Engine Swap Woes

Hello everyone,

My father and I are in the middle of diagnostics on my 1984 vf750, and we have found ourselves stuck on this swap.

It all started when I bought my bike off a Con man last year for $1000 in fair condition. Long story short, the tranny second gear was trashed from someone doing wheelies. I found this out after I bought it, because I did not test riding it hard enough. Well I needed a few parts so a 1985 vf700 California parts bike popped up. The 85 supposedly sat for three years after it slid under a semi's trailer. Only visible damage was to the top of the bike so all the plastic was trash. My father thought it would be a good idea to grab this parts bike and just change the engine into my 84 750, so I bought it for $300.

I'll add that the 750 sat for about 5 months before we started the overhaul, and the engine ran fine however the tranny was missing a tooth on second gear. We pulled the 750 engine out, replaced with the 85's 700, and we hooked up the 750's carbs to it.

Now here's the problem. When we attempt to start the bike we are getting a lot of blobs, pops, burps, and bangs throughout the exhaust. We have the air box off, and we are having misfires out of the carbs, and misfires out the exhaust pipes. The bike will run fully choked forever, but even when it warms up when the choke it turned off the bike dies. When we attempt to give the bike throttle, it behaves like it only wants to rev to a certain limit, as if it is governed, it will only run about what seems 40% throttle. When we do get it running, it loves to puff white smoke.

We thought the carbs were sucking air in from somewhere but that isn't the case.We are at a loss here fellas, we would appropriate any help we could get. Thank you. Now my father want's to clean out the carbs off the 85 and try using them after they sat for 3 years, but I don't see how that is going to help. I'm wondering if there is something we overlooked that might not be compatible because the 85 700 engine came from a California bike.

Thank you
Attached is the mess we're trying to make work.

 

You have a known runner and it's a 750. Why not just swap the good transmission from the 700 into the 750?

 

:welcome: to the madness hope you will sort things out soon

 

I would suggest cleaning the pilot jets before trying to run it, I would expect them to be clogged after sitting for such a time. It is going to cough, fart and pop withoot the air box, I would install that before running it again.

 

From my understanding the 85 700cc tranny has a final gear ratio that is different then the 84 750's Final gear ratio. I wasn't aware if they would inter-match. However I think it would be possible to take the second gear from the 85's tranny and throw it in the 84's.

But that's a PITA...

 

I'm planning on rebuilding and cleaning the Carbs from the 84, but my father want to skip that and just run the 85's carbs...

 

Ah... This I did not know.

 

Your carbs need cleaning. Most likely the jets are plugged. additionally the thing will run like crap without the airbox. if your running without mufflers it will be even worse. Biggest clue u gave was it will kind of run with the choke all the way on. that is dead givaway for this problem .

 

Okay, i'll tear them apart this weekend, and actually connect the airbox instead of just letting it collect dust on the ground. I thought it might be pulling in too much air and throwing the a/f mixture off.

I'll report back my findings. Thank you.

 

WAIT!!! U mean you did not have the airbox on when you tested this thing? Don't tear anything apart till you run it with the airbox and filter! You may be ok. If the jets are not plugged up you may be able to run some SeaFoam thru a few tanks of gas and clean it that way. Most bikes are calibrated to run with the resistance of the airbox and filter (and exhaust system).

I'm not saying it's a bad idea to clean your carb's, but that's a winter project if you have a choice... It's riding season right now!!! GET-ER' DONE (and on the road)

 

lol, Yeah I was going to attempt the simple fix first. If just a few screws fix this bike i'm going to feel so dumb.
I still have my little 250 this riding season, but its a pain since I do 85% highways.

 

Update after a month of silence and vacations... After getting feed up trying to diagnose the issue. I felt the front headers after it ran for a while. They were cold, as were the heads. I guess it was running on two cylinders this entire time -_-
I checked the wiring diagram and the only fault i could see was the CDI, so I ran to my parts bike and snatched it off and exchanged them. After that It started up and ran fine... Now all I need to do is adjust the carbs, find out why my tac is dead, and paint!