Exhausting out the intake

[preachan]

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

[steveloomis]

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

[preachan]

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!

[Jens Eskildsen]

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.

[preachan]

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

[steveloomis]

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