Absolutely puzzled...engine problem 750

I throw myself upon the mercy of the court yer honour...I'm right and truly puzzled.

Situation: one cylinder runs normally in every way. The other does not. It misses
occasionally backfires in the silencer it will idle because the other cylinder does the work
it will not run a small throttle at any speed but it seems fine quarter throttle and above.
I have:
Swapped plug leads, plugs and caps. Replaced coil with new one checking spark which is good.
Replaced EI , Pazon SureFire, with known good. Replace carb with two other both nearly new
and off running bikes. Pulled head removed valves, clean and reground valves just to insure
contact. Valves slide easily in guides valves not bent. Stems not loose in guides. Combustion
chamber was heavily coked up. No signs of oil leakage on head cylinder etc. No signs of
overheating on rocker spindles.
Compression test on room temp engine 85/90 with line air at 100. Both sides within one pound
of each other. Repeated test several times same result.
Timing checked with degree wheel. 2 degrees retarded. I've tried adding and subtracting a few
degrees makes no difference.
Engine will start one to three kicks.
Right side will warm quickly(hand on front of silencers) left hand, the bad one, will warm noticeably
more slowly. Engine does not smoke at all. After a run the bad side is about 15 degrees or more
colder than good side.
If the idle is enriched it will help to some small amount. But it is clearly not burning the fuel it
gets. When it is up to operating temp it will be nearly unmanageable at low speeds burbles and
backfires in the silencer, not huge explosions though.

What is left to look at? I'm really missing something I am certain but no smoking gun so far
has been found. Wiring is all new and secure and checked. Battery shows 12.4-6 charge and
the voltmeter shows 15 volts above 2200 rpm.

So what have I neglected to spot?

Check your cam for worn lobes, remove rocker covers and use a DTI to check the rocker movement, the inlets should match and the exhausts should match at least depending on the cam used.

You seem to have done everything that the collective would have suggested. When you say the combustion chamber was heavily coked up, was it just the side that is giving problems?

I am going to assume the offending cylinder chamber was heavily coked up.

That would be in keeping with all other signs and symptoms you have described.

You have correctly stated the problem: the cylinder is not correctly burning the fuel it gets.
Now the question becomes ...... Why?

Kommando's tip on cam lobes should be pursued. However, if a cam lobe is worn, I would expect you would know it, as the engine would not run well.

Other possibilities are weak spark on the one cylinder, retarded timing on that cylinder, over-rich mixture on that cylinder. You have addressed each of these conditions, and have taken the steps to rule each out. I repeat these here only to keep the focus on the obvious.

I am not familiar with Pazon E.I. Is it possible the timing can be correct on one cylinder, and be way off on the other (like a Lucas K2F has been known to do)? In this regard, have you checked timing with a strobe on each cylinder?

All the swap outs you have done would eliminate weak spark and rich mixture peculiar to one cylinder. All that is left is timing.

Slick

PS: This is a most interesting case, keep us informed.

Do you have the ability to post a short video of the bike running with the problems described

The engine is a low time rebuild. EI-wise I installed a Pazon Altair. I pulled it out and put in a SureFire the first is digital second is analogue -think Boyer. I did this because of hard starting and persistent kickback. The analogue almost never kicks back and when it does it is mild. Starting now seems sorted. Both are a wasted spark setup so timing should be equal no matter what. Timing check with a light several times. Timing also check with the degree wheel as stated.
A quick visual check shows the valves on each side to descend the same amount as viewed through the spark plug hole. A low time
cam should not be an issue wear wise. However at this point I agree nothing can be ruled out.
It was so over rich that one would think that leaning the idle out would assist but it is rather the opposite which puzzles me.
We don't know if the spark is occurring whilst under compression I have ordered a set of those little winkies that are on a plug cap perhaps that will show an ignition difference twixt the two sides.
And no I cannot do a video but perhaps I can try to do that at some point. Modern era and all that

storm42 said:

You seem to have done everything that the collective would have suggested. When you say the combustion chamber was heavily coked up, was it just the side that is giving problems?

Click to expand...

Both sides but the bad side was worse. Plug colour on the good side is quite clean. On the bad side less clean but after a good run you can see it burns away a lot of the soot. The bike seems to run well above 1/4 throttle at any speed. It likes load doesnt like no load.
The oil has gotten black fairly quickly which I put down to fuel dilution and carbon. Ive seen this before on bikes with over rich
mixture as in the choke was not lifting completely.

Check carb on bad side, blocked pilot jet holes in carb blocked, needle wrong position, main jet unscrewed, I could go on . . . . .

Please clarify: one or two carbs? Original post said replaced carb (singular) with two others..... i assummed one carb replaced twice.

If dual carbs, we open up a lot of possibilities, as Bernhard suggests.

Slick

@Onder you have no idea how much I ephasise with your plight. I have been chasing an idle misfire on one cylinder for 10 years. Like you, I've changed/swapped/replaced/measured everything to no avail.

This doesn't help much, other than to offer the old addage that misery loves company.

You are certain that it's not an inlet air leak I suppose? (sorry but had to ask)!

Twin carb. The bad side carb has been changed out THREE times. Each one was minutely inspected. Each one runs fine
on another bike. The coil was inspected, tested fine. Second coil as well. Both were new as one I got as a back up because one time
I had to walk ten miles home.
It isn't simply a miss. It is so bad you cannot ride the bike in town especially once warmed up.
Biggest clue is upon startup you have a distinct difference between sides in heat. Bad side cold.
Bad side coked up.
At speed I suspect it is still running weak but you dont feel the miss or hear it. Low load downhill
backfires.
I guess the cam may be an issue I tried to mark the events on the alt rotor but it is hard to do, that is
figuring out exactly where the events are I don't have a good dial indicator setup. Not to mention
running the cam around and around wondering what you are doing to your followers and cam :-(

daveparry said:

You are certain that it's not an inlet air leak I suppose? (sorry but had to ask)!

Click to expand...

If there is an air leak it would have to be a compromised head the surface of the
carb (new) and the head are fine. I would die before I overtighted an Amal.

texasSlick said:

Please clarify: one or two carbs? Original post said replaced carb (singular) with two others..... i assummed one carb replaced twice.

If dual carbs, we open up a lot of possibilities, as Bernhard suggests.

Slick

Click to expand...

Slick, the carbs are all new or nearly new and off running bikes.

Hi Onder,
I just went through this with my 850 and a brand new pair of Premieres. Left side good , nice plug-- even pulse at idle. Pipe warms up in 20 seconds. Right side cold and sooty , weak pulse runs like poo. Any reasonable settings did not help. Checked everything on Premiere three times. Swapped ignition left for right. No dice. Dug out my old standard Amal 932 from my storage, cleaned it and checked it , put it on the bike and problem solved. Warm pipe, decent idle pulses and good looking spark plug--runs well for a quick setup . It would be nice to have matching carbs!
Sincerely,
Kara

Have you also swapped the intake manifolds?

Fit a spark tester on the offending side so you can see if its fully sparking all the time.

Other possible issue which effects all wasted spark EI's when using two coils is the earth path from engine back to battery, if this cannot handle the current of 2 sparks then on mine the left was intermittently firing, no amount of EI box changes, coil HT leads etc etc would cure it, only running a dedicated earth wire from headsteady to +ve battery terminal cured the issue. I twigged the issue when I left the right side HT lead off and the left chimed back in perfectly, only thing left to try was the earth. If you have a dual ended coil then no need for earth as spark current runs from one side of the coil to the other so ignore the suggestion for dual ended coil.

Onder said:

Both sides but the bad side was worse. Plug colour on the good side is quite clean. On the bad side less clean but after a good run you can see it burns away a lot of the soot. The bike seems to run well above 1/4 throttle at any speed. It likes load doesnt like no load.
The oil has gotten black fairly quickly which I put down to fuel dilution and carbon. Ive seen this before on bikes with over rich
mixture as in the choke was not lifting completely.

Click to expand...

I asked if it was just the bad side as I was thinking cracked head, especially if it is worse when hot, could you try a compression test when the engine is hot?

When you say compression test, are you doing a leakdown test?

kommando said:

Fit a spark tester on the offending side so you can see if its fully sparking all the time.

Other possible issue which effects all wasted spark EI's when using two coils is the earth path from engine back to battery, if this cannot handle the current of 2 sparks then on mine the left was intermittently firing, no amount of EI box changes, coil HT leads etc etc would cure it, only running a dedicated earth wire from headsteady to +ve battery terminal cured the issue. I twigged the issue when I left the right side HT lead off and the left chimed back in perfectly, only thing left to try was the earth. If you have a dual ended coil then no need for earth as spark current runs from one side of the coil to the other so ignore the suggestion for dual ended coil.

Click to expand...

I've had very similar to this
The right cylinder was cutting out so I changed the right coil
It turned out the left coil was failing but still sparking
However it wasn't feeding the right coil

kommando said:

Fit a spark tester on the offending side so you can see if its fully sparking all the time.

Other possible issue which effects all wasted spark EI's when using two coils is the earth path from engine back to battery, if this cannot handle the current of 2 sparks then on mine the left was intermittently firing, no amount of EI box changes, coil HT leads etc etc would cure it, only running a dedicated earth wire from headsteady to +ve battery terminal cured the issue. I twigged the issue when I left the right side HT lead off and the left chimed back in perfectly, only thing left to try was the earth. If you have a dual ended coil then no need for earth as spark current runs from one side of the coil to the other so ignore the suggestion for dual ended coil.

Click to expand...

Dual HT lead coil. :-(
3.9 ohms tests out.
bench test with two grounded plugs and direct + and - leads gives an outstanding spark.
I need to know if it will spark inside the head under compression.