250 Monza backfiring thru carb

[cooperplace]

I finally put it all together. Filled with Penrite 40w oil. The tank is fixed: it no longer leaks. Fuel taps no longer clogged. Timing is statically set a few degrees BTDC. Brand-new Mikuni VM26, with (hopefully) correct jetting for a Ducati 250 -as supplied by Sudco. Lots of compression. Brand-new NGK BP7HS, gap set at 0.015". The good news is that it starts and runs, first kick. The bad news is that it constantly backfires through the carb, when running at a constant 2-3,000rpm. Pop pop, bang bang.

According to the factory manual, the automatic advance system should be checked. The carb of course might not be jetted right.

Any suggestions for what I could do next would be VERY much appreciated!

[graeme]

Aren't 250's supposed to be 20 or 30 deg before TDC?

Graeme

[cooperplace]

5-8 BTDC it says in the book.

[cooperplace]

I've just gone thru the procedure in the book that (I think) checks the automatic advance:
1. install a timing light, bring to TDC.
2. turn the crankshaft clockwise (I'm doing this as viewed from the RHS of the engine) about a quarter turn, light comes on.
3. Rotate the crankshaft counterclockwise until light just goes out.

this gives a reading of 30 degrees, which I think is within the specification.

Does this mean that the automatic advance is OK?

[cooperplace]

I think there might be an air leak between the carb and the inlet manifold. It's a very tight fit there.

[double diamond]

You can’t check the advanced spark timing statically unless you have somehow fixed the AAU in fully advanced position. Doesn’t sound like you have done this so you’re only checking the spark timing in retarded mode. Typically, you have to have the engine running at an RPM past the full advance RPM to check advanced spark timing with a stroboscopic timing light. .015” seems pretty small for spark plug gap. There have been a few posts on installing a Mikuni VM26 on this forum. You might want to compare the jetting that came from Sudco with what has worked for others.

[Rocla]

Hello,

I've had this kind of problems (back fire) on my 350 Scrambler during the start or at low engine speed and it was due to the fact that the bevel and ignition gears on the crankshaft were not perfectly aligned (with the spot, there were few teeths gap).
In parallel, the condenser was worn and didn't work properly. When the condenser is going to collapse, the ignition timing is significantly and randomly modified...

In fact, both reasons can explain such back fire problems...

[cooperplace]

finally got the studs out using heat, installed Sudco manifold using Allen bolts, Mikuni now seals well in manifold. Still backfires and kicks back when I try to start it.

[StewartD]

Hi Cooperplace,

Double Diamond has given you the answer: It sounds like you checked the timing without locking the auto advance unit in the fully advanced position, and found it to be 30 degrees before top dead centre.

You need to recheck and use the static setting. The reading, according to the Clymer manual, should have been - (Advance with engine still) 5 − 8 degrees.
Also it gives: extent of automatic advance - 28 degrees.
Total advance with engine running at 3000 rpm: 33 − 36 degrees.

You say that you rotate the engine clockwise looking at RHS of engine, and then counter clockwise until the light goes out.

I assume you are following steps 4 and 6 in the Clymer manual. The Clymer manual however is referring to diagrams that are looking at the left hand side of the motor.

Cheers,

Stewart D

[cooperplace]

I don't think the timing is the cause, but I admit I don't know the cause. The static timing is a few degrees BTDC. Every time I check it, I get a slightly different reading, but always in the 0-3 degrees BTDC area. So this might be retarded by a couple of degrees, which I doubt would cause massive backfiring.