After recently rebuilding my 84 XT600 it ran okay, just some popping on deceleration. The rubber intake boots that go from the airbox to the carb weren't mounted properly though and when i put them on properly the bike wouldn't start. I thought the air/fuel ratio was changed so started adjusting the pilot screw and idle screw but still no joy. The only thing that worked was starting the bike with the boots not fully on then trying to inch them on but the bike would just die with them on. I noticed the carb making very slight in-out movement in sync with the engine which got greater the more the boots were on leading me to believe I had messed up the timing and the intake valves were open on the exhaust stroke. The exhaust also got smokier the more the boots were on and the carb was also getting warm. I figured with the boots off it gave it enough room to suck in air while also letting the exhaust out but with them on there was nowhere for the exhaust to go.
Took the head off today, removed cam chain tensioner and front cam guide, checked the timing and it's spot on!

All I can think is the intake valves aren't seating properly but I did grind them when rebuilding and although I hadn't done it before I think they were done okay.

I've spent a long time at it now trying to get it going and I'm totally stumped! Any suggestions?

P.S. Engine was getting really warm/hot - after a 30 km drive averaging 80kmph the cylinder was over 100° C - oil is flowing good tho.

P.P.S. Also, bike's used (burnt?) too much oil since rebuild

You say you ground the valves, did you do the valve seats too? Did you lap the valves in with valve grinding paste? Did you pour gasoline in the combustion chamber with head upside down to see if any leaked thru? If any gas leaks thru the valves then you are not done lapping them. They MUST seal tight.

I just had a local bike shop regrind my intake seats because of pitting, the intake valves were pitted as well. He said NEVER grind the valves, but replace them. Didn't say why but I expect there is not enough material to grind away safely. He replaced the valves and lapped all the valves.

When it came to starting, it fired 3rd kick and ran perfectly 4th kick.

Other info:
Since the USA carbs fuel adjustment is plugged with a brass plug, and because I did not even know of it until I read about it, how it works and how to adjust it on one of these forums, I'd never adjusted them before. Yesterday I adjusted the 84 Xt outwards for peak rpm and turned it back about 1/4turn. Now the pop, pop, pop backfiring is GONE. I have to adjust my 86 as well as it like to pop too while blipping the throttle. I had to reduce the idle rpm from 2000 back to 1200 it made so much difference. Both sets of carbs came from eBay, I rebuilt them after a Chemdip. All rubber parts were replaced including the shaft seals in the primary carb, all jets etc had been removed including the brass plug on the enrichner. I really did not know where to set the lo speed setting so I set it at 1 1/2 turn out, seems it needs to be at least one more turn out from that.

These forums are great sources of information from some really great people that like to share in their successes and failures...

Steve

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

Hi Steve, I thought the terms grinding and lapping were interchangeable, looked it up there and what I actually did is lap the valves ie spun them in their seats with grinding paste. Didn't try the trick with petrol in the upside down head, will take it off now and give it a go. Why petrol? Won't it start evaporating quickly looking like it's leaking through?
Re: the carb adjustment - yeah I was just thinking today what did people do before google and forums? I had to look it up here to know what to adjust!

Sounds more like a carb/airfilter problem than an engine problem. Check "easy" things first, like clean airfilter and carb, make sure the carb is setup like stock, and not altered with other jets, ect.

Air filter is new, I removed it and was going to slowly put it back in as I adjusted the air-fuel mix on the carb when I thought the mix had changed by putting the boot back on but even with the filter completely out it was doing the same when I put the boot on.

edit: I'll double-check the carb now though

Other than checking for leaks you are good. You are not looking for the gas to disappear, you'll actually see it in the intake and exhaust passages if it is leaking. Gas is very thin and is a good test, do it outdoors (there, my disclaimer is there) don't want you burn down your home.... Won't take long as you will see it right away.

I'd certainly check everything I could before pulling the head off again. Did you check the vacuum line from the primary carb to the secondary? There is a really short line between the carbs at the front of them. Did you check the diaphram on the left side of the left carb under the round cover? This is the deceleration backfire circuit. The diaphram can go bad. What this does is the diaphram is connected via the small vacuum line going over the top of the secondary carb to the intake tract right at the head and senses hi vacuum from high rpm and throttle closure. This high vacuum is routed to the diaphram under the round cover and pulls the diaphram which in effect closes off air to the pilot circuit making it run rich. A rich mixture in the exhaust will inhibit a backfire. Anyway if this circuit is leaking air or any of the vacuum lines are rotted may contribute to the overall problem.

I just rebuilt 2 sets of these carbs and both sets are in service with very good results. I just finished a top end on my 84 XT as I mentioned in my signature and have been riding just this morning an a extended tryout ride in the country on dirt roads. Once I got the oil leak stopped on the filter cover I am just looking for anything else that needs attention as my son's and I are going to Ouray Colorado in Sept.

Your oil burning is a worry, did you put in new rings, check piston clearance and hone the cylinder just a bit so the rings can seat? This is necessary or the rings may not wear in and seal properly. If not, you can still do it and try again.

I am mentioning things you probably know just to cover all bases, please don't get offended. There are 2 seals on the right side of the cylinder between the crank case and cylinder and between the cylinder and the head that seals an oil passageway going to the rocker box to feed the cam and rockers.

I assume you adjust the valves, I set mine at .003" intake and .005 for exhaust. I had to reset the exhaust after riding a bit, I think I was not top dead center when I adjust them as they were both loose about 3 thousandths each. I reset them making sure I was right and they are fine now.

Lets us know how it goes, we'll help where we can.

Steve

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

Line checked and clear

Checked diaphragm and it looks a bit battered but is fully intact.

Didn't put in new rings, piston and cylinder were in okay condition. Left as-is to return to if necessary and when I had more money!

No offence taken at all, thanks for all the help! Information is never a bad thing. I was aware of the passageway alright when doing the rebuild, it was clear and o-rings inserted. Is this what you mean by seals?

Valves adjusted and readjusted, no harm to do them again though.

Have the carb apart and cleaning it now. My primary needle says 5C35 (manual says 5C38) and circlip is on the middle notch. My secondary needle says 4A70 (manual says 4A72) and the circlip is on the bottom notch. Will reassemble carb and check fuel level, check the valves for leaks and report back.

I just say when Jens has done 150k with same valves i dont understand why bother doing anything to them, seems like you can do more damage trying to fix things here. What happens when you have new valveseats and old valves, no go or?

Okay, I had to leave the carb for the time-being as it was getting dark and I have to work tomorrow and won't be around the bike so I took the head off, turned it upside down and filled it with petrol. Left it 20 minutes and the level had dropped a few mm. I haven't done this before but it seems if there is a leak I don't think it's big enough to be causing the venting out the intake valves. Correct me if I'm wrong but if not at least we can rule it out as a cause of the problem.
I'll take the carb to work with me tomorrow and check it out.. Right now...

damn, forgot these pics of the head & cylinder when i took them apart...

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That piston crown has been washed by excessive oil leakage past the rings.
Often if the rings are worn and not replaced, but the valves are renewed or ground, this will increase the vacuum within the combustion chamber because the valves are now sealing well, and cause more oil to be sucked past the rings.
That would explain if your oil consumption was say moderate before replacing the valves and now is excessive.
Now you have the head off is a good time to renew the rings & hone the cylinder if it's not too worn.
But first check that your crankcase breather is not blocked as that can cause the crankcase to pressurize and force oil past the rings.

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Remember that Assumption is the mother of all f**k-ups.

Have you done a compression test, a good engine will run 150 to 170 pounds of compression. You have to disconnect the compression release. The range is from 128 to 171 pounds at sea level, average is 156 pounds.

Measure your piston skirt to cylinder wall clearance with a feeler gauge. It should be approx. .045 to .065 mm. This is not the exact way to do it but will give you an idea of how worn your cylinder is. If the clearance is close to this, then a light hone and new rings will help raise compression to a good level. If your piston is pretty loose then it will obviously need boring to the next over size with a new OEM piston and rings.

My 86 xt has 170 pounds as measured with a new gauge. It has never been apart and has maybe 13000 miles on it. I just rebuilt an 84 xt that has been sitting for 25 years or so and has 9000 miles on it. Piston to Cylinder wall was right in spec, new rings and a light hone and it is running well, no smoking. I have not checked the compression yet as I wanted to get the new rings broken in, they are now so I will check it soon. From starting, even with the compression release it feels like the 86 and is likely close to 170 pounds.

Let us know how it goes.

Steve

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

That makes total sense, valves weren't in great condition before rebuild so that could be exactly what's happening. The breather was kinked before but I gently heated it with heat gun and 'massaged' it into nice round shape again, it's after re-kinking a bit since installing but should still be plenty of space inside for gases to move.

@steve I don't have a compression tester, I did buy a cheap 'motorcycle' compression tester off ebay a few months ago but it wouldn't fit the spark plug hole (surprise surprise), I might be able it adapt it to fit.

I can't get back out to the bike til Friday morning now but will check the rings, cylinder then, if needed I can bring it in for honing.

So, that could solve my high oil consumption and smokey exhaust but still not my other problem. I'm happy timing was correct and we can rule out the valves not seated?

That leaves the carb which I'm about to get stuck in to.
Clymer manual shows specs for carbs with ID mark 49N 00 & 49R 00 (XT600) and 34K 00 (TT600 83-83) and 55U 00 (TT600 85-86) - my carb says 34L 00. This thread has specs for a 43F carb but I can't find the specs for a 34L carb, anyone have them or know which one I should be following? I can't find reference to which notch on the needle to put the circlip.

One other thing - turning the crank for adjusting the valves it felt like it wanted to 'pull' past the timing mark. As I was doing it with a rachet spanner it was difficult to get it dead on without it overshooting by a degree or two. As the crank turns twice for every turn up top that would only be half a degree or one too much that the cam sprocket rotated. And (if my brain is working) I think this would only delay the intake valves opening not open them early. Nonetheless, I'll turn it with a non-rachet spanner after reassembly to rule out valve adjustments as a problem. Plenty of work to do before that anyway!

I just had my top end off so I marked the flywheel (since I could not get it off) where the horizontal lines on the cam sprocket were even with the gasket surface of the rocker box and the dot on the sprocket is up. This way I know the cam timing is dead on the same. With the flywheel off there is a mark on the crankshaft that matches a corresponding timing mark. With all aligned as above you know your cam is correct and not one tooth off.

I hope you can get by with just new rings and a light hone...

Steve

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.

Could you not simply use the TDC mark on the flywheel?

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Yamaha XT600E '01, Honda XR400 '03, Yamaha XT550 '83

I would think so, I used my marks so I know it was right.

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'84 XT600 is now bored to 2nd oversize and new OEM pistons and rings installed. No more smoking.