How big is too big?

[biker28169]

Hello. First let me say that I'm brand new to the forums. My questions may have been addressed by someone in the past, but so far I haven't found them.

I'm building a '78 CB750A at the moment. It will be a full custom job when I'm done, but right now it's a motor in a frame with other parts nearby. It hasn't run in probably a decade at least so my plan is to tear everything apart and start fresh. Being the type of person who hates to leave anything stock if I can help it I would really like to punch the motor out a bit to get more power and performance. I'm thinking something around 850cc should be doable without being too much for the block/other parts to survive. I'm also going to go with the 2 carb kit instead of the 4 so tuning will be slightly easier. Considering this is the automatic model I don't know if I'll have issues with the torque converter or tranny in relation to reliability. If anyone has built one of these I would love to hear how it went and what advice you may have for me. This is my first build ever so my progress is slow.

Thanks in advance for any help you can offer.

 

[dirtdigger]

It seems the auto motor will take anything the standard motor will. There are several guys on the sohc forum that drag race the autos with 915 and bigger cylinders with no issues with the tranny. The 850 will be totally fine and cause you no issues. In all honesty the 836 to 850 motors dont add a large amount of HP, unless you do a lot of work to cylinder head and use a bigger cam. There is a section on the sohc forum that deals specifically with the auto as many of the gaskets etc are getting hard to find. As far as the 2 carb setup I am not a fan and really they are not any easier to tune then the four carbs because you still have to do the same things, syncing, jetting etc. You will probably make less power with the two rather then the 4 carbs, these motors work best with 4 carbs and have seem fitment issues with the 2 in stock frames....and I think they look funny.

 

[biker28169]

Thanks for the info. I only said 850 as an example. I didn't know there was that much "room" to punch out the motors. And I was concerned previously about the tranny's ability to hold up. At the risk of being shunned by the members of the forum, I will say that I'm chopping this bike and making modifications to the frame. I think the 2 carbs will look better. But it's good to know that it could cut down the power. I certainly do want to find out more about what cams may be available, etc. I wasn't aware that there were people who drag race these bikes which I admit is pretty cool. Obviously I've got a lot of research to do in the forums. I'm willing to modify the motor as long as it improves the bike. I just want to avoid doing stuff that will slow me down.

 

[dirtdigger]

It all depends on how much power you want and what you are going to do with the bike. You can take these motors over 1000cc. I have built several drag motors, my current ones are 1026cc. Going bigger that 850 cost considerably more money as you have alot more machine work and have to upgrade various other parts to handle the power. You can make decent power with these old motors but it will cost you $$$. You can look at a couple of my motor builds and pictures I have on this site that shows some of what goes into bigger motors. If you want more power but very reliable you need to look at 836, 850cc and have a head ported with good valves etc which is where the power is. but doing this and putting twin carb setup on will really hold you back on the power side. If you have any motor questions dont be afraid to message me, I can help you decide and give suggestions.

The guys that are racing the autos are doing so because they are dead on consistent, the auto sucks up a lot of power so they aren't as fast but you cant beat them with there consistency.

 

[amc49]

'...the auto sucks up a lot of power...'

Depends................we had a single 4 bbl. BBC Corvette that ran 9.60s @ 140+ mph using a TWO speed Powerglide ATX.

As said, the twin carb thing will lose 10-15 hp or so. Twin carbs by their very nature have lopsided intake events that inline fours do not like powerwise, they pretty much detune a four carb bike. And if one can work on two carbs competently then there is zero issue with four. Two bigger bore carbs to make up for the loss in how many is a mistake on ATX, the 750A already has very small carbs compared to the others (26?) and the two may well be too big since designed for the four bigger carb (28) motors. Two carb bores vs. four are always bitchier about lower rpm driving quality, it's harder for the motor to flow them at lower flows. Of course they won't tell you that when they tell you how well they run. More carb bores if set right pretty much ALWAYS drive better than lesser bigger bores. The air is more active in them. It's a driveability thing.

If one needs them due to the chopper thing fine, but be aware they extract a cost there.

 

[dirtdigger]

'...the auto sucks up a lot of power...'

Depends................we had a single 4 bbl. BBC Corvette that ran 9.60s @ 140+ mph using a TWO speed Powerglide ATX.

Wasn't talking about a BBC with an auto...I was saying the 750 auto trans sucks up a lot of power. They wont put as much power to the ground as an equally prepped manual trans 750 will.