SBN Mikuni carb lean condition

[itsmarta]

The needle settings for a SBN mikuni carb on a 93 xp is 1-1/2 out low needle and 0 out on high needle. If the engine had a mag cylinder lean condition would it help to back the high needle out a little to correct it, say 1/8th or 1/4 turn out? or does the low speed jet need to be backed out a little more. Pop off pressure is 23psi in both carbs and filter is clean.

[Bill O'Neal2]

Re: SBN Mikuni carb lean condition

Of course if you open the high speed adjuster it will richen your mixture, but if all is well with the carbs, why does it have a lean condition in the mag clyinder? Is the intake manifold cracked?

[itsmarta]

Re: SBN Mikuni carb lean condition

Bill, My returned engine from SBT under warranty was checked with a detonation failure and replaced by SBT (great warranty SBT). SBT tech stated it is a lean condition in previous post. used 87 octane gas, BR8ES plugs, pre-mix oil with gas, timing adjusted on stator mark to align with engine mark. Carbs were rebuilt in May 04 with SBT kits. I just need to identify the problem so the new engine does not do the same detonation thing again and was wondering how to richen the mixture. the manifold is not cracked.

thanks for any advice

[Technical Support]

Re: SBN Mikuni carb lean condition

You do not correct for a problem with the carb mixture screws. They are for adjusting to aftermarket parts and fine tuning only. If you have detonation, something is wrong that needs to be fixed, not adjusted.

[1fastski.com]

Re: SBN Mikuni carb lean condition

you should still check the timing , the mark on the case is not perfect , it just gives you a starting point .

[itsmarta]

Re: SBN Mikuni carb lean condition

How do you check the timing, there are not timing marks or anything like that?

[Technical Support]

Re: SBN Mikuni carb lean condition

Procedures and tools needed are in the OEM service manual. Timing is rarely ever an issue.

[Bill O'Neal2]

Re: SBN Mikuni carb lean condition

If you aligned the timing mark on the stator plate with the timing mark on the cases, you will be fine, as it could not be off enough to detonate a piston.

[itsmarta]

Re: SBN Mikuni carb lean condition

What exactly is detonation of a piston. I looked at the piston pictures and it shows the detonated piston. Does the piston just get too hot or does the ignition of fuel occur too early. Please explain.

thanks

[Technical Support]

Re: SBN Mikuni carb lean condition

Detonation is melting of the aluminum piston, yes. It can be caused from too lean (hot) a mixture, timing too early, or too much compression for the octane (pre-ignition).

[Jamie in NY]

Re: SBN Mikuni carb lean condition

Bottom line, your combustion chamber is running hot, causing pre-ignition, which in turn can cause detination (very bad!!). Hot conditions are caused by advanced timing, restricted cooling, wrong plug temp range, or lean mixture (which can be carb or air leak).

If me, I would try a NGK#9 plug which would run about 70-100 deg C cooler, then adjust carb for correct mixture. You may still have a fundemental issue with timing, air leak, cooling ,etc, but if you don't think so, then try a cooler plug.

Have you mad any mods...ie, higher compression?

[Jamie in NY]

Re: SBN Mikuni carb lean condition

Pre-ignition versus detonation from NGK website:

Pre-ignition

Defined as: ignition of the air/fuel mixture before the pre-set ignition timing mark
Caused by hot spots in the combustion chamber...can be caused
(or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
A change to a higher octane fuel, a colder plug, richer fuel mixture,
or lower compression may be in order
You may also need to retard ignition timing, and check vehicle's cooling system
Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events

Detonation

The spark plug's worst enemy! (Besides fouling)
Can break insulators or break off ground electrodes
Pre-ignition most often leads to detonation
Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
Most frequently caused by hot spots in the combustion chamber.
Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward motion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
Most of the damage than an engine sustains when "detonating" is from excessive heat
The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion

[itsmarta]

Re: SBN Mikuni carb lean condition

Jamie, This sounds like what I had. The engine that had the detonation was a stock SBT 657 engine without any modification, carbs rebuilt in May 04 and using premix. Early in the summer I did replace the Mag side plug BR8ES since the ground electrode on the plug looked like it melted on the tip (looked like weld splatter). After the engine broke, I removed the front plug again and the ground electrode on the plug was actually squashed against the center electrode giving me a gap of 0.000". SBT provided me under warranty a new engine and I dont want to repeat the same thing again. I cannot see any signs of a loose bolt, gasket or anything that would lead me to believe I have an air leak. The PTO side cylinder looked great and undamaged, full compression, and great plug color. It is a 93 xp which has two carbs. It sounds like the mag carb may need to be rebuilt again. What are your thoughts?

[Bill O'Neal2]

Re: SBN Mikuni carb lean condition

A twisted crankshaft can cause the timing to be way off on one clyinder.......

[itsmarta]

Re: SBN Mikuni carb lean condition

Bill, do you mean it could have been a twisted crankshaft originally or would the detonation have caused this?