Topic: Basket Case 250 Nighthawk

[Alan F.]

I'm noting this here so I don't lose it. I'm told that there were three different pistons for CB/CL72s in three different compression ratios, I'll have to find part numbers and ratios.

The point is that those 250cc bikes use the same or close to the same bore and stroke as these 250cc bikes, but their combustion chambers were much deeper to keep compression low. I'll attach photos.

Why do I care? I've been researching Terry's use of CB77 pistons in his stock bore K0, this would appear to be a scaled down version of that modification (117cc/cyl vs 184cc/cyl) nobody's ever done it before, I've got a spare 250NH engine to experiment with and I'm curious if it can be done. It might not add much power, but this little twin already revs to 10K and has 9.2:1 compression, bumping it up to 10.2 might make 15-20% more useable power.

[grcamna2]

I can see how much less cc the Nighthawk combustion chamber has in the pic.

[Alan F.]

Yes, I think this one will be worth following up on when I get the chance.

[Alan F.]

http://forums.sohc4.net/index.php/topic,189492.new.html#new

Kudos to Kenzo1979 for reminding me that I've got the diodes set aside to wire my LED turn signals up to the single indicator bulb in the chi-mini-speedometer I'll be using on this bike.

[Alan F.]

I'll park these pics here too so I won't have to search my archive later.

[grcamna2]

Nice gauges

[Alan F.]

Thanks, it's an experiment actually. I bought the absolute cheapest cable driven speedometer with odometer & idiot lights, and the absolute cheapest electric tachometer I could find on ebay. Combined prices were less than $20 shipped, but they were on the absolute slowest boat from China. The Speedo I got in 2017, the Tach arrived in early 2020, they were separate purchases. Earlier in this thread I mentioned some vibration damping plates (designed for mounting circuit boards or cameras to flying drones) that I'd found for really cheap on ebay, they took 8 months to arrive but they cost 12 times more today so i win. These arrived late last summer.

The plan is to mount the cheapie clocks on the top triple with these vibration dampers and see if they can last a while.

[grcamna2]

Ok,from what I remember about that Nighthawk 250,doesn't it have a set of idiot lights inside a tachometer shaped pod rather than an actual mechanical tach ?

[Alan F.]

Yes. I have that idiot light pod but it's damaged, I should be able to use the wiring from the pod (so I don't have to cut my good wiring harness) to connect the idiot lights in the speedometer I'll be using.

And the electric tachometer, there are a few threads on the Gen1&2 Honda Rebel forum where these have been added successfully, I'm confident that I can get it to work.

The only down side is the cheap Speedo is in kilometers, not miles. I'm going to ride it on a measured mile to see if the odometer is in miles or kilometers. I haven't determined if I can pass a State Inspection with a speedometer reading in Kilometers though.

[grcamna2]

Yes. I have that idiot light pod but it's damaged, I should be able to use the wiring from the pod (so I don't have to cut my good wiring harness) to connect the idiot lights in the speedometer I'll be using.

And the electric tachometer, there are a few threads on the Gen1&2 Honda Rebel forum where these have been added successfully, I'm confident that I can get it to work.

The only down side is the cheap Speedo is in kilometers, not miles. I'm going to ride it on a measured mile to see if the odometer is in miles or kilometers. I haven't determined if I can pass a State Inspection with a speedometer reading in Kilometers though.

You didn't originally have the stock speedo ?

[Alan F.]

No it didn't come with the bike and neither did the 2 parts Bikes I picked up along the way.

[RAFster122s]

So, imagine the RE350 has sidetracked this project for a while and it is fun to think things through and wait for the next batch of inspiration and time to address this or other things…

[Alan F.]

I'm parking the RE650 right next to it actually so it never really leaves my thoughts. I find it funny how similar the 250 I envisioned in 2017 is to the Interceptor.
The 55" wheelbase, 100/90-17 front 130/70-17 rear, (same sizes but 18" on the RE) the 41mm forks, single front disc, the dual exhaust....
It's going to be a fun bike when it's done. It'll have about 40% of the Interceptor's power but it'll also weigh 150lbs less so who knows. I think it'll be great fun when I do finish it.

[Just John]

I found out today that there are big bore kits for these engines which takes them to 280cc.  I talked to a young man who had one in his Rebel.  He got it on EBay and they are made in China. 

[Alan F.]

Yes I wondered about those myself but decided that I'd keep stock for now, 30cc increase may or may not be a noticeable seat-of-the-pants increase, especially for the cash involved. If I had a siezed engine needing overbore pistons, rings and boring the cylinders it may be cost effective then. I spotted a video once of a British guy installing one of these kits, it ran on the first try for him.