rope presses up against the valves.

[LiveeviL81]

I'll have to ask him next time I see him, GT500 or GT350 is my guess, cause I got 8 of them, so I don't think it was for a I4 or V6. The part number doesn't show up on the internet, but they are very close in part number to these https://www.ebay.com/itm/293687554519

I sent them to get flowed and they came back as 48lbs, and all within 1% of each other. They are a bit larger than I wanted, I was originally targeting 42lb, which should be close to maxing out if I switch to E-85, but free.99 is hard to beat.

I like the rope idea. What do you use to compress the springs then though? My compressor requires the brace against the bottom of the valve, and I've seen some LS ones that used the bolts for the followers. Not sure if I trust the M6 bolts for that!

[scottly]

You'll have to get creative for a spring compressor.

[HondaMan]

I still have one of the old "top use only" valve spring compressors, but I found that with the thicker Honda valve springs in the F2/3 heads there isn't enough room to R&R the keepers out of the F2/3 style deep-dished top spring retainer. When this spring compressor was used on those springs, even wiggling the top plate around wouldn't give enough room.

It does work on the earlier K0-K6 springs and up to the F1, and on the K7/8, just not on the F2/3 type with their weak, deep, stamped-steel top retainers.

As I don't build racing engines anymore, I usually swap the F2/3 springs out to F0/1-K4-6 tapered types instead when building those heads, then use Kenny's marvelous top retainers for the springs. This makes for an excellent, wear-reduced Interstate touring version of the F2/3 bikes' engine with today's speed limits, a larger (as in, metric) rear tire and today's Regular grade gas, whether using Honda's cam or the Megacycle 125-00. For an earlier K engine that's going to the Interstates coast-to-coast alongside Goldwings, etc., the same Megacycle cam makes for a nice powerband in the 80+ MPH, surprising the GoldWingers it meets.

I've also built one F2 head with some K3 valve springs, as the owner also swapped to the earlier cam. It still holds its own, coming up on 30k miles this year on US Interstates across the southern USA during the summer months. He wrote that it easily clears the ton, fully loaded.