Absolutely puzzled...engine problem 750

storm42 said:

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?

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Yes a differential test. 100 line air 90 psi on the in side and 85.5 and 84 on the out.
But 90 psi you will get a leak from a crack.
5% loss isnt all that bad but and it is ring loss not valves.

Tinkerbell said:

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

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Exactly my problem. But THREE different carbs? Cannot believe it. Last one came directly off another running perfectly bike with
no changes. Gee swap carbs side to side? That is nuts

Did you try putting your suspect carb on one of your well running bikes?

After reading all of this I keep going back to soft or inadequate spark at the cylinder in question. If the plug comes out wet or soot laden at low rpm and there is no boom recheck the ignition/electrical influences on that side with all components in place on the bike.

Just wondering, did you check the spark in the dark? Comparing the spark on each side with the plug touching the head? Ground is rock solid? Dedicated frame to engine ground lead....just remembering some of the issues I had similar to yours.

I know that you have been-around-the-block technically/mechanically enough times to have worn a path; I have benefited from the advice you have given me on my 441, so I hope I can return the help here, thanks.

Given the data you have supplied and the trials you have done with single variables my first thought was that the coils primary resistance was high, but, after reading the instructions for the Pazon I see that what you measured should be ideal. I suggest that you try altering the carbs sync bias, as in raising the slide on the lazy cylinder and lowering the slide on the other to regain a proper idle RPM and checking that you have a bit of slack in the cables such that the slide height adjustments are in full control of the slides.

Best.

Chased a similar issue on left , a while back , I think it was the dedicated ground that cured it ..... the symptoms were very similar to yours and I went through all the same stuff as you have with same results , actually only tried 2 different EI’s , anyway will watch this one with interest .... good luck

If you haven't already, try this: Start the engine and warm the engine a bit. After it is running "normally," as normally as it can right now, turn off the fuel petcock with the engine running. Is there any improvement in the bad cylinder as the fuel level drops in the bowls?

You don't by any chance have a 2 into 1 carb manifold you could try?
I realize it would be long winded but with a single carb fitted it would definitely highlight a carb problem

Onder said:

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.

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As Kommando said, get one of those spark flashy thingies.

Slick

I had a spark plug that I believe had a cracked insulator, up inside the tip. I used spring clamps to pinch both plugs to the outside of the head as a ground for testing them... Both plugs tested fine clamped to the head. Installed in the bike, I had an intermittent miss on one side, with fuel charges igniting in the exhaust pipe when a miss pumped unburned fuel into the exhaust pipe. The next stroke that fired correctly would ignite that fuel in the pipe when the exhaust valve opened giving a "POP" out of the tail pipe.

It tortured me for a full day. I assumed that the plugs tested good outside the head, so they were good inside the head under compression. My buddy saved me by offering to pick me up a pair of new plugs when he went out to pick up lunch. After I put the new plugs in, the bike wanted to start on the first kicks, but the carbs were not set up correctly yet. On about the third or fourth kick, I goosed the throttle and it roared to life solidly and ran.

Lesson learned: If a spark plug is suspect, don't test it, change it because it's not an expensive part...

Besides that, swapping a known good carb on to the misfiring cylinder generally eliminates the carb as the cause of the issue. That still leaves a cracked head, or intake manifold leak as a possibility.

I had an invisible crack in one of my intake manifolds that led to popping and a horrible idle on one side. I got a stumble when I sprayed it with carb cleaner when the engine was idling, so I took the carbs off. When I unbolted the manifold in question, it broke into 2 pieces. I assumed that I over-tightened it and broke it during a past assembly. The crack was completely invisible when the carbs and intake were mounted on the bike... Of course a new manifold fixed the issue...

texasSlick said:

As Kommando said, get one of those spark flashy thingies.

Slick

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..ordered last night!

When I bought my commando, I thought I would treat it to new Amal premiers rather than mess about with the old carbs. I then spent weeks ‘fixing’ everything else to try and solve the poor running and starting on one cylinder. I ended up sending the carbs back and it transpired that the brass plug at the bottom of the pilot chamber was not pressed in completely and was causing an air leak into the chamber. Something else to check if you have new carbs!

I also had cool running on one cylinder after fitting Pazon Altair.
problem was solved when I fitted old type champion plugs I place of the latest NGK’s.

It sometimes seems that the more we improve our old machines the greater the list of potential problems! It is half the fun though.

Onder said:

..ordered last night!

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an induction timing light would probably work in a pinch.

I have just had the same issue with my sons racing Suzuki 250. Long story but..... change the carbs over, see if the issues the same or if it’s reversed cylinders. It did with us. Carbs stripped so many times. I eventually realised I’d had a true should have gone to spec savers moment. Small pilot jet in side that was overheating. Tiny no’s bad eye sight and bad light! 50 in one side 70 in other. Changed them and alls well.

MexicoMike said:

If you haven't already, try this: Start the engine and warm the engine a bit. After it is running "normally," as normally as it can right now, turn off the fuel petcock with the engine running. Is there any improvement in the bad cylinder as the fuel level drops in the bowls?

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Good idea and I've tried a lot of things so one I haven't yet tried is what I'm after here. In fact I, like most, don't run
the original single pipe type carb feeds but have some and will rig them up so I can feed each side by its own tap.

Deets55 said:

an induction timing light would probably work in a pinch.

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Another useful suggestion. I have been timing on the RHS wire because it is easier.

This type of spark checker works in-line with the high tension lead. The HT energy must pass thru the device and make a path to ground by sparking the plug. If the plug does not arc, there will be no flash.

The brightness and size of the flash is indicative of the spark quality.



If you have a ground problem, as has been suggested by some, this device will bring it to light (pun intended).

Slick

Thx I'll pick one up. I doubt the ground problem. A ground wire runs from the head to the + terminal!

Sounds like fuel to me. Have you checked the floats by putting then in hot water to see if one of tem has a hole in it ? Also the float needles should be checked see that there is not a piece of something holding one of them off it's seat. Also, you might have a carb touching the frame. If the extra coke in the cylinder is not due to fuel, it might be due to oil - rings ?

I wish it was so simple. I repeat that THREE carbs, all work fine on another bike, tried and make NO change at all to the problem. :-(

I have seen cracked inlet manifold photos on here . Have you tried spraying WD40 or carb cleaner around the the inlet joints when the bike is running ?