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Ported cylinder head

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I am running a 130 on a 900 turbo snowmobile. After installing a ported head with larger valves it doesn’t start as well. I know it’s probably An issue with fuel delivery but have no idea how to correct it. Your thoughts?

Likely intake air velocity dropped. Observe mixture ratio during cranking and startup, worth trying to add initial fuel.

Before starting to mess around with the tune, I'd strongly suggest carefully going over the electrical/electronics wiring and connections first, then use something like engine start or brake-clean sprayed around the intake joints and flanges to check for air leaks.

While the port work may affect the fuel demand, and timing, under load, the air drawn into the engine and the resulting fuel demand when starting, and at idle, should be approximately the same.

Exactly what model, and engine, is this? You said 'valves', so it's unlikely to be a ROTAX 2-stroke - if a 4 stroke, you may have the cam' timing out a tooth? Did you check for clearance with the larger valves?


Good points you mentioned.

I took for granted that the post opener has covered the basics.

Its an Ace 900 Turbo with a Motec ECU. Probable a simple fix by just adding fuel but Im not sure how.

What ECU exactly is it?

Do you have a wideband O2 sensor fitted to it?

How is the starting behavior when the motor is warm and being restarted ?

Its a Motec 130. It seems to start a little quicker cold then hot. On a restart, I will try it four or five times before it starts. Once it starts it runs good. It seams to help to give it a little throttle when turning over. Yes its a wide band. Thanks

1. Are you 100% certain that camshaft timing is correct and there are no vacuum leaks, like Gord pointed out?

2. Is the ECU reporting any relevant errors, triggering etc (can be hard to spot If not very familiar with the M100 series ECU).

3. Can you record a datalog to show Lambda values, manifold pressure, rpm, injector duty and ignition angle during cranking?

If you still find yourself stuck, I could take a look over Teamviewer, although the best person for remote support I could recommend for this is David Ferguson (David is a member here).

I would look at your Engine Crank Fuel Volume Compensation. You should have a table with Coolant Temperature vs. Engine Crank cycles. I would add 10-20% more fuel (above the current values) when warm for the first few crank cycles and see if that improves your starting.

Typically opening the throttle will supply more fuel, and this is a hint that it wants more fuel -- give it what it wants. For example on the Honda K20 I'm working on right now, at 100degC and above I have 137% fuel for the 1st crank, 67% for the second cycle, 18% third, ramping to 0% after 5 cycles.

Put more fuel in as David suggested, also look at the Post Start Compensation table, as this is used to phase out of the Cranking compensation.

Thanks, Appreciate the help

I would repeat what I said earlier, just porting the head and bigger valves should make SFA difference for starting- if you have an issue, the FIRST thing is to go back over the camshaft timing, wiring, connectors, hoses, etc, and check for vacuum leaks at all the flanges and fittings.

If AFTER that is done, and you have CONFIRMED they are not the problem, then, and only then, should you start messing around with fuel trims, etc. Heck, if it's easier cold than hot it may already be too much enrichment?

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