Poor sparks on front cylinders

[MadDogRoger]

Hi silverstrom,

Thanks for replying. It has been quit a journey to get to the point I am now. Somewhere along the way the float valves (needles and seats) have been replaced by new, original ones. That didn't cure the problem.
But because now nothing seems to "cure" the problem I got to thinking: what if the "problem" is fixed but the crankcase is still filled with petrol/oil which still causes the spark plugs to get (almost instantly) wet and stop firing.
If I clean the plug (basically blow them dry with compressed air) and start the bike, I can feel the left pipe get warm for 5-10 seconds and that's it. The right pipe doesn't even start to get warm. Then the spark plugs are fully soaked again so they can't fire.

I will try the pressure test and leak down test again but have to get a valve first to make it easier.

Greetz,
Roger

[silverstrom]

You likely don't have much fuel in the crankcase because if you did you'd likely hydraulic lock the motor and bend the rods.

I'd still check the float valves and I'd also verify float height. The only way to have fuel get into the cases is through the carbs.

Remove the lower pipes and drain any fuel in them. Remember that the pipes cycle air both directions. If your pipes are half full of fuel you'll obviously have problems.

Have you verified the coil to the lower cylinders is good? You can swap the coils and if the problem follows the coil you have a your answer.

Others have had issues with lower cylinders not working properly. you should be able to find details by doing a search of the forum posts. In some cases it was discovered that the gap between the pick up coil and the flywheel was too big. A simple adjustment solved that problem and the bike ran normally afterwards. The information about the pick up coil gap should be in the repair manual.

Note that a cold lower right pipe could be due to the crankcase sucking gearbox oil through a failed lower right crank seal. That oil ends up in the cylinder and on the spark plug. Coolant from a failed head gasket or oil from the gearbox via a failed crank seal will cause obvious running issues. Check compression on the lower cylinders if problems persist.

It's worth checking your carburetor air vents. Those vents allow air to get in and out of the carb bowls. If the vents are plugged fuel will not fill the bowls properly because the air inside cant be displaced. That would mean cold pipes on affected cylinders.

[MadDogRoger]

Hi Silverstrom,

Thanks again for replying and trying to help.
Like I said earlier, it has been quite a journey to get to the point I am now. Maybe it is good to summarize what has been done so far:
- it started with poor spark on lower cylinder. Checked were, spark plug (replaced) HT leads (replaced), coil for lower cylinders had bad connection (replaced), pick up coil checked (was ok), CDI was checked (on multiconnector ignition between front and rear cylinders was switched, no change so CDI not at fault). If the spark plugs were dried (blown with compressed air, they sparked. When mounted in the bike: no ignition and wet spark plug.

Compression was measured and 94 and 99 psi for the front cylinders (see page 2 of this thread).
Then attention turned to carburetors.
Float valve and needles were replaced by new original ones after damaged O-rings were detected.
Fuel level was lowered for the front cylinders.
Floats were swapped between carburetors.
Nothing cured the problem.

Then I did a pressure test showing leakage at the reed valves intake. Finally thought I had found the problem. Fixed that. No solution.

Did another pressure test this evening. It holds pressure well at 5 psi, no problem.

Did another leak test. Only have a cheap vacuum pump from a brake bleeder kit. If I pump with the end of the hose closed I can easily get it to 25 inHg and it will hold.
If I hook it up to the bike, I start pumping, after a while the needle starts moving (building vacuum), with every pump the vacuum gets a little bit bigger. Until 7.5 inHg. I can't get it higher than that. If I stop it quickly drops to 7 and in the next 5 minutes it slowly drops to 6.5.
It bothers me that I can't get the vacuum any higher.

Another thing that bothers me is that I noticed today that the gearbox oil level was below the inspection glass. I have the bike on a lift, it is level. But no oil. Added a little, no oil. Added more, no oil. Decided to drain the gearbox, maybe it's overfilled and the glass is completely full.
Only half a liter comes out, where it should be 1.5 liter.
I do think I checked the level sometime last year and added it to full. But with so little oil in it I'm doubting myself.

Could it be I have a vacuum leak that manifests itself at let's say 7.5 inHg, causing the vacuum to not get higher than that and also causes gearbox oil to enter the crankcase causing all kind of fueling issues?

Greetz,
Roger

[silverstrom]

Electrical problems often look like fuel problems, and that can make it difficult to get things sorted out.

I think these may be helpful

[phildu31]

- it started with poor spark on lower cylinder.

Why do you say this ?
There can't be any "poor" spark.
You have a spark or you don't have a spark ( and it will be on both cylinders ).

[MK]

One litre of vanished gearbox oil is a pretty strong indicator for a faulty crank seal.

After filling up, check for excessive smoke.
And leaning the bike to the extreme left will take away oil level from the seal - if it then quits smoking too much, then you have an exact diagnosis.

Just two years ago I had a broken cage on the outer right crank side of my 350. Bike ran fine, just noticed quite some smoke when accelerating out of the villages.
At home I checked gearbox oil level and it was low.
So I filled up, did the above check and then got the right engine cover and primary gear out. Then the issue was obvious and a crank rebuild was due...

[MadDogRoger]

Hi all,

I have an update on the engine problems. After trying to figure out what was wrong with the engine, after much thinking/doubting, I finally bit the bullet and decided: This is out of my league.
I decided to search for some help and the only guy I trust with my engine is Stan Stephens. So I gave him a call and one thing led to another.
So one fine Saturday morning in December, I loaded the engine in the back of my car and took off to England. Dropped off the engine at Stan's and headed back.
Today I picked up the engine, it looks absolutely new:
[]

Now to sum up what was wrong with the engine:
1. The rear pistons were a different size to the front ones.
2. The rear ones are from an RD125LC and it was the expander rings which had broken and caused the scoring on the cylinders.
3. The front crank seal were ok, worn but ok. The rear seal was a mess. Someone had rebuilt the engine and forgotten to fit the seal. Then they had driven it in from the outside, which cut the retaining rib off and because it wasn't in far enough, it had been rubbing on the drive gear. The little spring around the seal was missing.
4. The center nut of the clutch was not tightened which meant the entire clutch housing was loose on the shaft, causing the clutch to wobble, wrecking the clutch boss. It needed replacing. Oh and new plates.
5. Finally to top things off, while reboring the cylinders it turned out they had been bored lop-sided. It would not clear one side at the bottom and the other side at the top. It needed 3 bores to set straight.

It's a good thing I took the trouble to get the block over to Stan, because I would not have been able to set that block straight.
Well finally I know that the engine is alright.

But one thing led to another so now the frame, subframe and wheels are at the powdercoater to be redone. And I'm left with lots of pieces that need rework.
Let's hope I get the thing back together again before the summer is over.

Greets,
Roger

[MK]

Summer - which year?

Just kidding.
The piston issues despite from broken rings actually are no real "issues".

#3 perfectly explained your gearbox oil consumption

#4 happens more often than you would imagine (but not that some jerk plainly forgot to tighten it - they come loose, if you don't tighten & secure them properly)

#5 is pretty common for cheap rebuilds. Honing can be done by moving the cylinder up and down the rotating mandrel by hand without previous boring. Takes a few passes more, but it's faster than doing it properly - thus the "good" price for a rebore.
Unfortunately this gets the bore center axis out of the required perpendicular position and the bore shape out of round/cylindrical.
If you later try to fix that with boring, you need multiple passes / oversizes to get a clean state. This is what Steve was showing you.

[lost1750GTV]

"One thing led to another"
There's always something ...

[MadDogRoger]

"Summer - which year?", no kidding, I'm hoping this year. But it's gonna be close. I'll give it my best shot
@#4: well the "funny" part is that the nut was secured So it just wasn't tightened properly.

Got word from the powder coater, I can pick up the parts tomorrow
Bad news is that it's gonna freeze again here, so many cold hours in the garage await me.
I'll just have to keep in mind that it's for a good cause (namely me ).
Greets,
Roger