R90S misfiring

[tiktok]

I took delivery of my R90S yesterday afternoon. I’m going to spend this morning going through the bike and tracking down the cause of the misfire.

The bike starts, but needs the throttle to be held slightly open as it won’t idle on a closed throttle. Choke doesn’t make a difference. Idle is not steady, it has around 500rpm variance.

So far I have drained the fuel (including float bowls), and added new fuel. The inside of the tank is very clean with red paint. Plugs are now gapped to 0.6mm.

Next is to check for air leaks to the carb; check the points gap; then I was thinking to reset the idle screws to the same position and see if the carbs needed balancing (never done this, as my Harleys have a single carb).

I’ll check the HT leads when I take the tank off to inspect the coils and flush the brake fluid.

Anything else I should include? (I have a Haynes manual, and will be ordering parts on Monday).

 

[tiktok]

Valve gap inspection to follow.

 

[DrFarkoff]

What carbs are fitted?

 

[Gelandestrasse]

When getting a bike like this I ignore minor ailments and do the following:

Change the engine and transmission oils and the engine oil filter, taking off the sump and giving it all a good look. It's like reading tea-leaves, but more scientific (the sump bit).

Next I check/set the valve clearances (like, do this first, not last).

Those 2 operations are really the minimum to avoid damage whilst the next bits are done.

Check the timing. I use a xenon strobe, but use whatever you have. At idle and at full advance (~3000rpm)

I attach a balancing tool (I use a carbtune at the moment and lament the day my mercury manometer fell over and spilled) to the carbs - some Dellortos do not have a vacuum take-off spigot so I use vacuum gauges stuffed in the (elbow-removed) intakes. I set the tickover to be level and then adjust the cables to give a smooth pick-up as you operate the hand throttle. about 1 - 2mm free play.

If it won't idle and misfires after all that, at least the basics are right and further running won't harm the engine. For a misfire or idling issues the most common cause on Dellortos is a gummed up idle circuit. This connects the idle jet in the float chamber with a small hole at the intake mouth and the outlet perimeter of the carbs, also the idle mixture screw. If I can't blow this clean with air or a propriety aerosol spray then it's off to get them processed (Ultrasonic or vapour blast). Blowing this clean myself involves blocking the various orifices with my fingers and making sure there is a clear path linking each tiny orifice with the jet and/or the mixture screw tunnel. If the mixture screw is missing its spring or the small o-ring and washer/gasket then you can judge how well the bike has been maintained in the past.

Referring to the diagram below (from the Dellorto Technical Manual v 30.1), "Fuel from the float chamber is regulated through the idle jet (22) and mixes with air from the inlet (17) and, passing along the channel (19), reaches the mixture screw (21); from here, the controlled amount of mixture flows into the main barrel (1) below the throttle slide (2). The screw (18) adjusts the position of the throttle slide at idle." Hope this helps.

 

[tiktok]

I appreciate the advice @Gelandestrasse , thank you.

I may have a valve cover gasket pack in the folder that came with the bike, if there's a pair, I will take a look at the valve clearances today. I'll drop the sump etc once I have my ramp clear and the necessary parts and fluids have arrived.

The left side is misfiring. There's no air leak (on either side). I'm not seeing a spark from the left (Bosch) coil when cranking, even after swapping the HT leads and plugs.

Right side seems to run fine, but won't idle.

I'll clean the carbs asap.

Regarding ignition systems, I'm happy to run points and coils - unless there is a massive benefit to something like the Boyer Bransden kit?

 

[tiktok]

There must be a spark, I'm not seeing it though.

 

[Gelandestrasse]

KBN373P?
If you remove the rocker covers carefully you can reuse the gaskets.
Stick with ignition system you have until you have it running correctly. If one side has a weak spark then check the HT lead connections and the plug gap. The ignition system has a lazy spark so the points don't affect one side in particular. If a swap of leads (make left coil do right cylinder and vice versa) moves the weak spark over, suspect the coil.

 

[tiktok]

I've removed the left carb, cleaned and blown through it with carb cleaner/air - all seems clear.

I'm leaving it now until I receive a delivery from Motorworks and can finish rebuilding the carb and carry out a service.

 

[Kenny]

Could simply be a duff spark plug. Simple things first !

 

[tiktok]

Some progress today - kinda.

Drained the oil, dropped the sump and saw absolutely no evidence of glitter - everything looks pretty clean.
Fresh filters, fresh oil.
Adjusted the valves - they were slightly out, but the valve covers were pretty loose.
New plugs, correctly gapped.
Rebuilt left hand carb (bike won't run on this carb, but runs on RH carb). Idle mix at 1 1/2 turns from seated and same for throttle stop.
Started her up - still misfiring.

Tomorrow I will inspect the points and fit a new condenser to rule these out.

 

[mikeyboy]

It’s not the points or condenser making it only run on one cylinder,
Stop and think about it.

 

[mikeyboy]

LH cyl check compression.
Check spark.
Good compression,
Ok spark.
Carb issues

 

[Stirlingmoz]

Assuming you tried transposing the HT leads as @Gelandestrasse suggested (to rule out a duff ignition coil), a compression test may help.

If the bike has been standing for a while you may have stuck piston rings or a valve not able to fully close, on the left side resulting in low compression ?

A good strong spark on the right cylinder (and at the right time) suggests that the points and condenser are good.

 

[Stirlingmoz]

Oops. Mikey beat me to it

 

[littleade]

Are the carbs the same? If so could you swap them over to see if the problem moves over to the other cylinder.

 

[mikeyboy]

Are the carbs the same? If so could you swap them over to see if the problem moves over to the other cylinder.

Fuck me,
It’s not fucking brain surgery.
Suck squeeze bang blow.
The other side seems to be working.
Has it got compression?
Has it got a spark.?
If it has compression and spark on the bad side,there’s only fuel that’s not happening.
If this is the case with Dellorto carbs they need cleaning properly.

 

[DrFarkoff]

I haven;t read it all but have you already done a compression test and verified cylinder pressures are "close"
Have you stripped and cleaned out the idle circuit in the Dellortos and if sop have you set them up correctly ??

If it really is only running on one cylinder ....

Is one header noticeably cooler than the other ? (IR thermometer is your friend here!)

If above is verified .............

Change the plugs over does the fault follow the plug ?

Change the leads over ... same again does the fault follow the lead

You could try swapping carbs over, I think you can just manage it with Dellortos to get it to fire and run

If you want to really sure it runs okay fit a set of Bings!!!!

 

[DrFarkoff]

Fuck me,
It’s not fucking brain surgery.
Suck squeeze bang blow.
The other side seems to be working.
Has it got compression?
Has it got a spark.?
If it has compression and spark on the bad side,there’s only fuel that’s not happening.
If this is the case with Dellorto carbs they need cleaning properly.

Nurse Nurse He's off his meds again !!

 

[theoneandonly]

spray some carb cleaner , into the non working side , if it perks up ....

 

[littleade]

Fuck me,
It’s not fucking brain surgery.
Suck squeeze bang blow.
The other side seems to be working.
Has it got compression?
Has it got a spark.?
If it has compression and spark on the bad side,there’s only fuel that’s not happening.
If this is the case with Dellorto carbs they need cleaning properly.

I was only trying to help