Topic: RC motor heat seizing..Anyone care to guess????

[scondon]

I.M.O.,...mechanical seize problems don't fix themselves after cool-down....too lean a fuel/air mix might...if yer lucky. I'd be looking at that again real careful...I.M.O.

   That's why this one has me baffled. It gets super tight when hot, yet still runs fine. Mike's got his $ on compression, but I have a feeling that I'm going to pull the plugs to no effect. Bike has been ridden around for 1/2 hour rides on several occasions and will idle at 1000rpm when the ride is done, just don't shut it down. No smoke from pipes, no instant lock of engine when ignition is cut, or any other symptoms that I would correlate with a piston near seizing point or damage.

     I've been inside these motors and know most every detail, and have experienced failure of components of varying degrees(seizure, transmission, damaged bore/piston, rod and crank failure, clutch issues,cam failures, etc..), but this is just plain weird. I'm sure I will be pulling the top end apart, but I'm also trying to think of all the points in the motor that could cause binding, yet cause no obvious damage,just in case the pistons and jugs check out like the bottom end did(pristine).

     I don't know whether RC used teflon wrist pin buttons, that's an unknown factor for me since I've never seen those. RC rods have a insert on the small end? if so, could it be damaged or spun yet still function? yet another unknown for me. RC lightened rotor, did they modify the crank end or starter clutch?

    Anyway, you get the idea. I'm just trying to think of what else, besides the usual suspects, could be causing this symptom since it's like nothing I've encountered before. I'm just afraid of getting the top end apart to check bore and clearance, and having no more alternatives in case they check out fine. My money is on something inside the cylinders because it's the only thing I haven't checked out yet, but I don't see how pistons could get this tight without seizing or at the very least causing the bike to stall at low idle when hot.

   And Mike, I've never dealt with compression lock either, so yer $$'s still good until I yank those plugs hot

[MRieck]

 If you build an engine with a lot of compression it will be very difficult to turn over when hot. If that engine has 12:1 pistons plus head milling and cylinder decking you could be around 13:1. Long duration cams make it easier to turn over especially when the engine is cold but even the cam can't compensate for too much compression when the engine is hot. Trying to spin a hot, high compression engine by hand can be almost impossible as the cycle is so slow the engine can't bleed off pressure fast enough. If the bores are to tight you can run into problems when hot but I'd start with a possible compression issue.

[City Boy]

Hi Scondon.I have exactly the same condition with my 1000 Cobra.It has done this ever since it was built.If I stall the engine it will not turn over with the starter but will instantly start with the kicker.A delay in restart of even a few minutes and the starter works fine.I stopped worrying about it .I think the high compression,higher heat retention at low road speeds,and a starter being asked to turn over an engine 25% larger than before are the root of the "problem".The only inconvenience I find with this setup is that if I get caught in a traffic jam,I must seek another route or park as I feel seizure is a definite possibility in those instances. Rock On

[1080]

I agree with Spanner1, tell us the jetting being used.

I agree with City Boy as well. Slow traffic is not good for these Big bore engines.

[nippon]

Hi,
in order to turn the crank over the dead point, press the starter button. At the same time kick the
kickstarter softly. Normally the crank will move, the kickstarter will move down step by step, too. Now you should be able to kick completely. I had this problem a year ago and i was not able to turn the crank with a nut on the ignition side.
Readjustment of the ignition was the key for me. Use a flashlight and check your timing again.

No R.C stuff, just a 836 engine.

nippon

[754]

My biggest concern is that it was set up too tight on the bores, if it is fresh.(or from origan boring if it was not run)

Older forged pistons ran fairly big clearance while some made today, run tighter. (just saying , in case a decision was made to run it tighter than it was made for)

Next, it is still cool out (LUCKY, cause HOT outside is far less forgiving) .., if it is tight, it is probably scuffing the pistons. ,, if this is the case, repeating it is very risky..
If you are LUCKY, you may save piston & bore.. if not, you may  be looking to resleeve as a fix or replace pistons.. both costly.

This may help, but its a bit of work, figure out actual chamber volume, and see where you are at..ie.. if it is 13:1.. then you could cut a fair amount off the pistons to fix it..

[City Boy]

Hi Scondon.As to the rods,RC steel rods have a bronze small end insert.The golden alum. rods which are in my unit have no small end insert but do have a pin in the big end to prevent bearing turning.The 1000 pistons use wrist pin clips.    Rock On

[PJ]

I think I would take a very close look at the plugs with a magnifying glass. If their bone white or have silver specs I would not restart until you have made adjustments. Take it from someone that made his own Nitrous "kit" out of soda fountain solenoids and a holley 1" spacer plate in 1989. Not an expert on these things but if after it cools it turns freely I would think that it's a very high compression issue.

[NitroHunter]

My big bores were also hard to turn over when hot, but the kick starter worked every time.

There are great on-line engine calculators that can give your true static compression ratio, based on engine bore, stroke, rod length, cam timing, and observed cranking pressure. Just play with the numbers. You might try this one:
http://www.rbracing-rsr.com/comprAdvHD.htm

For what it's worth, I just looked at some 1980 RC F-2 piston installation instructions, it calls for .0055" #1 & 4, .006" #2 & 3, add .001 for sustained high RPM use. This is similar information to what I was told by Venolia pertaining to turbo / blower pistons. The low silicone content pistons grow more, but are stronger in high heat applications.
The RC pistons in my GS 750 called for much tighter (.0025") clearance.

[jonbuoy]

Hot starting issue's not uncommon with charging/startermotor failure/voltage drop - I had  very simillar symptoms with my VTR1000.  You don't get much leverage trying to turn the motor over at the crank by hand.  If it was hot siezing from metal on metal contact wouldn't the engine sieze up underway?  I know two strokes can recover from a sieze up by being left to cool but  the motors usually struggled to turn the back wheel by then 

[jhasewhite]

Has anyone thought of a too small ring end gap? It could be the problem..The fact that the engine is "soft seizing" points to rings, as too large pistons seem to stick solid to the bore when they reach zero clearance.

just my $.02

-j

[scondon]

 Well Mike, you didn't win the pool this time, but your $$'s still in if you wish another crack   And thanks to everyone who's added their two cents. As usual, really good info that helps me get a clearer picture

     Wednesday:

    Went to the shop and did a cold compression check, 155-162 range across all four. Spun motor by hand with all plugs removed to get a feel for tension, a bit tight compared to what I'm used to but not too worrisome.

     Owner of the bike requested that I give it a spin and get the motor "hot" for further diagnostics. Fine by me, as long as I have permission from someone who knows the risks. I set out through city streets and stoplights(good way to get hot without too much rpm), and made it to a 2 mile strip of frontage road. Engine good and warm, I "hot lapped" it back and forth in 2nd-3rd gear. Great power with wheel lifting capability, rpms up to 8000, about 5 miles of abusive riding all told, then turn around and head back to shop. I revved open, then closed throttle all the way back, giving as much heat abuse as I could. Remember, I had permission from the owner

    Rolled the bike back into the shop, and before shutting down I closed the throttle as far forward as possible bringing the rpm's down to 850. Not even a hitch or hiccup, engine kept purring.

     Pulled all plugs and did a hot comp test, same 155 average across all four. Plugs out, I tried to spin the motor from the crank and it wouldn't budge. Starter motor strained and wouldn't budge it either, kicker the same.

I fully expect that the motor should have seized from the abuse if this is a piston/bore clearance problem. Not a compression issue since "plugs out" makes zero difference.

  Starting from scratch the symptoms are: Motor very hard to spin when hot, either by starter, hand or kicker. This is what's "known". My intuition keeps pulling towards a starter clutch issue, but I can't quite grasp what type of failure there would cause these symptoms.

   I'll be pulling the rotor before pulling the engine, and I'll get to the bottom of this one way or another. Just posting an update

EDIT: changed that last rpm report to read "rpm's down to 850" instead of the "8500" I tapped out

[754]

If crank was previously used and run without dyno, they may have plugged (or welded), the oil hole for the E-start gear.

 Or it may have just got plugged up, worth a look.

 If the bushing was changed or damaged in the starter gear, it may tighten up in use..

[scondon]

If crank was previously used and run without dyno, they may have plugged (or welded), the oil hole for the E-start gear.

 Or it may have just got plugged up, worth a look.

 If the bushing was changed or damaged in the starter gear, it may tighten up in use..

   ...And look I shall. Thanks 754

[cafebob]

I like the clutch idea a lot.  It is a easy thing to check out.  I have seen other bikes the have clutch binding issues.

Also, you said the tappet adjusts had fallen out. I'm not sure if you rebuilt the motor after this was fixed or not?  Did you check to see if all is well in the top end? 

[paulages]

been watching this thread, waiting for the answer...

anyway, hot seizure tells me it's a clearance issue (which has already been opined), but i can't imagine a smoking gun, given that it runs so well until shut off. rings could be really tight and still run, but it seems like something would give at 850 RPM or at least it wouldn't be so smooth. were the main bearings replaced recently? oil clearances could be tightening up once the engine comes to temp.

i've seen a starter clutch seize in the wrong direction (making it no longer a clutch), but not actual bearing failure. if the oiler were plugged like frank says, that bearing could go bad, but only when hot? it's gotta be a clearance issue somewhere, and my money is on piston/bore or main journals/ bearings. if the pistons were running tight, you'd think you'd see a compression drop eventually, as the skirt will begin scoring the cylinder walls. if it's tight main bearings it seems like it would be showing fine metal dust in the oil. nothing there?

it wouldn't be the easiest process in the world, but you could pull the clutch basket to ensure that the friction plates aren't sticking when they get hot. even if they were, it seems like it would take some form of binding in the gearbox to still be doing it in neutral. it might at least eliminate the rear half as a possibility. 

[TwoTired]

The symptom sounds like what I had with an old 327 Chevy V8.  The previous owner put in new rings and didn't check the end gap.  Yeah, it had higher compression with the smaller chamber heads he used on it, too.  But it didn't run hot (water cooled).  Tried 3 different starter motors, two batteries, solenoid, no change.  Spun over easy when cold.  Starter barely turned if over when hot.  If you got lucky it might start if it caught in 1 rev.  Otherwise it would just stall the starter motor.  Wait 15 minutes and it would crank plenty. And then start like there was never a problem.  You could bump start the car, popping the clutch, chirping the tires.

I never did fix it, though.  Not enough money, and then the draft board notice came...  The guy I sold it to fixed it, new rings with proper end gap.

Anyway, that's my guess with your motor, Sean.  Too small an end gap on the rings.  Rings jam in the bore with hot expansion.

Cheers,

[Soos]

I never did fix it, though.  Not enough money, and then the draft board notice came...  The guy I sold it to fixed it, new rings with proper end gap.

As with paul, been watching, hoping to see a easy fix for you, but all along, i've been guessing what TT said is going on.




When I worked the engine machine shop years ago, one of the guys bought a 'cuda for a pittance.
The motor had been stroked, ported heads, hot cam, all the goodies you could throw at that old motor.
Body was crap, but the motor supposedly had $10k+ into it.(and thats just the motor!)
Dan bought it for 5 grand I think?


He pulled the pistons out, checked the endgap(there was around 2-3 thou clearance only!!), filed them a bit, stuffed it together, and he still drives that thing to this day.



REALLY hope it's simply something easy/cheap for you to fix though!!



l8r

[754]

Try to get or make a cylinder light, pop a plug and have a look inside. Just thinking if the skirts are scuffing you may see aluminum on the walls.

I think a leakdown test may tell if the endgaps are tight.

Pulling the dyno cover off would be quick, but have a gasket ready..

[Jerry Rxman Griffin aka MuthaF'er]

Hey Sean, have you taken an infrared thermometer to it and compared the temps with yours?

TT - another U.S., huh. That damn draft board!

[Jim F]

If you pulled the plugs I would just take a look at them to check your color

(this is a seat-of-the-pants way if you don't have a dyno)

If there close to being white on the insulators then your lean and you need to go up on your jets
but if your a good sandy brown color then your fine.

Just a thought