R90 doesn't run well

I can't get my 74 r90 to run smoothly. Sounds and feels like it is mostly running on one cylinder. Any ideas what I should check next? So far I have:

checked the coils
replaced spark pug wires
removed jets and otherwise cleaned carburetors
replaced plugs, condensor and points
checked the timing (statically)
adjusted the valves

When you say "check the coils", do you mean that you electrically tested the primary and secondary coil connections for the proper resistance, or just visually inspect for cracks/bad connections, etc?

start it up and let it run a bit. then check (carefully) to see if both header pipes are the same temperature. if one is cooler at least you will know what side to look at...

I just solved the same issue with my R75/6. Checking the quality of the spark, the left was much weaker than the right. After rebuilding the carburetors, cleaning all the contacts and replacing the plug wires, I pulled the PO's aftermarket electronic ignition (Dyna III) and went back to points with a booster. Runs like a top now.

I have been told that coils can be temperature sensitive, working well cold and failing and operating temp.

Did you blow out all the passages when cleaning the carburetors?

Have you checked the floats? If they fail, they can cause flooding.

checked

When this started, the left cylinder was not getting as hot as the right. so I "checked" the coils by switching them, and that didn't changed to problem to the right side. So, I replaced the spark plug wires and that may have help a little. I removed the jets and none were clogged, and there was no crud in the float bowls, and the floats seemed to be moving normally.

Maybe I should take the carbs off and really cleaned them and replace the soft parts.

Does the poor running cylinder improve if you pull on the throttle cable slightly?

Where are your initial carb settings? You could be way off on one side so that it doesn't run. If you provide the carb numbers, we can provide the initial settings per the Bing manual.

Also, what about the floats? How much gas is in each float bowl after you let them fill and then turn off the petcock? Carefully remove the float bowl and measure the height of the gas in the very center. Should be something around 24mm high.

radiofun1 said:

When this started, the left cylinder was not getting as hot as the right. so I "checked" the coils by switching them, and that didn't changed to problem to the right side. So, I replaced the spark plug wires and that may have help a little. I removed the jets and none were clogged, and there was no crud in the float bowls, and the floats seemed to be moving normally.

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When you say "runs" poorly, do you mean poorly at idle, or also when at part throttle when driving?

If only at idle, I'd suspect that the left carb's idle passages are clogged -- the idle jet may be clear, but the fuel and air passages leading to the outlet port at the throttle plate may not be flowing.

My suggestion is to remove the carb from the head (you can leave the cables attached if you are careful about not bending them as you manipulate the carb body), remove the float bowl, and blow through all the idle related passages -- the air bypass from at the inlet side of the main bore, the fuel entry point (remove the idle jet and blow up the tube), and at the idle mixture needle (remove needle, blow though), Also, double check the idle jet the cross- jet holes (for entry of air for fuel emulsion) which are absolutely microscopic -- verify they are clear. If you put your fingers over the idle jet and idle mixture tubes (jet/needle removed) and blow air into the idle bypass inlet, and you don't get air flow out of the outlet port at the throttle blade, you've found the problem.

dellorto

These are Dellorto carbs. Both the pilot jet and the idle screw are set the same. The left cylinder runs poorly at idle and when the throttle's opened.

did you check the petcock?

gas

there's plenty of gas going to the carb.

Dellorto

Radiofun1,
I had just this problem and finally traced it to my Dellorto carbs. I'm restoring a '75 R90S and rebuilt my Dellorto's with a rebuild kit plus new floats from Bevel Heaven. If you've rebuilt a four barrel carb these are about the same level of complexity.

The other suggestions you've received are excellent.


DaveM

radiofun1 said:

These are Dellorto carbs. Both the pilot jet and the idle screw are set the same. The left cylinder runs poorly at idle and when the throttle's opened.

Click to expand...

Ah -- the addtion of the "S" makes all the difference! Sorry, I'm strictly a CV guy.

I suppose I could add one more thought -- does the left side still run poorly at WFO/higher rpm? If yes, that points to electrical, as you would not have any of the three ranges (idle, mid-range and full main jet) good while the others were bad (i.e., if one was good, that would point to clogging in the other two circuits). Based on your report of the carbs being clean, I'm hard-pressed to think of a failure mode (other than fuel starvation -- which you say isn't an issue) which could take out all the fuel circuits at once. So, I starting looking to the ignition circuits.

How old are the plug wires? Have you measured their resistance? Coil primary and 2ndary resistances?

wires

I've replaced the spark plug wires and still have low spark on left side.

radiofun1 said:

I've replaced the spark plug wires and still have low spark on left side.

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You never responded to the Q about checking the coils -- only that you swapped them. If they are both marginal, swapping likely won't give a reliable indication -- time to ohm-check them boys

coils

True...time to really check those babies!