Ignition timing vs compression.

[Blind_Mel]

Hi i have a question aboute ignition and copression on a Tohatsu 50d2 i have shaved the head and I have 170psi what should I do with the timing? more,less the same? and can you run normal pump gass on 170psi.. and what jets should i run original 132 or 125? Do you need larger jets with higher compression? the reson I wonder is I'm running 125 jets and advanced timing but i cant get the revs up. i't feels like the engine is running slower now than before i shaved the head???? :eek:

As you can see i'm kinda green in this area :rolleyes:

Thanks... :confused: :confused: :confused:

[Mark75H]

Shaving the head usually brings the peak pressure on sooner in addition to making it higher.

Most motors want the timing back a little.

[JohnsonM50]

[QUOTE=Blind_Mel;93764]Hi i have a question aboute ignition and copression on a Tohatsu 50d2 i have shaved the head and I have 170psi what should I do with the timing? more,less the same? and can you run normal pump gass on 170psi.. and what jets should i run original 132 or 125? Do you need larger jets with higher compression? the reson I wonder is I'm running 125 jets and advanced timing but i cant get the revs up. i't feels like the engine is running slower now than before i shaved the head???? :eek:

As you can see i'm kinda green in this area :rolleyes:

Thanks... :confused: :confused: :confused:

A safe bet is to start at the normal time then experiment a couple degrees at a time. The motors I use, OMC 25/35's are spec'd to 35 deg. for 25hp & 30 deg. for 35hp. [varied by year] From milling the head they seem to run best overall at 28/29 deg. Compression above 150 might run better on premium, More compression is more likely to add acceleration than significant speed.

[Seagull 170]

More compression, tends to give more heat & a faster burn so you might well benifit from slightly reduced ignition advance.
The pistons will also run hotter so increasing a jet size could have a positive effect, check your pistons for any signs of rubbing & relieve if needed.
Your powerband will get narrower & might move up the rev range a little.
Premium fuel will slow down the start of pre ignition, or at least it does here in the UK where we use very little plant extracts.

[Fast Fred]

max rpm is a major factor, so if your still runnin the rev limiter, stock timein is good, she'll pull hard to the rev limiter, after that or after 6000rpms timein needs to go down or she'll burn down.:cool: