Suzuki GR 650 Tempter problem

[GuyRDag]

I'm having some trouble with my 1983 GR 650 Tempter, and I just can’t figure out what’s causing it. It runs poorly at any RPM above idle. As soon as I give it some throttle, it starts sputtering, popping, and blowing back through the carbs. It was running fine, and then suddenly, overnight, it started running like crap.
What I’ve checked and found to be in order:

• Ignition timing is advancing as it should
• Charging system is working properly
• Coils and caps are OK
• Inlet manifolds are new
• Spark plugs are new, and the spark is strong and blue
• I'm using the standard exhaust mufflers, so they shouldn't be causing any issues

This led me to suspect the carburetors, since it runs fine at low speeds/idle. At idle, the butterfly valve is closed, and fuel delivery goes through the pilot jet. I’ve cleaned the carburetors multiple times, including ultrasonic cleaning, but there’s been no improvement. Since the carbs are CV and I’m running pod filters, I thought the issue might be jetting. I upsized the jets based on a forum thread I found, but that didn’t help either. I then swapped in some VM34 carbs I had lying around to see if that would make a difference, but the improvement was minimal. I tried several different jet settings, but noticed no significant change.

With the carbs off, I noticed that when I turn the engine manually using a socket wrench, it sometimes blows some air back through the inlet on both cylinders. This made me think I might be dealing with leaky valves or a cam chain that had skipped a tooth. Compression was fine, so I removed the valve cover to check cam chain timing and valve clearance. The valves were within spec. I followed the procedure for checking cam chain timing: I set the crankshaft at top dead center (TDC) with the "T" mark aligned with the timing mark on the engine case (see picture). The exhaust cam’s mark 1 was aligned with the engine case or slightly above. However, the mark 2 arrow on the inlet cam was slightly below the engine case (see picture). The manual specifies there should be 24 pins between mark 2 on the exhaust cam and mark 3 on the inlet cam. I counted (multiple times), and this also seems correct (see picture). Everything looks good, except for the mark 2 on the inlet cam not quite lining up with the engine case.

I then checked the cam chain itself. The manual states that a 21-pin length should be a maximum of 128.9 mm. Mine measured 128 mm, so it’s within spec. The cam chain tensioner also appears to be functioning properly.

At this point, I’m at a loss and feel like giving up. Any help in pinpointing the culprit would be greatly appreciated.

 

[Downeaster]

What color are the spark plugs after a run? They should be a tan/light brown color. Too light indicates a lean condition (too much air, not enough fuel), too dark indicates rich. My first suspicion would be an intake leak causing a lean condition.

 

[GuyRDag]

With the original carbs, one of the spark plugs did show a color consistent with running too lean. With the VM34s installed, it ran a little better, but I had trouble synchronizing the carbs since they aren’t linked and had to be operated using a split-type throttle cable.

Is it possible that all the popping, hesitation, and sputtering is caused by the carbs running too lean?

 

[Wingedwheel]

Will it smooth out once you get past the mid-range?

 

[Its been a long time]

With the original carbs, one of the spark plugs did show a color consistent with running too lean. With the VM34s installed, it ran a little better, but I had trouble synchronizing the carbs since they aren’t linked and had to be operated using a split-type throttle cable.

Is it possible that all the popping, hesitation, and sputtering is caused by the carbs running too lean?

Yes, it's possible.

 

[Highpower]

When I run into problems like this, I always got the the last thing I did before the issue appeared. If it was running fine and then overnight starting running like crap, the very first thing I would suspect is the ignition system. I would almost bet money that the coil(s) are the issue. If not then the ignition control box is failing.

 

[GuyRDag]

Will it smooth out once you get past the mid-range?

Good question. It was jerking so bad that I haven't had the guts to try and push it too far. I will clean and remount the original carburetors and see what happens if I try to push i past mid-range. I will also check the inlets and see if I have any leaks. I'm thinking maybe a larger jet also just to make sure it doesn't run lean.

 

[GuyRDag]

When I run into problems like this, I always got the the last thing I did before the issue appeared. If it was running fine and then overnight starting running like crap, the very first thing I would suspect is the ignition system. I would almost bet money that the coil(s) are the issue. If not then the ignition control box is failing.

I really hope you are wrong (since this sounds expensive), but I agree that this might be the problem. Does anybody know of a decent, but not too expensive aftermarket electronic ignition that would fit? I already bought an original used ecu on Ebay, but this unit definitely was broken as the bike stopped a hundred meters down the road and I had to push the bike back home…

 

[Raymond]

Just a thought, have you checked the carb diaphragms? Even a tiny tear or hole will cause mayhem.

 

[Highpower]

I think you are barking up the wrong tree with the carbs. Since you changed them and the problem didn’t materially change, it’s almost assuredly not a fuel delivery issue. What I believe is happening is that the ignition system (probably the coils) can’t handle higher RPM’s and is breaking up. See if you can borrow a couple of coils from someone and plug them in and see what happens before you spend money one an entire new ignition system. If it is one or more coil, all the new ignitions in the world isn’t going to fix it.

I speak from experience here as I have chased the same gremlins before and one time it was a coil and another time it was a bad connection.

When working on any mechanical system it is always better to work off of the principle of Ocams Razor. The simplest solution is usually the correct one.

 

[GuyRDag]

Just a thought, have you checked the carb diaphragms? Even a tiny tear or hole will cause mayhem.

I checked them the last time I was cleaning the carbs and they were in very good condition. I guess a previous owner must have changed them at some point.

 

[GuyRDag]

I think you are barking up the wrong tree with the carbs. Since you changed them and the problem didn’t materially change, it’s almost assuredly not a fuel delivery issue. What I believe is happening is that the ignition system (probably the coils) can’t handle higher RPM’s and is breaking up. See if you can borrow a couple of coils from someone and plug them in and see what happens before you spend money one an entire new ignition system. If it is one or more coil, all the new ignitions in the world isn’t going to fix it.

I speak from experience here as I have chased the same gremlins before and one time it was a coil and another time it was a bad connection.

When working on any mechanical system it is always better to work off of the principle of Ocams Razor. The simplest solution is usually the correct one.

Well said! Overcomplication should probably be my middle name. I will remove the coil from a Honda CX 500 I have and see if things get any better. I also suspected a bad connection and I checked the whole wiring system just to make sure the problem wasn’t electrical. The bike was in very poor condition when I bought it and I feel it would be a shame to give up after all the hours (and money) I have put into it.

 

[Wingedwheel]

How about a pic of what you’re working on.

 

[GuyRDag]

How about a pic of what you’re working on.

Fair question. Since I’m new to this forum, I’m not sure how people feel about custom bikes—but I hope you’ll bear with me. The motorcycle was in rough shape when I bought it, and I kind of got carried away trying to bring it back to life. There were more worn-out parts than ones in decent condition, so instead of restoring it to stock, I ended up creating something completely different.

The original exhaust pipes were rusted through, so I fabricated new ones and welded them myself. Since I’m not exactly an expert welder, I wrapped the pipes to hide the imperfections. The mufflers are original (as far as I know), but I powder-coated them black to match the overall look. I also powder-coated the frame, rims, triple tree, some engine covers++. I built the seat from fiberglass, and since I don't know which color to paint it yet, it is vinyl wrapped for now. It is definetely work in progress and I'm still not quite happy with the way it looks. All of the powder-coating and painting was done in my own garage by the way. I built my own powder-coating cabinet and after that most things somehow end up beeing powder coated...

 

[Highpower]

Nice little project. It’s not my style, but I really admire it when someone is willing to tackle a project and do their own work. I painted both my XS650 and my Triumph using my bathroom for a paint booth.

 

[GuyRDag]

Nice little project. It’s not my style, but I really admire it when someone is willing to tackle a project and do their own work. I painted both my XS650 and my Triumph using my bathroom for a paint booth.

Thank you! My wife is quite tolerant when it comes to my hobbies, but the bathroom idea would probably put even her limits to the test

 

[arcticXS]

Nice! I love the tank color! I see that the awkward subframe shape gave you a challenge....Being a single shock "Full Floater" setup, fabricating a new subframe should not present any serious issues regarding structural integrity. All it needs to do is support rider (+passenger) weight. No suspension forces to take into account.

 

[gggGary]

What mufflers are those? do they have 7 pipes in the outlet?
run it with out mufflers as a test.

 

[GuyRDag]

Nice! I love the tank color! I see that the awkward subframe shape gave you a challenge....Being a single shock "Full Floater" setup, fabricating a new subframe should not present any serious issues regarding structural integrity. All it needs to do is support rider (+passenger) weight. No suspension forces to take into account.

Thank you! The subframe is a disaster when it comes to making ANY seat fit (except the original). Nothing I could find would do, so I decided to make my own. The reason for not modifying the subframe is that the rules are quite strict here in Norway when it comes to making bigger changes to the frame. It’s not impossible, but it takes some effort. The color was borrowed from Mini and is called WC3B British Racing Green.

 

[GuyRDag]

What mufflers are those? do they have 7 pipes in the outlet?
run it with out mufflers as a test.

I thought they were the originals, but maybe I'm mistaken. They came with the bike when I bought it and only have one hole in the outlet

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