Part throttle hiccup

[BrianG]

OK... I'm stumped.

 

I have recently acquired a part throttle hiccup that is pretty pronounced and VERY annoying. It almost feels like a backfire, and it is ruining the quality of the MG ride!

 

It occurs while at very low throttle opening and is most pronounced between 2000-3000 RPM.

 

This, together with a very poor idle quality, motivated me to do a "tune-up" last week-end, where I found the valve lash way too tight (0.0015/0.002). Rectifying that smoothed the idle out fine, and elevated the idle speed to about 1500+ RPM. I set the idle speed down to 1000+ RPM and sync'd the throttle bodies. That looked good. I did not set the TPS, or ign. timing, figuring that these items would be unlikely to have changed since the last setting a year ago.

 

I'll check those 2 items this coming week-end, but I'm canvasing the Guzzisti here to see if there is any consensus on the origin of this irritating hiccup thing that I've never experienced before.

 

Any ideas?

[Guest Megarad]

Compression test both cylinders while the engine is hot.You may have a burnt valve.

 

Len

[Admin Jaap]

Sounds like the infamous cracked rubber intake hose... Check them out first before you start fiddling with settings.

[Guest ratchethack]

Brian, this sounds very much like a loose timing chain to me.

 

See thread here: http://www.v11lemans.com/forums/index.php?showtopic=11660#

 

The key words I pick up on in your post are "part throttle opening" and "2000-3000 RPM", which is EXACTLY the same symptoms I have of late. I have NO symptoms whatsoever under load and at higher RPMs. I have a new tensioner ready for installation.

 

One sure-fire check is to use a timing light to get a read on signal scatter, as discussed in the thread above.

 

Good luck.

[BrianG]

Compression test both cylinders while the engine is hot.You may have a burnt valve.

 

Len

 

My experience with valve sealing issues is that the "miss" is most pronounced as the lowest RPM (at idle) and that the miss is pretty rythmical due to the consistency of the problem's repetition cycle.

 

In this current situation, idle is rock steady.

 

Sounds like the infamous cracked rubber intake hose... Check them out first before you start fiddling with settings.

Are you taking about the rubber boot between the throttle body and the head spigot?

.....or the tubing section between the airbox and the throttle body?

 

The reason I ask is that I'm not all that impressed with the interface between the tubing section and the airbox.

[docc]

Jaap is referring to the intake boot between the throttle boby and the intake to the head. Sometimes they just come loose with no crack or split.

 

New plugs and TPS setting are a crucial part of the tune-up. Check your TPS at an idle of 1100 remembering that the tach probably reads fast. If it indicates '1000' then it's actually idling at 700. An indication of 1300-1400 will get your TPS, and the idle, set 'where the music is.'

[Dan M]

Sounds like a lean misfire to me. Check the boots like Jaap & others suggested. Also, if you still have your canister installed check those vacuum hoses.

What mods have you made?

[BrianG]

Brian, this sounds very much like a loose timing chain to me.

 

See thread here: http://www.v11lemans.com/forums/index.php?showtopic=11660#

 

The key words I pick up on in your post are "part throttle opening" and "2000-3000 RPM", which is EXACTLY the same symptoms I have of late. I have NO symptoms whatsoever under load and at higher RPMs. I have a new tensioner ready for installation.

 

One sure-fire check is to use a timing light to check for signal scatter, as discussed in the thread above.

 

Good luck.

 

I was hoping that you were asleep and would not see this thread and make this suggestion... it is indeed what I suspect as well. I have now got 20,ooo miles on the donk and guess that this is not totally unexpected.

 

My question in this regard is..... With an upcoming 2000 mile trip coming up in September, is this something that is critical to attend to before such a venture, or is this still a winter project?

 

PS... I am having some trouble checking the timing because I can't find the timing marks. Apparently some donut clocked the flywheel when he did the clutch R&R last winter....

[Guest ratchethack]

Brian, if a wonky tensioner is in fact your situation, I reckon this isn't as "critical" a problem as others and is often mis-diagnosed. Again -- since riding under load gives no symptoms, I reckon it's the kind of thing that could be (and often is) postponed indefinitely with no additional problems over and above the rideability symptoms you've undoubtedly become as tired of as I have lately. Mine's been getting progressively worse. If you're doing a long trip, however, I'd be apt to get it done before, simply because the off-throttle Herky Jerky is so godawful annoying mid-sweeper, and in many transition situations when the terrain is most interesting, where riding would otherwise be most enjoyable. . .

 

Again, I'm not speaking from a perspective of having DONE THIS already (ask Raz wot he thinks), but man, everything I know sure points straight to the tensioner, and the signal scatter pretty much confirms things.

 

BTW -- I didn't use timing marks on the flywheel to check scatter. All you need is a reference mark of some kind. I used wot looked like random marks on the flywheel left by who knows wot for both cylinders. . . If you try to use the teeth, by my experience, they "overlap" with the strobe and you can't get a useful read.

 

Good luck.

[BrianG]

Sounds like a lean misfire to me. Check the boots like Jaap & others suggested. Also, if you still have your canister installed check those vacuum hoses.

What mods have you made?

No carbon canister is present.

I have Mistral slip-ons and a drilled air box.

I have installed a PCIII that was tuned on a Dyno-jet dyno.

It ran well after this.

 

This issue began late last season...... 2 years after the mods.

 

New irridium BPR6EIX plugs were installed along with the valve adjustment las week-end, and that smoothed the idle right out and elevated the idle speed about 300RPM

[BrianG]

BTW -- I didn't use timing marks on the flywheel to check scatter. All you need is a reference mark of some kind. I used wot looked like random marks on the flywheel left by who knows wot for both cylinders. . . If you try to use the teeth, by my experience, they "overlap" with the strobe and you can't get a useful read.

 

Good luck.

OK, that seems doable......

 

In this particular ign system, how "stable" ought the spark signal to appear? How would one determine the acceptable threshold for scatter? Frankly, I have not bothered to time many pointless ign systems since I see little reasson for variation from initial settings.

[Guest ratchethack]

In this particular ign system, how "stable" ought the spark signal to appear? How would one determine the acceptable threshold for scatter?

Unknown, but per my posts on this, under throttle or at higher than 3K RPM, there is NO visible scatter on my strobe. It's as steady as a rock. At fully warmed <3K RPM, the reference point on the flywheel jumps erratically by 1 mm to as much as 6-7 mm. This is very clear. I'd asked wot's acceptable before, and since I got no response I reckon there's no hard range of acceptability, and/or it's relative to other state-of-tune parameters, and/or no one who happened to see my post knows (or cares). . .

Frankly, I have not bothered to time many pointless ign systems since I see little reasson for variation from initial settings.

Same boat I was in before Docc suggested using the timing light way back when to check for scatter. . .I'd always considered using a strobe on an EI motor, well. . . pointless!

[BrianG]

. . .I'd always considered using a strobe on an EI motor, well. . . pointless!

 

You, my friend, are on waivers!

[BrianG]

Very early on, I'd suspected the signal sensor and eliminated this from consideration.

 

OK... so how did you do this?

The big Q's still remaining unanswered for me are:

 

1. Is it possible, without too much mechanical histrionics and cost, to improve the "scatter" that I've directly measured above by replacing and/or modifying the tensioner? I've progressed in my understanding (with your guidance and confirmation, thanks again!) to the point where I believe that if I could reduce the erratic jumps of the strobe on the flywheel of up to ~7 mm I observed to half that or less, it would make a considerable improvement on the road, and be well worth considerable effort, if necessary.

 

2. If it IS possible to achieve the above, wot's the best way to go about achieving it?

 

I would venture that if Pete Roper is correct about the mechanics of chain wear (and I believe that he is correct) it would be my suggestion that a better tensioner will not resolve a worn chain.

 

This along the lines of hoping that more rear wheel adjustment would help a worn drive chain issue. It might seem to reslove the "slack" but it won't help the sprocket tracking.

 

If mis-engagement of the rollers in the teeth is a significant part of the issue of the instability causing the scatter, the right resolution is chain replacement.

[Guest ratchethack]

OK... so how did you do this?

My first inclination was to make sure the sensor hadn't become fouled with debris from the phase (Hall Effect) wheel. So out it came for inspection. The "aperture" was clean as a whistle.

 

My next inclination was to carefully observe under wot conditions the symptoms appeared. Since they NEVER appeared under engine load or above 3K RPM, regardless of ambient or relative engine temp (or any other variances on the road), I was confident in concluding there was ZERO probability that the sensor could be the point of origin, otherwise I'd have some expression of the symptoms under load and/or above 3K RPM.

 

I would venture that if Pete Roper is correct about the mechanics of chain wear (and I believe that he is correct) it would be my suggestion that a better tensioner will not resolve a worn chain.

Brian, we covered this in some depth in the other thread. I believe he's correct also. Pete suggested that the driven cam sprocket is so close to the drive sprocket that the few links separating them could virtually NEVER result in retarded timing due to a worn chain. I might add that neither would slightly retarded timing result in the observed symptoms. A LOOSE chain due to a wonky tensioner, however, very well COULD -- dare I say WOULD cause the chain to whip and jump erratically, shortening up the normal chain length between drive and driven sprockets by folding back on itself (as they will!), thereby effectively advancing the timing on an erratic basis, which would seem to explain the symptoms perfectly.