Compression Issues

I have a 1986 XJ700c Maxim with 18000 on the odometer. Most times I can get it to start but it is a hit or miss situation. I have replaced the air filter and plugs with no change. It is way underpowered. Trailer trucks push me up hills. I took the carbs apart and cleaned everything but found nothing that would explain the lack of power. I did a compression test cold as the bike wouldn’t start to warm it up. Here are the results;
Cyl 1 100 PSI
Cyl 2. 100 PSI
Cyl 3. 75 PSI
Cyl 4. 100 PSI
I know they should be around 165. Any guesses as to what my problem might be?
Thanks for any insight you can provide.

 

Have you checked the valve clearance? In the how to section in a thead by Bigfits on this subject.

 

I have not but that was probably next on my list. I will post again once I complete that.

 

Two items when you did compression test did you do it with throttle wide open? Were all the plugs removed and was battery fully charged? And did you add a little oil and tested after? Agree with lost boy check valve clearance.

 

Throttle was wide open. I did put some oil in the 75 PSI cyl with little to no change. I will try to repeat it with a warm engine.

 

I'd do the valve clearance check before bothering with another compression check.
That the oil didn't improve things is a clue.
1. the valves are out enough that compression is low.
2. the rings are stuck,
3. the bores are worn beyond spec.

Given the mileage, 1 and 2 are the most likely, and you probably have both conditions (valve checks and adjustments are often neglected).

A good soak with a 50/50 mix of automatic transmission fluid and acetone will free up the rings if they are stuck. Put a tablespoon down each plug hole and wait a few days.

 

I was talking to a friend today and I was explaining my lack of power. He said “Did you check for spark?” My answer was no. After I returned for a short “Goin up the country”
Hey, my name is Gary. Anyway, I thought I should check for spark. I had it idling so I pulled the wire on cylinder 1. No change at all. I pulled the wire on cylinder 2. Still no change. No wonder I have no power. I am only running on 3 and 4. Now I need to figure out why. Is this a common problem? Any guesses as to the defect?

 

Could be bad, or oil-fouled plugs. Could be bad resistors in the plug caps. Could be corrosion in the plug wire ends where the caps screw on.
Corrosion is the most common. Unscrew the caps and cut the wires back 1/8" to get to shiny copper.

The coils feed two cylinders each. There is one coil for #1 and #4, and another coil for #2 and #3, so your coils are good or you'd not see the combination of spark that you have.

You still need to address the low compression though (unless your compression gauge is reading low).

 

Honda CB750 coils will directly swap in and have replaceable plug wires , can still be bought new . Cycle Recycle part 2 sells them for 44$ each and they have the wires and caps as well. I have been running them for about 2 years with good results. But as kmoe stated did you try another known compression gauge?

 

Now I am really confused. Are the cylinders numbered from 1 to 4 from the left to right. It is the 2 cylinders on the left side (same coil) that are not firing.

 

Correct. As you sit on the bike the #1 cylinder is on the left.

Think of the engine as a pair of parallel twins. Each coil fires both sparkplugs at the same time (one spark being "wasted" on the cylinder that is at BDC), so one coil will ignite cylinders 1 and 4 (the outboard cylinders), while the other will ignite cylinders 2 and 3 (the inboard cylinders).

 

Could my plug wires be wrong? The coil on the left side has a wire labeled 1 going to cylinder 1. The second wire is on cylinder 2. Is this incorrect?

 

No that is correct left outboard cylinder is number 1 and inboard is number 2

 

If the second wire from that coil is on #2, then the wires are hooked up incorrectly.

The left coil feeds #1 and #4. The right coil feeds #2 and #3.

The easiest way to make sure is to remove, or lift, the fuel tank so you can clearly see the coils and the plug wires.

The wires are labled from the factory as to which cylinder they go to, but the labels can come off or fade.

 

Is it any CB 750 coils Jetfixer, I had a K2 years ago I should have kept it.

 

So, I corrected my wires and went for a short ride to warm it up. I pulled the plugs an did another compression test. Even though they are still low, at least they are still close together. They read:
Cyl 1 - 115
Cyl 2 - 110
Cyl 3 - 110
Cyl 4 - 115
I didn’t have a chance to go for a longer ride to see if I have more power. Hopefully,I will check it out tomorrow.

 

That points to the rings just being stuck a little. They are gradually freeing-up and removing the rust that may have formed on the cylinder bores when the bike sat.
You should still do the valve clearnace check. It's not uncommon to find that they were never done at all, and are well out of spec.

 

I am having the valves adjusted. I thought about doing it myself but decided to bring it to someone that knows what they are doing and has the spacers on hand.

 

Get him to list all the sizes of the shims for your own records. That way you know he did a complete job.

 

I picked up the bike today. All valves were in spec. He did some checking and discovered that cylinders 1 and 2 are not igniting anything. He told me they have spark but you can pull the wires and the bike doesn’t change. My lack of power is due to the fact that I am running on only two cylinders. My guess is that fuel is not reaching these two cylinders although I can’t imagine why. Any guesses? Are there float bowl drains on the bottom of the carbs. If cyl 1 and 2 are empty I can be sure that that is the cause but then why.

 

Yes each float bowl has a drain screw, be careful opening use a JIS screw driver may look like a Phillip's buy is not, if you have tool kit with bike should be one in the pouch. You may have a bad coil wire , if you remove plug cap and trim a 1/4 inch off thread back on see if this helps . If there is no change 81 to 83 Honda CB750 coils will swap right in and Cycle Recycle part 2 has them new for about 44$ each and they sell the plug wires and caps . They have worked great on my Seca 750

 

The float bowl screws are straight slotted heads.

If there is spark then the carbs need to be cleaned thoroughly.

IN THE CHURCH OF CLEAN

 

I have stripped the carbs all the way down and cleaned everything. I put new plugs in about 50 miles ago. Plugs in cyl 1 and 2 still look brand new.

 

Are the plug wires on the correct cylinders? (left coil wires go to the outer cylinders, right coil wires go to the inner pair).
Are the spark plugs on the dead cylinders wet with fuel?

 

The wires are correct. The plugs are dry.

 

I removed the drain screws on cylinders 1 and 2 and the bowls are bone dry. I cracked the one on cyl 4 and fuel came out. Could the fuel rail that supplies all cylinders be plugged? That seems unlikely but.... when I cleaned all of the carb parts I don’t recall removing any fuel/air mixture screws. Do these carbs have such a screw? Could those be plugged or bottomed out?

 

Could be, but doubtful (those are pretty large passages). More than likely, the float needles are handing up in their brass seats on those 2 carbs. You can try tapping on the bowls with a screwdriver handle, etc. to see if that can free them up, or take the bowls off and manually manipulate the floats to make sure the needles are dropping down (and thus allowing fuel to enter the carb bowl).

Yes, they are at the very center front of each carb,, sometimes hidden under a small (1/4") brass or aluminum anti-tamper plug. If the tamper caps are still in place, they need to be VERY carefully drilled out to remove (VERY carefully.....the heads of the mixture screws are right under those caps!). But if the caps are still in place, then most probably the mixture screws are not bottomed out (since they would not have come that way originally).

 

I know Len said likely not, and I agree, BUT I have had several plugged with rust blockages over the years of carb rebuilds. So it's not unheard of.

 

Interesting that no fuel is in #1 and #2 but #3 & # 4 are getting fuel.......I am inclined to agree with Chacal in regards to stuck float needles or clogged float seats not allowing fuel in.....

GaryJ.....it is in your best interest to bite the bullet, & perform a full teardown ( break the rack ) & cleaning of your carbs........do it once and do it right......

I and others have been guilty in the past by circumventing from doing the whole 9 yards ( Rick Massey write-up ...if I recall ) only to perform full tear down in the end.....

Carbs off, carbs on, carbs off, carbs on......a never ending process if the Church of Clean or the Whole 9 Yards is not followed.....

Trust me.....I know first hand.....

 

Gary, I offer rebuild services---contact me if interested in details.
I'm near utica NY

Dave Fox

 

I thought the same, then I remembered the the fuel T on the 700 carbs feed #2 and #3.
He said the both 1 and 2 were dry, so if the T was plugged on that side....

 

I should have gone back to first post....I was assuming the fuel inlet was between #1 & #2 carbs but didn't realize it was different for a '86 XJ700 Maxim ( between #2 & #3 )

 

It can happen especially when the bike sits on the side stand long enough for dirt to collect and plug the side of the T. I've also found some clogged with spider webs....

 

Thanks to all of you that have taken the time to help me out. The issue has been resolved. I pulled the carbs back off the bike. I removed the covers, floats, and needle valves on cyl 1 and cyl 2. I also removed the 10 mm fitting that the needle valve sits in. I shot cleaner through the screens on the bottoms of those fittings. I am thinking they were plugged although it wasn’t obvious. I also ran a guitar string through the metal feed line to make sure it was clear. I reassembled and put fuel into the supply line and opened the drain plug and fuel ran out. I was fairly confident at this point. I put everything back together and hit the starter. It fired up and revved about 4K so I had to adjust the idle down. I took it out for a ride and found all that power I have been missing. Holy crap! Roll the throttle and hang on. I won’t be embarrassed going up hills anymore.

 

Good deal !!

 

And then just when you think you found the missing power, you'll start rolling above 6k rpm, and find out where the POWER BAND is and that it stays in the band til redline. You thought you were hanging on now....just wait til you redline it

 

He means it. I have first hand knowledge of trying to keep up with Hogfiddles on a ride!