Left cyl not working correctly, right one is - what's wrong?

[Martino]

I'm puzzled and could use some advice. '75 850-T. Not ridden in 12yrs, I get the bike and start to bring it to life. In terms of this issue, here is what I did and what I know.. ..

Issue:
- When bike is running, the right cyl and exhaust pipe gets hot, left cyl is only warm. When riding it, I believe that sometimes, the left cyl kicks in as you can feel the power boost but that boost is not always present. So I thought it was an electrical issue.

Observations:
- The coil feeding the right cyl was Marelli OEM. The one feeding the left cyl was a generic no name.
- The plugs were the wrong ones, they were the short version NGK BP6HS
- The plug wires and end caps were worn and generally in a very used state

What I've done so far:
- Bench tested the coils, they both produce spark at the spark plug, though neither produces what I would call a strong blue spark. The OEM ohm values matched the shop manual specs, the generic coil matched the primary but the secondary values where higher.
- Bench tested the condensers, got slightly diff readings between the two but no shorts or opens
- Replaced the plugs with new NGK BP6ES
- Replaced the plug wires
- Replaced the plug end caps with NGK LB05F
- Pulled and cleaned the breaker points and set their gaps
- Verified operation of the breaker points while hand cranking the engine
- Swapped coils and condensers when reassembling so the coil/condenser that fed the right cyl now feeds the left, etc
- Checked again for spark at the plug, got it but again, not a strong one in my opinion
- Cleaned all quick disconnects before reassembly
- Replaced the three fuel lines with clear tubing so I could see the gas flow

At the bike:
- Opened the fuel petcocks, watched gas feed both carbs
- Turned on, flipped up the chokes and pressed start. The bike starts so fast, you hardly hear the starter motor
- While chokes are engaged, rpm is around 2K and constant
- Wait a few minutes and turn off the chokes, rpm drops to 1K and is constant
- Feel exhaust pipes, right cyl is hot, left is only warm - same problem
- Start to engage the left cyl choke again, rpm's rise
- I pulled the left plug end cap off, bike continues to run like nothing happened. Pushed it back on.
- I pulled the right plug end cap off, bike quits running.
- I turned the bike off, pulled the plugs, both are black

Stumped in upstate NY

 

[V700Steve]

The plugs are black cause you had the chokes on, that's usually an 80 jet. Idle mix jet is more than 20 smaller.
If the T has been sitting that long and still has stock cylinders the chrome may be flaking off.
I would do a compression & leakdown test. You also didn't say if you timed the ignition but got to be close if it jumps to life cold.

 

[Martino]

Thanks V700Steve,
I have not yet done a compression test, don't have the tool but hope to borrow one later today. I did check the timing and it appears to be ok or at least very close and yes, I mean it really fires right up and never backfires. After posting, I downloaded a Dell'Orto pdf on carbs and am now being educated. At this point I'm going to assume the electrics are ok unless I left a stone un-turned and will turn my thoughts to the carb. I have no clue what jets are in them (VHB30CD/CS). I took them apart early on and replaced the o-rings, gaskets and did a general cleaning with solvents and an air compressor. I just got the bike so don't know the history.

 

[jr1967]

Sounds to me that your idle circuit is not flowing on the one side. If you open the choke the rpms rise so it makes up for fuel not delivered with the choke off. if you get the bike to run at 1000rpm with the chokes off, it means that the other cylinder is taking the load, so the idle speed adjustment screw on that side must be way higher. If you swap carburetors from one side to the other you'll know for sure.
Perhaps your tank is dirty so you have dirt in there again?

 

[Martino]

jr1967,
Interesting you mentioned that, I was actually just thinking about swapping the carbs around as another test. Was going over it in my head. I had to do that once already because I accidentally put them on the wrong sides when reassembling the bike and didn't notice it till it was time to start it and I couldn't find the choke lever - ha ha.
Fuel is ok, I have clear lines and can see it in both tank lines and the connecting line between the two carbs. Right you are, the rpm's rise when engaging the choke on the left cyl. So initially I thought that carb was ok. I am reading up on carbs and will have to figure out what the adjustments do and how to properly set them up but a simple swap should tell me right off the bat. This is my first endeavor into bikes.
Thanks!

 

[jr1967]

No problem. You can still have fine dirt/rust or water from your tank in your carburetors though. Don't dismiss that as a possible problem. Draining some fuel into a glass jar after shaking the bike(or tank) will give you an indication. I did a few carb cleans of a 55hp two stroke outboard on my boat that was supposedly always stored inside. I eventually drained the tank of at least a gallon of rusty water...... And yes I have been boat mechanic in a previous career so should have known to start with the basics......

 

[Dinsdale Piranha]

I agree with the consensus on the carbies. A couple of possibilities, easy to check:

1. Idle mixture screws/jets. If the lefty comes to life with more revs/throttle opening, perhaps the idle cct is closed off/clogged on that side. So that once you get onto the needle cct it begins to go. Check idle mixture screws and ensure they're both set about the same, and somewhere near default position.

2. For cleaning carbies you can get aerosol cans of "Carby Cleaner". I find that one of those with a plastic proboscis is the ultimate tool for cleaning some of those smallish passages etc. Depending on the carby you can often leave jets in place and clean them at the same time.

3. Pull the idle mixture screw out altogether (there'll be a spring and possibly a washer and O ring in there too) and hose it out thoroughly with the aforementioned aerosol can. Set it at the default starting point when you're finished.

4. Check that the choke mechanism is functioning properly. I might be staying on on the left. The aerosol can again!

After that if it were I, the floats, float level, float needle/seat would probably be my next stop. I have a compression tester so I'd already have done that. Worth doing it when you can I think, even if just to eliminate that from your reckoning.

Best o' luck.

 

[Martino]

Ok, well I tested the compression today, right side 165, left side 135. Looked in the shop manual and Haynes but couldn't find the spec. That's ok, I know the right side works well so just figured something near that would be ok, though I would have liked to have seen a range specified. So the left is 18% less than the right. Stake in the ground.

Next, checked the timing but the flywheel marks don't match the manual, so borrowed a timing light, while a slight adjustment might be in order, I'll wait till its running and check again, I believe it was pretty dang close and once again, no backfiring, no dieseling, no outward signs the timing is off.

Next, the carb. I was going to swap them but decided to simply tear the left one down and inspect and clean every orifice, nook and cranny. The Main jet seemed ok, I mean a little grimy but the hole was pretty clear. Ok, so when the throttle is opened up to that area where the main jet simply takes over, the bike seems to respond better. (only rode it for two short round the block trips).

The idle jet was totally clogged, .. solid. Once I cleared it, you could easily see a clean hole, nice. This might explain that when idling and I removed the left spark plug, the bike kept on idling without an rpm or sound difference. Also explains why when on idle, the left pipes are only warm and not hot. Could this have been the smoking gun?

The float looks good, no issues but was a tad high according to the manual, I made a slight adjustment there and checked the seat on the float needle, looked brand new.

Next, the choke. Well, I knew that worked but took the opportunity to remove and clean it too. Plus, I wanted to blow compressed air through all the channels, holes, orifices in the carb.

Next I took the accelerator pump apart. Looked a little grimy but you could easily see through the holes and I knew it worked ok as gas sprays out when twisting the throttle when the bike is turned off.

Checked all the jets numbers, they match OEM spec yet this bike has non OEM exhaust, ok, it is what it is. Taking it apart also gave me the chance to see how the idle adjustment screw interacts with the slide and I was surprised it really only has impact at the last turn or two before being totally turned in and tight against the spring.

Once done, I set the idle mix on both carbs to spec, turned in both idle adjustment screws and connected the fuel lines. Now, time to eat, I'm starving.

The suspense is killing me but the stomach won out More later.. ..

 

[Raven]

It sounds like you found a few problems and cleaned them up. It should be smooth sailing (fingers crossed).

 

[Martino]

Well, looks like this left cyl not firing problem has been solved. It was that clogged idle jet and just an overall grimy carb. I gave it a good root canal. Sounds better too, deeper sound, richer. Both cyl's get as hot as one another and when I pull the spark plug off the left, the bike now wants to die but somehow manages to just about hang on which tells me the carbs are not set correctly. I tried to set them up today for proper idle but just could not get the bike down to the 900 rpm mark, it likes being about 1,100 - 1,300. I have adjusted the throttle cables all the way down on top of the carbs and there’s no adj left at the throttle itself. The slide is as far down as it is going to go and a visual says both left and right slides are even when the throttle is closed.

The bike has non OEM exhaust pipes, pretty straight pipes with a very small muffler yet when I had the carb apart, all the jets and slide needle are OEM spec.

I had the idle screw backed all the way out which allows the slide to be down as far as it will go but then I got to thinking that with a smaller area in which to pull air through, the Venturi affect will be increased, right? That might be pulling too hard on the idle jet so I still have some learning and playing to do. Back to reading the 'How Carbs Work' book.

Thanks to V700Steve, jr1967, Dinsdale Piranha and Raven for giving me suggestions - you were all on the right path. Well, at least I know my electrics are all in good shape

 

[john zibell]

I like to see compression figures within 10 PSI. 30 PSI difference is a bit much. Run it for a few days and check compression again. If it is still off by more than 10 PSI you need to do some investigating on that left cylinder.

Just a note for future reference, when turning in the idle stop screw, do raise the slides with the twist grip first. Just turning in the screw will cause damage to the slide.

As to the not stock exhaust, the previous owner may have compensated by raising the needles. If she runs well, no other changes many be needed but do read the plugs after you get the bike running to make sure you aren't foo lean.

 

[Martino]

Right, was thinking the same thing. I don't know the bikes history but the state inspection sticker was 12 years old so I too am hoping it normalizes after running it for a while. Twist the throttle to raise the slide before turning in the adj screw - got it. The needles are V9's and clipped on their second (middle) slot, per the manual, so no compensation was done there either. I will try to get the idle down to the 900 range and then take it for a spin. Thanks for your feedback John!
Any idea what the compression spec is for this 850-T? BTW, your thumbnail pic, is that the Saturn V?

 

[Dinsdale Piranha]

I like to see compression figures within 10 PSI. 30 PSI difference is a bit much. Run it for a few days and check compression again. If it is still off by more than 10 PSI you need to do some investigating on that left cylinder.

I thought of this today too. It's quite possible that the rings are gummed up/stuck after such a long rest. Get out the aerosol can again and give the piston crown a thorough hosing down. leave it to sit and soak while you have lunch. Use some over-the-counter upper cylinder lube in your fuel for a few hundred miles, or one of those put-it-in-your-fuel decoke additives. Give it a thousand miles and test it again. I'd give you 80%+ chance of recouping the lost compression.

 

[john zibell]

Right, was thinking the same thing. I don't know the bikes history but the state inspection sticker was 12 years old so I too am hoping it normalizes after running it for a while. Twist the throttle to raise the slide before turning in the adj screw - got it. The needles are V9's and clipped on their second (middle) slot, per the manual, so no compensation was done there either. I will try to get the idle down to the 900 range and then take it for a spin. Thanks for your feedback John!
Any idea what the compression spec is for this 850-T? BTW, your thumbnail pic, is that the Saturn V?

Hard to give a specification for compression as gauges are usually not certified calibrated thus readings vary. Take the compression ratio and multiply the higher number by atmospheric pressure. eg. 10 to one compression, so 10 times 14.5 gives you a calculated reading of 145 psi. That would be the calculated compression reading. As long as you are close to that, you are good.

Yes, that is a Saturn V mock up outside the Space and Rocket Center in Huntsville AL. They have a real one inside the Davidson Center at that facility.

 

[Martino]

Ok, thanks! Well, I bought the bike as a project with the intentions of revitalizing it and then selling it. For a 42 yr old bike, its in surprisingly good shape. I suspect that idle jet just dried up and the gunk in it solidified over the years. Its been a fun project. I was just curious about the psi range. I will be test riding it and will recheck the compression on the left to see if it changes. For me here in NY, your formula works out to 132psi. Maybe both cylinders will normalize after a little use.

I almost made it down to the Huntsville space center back when I was involved in Space. I ran a couple programs responsible for the mfg/test/support of the booster power supply for Lockheed rockets and the device responsible for blasting the bolts that held stages together, thereby allowing each stage to drop back to earth. Our main interface was with Lockheed in Denver, this was all before the Lockheed/Boeing merger and the United Launch Alliance name.

 

[Martino]

Took the bike out today after adjusting the carbs. Runs quite well. Checked compression when I got back and found both cylinders to be at 130psi +/- a couple. Thanks to every one here who pitched in to help diagnose what turned out to be a clogged idle jet - much appreciated!