81 rough idle, tough start, and wont rev...

[TheTeZ]

Ok, so ive got a 1981 honda cb750k

The bike didnt run when i got it, i was told carbs needed to be cleaned, So i did ... 12 times....

they are extremely clean, and now polished, everything inside is correct and correctly set...
I had another guy who works on engines and (solar turbines) check out the carbs, he gave them a thorough look through.

So when i go to start the bike, it ... does, bike starts, and will only start with the choke ALL the way on, it will idle a little high, if i try setting the idle lower it dies. The bike will idle and warm up, and start to rev up around 3500 or 4000 and i still cant take the choke off at all, otherwise it dies.

Now when its idleing, if i rev it it will rev up to about 5000 rpm's sometimes higher... and just die off, go to stall or sputter down then go back to its original idle speed

I am pretty sure its a vacuum problem. Im sure most of you reading agree...
I took carb cleaner and sprayed it onto the boots (carb-engine) and the engine lets out a nice smooth rev up, and durring that time i can gve it throttle, then of course it dies down.... So obviously its a leak there.  I put High heat sealant on the boots carb side, let it sit for 24 hours, went ot start it.... didnt get any better,r sprayed it again, still leaking, took the boots off, looked at them, plenty of dry rot, putting that sealant on both ends of the boots, and thats about where i am now... waiting 24 hours....

I am hopeing this fixes the problem, but incase it isnt, does anyone have any ideas?  

i just dont wanna go spending 80 bucks on the boots, cuz i also need to get a new starter one of these months, redo the fork seals, and do the breaks... oh and a rear tire... Besides that its nice.... No rust, and pretty good condition

[Polymer-2]

Those boots are sometimes a necessary evil. They shouldn't be so hard that the clamps wont compress them to seal. Also the aluminum spouts on the engine cant be really corroded.
When you rebuilt the carbs did you run a wire through the non removable jets?
It's a .0125 diameter jet and MUST be cleaned.

[Cricket]

TeZ,

Assuming that you're getting good fuel flow from the tank and your air cleaner is all good, sealing the boots might be the trick. When inspecting the boots did you see any cracks in the rubber? These can be fixed from the outside with rtv too.

When you sprayed the carb cleaner on the boots, were you able to find out if one boot was better or worse than any others? It would be odd that all your boots are jacked up.

One nice thing about doing a quick fix is that you will at least find out if that is the problem before investing in new boots. Although I would suggest getting them as soon as you get the scratch.

[TheTeZ]

The boots are cracked, near the engine side, all of them are dry rotted on that end,  i put rtv on both sides, near the engine and near the carbs this time, gonna go see if i cant get it running later at 3pm

i didnt stick a needle through, but i did make sure that fluid would dgo through all openings including those non removable jets, i ended up using compressed air to clean out some of the stubborn clogs.

My boots are pretty hard, and i am hopeing this fix works, at least long enough for me to know that this IS the problem, then ill buy boots when i get some money.  They are pretty hard, almost plastic like,  (but not quiet) I had a feeling they werent compressing with the clamp...   The engine part they connect to isnt rusted at all, the engine is in pretty decent shape (with compression readings flat across the board)  im really hoping this fix works, but ill find out around 3pm est

Thanks

[Cricket]

TeZ,

Those boots sound pretty messed up. Even if you were to seal them I would worry about bits of deteriorating rubber going into my engine.

I totally sympathize about managing the costs of repairs, but once you prove that the boots are the problem I wouldn't run the engine very much (if at all) until those boots are replaced. I can't imagine the boots are more expensive than having to repair your valves and cylinders. In fact I doubt any of your repairs are expensive as that.

Jimbo

[TheTeZ]

ha ha, your right, they wont be that expensive.... its just being out of work, ... leaves me with no money....

so i mean even 80 bucks for a set of new boots is painful to swing...

Well they are pretty much cracked, and the sealant is covering all that area, so i would be surprised if any rubber fell into the engine,

But at the same time, if this does fix the problem, i do plan on getting boots, Because incase i have to  take the carbs off again, it'd be nice to not have to re-rtv them

[TheTeZ]

Ok.... so what if that is not the problem? i sealed them up real nice... where else could that vacuum break be coming from..?

[DWightSperber]

If you are keeping the bike, I would recommend buying new boots. It made a world of difference for me. I got a set of 4 from bikebandit.com for $71.04 after shipping. In the mean time while you are still tuning, I'd recommend completely lining the inside of all 4 boots with a clear silicone or black rtv. This worked temporarily, but I had to coat the entire inside, not just where they seat on the carbs and intake.
My carbs were completely rebuilt with a stage 3 jet kit put in and k&n air pods. On really warm days, I can start the bike without the choke, but doesn't run 100% smooth until the bike idles for a min or two. If I start the bike with the choke, I just barely have to tap the start button and it fires right up and idles @ 3000 and slowly works its way down. Once the rpm,s level off (20-30 seconds), and I can push the choke back in, and the bike runs fine.

[TheTeZ]

so, i did get new boots.... and nothing, nothing changed, exactlly the same, their is nice new rubber,  and nothing has changed.... what else could cause these problems?

[danfr]

what about the other boots (carbs-air box)? How are they looking?

[Cricket]

Hi Tez,

Do you still get engine revving when spraying starter fluid around the boots?

The reason I asked is because I'm getting ready to sync my carbs and thought I'd try the same trick with the starter fluid. Getting revving on #1 and #4 boots. Made me think of you rtv'ng the daylights out of your boots.

I'm going to remove them and make sure they're seated properly. It looks like each one has to be positioned in a certain way and then well clamped. I noticed my clamps appear to be a little off of the lips of the carbs and intake manifold. If that don't work, what did you do with your old rtv'd boots?

I will say that even with my leaking boots, my bike runs fairly well. So I'm thinking your problems may be in your carbs proper.

Anyway, I know you've done a lot of work. I would hate to see you have to take it to a shop and get pants for a couple of hundred bucks... but that may be your solution.

What's your location maybe somebody on this forum is near enough to stop by and wrench on it with you? Do a shout out, what have you got to lose? Your manly pride, screw that, its riding season.

Jimbo

[blueboy2679]

i think your problem might be in the air box...their are a few lines for air intake, plus the boots inside.It might not be the problem...but its easy to check out and fix with sealant. Hope this helps.

[TheTeZ]

Well i didnt put them on, i dropped them off with my friend and he said nothing changed. so i assume yes.

The old boots are sitting near the bike hard as a rock.

The problem is definitely not in the carbs, I dont mean to be all "I know what im doing"  but i had a few people look at them, and one of the guys cleaned them, pretty sure that makes about 11 cleans.

I really dont want to take it anywhere, it would be nice to fix on my own, but honestly man, im out of ideas... in rockland county NY, if theirs anyone around who would like to give it a try, id be more than happy.

By the way i just remembered my buddy told me he thinks the spark is a little low, its not getting a very strong spark....  thats what he said anyway... which sounds like it may be the cause....

[free2ride]

good luck.

and while it might not be the carbs, what you are going through exactly describes what I wen through when I first got my bike. was told "runs rough on one carb". eventually took it in to a mechanic [I'm in awe of all you guys who do your own work] and all 4 carbs were basically rebuilt [don't know what the mech did -- just that it ran perfectly once he was done with it].

[TheTeZ]

i wish i had the money to bring it to someone, i'd say the heck with it, you fix it!

[OTTODOCTOR]

    I have to agree with the carbs. being most of the problem. -- Many small passages that must be cleaned out . -- Did you soak them in an actual carb. cleaning solution , and use high pressure air to blow them off / out ?? -- This is a MUST to get them REALLY clean .

[Grussell]

Hey Tez,

        I’ve got a 1980 CB 750 that I rebuilt.  I was having problem that sounds similar to yours, but not exactly the same.  It would idle ok and run reasonably at low speed,  but if I took it up to 4500 or 5500 RPMs it bogged down, and would die if I didn’t back off.  I spent over $750 in carb work, and it didn’t do a thing.

        I ended up cleaning off the metal prongs on all of the connectors for my regulator, spark units, the pulse generator, and my coils.  Then, I snugged up those connections and replaced the main fuse.  Somewhere in there was the solution to my problem.

        It may not do the trick, but if your connections look a bit worn, loose, or weathered it’s easy and cheap so you might try it out before you throw more money at the fuel/air system.

        Good luck man.  It’s a great looking bike.  I hope you get it up and running.

Cheers,
Greg

[OTTODOCTOR]

    Just remembered something about low voltage causing problems like you're having . --- Make sure your alternator is putting out at least 14 volts @ 3000 rpm . -- Hope this helps -- Otto

[TheTeZ]

i have cleaned the carbs with carb cleaner, didnt let them soak exactly, but i used compressed air and lots of carb cleaner.  every little passage has been cleaned and fluid flows through no problem. ive gone  through every little opening multiple times, and had a 2nd set of eyes do the same

[TheTeZ]

Yeah, my buddy thinks the sparks look a little weak, havent tried much yet, but probably the next thing i do.  Im probably going to end up selling it real soon, and just buy a bike that works... i just wanna ride!