Ignition Timing

Production K Models

Ignition Timing

Postby wz507 » Wed Oct 10, 2018 5:50 pm

Was looking through my 1955 KH Rider’s Hand Book for ignition timing specifications and found them given in terms of piston position (inches BTDC) for both KH (19/64”) and KHK (11/32”). I was interested in knowing what crankshaft angle these down-bore distances correspond to, so calculated the crankshaft angle corresponding to the piston position, which is shown in the table below.

Thought it would likewise be interesting to check the timing provided in the KR Competition Racing manuals and found a discrepancy between a couple manuals. I have one “early” and one “late” Competition Racing manual for which I have no issue date. The only means I have of estimating the date of each manual is to note the cam offerings described in each. The early manual provides specifications for H and J cams that have -66R and -67R part numbers respectively. The later manual provides specifications for J/K and J/L cams that both have -69R part numbers, so it would appear the manuals are perhaps a couple years apart. The early manual is consistent in it’s written description of ignition timing, providing piston position of 7/16" BTDC and crankshaft angle of 36 deg BTDC, along with an image of the flywheel mark at the edge of the timing hole. However, in the later manual the written description calls out the same piston position (7/16” BTDC) but couples this with a crankshaft angle of 38 deg BTDC along with the same image of the flywheel timing mark at the edge of the timing hole used in the early manual. Thought this was an interesting disparity so again determined crankshaft angle vs piston distance for the KR, which is shown in the table below.

KR Ignition Timing.jpg
KR Ignition Timing.jpg (31.42 KiB) Viewed 323 times


Ignition timing table.jpg
Ignition timing table.jpg (10.71 KiB) Viewed 323 times

Placing a degree wheel on my KHK flywheel confirms a difference of ~ 28 deg between the TDC dot and timing mark line.

Found it curious that the KH and KHK were timed considerably later than the KR. Perhaps the K Model ignition timing was set later than the KR to avoid detonation, as these engines are stuffed up with their small diameter 2-into-1 pipe set up relative to a KR with open pipes, possibly operate at a “significant” temperature and are often subjected to low speed lugging all of which is a perfect recipe for detonation to occur. In light of the possibility of lugging operation and fuel quality of the era the modest timing may have been required for survival. I would guess that if a K model consumer engine were operated without lugging, i.e., run free and fed modern high octane fuel it could tolerate quite a bit more spark advance than the specification calls for.
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Re: Ignition Timing

Postby Ferrous_Head » Wed Oct 10, 2018 8:54 pm

Different strokes. The K amd KH are different engines. 750 vs 883
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Re: Ignition Timing

Postby Rubone » Thu Oct 11, 2018 10:09 pm

Late KR pistons are a greater pin to crown height, so even though the measurement is the same, the crank position is different.
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Re: Ignition Timing

Postby wz507 » Mon Oct 15, 2018 8:11 pm

Rubone wrote:Late KR pistons are a greater pin to crown height, so even though the measurement is the same, the crank position is different.

Not to disagree, but I respectfully disagree. For this timing method the piston position is given relative to TDC, i.e., the timing method requires that TDC be located and thereafter the crankshaft rotated to move the piston down-bore the specified distance BTDC. Regardless of the piston compression height, top shape (flat, concave, convex or anywhere in between), or deck height, once TDC is established, and the piston subsequently moved the specified amount down-bore from TDC, the crankshaft angle is always the same.
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Re: Ignition Timing

Postby Lisa » Mon Oct 15, 2018 10:02 pm

Use a quickstart 2000 and you won't be sorry they work great with a lifetime guarantee. My opinion
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Re: Ignition Timing

Postby Noot » Thu Oct 18, 2018 9:20 am

I like wz507 . . . But anyway, I bump my timing up on these Ks and KHs (timing mark just entering the hole to the right side) and they run better. A bit more crisp. I run a solid mount FM magneto, and still starts great with this advanced timing. No kick back, no pinging. There's only about 5-7 degrees across the entire window of the timing hole anyway....
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Re: Ignition Timing

Postby wz507 » Thu Oct 18, 2018 6:08 pm

Noot wrote:I bump my timing up on these Ks and KHs (timing mark just entering the hole to the right side) and they run better. A bit more crisp. I run a solid mount FM magneto, and still starts great with this advanced timing. No kick back, no pinging.


I'm in your camp too Noot - a little earlier timing for me.
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Re: Ignition Timing

Postby wz507 » Thu Oct 18, 2018 6:25 pm

Thanks to those people both on and off the forum that have contributed varying views on ignition timing. Your input caused me to revisit the issue and hopefully bring more clarity to some topics, but digging a little deeper has also introduced new uncertainty on a least one front.

The thing that got me interested in continuing the discussion here was the factory HD document Starcain provided on ignition timing for several different K Model engines. The document, shown below, provides ignition timing in two ways; 1) as a function of piston position BTDC and 2) as a visual method showing where the flywheel timing mark should appear in the ignition timing hole. Due to the quality of some parts of the image, I want to clarify the 1st line of text which is challenging read, and reads "K Timing 24 deg to 27 deg"

K Model Timing sheet.jpg
K Model Timing sheet.jpg (226.92 KiB) Viewed 102 times

Upon seeing the images of the flywheel timing mark in the ignition timing hole I decided to measure the timing mark on my own KHK flywheel (again), observe where the TDC mark appears in the timing hole, and estimate how many crankshaft degrees appear in the engine case timing hole. A degree wheel having slightly smaller diameter than the flywheel was printed out for measuring these areas. As shown below, the timing mark on my KHK flywheel is located between 27-28 degrees from the TDC dot. Placing the same timing scale across the engine case timing hole revealed that ~ 8 degrees of crankshaft rotation can be viewed across the center of the hole.

KHK Flywheel Timing Marks, TDC in Timing Hole, Span of Timing Hole
Tming collage.jpg
Tming collage.jpg (23.52 KiB) Viewed 102 times

With that background the next step was to examine the factory document above to better understand its content. It is a busy document with considerable information and for me (Mr Dyslexia) to fully grasp it required putting the information into a table. To that end I assembled a table that provides the following information for a variety of K Model engines.

• Engine model
• Engine size
• Relative size of cam lobe employed, i.e., small medium, large
• Factory ignition timing specification, given as a fractional value BTDC (e.g. 11/32”)
• Factory ignition timing specification, presented as a decimal equivalent
• Factory range of ignition timing given in degrees BTDC
• Calculated ignition timing for each engine using the factory piston position specification and taking into account the stroke and rod length of each engine.

A summary of this information is shown in the table below, where KR data from the factory race manual is also included. To provide a qualitative sense for the difference in cam lobes employed in the K Model engines discussed here, Starcain sent the image below of 4 representative K Model cam lobes.

Ignition timing table and cam image.jpg
Ignition timing table and cam image.jpg (71.46 KiB) Viewed 102 times

The data in the table above suggest there is no relationship between ignition timing and engine size. There is however a strong relationship between the cam employed (governs maximum rpm range of engine) and ignition timing. All the engines with either the very small or small cam have late ignition timing (~ 26 deg). All the engines employing the medium cam employ earlier ignition timing (~ 31 deg). The KR engine which employs the largest cam (5000-7000 rpm) employs the earliest ignition timing (36 deg).

I believe the above information provides a reasonable relationship between the various K Model engines and the factory ignition timing preferred for each.

At this juncture I have posted 3 pictures so cannot add any more content here. There is one more brief point that will be made in the subsequent post found immediately below.

As always, interested to hear others thoughts on any of the above. Thank you.
Last edited by wz507 on Thu Oct 18, 2018 6:38 pm, edited 1 time in total.
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Re: Ignition Timing

Postby wz507 » Thu Oct 18, 2018 6:31 pm

It was mentioned above that although reasonable clarity was brought to the subject of K model ignition timing, one new uncertainty arose – namely the fact that multiple HD ignition timing specifications exist for the KHK model. The 1955 KH Rider’s Hand Book specifies KHK timing of 11/32” (calculates to 28.0 BTDC), whereas the factory document page shown in the post above specifies KHK timing at 13/32” (calculates to 30.5 deg BTDC). The images of the timing mark in the timing hole in each document (shown below) appear consistent with the given specifications (assuming flywheel timing mark is somewhere in the neighborhood of ~ 26-27 deg BTDC).

Image from HD 1955 Rider’s Hand Book
KH and KHK Ignition Timing.jpg
KH and KHK Ignition Timing.jpg (91.41 KiB) Viewed 101 times

Image from Factory Ignition Timing Sheet
KHK timing from sheet.jpg
KHK timing from sheet.jpg (30.83 KiB) Viewed 101 times

With that I’ll leave the KHK timing conundrum to KHK owners to sort as they see fit.

As always, interested to hear others thoughts on any of the above. Thank you.
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Re: Ignition Timing

Postby Noot » Fri Oct 19, 2018 9:11 am

I'm gonna print that chart - neat. The faster timing "gets that fuel burning a bit earlier" since we're dealing with a pretty in-efficient combustion system here. Everything is moving so fast, it takes longer to get that fuel ignited and explode ! I like Autolite 4316 Spark Plugs for my K and KH (FYI) ;)
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