Overheating Problem-Stumped and Need Help!!

[NOTFASTENOUGH]

1980 gs 750E 16 Valve

Recently I got back from a 5 day trip to The Smoky Mountains. I took this bike down on a trailer from Illinois and went to race the hills and twisties at and around Deals Gap US Rt. 129 The Dragon. Over the trip I put on 750 miles and this bike ran great. I decided that I was going to put more into this nice old beast.

About 500 miles later a great opportunity came. The cam chain needed to be replaced. As soon as it started making a little grinding at low rpms, I took it off the road and tore it apart. I decided not to try to get a linked chain but get the stock 1 piece chain since I was going to paint, polish, etc. go thoroughly through the bike restoring it. When I got the bike it had a 4-2-1 exhaust, paper filter, stock box, and stock 112.5 main jet, so I decided to leave the filter and box, but increase the main jets 2 sizes for a little better performance and to keep the bike a little cooler. Other than the chain noise, the bike ran well before I tore it down. I figured that While I had the engine apart, even though the compression was fine, I would put some new stock rings on.

So, new rings, cam chain, larger jets (also replaced other parts that were in carb kit), and when I put the bike together, valves cl. adj., synched, new plugs, wires,etc. temps were uneven significantely accross the cylinders. Header temps read 1-2-3-4 400F, 350, 230, 150 with a infra red thermo. Bike runs decent but blows white smoke out after a few min after it heats up and continues when I drove it just a little. Oh yeah, and the fuel mix screws on these non cv carbs adjusted to rpm are set (1-4) to 4 tuns out, 3.5, 2, 1. Before this project all were basically 1.75 out. So something is really wrong. Plugs 1-4 are white, white, white, black on the ceramic but all have carbon all over the rest of the plugs. My guess is that even though 3 of the 4 cyliners are running real hot, carbon is all over the plugs because the adj on the fuel mix screws needed everything ran rich to help alleviate the other problem(s)

I know a few mechanics and we try to trouble shoot this starting with the simple stuff of coarse. Ok, so we are pretty sure carbs are fine. I looked over them again later on to be sure. No air leaks around boots, petcock works fine. cam timing and valve cl. recheck fine. Oil passages were cleared when I rebuilt engine and while I was rechecking clearance, I cranked engine and oil was dispensed evenly over all intake and ex. cam lobes. My favorite and best exp. old school mechanic says that the reason they are running hot should have to do with a fuel air mixture problem. Others favorite answer is that I have too much top end wear. All this doesn't make sense since I had a working bike that didn't burn excess oil, run significantly uneven temps, or pass noticable smoke from the pipe. Especially since I increased the jet size which by the way caused the bike to have a lot more power. Any Ideas? Anything recommended to recheck? Currently the valve cover is off... Thanks so much for reading!! Alex

[Nessism]

The smoke could be oil, did you hone the cylinder and replace the valve stem seals when the engine was apart?

Regarding the carbs, it's generally adviseable to always use the stock Mikuni jets when possible. Those aftermarket carb kit jets sometimes lead to problems.

Good luck.

[robertbarr]

larger jets (also replaced other parts that were in carb kit),

You would not -- by a long shot -- be the first person whose problems began when you replaced OEM parts with those from a carb kit.

At the very least, I'd swap in the original pilots and mains, (even though the replacement main jets are supposedly larger), and then adjust the bleed screws again. For now. This, unfortunately, will necessitate a repeat of your sync efforts.

If you then decide to swap the larger mains back in, it shouldn't mess with your idle mixture or your carb sync, but you'd have the opportunity to eliminate these as a variable.

But my theory is that the pilots are either poorly machined or just plain wrong for that bike, and probably not even uniform from one carb kit to the next.

[tkent02]

Oh yeah, and the fuel mix screws on these non cv carbs

What non cv carbs are you running?

[rapidray]

Yeah, that was my next question. Ray.

[TheCafeKid]

Im still trying to figure out why the cam chain needed replaced? I have bikes with retarded amounts of miles on them that have never had the cam chain replaced, and all still measure well within specs. Good onya for rebuilding her, and assuring the oil ports are clear, as those motors are notorious for poor oil flow (having examined one again, suddenly it dawned on me that the oil gallery is at the very very bottom of the motor, and I wonder if this design has alot to do with delivery problems.)

Yes, what NON CV carbs are you running? Even increasing the jet size on a set of carbs that are simply too big may allow too much air into the set up, causing lean-ness and your overheating issue, but i suspect something else to be honest. Did you replace the oil pump when you rebuilt the motor?

[NOTFASTENOUGH]

The carbs are the stock Mikuni VM26SS. It does make some sense to me that the reason for differences in the turns in all the pilot screws could very well be due to differences in the pilots. And if they were not machined correctly then the main jets could b off too. I forgot which brand I used for the carb kit but I remember they were a fairly large reputable company and not mikuni because they did not have those jet sizes 4 me that I could find. I was talking to my mechanic back in Illinois and he said that he fixed a very similar problem with the engine running hot and unevenly in an old 550 (but no white smoke) by drilling out the main jets on the hotter cylinders.

I have not heard of such an idea that by increasing the mains (especially only 2 sizes) allowing more gas would cause a lean condition. I don't see how this might cause additional air to pass through a properly working carb. Maybe my carbs are not working properly now, but I did minimal cleaning since they were working great before the project. I do like the idea of eliminating variables though but am hesitant becuase I need to buy a manometer and cant find a decent inexpensive mercury one to buy. I used a home made one, which is Thousands of miles away and a pain in the Arse. I just relocated to Arizona. maybe Ill go that route again since it will b as inexpensive as shipping the old one and faster.

I did not hone the cylinder or or replace valve stem seals due to low miles 34K and performance of bike before froject. I also did not replace the oil pump. I was doing a partial rebuild and figured that this bike would be fine for enough miles and years to come for my satisfaction. As for the reson behind the replacement of the cam chain. The automatic tensioner was fully extended and the bike was running a little less smooth because of the increasing slack. When I shifted to a shorter gear, lets say from 2 to 3 and throttled it through from lets say around 2k even though that is not how I drive, there was grinding/ rubbing noise in the engine. This sound and feeling of slop and slack in the engine progressed increasingly stronger and quickly in about 100 miles. I've had this bike for about 2 years now and the bike came with the noname 4-2-1 exhause with stock jets. My theory is that this exhaust had been on this bike for quite a while and that since they did not increase the jet size to compensate the bike did run hotter. I assume that atleast one of the 2 previous owners also drove tahis bike hard. This extra heat and force led to significantly decreased cam chain life. Thanks for all your imputs so far.

Does anyone reccomend a specific site and manufacturer for pilot and main jets?
Also, does anyone know the standard hole sizes in the bottom of this stock air box? There are two identicle holes side by side but look large to me and possibly drilled out.

[TheCafeKid]

The carbs are the stock Mikuni VM26SS. It does make some sense to me that the reason for differences in the turns in all the pilot screws could very well be due to differences in the pilots. And if they were not machined correctly then the main jets could b off too. I forgot which brand I used for the carb kit but I remember they were a fairly large reputable company and not mikuni because they did not have those jet sizes 4 me that I could find. I was talking to my mechanic back in Illinois and he said that he fixed a very similar problem with the engine running hot and unevenly in an old 550 (but no white smoke) by drilling out the main jets on the hotter cylinders.

I have not heard of such an idea that by increasing the mains (especially only 2 sizes) allowing more gas would cause a lean condition. I don't see how this might cause additional air to pass through a properly working carb. Maybe my carbs are not working properly now, but I did minimal cleaning since they were working great before the project. I do like the idea of eliminating variables though but am hesitant becuase I need to buy a manometer and cant find a decent inexpensive mercury one to buy. I used a home made one, which is Thousands of miles away and a pain in the Arse. I just relocated to Arizona. maybe Ill go that route again since it will b as inexpensive as shipping the old one and faster.

I did not hone the cylinder or or replace valve stem seals due to low miles 34K and performance of bike before froject. I also did not replace the oil pump. I was doing a partial rebuild and figured that this bike would be fine for enough miles and years to come for my satisfaction. As for the reson behind the replacement of the cam chain. The automatic tensioner was fully extended and the bike was running a little less smooth because of the increasing slack. When I shifted to a shorter gear, lets say from 2 to 3 and throttled it through from lets say around 2k even though that is not how I drive, there was grinding/ rubbing noise in the engine. This sound and feeling of slop and slack in the engine progressed increasingly stronger and quickly in about 100 miles. I've had this bike for about 2 years now and the bike came with the noname 4-2-1 exhause with stock jets. My theory is that this exhaust had been on this bike for quite a while and that since they did not increase the jet size to compensate the bike did run hotter. I assume that atleast one of the 2 previous owners also drove tahis bike hard. This extra heat and force led to significantly decreased cam chain life. Thanks for all your imputs so far.

Does anyone reccomend a specific site and manufacturer for pilot and main jets?
Also, does anyone know the standard hole sizes in the bottom of this stock air box? There are two identicle holes side by side but look large to me and possibly drilled out.

THose carbs are not stock to that bike. The 16v 750 used BS32SS carbs. The VM26SS were used on the 77-79 8v 750. I am not even sure how you managed to get those carbs to fit, but i AM positive that this is your problem, or one of for sure.

[NOTFASTENOUGH]

I must have provided the wrong information sorry. The carbs on this bike are very likely the stock but I was not sure of the model since I don't know where the model # is on the carbs so I did a quick search online and that is where I came up with that model. I must have misread or got the wrong information, sorry.

[SqDancerLynn1]

Assuming the engine work was ok, You have carb problems. Sugest a good soak & clean and return to original jets. Make sure the carb boots are OK and air box is sealed good

[NOTFASTENOUGH]

Engine was running evenly hot before but not overheating. Either I can try to find some original exhaust or I will need larger jets. It is possible that the replaced jets were drilled out more for 1 and 2 by a previous owner to compensate for upper engine wear problems, and in that case I did not label which carb the original jets came from. I don't think that is a common fix at all and is actually quite difficult to do right according to my old school mechanic who has done this before. If the cam chain has worn that quickly on this bike, unless I get it running cooler than before my overheating problem, I can't expect the rest of the engine to last that long in my opinion even though these engines are tough. Thanks for the imput though and I take all advice into consideration. Thanks