This is a reprint of my service notes on my bike pretaining to spark plugs. You may find it helpful.
Date 2-12-10
Mileage 30902
Spark Plug Replacement Autolite XP 3924 from AutoZone $ 28.00
Removed the original OEM spark plugs NGK IFR6L11 (Iridium .040” gap) and installed
Autolite XP 3924 (Iridium) with a .040” gap, which are one range hotter than original OEM and
are equivalent to the NGK IFR 5L11.
This Autolite XP 3924 (XP denotes Xtreme Performance Iridium Enhanced) is a Copper Core, Double Platinum (ground side wire), Fine Wire Center Electrode, Iridium Enhanced, Nickel Plated Shell, Trimmed Side Wire (ground side wire) as per the Autolite description at
http://www.autolitecatalog.com/PartDetail.aspx?b=A&pn=XP3924The original IFR6L11 was too cold and had tremendous fouling and build up on all plugs, especially on the deep socket location plugs on both cylinders and it was the worst in the rear cylinder. When comparing both plugs from a given cylinder, the shallow socket location burns cleaner than the deeper socket. The more fouled plug was noticed to be installed/located in the deep socket location in each cylinder. It is suggested by some that this difference in burn cleanliness is a function of the “Wasted Spark” design Honda uses on the bike or a supposed “Reversed Polarity” ignition of the two plugs in each cylinder. This “Reverse Polarity” theory is non-sense as there is no evidence to support this claim. The VTX uses a conventional wasted spark design used on many current bikes with no issues. I don’t believe either theory to be the cause and believe poor head turbulence and overly rich A/F fuel curve as the likely causes. BareAss Choppers suggests the carbon fouling of the deep socket plug is a result of the oil jets under the piston. I disagree with this theory also. Oil jets are standard on many for stroke engines and the VTX's jet system is no different. The carbon fouling on the plugs is a result of over fueling and cool flame fronts, not over oiling. Some minor carbon fouling was noticed on the piston crowns and valves but was not very heavy. It was heavier in the rear cylinder than in the front. Honda made a mistake and chose to install the colder 6L11 on the 2002 (first year) VTX when they should have used the hotter 5L11. Honda made a change in 2003 and speced the 5L11 as the standard plug for models after the 2002 model to correct the fouling problem so prevalent on the 2002 VTX. The OEM rich fuel mapping and poor burning habits of the deep socket location compounds the fouling. I also contributed to the fouling build up due to my mistake of running premium gas most of the time these plugs were in use. Honda specs call for regular grade fuel which burns hotter than premium and thus would not have fouled the plugs as bad as the premium fuel did. While running premium fuel has contrubuted to the fouling due to it's cooler flame front, the major fouling factor is the too cold plug heat range and overly rich fuel curve honda designed in the bike.
See hand written notes on plug socket to valve measurements and actual plug measurements that were taken to compare plug to valve clearance relationship. The Autolite XP3924 project into the head .025” more than the 6L11 plugs. They are still missing the intake valves by .077”. OEM 6L11 plug projection clearance to valve is .102”. These Autolite XP3924 plugs have a projected tip design which extends farther from the plug body than the OEM NGKs. This added projection should help clean the electrode of debris from gas turbulance as is its intended purpose.
Also purchased Bosch 4312+2 (Platinum) as spares. These have a gap of .055” out of the box. $ 21.61
Notes:
- Do not attempt to adjust gap on Bosch plugs with multiple ground elctrodes. Ground arm may break off.
- The NGK BKR5EIX11(Platinum) is functionally equivalent to the IFR5L11 but cheaper.
- Do not use the Autolite XP5224 because it projects too far into head.
Recommended plugs can be found here:
http://www.rattlebars.com/vtx/xplugs.htmlhttp://tech.bareasschoppers.com/engine/spark-plugs-and-the-vtx-1800/*I tried to verify as many plug numbers as possible but ran into problems finding any concrete answers from many plug manufacturers. The plugs with “*” next to them are verified by the manufacturer as a direct cross-reference, the others are old numbers that were floating around the VTXOA.
Spark plug cross reference for the VTX 1800
Manufacturer OEM (03+ stock) Cold (02 stock)
AC Delco* 41-806 41-801
Autolite (these are double Platinum) APP3924 APP3923
Bosch Platinum +4* 4419 4418
Champion Copper* RC9YC4 RC9YC4
Champion 7346
Champion Double Platinum RC10PYP4
Denso VK16PRZ11 VK20PRZ11
Denso PK20PR11
Denso Iridium* IK16
NGK* IFR5L11 IFR6L11
NGK Iridium BKR5 EIX-11 BKR6 EIX-11
Splitfire* SF522D SF392C
Splitfire Platinum* TP522D TP392C
Uber Power* FZX5iL FZX6iL
OEM Reccomended Gap 1.0 – 1.1mm (.038″ – .043″)
