Running lean

[Iain Cameron]

To start this DT1 motor ran very nicely , I added a high comp head and a DT400 cdi system . checks already done are no air leaks , inlet ,mains seals . Ive raised the main jet . pilot jet up from 40 to 60 ,lifted the needle to bottom clip . still runs lean . Im buggered if I can find the answer . Any help appreciated Iain .

[evo550]

What are the "running lean" symptoms, a carburation mixture problem will sometimes present different to a air leak problem. If it started happening straight after adding the extras try going back to original equipment one item at a time, if the problem disappears after replacing one, then hey presto there's your problem.

[80-85 husky]

float level or air screw?

[Iain Cameron]

Im wondering timing would too far advanced or retarded be the cause as its set per original dt1 maybe with a hi comp head it needs a different timing setting . . Ive tried everything . Iain

[Iain Cameron]

bump

[GD66]

Had a friend with a DT2 way back whose rubber carburettor inlet manifold had perished and had a tiny crack, that took some finding !

[bazza]

Would suspect main seal or rubber inlet but you have checked them. suspect H/comp head. You dont mention a richer main jet??

[Andrew L]

Ian my rt360 would ping its a$$ off I tried richening to the point it wouldnt run clean but it still pinged I tried leaning it off a bigger mikuni carb all still had the same result. I always assumed the head was standard but after checking someone in the past had machined the head to the same as the rt2mx compression but hadn't changed the non adjustable timing plate so it was still firing 2.9mm before rather than the MX's 2.1mm , fitted a later dt400 ignition which I made adjustable and timed it to 2.1mm before tdc and it stopped the pinging maybe try retarding the ignition to the specs for your MX version , needed to do allot of rejetting as it lost a bit of punch but better than seizing and a dnf as it had done before.

[alexbrown64]

Iain, doesn't higher compression cause a leaner condition and pinging.  An example is Diesel engines.. knock..knock..knock... is that the lean condition you are getting?

Taken from thumpertalk  "Several things cause pinging...

1. not enought fuel (lean)... amount needed varies with temp and altitude
2. too much air, air leak = lean
3. ignition too advanced
4. squish too large so hot end gasses pre-ignite fresh charge
5. compression too high/head poorly designed... during combustion pressure increases too rapidy causing spontaneous combustion of charge = detonation.

1-4 are all 'pre ignition' caused by chamber temps which are too high
5 is 'detonation'

Both detonation and pre-ignition sound the same while you're riding." http://www.thumpertalk.com/topic/689696-what-causes-pinging/

I think a fix for this is running the best high octane fuel you can get.  An example is my Jeep V8.  I have to run Premium 98 or my knock sensors go crazy.  This is because i have the more powerful HO 4.7 which runs higher compression.

This is also interesting.. you have just rebuilt your bike... are you running more oil?  This can lead to a lean state and i have read this in many places...
"When talking about being rich or lean, we're actually referring to air/fuel ratio.. not oil/fuel ratio. When you add more oil to your fuel (eg. going from 40:1 to 30:1) your actually leaning out your air/fuel mixture because the extra oil takes up the space that the fuel would have otherwise taken up. When going from 30:1 to 40:1 you're richening up the air/fuel ratio because now the fuel is taking up the space that the oil used to take up."  http://www.dirtbikeworld.net/forum/archive/index.php/t-2463.html

So, Iain, due to the higher compression head, your bike is running leaner..... all the readings suggest that to fix this problem, you can try retarding your timing slightly (going closer to TDC), use higher octane fuel, run less oil in your mix, jet your bike richer, or find another standard head. 

You could eliminate one problem at a time.  Go through the timing, fuel, oil changes above... You have changed ignitions so maybe the new ignition runs a little more advanced than the old one.  Swap just the ignition first. 
If no changes, put the ignition back and swap out the cylinder head.

Looking forward to finding out the answer..

Cheers,
Alex

[Lozza]

Umm your NEVER going to hear pre-ignition because your engine will be destroyed before you can grab the clutch. A too advanced ignition will burn a hole in the piston under the plug before anything. Pre-ignition as the name suggests happens before spark event detonation after the spark. Pre-ignition is far more damaging and it's uncontrolable.
Changing your oil ratio from 40:1 to 30:1 represents a 0.5%(2.5 to 3%)change in the amount of oil per litre not many vmx bikes run to that level of tuning.

Iain what throttle % is it lean at and how are you certain it's lean?

[Iain Cameron]

The motor runs very hot even at idle ,  the plug is white , once the motor is warm it runs better with the choke on . Im thinking Ive got the timing way to advanced .

[Andrew L]

Sounds exactly like mine was

[sleepy]

The motor runs very hot even at idle ,  the plug is white , once the motor is warm it runs better with the choke on . Im thinking Ive got the timing way to advanced .

A retarded ignition will make the motor run hot and the high temps can make it ping. Have you had a timing light on the motor to see where it is actualy sparking? Have you any idea what the compression ratio is?

[crossedup2]

Ian
Put a standard head back on and see where that goes. The timing verses compression ratio makes a big difference. If the bike runs better than work out the difference in the combustion chambers and work towards a happy medium.

Also, are you using the standard DT CDI box. Strangely enough, an IT or YZ  CDI may make a difference.

Peter B

[Iain Cameron]

Ok thanks for the help , I don't know who is more retarded me or the bike . I had set timing at 2.1 btc on assembly on checking with a strobe light almost 4.6 btc . I knew the head and jetting would be very close due to using a DT1M head and cylinder ported to DT1M specs, the carb was new from Hatricks jetted to DT1M specs . pipe built by Peter Worrel . Thanks again gents , Iain