anyone know their way around late 70s suzuki gs750 engines (in a chopper)?

i’ve asked a few people so far, but knowing there’s generally some bike experts on here (i mean that in a good way) id be interested to hear from anyone who knows the engine.

basically i’m looking for a bike project for myself and have seen an old build chopper thats been sat for 15 years with a ’78 gs750 motor. its a bizarre bike, but it interests me.

apart from checking the motor turns over, no kick start and no electrics so i’m going to have to try and do it by pushing in gear, there’s not much i can do to check it so i’m wondering what are these motors like to work on (for someone with limited experience), how available and pricey are parts if needed, and what should i look out for when looking at it?

if anyone knows about choppers specifically i’d also be interested in what you can tell me. i come from dirt bikes so this is a new one on me.

should say, ideally id like a bsa b40/b44/b50 as a first project like this but not a hope in hell can i afford one from what ive seen them go for. crazy world bike prices these days!

The nicest bike I ever had was a ’78 GS 750 & it was absolutely bulletproof. Choppers however, are a different kettle of fish, usually because the builder extends the forks without raking the frame, so the carbs suffer from the change in angle and to add to this, the exhausts are usually messed about with so the jetting goes all to pot. Other than that all I can say is ,non-runner avoid it like the plague, it’s a forty year old bike that’s been screwed with.

Jap bikes are simple enough to work on. Parts are expensive. Suzuki
were destroying all five year old parts at one one time!
Been standing a long time, sometimes they run fine after cleaning the carbs.

I know a bit about chops. But saying it’s a chop doesn’t really mean a lot,
it’s a wide ranging name, at one end you have a stocker that has been attacked with a hacksaw and the other end a custom framed bike built from scratch.
The only thing I can say for sure is an unsprung saddle on a hard tail can be painful. To me they have to be hard tail.

At the time, those engines were regarded as bullet proof – probably the best designed engine at the time. However, that’s 35 years ago.
Roller bearing bottom ends made them very long lasting (I did 160k on a GS650 without any engine work other than cam chains). The cam chains need changing every so often, and a decoke makes sense too, along with cam shim adjustment.
But, as its so old, anything could be wrong with it.

Of all the things to put you off buying a chopped bike a GS750 engine is low on the list of concerns. The engine was considered bullet proof and now that Suzuki GB have started their Vintage Parts Program, bits for certain models are being manufactured again.
However, all the normal old bike engine induction and exhaust woes will be magnified on a chopped bike.

gavtheoldskater – Member
… there’s not much i can do to check it…

Basic compression check:
Remove the spark plugs – spray some light oil in the cylinders (to provide upper cylinder lube and seal the rings).
Put wood / blocks / plastic recycling-box under the frame to raise the back wheel off the ground (whilst your assistant holds the bike).
Select highest gear – turn back wheel to turn the motor over.

If it turns OK (I.e. not seized), test cylinders individually for compression.
Ideally, by using a compression tester – but you’ll get an idea if you replace spark plug 1 – and, for part of the cycle, the motor is now harder to spin.
You can also screw the plug in by the smallest amount possible (e.g. one thread, and listen for a hiss.

Remove plug 1, repeat for 2, 3 and 4

If there is compression on all cylinders, replace all plugs and turn the back wheel – if compression is good, the motor should be turnable but require some force.

Basic HT ignition check:
Wasted spark ignition, so one coil feeds (I think) cyl 1 and 4 one feeds cyl 2 and 3.
Check total resistance from plug cap 1 to plug cap 4
Compare total resistance from plug cap 2 to plug cap 3
They should be similar – if not, or no reading / open circuit, you’ve got problems.

and what should i look out for when looking at it?

Regarding potential: Insurance / MOT / legality
Check registration documents / Frame number / Engine number etc.
I.e. Is it an original Suzuki frame (with numbers) that’s been altered, or is it a fabricated frame that’s had numbers stamped on it? (Not the end of the world, but can be MOT hassle).

sat for 15 years

Drain / discard any fuel, check tank for rust
Carb strip-down: clean float-bowls / jets / de-gunk
Condition of throttle / clutch cables
New pair of tyres

It’s a bizarre bike, but it interests me.

Photos?