Petcock Vacuum

Hey guys what's up, I have an 82 750 Maxim that I'm dealing with. I got it running and it sounds normal, but it has no acceleration Power and it will not idle. I've rebuilt the synced the Carbs, replaced the Valve stem seals, and have the valves adjusted with no change. I was all outta idea's when I discovered I have a Broken Petcock Vacuum. I've ordered a Replacement, but I've never delt with a Vacuum Petcock before, could this have been the problem this entire time? What kind of problems is no petcock vacuum known to cause?

 

It depends on where the break is. If it’s the vacuum diaphragm you’ll get excess fuel in one cylinder (and possibly into the crankcase). If it’s the vacuum line you’ll have a big vacuum leak in that cylinder. In either case she’ll run like poo, exactly as you describe.

 

The Vacuum Prong on the Petcock has been broken off, so there's no Vacuum line running from the Petcock to the Carb boot. There just the Fuel line running to the Carbs.. Thank's for the Knowledge Brother. I appreciate it.

 

Pity you never made clear that bit about the broken petcock barb earlier, would have saved some guessing.
And, yes, unless you plug the vac line to the inlet rubber it will run like crap. However, this would generally only be at low speed, once on full throttle I wouldn't expect it to make a huge difference. However, I'm not sure if prime allows ll

 

i think you loose the reserve setting, thats good to have

 

Sorry, still having freezing up issues..
I meant to say, not sure if prime allows full flow? Even if not, I would expect full performance for a little while before the fuel in the bowls drops off.

 

Prime allows full flow. Remove you fuel line and try it.

 

I'll take your word for it

 

Of course you would need to add a temporary line to empty into a container. No one need gas all over the engine, frame and floor. A good way to make sure no fuel flows on reserve or 'on' positions too.

 

could always set to prime and ride..
there is a thread on converting petcock to non vacuum

 

New Petcock arrived, And now it will idle with the Choke on. But with no choke I have to apply slight pressure to the throttle or it will instantly stall out. And it still feels very weak at higher RPM's.

 

I had brand new Plugs in it, and I checked them after about a half hour of accumulated running. And the No 1 plug was a little dark but looked pretty good overall. The number 2 plug still looked pretty much brand new and according to it's pipe it was firing. But 3 and 4 had heavy black sooting, and looked to be close to fouled.

 

Oh, and I thought I should mention that the factory exhaust was toast when I bought it. So it now has a 4 into 1 header with a slip on muffler. It's running the factory airbox with a new filter..

 

Alright, found my issue. Been studying up on syncing the Carb's. And the pilot screw in my in my number 3 Carb is seized and stripped out pretty good. And that's the piston that has the brand new looking spark plug with no sooting. So I'm guessing that it's seized all the way closed.

 

Been doing some research on my power loss at high RPM's, And I'm pretty sure my valve's are floating. It's the only thing making sense at this point. So it looks like I'm going back in...

 

So you've gone from not running too good to now running so good you think it's valve float.
Sorry, Jerrod I can't agree with your diagnosis.

Where are you with this?
You need to extract this pilot screw. Sounds like you might even need a replacement carb body for #3 if you're not successful.
Get that sorted. If it was me I would start with a complete rebuild of all 4 carbs.

 

Sorry Jerrod, I just went back to your original post,

Explain "rebuilt the synced carbs", if you rebuilt the carbs you would have spotted #3 pilot screw seized then.
What has been done to the carbs so far?

 

Yeah, I asked the guy I payed to adjust my Valve's and sync my Carbs a very similar question. He's not responding.

 

He had the Bike last Fall for like 2 Week's cause he said he had to order shim's. I bought the Carb rebuild Kit myself so he probably put the new Jets and Gasket's in it and called it a Day. But Im not sure cause he won't respond.

 

The Float Gasket's were replaced, and the screw that was seized was Stripped too. Which I think he did, but instead of telling me he couldn't get the Screw to break loose and working out a refund. He just gave me the bike back and kept my money.

 

Yep, it’s not so much the screw needs adjustment, it probably does but until you remove it you can’t properly clean the idle circuit there. If you take care you should be able to drill into the screw and try a screw extractor.

 

I was able to free it up last weekend with a Propane torch. And just did the adjustment myself. It will idle all day now, without any Choke. But still no power at high RPM. I'm almost positive my valve spring's are worn and their floating at high RPM.

 

It feels like it has no compression above like 4k. Has no trouble reving, just doesn't accelerate it's a really weird feeling.

 

Only problem I'm having now is finding a set of Valve springs. Guy's have like 1 or 2 for sale on eBay. But it's starting to look like I'm gonna have to buy a Head to get a whole set.

 

I would hold off on the head purchase, make certain the carbs are fully sorted.
This you will have to do yourself or send them to a trust worthy source like @hogfiddles
After that you need to verify the fuel level in the bowls with a wet set.
Next is a running sync, small adjustments can make a big difference.

The valve train in these XJ's is good for 10,000 RPM +, you won't see valve float until you're beyond that.

 

Are all 4 pipes hot at idle?

 

Yeah, it's fireing on all 4. It pass's the Ear test. Rev is very responsive and quick. And it doesn't hang at all since I tuned the Carbs. It sounds great. While riding, it has good power down low. But once above 4 or 5k power just disappears. The motor is not struggling though. It's reving normally, you just don't accelerate if that makes sense. It feels really weird.

 

check your jets location fuel and air.
are your emulsion tubes clogged.

sounds like the carbs need to be cleaned. cant count on him having done it correctly

 

All of the issue's at Idle have been sorted. And I have an even build up on all 4 plugs now, slightly dark but acceptable considering I've never tuned Carbs before lol. It will sit at idle all day.

 

But the drop in Power at Higher RPM has not changed at all. I read that your valve springs can get worn out, and it will Cause float at lower RPM then normal. I'm not positive this is my problem, but it's my main suspect at this point.

 

Forget your valve springs, listen to @Simmy..
Carbs

 

Is your clutch slipping?

 

put a number on "higher rpm" same in all gears?

 

Mine has well over 100K on it and I knew one with over 300K up, neither of them suffered from valve float.

 

The guy I bought this bike from roached the original Motor by downshifting to 1st on the highway I guess. Not sure how you do that, but that's what he said. The current engine is from a wrecked bike he bought as a parts bike. And he apparently has never had it running with this motor in it. I gave next to nothing for it though

Reasonable suggestion, but i don't think so. It's hard to explain, best description is it feels like it looses compression above 5k. The Rev sounds normal, but there's just nothing there.

From around 5K till I get out of it probably around 8 or 9K. And yes it starts at the same point in every Gear. It seriously feels like I loose all Compression above 5K, and it just stops pulling all together.

 

Im sure, but i think we can all agree that 2 of the exact same engine's can end up in very different condition 40 year's after they rolled out of the Factory.

 

I could definitely be Wrong about this, but after learning the symptom's of Valve float it has become my primary Suspect. It's just too close to ignore. But I have no issue putting it on the back burner to insure I've covered every possiblity.

 

Compression can't dissapear, valve float sounds horrible, like a million hammers banging away in the headunmistakable. You have an issue with your carbs - or the tci, forget valve fl

 

My point being, that age and normal wear and tear don't seem to affect Yammy valve springs much.
If the PO had over-revved it enough to damage the springs, all the valves would be quite well bent or trying for freedom out the top of the head.
What you might have is a few slightly bent valves, but if your compression is ok, it's not really likely.

 

Okay, that's a relief cause I didn't wanna pull the Head apart again. It's funny that you mentioned TCI because I lost Spark the night before last. The Bike came with a box of Parts that included a Stator. I installed it with no change. I ordered Coil's and they will be here on the 24th.

 

Did you test the coils first, or did you just make a WAG and buy parts?
If you just buy parts you'll be taking a long and expensive trip to having a reliable motorcycle.

The stator has nothing to do with the ignition system, except to make sure that there is voltage for it to function.

Please stop swapping out parts and start diagnosing to see if they need to be changed or not.

 

After replacing the Stator did nothing I kinda went off assumption. They were only 20 bucks. And an extra set of Coils laying around never killed anybody.

 

But I made a trip over to the guy I bought the Bike froms place tonight and brought the Original Engine home. Plus a whole bunch of parts. The OE is missing 2nd gear but runs good I guess. I have another rack of Carbs several Petcock's including what appears to be a Gravity feed. The other 2 look to be factory. But they both have RES which my current Petcock does not

 

The box he gave me has a single Coil in it lol. But I have a spare set of almost everything now. Apparently he bought a crashed parts bike. And the motor my bike currently has came from it. But the whole top end on the Original motor is supposed to be Good. I'm about as happy as Campers come right now lol.

 

Ok guy's, so I learned about the high speed Stator Coil last night. So I decided to go back over my Stator install real fast and found out that the little plastic piece that screws into the Stator cover was actually backwards. Which obviously is not gonna work. So I installed it correctly. And I'm currently at moment of truth part 2 lol. I know this was a problem, hopefully it's THE problem.

 

go up one size on the main jets

 

Alright.

 

So I've done some more reading, and learned that the Chance Bad Coil's are responsible for 4 dead spark Plugs on an XJ-750 is slim to none. And seems how I replaced the Stator the only thing left is the Ignition Module. So it looks like I have a spare set of Coil's showing up later this week. But that's ok, from my experience riding old bikes so far, a spare set of Coil's ain't a bad thing to have.

 

Fault finding by replacement is ok if you have known good replacements to hand....
Oh, I'd go up at least 2 main jet sizes for a 4 into 1 straight through. Might not be right though, just for starters.

 

Well, I noticed all 4 of my Plugs were dead the other Day. And it seemed unlikely that both Coil's would go at the same time. So I ended up ordering a replacement Ignition Module and it will be here tomorrow.