Topic: Building a hot-rod engine: does the donor motor year matter?

[GroovieGhoulie]

I know how Honda 'detuned' the 750 over the years, as outlined by HondaMan and others on this forum.  It's no big secret.  If one wants to build a modified engine, with all the bells and whistles, does it matter if the donor engine is a mildly hotter "F" or early "K" engine, or the more sedate mid to later K engine?

I mean folks said that the cams got milder, the carbs got leaner and the intake and exhaust got more restrictive, but if one is going to massage nearly every part anyway, with big CRs, a hot cam, big-bore kit, port the heads, I mean REALLY take it to town, does the base motor really matter as long as it is solid and not damaged at all?

[bryanj]

If you are altering everything just look for good castings and tranny

[Geeto67]

I know how Honda 'detuned' the 750 over the years, as outlined by HondaMan and others on this forum.  It's no big secret.  If one wants to build a modified engine, with all the bells and whistles, does it matter if the donor engine is a mildly hotter "F" or early "K" engine, or the more sedate mid to later K engine?

I mean folks said that the cams got milder, the carbs got leaner and the intake and exhaust got more restrictive, but if one is going to massage nearly every part anyway, with big CRs, a hot cam, big-bore kit, port the heads, I mean REALLY take it to town, does the base motor really matter as long as it is solid and not damaged at all?

Most of the internal performance stuff out there is designed for the 1971- 1976 K stuff (big bore kits, valves, etc). Assuming they are compatible you are going to want the strongest bottom end you can find which would be a 78F coupled to K cylinders and head. I have heard that the A automatic cranks are stronger, but the 78F has the larger output shaft and bigger con rods. Actually if you are going larger than 836cc then you are going to need to sleve the cylinders anyway so the cylinder bank doesn't matter as long as the oil passages line up.

I'm sure there are others with way more expirence but this is a good start. I have known quite a few cb750 drag racers and the K engine seems to be the hot rod mill of choice. Even when running F bikes there is usually a big bore K mill in the frame.

[RSV12K]

I know how Honda 'detuned' the 750 over the years, as outlined by HondaMan and others on this forum.  It's no big secret.  If one wants to build a modified engine, with all the bells and whistles, does it matter if the donor engine is a mildly hotter "F" or early "K" engine, or the more sedate mid to later K engine?

I mean folks said that the cams got milder, the carbs got leaner and the intake and exhaust got more restrictive, but if one is going to massage nearly every part anyway, with big CRs, a hot cam, big-bore kit, port the heads, I mean REALLY take it to town, does the base motor really matter as long as it is solid and not damaged at all?

This is the first I've heard the 750 was detuned over the years, so for me it's a big secret. All along, I thought the 77 and 78 engines had slightly more HP than earlier versions; bigger-mouth carbs too.

[dusterdude]

might have,but they were tug boat sized bikes so they needed it.

[kghost]

Was a cam profile and advance thing. But your gonna change that anyways.

F models valves are problematic parts wise.

Heads are not compatible compression ratio wise between the K and F.

They did get leaner and more restricted as the years progressed.

[eldar]

Tug boats? This from a guy on harley!  Actually they really were not any heavier than the earlier Ks. Besides than, lean is mean as some have said here. The years did get detuned. From 71 - 74 the models were progressively detuned. K5 and 6 were about the level of the 73. 77 and 78 had some of the oomph brought back.

[seaweb11]

 

[eurban]

Well Eldar, everything I have read puts the late K (77/78) models about 40Lbs heavier than the 69s and the F2/F3s about 10lbs heavier than the K7/K8s.  However inspite the load of bricks in the pants these models turned in quicker (at least as tested in period magazine articles) quarter mile times than the sandcasts.  Acelerator pump carbs, higher compression ratio for the (probabably a bit better flow in the for the head too) K, bigger valves and better flow for the F all made for better performance.  Sandcast nostalgia aside, these late engines probably made the most stock HP.  The F2/F3s do have valve train reliabiltiy issues.  . . . Certainly if you are going to add a whole bunch of performance goodies then starting with a solid engine of just about any year is a wise choice.  Sandcast engines are probably too valuable to be messed with. The 77/78 K engines are probably the most plentiful, and have many reliability oriented upgrades so I would think that they would be a wise choice for a performance build.  This has acutally been previously suggested on this board and the fellas at Cycle Exchange seem to think so too.

[Jonesy]

I read that the reason why the 750A crank was popular was because the primary drive sprocket on it was cut for use with a BorgWarner Hy-Vo drive chain, which was much stronger than the double-row roller primary chains like the K and F models. I think you had to remove the rivets from the A torque converter to free the other primary sprocket and then somehow rivet it or weld it onto a modified K clutch outer. Aside from the heavy mods required for the clutch, the parts essentially drop-in.

[MRieck]

I read that the reason why the 750A crank was popular was because the primary drive sprocket on it was cut for use with a BorgWarner Hy-Vo drive chain, which was much stronger than the double-row roller primary chains like the K and F models. I think you had to remove the rivets from the A torque converter to free the other primary sprocket and then somehow rivet it or weld it onto a modified K clutch outer. Aside from the heavy mods required for the clutch, the parts essentially drop-in.

Pretty much it. I'd add use a 1974K- on or F head. a heavier casting with the oil jets. If you are going over 836 you need an earlier K cylinder (no oil return cut outs for dowels and sealing rings).

[seaweb11]

Well Eldar, everything I have read puts the late K (77/78) models about 40Lbs heavier than the 69s and the F2/F3s about 10lbs heavier than the K7/K8s. However inspite the load of bricks in the pants these models turned in quicker (at least as tested in period magazine articles) quarter mile times than the sandcasts. Acelerator pump carbs, higher compression ratio for the (probabably a bit better flow in the for the head too) K, bigger valves and better flow for the F all made for better performance. Sandcast nostalgia aside, these late engines probably made the most stock HP. The F2/F3s do have valve train reliabiltiy issues. . . . Certainly if you are going to add a whole bunch of performance goodies then starting with a solid engine of just about any year is a wise choice. Sandcast engines are probably too valuable to be messed with. The 77/78 K engines are probably the most plentiful, and have many reliability oriented upgrades so I would think that they would be a wise choice for a performance build. This has acutally been previously suggested on this board and the fellas at Cycle Exchange seem to think so too.

Ya Duster....what he said

[my78k]

"This is the first I've heard the 750 was detuned over the years, so for me it's a big secret. All along, I thought the 77 and 78 engines had slightly more HP than earlier versions bigger-mouth carbs too."

By the way...as a fellow K8'er I have to clarify something said a little earlier...it's not the carbs that were bigger mouthed it's just us owners!!!  Right Eldar???        

[seaweb11]

I read only what I want to hear.

"Well Eldar, everything I have read puts the late K (77/78) models about 40Lbs heavier than the 69s and the F2/F3s about 10lbs heavier than the K7/K8s.  However inspite the load of bricks in the pants these models turned in quicker (at least as tested in period magazine articles) quarter mile times than the sandcasts."

[RRRToolSolutions]

I took a long nap today so I can ramble here a bit. 

Seriously, I've just done one this summer and I learned the "hard way" what I'll do next time. This advice is free -

1) The Wiseco 836cc kit is kick-a$$. Forged pistons, new pins, special head hasket for the big bore and replacement rings only $75 a set - Sweet.

2) Megacams makes a 375 lift cam with 256 degrees of duration at .40 made just for the 836 # 12565 - Sweet.

3) Absolutely have your heads ported. Mike does pretty work, but whomever does it - have it done.

4) Brass valve guides - Dynoman has them - Sweet.

5) Stainless valves - Dynoman has them - Sweet.

6) Titanium retainers - again, Dynoman has them.

7) Performance dual valve springs - Dynoman has these too.

New, stock clutch fibers and steels (ain't cheap, but produce the smoothest shifts) and with Barnett's springs will hold like a Mack truck.

9) Rods - Good Rods and bolts. This is where I messed up. Horsepower is a mathematical result derived from pounds/feet torque x rpm divided by 5252. With all this 836cc, 10.25:1 compression, ported heads, titanium retainers, performance springs and other goodies, there is still a finite amount of torque available. Rpm is the only way to "raise the bar" - horsepower bar that is. With stock rods and those newly installed Wiseco forged pistons at 836cc - the stock K0~K6 is only safe to 8,500. This is the 1 single thing I wish I'd done - added the Carillo rods and bolts. Once you get them built like described above, they will turn. With performance rods I'd spin this thing to 11,000 if she'd pull it.

10) Dyna 2000 with a built-in rev limiter is going to be very desirable. In less than 200 miles I found myself missing a few shifts and blistering the clutch, so mine saw the 10 + mark more than it should have. Problem solved - Dyna was installed this week.

Ok, I've rambled. But seriously, if you want to impress your friends, out run any stock H2 Kawasaki out there, and smile at every light - this old CB is a great, cheap way to do it. It's the perfect sleeper. Mods are going to run $3000 is my estimate plus exhaust, dyna, and filters. For $1,100 more you can add insurance and 3,000 rpm to the mix. If 80% of the torque is maintained, that's worth 20 extra horsepower - $55 each, not bad.

[kghost]

I'm with ya.

But we just have to get cheaper rod's then those. Mother of god those things ought to be gold plated at that price.

[eurban]

Heavy duty cylinder studs are also a very wise choice if you don't want your newly rebuilt motor to leak.

[MRieck]

I'm with ya.

But we just have to get cheaper rod's then those. Mother of god those things ought to be gold plated at that price.

Well...the small end is bushed. It does kinda look like gold.