Time to rebuild?

kevbo82 · May 23, 2012

so i think i'm living in denial, and need to hear someone else tell me the truth. short back story, 73 750 that was my fathers, basically untouched since 1991, i just got it running by rebuilding the carbs and changing plugs and cleaning things up a bit. seemed like the motor was running a bit weak, today i checked the timing, the left side which seemed to run and sound good, was right on, the right side which ran like poop was around 20 degrees around 3000 rpm's. So i adjusted the points and got the timing right, it seemed to run a little better. Exhaust from that side sounded and felt weaker, and when i pull the plug wire on the left side she dies right away unless i give it throttle. but pull the right plug it stays running. i also saw small clouds of white smoke coming from the right side exhaust. so i also checked compression today. warmed the bike up for 5 minuets or so, took both plugs out, tested the left side a few times and usually got reading between 90-100. right side, 75-80. For these isn't 90-100 low as it is let alone the drop on the right side? i'm thinking that right side has a stuck ring :cry:

ML · May 23, 2012

Fresh plugs, oil and hi octane. Get on-board and thrash the max out of it. If runs betters leave it alone, if not start saving.

Mick

montelatici · May 23, 2012

kevbo82 said:
so i think i'm living in denial, and need to hear someone else tell me the truth. short back story, 73 750 that was my fathers, basically untouched since 1991, i just got it running by rebuilding the carbs and changing plugs and cleaning things up a bit. seemed like the motor was running a bit weak, today i checked the timing, the left side which seemed to run and sound good, was right on, the right side which ran like poop was around 20 degrees around 3000 rpm's. So i adjusted the points and got the timing right, it seemed to run a little better. Exhaust from that side sounded and felt weaker, and when i pull the plug wire on the left side she dies right away unless i give it throttle. but pull the right plug it stays running. i also saw small clouds of white smoke coming from the right side exhaust. so i also checked compression today. warmed the bike up for 5 minuets or so, took both plugs out, tested the left side a few times and usually got reading between 90-100. right side, 75-80. For these isn't 90-100 low as it is let alone the drop on the right side? i'm thinking that right side has a stuck ring :cry:

Check to see if the pushrods are opening the valves completely. The cams on the 73 and later had wear problems. The cam will wear down and won't lift enough to open the valves. Megacycle used to weld up the original cam and it would last forever afterwards.

1973-mk1-camshaft-wear-t8809.html

shrugger · May 23, 2012

If you would. Please explain how a worn Cam lowers compression. :?:

Likely Culprit #1 Valves
Likely culprit #2 Rings

montelatici · May 23, 2012

shrugger said:
If you would. Please explain how a worn Cam lowers compression. :?:

Likely Culprit #1 Valves
Likely culprit #2 Rings

Please explain how accurate compression testing is.

Guido · May 23, 2012

Let me get this straight.
It has been sitting for 20 years, you "rebuilt" the carbs, change the plugs and regapped the points and now it runs like poop.
Did you change the oil and filter?
Did you boil the carbs in carb dip?
Did you make sure the idle jets are cleared?
Are the point in good shape?
These are the easy things to check.
Do a leak down test and listen for air in the exhaust and intake ports.
The rings could be stuck, the guides could be sticking. IT'S BEEN SITTING FOR 20 YEARS FOR CHRIST SAKE MAN!!!!!!!!!!!!!!!!!
You don't just gas it up and expect it to be hunky dory. I don't care how many miles it has on the clock.
ok, I'm done.

kevbo82 · May 24, 2012

Guido said:
Let me get this straight.
It has been sitting for 20 years, you "rebuilt" the carbs, change the plugs and regapped the points and now it runs like poop.
Did you change the oil and filter?
Did you boil the carbs in carb dip?
Did you make sure the idle jets are cleared?
Are the point in good shape?
These are the easy things to check.
Do a leak down test and listen for air in the exhaust and intake ports.
The rings could be stuck, the guides could be sticking. IT'S BEEN SITTING FOR 20 YEARS FOR CHRIST SAKE MAN!!!!!!!!!!!!!!!!!
You don't just gas it up and expect it to be hunky dory. I don't care how many miles it has on the clock.
ok, I'm done.

okay, yeah i changed all the fluids in bike, and the oil filter. put new plugs in. took the carbs off, soaked them in a gallon of carb cleaner for a day or so, got a rebuild kit from amal including all new jets, needle, clip, float. cleaned out the carb and idle circuit best i could with some guitar strings and a spray can of carb cleaner, when i sprayed everything was flowing after i cleaned it so that looked good. i wasn't expecting to start up and ride off, just want more opinions if it seems like time to pull and rebuild the motor. obviously a loss of compression and uneven compression which is more than old points or fuel issues.

bwolfie · May 24, 2012

The good thing is it's a 750. you could pull the barrel and head off as an assembly to confirm the ring condition. You would also be able to inspect the valves if you fully remove it. It would be some work, but not too bad.

mikegray660 · May 24, 2012

did you check the compression with the throttle wide open? these readings seem pretty low, but as alluded too compression values from individual testers can be misleading, you'd just like them to be within about 10% of each other, which yours are not. If you didn't check with thew throttle open i'd retest and if the reading are still low you can add a bit of oil (teaspoon)to each cylinder to see if the reading come up dramatically suggesting rings are shot. one thing you haven't told us is how many miles on the bike? with proper maintenance and not being trashed too frequently you should about get 15-20k outta the top end before signs of wear appear.

kevbo82 · May 24, 2012

ya know what mike, the first time i tried to check compression i was using an older rubber type tester you hold in the plug hole, and trying to hold the throttle open, hold the tester, and kick, was not working out for me. I went back the next day with my thread in set and got those numbers, and i do think i forgot to hold the throttle open that time...but your right the difference is still pretty dramatic. The bike belonged to my dad, he bought it new and road it A LOT until 91 when he bought a new bike. I keep forgetting to check the mileage on the bike, but i'm pretty sure it's up over 100k. Not sure if he ever rebuilt anything in the motor at any time? He passed away earlier this year so i can't ask him directly, i do remember him years ago telling me something like "i thrashed that motor" he wasn't a casual rider, more of a hell raiser (i have newspaper articles about high speed chases involving this bike and the police lol) it's at my mothers house now, i think it's time to trailer it to my garage so i can get to work on it.

kevbo82 · May 24, 2012

okay i was being dramatic, i found this on my computer, looks like 21K to me

mikegray660 · May 24, 2012

Kevbo
Sorry about your Dad - 20K is border if thrashed (maybe) - i'd retest w/ throttle open - if the readings go up (you can do the oil trick too) i'd look else and be methodical about it, easy get caught up trying to get it running quick thinking - rings/top end no big deal in reality though

sounds like the bike will have a lot of fond memories 4 u -

best of luck - when you get it going you be like -

" that wasn't so bad"
m

kevbo82 said:
ya know what mike, the first time i tried to check compression i was using an older rubber type tester you hold in the plug hole, and trying to hold the throttle open, hold the tester, and kick, was not working out for me. I went back the next day with my thread in set and got those numbers, and i do think i forgot to hold the throttle open that time...but your right the difference is still pretty dramatic. The bike belonged to my dad, he bought it new and road it A LOT until 91 when he bought a new bike. I keep forgetting to check the mileage on the bike, but i'm pretty sure it's up over 100k. Not sure if he ever rebuilt anything in the motor at any time? He passed away earlier this year so i can't ask him directly, i do remember him years ago telling me something like "i thrashed that motor" he wasn't a casual rider, more of a hell raiser (i have newspaper articles about high speed chases involving this bike and the police lol) it's at my mothers house now, i think it's time to trailer it to my garage so i can get to work on it.

Time to rebuild?

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