Author Topic: Hondamatic 750 Camshaft (Read 3269 times)

JerrodR · « **on:** July 04, 2020, 05:56:30 PM »

Head gasket on my 74 750 four let go not to long ago, was forced to ride it home about 15 miles and wiped out my cam and one of the Rocker towers. I ordered a complete head assembly from eBay, and found out after it was in the mail that it's out of a Hondamatic. The seller told me it should work fine, and I can't see a reason why it wouldn't, but what I'm worried about is loosing power because of the difference in came. The Hondamatic claims half the power that the standard 750 has, but I was thinking that could possibly be lost in the transmission, does anyone know the difference in cam's between a Hondamatic, and a stock 74, 750 four?

bryanj · « **Reply #1 on:** July 04, 2020, 11:56:42 PM »

It will be lower power but not half and it will run fine if you use your original head and valves

JerrodR · « **Reply #2 on:** July 05, 2020, 12:14:59 AM »

That's good news, and i will be using my original head and valves. Thanks.

ekpent · « **Reply #3 on:** July 05, 2020, 06:45:07 AM »

Supposedly the automatic had "revised combustion chambers for lower compression, smaller carburetors and a revised camshaft for better low end torque" to work better with the transmission. If it was me and I had it all apart I would probably look for a K model or F camshaft to plop in there. https://www.motorcycleclassics.com/classic-japanese-motorcycles/honda-cb750a-hondamatic

JerrodR · « **Reply #4 on:** July 05, 2020, 01:47:13 PM »

Thanks for the link, and I would very much like to, but I'm just about at the end of my budget for this surprise expense, and honestly I wanna get it back up and on the road while I still have some summer left, what I found interesting while reading the link is I have the accelerator pump carbs, and a 4 into 1 header, the carbs have also been jetted up 2 sizes I believe, and a couple holes were knocked into the stock airbox. Going based on what the seller told me the guy he bought it off did.. But I guess I'm hoping, because the carbs have been turned up some and I have a little better air flow, that even with the detuned camshaft, I will make around factory output when all is said and done. But will be happy to just be on the road again honestly..

JerrodR · « **Reply #5 on:** July 05, 2020, 01:51:39 PM »

Speed isent exactly my top concern, I like to run through the gears every now and then, but this is more of a Sunday cruise kinda bike, I mean it's like 50 years old lol. So if it's not making all 67HP at the end of the day I'm ok. Just want it to ride well. I'm looking into something a little more modern for my go fast bike.

ekpent · « **Reply #6 on:** July 05, 2020, 02:01:25 PM »

Sounds like maybe you might have later 77-78 what we call pumper carbs on it then. Should be a model number stamped on them somewhere if you wanted to know exactly what they are.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #7 on:** July 05, 2020, 02:52:22 PM »

"So if it's not making all 67HP"

That's not going to be close.

JerrodR · « **Reply #8 on:** July 06, 2020, 08:49:04 AM »

Yes, the carbs are from 78, along with the gas tank and side plastics, Title for the frame claims 74, and that's what the serial number on the engine came up as.

JerrodR · « **Reply #9 on:** July 06, 2020, 08:51:14 AM »

And I was looking at the cam I have coming, and the one I took off my Bike and their the same camshaft. Both are stamped R2.

JerrodR · « **Reply #10 on:** July 06, 2020, 08:55:49 AM »

Unless all CB Camshaft's have R2 stamped on them. In that case I'm just stupid lol.

JerrodR · « **Reply #11 on:** July 06, 2020, 09:55:17 AM »

First pick is how I bought it, and the 2nd was taken a couple nights before the head gasket went. I had spent quite a bit of time on it before I completely tore it down.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #12 on:** July 06, 2020, 09:58:04 AM »

R2 has NO relevance whatsoever as to the grind of the cam. There is NO way to ID a stock Honda CB750 cam based on markings. Mic them when it arrives. If the lobes are the same height that also doesn't mean they are identical. You could set them up in the engine one at a time, install a degree wheel and see opening and closing timing/degrees as well as lifts. That is how you can discern the differences.

JerrodR · « **Reply #13 on:** July 06, 2020, 10:07:33 AM »

If you look close you will see it originally came with a 18" Lester mag on the rear, but my sprocket bearing came apart, and wiped the wheel bearings in it out. It needed some TLC anyhow, so I ordered a replacement. It was 16/3 when it got hear. But the 16 is what I'm going to stay with. It completely transformed the bike. Accelerate's harder, and handling was majorly improved, it was sluggish and had a poor turning radius with the 18", only downside is it turns about 300 more RPM with the 16", but honestly I barely noticed.

JerrodR · « **Reply #14 on:** July 06, 2020, 10:10:35 AM »

Gotcha, I have access to a micrometer, but I'm not going to be digging that far into it. As long as it runs, I will be happy. If I don't like the performance, I can address that this winter when I rebuild the carbs, and powder coat it.

JerrodR · « **Reply #15 on:** July 06, 2020, 10:13:31 AM »

It wasn't very difficult pulling the motor, and I did it on my own with a couple Jack's, this winter the guy that's powder coating it will be helping me, so pulling the motor again won't be a problem at all, if that's the route I end up taking.

Don R · « **Reply #16 on:** July 06, 2020, 12:12:24 PM »

Good luck with the motor, check the oil flow to the top end when you get it started. When it goes together I'd add a little assembly lube to all of the friction points to keep it lubed while the oil gets up there.

JerrodR · « **Reply #17 on:** July 06, 2020, 12:22:15 PM »

Thanks, yeah I have assembly lube and was planning on doing so. Just now got the Head torqued down in the correct order. So now I'm just waiting for my new Head components to get here on Thursday it's looking like. Hopefully everything goes to plan, I took another piece of advice from here and spent around 3 hours last night going through all the oil passages with q-tips. Definitely sucked lol, but hopefully it will be worth it.

JerrodR · « **Reply #18 on:** July 06, 2020, 12:24:34 PM »

I have at least a solid 8 hour work day in cleaning the head alone. That old gasket material was no joke.

JerrodR · « **Reply #19 on:** July 06, 2020, 12:33:16 PM »

Just kicked, well pushed it over a few times lol, and compression feels normal. And I felt a little puff of air out of each of the oil passages every kick, so as long as there's not a big chunk of nasty somewhere in the system that will come loose when I start it, it looks like I'm good.

bryanj · « **Reply #20 on:** July 07, 2020, 12:04:33 AM »

You have got the oil restrictor jets in the head havent you?

ekpent · « **Reply #21 on:** July 07, 2020, 05:43:48 AM »

Quote from: bryanj on July 07, 2020, 12:04:33 AM

You have got the oil restrictor jets in the head havent you?

with preferably fresh o-rings around them and two more for the other end of the cam towers when you put them down on the head.

JerrodR · « **Reply #22 on:** July 08, 2020, 07:31:55 PM »

Yes, and yes.

JerrodR · « **Reply #23 on:** July 08, 2020, 07:37:39 PM »

Tomorrow is the day guys, The replacement top end will arrive around 2. Never timed a Camshaft before, but ive gone through it in my Service manual, and checked out a YouTube video. And it actually was alot less complicated then I was expecting. The job isent done yet, but I'm confident. Hopefully the old girls back on the road by tomorrow night.

JerrodR · « **Reply #24 on:** July 08, 2020, 07:43:02 PM »

Also made some pretty significant cosmetic improvement's. I couldn't look at that old frame with no motor in it, and not paint it. The 16 on the back I bought, and it is a pretty decent looking survivor, (oughta be for 350.00 bucks). But I stripped all the old jacked up paint of the front rim, and im considering my options, may just leave it as is.

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Author Topic: Hondamatic 750 Camshaft (Read 3269 times)