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

[scondon]

     I've been helping a friend get a RC bike back in action and am kind of stumped at the moment. Bike starts up and runs fine, but when it gets hot the starter motor won't spin the engine after it has been turned off. In other words, the engine runs when hot, but friction is building up somewhere in the motor, not enough to seize the motor, but enough to keep it from restarting again until the motor cools off.

      I've had the bottom end apart already for an unrelated matter, and it passed visual inspection in regards to crank, bearings, transmission, clutch, etc.. I've also had the valve cover off, and cam/rockers/towers all look good as well. Things have looked pretty pristine inside this motor so far.  I am leaning towards tight bore clearances or some other issue causing pistons to tighten up in the bore.

    Does anyone know what type of pistons RC used(cast-forged) and whether or not these could be affected by heat more than Wiseco or other big bore pistons?

      I'm bringing a oil gauge with me next time to see if the pump is up to snuff, and would like to have a plan of action about this engine issue(my friend is bummed, and I'd like to cheer him up). Any input, wild ass guesses, or general wisecracks welcome

[MRieck]

 Too much compression Sean I'd say. Could also be a low battery/poor charging system and the compression issue. I'd check out the charging, switch to a DOHC CB900 starter, replace the battery to starter solenoid cable with an FJ1200 piece and see what happens.

[MRieck]

 A question Sean....does it spin about half a turn and kick back when trying to start it hot or does not simply not turn over?

[scondon]

 I was just over there and my friend(who owns a vintage jap bike shop) had just replaced the starter with a known good one. His first ride out, and the starter couldn't crank the engine hot. I pulled the points cover off and fit a wrench around the ignition nut, motor did not want to spin. I thought about compression, but I usually think of that as having difficulty spinning engine at certain points(when pistons are rising) and becoming easier at other points(pistons midway). The motor was at same tension through 360 degree spin(very tight, thought I might snap the ignition shaft).

  The first diagnosis was the starter, but I am pulling away from that fast and am trying to understand what's binding the motor when it heats up. I wanted to do a compression test right then and there, but not being able to crank the motor was a problem    I can do a comp test cold on this motor, do you know what kind of p.s.i. readings would indicate a problem, Mike?

A question Sean....does it spin about half a turn and kick back when trying to start it hot or does not simply not turn over?

    See above    It is tight as frickin hell through entire revolution. No kickback.

[Jerry Rxman Griffin aka MuthaF'er]

Sean,

Russ Collins responded to my email enquiring about the identity of my cast 73.75mm pistons. He told me that he only did forged pistons. What clearance do the pistons have? My Henry Abe kit has them rather loose compared with today's standards but they are cast. I would assume the looseness is due to the extra heat generated and the additional expansion.  

12.5 to 1?

Try synthetic oil. It has done wonders in my Fat Boy as far as the tranny goes and in long stop and go traffic.

Mike,

Are you saying the DOHC 900 starter is a direct swap? Are we going to have to do this on our Big Jay 1000's ?  

[MRieck]

I was just over there and my friend(who owns a vintage jap bike shop) had just replaced the starter with a known good one. His first ride out, and the starter couldn't crank the engine hot. I pulled the points cover off and fit a wrench around the ignition nut, motor did not want to spin. I thought about compression, but I usually think of that as having difficulty spinning engine at certain points(when pistons are rising) and becoming easier at other points(pistons midway). The motor was at same tension through 360 degree spin(very tight, thought I might snap the ignition shaft).

  The first diagnosis was the starter, but I am pulling away from that fast and am trying to understand what's binding the motor when it heats up. I wanted to do a compression test right then and there, but not being able to crank the motor was a problem    I can do a comp test cold on this motor, do you know what kind of p.s.i. readings would indicate a problem, Mike?

A question Sean....does it spin about half a turn and kick back when trying to start it hot or does not simply not turn over?

    See above    It is tight as frickin hell through entire revolution. No kickback.

Sean....pull the plugs when it is hot and see if it turns over easily or not. That will indicate whether it's some wierd hot seize thing IMO. Stuff in the 200 to 220lb cranking range cold will give you problems in my experience

[MRieck]

Sean,

Russ Collins responded to my email enquiring about the identity of my cast 73.75mm pistons. He told me that he only did forged pistons. What clearance do the pistons have? My Henry Abe kit has them rather loose compared with today's standards but they are cast. I would assume the looseness is due to the extra heat generated and the additional expansion.  

12.5 to 1?

Try synthetic oil. It has done wonders in my Fat Boy as far as the tranny goes and in long stop and go traffic.

Mike,

Are you saying the DOHC 900 starter is a direct swap? Are we going to have to do this on our Big Jay 1000's ?  

Jerry...what I'm saying is you need to pay airfare for me to come out and handle all aspects of your build. Liquor, food, lodging all included.

[754]

why are you not trying the kicker, when the starter does not work?


could be just lots of compression..

[Jerry Rxman Griffin aka MuthaF'er]

Sean,

Russ Collins responded to my email enquiring about the identity of my cast 73.75mm pistons. He told me that he only did forged pistons. What clearance do the pistons have? My Henry Abe kit has them rather loose compared with today's standards but they are cast. I would assume the looseness is due to the extra heat generated and the additional expansion.  

12.5 to 1?

Try synthetic oil. It has done wonders in my Fat Boy as far as the tranny goes and in long stop and go traffic.

Mike,

Are you saying the DOHC 900 starter is a direct swap? Are we going to have to do this on our Big Jay 1000's ?  

Jerry...what I'm saying is you need to pay airfare for me to come out and handle all aspects of your build. Liquor, food, lodging all included.

I'll keep my eye open for good airfares. What works for you? How much time do you have? 

[scondon]

Jerry,

      I've had everything apart except the top end. I didn't want to do it at the time because the seizing issue was not known then, and the head gasket was doing a brilliant job so I didn't want to crack it if I didn't have to  Now I'm thinking I may have to, but am trying to rule out other things before making that commitment.

    It's a bit intimidating working on this bike when there's 3-4 guys standing around who have been working on bikes a lot longer than me. But they are not the "go to" guys for cb750's, I am  A lot of opinions get thrown around, and I have to be the voice of reason and act like I know what I'm talking about. Couple of the guys are sure it's the crank, but I had that puppy apart and it looked great, bearings looked new, journals pristine, rod play checked out, etc...   One of the guys thinks it's the thrust bearing and asked me if I checked it when I was in there

Sean....pull the plugs when it is hot and see if it turns over easily or not. That will indicate whether it's some wierd hot seize thing IMO. Stuff in the 200 to 220lb cranking range cold will give you problems in my experience

    If it still won't turn when plugs are out(hot motor), then I'd be leaning towards a hot seize condition rather than compression, right. Everything I've seen so far(I've spent a bit of time helping out) leads me to believe that something is binding in the engine when hot, rather than an issue with compression, but I do not have experience with motors over 10:1 compression so I am open to everything at this point since I don't know anything about the pistons except that they are oversize and not stock(judging by what I could see from the bottom end).

why are you not trying the kicker, when the starter does not work?


could be just lots of compression..

   The kicker was why the bottom end was apart in the first place. It is binding heavily at the top of the stroke. Once past the first inch or two of travel it swings smooth as butter through the stroke. It is not an issue with compression, the kicker shaft is binding somehow. The kicker works fine with the engine cover removed so I am leaning towards an issue of alignment between shaft hole in cover and shaft hole in case.

     No matter where the kicker is positioned it will not spin this motor hot, at least not fast enough to get it started. The engine really is binding somewhere.

[754]

Well if its binding, it has to come apart..

 too bad you didnt check the piston clearance when the crank was out... with a feeler gauge..

[scondon]

Well if its binding, it has to come apart..

 too bad you didnt check the piston clearance when the crank was out... with a feeler gauge..

   If the "hot engine" issue was known at the time I certainly would have pulled the top end apart, since I had the engine out already. Are the clearances the same at the skirt/bottom of cylinder as they are at the crown/ top of the cylinder? What clearances should I have looked for in regards to these unknown pistons?

    The method I adhere to is to run through all the possible causes before yanking the motor. Sometimes it's something I haven't yet come across in my limited time as an engine mechanic. I'm steering away from compression issues, but I've never had any so what do I know. As far as my saying the engine is "binding", that is what it appears like to me from the info I have been able to attain. Where it's binding, and even if it's binding is up for debate and I appreciate all input as to possibilities

[754]

Pistons are measured on the skirts. older ones forged should have at least 2 1/2 thou or more clearance.

Try pulling the plugs, if it does not easily turn over there is a problem. With plug out and a light, you should be able to see if it has a reakl high dome on the piston, but it could get high compression from a decked head as well.

[scondon]

 I did have the plugs out when trying to sort the kicker issue and compression did not seem to be an issue at all, but engine was cold then and that was a couple months back. I will definitely try it again with the motor hot. I'm also trying to think of possible reasons that might cause excessive heat in the cylinder(s).

   I'm bringing comp tester, oil pressure gauge, tiny flashlight, and anything else I can muster when I go back next Tuesday. I plan on doing a cold comp test, then install pressure gauge and get bike running to how well oil is pumping, get engine hot and pull plugs to see if engine will spin better without'em. Trying to gain as much info from engine while it's together and running.

 Kind of frustrating that things have checked out so well inside the engine so far, yet it's not running properly. Trying to run through all the possibilities so that when it comes out again I can be as thorough as possible on every detail. Hopefully it is a bore/clearance problem as I suspect, but if that checks out OK then I want to have plan b and c ready.

[1080]

My 1080 was always harder to start when hot,but not to point where it could not be restarted. I always had to jump on the kicker with all my weight.
I gave up on trying to use a electric starter, so fix that kicker. There was also something done to the kicker mechanism to improve it's reliability. I had mine still stock.
I have MTC pistons.
Sorry I did not catch how big the RC motor is?
Make sure lots of oil is getting to the top end towers.

[MRieck]

Sean...pull the plugs when it is hot. This isn't a seizure problem....the engine runs. Don't look for to many answers......especialy around here. It's compression in general....that RC stuff...especially later units....were known for that.

[scondon]

My 1080 was always harder to start when hot,but not to point where it could not be restarted. I always had to jump on the kicker with all my weight.
I gave up on trying to use a electric starter, so fix that kicker. There was also something done to the kicker mechanism to improve it's reliability. I had mine still stock.
I have MTC pistons.
Sorry I did not catch how big the RC motor is?
Make sure lots of oil is getting to the top end towers.

   That's good to know and I'll pass the info on to my friend. I've been at a loss to explain why the kicker is the way it is, but if the hot start issue was always present in this motor, and it had to be jumped on to get started again, it might explain how the kicker got in the condition it is in now.

 I wish I could remember everything I checked two months ago, but I believe the liners were stock which would put the bore at 850 or below(I'll remember what I measured, just can't seem to right now). I hope the comp test will give me more info about them.    EDIT: just checked on Axl's site and he shows 67mm Russ Collins pistons(890cc?) and nothing lower, so I will definitely be jogging my memory around the block this weekend

       I checked the kicker shaft when the cases were split, and even though it checked out I replaced it with a known good one. The shaft shoulder in the case also checked out as having no obvious wear or play, as did the shaft hole in the engine cover. I'm going to bring a spare cover with me and see if installing it will help the suspected shaft alignment issue. Agreed that the kicker needs to function 100% while sorting out this motor, just wish it was an obvious problem with an obvious solution. I Hadn't been stumped by a CB750 motor until I agreed to tackle this RC

Sean...pull the plugs when it is hot. This isn't a seizure problem....the engine runs. Don't look for to many answers......especialy around here. It's compression in general....that RC stuff...especially later units....were known for that.

    You can bet that I will be doing everything possible to sort this problem before pulling the motor. It does run well and hasn't seized, or at least it hasn't yet, just gets real hard(and I mean REAL hard) to turn over once it's hot. I fully expect to have learned something new by the time I'm done with this one

[Gregorymoto]

Just how hot is the motor?

I have a temp scan over hear in SF if you want to use it. i can send it to work with my wife she is working in Emeryville.
It will help you pin point over heating.

[scondon]

 Thanks for the offer Greg. I'm going to see if he has one on Tuesday and I'll let ya know.

[Jerry Rxman Griffin aka MuthaF'er]

Sean,

Just to throw something at you. How is the jetting?  Not running lean (heat) are you?

[scondon]

Jerry,

      I didn't do the carb work on this resurrection project, the owner handled that, but I definitely plan on pulling a bowl on Tuesday to at least see what the jetting is. A bit of smoke in the oil tank after only 15 minute ride, and the valve cover was still too hot to touch 10-15 minutes after shut down, this is was got me wondering about excessive heat. Pipes are not bluing and there is no "ticking" when the bike is cooling off, you know, the obvious overheating signs, but I am still going to check it out.

     The owner is very competent and has been working on Jap bikes for many years, all SOHC bikes included, but he is "service" oriented for tune-ups, repairs, carb work etc.. on stock bikes and not too experienced with internal engine work and tuning for modified cb750's. He's seen what I've done with my bikes which is why he asked me to help tackle his RC. He has really made this bike shine compared to how it looked when he got it(left for dead by a customer), and all that's left is getting this motor sorted.

      It was left for dead because they thought the engine to be blown because it ran so very badly when it could be started at all, regardless of carb work,timing,etc... My first intuitive diagnosis was correct and I found both tappet adjusters absent from #4. Fished one out, replaced the other and tried a short run of the engine. It ran soooooo much better(duh!) that green light was given to yank the motor to attempt kicker fix and fish out the other tappet adjuster. Still runs good but this other issue has now become evident. Pooh!

[Gregorymoto]

Or if you have a cheep o meat thermomitor from the 99c store. You can take readings from different cylinders and see what what.
Good luck that a strange issue.

[MRieck]

Excessive heat=a ton of compression.

[scondon]

Too much compression Sean I'd say.

Sean...pull the plugs when it is hot. This isn't a seizure problem....the engine runs.  It's compression in general....

Excessive heat=a ton of compression.

   Just so I have this straight, are you saying there could be a problem with compression on this motor

    Consider your bet placed, I'll let ya know if you've won anything on Tuesday

[Spanner 1]

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.