RGS Cylinder Compression

K

Kristian

New member
Advice sought. Recently had a problem with No1 cylinder on my RGS not firing. Ignition OK, carbs OK , valve adjustment OK,  compression seemingly NOT OK at 130psi. No 2 is 140psi and No3 is 160psi. Obviously my first thoughts turn to valves / seats, but any words of wisdom before I take off the head? Thanks.

RGS 1983 Witt Ignition 7c cams  60'000 +  miles
 
could be Piston-rings, too... does it blow oil or mutch pressure out of the crank breather? what if you put some (!!!) oil into the cylinder - does it increase compression? How are the gaps between cam and follower? sometimes Valves do "grow"/get longer and all of a sudden, you don't have the valve closing up with the seat...
 
Ah, Piston rings, sorry I should have mentioned 'em.  Bike not smoking, crankcase pressure fine. I haven't tried the oil trick yet...
Re: Gaps. No 1 is  IN:20  EX: 30. So could do with tweaking but shouldn't be the cause of the problem. All other gaps within .5.
With regards to the compression readings across the engine, what should I make of them?
 
Compression low on both #1&2 cylinders. Even so, shouldn't of itself stop firing on #1

Assume you have checked carb slides are lifting fully when doing compression check? What do the plugs look like?

AFAIK, 7C cams do not work with a 120 motor... intended for 180. Profile not kind to valve train, so may provide a clue.
 
Yes, I agree, No1 should still fire despite the low reading. Carb slides operating smoothly, plugs just on the oily side but not too bad. I did the compression check with the carbs off anyway as it happens.

The 7c's have been on the bike since I bought over 20 years ago (bloody hell..) interesting what you say about valve train strain...I've not heard that before.
Thanks for your input!
 
throw a new set of plugs in it and try again, may just be a dud plug
if you have spark and it fires at the correct time, fuel and compression is okay, should fire
have seen engines with 40 psi compression and still running

out of curiosity, what brand are your coils ?

and what colour is your DMC Black box, Blue or Bronze ?  :D





 
First thing I did was swapped plug the out...yup, no change!
HT lead is cracking away (I can hear it and see it) whilst the accelerator pump is visibly injecting neat petrol into the bores. Maybe the Witt is at fault...?
Coils are low milage TEC MP08's and the Moto Witt Box is blue.
 
okay, you may have reached the end of the life span on the Blue DMC box but before you go any further, look at the wiring on your original Bosch pick ups
as the Blue box DMC relies on 35 year old ignition triggers then that is probably your issue in this case, especially being the RGS set up
wiring on the original pickups falls apart and then they start to cross short between each other
you need to carefully inspect the wires from the triggers and see if the insulation is totally shot

it is possible to replace the wiring totally but is touchy to do and sometimes it works, other times not so good
your best bet would be to buy a new Bronze box DMC2 unit which has a single Optical trigger that is designed to replace all of the original ignition system entirely

to test your spark plug circuit, grab a multimeter, set to ohms, jam one lead up the plug cap so it contacts the brass bit, other lead on the engine to provide an earth, in your case with a 120 you should read around 14 ohms
do that on all 3 leads, they should all read the same, +/-

the NGK spark plug caps do fail, they can be tested by unscrewing them from the spark plug lead and just do same test by holding one lead from the Multimeter on the brass screw end and other up the cap itself, being a 5 ohm cap it should read above 4 ohms, if not, ditch it
quite often you find nothing on this test

clear as mud ?
good luck


 
Rob said:
Compression low on both #1&2 cylinders. Even so, shouldn't of itself stop firing on #1

Assume you have checked carb slides are lifting fully when doing compression check? What do the plugs look like?

AFAIK, 7C cams do not work with a 120 motor... intended for 180. Profile not kind to valve train, so may provide a clue.
Click to expand...

They were available for the 120 Jota.  Work just as well/bad as in the 180.

piet
 
Manual of my DMC/2 (bronze box) offers a test option for the pickup-circuits, ingnition curve selector set to '0' position. With plugs out and properly grounded, they should throw a spark when pulling away a screwdriver blade from the iron core of the respective pickup.
Don't know if this applies to the blue box and the 120s, but might come in handy to locate a faulty pickup.

BTW, read kOhms for the resistance values listed by Red for testing the HT circuits.

Regards,
Stefan
 
As Red said, wiring from the pickups to the DMC should be checked.

Had a 1000 SFC Classic on the bike lift 2-3 years ago that I restored. It did not run properly, a lot of "bangbang" and running on 2 cylinders only, though I had sparks (hard to tell wich plug fires at which time though when you start the bike with the plugs out and grounded...). After completely rebuilding the carbs (which was necessary anyway after 15 years of not operating...), I checked the pickup-wiring and found the insulation was broken on all 3 and partly the wires were touching the primary case, partly they were touching each other...

I cut them off  roughly 3 cm away from the pickup, soldered new cables in and the bike started first push of the button.

I am not saying the DMC can't fail (some have defenitely), but I found 99% of the issues are bad cables or bad connections outside the box.
 
Thanks everyone for your suggestions. I'll be away from the bike for a few days now so temporarily can't continue to persue the problem. 
I can fully understand why pick up wiring would be a weak point. I resleeved all the wires a few years ago to hopefully pre-empt any problems. I haven't looked at them yet, so will do so asap. Will also attempt the DMC function test.
 
If you re-sleaved them, 80% of the potential problems have gone. However, as corroded as mine were (this green corrosion copper shows...), I was surprised there even was a spark...

If the cables touch eachother without proper insulation, the DMC get's confused as signals are coming potentially through more than one cable...

 
The DMC1 (blue box) uses DIP switches and you will find the test function is DIP#4 (from memory) The DMC2 (bronze) unit has a different arrangement, as described by Gravelroad.

I found the trigger wiring embrittled along much of its length, and amazed ignition still worked with bare strands of wire at the connector plug at rear of crankcase! Well worth checking that plug too for corrosion.
 
You can check the integrity of the insulation with an ohmmeter. If I remember correctly the resistance of the pickup is 220 ohm.  Test the leads from each pickup.
You should test each lead with one of the ohmmeter leads grounded to see if that wire is grounded, should be an open circuit. Also test for shorting between the pickups,one wire from one pickup to one of the wires to the other pickup should also be open. If they are shorted together you will probably see about 1/2 the pickup resistance. That is a common problem on the Ducati beltdrive ignitions. The solution is as the others have suggested, replace the wires from the pickup to the Dmc harness. Pretty simple, shouldn't take more than a few beers. 
 
sometimes the wires only touch when the vibration of the somewhat running engine sets in, which you can't find with the multimeter. Had that on the 1000 SFC I restored... Best way is simply to look at the wires and if the insulation is broken, humidity has creeped inside and the wires are corroded. I would replace them, too, just to be sure to eleminate one potential fault. Doesnt cost a fortune nor a lot of time. ;)
 
Ok,well, I think I've located the problem... inline between the pick ups and the DMC is a waterproof "DeRists"  multi connector. It has internal locking bullet fasteners which I crimped up to improve the connection. The spark on No1 is now looks a lot healthier than it was...perhaps previously it was being blown out by the compression? The wires checked out fine. However, the bikes not back on the road yet due to another problem:  No 3 carb slide appears to have fractured. Obviously the part that broke off was swallowed by the engine and spat out the exhaust...  The poor old carbs are so worn out!  A new set of Miks is looking like the way to go...this is the second time this has happened.

Thanks everybody for your suggestions, incredibly useful!
 
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