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post #1 of 11 (permalink) Old 01-27-2019, 11:38 AM Thread Starter
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Carb questions

I'm new to a dual carb setup but not new to carbs in general.

A little history on this 750. She's a 95 and the previous owner did an ear shave. However after completion he wasn't able to get her going again. He decided to let it go because he was over his head, his words. Now I've been able to sync the carbs and got them both primed. She'll run for a few and die out. Both lines are out of fuel at this point so I know it's a feed issue. I tested and cleaned the petcock and both sides flow perfectly. One cylinder runs (rear) but the other is weak and doesn't seem to run. In checking everything I found the front carb blows while the rear sucks. I'm fairly certain this is my flow issue as the vacuum, for the fuel, comes off the front carb.

My question is 2 fold. First is this normal? And second is not, fairly certain it's not, how do I go about fixing it. What would cause this? Oh also I know he had a leaking diaphragm, not sure which one, which he replaced.
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post #2 of 11 (permalink) Old 01-27-2019, 12:06 PM
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Not normal. Some reversion may be seen, but a larger amount of air coming from the carb is not normal. If there's excess air exiting the carb throat, that's a problem.

Maybe an intake valve stuck/leaking in the front cylinder? How is the compression on the front cyl.? Just a thumb check should show if there's an intake valve leaking. The valve problem would prevent the cylinder from making much vacuum.

Could possibly be valve timing.

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Last edited by Spockster; 01-27-2019 at 01:11 PM.
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post #3 of 11 (permalink) Old 01-27-2019, 12:50 PM Thread Starter
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Nail on head! 30-60 lbs per ft on the front, from right to left. 150 both sides on the rear. Guess I'm doing a valve job on the front, after I check lash. My manual should be here this week to verify against.

Thanks I'll post again once I fix this part.

The chain tensioners could have something to do with the timing though, yes?
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post #4 of 11 (permalink) Old 01-27-2019, 01:03 PM
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Originally Posted by Corvine63 View Post
Nail on head! 30-60 lbs per ft on the front, from right to left. 150 both sides on the rear. Guess I'm doing a valve job on the front, after I check lash. My manual should be here this week to verify against.

Thanks I'll post again once I fix this part.

The chain tensioners could have something to do with the timing though, yes?
Loose cam chains would affect valve timing, but it looks like a stuck valve on the front.

Have you been able to get it to normal operating temp yet? Sometimes the valve will free up if it's not seized bad, it's usually just varnish on the valve stem and sitting too long. The valve may have been sitting fully open, then stuck, so now there's no force from the rocker arm to get it moving again.

Seafoam in the oil would help if you could get the valve to start moving first. Maybe pull the valve cover and try to hit the valve stem with seafoam or cleaner, every day for a few days? You could crank the engine and see which valve isn't moving, but I'd probably try to spritz them all. Then most certainly run seafoam in the oil for 200-300 miles.

There a link for a repair manual to download, it's in my signature.

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Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #5 of 11 (permalink) Old 01-27-2019, 01:19 PM
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Edited my post #2 and took out half of the last sentence ... Didn't remember the engine layout. Separate chain systems and tensioners for each cylinder, so one won't affect the other.

Wouldn't hurt to check out the tensioner for the front cylinder. If the exhaust valves are also open at the wrong time, could be the tensioner. Maybe?

If an intake valve is stuck, there's also a good chance it's both intake valves. Four valves per cylinder on this baby.

2003-21k mi
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__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #6 of 11 (permalink) Old 01-27-2019, 01:26 PM Thread Starter
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I appreciate that. I'll work on that now. As for getting her up to temp, nope. Longest I've gotten her running is 2 min. Mostly due to the fuel feed issue. She's close though. If I can get the front sucking, then I'm fairly certain she'll feed fine.
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post #7 of 11 (permalink) Old 01-27-2019, 01:34 PM
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You can run the vacuum from the right side carb to get fuel flowing. The emissions valve won't affect much just testing.

Have to be wary of running it ... unburned fuel will be present in the cylinder and exhaust, and there can be some backfire into that left carb. Also possibility of fuel washing oil off the cylinder, and contaminating the oil in the crankcase. Any valves stuck open will have loose rocker arms, possibly some tapping.

A cycle or two up to temp might free it up though. I wouldn't rev it much, a reasonable fast idle should be ok.

2003-21k mi
Shaved w/UniPK92+Stock Jets-TPE/MOSFET-Shinko Tires-AGM batt-bags-chrome-LEDs...more
__________________________________________________ ____________
Repair Manual: http://www.mediafire.com/file/mj7z81..._Catalogue.pdf

Owner's Manual: http://www.mediafire.com/file/nscb5f...ers+Manual.pdf
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post #8 of 11 (permalink) Old 01-27-2019, 03:26 PM Thread Starter
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Ok so I found the left side front valve not under the arm. The arm was just pushing on the spring. Removed the cam cap and got it reseated. Got the cap back down and torqued. Still only getting 30lbs of compression but the front carb is now sucking. The batt is low so she's not turning over well. I'll let it charge and test again. Do you know if this is an interference motor? I'm really hoping not. I found some scoring on the bearing faces on the one cam but the cam felt good. I'm wondering if someone has been in there. Those arms don't just come off, typically.
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post #9 of 11 (permalink) Old 01-27-2019, 05:14 PM Thread Starter
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Also found that the front cylinder is about 90 out. Completed a leak down test and found this. At TF, it either leaks out of the carb or pipes. The rear is either pipes or crank case. I figure the rings are worn. Once I rolled it around more, them the front vented to the case. I figured someone had been in there. Once I get my book, then I'll fix that too.
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post #10 of 11 (permalink) Old 01-31-2019, 01:05 PM
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Do the tw200 petcock mod and be done with fuel flow problems.

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