View Full Version : CB750 Big Bore 1000 CC questions

04-06-2010, 1:42 PM
Ok here's the deal... I had a guy call me last week wanting me to work on his Honda CB750. The bike hasn't run since 1980 something... Right before the bike was parked, the PO put in a 1000 CC big Bore kit, I went over to work on it over the week end to try and get it started, when I found several problems... Problem one... The carbs that came with the bike weren't on, I cleanedthem up only to find that they wouldn't fit (the spacing between carb 2 and 3 was too wide for the stock air boots) Problem two.... the electric starter had been removed.
The guy had a running CB750, which I took the carbs off of, and tried to use to start the non running 750. and the bike wouldn't start with the kick starter... and we found out that the clutch wouldn't release so trying to push start it was out of the question for us.

So here are my questions...
Engine is bored out to 1000CC, Has 4 to 1 exhaust. What carbs would a person run on this, and what jetting? (we live at around 500 feet elevation)
Second question is, Can we just get an electric starter for a cb750, and put it into this engine, or was it removed do to other reasons? The starter assembly on the non running 750 is missing completely, the starter motor, flywheel gear (i guess that is what to call it) and the idler gear are all missing, the little hole for the starter drive gear to go through the starter cavity to the Stator housing, has been blocked off with a machined plug and silicone.

04-06-2010, 6:33 PM
they pulled the starter to save weight.
jetting? just pick a # , say 130, and start from there. there easy to change with the carbs on the bike.
carb boots, radiator hose works

so why woundn't the engine start with the kicker, what was it doing?

04-07-2010, 8:36 AM
I am not sure why it wouln't start... I took the carbs off of a running stock CB750, and put them onto the non runner. hooked up all the electrical, then kicked the $#!T out of it... I was getting spark, the carbs were filling up with fuel, but the fuel wan't even trying to burn, I then tried starting with starting fluid, and still no vroom...

The wiring on the frame is all jacked up, an aftermarket Ignition System has been installed, and am not sure if that is hindering us at all... my Plan this week end, is to rewire the whole bike, and add an electric starter to save energy on the kicks... (I am kinda small guy... 6' 128lbs) so kicking is very hard for me... I am also going to try to find another set of carbs off of fleeBay for the bike...

Any other suggestions?

04-09-2010, 9:04 PM
I've got a set of stock carbs from a running cb750 I took off you can have, with boots and everything. $25 would cover postage.

04-09-2010, 10:24 PM
sweet, I will send you a PM tomarrow after talking to the guy

04-10-2010, 2:42 PM
Won't kick start with out a charge. Do you have a good battery in it? Or a old one that will hold a little charge? If it "ALL GOOD" Cb's are very easy to kick. The alternater needs an excited field to work. at least hook a battery charger to it to get it to fire up. Good luck

04-10-2010, 3:38 PM
Brand new battery... 12.5 volts... I have 12.0V at the coil with key on. should I run a hot wire to the field just for testing purposes?

04-10-2010, 5:24 PM
Just wire it correctly and go from there. Go to Hondachopper.com and get a simple chopper diagram.

04-10-2010, 8:11 PM
wiring here

04-10-2010, 8:13 PM
this site has alot of manuals

a good 750 site


04-10-2010, 8:21 PM
The wiring for the bike it self is good... the aftermarket ignition system... I have no clue.. I am getting spark so that is good... I think my problems lay more in carb work than in electrical... tomarrow I am going to look at the electrical system better...

04-10-2010, 10:45 PM
fuel filters? change them out or take them out. check the fuel lines and the flow.

I hate to say it, but if it starts with a starter, it should kick over. Starter gets you more cranks, but if everything is there, it will kick.

Timing might be off.

Might need more voltage in the spark if there is higher compression. Might need higher voltage coils and different plugs. Speaking of which, have you checked the compression?

~Rev Mike

04-10-2010, 11:46 PM
Haven't checked compression yet... it should be good, its a brand new engine...less than 100 miles... I have fuel flow. No fuel filters... Brand new plugs... the engine doesn't ahve an electric starter, only a kick start. the Coils are aftermarket, along with some kind of electronic igniton system that looks like some one just hacked it in there... ( have seen better wiring jobs by blind, drunk monkeys.)

My plan is to take the ignition system from a running 750 and hook it up along with an electric starter, take carbs from running bike and put back onto this one, and see where that leads me... This thing just boggles my mind... The bike ran once... then new handle bars were put on, and hasn't run since... I found broken wires inside the handlebars, so swapped out handlebars and controls from a running one... I then Got spark, but it still acts like it doesn't even want to fire. The owner of the bike wants to put this non running engine into his daily rider, because his running engine is starting to burn oil... I know this thing will run...

and yes timing could be off... like I said this engine is no where near stock so I don't even know where to begin with trying to figure out where the timing needs to be... how do you adjust the timing with an electronic ignition... the pick up coils are permanently mounted to their brackets. Does the whole assembly rotate just like an old distributor?

04-13-2010, 5:05 PM
Oh, I didn't see that it had been started once already. That REALLY pisses me off. But it's gotta be something simple. I had a fishing line knot of wiring on mine I had to go through.

I had this happen once and I drained the gas, to include the bowls, and refilled with new and it worked out. If you had it running, that should eliminate most possibilities, including compression.... you could have a stuck valve, but probably not 2 or 3.... gas air compression spark, all at the right time in the right amounts....

If you weren't in NY I'd come take that 1000 cc off your hands and make life easy for you.


04-13-2010, 5:07 PM
Ignition depends. What type is it. You can prob download the inst sheet, although there are prob plenty here who can help you sight unseen. With the exception of me, but I can open a beer....

04-14-2010, 7:45 PM
the 1000 engine isn't for sale... we are trying to get it running so we can replace a stock worn out engine... he might be interested in selling the old engine after he gets done rebuilding it

04-18-2010, 9:20 AM
With a total top end rebuild and 0 starts. I'd check cam timing and valve adjustments before the ignition.

04-18-2010, 11:36 AM
it has started since the rebuild, not started since new bars were installed on the bike... I fixed the bar issue, the guy did a shoddy job of running the wires inside the bars, and cut every single wire from over tightening the clamps down on the risers... I think we will ahve this thing started next time I run up there... he got a new starter, and one of the members here sent him a good set of carbs, which he sent off to get cleaned up before we install them (just want to cover all the bases).

04-20-2010, 10:15 PM
that bar clamp pinch was probably a lot of it. still wondering what kind of after market ignition system. engine should not be hard to kick through, i used to show off by reaching over and starting engine with my arm. it HAS to have good battery or nothing happens, as someone already advised.