Exhaust Sucking In Air

[johne67]

Some Background: I have a 1980 650, which came with Mikuni VM 34's and straight pipes. I replaced the TCI ignition with a Hughs Handbuilt complete PMA/CDI Ignition. I adjusted the cam chain tensioner, then the valves. I followed the Carb Guide with respect to jetting my carbs including the above steps and set the ignition timing with a timing light.

The HHB ignition worked great, and my bike would start with a single kick every time, even when cold. Only problem was that the chokes needed to be on for the bike to start and idle. Switched to a 27.5 pilot and the bike started and idled nicely. Once the bike idled nice, I set the mixture screws using the dead cylinder method. I then put some brand new plugs in and let it idle for a while. Then I shut it down and read the plugs, left side plug was brown, maybe slightly lean but close. The right plug was pure white, as if it was a brand new plug, no discoloration at all, none.

I figured the right cylinder must be running way too lean so I really richened it with the mixture screw and let it run to see if the plug would show a rich condition. But, it was still white like brand new.

While fiddling with the carbs I put my finger next the exhaust to see if the amount/pressure coming out was the same, and it was. Except, I noticed that my finger would get sucked into/toward the exhaust in intervals that make me think it was happening on the intake strokes. The exhaust that's taking in air is the right side cylinder, same cylinder that is persistently lean.

I guess its worth revisiting the valves, but I'm 99% sure I adjusted them properly. Am I correct in assuming that this sounds like a top end job with new valves and/or seats?

 

[gggGary]

Carbs synced?
Compression tests. Then piston TDC on compression stroke put some air in the plug hole, listen, where is it coming out?
(don't go nuts with the air pressure it'll spin the motor!)
Read all about it here: https://www.xs650.com/threads/the-meatball-mechanic-low-compression-fix.11693/

 

[jpdevol]

Compression test, or better yet, leakdown test & go from there.......https://www.amazon.com/LEIMO-Cylinder-Compression-Leakdown-Diagnostics/dp/B07W9FXKTF

You might get lucky and just have some carbon (like in Gary's thread), but at least you'll know what you're looking for if a teardown is needed

 

[johne67]

Carbs synced?
Compression tests. Then piston TDC on compression stroke put some air in the plug hole, listen, where is it coming out?
(don't go nuts with the air pressure it'll spin the motor!)
Read all about it here: https://www.xs650.com/threads/the-meatball-mechanic-low-compression-fix.11693/

Carbs were bench synced with a drill bit but that was all. I read the other thread and I’ll keep my fingers crossed it all works out like that. I guess no need to panic just yet.

 

[gggGary]

I'd suggest you do a proper sync.
A shut butterfly could do what you are seeing.

 

[Bruno_c]

I'd suggest you do a proper sync.
A shut butterfly could do what you are seeing.

I think that before reading the plugs, you should ride the bike on open roads for a significant distance.

 

[gggGary]

I figured the right cylinder must be running way too lean so I really richened it with the mixture screw and let it run to see if the plug would show a rich condition. But, it was still white like brand new.

Don't know what you know; but the VM idle mix screws are backwards to the stock BS idle mix carbs screws, On the VMs tightening mix screw makes it richer, loosen to lean.
The rule is;
If the idle mix screw is on the engine side of the slide it lets in fuel when you loosen
If it's before the slide it lets in air when you loosen.

 

[johne67]

Don't know what you know; but the VM idle mix screws are backwards to the stock BS idle mix carbs screws, On the VMs tightening mix screw makes it richer, loosen to lean.
The rule is;
If the idle mix screw is on the engine side of the slide it lets in fuel when you loosen
If it's before the slide it lets in air when you loosen.

I did have the VM Carb stuff down.

I did a compression test and got 160 on each cylinder, so at least I got that going for me. I re-checked the valves again and the intake on the right cylinder was maybe a tad loose, but thats it. Then I tried the blowing air into the spark plug hole with the engine at TDC. I don't have a good method of blowing the air in so I'll have to figure something out. But before I do, would the compression test be indicative of something other than the valves causing my issue?

I'm now thinking I may have played it a bit fast and loose with finding TDC when I was installing and setting the timing on one of Hugh's ignitions since I got a slight kickback today when kick starting it.

 

[nj1639]

One thing you don't like to question is if you set the timing off one tooth AFTER it's all back together. How would I know? Kinda like finding that oil deflector internal part of the breather setup sitting on the bench after the engine is in the frame, though there's no guessing if you installed that.........

 

[johne67]

So I’m back at this, got busy with my final semester of law school.

The right cylinder is persistently running lean and is significantly hotter than the left. For example, I can grasp the left header even after the bike is running for a minute while water will sizzle off the right header.

I worked through the following above recommendations: compression test (160 left and right), tried blowing compressed air into the spark plug hole but didn’t hear air escaping, and a leak down tester is on the way from Amazon.

I did NOT do a true carb sync as recommended by Gary as my vm34s don’t have a port to hook up to a manometer.

All that said, I have some additional info which may be helpful in diagnosing this. I stated earlier that I adjusted the cam chain, set the valves, then did the ignition. However, upon revisiting the cam chain adjustment, I noticed that the plunger is not moving at all. I first tried just turning the engine over with the kickstart, but when it wasn’t moving I actually started it. There is no movement of the plunger. After searching the forum it is apparent that it should in fact be moving and its travel is used to properly set it. I did not however see any posts where anyone was trouble shooting it not moving at all.

My thought now is that the cam chain is either A: set way too tight, or B: the chain is shot.

I’m guessing this COULD possibly be the cause or at least be contributing to my problem as I did hear some rattling during a test ride although I couldn’t discern where exactly it was coming from. I’m thinking this is more indicative of a loose or worn out chain rather than an overly tight one though.

To further complicate this, I rechecked my ignition with a timing light. Now this is a HHB ignition system which comes with a sticker used to check timing (not sure if anyone’s familiar) which denotes where it should be firing. I don’t know if my timing light from Mikes is defective, but it’s not lighting up at consistent intervals, as in not lighting up at all sometimes. When it does decide to work it does show the left cylinder firing properly (at about the 6 o’clock position) it intermittently appears to fire at about 2 o’clock though. I hope this is making sense.

Does any of this point towards a culprit? Or should I report back after the leak down test and figuring out how to do a proper sync (will this entail drilling the carb and putting a fitting in there to run hoses to a manometer?). Would videos of some of these issues be more helpful than me trying to explain them textually?

I’m sorry about the very long post, but any input is greatly appreciated.

 

[gggGary]

I can grasp the left header even after the bike is running for a minute

Sounds like that left is not running, at start up anyways, a pipe should be too hot to touch after mebby 15 seconds of running.
If the cam adjuster stem isn't moving, back off the adjuster til it does. No movement means the cam chain is under too much tension.

 

[jpdevol]

To further complicate this, I rechecked my ignition with a timing light. Now this is a HHB ignition system which comes with a sticker used to check timing (not sure if anyone’s familiar) which denotes where it should be firing. I don’t know if my timing light from Mikes is defective, but it’s not lighting up at consistent intervals, as in not lighting up at all sometimes

Mike's sells a timing light?? Who knows what the issue may be.......Sometimes the clamp that goes over the HT lead is directional (often marked with an arrow) but you might try changing the direction to see if you get a consistent flash.

If you're getting 160psi comp. the leakdown should be good. Might be pointing toward it being a synch issue, but I'm with gggGary on the left cyl sounds as if it's not firing at idle. Maybe the left carb slide is too low

 

[AUTOMAN]

straight pipes

reversion is normal, but you may warp a valve if shut down with a breeze blowing into the tailpipe

cliff

 

[Slippery]

As mentioned previously a compression test will immediately tell you if your top end is good or not; regardless wether its the valves or rings, the compression test will indicate if you need a refresh - valve seals; rings or valve seats ... you would see oil and or exhaust deposits if your head gasket was gone ...