1981 CB750F carb idle problem

[SmashEmUp]

Hello! First off, I'm new to the site. I'm here to source the knowledge of the experts and veterans. Even the Joe Smoes that have learned a thing or two and might have experienced my same issue. I'm new to this forum but not new to motorcycles or forums in general. I know a little about motorcycles and a little about tooling but I don't nearly know it all! This is why I'm here.

Last year I bought a 1981 CB750F in pieces. I love motorcycles and was introduced to Cb750s and Cafes. I figured what the hell? I'll give it a go! The original bike I bought didn't have a title, I stumble across a 1981 CB750C on Craigslist will all kind of issue but had a title for $600. Talked the guy down to $400 and took her home. I swapped the motors and harnesses between the bike because I know my original bike ran and his motor didn't. The thing is, my carbs from the F are currently disassembled for cleaning and rebuild and I ran in to some seized parts and other issues. So I took his carbs from the C and put them on my motor from the F. Went to synce them and while trying to sync the idle will jump from 1k RPMs to 3k or 4k RPMs. It's a little erratic. If I blip the throttle when it jumps they'll drop back down. I have tried to do a little research, but have had no luck.

Thank you for your help and sharing your knowledge in advance.

[robs750]

Have you checked for vacuum leaks at the carb isolators?
If they are hard and cracked,time to replace them.Are you sure you have the carbs pushed all the way in.?

Does the bike have the stock airbox?

Maybe the carbs are dirty and need to be rebuilt also.

[SmashEmUp]

I haven't checked the carb isolators. Don't know anything about the isolators! Lol! But I will look it up tonight and check them. Yes, the carbs are pushed in all the way. I stole the carb boots from the new motor because they hold on the carbs and motor very solid. The old set I had were hard and didn't hold well, the carbs would start to back out just from the rough idle. The bike does not have the stock air box. I replaced it with pod filters I bought off of eBay. I know the change in filters can affect idle and performance, but could it cause a sort of intermittent problem like this? I assumed it would be a constant problem if anything. But I'm asking because I don't know, that was just my guess.

As for possibly needing a rebuild, I'm afraid that's what it might be. The bike supposedly ran a week or two before I bought it. I was hoping I could get the bike running first and could ride it do a little bit and completely overhaul the bike in the fall or winter, but it looks like I may have to do that sooner than later.

[rrgunslinger]

Do a bench sync.  Cured my issues like that. And make sure all the needles are at the same level. But do the bench sync first.

On Thu, Jun 28, 2012 at 6:09 PM, SmashEmUp [via Honda CB750'S] <[hidden email]> wrote:

I haven't checked the carb isolators. Don't know anything about the isolators! Lol! But I will look it up tonight and check them. Yes, the carbs are pushed in all the way. I stole the carb boots from the new moor because they hold on the carbs and motor very solid. The old set I had were hard and didn't hold well, the carbs would start to back out just from the rough idle. The bike does not have the stock air box. I replaced it with pod filters I bought off of eBay. I know the change in filters can affect idle and performance, but could it cause a sort of intermittent problem like this? I assumed it would be a constant problem if anything. But I'm asking because I don't know, that was just my guess.

As for possibly needing a rebuild, I'm afraid that's what it might be. The bike supposedly ran a week or two before I bought it. I was hoping I could get the bike running first and could ride it do a little bit and completely overhaul the bike in the fall or winter, but it looks like I may have to do that sooner than later.

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[robs750]

I would suggest doing a compression test.
If the compression is bad,your going to be chasing your tail!

The erratic idle does sound like a vacuum leak or something wrong with the carbs.

A lot of people say that the stock airbox and filter is the route to go.

[SmashEmUp]

What's different about a bench sync? I'm not questioning the validity of your solution, I'm just trying to learn and understand what is going on. Appreciate the suggestion. I'll give it a try.

[SmashEmUp]

I'll do a compression test first. Just see what's going on. Thanks guys!

[rrgunslinger]

To me it's like starting from scratch or the beginning.  Put things in focus for me. I had set mine by gauges also.  Mine are round tops so not sure it will even work for you. 

On Thu, Jun 28, 2012 at 7:10 PM, SmashEmUp [via Honda CB750'S] <[hidden email]> wrote:

What's different about a bench sync? I'm not questioning the validity of your solution, I'm just trying to learn and understand what is going on. Appreciate the suggestion. I'll give it a try.

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[SmashEmUp]

So I tore the carbs down. Completely. Soaked all the metal parts in some parts cleaner I borrowed from a friend. Inspected and cleaned the plastic parts, rubber parts and hardware with carb cleaner. It was hard to make time, but finally got them assembled, bench synced and on the bike. The bike runs great. Maybe about 90%. It being my first time and all, I'll have to go back and make some tweaks.

Thanks again for all the help guys.

[seestheday]

I was able to get my 81 DOHC CB750K carbs sync'd perfectly on the bench (I tested them later with a Morgan carbtune).  I think I just got lucky.  For most people they just get them "close" and then finish it with the gauges.

That said, the erratic idle sounds like a vacuum issue like many other have said.  You can confirm this by spraying wd-40 or starter fluid on the boots.  If the idle rises when you do this then you have vacuum issue.  Could be a crack or a bad seal.