756 gas tractor running rough

My '67 756 gas recently starting running rough...kind of chugging when running past idle up to full throttle. It starts very easily and runs perfect at idle. Also, it seems as though there is more blow by than usual. Maybe since it is now cold, it is more noticeable. I don't know if this could be related...I changed the spark plugs last year to NGK from Champion. I had the parts guy from Napa figure the spark plug cross reference. The first few times I ran it, a bunch of black soot/carbon blew out the exhaust. Seemed to run better, so, I just figured it blew out the cobwebs. Am I possibly running a too hot plug?

In the cold a carb need to be set a bit richer due the the cold and also simple but part of how a carb works carb icing can cause that. Try choking it a bit and also let ti warm up more before you start throttling it up

sounds like a plugged muffler take it off and see if it runs better

Plugs too hot won't cause that. Rather they may cause detonation/knock under heavy load. Also too hot plugs will wear out very quickly.

Sounds more like a plugged muffler as Donald suggests. Also carburetor icing if running under icing conditions (32 to about 50 deg with high humidity). Finally a stuck centrifugal advance can give poor running at speed.

Incidentally I've had excellent luck with the NGK cross to Champion D-21 plugs in my Farmalls.

does it only do it in the cold weather. probably the heat riser valve is not working. a spark plug is a spark plug, no matter what brand and is not going to do that. and when was a major tune up done? they are cold blooded engine in the cold.

Put a 9/16" wrench on manifold bolts and check tightness.

Thanks for the responses. I usually am using the tractor when it is cold...to move snow or take wood to the burn pile. It mainly just putters around and sits inside the barn with a battery tender on it. I am curious on how the muffler could be plugged. Are you thinking from mouse or birds nest..or do they collapse somehow?


I haven't done a major tune up for a long time...only because it does not get many hours of use. What do you mean regarding: 'a stuck centrifugal advance'? Is that something in the distributor? How would I check that?

I put a garbage bag on the front grill and let it warm up quite a while before I use it. It still barely gets past the cold range on the gauge. It definitely sputters more when it is cold.

I had a new exhaust manifold and heat riser installed when I bought it around 9 years ago.

A muffler can become partially plugged if the internals rust and collapse. Easy test is to remove the muffler then run the engine.

Centrifugal advance is inside the distributor. Check with a timing light - spark should be within a couple degrees of TDC at low idle, and advance to 30 deg or so before TDC at governed RPM.

I'm going to suggest checking the air cleaner for
plugging up.
If it is an oil bath cleaner, it may have ice in
the cup.
If a dry type, it could be loaded up restricting
air flow.
Check for a collapsed hose in the intake air
system.
Jim

Wrong , ing timing on a 756 gas is 18 Degrees before TDC at high ideal or full throttle . One real fast way if super heating and scoring sleeves on the higher RPM engines , this is not and old M and even on them with a over bore and step heads ya do not want the ing timing above 27 degree and more in the 22-24 degree range.

Well now that everybody has given you there advice and some of it totally WRONG . Here is my uptake on this . And i do believe i have a little better leg up on this tractor as i have worked on more of them then any one on this board. You say you now have more blow by , Well you now have scored sleeves , this is caused by the pistons becoming super heated and getting out into the cylinder walls . Ya can pull the plugs and take a look inside and see on the far wall . what caused this , well there are a list of things that start this , a couple covered them . (1) your manifold COULD be loose and she leaned out and she got hot , now you will NOT see the HOT on the Temp gauge as it takes time for this heat to soak thru to the coolant and by the time it has it is to late damage is done . (2) wrong heat range of the spark plug as ya want COLD . Next ya want to be using a LOW ASH oil , now the the oil thing is mainly for the valves so the exhaust valve does not COKE and burn and here again when they burn a valve they do it so fast it will make your head spin . Next on the list is proper Ing timing and ing. timing needs to be set with each points change. Timing on a 756 gasser is 18 degrees BTDC at full throttle . Any higher and she will super heat the pistons and score the cylinder walls of in this case the sleeves and pistons . All damage to the pistons will occur above the top ring and cause the top ring and sometimes the second ring to collapse and even break. . BUT the main reason for all these problems and this across the board with all the newer HIGH RPM gas engines in the I H line is the GAS the reg gas you run in your cars and pick up today burns way to hot for emission and is to low on octane . . MINIMUM octane requierments on the the engines built from 1963 up till they did away with the gas tractor was 93 , octane requirements for the MODERN gas engines made from 58 to 63 was 89 That covered the 460 thru 660 gas tractors . Yep there will be the one's that say they know more then i do , BUT not one of them has worked on them , not one of them has spent the money in lab testing the gas to figure out why and how come and non of them has rebuilt as many let lone bought and sold as many GASSERS as i have bought and sold over the years or for that matter farmed with A GASSER . As lone as we could get the GAS we got along just fine , yes we filled the fuel tank at the farm with 93 octane gas 350 to 500 gallons at a time and the extra cost of the 93 per year even over a four year time frame did not equal the cost of a rebuild . Three years ago we had both the 706 gassers that my two friend had go down a day apart and on both was after the new loads of gas were delivered from two different fuel suppliers and they brought out the NEW gas and it was NOT 93 but 90 . two days later again on another farm a 560 took a dump after they got a new load of 500 gallon , Then a 460 went to lunch while picking corn not five min. after they filled the tank and less then 500 feet into the corn . Ya want a better look inside your engine get a bore scope and take a look. You will NOT like what you see . . Posts like yours come up all the time on the gas tractors . How to solve the problem find the correct grade of gas and better pistons then and only then will they run like they are/were suppose to.

I would agree with vet, timing on high compression engine should max out in lower 20s.

Owning an otherwise great 756 with the C291 gas, I?m reminded first hand - even some 50 years later how IH could screw-up a design. I don?t know who dreamed-up the C291, a difficult engine to rebuild, the goofy pressed-in sleeves and the ONLY gas engine in a tractor that I?m aware of that can?t run hard on todays gas. I?ve still got my 756 gasser. I?m going to try to get the POS gas engine going enough to run a hay rake or power itself up on a buyer?s truck or put a Cummin 5.9 in it. Way to go IH on the C291, a real legacy builder. It?s IH crap designs like the C291 that helped people migrate over to JD and I?m feeling the urge every time I see the POS gas engine in my otherwise fine 756.

Off my soap box and back to lurking.

To the original poster - I hope you figure out and are able to fix your 756 gasser.

Bill

Nothing wrong with the design of the engine , it's todays gas . Just like todays diesel and the problems it caused when they started that junk . Hey i made money off pulling pumps and sending them in to be repaired and updated to run the new fuel , my pump man made a small fortune rebuilding them to the point he was abled to buy a Beechcraft turbo baron . You can not even run and old late sixty's early seveenty's performance car on todays gas . Clevette and TRW performance parts catalogue even tells ya that when ordering pistons for MILD street performance that 9 to 1 and max timing on a engine is limited to a max of 20 degrees at 2000 and a total max of 36 degrees on 93 Octane and this is for a car engine that usually is only around max load of 20-30 % . Now as to dropping a Cummins in one must set back and put the penile to it before one dives in with both feet . First ya got the cost of a wore out engine , then ya got the cost of everything to adapt to the carcass then ya have to replace or redo the injection pump for industrial ap. and not automotive . Like i aid BEFORE they changed the gas once again we had no problems with our C263's and C 291's it just cost us about a buck and a half more per year to run them . Now myself i don't have a problem with the press in and pull out dry sleeves but i have the STUFF to do the job with . When you work on a certain product for a living then it is wise to have the proper tools to do the job just like the dealer . Buying out the shop tools from a closed up I H dealer was one of the best investments i made.

When I was looking for another tractor, I wanted a gas model, larger frame like the 756. I had read that some JD owners had ignition issues with 3020 and 4020 gassers - but I've never read anything about those gas engines not running on today's gas or any other tractor gas engine. Read and even posted regarding larger IH gas engines and their reliability and maybe I didn't read enough, but none the less, I came away with the feeling that a gas 756 would be the cat's meow for no more hours than we'd put on it in a year. I liked the idea of using it in cold temperatures, tooling around and no wet-stacking issues. Once I started inquiring about C291 rebuilds, the whole gas issue and you ain't going to take the C291 to the local engine rebuilder and have it rebuilt because of the sleeve issues, specific knowledge required to rebuild this type engine, special tools required, etc., I'm thinking - what have I got myself into... The C291 is the ONLY tractor engine that I know of that is plagued with these issues. The C291 is truly a POS engine.

The Cummins 5.9... I visited a friend's farm several months ago and he had put a 5.9 into a 706. He did a GREAT job with it and didn't break the bank doing it. He took a 706 good tractor and made it a great tractor. As I mentioned earlier, I don't know that I'd do a 5.9 and ruled it out prior to buying a replacement for this tractor on the farm - but I've still got the 756, everything but the engine seems to be in good working order, and after seeing that 706 with the Cummins 5.9 - I'm intrigued about the replacement again. Maybe not this year, maybe not next year - maybe never, but I'd rather roll the dice and spend the $$$$'s on a Cummins 5.9 transplant in an otherwise good tractor than rebuilding a junk C291.

YMMV...

Thanks,
Bill

Well like said up till three years ago myself and ALL my friends had either a 706 with a C 263 or the C291 and several 756's and one 766 all with gas . As long as were were paying the 10-20 cents a gallon more per gallon we all got along just fine . Myself , Eugene , Bill Vernon , Ron all did what they were designed for . From heavy tillage pulling four bottom sixteen plows 12-14 foot disc , four row corn planters mowing hay baling hay grinding feed once a week cleaning out the barns and moving snow . Euegen's fuel supplier brought the wrong grade and this trashed his engine just after a rebuild , i knew what i was looking at when i first opened the engine up . To prove my point to Eugene that it was fuel related we hand delivered one gallon of his gas straight out of his fuel tank in a new gas can to the lab in Columbus O for testing . we did not tell the lab who's gas it was , After testing the Lab told us who's gas it was and what octane it was and what all it had in it and how when it reached peak combustion just how HOT it got and how determental it was to a working engine under load as in Industrial app. . The fuel supplier ate the bill as they DID NOT deliver what was ordered . Once thqat probloem was solved all was well , till the next time when a new driver hauled a load of FARM gas and i got to put another set of pistons in . I myself never had and on farm tank and hauled gas to the tractor and combine that was also gas and it ran extremly HOT on reg grade aws well but it did not eat pistons just got hot , that one was powered by a 225 Chrysler slant six . And i fought that one the first year i had it , recored the rad installed a seven blade fan , change the water pump out to and A/C water pump as they move more water and have less drag. and old drag racers trick. . Nothing i did worked till i happened to buy some gas out of state coming home from my bi monthly trec to a sale in In. and i had bought and L shape fuel tank and a square tank for resale CHEAP with pumps and while filling my truck at this one station in Mt Co0mfort they had a pump labled FARM GAS and it was alot cheaper then gas back home and i was starting corn shelling . SO Cheap gas for the combine adds to the bottom line when doing custom work so i filled both tanks with a little over 200 gallon back then for like 65 cents a gallon compared to the 92 cents a gallon back home . The combine was running great and OH guess what instead of running at 220-240 it is now running at 185-195 . I( was being cheap with the combine as normqally when i went to go play farmer i would fill my pick up up with gas and fill my 105 gallon tank with the same hose qand i ran HIGH TEST in the truck due to the 460 i had stuffed in it and it was built to 1970 spec.s so the tractor as i had only one tractor of my own at that time frame i had no problems with the C 291 . Yes I H could have buiolt a better gas engine , they could have put seven main bearing in it , they could have added in a half dozen more head bolts , they could have made it across flow head , they could have taken a lesson from Chrysler and made them a Hemi head with four valves and might as well go with dual dist. and dual plugs per hole add in side draft webber carbs as well. I think three would be good. The only thing we need to get these old gals back to work is a good gas , don't even need the lead just good gas of 93 octane 95 would be a lot better and you could bump the ing timing up five degrees more and watch it come alive . Yes i have played with them seeing how much more ya could get out of one just by tuning and tweaking here and there. My late model 706 with a C junk 291 would NORMALLY pull my 710 4x16's in High one in sod and hardly had to use the T/A . If you were in last years corn ground and could stay in the seat then ya could run second high , little hard on shear if ya had rocky ground but she would run with it . I had a ton of money in that one as i had to give 950 for it in the DEAD ROW .

the C 291 sets at 18 a C263 on 706 sets at 23 degrees . Ya can't just go by one setting here . They all set different . 560 sets different and it has a C 263 . The C263's that i made into C 291s are changed for the 18 degrees and this is done inside the dist. and the work is done on a Dist. Strobe so i can check the curve at the different RPM's . One of the OLD lost art things recurveing a dist . . Something i use to do every day while working in a dealership , we use to pull the dist and place it on the strobe and install the new points and cond on the machine and then run it and check how the advance was working and correct any problems or tweak them for the young guys to give them a couple tenths less down the quarter and with a little more work might even squeak out a second and a half for them . If i really did not like you then you may end up with a DOG . Always made sure that the state boys and there 440 interceptors in the Fury II did not run as well as my 383 did .

I think you should give some info here on how long you have owned this tractor, AND how hard you have worked it, and what you actually do with it. I do know in cold weather they need to be warmed up. AND this blow by thing? do a compression test on it and post your results. this gas thing applies to hard working engines, running around the yard using it as a quad I hardly think you have scored pistons. but need more info on this.

I have read The Tractor Vet's theory on this motor for years regarding the gas, oil, and scored cylinders. I believe what he is saying and have tried to follow his advise. I use premium fuel and IH 30 wt oil. I am not a mechanic....just an acreage owner that loves old tractors. I am just hoping his theory is not correct in my situation!

Background: I have owned this tractor for 9 or 10 years. Right after I bought it, I found out an exhaust valve was torched. I had an experienced IH mechanic nearby re-do the valves. At that time he said the cylinder walls looked very good. I had it completely tuned up: plugs, wires, cap, rotor and he set the timing for me. It has started and ran very well over the years. I know it is very cold blooded in the cold. I usually plug it in for a while on real cold days. I let it warm up before I use it.

It does not ever get worked hard. It is used mainly for pushing snow in the winter and to take tree limbs and brush to the burn pile. I use occasionally to help a neighbor move stuff around. I do not pull a plow, cut hay, rake or anything strenuous with it. I have put probably less than 150 hours on it in 9 or 10 years.

It seemed like it was down on power last year before winter, so, I changed the spark plugs to NGK. It immediately pepped up...started and ran great with lots of power. Like I mentioned earlier, it blasted out a bunch of black soot/carbon/??? out of the exhaust. Still ran great. Earlier this year, I noticed it seemed to chug a little at higher rpms. Last weekend I moved a bunch of tree limbs in @ 20 degree weather, it seemed like there was more blow by coming out the tube. I was not running it hard...just slowly dumping debris into a burn pile. I don't know if it was oil...or possibly condensation in the oil burning out?? It was running for about 1 1/2 hours while I chain sawed and piled on tree limbs. The temp gauge barely got above the C on the low end.

I do not have a compression tester or any scope to check cylinder walls. I plan to check out some of the suggestions given: check the manifold bolts, check the muffler to see if it is clogged...etc. I did check the air filters..both inside and out and they both look good. What spark plug is recommended for the c-291? It ran better with the NGK...but, am willing to try something else.

Stock muffler is basicly a straight thru design . so unless someone stuffed something in there .

Kind of what I thought. But, I am looking into the top and see a metal plate straight down...with holes on both sides of pipe where exhaust blows through....suppose it could be partially blocked.

NO , after i said that about almost straight thru old age here i was thinking 706 muffler , on the 56 they went oval to reduce noise levels . so i9t hqas a little bit of baffling and NO i don't think or ever hard of a tractor muffler plugging . For it to plug it would have to burn oil so bad that you would be hauling oil to it by the tanker load and as far as old lead deposits Nah ain't happing . In here prime you can bet she got the snot worked out of it and at night the exhaust pipe and manifold had the high pro glow . I did a lot of night farming due to keeping everybody else in the field during the day and i could do mine under the cover of darkness . When plowing my exhaust manifold waws glowing bright cherry red and half way up the muffler and standing 3-6 inches of flame out the top . Gassers run way hotter then diesel even if you turn up the wick . To get any color on a diesel manifold you really got to turn up the wick and leave a trail of black they can see for miles like my old 450D would put out as i really had the wick turned up on it .

i gave my reply on the info you provided. my first thought was carbon build up wasp nest or mud daubers. working on tractors for 45 years you run into a lot of unusual problems. i once drove 40 miles on a service call on a leyland that wouldnt start after checking on fuel i figured out it wasnt getting any air. i took out the air filter to find a dead rat in the air intake hose to the filter. the boss told me he had a leyland that quit running whenever the fuel got to a 1/4 tank every time they hauled it in it would start right up. this happened several times. he said that he finally found a dead fly in the fuel tank it would get fall in the inlet on the sediment bowl and float out on the truck into the shop. we had a 560 gas that was doing about the same thing as your 756 . the boss asked where the customer was getting his gas we thought he was keeping it in a rusty fuel tank. he said he brought it from town in plastic fuel cans. i drained the gas and found pieces of plastic in the inlet for the sediment bowl. so i think you should check your sediment bowl also

I took the muffler off and drove around a bit yesterday...with ear plugs...it was loud! I didn't detect any difference in the way it ran. The exhaust seemed to flow fully when I put it back on.

I took the spark plugs out and cleaned and set the gap again, too. All bolts to the intake and exhaust manifolds were tight. I saw your idea regarding the sediment bowl today. I looked and it had a 1/2 of crud at the bottom. I cleaned the crud out as well as the screen. That just may have been the issue. I will definitely monitor that better in the future. It was too muddy and sloppy to take out for a test drive.

I am still unsure on how much blow by is acceptable. There is a steady stream of air blowing out....just more noticeable when it is cold?? It doesn't seem to be losing a lot of oil when I check the dipstick.