HarleyTechTalk

This is something that I've seen only twice, but it just doesn't make sense to me.

Bike has a slight bog when coming off idle. That happens when opening the throttle a little fast. It only seems noticeable when the bike isn't moving, but some times in a hurry trying to take off fast the bog will happen.

What I see, and also saw the last time I had seen this is when the throttle is opened quick, the spark goes to a negative value. Based on the kPA reading it should be in the range of 4° or 5°.

I attached the scan data. In the images I attached below, this occurred at record #217. Right after this as the RPM recovers the spark raises.

I assume this negative spark is causing the bog? Any idea what's causing that? The closed throttle spark is the same as the idle spark. So I assume this is making a smooth transition.

Any ideas as to what causes this?



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Well, maybe I found it. At the temperature the motor was at record #217, within the "Spark Temperature Correction" table it says -1.00° to -2.50° in timing.

Is that how this table would work? As I remember the throttle response is pretty good when the motor is cooler.


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This would be taking 1 degree off what ever is commanded in your spark table at that specific range.

When the bike is back together I can try that (or he can) and see if it fixes the bog off idle. In my mind it starts to make sense because if the motor is cooler, there is no bog. It may be something else, but the temperature thing had me stumped. The data file is from him, when he was trying to find the source of the bog.

Quote from: Ohio HD on August 19, 2023, 01:23:08 PMWhen the bike is back together I can try that (or he can) and see if it fixes the bog off idle. In my mind it starts to make sense because if the motor is cooler, there is no bog. It may be something else, but the temperature thing had me stumped. The data file is from him, when he was trying to find the source of the bog.

It seems when the low temp is typically calling for more fuel than an up to temp engine it can handle more spark advance.  Otherwise at operating temp it would be too lean to allow more advance.  Can that area be tuned (temperature sensitive) in open loop?  

The MAP is open loop until 2,250 RPM, then the cruise areas are setup as closed loop. I know he tried to add fuel in the VE tables but that didn't effect the bog at all.

The issue as I think it is, the timing drops low and to a negative value at that temperature and at the higher kPa. I think that's the issue with the bog off idle. When the motor is done (just being rebuilt) after a few miles on it I'll test the theory by upping the timing around 750 to 1,250 RPM at 70 kPa and higher in the front and rear cylinder timing MAPS. I still want to keep the temperature reduced timing in the table for other conditions that may require the timing to be reduced. High kPa and heat makes detonation.

That may work.  With the lower RPM (and velocity) it may need the timing to light the fire earlier.  Is it a long duration cam with more than average overlap and a later intake close?  Maybe a product of cam / air temp due to a cold head and combustion/ low RPM.velocity

I would also look at IAT temp correction's table.  Your IAT is hot enough to activate this table (rec 217) as well and is probably pulling a couple of degrees also.

I don't see an IAT table in this MAP. I loaded a newer MAP into Mastertune BT and then see the IAT spark table is active. This is an older .MT8 MAP.



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Quote from: kd on August 19, 2023, 10:11:12 PMThat may work.  With the lower RPM (and velocity) it may need the timing to light the fire earlier.  Is it a long duration cam with more than average overlap and a later intake close?  Maybe a product of cam / air temp due to a cold head and combustion/ low RPM.velocity

This is a simple motor, it's a 96 inch with Andrews 48 cams. He bought this bike used and has ridden it a few years like this. The crank slipped a few months ago and caused the oil pump to wear out quickly. So it's rebuild time mid season for him. The only thing that will be done other than stock is boring the cylinders to 103". He doesn't want or really need head work, so just a clean up of the heads and DH crank rebuild.

Previous bike owner gave him the TTS module along with the MAP. No idea at all who tuned this originally. The current owner had been trying to eliminate the lag with the throttle off idle. I told him I'd look at it since I'm rebuilding the motor.

Wrong base map, IMO. Base map is for a 113 high compression build with a big TB, used on a 96" stock compression?
I'd start over with a DAC205-04 after putting it back together.

I see that now. I wonder how much that's playing into his lag off idle? Other than that he said it ran fine. But fine to one guy may not be fine to another guy. I'll let him know that maybe a new dyno tune is needed. 

Quote from: Ohio HD on August 20, 2023, 08:13:53 AMI don't see an IAT table in this MAP. I loaded a newer MAP into Mastertune BT and then see the IAT spark table is active. This is an older .MT8 MAP.



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Because you don't get access in the calibration.  Doesn't mean the table isn't active.

Probably true enough. I imagine there's a lot of data that's imbedded in an EFI MAP that isn't accessible by the end user.

I see the VE table in the circled area (15%>, 1000-1750) is unaltered from the original map. That will contribute to the problem.You cannot view this attachment.

Oh wow, I didn't even think to look at that. Quite a lot of change in the 750 RPM above 15%.

That could be some issue too. The cells around a cells value can be effected to some extent I think as well.


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Quote from: rigidthumper on August 20, 2023, 10:19:31 AMI see the VE table in the circled area (15%>, 1000-1750) is unaltered from the original map. That will contribute to the problem.You cannot view this attachment.

When the bike is back together,  maybe I should try reducing that area by 5% at a time and see what happens.

I think I'd be telling him that whoever tuned it previously started with the wrong base cal and didn't do a thorough job on the whole so best bet is straight onto a good tuners drum after the rebuild, you be chasing ghosts Brian, you may well make it better in one area or another but that tune is built on sand.

I've already discussed the findings with him. He's actually looking for another used bike right now as a backup and to ride right now.   

I chuckled at him, he said, well you have a backup. I couldn't argue about that.   

Two is the go, wrench one ride the other, wrenching is fun too