Low MAP stumble

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Today | 11:44 AM

OakMountainRider

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Joined: Aug 2018

Posts: 593

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From: Stockholm

Low MAP stumble

Hello,

I thought I would get an advice from you guys here.

Dyna 06, 98 ci (bored 2023), Andrews 57 (2026), Feuling OE+ cam plate and oil pump (2022), Fueling HP+ lifters (2026), MLS 0.030” (2026), new exhaust valves, springs and seals (2026) Power Vision Target Tune, new IAC (2026)

Crankshaft runout 0.0021” (2026), Compression front 202 psi and rear 212 psi (2026), Leak down front 6 % and rear 5,5 %, MAP at warm idle 35 kpa

When twisting the throttle the bike runs well. It has an excellent throttle response, and from 3000 rpm up to the red limit it shines.

But here are the issues:
1/ A bit of stumbling and jerking at low throttle (below 40 MAP),
2/ Cold start idle is very jerky, like it is suffocating, (began 2025 with previous cam)
3/ Some ticking noise, tracked to front exhaust valve, but had this kind of noisy valve train since three or for years, even with the previous cam

Intake leaks are ruled out with 35 kpa at warm idle. IAC was the suspect, but I changed the IAC this spring and it did not help.

I lean towards that it is a tuning issue. However this does not explain the cold start idle problem, as it began last year.

Tuning: Tune from Dynojet, autotuned 8 times, AFR put at 14,2 from idle to cruising, pulled timing 20 % at idle/cruising. I am in the process of data logging the cells in this area, but no real change yet.

If someone has a clew here, it would be great.

Thanks a lot
Oak Mountain Rider

Last edited by OakMountainRider; Today at 01:27 PM.

Today | 12:21 PM

Max Headflow

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Joined: Dec 2005

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From: poway

Does it cough? May increase the timing at lower MAP. Autotunes don't adj timing.

Today | 12:40 PM

jbarr1

Road Master

Joined: Nov 2015

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From: georgia

I hope your crank runout is .0021 instead of .021!!

Today | 01:01 PM

OakMountainRider

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Joined: Aug 2018

Posts: 593

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From: Stockholm

Quote:

Originally Posted by Max Headflow

Does it cough? May increase the timing at lower MAP. Autotunes don't adj timing.

Hello Max, thanks for your input. No coughing. I pulled timing the other day from Dynojet’s tune with 20 % at all MAP up to 2500-3000 rpm, and it did not make it worse, rather a bit better, I would say. So I will keep the timing there. Just came in from a test ride, (after I recycled the IAC and let it idle for ten minutes). I feel happy about how it runs, except for the issues mentioned. Twisting the throttle and overtaking cars on the motorway is deeply amusing. I just want it to get a better cold start idle (it jerks when cold and the compensator rattles), and that it would run more smoothly on low throttle. (When warm, the idle is fine and the compensator quiet.)

Last edited by OakMountainRider; Today at 01:06 PM.

Today | 01:04 PM

OakMountainRider

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Road Captain

Joined: Aug 2018

Posts: 593

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From: Stockholm

Quote:

Originally Posted by jbarr1

I hope your crank runout is .0021 instead of .021!!

Thanks for observing this, it is corrected to 0.0021”.

Today | 01:09 PM

Max Headflow

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Joined: Dec 2005

Posts: 19,989

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From: poway

Sounds like you are headed in the right direction. I suspect that you need to tweak the cold start timing tables some. I'm really not all that familiar with PV.

Today | 01:15 PM

OakMountainRider

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Joined: Aug 2018

Posts: 593

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From: Stockholm

Quote:

Originally Posted by Max Headflow

Sounds like you are headed in the right direction. I suspect that you need to tweak the cold start timing tables some. I'm really not all that familiar with PV.

Yes, it is a logical step, I will look at this. A couple of days ago I increased the IAC warmup steps by 20 %. There were no radical improvement, so I put it back to where it was, but I may try 50 % and see what happens.

Last edited by OakMountainRider; Today at 01:34 PM.