1981 Bobcat 743 Smokes Continuously While Running

[dwcrigler]

Hi guys, I just recently bought a 1981 Bobcat 743 that has around 5,000 hours on it. The seller did say that it recently had new piston and rings installed along with a new head. The machine does not like to start from cold but will after warming plugs and cranking for a couple minutes. After that is runs good and strong with out problem, but smokes continuously. It's enough smoke to quickly fill my 30x40 shop with grayish smoke after running for about a minute. The smoke just smells like diesel. I don't think it's burning coolant.

In hopes to find the issues I checked the glow plugs, injectors and did a compression test. I found one bad glow plug which should fix the cold start issue. The injectors seemed a bit old so I just ordered a whole new set.

When I did the compression test the machine was just room temperature. All four cylinders had compression around 345psi. I did the oil test where you add 1ML of 10-30 to each cylinder and doing so increased the values to around 450psi. This tells me that the piston, rings, cylinder and valves should be within spec hopefully.

Does anyone have any other ideas of what might be causing the continuous gray smoke while the machine is running? Any advice is truly appreciated!

 

[brdgbldr]

Bad injectors or possibly a clogged air filter.

 

[Tazza]

345 PSI is out of spec, adding oil and it rising to 450 means your cylinders are worn, a wet test is what you did, the oil creates a better seal and if the value did not change, your issue is not pistons and rings. As your value changed, you have a cylinder issue.

The person that did the work before you told you lies, or they just installed new parts into worn cylinders.

I'd put money on that your cylinders are worn and need to either be sleeved or machined to over size and install new pistons and rings.

 

[Wayne440]

(Disclaimer- my opinion follows- I claim no expertise)

If it runs and does what a Bobcat is intended for well enough to beat a shovel it is worth $3K in my area (not too far from you).

My guess is that the seller tried a "new head" to clear up the low compression/smoking, then decided to sell it instead of
doing more extensive engine work. The slow starting and smoke you describe are typical of a high hour, worn but not worn completely out
engine.

 

[dwcrigler]

345 PSI is out of spec, adding oil and it rising to 450 means your cylinders are worn, a wet test is what you did, the oil creates a better seal and if the value did not change, your issue is not pistons and rings. As your value changed, you have a cylinder issue.

The person that did the work before you told you lies, or they just installed new parts into worn cylinders.

I'd put money on that your cylinders are worn and need to either be sleeved or machined to over size and install new pistons and rings.

OK, thanks again Tazza. I was hoping that wasn't the case. I'm going to put in new glow plugs, injectors and new filters all around and see how it runs. If it persists to smoke I suppose I'll have to pull the engine for an overhaul.

If I get it back to gether and it does continue to smoke, is it a bad idea to run the machine with worn cylinders?

 

[Markle]

If it runs ok (but smokes) and does what it needs to, I suggest just use it. You're still looking at a rebuild at a later date. And there are rebuilt crate engines ready to drop in ( i don't know if I can state a vendor name per site rules)….but if I can, I will tell you the firm I used. I had all cylinders with low compression on a non running 743 i bought for $1500. It's now at my sons farm getting used with new life. Good luck!

 

[dwcrigler]

Thanks Markle. Is it these guys? https://dieselexperts.net/product/kubota-v1702-engine-bobcat-743-engine/

 

[Tazza]

Honestly, you are throwing good money after bad. Glow plugs will not fix your issue, if you remove them and use a battery and cable, see if they glow to the tip, if they do, your glow plugs are fine.

We see it often at work, customers will buy parts off us, it fails to fix the issue, and they send their engine up to us for a rebuild. Put your money towards machine work.

At the very least, pull the head and feel if there is a ridge at the top of the bore, if there is a lip, you need machine work doing, take it to .5mm over size, or if it's too far gone, get it machined to fit 1902 pistons, it will give you a little more power.

 

[dwcrigler]

The glow plugs I installed were well worth $36 set as at least one of the old ones were dead. She now fires right up with the new set with just a 10-15sec warm up with the plugs.

The smoke is still there but I don't plan to use her more than a few hours a week so will probably hold off for a year or so and then buy a rebuild crate motor. I've seen a place near me that has them priced for $2750 + core. That seems like a pretty decent deal. If anyone know of better pricing or motor upgrade options for a '81 743 please let me know.

 

[polishpete]

SOUNDS like u need new rings

 

[laurencen]

had a similar issue on a 843 but its Isusu engine, we picked up a rebuild kit including cylinder sleeves for under 1000, not a hard job we sat the new cylinders in Nitrogen prior to installing, worked great still used a aluminum block to hit the sleeves after inserting them, not sure can you re sleeve a Kubota motor?

 

[Tazza]

If it's got blow by, with the smoke, i'd guess it would. Change your oil more often. The more exhaust gasses that get into the crank case, the more carbon you'll get in your oil.

If you plan on running it like this, may as well make it run as long as you can.

That price seems reasonable for an exchange, it's close to what we charge over here in Australia, but as our dollar is lower than in the USA, it's kinda expected.

 

[Markle]

Thanks Markle. Is it these guys? https://dieselexperts.net/product/kubota-v1702-engine-bobcat-743-engine/

No…I sent you a message.

 

[oiu789]

How many hrs on it after the rings and head job. They may have not broke them in correctly. If low hrs run break in oil and procedure to see if the rings seat in. This worked on a JD tractor I had. I switched to JD plus 50 oil after 500 hrs and it started using oil and smoking and hard to start. Switched back to JD break in oil then ran JD supreme 30 weight for 1000 hrs and back to plus 50 and it ran fine no smoke on oil usage and started right up. Also check the injection pump timing. Check that they put the right head on it. I do not know if there was a DI head and a IDI head. If so the wrong head could be why it is hard to start. How much blow by does it have?