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1982 Honda Magna VF750C w/40,800mi

Bike's been giving me some running issues I can't figure out. This past summer I took her out for a ride over the weekend to church and approaching a stop sign, RPMs suddenly jumped way up and wouldn't come down. Long story short I had to trailer the bike home.

Since then I rebuilt the carb, and made sure everything is moving as should, nothing sticking, but I can't get her to idle at normal 1000 +/- 100rpm. The moment I start her up she immediately jumps to about 4k and won't come down. I've tried pulling back the idle adjust screw ALL THE WAY so that it's at the very bottom of the adjustment and no longer raising idle speeds. Tried setting the pilot screws all to the recommended 2.5 turns from bottom, and now even to just cracked open about 1/4 to 1/2 turn and neither do anything at all. She still screams at 4k RPM the moment she starts up.

I thought perhaps the low compression is caused by perhaps a blown head gasket near an oil hole, but so far just can't figure anything out. Just wondering if anyone might have any ideas as to why I can't get her to settle down.
66491
66492
 

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With the rebuild, did you replace the CV piston/diaphragms? It could be a leak in one or more diaphragms, or a sticking piston.
Low compression could be valve tappet(s) adjusted too tight, keeping the valve(s) from closing all the way. Was the engine warm when you did the test? Throttles held open all the way?
 

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What you described sure sounds a little bit like an intake vacuum leak that is getting fuel from somewhere.

On the low compression, when was your valve clearance ever checked?

... is this one of those Honda's that has a fuel pump to feed the carburetors?
Make sure it is not introducing air as well.
 

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What you described sure sounds a little bit like an intake vacuum leak that is getting fuel from somewhere.

On the low compression, when was your valve clearance ever checked?

... is this one of those Honda's that has a fuel pump to feed the carburetors?
Make sure it is not introducing air as well.

I know the previous owners had stated that it had valve work done on it somewhat recently while they had it, but they didn't have the paperwork anymore to show what all was done. I'm going to attempt taking apart and looking at the carb again. Perhaps I accidentally missed something or didn't quite align something right when it was put back together. Other than that, I haven't been able to see any leaks yet. Perhaps there's a way to do a version of a smoke test on the carb? If I can figure something out or make any progress with it I'll make an update
 

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Here's the thing with valve clearance adjustment; When your bike is new everything is set theoretically correct. With normal wear on the engine your valve clearances will tighten as the valve seat becomes depressed as a result of the valves banging up against the valve seat a few million times. Now when your engine gets hot your valves do not close completely or at the right instant. The engine becomes harder to start, combustion compression and intake vacuum are reduced and your bikes runs tardy.

Yes, check the carbs again and make sure none of the slides is installed incorrectly. Look inside the carburetor throat with the air intake rubbers removed and see that they are all closing when you release the throttle. You would not be the first person to install a carburetor slide rotated 180 degrees from where it should be.

Re: Vacuum intake leak (smoke test) you are very close, if you spray a little quick start (ether) around the intake manifold where there is not suppose to be an intake hole and there is a vacuum leak there, your engine will rev like crazy once it starts to draw starter fluid through any leak.
 

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Was the valve work perhaps related to camshafts? I know the early V65 engines had cam problems related to poor oil supply to the front cylinders. I wonder if the V45 suffered from the same problem?
 

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Discussion Starter #7
Here's the thing with valve clearance adjustment; When your bike is new everything is set theoretically correct. With normal wear on the engine your valve clearances will tighten as the valve seat becomes depressed as a result of the valves banging up against the valve seat a few million times. Now when your engine gets hot your valves do not close completely or at the right instant. The engine becomes harder to start, combustion compression and intake vacuum are reduced and your bikes runs tardy.

Yes, check the carbs again and make sure none of the slides is installed incorrectly. Look inside the carburetor throat with the air intake rubbers removed and see that they are all closing when you release the throttle. You would not be the first person to install a carburetor slide rotated 180 degrees from where it should be.

Re: Vacuum intake leak (smoke test) you are very close, if you spray a little quick start (ether) around the intake manifold where there is not suppose to be an intake hole and there is a vacuum leak there, your engine will rev like crazy once it starts to draw starter fluid through any leak.
Thanks, I appreciate all the input and advice. I'll take a look at all and see if I can't make any progress on it on Sunday if possible. I was fairly certain I had gotten it right, but it's easy to make mistakes when you're tired and are getting frustrated with parts.
 

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Had to field clean a mikuni carb seven times in a row, on the eight attempt it started working great again.
... swiss army knife and a thorn bush (y) always carry one. The knife, not the thorn bush.

Ripping apart a friends Beta tomorrow morning, something boogered inside the kick start. **It happens
 

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Ya know on some bikes, you can just suck on the little black rubber hose or spigot attached to the top of the carb and the carburetor slide will go up and down
... jus sayin ;)
 

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Update: finally got her running again, and man does she have some power compared to before. With the way she rode I don't think there's anything wrong engine wise with the exception of some bad seals, second gear is a hard shift (clutches are likely very worn) and it could use new plugs and wires. I had to clean the spark plugs off with carb cleaner and very gently using some 400 grit sandpaper to clean the terminals on them. I think the throttle cables have stretched a little over the years, I got it so that it idles perfectly at about 1100rpm if I hold the gas forward (as if to remedy a sticky throttle), but the moment I let go she rides up. From what I can tell it seems to be that the main carbs air luvre is stuck just slightly open. Perhaps I didn't quite put enough turns on the throttle pulley spring when I reinstalled it, though I thought I had it enough as I remember a couple times I accidentally let go and it spun around in my hands giving me some harsh reminders on my knuckles).

All said and done, she still has a few ticks, but she runs strong, and is ridable. Which mean now I am left with the final unfortunate task on the list of seeking to sell her to someone else who could enjoy a bit of a project bike, and perhaps have more luck than me.
66670

Any thoughts on a good solid bike to look for around the $2000 price range? In my area there seems to be an excessive amount of the Yamaha V Star 650's. Not quite my style, but if they are fairly reliable, hold their value and pretty easy to find parts for if be willing to look more at them for the time being until I can find something better.
 

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So you're saying your carburetor slides are sticking for sure
and maybe your cable routing is in question or cable pinched.

Hard shifting is not a clutch issue unless it's dragging, that's shifter forks and drums issue.
Has the bike been crashed on the shift lever or has the lever been stomped on lots?

Hydraulic clutch, if it's dragging or mushy it has air in it.

Is the right fork seal shot? looks it.
 

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Your cables are at the least not adjusted correctly. With a push/pull cable your description says exactly that.

Ditto on the fork seals. Order some new springs from racetech while you're at it. Theyre cheap. Call em. The difference will be night and day.
 
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