bad overheating

[intruder]

gs 850 gt 1980 . engine overheating smoke coming from it . breather not blocked , smell of burning oil but no oil leaks , starts good when cold but difficult to start when hot . the ignition timing pick ups plate is pre set so theres no adjustment on that. the front camshaft was "jamming " in the head causing a build up of pressure that was blowing the cam end plugs out . so i sent the head off to a company that specializes in doing up cylinder heads , they re ground camshafts etc to make them run "true " in the head. upon rebuild i renewed valves, guides, seals ,camchain, gaskets , all seals, valve shims , even piston rings . so all top end is like new now.it starts and runs well until it gets hot ( approx 5 - 10 miles) whilst engine was red hot + smoking i removed the pick ups cover to try and turn engine by means of the 19mm nut on the crank ( to see if it was stiff ) it turn over easily even with the spark plugs in . so nothing seems to be locked or jamming up inside . i have been working on these gs engines for years but this has me baffled . any help / ideas much appreciated . thank you .

[keman]

How's your carb and intake set up?

What exhaust??

I know over heating can be an issue if you are running Pods on the intake, and sometimes with aftermarket pipes.

Have you looked at your plugs?? It sounds to me like you may be running too lean.

Have you changed your oil since the top end rebuild and ring break in??

When were the carbs last fully cleaned??

Did you do a vacuum carb sync?

Are there any air leaks? how are the intake boots?? Nice and soft?? or brittle??

Are all four pipes glowing, or just some of them.


Did you replace the intake manifold O-rings when you had the head done??

[Steve]

I am still trying to figure this part out.

... the front camshaft was "jamming " in the head causing a build up of pressure that was blowing the cam end plugs out . ...

There is nothing there for the camshaft to "jam" on, especially anything that would cause a buld-up of pressure. Any pressure that is in the cam area is also in the lower crankcase, and would be a result of blow-by from the rings. The breather on the top of the valve cover should handle any pressure build-up as it happens, keeping pressures very low, basically the same as outside the engine.

If your end plugs were blowing out, your rings are shot, your breather is plugged and your plugs are old and hardened.

But, it sounds like you have addressed all of them.

Now, to your overheating problem: A 'new', tight engine will run a bit warmer because of all the friction of new parts. However, it should not be as warm as you are describing.

Have you verified oil movement in the upper end? There might be just enough to keep it from seizing, but not enough to cool it.

Even though the timing appears to be 'preset', there is some adjustment, so you should check your timing to verify. Simply looking at something to evaluate its condition is seldom enough. User "Nessism" has a golden nugget in his signature: "To measure is to know." Keep that in mind as you work.

.

[BassCliff]

Greetings and Salutations!!

Hi Mr. intruder,

Yes, perhaps a little more information is needed. A lean fuel/air mixture is also my first thought. Has the intake or exhaust been modified? If stock, is the air intake system tightly sealed? Any air leaks will cause a lean condition. Check the oil levels in the crankcase and secondary gear case. Might as well check the oil in the final drive unit too. Forgive me if I state the obvious.

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[intruder]

spark plugs new , brown/tan in colour that signifies correct running , carb to head rubbers are new as are the sealing rings in them ,.carbs have been cleaned + blown through, . jets etc are new , carbs have been vax/balanced + are correct, inline filter fitted to catch any dirt that may be in the fuel but this is clean + correct way around .. the camshaft had jammed because of a slight twist in the rev counter worm drive this dragged the camshaft off centre this has now been sorted by the specialist cylinder head company. head has been re ground , cams re ground + re hardened + head skimmed. i ran some semi synthetic oil but it got red hot so dropped that oil + replaced with fully synthetic . same problem then i tried mineral oil same problem so it aint the oil . this is what has me baffled i have renewed all parts , plugs read a good colour , oil getting up to top end in good quantity, exhaust is a 4-1 motad but this has been on a few years with no problems. all this started when the `new ` head was fitted , so it was removed + sent away to be " sorted out " but on replacing it yet agen i have these problems . i suppose i could fit the old cylinder head + if problems are no more then it would prove that this `new` head must have something seriously wrong with it ?

[49er]

Just how hot is the oil getting? Have you taken a temperature reading? These old air cooled engines do get hot under elevated ambient air conditions. It is normal to burn your skin on hot oil, under normal running conditions. If your engine is "red hot", I would expect you to be experiencing detonation, which should be quite audible.

A couple of things to consider:

1. When an engine starts better from cold than when it warms up, it's a sign that it is tuned over rich on the idle circuit. Your plug colours sound as though your needle and float levels are ok though.

2. How much metal has been machined off this head? You could be running quite a high compression ratio, if it has had several resurfacing attempts. In stock trim, the 750/850 chambers should measure close to 35 cc.

3. Did you have the cylinders honed after fitting the new piston rings? If not, this could be causing poor cylinder sealing, and excessive blow by.

4. When pressure is building up to the extent that you mentioned, you are getting excessive blow by at the rings, or the venting system that is plumbed through your airbox is blocked. This is a common problem on the early 750/850 metal boxes, as the thin wide tract inside the box closes with rust. Other symptoms when this occurs include sudden oil leaks around the cam chain tensioner and the tacho drive cable seal.

I would re-check your airbox vent, because it should handle any blow by conditions that may eventuate. If it is good, I would check the cam cover internals to make sure that the area around the venting hole hasn't been inadvertently sealed off.

[keman]

I remember reading a post somewhere about someone resurfacing a head for better compression ratio and having an issue with overheating as a side effect. If I remember, he ended up adding an oil cooler, and drilling the oil access passages on the head to add a top end oiler.