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Can't Get My Bike To Turn Over

Midwest750

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I have a 1981 CB750K. It's the first bike I've owned and worked on. I have done plenty of work on cars and worked at an auto parts store for a few years. I bought it a couple weeks ago and have been doing all the little maintenance since then. I changed the plugs and battery, and did a lot of cleaning but it still won't turn over. I took off the air cleaners and realized it was missing a choke plate in carb number 1 so I bought a used one on ebay and got it all put together. I was hoping that was it but it still won't fire. It sounds like it wants to which leads me to wonder if it isn't getting enough fuel but since this is my first bike I don't know how much fuel should be running through the fuel line out of the petcock. I'm just looking for advice on what I might be missing if anyone has anything. Thanks so much!!! Really excited to get her running and out on the road.
 
Be sure you have the airbox connected and sealed... paper filer in place and the sides sealed with the covers. CV carbs sucking air are a severe pain, and may cause the bike to not start at all if sucking enough air.

I didn't know that the first time I worked on an '82 CB900, and it just wouldn't start. If I remember correctly, I folded a towel and put it over the carbs inlets and the bike started.
 
If your carbs are adjusted close and your choke is working you do not need the airbox for it to start and run. If you have to restrict it that much to get it to start then you are extremely lean and need to look for the issue. The choke and fuel from the accelerator pump should be more than enough fuel to get it started. The airbox problem is more of a 1/4 throttle up issue when the vacuum slide comes into play. Need to start looking for idle circuit fuel passage restrictions along with vacuum leaks at the boots and in the vacuum part of the throttle. I have on several occasions gotten these started without the airbox on after carb rebuilds in order to get the engine running to check the general condition of the motor. Not trying to bust anyone down just saying that if the carbs are in good order you should be able to start the bikes without the airboxes but you would definately have running issues at part throttle and above.
 
I won't disagree with dirtdigger that if the carbs are in order the bike should probably at least start. However, I get the impression that this is a new-to-someone bike that may not have been run recently so carbs may be an issue. If that's the case, it might be worthwhile to be sure everything is in place just to be sure. Could be it has nothing to do with the issue, but you have to start somewhere. In my case with my 900, the bike tried to start without the airbox on, but actually did start when I had it connected and sealed.

In the end to get it to run strong I had to remove the carbs and clean them thoroughly... and in my case once wasn't enough!!
 
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I've been doing some more work and checking and I'm starting to think it is a vacuum issue or something to do with the fuel supply line. My one issue is since I'm new to working on motorcycles, I'm not sure how much fuel should be running through the fuel supply line. I can see some fuel but not much and it doesn't seem to be moving at all. I'm just about to start looking for vacuum leaks now. I'll let you know how it goes. Thanks for the help!
 
I doubt very much that you'll see any fuel moving, especially since the bike hasn't started yet. You can check to see that the bowls are at least getting gas just by removing the bowl drain screws and confirming that gas runs out. Actually, I suppose you could look at your fuel lines to see that gas is moving, assuming that your lines are clear. You should get a decent flow with the drain screw out, but don't expect to see that same flow just from starting the bike.

In my opinion, you picked a pretty hard bike to work on as a first bike. I think the older 750s were much easier overall... again, my opinion. If you think you have a vacuum issue such as sucking air, you can check by spraying starting fluid around the connections at the insulators (between the carbs and head) and between the airbox and carbs. You can also use a propane torch like for soldering copper water lines (just turn it on and aim it at the possible source of a leak.don't light it! lol) If rpms increase, you have a leak. Problem is, the bike needs to be running and you're not there yet.

You've had the carbs off... how did the insulators look? Hard as rock? Any cracks? Are you confident that the clamps pulled them tight around the carbs?

When you twist the throttle can you see any squirt of gas in the carburetors? You should look closely into the throat of the carbs and see a small nozzle sticking up at the bottom... you should see a squirt of gas when you twist the throttle or your nozzle(s) might be clogged or the accelerator pump isn't doing its job.

I assume you've checked the plugs to be sure they're firing. Even though you've replaced them, if you've been trying to start it for quite awhile you might want to get another set of new ones. You might also get by with just removing and cleaning them with a wire brush or compressed air, or both, but don't overlook the fact that you could have fouled plugs.

I just re-read your initial post and see that you said you removed the "air cleaners"... are you saying that the bike has pods instead of the original airbox?

Have you thoroughly cleaned the carbs, or just removed the bowls maybe and sprayed carb cleaner in them? Here's a link to a carb cleaning manual available from another forum... a very good source of information. Pay particular attention to cleaning of the jets and carb body orifices. I actually won't recommend that you take the carbs completely apart at this point, but you can remove the jets and do the majority of cleaning with the carbs still left on the mounting bracket. This is a good read and you might get some feel for how well you cleaned your carbs. I wasn't very good at... I think it took me 3 times (or was it 6) to get my 900 running well, and each time after cleaning I'd swear I did a good job. Here's the link: http://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revF.pdf

From that same forum is a checklist for new owners of DOHC bikes. Maybe a bit of overkill, but another good source of information. Here's that link: http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=68364

I'm sure you'll find others who know way more than I do about these bikes, and my only experience with the 80's CB models was with a couple of 900's. Someone will surely chime in and offer some helpful suggestions. For now I've hopefully give you something to work with for starters... yeah, pun intended.
 
Wow, where was this forum when I needed it last year? I had almost exactly the same problem; new rider, bought a neighbour's 1980 CB750 Custom, and drained the battery 4 times just trying to start the damn thing. A friend of mine came over and got it started and now that I've done it a lot more, I've learned that it takes a little bit of 'feel' to get it right, at least on my mine.

I've also heard that this generation of CB750 (the early DOHC's) had carb issues, hard starts when cold, etc. As it is now, I can get mine started but it still takes a few minutes, unlike my Suzuki GS500 which always fires up on the first shot even after a week of sitting in the garage. The choke doesn't really choke that much anymore and once it's warm, it idles on the high side (just under 2k rpm) which I'm sure is not good. I plan on getting it looked at soon and might even work on it over the winter myself if I can get the time.

I agree with tommycarl though, it is a hard bike to work on as a first bike. I'm going to take the carb issues in and let the pros handle it for now since I'd rather spend more time riding it. I'm curious to know what you find out about your carbs though since it might be the same as mine. Good luck!
 
Try some starting fluid. Put the air cleaners back on, hose them a bit with some good ole ether. If it fires up on that but won't stay running you know you got a fuel issue, probably gunk inside the carbs.
 
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