Engine swap for 77 GS750

[Guest]

My GF's son wants to buy a used '81 GS750 engine and put it in his '77 GS 750 (chain drive). He is also planning to re-use the carbs from his 77. I have tried to discourage him, but he seems hell bent.

How bad an idea is this and what can I tell him about the problems he will have? I have already mentioned carb spacing might be an issue, engine mounts and even carb size to feed the 4 valve engine, but...

[tkent02]

Why would he want to pull out the good engine and put in the bad one?

[Big T]

What Tom means is that the early 750 16 valve motors are prone to oil related failures - like the crank and cams

The 750 8 valve motor is nearly indestructible

Who's paying for this? You? Him?

If it's him, let him go to it. Once he gets bogged down because the carbs don't match and he needs to fab new engine mounts and etc etc, it sits in the garage all summer and you don't have to worry about him getting killed on the bike.

Plus, he learns that he should listen to your infinite wisdom

I'm sure it can be done, but in May? He'll lose the riding season

Bad idea rating= pretty bad idea

[TheCafeKid]

The carbs for his engine won't work in the later engine. They won't even fit. He will need a set of BS32SS carbs. Could they be made to work? Yes. But it will involve a lot more work and modification.
Also the engine mounts will need to be changed out. The 77 motor is longer because of the kickstarter assembly.

It could be made to work, but as already stated, the 81 motor is junk comparatively speaking from a longevity standpoint. Plus the HP is not all that different.

If he wants to do a swap, tell him to find a 1000E motor, or an 1100E motor. That would be worth swapping.

[tkent02]

If he wants to do a swap, tell him to find a 1000E motor. That would be worth swapping.

And it's bolt in easy, too.

[Wingnut1]

Why would he want to pull out the good engine and put in the bad one?

Apparently the 77 motor isn't so good. I know he has had problems keeping his timing plate in place and he told his mom tonight he tore into it and its messed up. I don't know the details but it had a bad knock on Saturday.

Shop with the used '81 also has a used 2 valve motor I think, so maybe I can convince him to go that way or 1000E.

[Wingnut1]

What Tom means is that the early 750 16 valve motors are prone to oil related failures - like the crank and cams

The 750 8 valve motor is nearly indestructible

Who's paying for this? You? Him?

If it's him, let him go to it. Once he gets bogged down because the carbs don't match and he needs to fab new engine mounts and etc etc, it sits in the garage all summer and you don't have to worry about him getting killed on the bike.

Plus, he learns that he should listen to your infinite wisdom

I'm sure it can be done, but in May? He'll lose the riding season

Bad idea rating= pretty bad idea

I'm not paying for it until he borrows my 1100

[tkent02]

I'm not paying for it until he borrows my 1100

Bad idea rating= really really really bad bad bad bad stupid idea

[Wingnut1]

Bad idea rating= really really really bad bad bad bad stupid idea

LOL, I picked up a used HD last summer and he may have put about 1,000 miles on my GS riding with me, but no he doesn't get to take it home

[7981GS]

Make HIM ride the hog.

Eric

[1_v8_merc]

Yes, yes...Those 77 8-valves are SOO terrible.
Just sell it to me as quickly as possible and I'll take it off
your hands!

[TheCafeKid]

The knock is likely not as bad as one might assume. The 77-79 750 has a roller bearing crank, so it's likely NOT a rod knock you are hearing. However, they, like many Gses have a penchant for snapping the starter clutch bolts allowing for a rattle that to some sounds like its possibly coming from the crank since its hanging on the end of the crank.

Timing plate could easily be replaced with a Dyna system. Don't give up on that 8 valve yet...they're tanks!!