Timing a modified bike

[Blurredman]

Hey guys.

I'm using a 300cc re-bored 250 barrel and head on my ETZ 250.

Before re-bore the old piston was 0.4mm clearance to barrel, and I used 2.5mm before tdc, as standard, and the bike seemed to work pretty well.

But to accompany re-bore I had to replace bottom gasket, which wasn't as thick as the top gasket. Which was approximately 1mm in thickness, I therefore put an extra 1mm squish band (to make 2mm) at the top of the barrel to accompany this.

Anyway, When I set timing to 2.5mm before TDC the bike runs backwards in 50/50 chance !!


What should I do!? My brother and I have different theories. I have it currently at 3.5, and although it runs crappy it doesn't go backwards, but he thinks I should put the timing at 1.5mm before TDC.. what should I do!?

[Skorpion]

Hi.

The ETZ300 timeing is set at 2.5 to 2.75mm.

You check the squish band clearance using strip of 2mm solder inserted through the plug hole, when compressed by the piston you need it to measure 1.0-1.2mm,
check in more than one place around the piston crown.

[Blurredman]

When the squish band is 1mm, and the timing is set to standard 2.5 BTDC.. it runs backwards!

[Skorpion]

Hi.

The ETZ300 timeing is set at 2.5 to 2.75mm.

You check the squish band clearance using strip of 2mm solder inserted through the plug hole, when compressed by the piston you need it to measure 1.0-1.2mm,
check in more than one place around the piston crown.

If setup like this it should not run backwards.
Recheck any other thing you have removed/changed ie contact breaker/cam.

I think as your brother says it would run better by retarding the ignition, BUT that is only MASKING the PROBLEM.

[Blurredman]

Tonight, I will be making a timing disc and setting up timing with it running. 22* is what the official manual states, I will be interested in seeing how much the points carrier is out. Probably not by all that much in reality.


The only thing that is different would be the crank. As I had installed a new one (the big end disintegrated on old crank). I replaced the crank with a re-built item as this was a full engine re-build. I have used the original lobe, but I have tried different contact breakers. I'm guessing it is a possibility that the crank is a little off from the lobe, However, I do have my old crank and can compare the key angle.

[Blurredman]

So..

Tonight i used a timing light with a programmable offset. The end result is my required 'plug hole measurement' is 1.5mm, not 2.5.



Running lovely now..! Cheers Skorpion.

[DAVID THOMPSON]

as a point of reference most of the pump jack oil field engines 4t here in USA
are fixed at .020 gap and fire at 20 deg before top dead center
at 28oo to 3800 rpm any thing over 4000 and they do not last very long (run on natural gas)
old BMW was about 1000 rpm at idle and 9 deg before TDC
getting the dwell to long makes coil over heat
(points closed to long)

good read for the do it your self types is
http://www.gofastforless.com/

how to make it all run on the cheap
dave

[AlanJ]

Hi All,
Great post Scorpion. Just to a few pennoth to the mix if I remember rightly the Messerschmitt 3 wheeler had 2 sets of points, a standard set and an advanced set, you switch to the advanced set to fire the engine backwards. In theory you could do 65mph backwards using all 4 gears. That would have given a few onlookers a shock. I remember way back, I was up behind my bruv on his BSA Bantam going to work and we were on a slight slope opposite a bus stop,he stalled it so he clutched it and started it again, let go the clutch and backwards we went, much to the surprise of those at the stop. So, just a little too far advanced can certainly give some odd results. Just to add a little more, if you keep getting kick back then you are probably a tad too far advanced.
Take care All Alan

[Blurredman]

Just to note, I did end up retarding the ignition by another half a mm, approximately 7 degrees? As it was still starting up backwards occasionally, most notibly when I applied choke on a running but still cold engine, or when it had stalled..

Seems good so far..

[dirtsurfer]

I wonder if your base gasket has altered your port timings slightly?

[Puffs]

Might it be Dieseling backwards due to too high pressure? Very unlikely, but you can try adding a thick cylinder head gasket.

My brother had a 250 Husky MX ('64 if I remember correctly) that had the same problem. Can give a rather unexpected result at the starting grid.

[Blurredman]

I think it is high pressure?

New piston installed in re-bored cylinder with 2 thou clearance.

Original base gasket was thicker yes at around 1mm, but upon re-build made note where the piston aligns with the inlet ports and it lines up nicely with thinner base gasket.

To counteract this compression a bit I have put 2mm squish band up top, so had extra gasket up top to accomodate this fact that lower gasket was thinner.



Didn't overheat/run backwards before because old piston was very low on compression (0.4mm piston/cylinder wall gap).

[Puffs]

Well, I don't actually know what the term "squish band" means in English. Previously I figured it was something like Plastigauge as Japan Inc. use, or a piece of solder, you put for instance between the piston & head (through the plug hole) to measure the gap between those, but now I guess not.

But I do think the ignition timing values you mention are very late.

Yes I suspect it is due to too high pressure, which makes the engine kick-back when you start it. And then it happily runs backwards, as two-strokes do (if the ignition allows that).

I don't know, just shooting from the hip. Did you use the original 250 head? The volume of the combustion chamber in that is likely smaller than for the 300 head, so you're compressing a bigger volume (bore increased) into the same (or at least very similar) volume when the piston is at TDC. Hence the pressure is higher.

You could measure the compression pressure, but I do not know what value you should get for such a bike. You can also just take off the head & put some more gasket below it, if you have those. That will increase the volume left @ TDC: You can estimate the thickness as follows:
Haynes specify a 10.5:1 compression ratio for both the 250 & 300.
The 250 is 243cc, so the volume of the combustion chamber is 23.14cc.
The 300 is 287cc, so the volume of the combustion chamber is 27.33cc, so you need 4.2cc more. At a bore of 76mm that is about 1mm. So you need about 1mm more gasket than before. If you leave it as-is, you'll have a compression ratio of about 12.4:1

Fyi, Haynes also specify 0.035mm / 0.0013in piston-to-bore clearance for the 300 (and, surprisingly, 0.04mm / 0.0015in for the 250/251). Yet 0.06-0.07mm would do me fine.

[Puffs]

OK, now I've looked up "squish band", and it appears you do mean the clearance between head & piston, as measured w/ a piece of solder. The standard value is about 1mm. So with 2mm you should be about right, if my earlier estimate is anything to go by. But that estimate might not be accurate, and I'd still try to bring down the compression a bit & see if that helps. Is easy, just cut it from 1mm alu sheet, or use a couple of thick gaskets.

Yes, the standard 21°-22° preignition might enhance the engine's tendency to run backwards, but that timing recommendation comes with a very long history behind it, and to deviate from it now, in 2018, in an otherwise standard engine, seems a bit odd. Something else is going on.

[Blurredman]

The only thing not standard is the 300cc piston and cylinder.