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Discussion Starter #1
Hey All,
I recently bought a 2002 Yamaha 650 vstar custom and it has some issues. When I bought it I did some routine maintenance like replacing the spark plugs that were worn. The one had carbon build up which I contribute to the bike probably running lean. The other had what looked like oil all over the tip. I replace those then I replaced the battery. The bike came with air filter pods instead of the stock air system and the carb is pretty clean for a 2002. When I first put the bike back together it started up pretty easy and then I started to hear a small popping in the exhaust. I looked past that for a while and let it warm up and then went for a ride. I got nowhere because I would put it in gear and the bike would instantly lose power backfire like a gun shot and die. Since then I have drilled a hole bigger in the main jet and not messed to much with the float. The float needle is for sure bad and I have ordered new needles and gaskets for the carb. It was leaking fuel yesterday when I installed it so I hope that fixes that. But when I started it up it would not start without the choke and even then the backfire was louder than ever. It does have some aftermarket exhausts that I think might not be sitting right or have a gap where too much air is getting out. idk. My next step is to just remove the AIS and see if that has any fix to the pop. I have gasket sealer that i'm going to apply around the exhaust but I don't know what to do if that does not fix the issue. Honestly I'm desperate or answers.
 

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Those pod filters without proper retuning with properly sized jets and timing are more trouble than they are worth from what I've seen. But you may be on the right track looking air leaks. Not just exhaust but intake as well. Check the carb boots for cracks or pin holes. Maybe others have even more suggestions.
 

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Carbon on a plug likely means that cylinder is running rich. Oil on a plug likely means oil is getting into the combustion chamber for that cylinder. What you describe sounds to me like more than one issue.

Backfiring can be caused by various issues including fueling and air leaks but also valve timing. Have you run compression tests of the cylinders? Leak down test? Any visual inspections that would give an idea of how much carbon might be in the combustion chamber or on the valves?
 

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Discussion Starter #4
Carbon on a plug likely means that cylinder is running rich. Oil on a plug likely means oil is getting into the combustion chamber for that cylinder. What you describe sounds to me like more than one issue.

Backfiring can be caused by various issues including fueling and air leaks but also valve timing. Have you run compression tests of the cylinders? Leak down test? Any visual inspections that would give an idea of how much carbon might be in the combustion chamber or on the valves?
I have not tested the compression for that yet. Is there a way to do that without going to a shop? I do not see any places where there is build up but I haven't pulled the cylinder out yet.
 

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I have not tested the compression for that yet. Is there a way to do that without going to a shop?
Yes, you need a compression tester:
Amazon.com: Engine Compression Tester Testing Gauge Gage Check Test Tool Kit: Kitchen & Dining
Make Waves Compression Tester #7110 0-300 PSI Push In Style Free USA Shipping! | eBay
Compression Test Kit 8 Pc.

Disconnect ignition wires. Remove spark plugs, Install the compression gage. Turn the engine over 6 or 8 times. Record the result. If compression is low, add an ounce of engine oil to the cylinder and repeat the test. This will help identify the source of low compression. You can google "engine compression test" and read lots more about this online, or consult any Chilton's, Clymer's or Hayne's auto manual.

If compression is high, suspect excessive carbon buildup.

Carbon on valve seat can mean the exhaust valve is not seating. This would result in low compression and could cause exhaust backfire.

Are you sure that backfire is out exhaust and not out intake?
 

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Discussion Starter #7
Yes, you need a compression tester:
Amazon.com: Engine Compression Tester Testing Gauge Gage Check Test Tool Kit: Kitchen & Dining
Make Waves Compression Tester #7110 0-300 PSI Push In Style Free USA Shipping! | eBay
Compression Test Kit 8 Pc.

Disconnect ignition wires. Remove spark plugs, Install the compression gage. Turn the engine over 6 or 8 times. Record the result. If compression is low, add an ounce of engine oil to the cylinder and repeat the test. This will help identify the source of low compression. You can google "engine compression test" and read lots more about this online, or consult any Chilton's, Clymer's or Hayne's auto manual.

If compression is high, suspect excessive carbon buildup.

Carbon on valve seat can mean the exhaust valve is not seating. This would result in low compression and could cause exhaust backfire.

Are you sure that backfire is out exhaust and not out intake?
I am pretty sure it's coming from the exhaust. Its loud enough to where I can hear It coming from the tailpipes. I have not really considered it coming it from anywhere else... I only think its coming from there because when it first started happening I could turn around and hear it. When you say its not seating what do you mean? im just trying to get a list of things I need to check. I already just ordered an actual jet kit that I can play around with. Is there a valve that I can adjust without pulling the carb off? I saw one guy on youtube adjust a screw that I guess factory is 2 1/2 turns that adjusts the airflow/fuel levels.
 

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You might be able to borrow a compression gauge from you local auto parts store. Most have loaner tools. Or you can buy one if you are building up a tool kit. The AIS can cause back firing but normally only on deceleration. You can also just plug it if it isn't already.
 

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When you say its not seating what do you mean? Is there a valve that I can adjust without pulling the carb off? I saw one guy on youtube adjust a screw that I guess factory is 2 1/2 turns that adjusts the airflow/fuel levels.
An exhaust valve can be physically prevented from seating because carbon build-up has flaked off from inside the combustion chamber and gotten stuck on it's way out, past the valve. The valve can also warp or burn from heat. Any of these issues means the valve doesn't close entirely, and combustion gases then leak through. A compression test will reveal this.

Carbs can be equipped with "mixture screw" which can adjust the amount of fuel, or air, available at idle. This will generally only affect idle speed operation.

I would expand your analysis from thinking strictly about fuel ratio issues ( the carbs, the jets, the needles) and take a look at the more basic mechanical elements of the engine, like compression, valve functioning, timing. Once you verify that those elements are present and are correct, you can worry about refinements like correct fuel ratio.

How many miles are on the engine?
Had the seller or other previous owners modified the engine?
Did the seller tell you anything about what issues might exist with the engine?
 

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Discussion Starter #10
An exhaust valve can be physically prevented from seating because carbon build-up has flaked off from inside the combustion chamber and gotten stuck on it's way out, past the valve. The valve can also warp or burn from heat. Any of these issues means the valve doesn't close entirely, and combustion gases then leak through. A compression test will reveal this.

Carbs can be equipped with "mixture screw" which can adjust the amount of fuel, or air, available at idle. This will generally only affect idle speed operation.

I would expand your analysis from thinking strictly about fuel ratio issues ( the carbs, the jets, the needles) and take a look at the more basic mechanical elements of the engine, like compression, valve functioning, timing. Once you verify that those elements are present and are correct, you can worry about refinements like correct fuel ratio.

How many miles are on the engine?
Had the seller or other previous owners modified the engine?
Did the seller tell you anything about what issues might exist with the engine?
The bike has about 20,000 on it and the seller did tell me it was popping before but he said he rode it while it would pop. He had the carb pulled off when I got there to pick it up so I never heard it run. When I messaged him about it he said his mechanic said it needed more fuel because of the aftermarket exhaust. Other mods include the pods for the air filter replacement. I just looked and the AIS is already removed. The bike was made to look like an Indian bobber so it has the back chopped and an aftermarket seat. Other than cosmetics, the air filter and the AIS there aren't any major changes.
 

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Sounds like this bike needs to be both fixed and tuned. You'll need to learn how to "read" spark plugs to get it right.
Luckily the spark plugs are easy to get to on your V-Star...

The one had carbon build up which I contribute to the bike probably running lean. The other had what looked like oil all over the tip.
Carbon on the plugs would be due to burning oil or running rich with fuel, maybe both. What looked like oil all over the tip could be just that, or it could also be a rich condition.
One thing you can be sure of is that if the carburetors were overflowing and leaking fuel the bike IS running WAY rich, so there is that.
The bike came with air filter pods instead of the stock air system and the carb is pretty clean for a 2002.
his mechanic said it needed more fuel because of the aftermarket exhaust. Other mods include the pods for the air filter replacement.
Pod air filters make a VERY big difference in carburetor tuning. The aftermarket exhaust is of secondary concern when you open up the air inlet by installing pod filters.
I already just ordered an actual jet kit that I can play around with.
The adjustable jet needles will be of some use but the main jets that come with the kit will be too small for use with your pod air filters.
Drilling jets will just add confusion to the tuning process. You will need to buy a selection of main jets.
With an open air inlet (pod filters) you can expect to end up somewhere around five ~ six steps larger than stock on the main jets with the V-Star 650.
But when I started it up it would not start without the choke
^ Classic example of carburetors with plugged jets and probably plugged ports (passages) inside the carb body itself.
I'm thinking make sure the carburetors are good and clean, inside carb body too, then start working on tuning.

S F
 

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Wot the others have said above. Plus. Open the throttle when testing for compression. Find out what the stock main jets are, and put those in. How many carbs does it have ? Throw away the jet you drilled. Get some large and some smaller main jets. But before I want to talk about jets, I would like to know the compression. I have an XS400 with foam pod filters, and non stock mufflers, running the smallest jets that were stock. There is a Mikuni carb tuning guide on line. It works well with reading the plugs.

Bon chance. UK
 

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Discussion Starter #14
Wot the others have said above. Plus. Open the throttle when testing for compression. Find out what the stock main jets are, and put those in. How many carbs does it have ? Throw away the jet you drilled. Get some large and some smaller main jets. But before I want to talk about jets, I would like to know the compression. I have an XS400 with foam pod filters, and non stock mufflers, running the smallest jets that were stock. There is a Mikuni carb tuning guide on line. It works well with reading the plugs.

Bon chance. UK
Wot the others have said above. Plus. Open the throttle when testing for compression. Find out what the stock main jets are, and put those in. How many carbs does it have ? Throw away the jet you drilled. Get some large and some smaller main jets. But before I want to talk about jets, I would like to know the compression. I have an XS400 with foam pod filters, and non stock mufflers, running the smallest jets that were stock. There is a Mikuni carb tuning guide on line. It works well with reading the plugs.

Bon chance. UK
I just ordered some new jets so once those come in I will update how they work. It has two carbs and I plan on doing the compression check today. I picked up the pressure tester yesterday so again ill update that as soon as I get some results. Yesterday I also went ahead and adjusted the timing on the engine. The front seemed to be pretty off and I did not have time to look at the rear.
 

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I also went ahead and adjusted the timing on the engine. The front seemed to be pretty off and I did not have time to look at the rear.
On most twins I've worked on, you adjust the ignition timing for one cylinder and that's it. The solid crankshaft ensures that the others remain in the same relationship to it. Some older bikes had two sets of points, but I'm pretty sure yours isn't like that. If the timing seemed like it was more than 15 degrees off, and the engine had been running, albeit poorly, I'd be wondering if I misread the instructions.
 

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The compression gauge may have a screw in hose for the plug hole, or it may have a tapered rubber piece. If it is rubber, you need to push on it fairly firmly to prevent leakage. The manual should have the pressure listed. My older Yamaha's are around 150 from memory. My 41 Ford tractor is 114 when new, and a bike we ran at the track was 180 and up. There is always a warm fuzzy feeling when we get these older bikes running and serviceable. My winter daily ride is an 83 XS400, that I spent a long time getting the jets correct.

UK
 

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Yesterday I also went ahead and adjusted the timing on the engine. The front seemed to be pretty off and I did not have time to look at the rear.
Confused am I.
The V-Star 650 is a very popular machine in my area. I've serviced and performance tuned a bunch of them.
There is only one 'pick up' that triggers the ignition timing (spark) for both the front and rear cylinders. The one and only pick up coil is bolted in place and non adjustable.
Or, are you saying the cam timing is out?
Are you sure this is a 2002 Yamaha V-Star 650 Custom we are talking about? (XVS 650)

S F
 

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Discussion Starter #18
Ok so for all you still wondering I’ve narrowed down where the exhaust is popping. It’s the rear cylinder that I’m having the issue with. Today after I messed with the carbs a little cleaning them and whatever I tried to start her up. She still won’t start without choke but it also hasn’t started well before so it’s cold and hard to start. I got it started on choke and had to hold the choke down to keep it running. The rear exhaust was shooting small flames out and making the worst pop ever. I ended up doing a compression check and found that the back cylinder was about 50 psi vs the front which was about 150. I added some wd40 to just get the cylinder back and it went up to 150 as well. I put it together and still pops. I tested the pressure and there was no loss of pressure even after I let it sit. So now I’m at a loss for where to go now. My timing should be good I made sure all points align at top dead center and my carbs are cleaned.
 

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Usually if you have to use choke to keep it running, that suggests the small jets are plugged. Not uncommon, even after cleaning the carbs.
UK
 

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I added some wd40 to just get the cylinder back and it went up to 150 as well
The usual interpretation of compression increasing in a cylinder after adding oil is that this has managed to seal the rings which were previously leaking. One cause of this is that, if the engine has been sitting, the rings have "set" in a compressed state, and aren't properly riding the cylinder wall.

Did the seller tell you whether the bike had spent substantial time sitting still, prior to his selling it?

My pet theory is still that the exhaust popping and the fire show is being caused by a leak at the exhaust valve, despite the suggestion of a leak around the piston rings. Even if the popping is a fueling issue, you're going to have to fix that compression issue before the engine will run right.

One thing I might try before deciding I needed to pull the head off, would be to douse the piston and the exhaust valve with an aggressive penetrating oil, (I like Kroil, but 50/50 ATF and acetone is not bad, and a lot better than WD40, IMO) and letting them sit for a day, then kick the engine over a few times without starting it, and repeat. The idea is that the penetrating oil can free the rings and soften any carbon on the valves, so when you eventually start it, any carbon on the valves flakes off and gets blown out. The exhaust valves can be accessed by through the exhaust ports by pulling the exhaust pipes off. You want to get the exhaust valves in a closed position so the penetrating oil will sit on the top of the valve and penetrate over a period of time. Vibration helps it penetrate so if you can figure out how to inject some vibration into the engine, that will be helpful. Maybe put a speaker on the cylinder and play John Bonham drum solos for a few hours. That should teach it who is boss!

There are other techniques for getting the valves to seat without surgery, including the "Italian tuneup", where you flog the engine at wide open throttle for a painfully long few minutes, the "water injection de-carbon treatment", and the "Seafoam top end cleaning". The Seafoam is the least brutal of these.

Good luck.
 
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