1983 VT750 Project - Honda Shadow Forums : Shadow Motorcycle Forum

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Old 07-31-2012, 05:55 PM   #1 (permalink)
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Default 1983 VT750 Project

I recently bought a 83 VT750. The story behind it is, the guy I bought it from bought it without seeing it running. He brought it home and realized it wasnt running right, so he brought it to a mechanic to get the carbs rebuilt. After doing so, it was still not running correctly, so he did a compression test and the front cylinder has low compression. He didnt want to sink any more money into it. He left it for a year and now decided to put it up for sale.
I bought it, and I'm now in the process of trying to get it to fire up. I am picking up a new battery today, and will go from there.
I have a couple question for you folks.
First, what are the positions for the petcock? I have heard back is on, down it off, and front is reserve, but I'm not sure what year of shadow that is from.

Second question is about the compression issues. The mechanic told me that he thinks it was overheated which caused the low compression. He also said it's hard to start becuase of it, but once it's running it's not too bad. I'm thinking rings, possibly valves, but if anyone has some advise on this, it would be appreciated.

Below are some pictures.



Last edited by corner27; 07-31-2012 at 05:58 PM.
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Old 07-31-2012, 07:57 PM   #2 (permalink)
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That looks nice. A great project.

The petcock only has two positions: open and closed. The reserve is a small tank that sits below the front seat, behind the air box.
Get the shop manual. It's a real treasure.

You may have a stuck exhaust valve, or a blown gasket.
There is a test that can be done which checks the content of the coolant for carbs. If that is the case, the gasket is blown. Changing it is a bit of trouble, but no biggie. You'll have to remove the engine in either case.

Good luck and keep those pics coming.
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Old 07-31-2012, 10:53 PM   #3 (permalink)
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That's a nice lookin bike! I'm not aware of any "reserve" tank or reserve position of the petcock. There is a secondary tank under the seat but does not function as a "reserve" in any manor. It does have a fuel gauge and lights up on your panel when the fuel is low, but it's not really a reserve.

Before pulling the engine and tearing it down I'd suggest the following. Test the compression and see what it reads. Then pour a teaspoon of oil into the chamber via the spark plug hole and check the compression again. Is there is a considerable difference (ie far from spec dry and close/on spec with the oil added) that's good indication that the rings need replacing, and maybe a cylinder honing.

I'm currently rebuilding my 83 750 engine. I didn't have any major problems that really required a rebuild but wanted to go for it anyway and learn about engines. Turns out it was good decision because the head cover was cracked and the oil pump was cracked. I have some good pics that may help you out. I don't how engine savvy you are but pics are fun anyway. engine rebuild and more...

Luis also has a great thread on his "restoration" which is more accurately described as a resurrection and complete overhaul. Great pics and lot of good information here. 86 Honda VT750C Restoration log

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Originally Posted by luisr320 View Post
There is a test that can be done which checks the content of the coolant for carbs. If that is the case, the gasket is blown.
Can you explain that more, Luis? Coolant for carbs? I'm confused. Do you mean testing for coolant in the oil?

Good luck! Keep us posted!
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Old 07-31-2012, 11:42 PM   #4 (permalink)
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I got the new battery charged up and after a bit of cranking, she slowly came to life. There are a few things I have noticed.
1) It looks like some of the covers have been replaced by a previous owner, but instead of using gaskets, they opted for the redneck fix of using gasket in a tube. The round cover, and larger cover it attaches to on the right side of the bike has oil seeping from it. It's not a huge leak, but definetly something I will fix when I pull the engine.
2)There is definetly an issue with the front cylinder. As the mechanic stated, it is a bit hard to get started. When I have the bike running, you can feel the exhaust is hot from the rear cylinder, but just air blowing from the front. If you give it some throttle, the front cylinder will fire off an on, and "heavy exhaust" comes out the pipe.

I don't currently have a compression tester, but I plan on picking one up this week. Is there a way to test if the issue is in the valves, or with the rings? I've replaced rings on a 2 stroke before, but dealing with heads on 4 stroke will be a new adventure for me.

Corey
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Old 08-01-2012, 12:13 AM   #5 (permalink)
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I quickly ran down to Canadian Tire before they closed and picked up the compression tester. I did the test on the front cylinder and it showed 60 psi. I put a bit of oil down the cylinder and it went up a little bit to about 70, but after testing it a few more times, it went back down to 60. One thing I did notice is, if I held the throttle open while doing the test, it went up to 120.
On a side note, the rear cylinder tested at 140.

Last edited by corner27; 08-01-2012 at 12:23 AM.
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Old 08-01-2012, 01:21 AM   #6 (permalink)
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I believe compression testing the cylinders the throttle is supposed to be open. I made that mistake on an intruder I picked up and it was real low, after I read to do it with throttle opened it was 220psi in each cylinder and 190-225 is the norm. Might be why the PO thought it was bad. Do you know what your numbers are suppose to be at?

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Old 08-01-2012, 03:53 AM   #7 (permalink)
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Quote:
Originally Posted by mattyo View Post
Can you explain that more, Luis? Coolant for carbs? I'm confused. Do you mean testing for coolant in the oil?
Good luck! Keep us posted!
You test the presence of hydrocarbon in the cooling fluid and or of exhaust fumes on the radiator (not sure which):


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Old 08-01-2012, 04:19 AM   #8 (permalink)
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Quote:
Originally Posted by luisr320 View Post
You test the presence of hydrocarbon in the cooling fluid:

Oh yes, test for hydrocarbons in the coolant. The oppisite of my "test for coolant in your oil". Your carb reference really threw me off there. Apparently "carb" was referring to hydro"carb"ons? But yes, that would be a quick and easy test of the head gasket integrity. I agree that would be a good test to do.

Your compression does sound a bit low. If I remember correctly (I could be a little off here) I think the 83 750 calls for around 170-190psi on the compression. So that one cylinder (at 120) definitely sounds low and the other cylinder (140) also seems low. I'm not sure if that's totally due to bad rings or pitting/scratches in the cylinder but the low compression should be addressed and looked into. The head gaskets could also be the problem. Oh well, not a big deal but you'll probably want to rebuild those cylinders, get new rings, and also replace the gaskets anyway since you'll have it torn down.

I'm sorry, but at least you went into the deal knowing it might be a bit of a project bike.

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Old 08-01-2012, 02:07 PM   #9 (permalink)
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Why would there be such a difference in results with WOT and closed throttle? The 140 I got on the back cylinder was with the throttle closed. I will check it at WOT tonight and see what it reads.

I was originally thinking if it would run half decent, I would just ride it as is for the summer, and tear it spart this fall, but I'm starting to lean towards just tearing it apart now.
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Old 08-01-2012, 10:01 PM   #10 (permalink)
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Quote:
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Why would there be such a difference in results with WOT and closed throttle? The 140 I got on the back cylinder was with the throttle closed. I will check it at WOT tonight and see what it reads.

I was originally thinking if it would run half decent, I would just ride it as is for the summer, and tear it spart this fall, but I'm starting to lean towards just tearing it apart now.
I have no clue why, but when I bought the intruder I thought I was screwed and gonna have to replace a head, front cylinder was 120 rear was about 140 but then I read on a forum to be sure throttle is wide open while testing, and both cylinders hit 220.

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Last edited by zer071; 08-01-2012 at 10:07 PM.
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