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Old February 17th, 2020, 12:30 AM   #1
nocturncal
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Can An Intake Leak Cause Hard Starting?

I had a hard starting problem for a while, chalked it up to dirty carbs/being cold originally.

Whenever I started the bike, it would sputter/have a hard time getting going.

After having the carbs cleaned, the bike ran perfect! Ride it around for a day, then today the sputtering returned! The first couple of pistons firings sounded super weak. But all it took was a second of winding the throttle or pulling the choke to full on startup to get the engine going, then perfect idling, no idle troubles for today.

Took a look at the carbs and one of the intake rubbers from the airbox to the carb had unseated itself. So I reseated em and now the bike starts up without a hitch, no weak piston firing during the first couple of cycles.

Does my fix sound plausible? I did this with the bike not cold, but not totally warmed up yet so I'm wondering if there's something else that could be going on.

Also trying to understand how a vacuum leak could lead to hard starting
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Old February 17th, 2020, 09:18 AM   #2
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Quote:
Originally Posted by nocturncal View Post
I had a hard starting problem for a while, chalked it up to dirty carbs/being cold originally.

Whenever I started the bike, it would sputter/have a hard time getting going.

After having the carbs cleaned, the bike ran perfect! Ride it around for a day, then today the sputtering returned! The first couple of pistons firings sounded super weak. But all it took was a second of winding the throttle or pulling the choke to full on startup to get the engine going, then perfect idling, no idle troubles for today.

Took a look at the carbs and one of the intake rubbers from the airbox to the carb had unseated itself. So I reseated em and now the bike starts up without a hitch, no weak piston firing during the first couple of cycles.

Does my fix sound plausible? I did this with the bike not cold, but not totally warmed up yet so I'm wondering if there's something else that could be going on.

Also trying to understand how a vacuum leak could lead to hard starting
That's not really a vacuum leak at that point. Being unseated from the boot to the airbox may certainly disrupt airflow into the carb and cause issues, but there's no vacuum on that side of the carb.

A vacuum leak can affect starting as well as running at any RPM.
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Old February 17th, 2020, 09:40 AM   #3
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The factory airbox is designed to allow a predetermined, metered, intake airflow....having 1 airbox boot disconnected would create an imbalance between it and the side that remains connected, directly affecting carburetion, IMHO.
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Old February 17th, 2020, 01:14 PM   #4
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There's two concepts involved here:


vacuum leak would occur between head and carbs since restriction of throttle generates vacuum. Leak in rubber manifolds would suck in extra air that didn't go through carbs. This leads to lean mixtures that indeed can cause difficulty starting.

pre-carb leaks wouldn't really be classified as vacuum-leak since there's no vacuum ahead of throttle-body/slide restriction. But yes, there's configuration of air-box and velocity-stack style intakes that shapes air-flow pattern before carbs. Any distruption of that flow would change air-volume going into carbs and cause difficulty starting.


Let's see how it starts 1st thing tomorrow morning.
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Old February 17th, 2020, 09:12 PM   #5
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So I tried starting it again this morning, and still the same thing. Thud thud, dies. Try again thud thud, give it a little throttle, bam, fires and idles perfectly.

When it's warm it's a bit better, I don't need to give throttle but it takes a couple seconds to get to idle

Link to original page on YouTube.
Video is with the engine fully warmed up

The thing is that it's inconsistent. I've had some start ups where they are perfect. I will be taking a look at the ignition coils soon


Last futzed with by nocturncal; February 17th, 2020 at 10:19 PM. Reason: Deleted the word choke
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Old February 17th, 2020, 09:32 PM   #6
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Thud, thud dies with full choke?
Is your bike parked inside or outside?
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Old February 17th, 2020, 09:38 PM   #7
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Quote:
Originally Posted by DannoXYZ View Post
Thud, thud dies with full choke?
Is your bike parked inside or outside?
Bike is usually parked inside. Full choke or any choke makes the bike die at startup. Only blipping the throttle right after I hit the starter button helps

@DannoXYZ sorry my comment before this mentions me using the choke and it starting up, to correct myself I don't use any amount of choke because it will die out
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Old February 18th, 2020, 12:11 AM   #8
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Quote:
Originally Posted by jkv45 View Post
That's not really a vacuum leak at that point. Being unseated from the boot to the airbox may certainly disrupt airflow into the carb and cause issues, but there's no vacuum on that side of the carb.

A vacuum leak can affect starting as well as running at any RPM.
Ah gotcha that makes sense. About a quarter of the boot was off of the airbox side of the carb. Funny thing is I had a have a flat spot when at 3/4 to full throttle at around 9k-11k and after the boot slipped off off the flat spot went away. Now that the boots fully seated the flat spot is back :-(
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Old February 18th, 2020, 06:31 AM   #9
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fresh fuel?
innards of fuel tank clean?
have you set each cylinders' pilot screw for best running, found each "sweet spot"?
Spark plugs correct, relatively fresh and properly gapped and torqued?
What RPM is idle set at?


I'd contend there are folks that would kill for such starting as on your vid
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Old February 18th, 2020, 07:22 AM   #10
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Quote:
Originally Posted by nocturncal View Post
Ah gotcha that makes sense. About a quarter of the boot was off of the airbox side of the carb. Funny thing is I had a have a flat spot when at 3/4 to full throttle at around 9k-11k and after the boot slipped off off the flat spot went away. Now that the boots fully seated the flat spot is back :-(
Have you shimmed the needles?

The carbs are usually lean in the mid-range, and shimming the needles richens it up and smooths-out that area. The Main Jet is typically plenty rich.

As ducatiman noted, take some time and get your idle mixture screw adjusted precisely. Make sure the engine is fully warm and the gas is fresh. That adjustment will change with weather, so know that you may need to make additional adjustments if the temp changes significantly.

Make sure your air filter is clean - and not over-oiled. Carbs synced and valves adjusted as well.
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Old February 18th, 2020, 03:41 PM   #11
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Quote:
Originally Posted by ducatiman View Post
fresh fuel?
innards of fuel tank clean?
have you set each cylinders' pilot screw for best running, found each "sweet spot"?
Spark plugs correct, relatively fresh and properly gapped and torqued?
What RPM is idle set at?

I'd contend there are folks that would kill for such starting as on your vid
Found the problem:

Spark plugs gaps 0.7mm each, no rust in gas tank , verified by boroscope.
Fresh fuel, filled up twice with same issue
Adjusted pilot screws to 3 out each. I used idle drop procedure. I can find the idle drop when turning the pilot screws in, but when turning them out, the idles keep on rising to 4+ turns out with no end in sight. 3 seemed to be where the idle would come down smoothly, with the rpms being a 1500rpm. This was a quick fix to get the bike
in decent shape to run cuz I needed it, but now that the petcock is fixed I'll be redoing it.

Petcock Problem:

I have my original petcock and a spare that I bought off of eBay. The riding I've done with my startup issue has been with my original petcock (was using eBay one and had my original sent off with my carb clean, then when original came back I put it back on) and wondered maybe I should take a look at the original petcock that is currently on the bike. Vacuum tested the petcock hooking up a syringe and pulling. When under vacuum it pulls gas but does not hold a vacuum. While pulling on the syringe, I pinched the vacuum line to hold vacuum, but the the fuel stops. I'm guessing air is leaking in from somewhere which kills the vacuum. Also is leaking fuel.

I also tore down my spare petcock to clean it and use this one instead, the diaphragms were stuck inside. I don't have a rebuild kit, so I unstuck them and the diaphragms didn't have any holes or tears in them, so cleaned em and put it back together. This petcock pours fuel under vacuum, and will hold a vacuum when I pinch the vacuum line.

The bike fires up no problem now. Choke is usable now, and won't immediately kill the engine even if I just go out a small amount on it.

Video with completely cold engine with no choke:

Link to original page on YouTube.

Video With Bike Warmed Up:

Link to original page on YouTube.

Just finished readjusting idle mixture screws and went for a test ride. The idle drop procedure makes more sense now. Now when I turn the fuel mixture past a certain point, I can find the peak idle and back off of it a bit, in my case I set the idle mixture screws to Left - 3 turns, Right 3.5 turns.

My flatspot between 9k and 11k has also disappeared after fixing the petcock


Went for test ride and everything checks out so far.

Sad part: I just discovered a crack in my generator cover that's slowly leaking oil. Damn it...


Last futzed with by nocturncal; February 18th, 2020 at 06:46 PM.
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Old February 18th, 2020, 04:10 PM   #12
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Quote:
Originally Posted by jkv45 View Post
Have you shimmed the needles?

The carbs are usually lean in the mid-range, and shimming the needles richens it up and smooths-out that area. The Main Jet is typically plenty rich.

As ducatiman noted, take some time and get your idle mixture screw adjusted precisely. Make sure the engine is fully warm and the gas is fresh. That adjustment will change with weather, so know that you may need to make additional adjustments if the temp changes significantly.

Make sure your air filter is clean - and not over-oiled. Carbs synced and valves adjusted as well.
Carbs were just cleaned via ducatiman, no shimming, everything's stock except newly drilled idle mixture screws. I'll be doing more testing today to see if my idle mixture screws feel good - I'm at Left 3 and R 3.5 as of today. Swapping out my petcock for a spare I just cleaned helped everything, see above post. Old one would work initially under vacuum application but wouldn't continue to pour fuel with continuous vacuum application.
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Old February 18th, 2020, 05:57 PM   #13
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I had some wierd*ss symptoms similar to yours when I had leaking petcock.

1. it had difficulties starting in winter, even with full-choke. Finally started after 10-sec. or so of cranking.

2. took long time to warm up, then would die if I gave it throttle too quickly

3. strange thing here is if I reduced choke to halfway while simultaneously giving it throttle, it would rev up and let me take off.


Bizarre.. This was also petcock that would dribble gas into carbs while sitting. Before carbs refurbed by ducatiman, it actually hydrolocked my engine and snapped teeth off starter ring-gear!
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Old February 18th, 2020, 06:28 PM   #14
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Quote:
Originally Posted by DannoXYZ View Post
I had some wierd*ss symptoms similar to yours when I had leaking petcock.

1. it had difficulties starting in winter, even with full-choke. Finally started after 10-sec. or so of cranking.

2. took long time to warm up, then would die if I gave it throttle too quickly

3. strange thing here is if I reduced choke to halfway while simultaneously giving it throttle, it would rev up and let me take off.


Bizarre.. This was also petcock that would dribble gas into carbs while sitting. Before carbs refurbed by ducatiman, it actually hydrolocked my engine and snapped teeth off starter ring-gear!
Damn petcocks, who knew premature dribbling could cause so many problems...
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Old February 19th, 2020, 08:07 AM   #15
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Quote:
Originally Posted by nocturncal View Post
Found the problem:

Spark plugs gaps 0.7mm each, no rust in gas tank , verified by boroscope.
Fresh fuel, filled up twice with same issue
Adjusted pilot screws to 3 out each. I used idle drop procedure. I can find the idle drop when turning the pilot screws in, but when turning them out, the idles keep on rising to 4+ turns out with no end in sight. 3 seemed to be where the idle would come down smoothly, with the rpms being a 1500rpm. This was a quick fix to get the bike
in decent shape to run cuz I needed it, but now that the petcock is fixed I'll be redoing it.

(SNIP)
That is odd. Typically the RPMs will drop off as you go past the optimum adjustment - which usually isn't that far out.

Danno, ducatiman - thoughts?
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Old February 19th, 2020, 07:13 PM   #16
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Originally Posted by jkv45 View Post
That is odd. Typically the RPMs will drop off as you go past the optimum adjustment - which usually isn't that far out.

Danno, ducatiman - thoughts?

Yeah it was really weird.. fixing the petcock fixed the forever climbing idle as a turned the idle screws out and allowed the idle to drop past optimum adjustment. I'm curious to the reasons behind this as well
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