Jump to content

AArrgggh!! More problems - possible piston heat seize when idling setting carbs


Recommended Posts


I want to start by thanking you for indulging me with my other questions. I really appreicate your input.

I've had this Dnepr K750 for a couple of months (supposed to be 1966 but plate on the headstock says 1962? I realise that these can be changed but looks original but mightr not refer to the manufacture date anyway??) But I digress.

It was bought as a non runner. It wouldn't start so I cleaned the carbs, checked the points, plugs and valve clearances, bought a new battery. Could get it running on occasion but, even when it seemed to be running OK, it would suddenly stop, always after maybe 2-3 mins.


So, new: battery, electronic ignition, coil, leads, plugs. Now starts with one or two kicks, which is great.

However, carbs need setting up. I've been trying to do this but, when idling and seemingly running ok as I adjust, it will suddenly stop, as above after maybe 2-3mins. It does seem to get really hot on the pots - they smoke.

Now, when I bought this, I was told the engine had been rebuilt but not run. I bought it as a trade-in from a dealer, so he wasn't the one who rebuilt but, looking at how clean it is and the fact it has all new mounting bolts, this is possible. Also the oil is crystal clear

So, my thought is that, at idle, it's getting really hot and doing a light seize of the piston (perhaps because of new rings?) - if there is such a thing. What I mean is, it's not a high rev overheat and isn't pushed futher once in this state.

I've removed both heads and it looks OK to my untrained eye. When cool, it turns over easily. When it's hot and stalls, it's not as easy.

So, questions:

1.Does this sound likely?

2. If this is the case, might it just be that a new, tight piston might just need higher revs to run in and idling is not a good idea?

3. I'm worried both idle and higher revs might be damaging the engine.

4. Is there something else or better to check?

5. Might something just be damaged or wrong?


I'm in a bit off a viscious circle -  I want to test ride it for the first time but need to get the carbs setup properly but might not be able to do that by letting it idle and also don't want to do it at higher revs in case this makes it worse.

Any insight or advice very gratefully received.


Link to comment
Share on other sites

I suppose the question is why it’s running so hot after only a few minutes. Ring gap, piston clearance come to mind  but maybe it’s running so lean the valves/heads skyrocket in temperature.  Vacuum leaks?  Or lack of lubrication, or a combination of all these things.  If it’s difficult to turn over by hand after a few minutes the problem seems serious, hopefully this means it’s easy to find.

If it starts easy enough perhaps someone who knows your carb type can suggest how to check and ensure rich running to eliminate this variable.   It’s easy to check oil pressure too to get this  variable out of the way.  If it’s not running lean and you have lubrication it seems to me it’s down to cylinder/piston/ring tolerances.  ND


Link to comment
Share on other sites

Thanks for the replies.

I spoke to someone earlier who suggested crank endfloat; ie not enough, could cause a mild heat expansion.

Also, another suggestion was back to carbs being too lean.

I need to do some more investigating but have little experience with engine internals. It's frustrating as the mention of the engine having rebuilt should be a positive thing but I'm just wondering if something was put together wrong. I really don't want to have to tear the engine  down. I just want to ride it!!


Link to comment
Share on other sites

When testing the engine you should consider having fans blowing lots of cooling air over the cylinders.

This is the normal testing set up at the Ural factory in Irbit, RU.  I do not know what was done at the Dnepr factory in Kiev.

Then perform the suggested tests and measurements, cold and hot. 

Food for thought.



Link to comment
Share on other sites

Don’t give up!  You’ll figure it out.

Fans are a good idea but from experience 2 to 3 minutes before overheating is indicative of another problem.   

Of all the possible culprits I think the easiest to eliminate is ensuring the mixture is not too lean, you can do this without disassembling the engine.  I’d start there because carb setup should be part of putting on the road anyway.  I don’t know if you have checked flanges for leaks, jets cleanliness, float height, needle setting etc but if fuel is not flowing adequately at idle or revs it can get v hot and destroy an engine.  Others have had hot running idle issues with PZ28 carbs, you wouldn’t be the first.  

Fuel flow may not be the problem or only part of the problem, but you need to set up carbs no matter what.  2 to 3 minutes is fairly rapid heat build up and maybe it points towards piston and ring clearances but approach it stepwise and maybe it’s not a more serious issue.

I’d still check oil pressure if you can,  even with overheating issue solved this is an essential test for rebuilt engines. Easiest way is to screw out the oil pressure switch and screw in a gauge and start the engine.  They are standard threads. 


Link to comment
Share on other sites

First pull the generator off the engine, make a cover to cover the hole left by removing the generator, and crank the engine too see of that was the problem. If it's still doing the same thing you'll need to dig deeper. Pull the cylinders off and check the pistons and rings. If the pistons are scuffed or the ring gaps are too small those will cause your problem. If the pistons and rings are OK pull the pistons off the rods and check the rods for binding or play indicating bad rod needle bearings on the crankshaft. The K750 engine uses pressed together crankshaft components like the crankshaft used in Ural engines. It's possible the crankshaft components have slipped in relation to the crankshaft rod pins which will cause binding. You can take the ends of the rods in hand and use them to spin the engine crankshaft. It should spin freely as you spin the crankshaft by pushing and pulling the rods in and out. If it does not spin freely your crankshaft is bad. You could also have bad crankshaft bearings which would also cause the crankshaft not to freely spin in the engine block. Obviously if you find crankshaft binding you will need to remove the crankshaft from the block so you can inspect it to see if the crankshaft components have slipped or you find bad crankshaft bearings. It's entirely possible the "rebuild" was just new or reworked cylinders, valves, pistons, and rings and the bottom end was not touched, which may have been the original problem to begin with. RussN is correct about setting up a fan in front of the engine so air can move across the cooling fins if you are going to run the engine for more than 10 minutes from dead cold. KMZ produced engines and transmissions for railroad maintenance of way "speeders" which are the same engines used on motorcycles except the front of the engine had a cooling fan and ducting to direct that air across the cylinders and heads. It sure would have been nice if that same cooling setup was done for motorcycles as well.


Link to comment
Share on other sites

  • 2 weeks later...

Hi there. Hope you all head a good xmas day. Had a bit of time this morning so took another look at the bike.

Checking the fuel mixture is what I've been trying to do at idle and am pretty confident it's not this as a turn of the mixture screw either way affects the idle.

Also, there's no oil pressure indicator that i can find. This is a mid 60s bike but I also think oil is circulating OK since it travels to the valve springs and is behind the pistons

I've now taken the heads and barrels off  a couple of times. The very first time, I found the top oil ring was tight in it's groove. The top and bottom of the piston, at maybe 10 o'clock to 2 o'clock and underneath from 4 - 8 o'clock,  had scuffs on it that had burred the groove the oil ring was in, stopping it move freely. The odd thing is, both pistons had this issues - almost identical. However, the bores don't seem to reflect these marks or damage??


So, I freed the oil ring both sides and cleaned / polished the burr so it all moved / rotated easily. I was pretty happy about this discovery as I though it must be my issue.

I put it all back together but there still seems to be an issue with overheating. So, I took it apart again as I wanted to check the ring gaps

They all seem to be 0.25mm, which seems to be in tolerance. I found this below - sorry, I forget who posetd it:


Piston ring end gap can be 0.15-0.6mm.

So, my gap is maybe on the lower end of this. Both top rings and oil rings have similar gap. Do these look OK for a sidecar bike?

Is this likely to get tight if idled for too long?


Pistons look a bit rough but, and seem to have been scuffed in the past but since it's the rings that contact witht the bore, can't see too much here that might be an issue, but I'm not expert. I found figures of 0.15 - 0.23mm for this. The bore does seem to have honing marks on it.

Piston to bore gap also seemed within tolerance, I found figures of 0.15 - 0.23mm for this - 0.2mm feeler went around the entire gap.

Does all this seem OK or would a larger ring gap be better?

Also, piston face has writing on it. Is this the piston size? It's a bit difficult to make it all out.

Thanks again in advance for any input. Cheers : )







Link to comment
Share on other sites

The last two photos are from the first time i removed the barrels. The marks on the piston are odd becuse they are around the circumference, so couldn't have been caused by the stroke. My thought is that someone has done something manually to this at some time in their life.

It's also odd that these makes are within the lateral marks I mention above, almost like someone has marked the piston then worked within this area.

It's also confined to the area below the top oil ring and above the lower oil ring.

I don't understand what has happened or why there marks are there?

Link to comment
Share on other sites


This is piston diameter marking.  P2 = second repair pistons (original, then (P1) first repair bore, then (P2) second bore diameter)

Generally, 0.004" per inch bore; 78mm is approx 3" = 0.012" = 0.3mm  <-- this is in range of what is mentioned int he Russian repair manuals.


Link to comment
Share on other sites

Thanks for the info on the piston markings. Is P2  the max repair size before affecting the integrity of the cylinder?

Also, the second part of your info, are you talking about the ring gap? I think you are. Are all 4 rings set with the  same gap? 

So, my gaps are potentially a bit tight?

I think I also read somewhere that sidecar bikes have bigger gaps than solo bikes? Although I don’t think this will have any effect initially in this case since I’m not riding it either way : )

Thanks again

Link to comment
Share on other sites

Yes, I was demonstrating ring gap calculation.  See attahced from Russian manual.  

I did not see P3 listed for K750, however it does not mean it is impossible.  May have to source pistons and rings from different manufacturer.  Now that I am thinking about it, I believe I saw one once that had 80mm pistons.


Link to comment
Share on other sites


Next time you have the cylinder of do a gas flow job on the ports.....don't worry you will not make them worse than they are! There are a lot of sharp edge that act as fins to soke heat into the engine. The "valve guide boss" can be shortened quite a lot. If you have a lot of valve rock get the guide holes Klined and seats recut. A light (c 3mm) radius on the top of the cylinder at the combustion chamber edge helps spent gasses on their way to the exhaust port too.

Ignition timing. Not many know that the fully retarded stop can be adjusted. This should be set so that the manual retard is somewhere between 5 & 10 before TDC. I set my advanced and then back it off until it stops kicking me back!

I can't remember off the top of my head what the advanced timing is in degrees but in millimeters it is 9.6mm before TDC. I have a piece of 10mm rod I poke in the plug hole that is bent to be parallel with the top of the piston when it traps against the cylinder head that I use for setting the timing. This accounts for the head gasket and a little bit of retard to account for modern unleaded fuel.

Fitting Russian electronic ignition with it's built in advance curve helps cool things too.


Link to comment
Share on other sites

13 hours ago, Serious Black said:


Fitting Russian electronic ignition with it's built in advance curve helps cool things too.


The best thing I did to my Dnepr was fitting the Microprocessor type electronic ignition system. Before I did that I installed new points, condensor, and made sure the gap was correct and maximum advance point setting was exactly correct. Double checking the timing with a timing light showed it firing the spark plugs all over the place until the engine speed was high enough for the timing advance weights to be pined to the stops. Now the timing is perfect across the RPM range and I never have the kick start lever kick me instead. 

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...