dgoodsy's '72 gt750 revival/log thread

[dgoodsy]

Yes, they are very different tests really.

In other news I got out to the garage last night but didn’t have the ambition to move things around and crawl on the floor to figure out the leak discussed above. Instead I worked for a bit at cleaning carbs. I found getting the needle jet out to be quite tricky until I figured out the washer under the main jet was still resting/stuck on the needle jet. Once it was removed all 3 needle jets move quite easily. I bought some wooden dowel (5/16 dia I think is the perfect size) and hacked a small groove along the length of it and it works great to tap the needle jets all the way out.

I finished one carb completely. Well, complete enough for me. All the jets and internal passages seem very clean and smooth to me. I did not have a magnifying glass though, I intend to pick one up before I do the other carbs. The outside of the carb I scrubbed with a nylon brush and got it cleaner but it is still stained in spots. Not nearly as clean as carbs I have seen on here. I did pick up some Pinesol the other day so I can get a little more aggressive with cleaning the outsides if I find the ambition, at this point though I’m not too concerned with the cosmetics of the carbs. Maybe one day.

And a side note/ramble question:
I knew the carbs were not all jetted identical and I did not mark what carb went where so I wasn’t sure which one was the center carb. The left and right are easy enough to figure out with some logic. I figured I could tell by the checking the main jets. Sure enough 2 carbs had 102.5 mains and one had 100. So I figured the 100 was the center one, good. The GT750 service manual states exactly this, and also that the left and right carbs have P-4 Needle jets while the center uses a P-3, and all three use 5F16-3 needle. That’s all fine and I trust the manual but it seems backwards to me that the center cylinder would use leaner jetting through the midrange and top end. Leaner = hotter, and as I understood it the air cooled 4 cylinder 4 stroke bikes that had the inside carbs jetted different than the outside carbs was to jet the middle cylinders richer to help the cylinders remain cool because of the poor air cooling on those cylinders inherent in the design, blah blah. So what’s going on with this bike, why is the center cylinder jetted leaner? I realize liquid cooling makes it different than air cooled in that respect, but I would have expected them all to be jetted the same.

[tz375]

Outside of the carbs are just for looks. What matters is the passageways. The easiest/best trick is to blast wd40 or carb cleaner through one drilling in one carb and then repeat on the other two and compare results. Repeat on the next drilling on all 3 carbs and so on. Same with jets. Try one and repeat on other two for comparison.

The hot running center cylinder is an old triple 4 stroke problem that doesn't appear to have a direct parallel on out triples. I have heard it said that the split exhaust makes the difference to running temps, but I'm not in a position to confirm or deny that.

With 3 expansion chambers we usually go to the same jetting across all three pots.

[dgoodsy]

Agreed about the outside being clean

Outside of the carbs are just for looks. What matters is the passageways. The easiest/best trick is to blast wd40 or carb cleaner through one drilling in one carb and then repeat on the other two and compare results. Repeat on the next drilling on all 3 carbs and so on. Same with jets. Try one and repeat on other two for comparison.

Agreed on the "just for looks" part. Exactly why i quit scrubbing the stains. Don't get me wrong, i do appreciate a set of carbs that is sparkling clean and unstained, but its not worth that effort at this point for this bike. I have read that suggestion about comparing the parts from different carbs as you spray carb cleaner through them to see if they are consistent, but hadn't thought of it while I was doing the cleaning. I want to do that now. Maybe i will get it done when i have the bike pulled away from the bench and have space to take a few carbs apart and keep organised.

The hot running center cylinder is an old triple 4 stroke problem that doesn't appear to have a direct parallel on out triples. I have heard it said that the split exhaust makes the difference to running temps, but I'm not in a position to confirm or deny that.

Aah, I hadn't thought about the exhaust making the difference. I wondered about the airbox but it seemed to me like there wouldn't be any bias towards or away from any of the 3 cylinders. Good enough to cure my curiosity.

[tz375]

I forgot to mention that the carbs are marked with their positions. They should have numbers like 310 12L or 12M or 12R, obviously left middle and right. On yours I think they should be 301-12 and 73's are 310-13s

[dgoodsy]

Remove the alternator cover first to see if it's dry in there and then remove the gear shift lever and the two left rear covers around the sprocket. Pour some more diesel in and see where it leaks out of.

I got around to doing this today. The alternator area was dry so that's a good sign. I took the two drive sprocket covers off, it helped confirm where the leaks were.

When I poured diesel into the right cylinder I found it dripping off the right exhaust pipe. I followed the leak from the exhaust pipe up the side of the engine and found that it was leaking out of the oil injection port (no oil injection pipes on there at the moment). Evidently the diesel in the cylinder was filled higher than that port and that is where it escaped. I found the center and left cylinders where leaking out of their oil injection ports as well, however they leak into the starter cavity, and the diesel runs down that cavity drain near the chain sprocket, which I could see with those covers off.

So I am much less worried now about major leakage in the crank seals. I did notice when I was pouring diesel in the center cylinder that a bit more leaked out of the right cylinder oil port. At first I wondered if it was leaking from the center to the right cylinder, but after thinking about it more I changed my mind. I had just turned the crank a bit to see the bottom of the center cylinder which I think caused the crank to displace some diesel and leak bit more. But this is all guessing and surmised again, a leak down test will confirm or deny these guesses.

I forgot to mention that the carbs are marked with their positions. They should have numbers like 310 12L or 12M or 12R, obviously left middle and right. On yours I think they should be 301-12 and 73's are 310-13s

I did see this in the manual when I was confirming my jetting was according to the manual but forgot to look for the numbers when I was in the garage. Next time!

[dgoodsy]

I had a few more minutes to work on the carbs again tonight. Hopefully soon I can spend some quality time with the project and make decent progress, hasn't really happened yet.

Last time I was working on the carbs I found one of the floats had a leak, which resulted in a stream of bubbles when the float was dunked in hot water. I accepted that it would need to be replaced but I also wanted to have a go at fixing it. The bubble stream came from a little crevice that is too small to get solder into really. I am not really sure how but after a bit of fiddling around with the soldering iron I tested it in hot water and it didn't seem to bubble.

I didn't add very much solder to the float but I realise it will have an effect on the relationship between fuel level and float height so I will have a close eye on it. I need to order some bowl gaskets so I might add a float to the order so I know I have one if the fixed one turns out to not be usable. I'll

[dgoodsy]

I spent a few more minutes in the garage last night and i replaced the banjo check valves in my oil lines. last fall i had purchased some kawasaki fittings to replace the ones that were not working properly, thanks to me and the air compressor. I removed the old banjo valves and replaced with the kawasaki items and filled the oil lines with oil and let it hang for the night. One of the new valves leaked a bit of oil but I'm not too worried about that yet, it looks like a tiny little bit of oil on the shop towel below that fitting. It could be just a bit of oil that dripped out from inside the fitting that i didn't wipe out prior to hanging, and maybe not actually leaking through the fitting. I bought a dark colored oil specifically so i could see it in the pipes and see where there were bubbles etc, but did not have it handy, so i used regular lighter colored oil and as a result i can't tell how much if any the oil level in the pipe has dropped.

The single check valve on the left-most oil line i did not replace. It is the one that is not in a banjo fitting, it is in the middle of the line. I thought I read recently that there was some outfit selling just the springs and balls for the check valves but can't find anything of the sort in a search. Does anyone else recall that (and have a thread link handy ) or am i losing my mind?

One more simple question. Where are the drain bolts for the crankcases? I tried the ones that looked logically like they might be drain bolts based on location (14mm and 10mm bolt heads) but they were long bolts that had dry threads. Not what i was looking for, since there is a bit of diesel in the crankcases.