Helped friend swap 79 kz750 engine into his 82 csr

Greetings everyone. New member here posting on a friends behalf. His motor gave out in his 1982 csr 750 twin. So he picked up a new one at a motorcycle salvage yard from a 1979 kz750 that the guy told him would work perfectly. Well, there are a few differences, at least. For one the top motor mount at the coils doesn't line up, but that is the least of the problems and easily fixed with a welder.

We got the bike back together and it will start and idle, but that's it. After anywhere between a few seconds and 30 seconds it will shut off. Rpms might change by the slightest margin with throttle... maybe 100-200 rpms at best. It does pop and almost sounds like an ignition miss. Any ideas on what we may have missed? I know the wiring harness are a little different. In the 82 engine there was 2 wires coming out of the stator, and there was only one wire coming out of the 79 engine. There is also a solid red wire in the headlight bucket that seems to not connect to anything.

We did have to use the old carb intake boots, which had a slightly smaller diameter to work with his carbs. The intake boots that came on the 79 engine were a bigger inside diameter. There is spark to both cylinders... but i couldn't tell if it was weak or not since it was light outside. But it looked kinda weak. It was blue spark, however. It's getting fuel. Gas comes out of the bowls when the drain screws are loosened. Any thoughts or suggestions? We are stumped. Thanks for your time.

Check your advancer to make sure its not stuck. Otherwise, the first thing I would do is spray some starting fluid in each carb. Open the throttle half way and hit the start button. If it roars to life and sounds strong with a lot of rpm's then you can pretty much determine it is a fuel issue instead of the electrics and then focus on that.

Forgive me, but where/what is the advancer? We tried starter fluid... it will rev with the starter fluid, but only idle without it. And the carbs were rebuilt a month ago and the old motor ran great with these carbs till last week.

So if it revs for a few seconds with the starting fluid, then it would tell me you have fuel or air issues. The starting fluid pretty much replaces your carburetors for those few seconds that it rev's. So that is most likely the issue. But you should do maintenance on the advancer anyhow. Here is a link of a post from on this site in which the member has a link to a service manual:

www.kzrider.com/forum/2-engine/203671-75...ervice-manual#203824

Here is a direct link to Page 52 on the manual:

biquetoast.com/assortedDocs/750manual/052-Disassembly.pdf

This page explains how to disassemble it. But you don't have to do that, just lube it with some cam grease.

There is also vacuum lines on the 82 engine that there is no place for on the 79 engine. There isn't one coming off the valve cover, and I have an extra vacuum line coming off the carbs. any ideas?

Someone will correct me if I'm wrong since there are others here who are more expert on the KZ750 twins in general. Anyhow, sounds like that is for emissions stuff. So you can plug that extra vacuum on the carbs. I'm sure that is part of the issue you are having if you left it unplugged. Be to do your tuning with the air filter or pods installed. That is another factor.

I have 3 hoses coming off the carbs... the fuel line, and what appears to be 2 vacuum lines off a 'T'. I hooked up one of the vacuum lines to the petcock, but the other one is just hanging there.

We have air pods installed... the bike didn't have the air box when he bought it. But shouldn't choking it reduce the airflow enough?

Again, I'm not an expert on the twins. But my bike uses an enricher circuit for a choke. Not sure with your bike. I posted the manual for for you on the 76 to 79 twins which will help you with the engine. But you will also need the service manual that has the original 82 since you used the carburetors off of it so you can reference information on your carburetors. It will also come in handy for the rest of the bike. The other vacuum can be plugged. As long as you are sure it is a vacuum line and not a breather hose for the bowls. Someone will need to chime in or you will need the manual. But it does sound like your petcock is vacuum operated. So most likely it is a vacuum line.
Before you friends original motor gave out, did the bike run right with those carburetors? Did the enricher circuit (choke) work properly? I guess, I just trying to get a base reference here since I have no way of knowing what the condition of the carbs are. Were they clean and operational before the swap?

Thanks for the links, I just can't look at them till I get home. The carbs were recently rebuilt and the bike ran great with the old motor. The choke also worked right.

This may help you.




You can download the complete manual at the following link:

bublicpucket.s3.amazonaws.com/Manuals/ka...llwith80and81sup.pdf

You will have to right click and "Save as" when you get to that link (note which directory you save the file so you can have access it). It will take about 10 min to download. This service manual has a supplement for the 81 CSR towards the back which should help you out with the carbs. That is where I got the above picture from.

That picture looks just like the old engine, the 1982. The 1979 engine doesn't have the vent port on the valve cover. So there is no where to hook up the vaccum switching valve. We don't even have it hooked up right now at all to the new engine.

OK, so I hope I don't lead you in the wrong direction. But the red X's in the picture above show what has been eliminated do to the engine swap. But it if you look at the picture, it looks to me like the once I circled in red is what you should have left. The ends marked as 'D' should go to each carburetor and the one I marked in red as 'F' should go to your petcock. As mentioned, I have never owned a KZ750 twin so its hard for me to be 100% certain. Maybe somebody like bounty hunter can confirm all this for you. Download that service manual that I gave the link for ( bublicpucket.s3.amazonaws.com/Manuals/ka...llwith80and81sup.pdf ). It will seem like its stuck once you get to the site. But point your mouse where the body of the text should be then right click and choose "Save page as" to a local directory you can access. That should help you out.